Krebs Cycle and Malate-Aspartate Shuttle Defects

Fumarase deficiency, 2-oxoglutarate dehydrogenase deficiency, malate-aspartate shuttle deficiencies (SLC25A12, MDH1, MDH2, GOT2), mitochondrial citrate carrier deficiency (SLC25A1), mitochondrial pyruvate carrier deficiency (MPC1), mitochondrial aconitase deficiency (ACO2), isocitrate dehydrogenase deficiencies and recessive complex II deficiency

Literature and regulatory information updated: 31 October 2026.

0. Initial Reference Table

The class brings together the inborn errors that interrupt the tricarboxylic acid (TCA, Krebs) cycle, the entry of pyruvate into the mitochondrion or the shuttles that transfer reducing equivalents and metabolites between the cytosol and the mitochondrial matrix. Orphanet groups them under code ORPHA:254749 (“Tricarboxylic acid cycle disorder”) and the European Reference Network for Hereditary Metabolic Disorders (MetabERN) includes them in the PM-MD subnetwork, dedicated to disorders of pyruvate metabolism, Krebs cycle defects, oxidative phosphorylation disorders and disorders of thiamine transport and metabolism [1, 2]. The first table identifies each entity; the second summarises the presentation, first-line tests and targeted treatment.

Abbreviations: AR, autosomal recessive; AD, autosomal dominant; OMIM, Online Mendelian Inheritance in Man; HLRCC, hereditary leiomyomatosis and renal cell carcinoma.

The data in the table summarise the following sections and the sources cited there [3–58]. The heterozygous forms with tumour predisposition (FH tumour syndrome and paragangliomas due to SDHx, MDH2, DLST and SLC25A11), pyruvate dehydrogenase and dihydrolipoamide dehydrogenase deficiency, pyruvate carboxylase deficiency, citrullinaemia type II and the mitochondrial DNA depletion syndromes appear here only insofar as they distinguish the class.

1. Introduction

1.1 Definition and delimitation of the class

The tricarboxylic acid cycle oxidises acetyl-CoA derived from pyruvate, fatty acids and amino acids and delivers reducing equivalents, in the form of NADH and FADH2, to the respiratory chain; it also feeds biosynthetic pathways such as those of aspartate, glutamate, cytosolic citrate and succinyl-CoA. Enzymatic defects of the cycle produce, together, two types of damage: energy deficiency due to interruption of flux and secondary inhibition of other enzymes by the metabolites accumulated upstream of the block, such as fumarate, succinate, 2-ketoglutarate and citrate [3]. The European reference textbook of metabolic diseases organises this group in a single chapter, with subsections for deficiency of the 2-ketoglutarate dehydrogenase complex (KDHC), succinyl-CoA ligase (SUCL), succinate dehydrogenase (SDH), fumarase, mitochondrial aconitase, isocitrate dehydrogenase, the components of the malate-aspartate shuttle (MAS), the mitochondrial citrate carrier and the mitochondrial pyruvate carrier (MPC) [3]. The paediatric approach of the same textbook classifies these defects among the defects of aerobic glucose oxidation that present with congenital lactic acidaemia, alongside pyruvate carrier, pyruvate carboxylase and pyruvate dehydrogenase complex deficiencies [59].

This article covers the class uniformly, according to the metabolic step affected: pyruvate entry (MPC1), cycle enzymes (aconitase 2, isocitrate dehydrogenase 3, 2-oxoglutarate dehydrogenase complex, succinyl-CoA ligase, succinate dehydrogenase, fumarase and mitochondrial malate dehydrogenase), MAS components (aspartate-glutamate carrier 1, cytosolic malate dehydrogenase and mitochondrial aspartate aminotransferase) and the mitochondrial citrate carrier. Each related entity appears here in summary form: FH tumour syndrome, hereditary paragangliomas and phaeochromocytomas, pyruvate dehydrogenase complex and dihydrolipoamide dehydrogenase deficiency (DLD, the E3 subunit shared with KDHC), pyruvate carboxylase deficiency, citrullinaemia type II due to citrin deficiency, the mitochondrial DNA depletion syndromes due to SUCLA2 and SUCLG1, optic atrophy due to ACO2, congenital myasthenic syndrome due to SLC25A1, the retinopathies due to IDH3A and IDH3B, Leigh syndrome and the serine synthesis deficiencies.

1.2 Historical overview

The first human disease of the cycle was described in 1982 by Kohlschütter and colleagues in two siblings born to consanguineous Tunisian parents, with slowly progressive extrapyramidal neurodegenerative disease, marked excretion of 2-oxoglutaric acid and 2-oxoglutarate dehydrogenase activity in fibroblasts of about 25% of control [60]. In 1986, Zinn, Kerr and Hoppel described fumarase deficiency as a “new cause of mitochondrial encephalomyopathy” in an infant with failure to thrive, hypotonia, cerebral atrophy, lactic and pyruvic acidaemia and fumaric aciduria, who died at 8 months; fumarase activity was practically absent in liver and muscle mitochondria and the deficiency involved both the mitochondrial and cytosolic forms [61]. In 1992, Bonnefont and colleagues reported three siblings with α-ketoglutarate dehydrogenase deficiency presenting as congenital lactic acidosis, with death at around 30 months, and proposed low molar ketone body ratios as an indication of cycle dysfunction in the newborn with lactic acidosis [62]. Kerrigan and colleagues extended the phenotype of fumarase deficiency in 2000 to brain malformations (diffuse polymicrogyria, open opercula), facial dysmorphism and neonatal polycythaemia in a consanguineous family from the southwestern United States [63].

The molecular era brought new genes and entities. In 2003, Brivet and colleagues demonstrated in permeabilised fibroblasts a defect of mitochondrial pyruvate import in a newborn girl with lactic acidosis with a normal lactate/pyruvate ratio and made the diagnosis in a fetus at risk [64]; the MPC1 gene was identified in 2012, when the mitochondrial pyruvate carrier was characterised in yeast, Drosophila and humans [65]. There followed biallelic IDH3B variants in nonsyndromic retinitis pigmentosa (2008) [66], deficiency of the aspartate-glutamate carrier AGC1 (aralar, SLC25A12) with global cerebral hypomyelination (2009) [67], the assembly factor SDHAF1 in leukoencephalopathy with SDH deficiency (2009) [68], infantile cerebellar-retinal degeneration due to ACO2 (2012) [69], deficiency of the mitochondrial citrate carrier SLC25A1 as the cause of combined D-2- and L-2-hydroxyglutaric aciduria, identified in 12 of 12 patients (2013) [70], MDH2 deficiency (2017) [71], MDH1 and GOT2 deficiencies (2019) [72, 73] and, finally, molecular confirmation of 2-oxoglutarate dehydrogenase deficiency due to biallelic OGDH variants (2021 and 2023) [74, 75]. Until 2020, the diagnosis of KDHC deficiency depended solely on the enzyme assay in fibroblasts [3].

1.3 Nomenclature and reclassifications

Current OMIM nomenclature uses gene-based or numbered phenotype designations: the neurological MAS defects have become developmental and epileptic encephalopathies (DEE) — DEE39 with leukodystrophy (SLC25A12), DEE51 (MDH2), DEE82 (GOT2) and DEE88 (MDH1, with an association that is still provisional) [44–47]. Recessive complex II deficiency is subdivided into four nuclear types by gene: type 1 (SDHA), type 2 (SDHAF1), type 3 (SDHD) and type 4 (SDHB) [39–42]. 2-oxoglutarate dehydrogenase deficiency was given the abbreviation OGDHD and fumaric aciduria the abbreviation FMRD [36, 43]. Combined D-2- and L-2-hydroxyglutaric aciduria was described in 2000 as the “third biochemical variant” of the 2-hydroxyglutaric acidurias and only linked to the citrate carrier in 2013 [13, 57]; the same gene causes, at the mild end, presynaptic congenital myasthenic syndrome type 23 [49, 76].

1.4 Zygosity and two diseases per gene

Several genes of the class cause different diseases depending on zygosity, and reading the genetic report must start there. In FH, biallelic loss of function causes fumarase deficiency, a recessive encephalopathy, whereas the heterozygous germline variant, with somatic loss of the second allele in the tumour, causes FH tumour syndrome (hereditary leiomyomatosis and renal cell carcinoma, HLRCC) [4, 77]. In SDHA, biallelic disease is a Leigh-type mitochondrial disease, whereas heterozygosity predisposes to paragangliomas, gastrointestinal stromal tumours and, in some patients, to a dominant late-onset neurodegeneration with ataxia and optic atrophy [78, 79]. In ACO2, optic atrophy 9 has recessive and dominant cases [16, 32]. In 2026 the first patients with monoallelic OGDH variants and late-onset peripheral neuropathy with ataxia and optic atrophy were described [80]. In MDH2, DLST and SLC25A11, a heterozygous association with paragangliomas has been proposed (MDH2 in a family with multiple malignant paragangliomas) but has limited or modest evidence, and DLST has no established recessive disease [81–84].

1.5 2-Hydroxyglutaric acidurias: history and classification

2-hydroxyglutarate (2-HG) is a five-carbon dicarboxylic acid with one chiral carbon, which exists as the D (R) and L (S) enantiomers; both derive from mitochondrial 2-oxoglutarate (2-ketoglutarate), an intermediate of the Krebs cycle, and both are normally reconverted to it by flavin adenine dinucleotide (FAD)-dependent dehydrogenases [54]. In 1980, two simultaneous publications in the Journal of Inherited Metabolic Disease described D-2-hydroxyglutaric aciduria (Chalmers and colleagues) and L-2-hydroxyglutaric aciduria (Duran and colleagues, in a 5-year-old Moroccan boy with psychomotor delay); Barth and colleagues defined in 1992–1993 the usual phenotype of the L form, with slowly progressive delay and cerebellar signs from the end of the first year, and Muntau and colleagues described the combined D,L form in 2000 [54, 58]. The genes were identified in 2004: D2HGDH, by Achouri and colleagues, and L2HGDH, independently by Rzem and colleagues and by Topçu and colleagues, who called it “duranin” in honour of the author of the first description [54, 58]. In 2010, the demonstration that about half of the patients with D-2-hydroxyglutaric aciduria had neither D2HGDH variants nor enzyme deficiency led to the division between type I (D-2-hydroxyglutarate dehydrogenase, D-2-HGDH, deficiency) and an “idiopathic” form with higher D-2-HG; in the same year, germline gain-of-function IDH2 variants at residue Arg140 explained that form, now called type II [85, 86]. The combined form was attributed to SLC25A1 in 2013 and is covered in the sections on the citrate carrier [70].

Orphanet brings these entities together in the group ORPHA:19 (“2-hydroxyglutaric aciduria”): D-2-hydroxyglutaric aciduria (ORPHA:79315, types I and II), L-2-hydroxyglutaric aciduria (ORPHA:79314) and D,L-2-hydroxyglutaric aciduria (ORPHA:356978); metaphyseal chondromatosis with D-2-hydroxyglutaric aciduria (ORPHA:99646; OMIM 614875), caused by mosaic somatic IDH1 variants, is a skeletal dysplasia of the enchondromatosis group [21, 29, 53, 54, 87, 88]. The inclusion of these diseases in the class is justified by the common metabolite: 2-HG is formed from 2-oxoglutarate by side reactions of malate dehydrogenase (L form) and of hydroxyacid-oxoacid transhydrogenase (D form) or, in type II, by a new activity of mitochondrial isocitrate dehydrogenase; L-2-hydroxyglutaric aciduria is the classic example of a “metabolite repair disease” [54, 57].

2. Molecular Genetics and Pathophysiology

2.1 Common biochemistry: the cycle, the shuttles and the consequences of the block

The inner mitochondrial membrane is impermeable to NADH. The MAS solves this problem by transporting reducing equivalents in the form of malate and regenerating NAD+ in the cytosol; it consists of six proteins — cytosolic (MDH1) and mitochondrial (MDH2) malate dehydrogenases, cytosolic (GOT1) and mitochondrial (GOT2) aspartate aminotransferases, the malate-2-oxoglutarate carrier (OGC, SLC25A11) and the two isoforms of the aspartate-glutamate carrier, AGC1 (aralar, SLC25A12, mainly in the brain) and AGC2 (citrin, SLC25A13, mainly in the liver) [11]. By keeping the cytosolic NADH/NAD+ ratio low, the MAS sustains glycolysis and de novo serine synthesis, whose first step is catalysed by the NAD+-dependent 3-phosphoglycerate dehydrogenase [11, 73]. GOT2 is also the main intracellular source of aspartate, a precursor of purines and pyrimidines and a substrate of argininosuccinate synthetase in the urea cycle [12]. In the brain, the MAS is coupled to the metabolism of N-acetylaspartate (NAA): neurons synthesise NAA from aspartate and oligodendrocytes hydrolyse it, recovering acetate for myelin synthesis [57].

The biochemical signatures of the class follow from these links. Cycle blocks accumulate the substrate of the affected enzyme and the upstream intermediates (fumarate, succinate, 2-ketoglutarate, citrate) and reduce energy production; lactate rises because of the lack of mitochondrial reoxidation of NADH [3]. In MAS defects, accumulated cytosolic NADH inhibits serine synthesis and diverts pyruvate to lactate; the lack of mitochondrial aspartate compromises argininosuccinate synthesis, with accumulation of citrulline and ammonia; the lack of neuronal aspartate and NAA contributes to hypomyelination [3, 11, 73]. The glycerol-3-phosphate shuttle, another route for NADH reoxidation, appears to act as compensation, which explains the elevated glycerol-3-phosphate in dried blood spots in MDH1 and GOT2 deficiencies and on brain spectroscopy in AGC1 deficiency [11, 12, 72]. Fumarate accumulation also has an oncogenic effect: it competitively inhibits 2-oxoglutarate-dependent dioxygenases that regulate hypoxia-inducible factor (HIF), activating pseudohypoxia pathways; the same mechanism, with succinate, operates in SDHx tumours [4, 89].

2.2 Mitochondrial pyruvate carrier (MPC1, MPC2)

The MPC is a hetero-oligomer of two obligatory subunits, MPC1 and MPC2, which stabilise each other and symport pyruvate with protons across the inner membrane; the metabolic derangement is that of pyruvate dehydrogenase deficiency, with elevated lactate and pyruvate and a normal lactate/pyruvate ratio [3, 7]. Pathogenic MPC1 variants destabilise the complex or abolish transport, and patients' fibroblasts have less MPC1 and MPC2 [90]. In digitonin-permeabilised fibroblasts, oxidation of labelled pyruvate to acetylcarnitine, cycle intermediates and CO2 is severely reduced, with normal malate decarboxylation and normal pyruvate dehydrogenase activity, a behaviour identical to that of normal cells treated with the specific inhibitor α-cyano-4-hydroxycinnamate [64]. Until 2022 no human disease had been attributed to MPC2 [11], and the gene's OMIM entry still has no associated phenotype [91].

2.3 Mitochondrial aconitase (ACO2)

Aconitase 2, encoded at 22q13.2, converts citrate to isocitrate via cis-aconitate and also participates in the maintenance of mitochondrial DNA; in deficient cells respiratory capacity, cellular ATP, mtDNA copy number and mtDNA transcription are reduced [17]. Biallelic variants cause a spectrum ranging from infantile cerebellar-retinal degeneration (ICRD) to isolated late-onset optic atrophy, without a consistent correlation between residual activity and severity [3]. Plasma metabolomics in eight patients showed a signature with decreased cis-aconitate, isocitrate, 2-ketoglutarate, phosphoenolpyruvate and hydroxybutyrate [3]. In European cohorts of unresolved hereditary optic neuropathy, 50 of 61 cases with ACO2 variants were monoallelic, which made the gene the third cause of autosomal optic neuropathy, after OPA1 and WFS1; the fibroblasts have less ACO2, lower activity and reduced respiration with citrate and pyruvate, restored by the addition of other cycle intermediates [8, 16].

2.4 Isocitrate dehydrogenases (IDH3A, IDH3B, IDH2)

NAD+-dependent isocitrate dehydrogenase (IDH3) is the cycle form. Biallelic variants in IDH3A and IDH3B cause nonsyndromic retinitis pigmentosa; in IDH3B, patients homozygous for loss-of-function variants had only retinal disease, despite a marked reduction in NAD-IDH activity [66, 92]. In the IDH3A series, seven patients from four families had typical retinitis pigmentosa or early-onset bilateral macular pseudocoloboma [93]. A single patient with the homozygous IDH3A variant p.Pro304His (published designation) had severe encephalopathy, hypotonia and dysautonomia [3]. The NADP+-dependent isocitrate dehydrogenases (cytosolic IDH1 and mitochondrial IDH2) have another role: somatic point variants in gliomas, acute myeloid leukaemia and cholangiocarcinoma confer a new activity, the conversion of 2-ketoglutarate to D-2-hydroxyglutarate (D-2-HG), an oncometabolite; the heterozygous germline variant IDH2 c.419G>A p.(Arg140Gln), present in 15 patients and de novo in eight of nine parent pairs, causes D-2-hydroxyglutaric aciduria type II, with no cancer described in these patients [3, 94].

2.5 2-Oxoglutarate dehydrogenase complex (OGDH, DLST, DLD, OGDHL, MRPS36)

The KDHC is a 2-keto acid dehydrogenase analogous to pyruvate dehydrogenase and branched-chain keto acid dehydrogenase. It oxidises 2-ketoglutarate to succinyl-CoA with production of NADH. The E1 subunit (2-oxoglutarate dehydrogenase, OGDH, 7p13) is substrate-specific, uses thiamine pyrophosphate (TPP) and, unlike the E1 of pyruvate dehydrogenase, is not regulated by phosphorylation; E2 (dihydrolipoyl succinyltransferase, DLST) has covalently bound lipoic acid; E3 (DLD) is shared with pyruvate dehydrogenase [3, 75]. Consequently, low KDHC activity in fibroblasts localises the block to the complex but does not identify OGDH: DLD deficiency (OMIM 246900) includes KDHC deficiency as part of its phenotype [9, 75]. The variants described in OGDH are concentrated in or near the TPP-binding domain; the gene is highly constrained (missense z-score 4.76; pLI 1). p.(Ser297Tyr) behaves as a severe loss of function in Drosophila and increases protein turnover in fibroblasts, p.(Pro189Leu) (published protein designation) is hypomorphic, c.935G>A causes loss of exon 7 and p.(Asn320Ser) reduces protein and activity in fibroblasts; total loss of OGDH is probably incompatible with life [74, 75].

In DLST there is no established recessive disease: OMIM associates only phaeochromocytoma/paraganglioma syndrome type 7 (PPGL7), autosomal dominant, with the gene, and no GenCC submission assigns a recessive 2-oxoglutarate dehydrogenase deficiency to DLST [83, 95]. OGDHL, a brain-enriched isoform of 2-oxoglutarate dehydrogenase, has been associated in homozygous or compound heterozygous form with Yoon-Bellen neurodevelopmental syndrome (YOBELN, OMIM 619701), with a very variable phenotype; a cohort of 14 individuals published in 2023 found in most a second likely pathogenic variant in another gene and called into question the existence of the syndrome as a monogenic disease [96–98]. MRPS36 encodes the E4 subunit of the complex; three patients with biallelic variants had Leigh syndrome with reduced KDHC activity [99].

2.6 Succinyl-CoA ligase (SUCLA2, SUCLG1, SUCLG2)

SUCL converts succinyl-CoA to succinate with substrate-level phosphorylation (GTP or ATP); it has an α subunit (SUCLG1) and two alternative β subunits, SUCLA2 (ADP-forming) and SUCLG2 (GDP-forming). Associated with nucleoside diphosphate kinase, it participates in mtDNA maintenance, which explains the mtDNA depletion; accumulation of succinyl-CoA raises methylmalonic acid and succinylcarnitine [3, 100]. The Faroese founder variant SUCLA2 NM_003850.3:c.534+1G>A causes skipping of exon 4 [5, 100].

2.7 Succinate dehydrogenase (complex II)

SDH is the only respiratory chain complex with all its subunits encoded in the nucleus: SDHA (flavoprotein that oxidises succinate to fumarate), SDHB (iron-sulphur protein that transfers electrons to ubiquinone), SDHC and SDHD (membrane anchors), with the assembly factors SDHAF1 and SDHAF2; because it participates in both the cycle and the respiratory chain, its deficiency more closely resembles the oxidative phosphorylation disorders [3, 101]. A 2020 compendium described 61 patients with isolated complex II deficiency and 32 pathogenic variants in SDHA, SDHB, SDHD and SDHAF1; SDHA is the most common cause, followed by SDHAF1; SDHB and SDHD then accounted for ten and two patients [3, 15]. No metabolic presentations have been described with SDHC or SDHAF2 [3]. The myopathy with combined SDH and aconitase deficiency described in Swedish patients is due to a defect in iron-sulphur clusters (ISCU) and not to SDH [3, 102].

2.8 Fumarase (FH)

The FH gene (1q43) encodes, through translation from alternative initiation codons, a mitochondrial isoform, which is part of the cycle, and a cytosolic one, which metabolises the fumarate produced in the urea cycle, the purine nucleotide cycle and arginine synthesis; the active enzyme is a homotetramer [4, 77]. The deficiency affects both isoenzymes, fumarate accumulates inside and outside cells and, in excess, is toxic to development [4, 61]. Pathogenic variants affect the structural integrity of the enzyme core or the active site and are concentrated at the C-terminus; complete loss of activity is probably incompatible with life [4]. The most frequent variant in fumarase deficiency, NM_000143.4:c.1431_1433dup p.(Lys477dup), accounts for about 30% of pathogenic alleles and appears in the literature under various historical designations (1302insAAA, 435insAAA, 435insK, 1433insAAA, insK477) [4, 103]. In a 2026 English series, the variant c.410C>T p.(Pro137Leu) accounted for 44% of mutant alleles [103, 104]. The GeneReviews text records the doubt as to whether certain variants in the biallelic state, such as c.521C>G p.(Pro174Arg) and c.923C>G p.(Ala308Gly), might cause the deficiency without the tumour risk of FH tumour syndrome [4]; the English series concluded that all variants found in patients with fumarase deficiency should be assessed independently for association with HLRCC [104]. The interpretation of variants of uncertain significance (VUS) has new functional tools: the p.Lys414Glu variant proved to be completely inactive, and a Caenorhabditis elegans model allowed 25% of 20 missense VUS to be reclassified as pathogenic [105, 106].

2.9 Mitochondrial malate dehydrogenase (MDH2)

MDH2 converts malate to oxaloacetate and participates simultaneously in the cycle and the MAS [11]. In patients' fibroblasts, MDH2 protein was apparently absent and activity was almost nil, with accumulation of malate and fumarate; lentiviral complementation with normal cDNA restored the protein and activity, and the three missense variants showed a pathogenic effect in yeast [71]. The variant NM_005918.4:c.398C>T p.(Pro133Leu) was present in all four known patients and appeared seven times in heterozygous form in about 120,000 ExAC alleles [11, 71]. MDH activity was also reduced in fibroblasts from the heterozygous parents [71].

2.10 Malate-aspartate shuttle components (SLC25A12, MDH1, GOT2)

The first variant described in SLC25A12 abolished the activity of the recombinant carrier and p.(Arg353Gln) reduced it to about 15% [67, 107]. In the 2022 series, p.(Asp540Asn), which affects the PX[D/E]XX[K/R] motif common to all mitochondrial carriers, caused complete loss of transport in liposomes, and the combination of a frameshift variant with a synonymous variant that alters splicing eliminated the protein in fibroblasts [11]. Respiratory chain assays in muscle may be normal [107]. In MDH1, the only variant described was published as NM_001199111.1:c.413C>T p.(Ala138Val); on the MANE transcript it corresponds to NM_005917.4:c.359C>T p.(Ala120Val), with the same genomic position NC_000002.12:g.63597558C>T, and it greatly reduces protein expression [11, 72, 103]. In GOT2 (NM_002080.4), all patients have residual activity, probably necessary for viability, and Got2-null mice are not viable [73]; the p.(Arg262Gly) variant recurs in unrelated Egyptian families [12]. In fibroblasts, the residual activity described was 8–21% of controls and, in a patient homozygous for p.(His369Pro), 27% in three independent experiments; this variant, absent from gnomAD v4.1, was first classified as of uncertain significance and became likely pathogenic on the basis of the enzyme assay [103, 108]. GOT2 depends on pyridoxal phosphate (PLP), the coactivator of all transaminases, which may stimulate residual activity [73].

2.11 Mitochondrial citrate carrier (SLC25A1)

SLC25A1 (22q11.21, in the 22q11.2 deletion region) encodes a 311-amino-acid carrier that exchanges mitochondrial citrate and isocitrate for cytosolic malate [13, 94]. When the exchange fails, the intermediates downstream of isocitrate, mainly 2-ketoglutarate, accumulate in the mitochondrion and feed the formation of D-2-HG and L-2-HG; in the cytosol, citrate depletion reduces NADPH production and fatty acid and sterol synthesis [57]. Transfection of deficient fibroblasts with the normal gene restored citrate efflux and lowered intracellular 2-HG, which confirms causality [13]. A functional assay applied to 17 missense variants from 26 patients showed residual activities from almost 0 to 85%; with a cut-off of 25%, patients with two severe alleles (truncating or missense below 25%) had a severe phenotype with death before 1 year or intensive care, and patients with two alleles of higher activity had milder forms, with exceptions [13]. The recurrent variant p.(Arg247Gln), with 52% activity, causes congenital myasthenic syndrome type 23; haplotype analysis indicates recurrent mutation and not a founder effect [13, 109]. A patient with a 22q11.2 deletion who has a pathogenic variant on the remaining allele may become functionally biallelic [10].

2.12 Normalised reference variants

The following variants were normalised with VariantValidator (GRCh38, MANE transcript) from the original description [103].

The SLC25A1 variant c.821C>T predicts, on the transcript, the substitution p.(Ala274Val), but the dominant effect is a splicing defect (r.820_821del, p.(Ala274Ilefs*24)), a reminder that prediction from the genomic sequence does not replace RNA studies [13, 103, 116].

2.13 2-Hydroxyglutaric acidurias (D2HGDH, IDH2, L2HGDH)

D2HGDH (2q37.3; MANE transcript NM_152783.5) encodes D-2-HGDH, a FAD-dependent mitochondrial enzyme homologous to D-lactate dehydrogenase, which oxidises D-2-HG to 2-oxoglutarate and probably transfers the electrons to electron transfer flavoprotein (ETF); it is highly active in liver and kidney [54, 94]. D-2-HG is formed mainly by hydroxyacid-oxoacid transhydrogenase (HOT), which converts γ-hydroxybutyrate to succinic semialdehyde with stoichiometric production of D-2-HG from 2-oxoglutarate, and has no known physiological function [54]. By 2012, 26 patients with type I and 31 variants distributed across the coding sequence had been described, 11 truncating and 20 missense at conserved residues; all patients with variants had reduced enzyme activity (fibroblasts 0–41 pmol/h/mg protein; controls 247–665) and no patient with normal activity had variants [54, 85]. Normalised examples include NM_152783.5:c.1315A>G p.(Asn439Asp), homozygous in a Palestinian family, with 13% of control activity on expression, and c.458T>C p.(Met153Thr), homozygous in the only type I patient with hypertrophic cardiomyopathy [54, 94, 103]. Inheritance is autosomal recessive, with frequent consanguinity [54].

IDH2 (15q26.1; NM_002168.4) encodes NADP+-dependent mitochondrial isocitrate dehydrogenase, which converts isocitrate to 2-oxoglutarate and contributes to mitochondrial redox balance through NADPH production [57]. Heterozygous variants at residue Arg140 — c.419G>A p.(Arg140Gln) in 14 of the 15 initial patients and c.418C>G p.(Arg140Gly) in one — confer a neomorphic activity: the mutant enzyme reduces 2-oxoglutarate to D-2-HG with NADPH [54, 86, 103]. In patients' lymphoblasts, the rate of this reaction was about eight times that of controls (14,400 vs 1800 pmol/h/mg protein), with normal D-2-HGDH; production exceeds clearance capacity, which explains D-2-HG concentrations two to eight times higher than in type I [54]. The mechanism is that of somatic IDH1 and IDH2 variants in gliomas and acute myeloid leukaemia, but no neoplasms have been described in patients with D-2-hydroxyglutaric aciduria type II [54, 57]. D-2-HG competitively inhibits 2-oxoglutarate-dependent dioxygenases, with altered histone and DNA methylation, and in experimental models it inhibited creatine kinase and complexes IV and V and activated N-methyl-D-aspartate (NMDA) receptors [54, 94].

L2HGDH (14q21.3; NM_024884.3), with ten exons spanning more than 75 kb, encodes the FAD-linked mitochondrial L-2-hydroxyglutarate dehydrogenase (L-2-HGDH) [58]. L-2-HG is formed by a side reaction of L-malate dehydrogenase on 2-oxoglutarate, a structural homologue of oxaloacetate, and L-2-HGDH reconverts it to 2-oxoglutarate [54, 57]. The lack of mitochondrial 2-oxoglutarate, required for the first step of lysine oxidation (saccharopine synthesis), explains the increased lysine in plasma and cerebrospinal fluid (CSF) [54, 57]. The 2010 variant update brought together 106 patients from 83 families, with 35 novel and 35 previously described variants; in 82 families the variants were found by sequencing and/or multiplex ligation-dependent probe amplification (MLPA), in one case MLPA was needed for the second allele and in another RNA studies with deep intronic sequencing were required [55]. In 2012, the LOVD database held 86 unique variants in 164 individuals, mostly missense at invariant amino acids [54]. L-2-HGDH activity was absent in 15 of 15 patients studied, except in one compound heterozygote for two missense variants with 20% activity [54]. There is no clear genotype–phenotype correlation; patients with two missense variants excreted on average 25–50% less L-2-HG than carriers of two presumed null variants (1431 vs 1916 mmol/mol creatinine), with no clinical difference demonstrated [54, 55, 112].

3. Clinical Spectrum and Manifestations by System

3.1 Common description and table of types

The neurological defects of the class share a core: encephalopathy with onset in the neonatal period or early infancy, hypotonia that may progress to spasticity, severe global developmental delay, frequently refractory epilepsy, microcephaly (often progressive), white matter abnormalities and lactate that is raised at least intermittently [3, 11]. The table distinguishes the types by what separates them in practice.

Sources for the table: [3, 4, 7, 11–14, 16, 31, 32, 39, 54, 55, 57, 66, 69, 71, 72, 75, 93, 94, 109, 117].

3.2 Fumarase deficiency

GeneReviews counts fewer than 100 patients, of various origins [4]. The classic form is a severe neonatal or infantile encephalopathy with poor feeding, failure to thrive, growth retardation, hypotonia (sometimes with no head control at all), lethargy that may progress to stupor and coma, and seizures; some newborns are symptomatic immediately after birth and others are discharged without problems and present between the first week and the first month [4]. In the published series, the frequency of the manifestations is as follows [4]:

The most common imaging finding is a small brain, owing to underdevelopment: cerebral atrophy in 73% and ventriculomegaly in 82% of the patients reviewed by Allegri and colleagues [4]. Because the small brain coexists with enlarged cerebrospinal fluid (CSF) spaces, head circumference may be normal; in Kerrigan's series, two patients received a ventricular shunt for suspected hydrocephalus, with collapse of the CSF spaces and secondary microcephaly, without clinical improvement [4]. In addition there is delayed myelination or hypomyelination, incomplete closure of the Sylvian opercula, hypoplastic brainstem, thin or absent corpus callosum and, in one report, hyperintense lesions in the caudate and thalamus with raised lactate on spectroscopy; diffuse bilateral polymicrogyria was universal in the eight affected members of one family and has been described in three further unrelated patients [4, 63]. The dysmorphism is considered cardinal, although perhaps not universal: in more than half, depressed nasal bridge, frontal bossing and widely spaced eyes; in fewer than half, cleft alae nasi or anteverted nares, ear anomalies and narrow forehead; in a family with eight affected members, seven had “relative macrocephaly” due to atrophy with mild communicating hydrocephalus [4]. Spasticity develops over time in severely affected patients, with contractures and scoliosis; there are reports of optic nerve hypoplasia, recurrent neutropenia, recurrent vomiting with hepatosplenomegaly and pancreatitis [4].

Pregnancy complications (polyhydramnios, oligohydramnios, intrauterine growth restriction, maternal intrahepatic cholestasis and pre-eclampsia) occurred in 23% of pregnancies and preterm delivery, usually at 33–36 weeks, in 30%; fetal ultrasound may show enlarged ventricles, agenesis of the corpus callosum or a Dandy-Walker cyst [4].

The spectrum extends to mild forms. There are children who walk and attend special education despite bilateral polymicrogyria [4], and the retrospective series from three centres in northern England, published in 2026, brought together ten patients (seven female) from seven families with a median age at diagnosis of 5 years (1 day to 16 years): all had developmental delay and learning difficulties of variable severity, five had neutropenia, four had abnormal neuroimaging, and three homozygous for p.(Lys477dup) had only mild intellectual disability [104]. A Portuguese case, followed in Coimbra, began at 3 years of age with asymmetric dystonia of the lower limbs in a child who was already walking and talking; the dystonia became generalised, versive seizures and cognitive regression appeared and, at 15 years, parkinsonism with akinesia and hypomimia, spasmodic dysphonia, head drop and inability to walk independently, with MRI showing only a thin corpus callosum and mild atrophy [110]. Acute metabolic crises with hypoglycaemia, ketosis, hyperammonaemia or acidosis are rare [4, 89].

3.3 Malate-aspartate shuttle deficiencies

AGC1 deficiency (SLC25A12, DEE39). In the 2022 series, 14 patients from 12 families presented at a median age of 2 weeks (neonatal period to 7 months); the phenotype was homogeneous, with early epilepsy in 13/13 and severe to profound global impairment in 12/12 [11]. There was hypotonia or weakness in 10/13 (three progressed to spasticity), regression or stagnation at seizure onset in 9/9, absent speech in 12/12, hyperkinetic movement disorder in 4/9, microcephaly in 7/10 (secondary in four) and failure to thrive or feeding difficulties in 5/7; the best motor milestone was weight bearing or sitting unsupported; one patient died at 7 years [11]. Epilepsy began between 40 days and 10 months (median 7 months) and was refractory in 7/11; seizures were focal, with apnoea, tonic or clonic components, odd laughter and status epilepticus in 4/10 [11]. Serum lactate was raised at least intermittently in 9/12 (maximum 7.2 mmol/L) and CSF lactate in 3/4 [11]. All untreated imaging studies showed progressive loss of brain volume and extensive abnormality of the supratentorial white matter with a thin corpus callosum, interpreted as hypomyelination secondary to the neuronal disease; spectroscopy showed reduced NAA in all studies and increased myo-inositol [11]. ClinGen also records optic neuropathy [107]. OMIM notes that serial imaging in one patient suggests a leukoaxonopathy-type mechanism [45].

MDH2 deficiency (DEE51). The four published patients presented between birth and 5 months with hypotonia, severe constipation, febrile seizures or macrosomia with macrocephaly and supernumerary nipples; all developed epilepsy in the first months, refractory in three; there was dystonia, dyskinesia, pyramidal signs, strabismus and, in one patient, retinitis pigmentosa at 4 years and, in another, a metabolic infarct of the lentiform nucleus [11, 71]. Plasma lactate ranged between 2.8 and 5.7 mmol/L, with a lactate/pyruvate ratio between 20 and 63 (normal below 18), and CSF lactate was raised; urinary malate was increased (56 µmol/mmol creatinine in one patient, normal below 7) and fumarate in some measurements, with normal succinate [11, 71]. A boy died at 20 months with decompensation (pH 6.97, lactate 9.0 mmol/L, severe pulmonary hypertension) [71]. MRI shows cerebral atrophy with anterior predominance, cerebellar atrophy and delayed myelination, with a lactate peak on spectroscopy [11].

MDH1 deficiency (DEE88). Two cousins from a consanguineous family had global developmental delay, epilepsy (with spasms and hypsarrhythmia in one), microcephaly (progressive in one), strabismus and dysmorphism (plagiocephaly, bulbous nose, deep-set eyes, frontal bossing, micrognathia); MRI showed abnormality of the corpus callosum in both and, in one of them, reduced posterior myelin, cerebral atrophy and hypoplasia of the pons and vermis [11, 72]. Routine metabolic tests were normal, and OMIM keeps the association as provisional [46, 72].

GOT2 deficiency (DEE82). The 2025 cohort brought together 16 patients (eight of each sex) from ten families, followed up to 3.5–30 years of age (median 10.2 years), of Egyptian, Iranian, Syrian, Pakistani, Yemeni, Turkish and Romanian origin [12, 73]. All had severe or profound intellectual disability and hypotonia in infancy; 94% developed spasticity, 93% progressive microcephaly and 88% behavioural abnormalities; the course was progressive in all assessable patients, with loss of skills in 13; only one patient achieved independent walking [12]. Seizures occurred in 94%, began in the first year in 13/14 (median 5 months) and were of varied types [12]. In addition there were incontinence, sleep disturbances, contractures, scoliosis, dystonia, ataxia, frequent infections and a dysmorphism with narrow forehead, broad nasal tip and high or pointed chin [12, 73]. In the original patients, lactate ranged between 3.0 and 5.7 mmol/L and ammonia between 110 and 143 µmol/L; in the most severe patient, at 14 months, plasma serine was 47 µmol/L (reference 70–294) and citrulline 89 µmol/L (reference 7–55) [73]. MRI shows cerebral atrophy, hypoplastic vermis, thin corpus callosum and, in the extended cohort, two severity groups according to volume loss and myelination defect, with relatively preserved basal ganglia, cerebellum and brainstem [12, 73]. In each of the three original families one child died in utero, at birth or very early [73]. In the other three original patients, plasma serine (114–171 µmol/L) and citrulline were normal at assessment at 4–10 years, two of them sisters who never received supplements, whereas lactate (3.0–3.6 mmol/L) and ammonia (70–79 µmol/L; reference 11–32) remained raised [73]. A Sudanese patient described in 2026, born to consanguineous parents, had abnormal visual behaviour at 3 months, prolonged focal and generalised seizures from 7 months, less frequent than in the original patients, and regression at 23 months, with loss of supported walking and vocalisation; MRI showed delayed myelination, symmetrical juxtacortical T2 hyperintensity of the white matter (a finding not previously described in this disease), ventriculomegaly and a thin corpus callosum, without vermis hypoplasia or cerebellar atrophy, and lactate, ammonia, plasma amino acids and urinary organic acids were normal [108].

3.4 Mitochondrial citrate carrier deficiency (SLC25A1)

Combined D-2- and L-2-hydroxyglutaric aciduria. This is a neonatal-onset encephalopathy with profound weakness and hypotonia, intractable seizures, respiratory distress and practically absent psychomotor development, usually lethal in the first years; imaging shows enlarged ventricles, delayed myelination and germinal layer cysts [10, 48]. By 2014, eight of the 13 patients described had died at a median age of 7.5 months [116]. In the review of 26 patients, the dysmorphism was recurrent (frontal bossing, midface hypoplasia, hypertelorism, downslanting palpebral fissures, low nasal bridge, low-set or rotated ears, micrognathia); six had microcephaly and one macrocephaly; 14 of 20 had cerebral atrophy or ventriculomegaly and 12 of 20 hypoplasia or dysplasia of the corpus callosum; two had optic atrophy, and epilepsy gains prominence over time [13]. Mühlhausen's patient illustrates the presentation: intrauterine growth restriction, episodes of tachycardia followed by apnoea with cyanosis and bradycardia from 10 days of age, increasingly frequent, which required resuscitation and intensive care admissions, severe truncal hypotonia, nystagmus and absent eye contact at 5 months [116]. In patients with alleles of higher residual activity, the course is milder and one of them had myasthenic-type crises during respiratory arrests triggered by infection [13].

Congenital myasthenic syndrome type 23. Six patients from four families (British, Indian, Greek and Pakistani) homozygous for p.(Arg247Gln) had non-progressive proximal weakness from early childhood, fluctuating and fatigable, extraocular involvement (from severe bilateral ptosis to subjective diplopia), normal respiratory function and mild intellectual disability; brain MRI was normal in the four studied and urinary 2-HG was normal in those tested [109]. The neuromuscular junction defect has been demonstrated across the whole spectrum, including in a patient with the severe metabolic phenotype [10, 76].

3.5 2-Oxoglutarate dehydrogenase deficiency

The early biochemical descriptions show two patterns: slowly progressive extrapyramidal neurodegenerative disease after normal early development [60] and fatal congenital lactic acidosis at around 30 months [62]. Saudubray's textbook describes, in families with isolated KDHC deficiency, developmental delay, hypotonia, ataxia, opisthotonus, less frequent seizures and extrapyramidal dysfunction, and bilateral striatal necrosis compatible with Leigh syndrome, with onset in the neonatal period or early childhood [3]. OMIM records two siblings born to consanguineous Portuguese parents with 2-ketoglutarate dehydrogenase deficiency, axial hypotonia without head control at 4 years and acidotic decompensations, sometimes with hypoglycaemia, during infections; the sister died suddenly after general anaesthesia [36].

With molecular confirmation, the first Syrian family had different phenotypes in the two siblings: the boy, with motor and speech delay, ataxic gait at 3 years and upper limb dystonia at 6 years, with lactate of 1.8–2.9 mmol/L and MRI with mild ventriculomegaly; his sister, normal until 8 months, lost her skills, became dystonic, non-verbal and wheelchair-bound, had seizures at 11 years, lactate of 6.8–7.2 mmol/L and frontotemporal and basal ganglia atrophy with cystic changes [74]. In the 2023 series, four patients from consanguineous families (two Iranian, one Ashkenazi Jewish and one Syrian) had onset in the first year and shared global developmental delay, hypotonia, metabolic acidosis and raised lactate; three had dystonia, microcephaly and an abnormal nasal bridge, three had hyperammonaemia and all those tested had high plasma glutamine (well above that expected for the ammonia) and increased urinary α-ketoglutarate (446–754 mmol/mol creatinine, reference below 117) [75]. MRI showed agenesis of the corpus callosum in two, severe pontine hypoplasia, delayed myelination and lesions of the dentate, pallidal and subthalamic nuclei; three had a picture comparable to the Leigh spectrum; there were acidotic decompensations during infections, one with hypoglycaemia, and one patient died at 12 months during pneumonia [75].

In 2026, two unrelated adults with monoallelic OGDH variants — NM_002541.4:c.1909C>T p.(Arg637Trp) de novo and p.(Ser54Arg) (published protein designation) — had late-onset cerebellar ataxia, axonal sensorimotor neuropathy and optic atrophy, with a progressive need for walking aids; the Drosophila model suggested dominant-negative and toxic gain-of-function effects [80]. These are two patients and an animal model, and the causal relationship has not yet been established.

3.6 Mitochondrial pyruvate carrier deficiency

The index case was a newborn girl of consanguineous parents with hypotonia, mild facial dysmorphism, periventricular cysts, marked metabolic acidosis and lactate of 5–14 mmol/L that did not correct with bicarbonate, thiamine, 2-chloropropionate or ketogenic diet; MRI showed cortical atrophy, periventricular leukomalacia and calcifications; there was progressive microcephaly, neurological deterioration and death at 19 months, and an affected fetus was identified in the following pregnancy [3, 64]. In the 2022 series, eight patients from six families had central nervous system involvement and developmental impairment, mild in three (one with an IQ of 56 at 12 years) and severe or profound in the others, with variable severity within the same family; hypotonia in 5/6, epilepsy in 4/5 (from myoclonus at 2.5 months and spasms at 8 months to tonic-clonic seizures from 6 years) and microcephaly in 3/5 [11]. Outside the nervous system there was peripheral neuropathy, visual impairment, congenital heart disease, mild renal failure and, in one patient, splenomegaly, antibody-negative insulin-dependent diabetes and fractures after minimal trauma [11]. Lactate was raised in 6/6 and pyruvate in 5/5, with a normal or low lactate/pyruvate ratio (5–10); in one newborn, CSF lactate was 5.8 mmol/L and the CSF/blood glucose ratio 0.41 [11]. One of the six patients described in 2022 by Jiang and colleagues had Leigh-like syndrome [90].

3.7 Mitochondrial aconitase deficiency

In the severe form, ICRD, eight patients from two families presented at 2–6 months with truncal hypotonia, athetosis, seizures and ophthalmological abnormalities, did not achieve milestones, progressed to profound psychomotor disability with progressive visual loss due to optic and retinal atrophy, and some survived to 18 years; serial MRI showed progressive cerebellar atrophy, thin corpus callosum, dysmyelination and frontal and temporal cortical atrophy, with normal routine tests [69]. There are intermediate forms with ataxia, developmental delay and behavioural abnormalities [31], and a 2025 case with hypotonia at 4 months and optic and cerebellar atrophy at 6–7 years [118]. In optic atrophy 9, loss of acuity and disc pallor begin in the first or second decade, with preferential involvement of the temporal quadrant; recessive cases are more severe (more than 60% of eyes with acuity below 20/200) than dominant ones (more than 80% above 20/200), and there was deafness and late cerebellar ataxia in some [16, 32]. A 2026 Franco-British series of 55 patients from 37 families with dominant optic atrophy due to ACO2 found a median age at diagnosis of 24 years, severity ranging from paucisymptomatic to legal blindness and acuity correlated with nerve fibre layer and ganglion cell thickness on optical coherence tomography, but not with variant type [119]. By 2024 about 100 patients had been described [17].

3.8 Isocitrate dehydrogenase deficiencies

Biallelic IDH3A variants cause early-onset retinitis pigmentosa, with night blindness and diagnosis between 1 and 11 years, with or without macular pseudocoloboma; two siblings with the same genotype had the two forms [33, 93]. In IDH3B, retinopathy was the only health problem in the two families described [34, 66]. D-2-hydroxyglutaric aciduria type II, due to IDH2 gain of function, causes developmental delay, macrocephaly, seizures, vomiting, cerebral atrophy and, in severe patients, dilated or hypertrophic cardiomyopathy; macrocephaly with subdural collections resembles glutaric aciduria type I and should be considered before suspecting child abuse [94].

3.9 Recessive complex II deficiency

The picture ranges from multisystem disease in infancy to isolated cardiac or muscular involvement in adults with normal development [3, 39]. Neurological findings are the most frequent: regression after acute illness, initial hypotonia and spasticity; dilated or hypertrophic cardiomyopathy is frequent, and there is ophthalmoplegia, retinopathy, nystagmus, optic atrophy and blindness [3]. Early-onset leukoencephalopathy with spasticity and motor regression occurs with SDHA, SDHB and SDHAF1; in a cohort of 19 patients, MRI patterns of the thalamus and brainstem appeared specific, and succinate accumulation in the white matter is specific but inconstant [3]. In SDHAF1, five children from three families had onset between 4 and 20 months with motor regression and spasticity after normal or near-normal early development; all had bilateral leukoencephalopathy with a succinate peak, one had Leigh neuropathology and one died at 18 months [117]. Biallelic SDHD variants may cause fatal infantile hypertrophic cardiomyopathy or an encephalomyopathic picture with episodic regression, and SDHB variants an early neurodegeneration with leukoencephalopathy triggered by infection or fasting [41, 42]. In SDHA, the course may be indolent, with survival into adulthood [78].

3.10 Succinyl-CoA ligase deficiency

In the largest series (71 patients; 50 SUCLA2 and 21 SUCLG1), median onset was at 2 months in SUCLA2 and at birth in SUCLG1; median survival was 20 years and 20 months, respectively; liver disease (38%) and hypertrophic cardiomyopathy (14%) occurred only in SUCLG1, and basal ganglia involvement was very frequent in both groups [14]. Among 61 patients with SUCLA2, there was developmental delay or intellectual disability in 95%, hypotonia, sensorineural deafness and growth failure in 90% and dystonia in 80% [120].

3.11 2-Hydroxyglutaric acidurias

Before the molecular separation, the international series by van der Knaap and colleagues (25 patients) described mild and severe forms that mixed types I and II and the combined form [54, 94]. The questionnaires on 14 type I and 19 type II patients published in 2012 give the current picture. In type I, onset occurred between birth and 6 years; 11 of 14 (78%) had developmental delay (5 mild, 3 moderate, 3 severe) and 3 were developmentally unaffected; 8 (57%) had hypotonia and 4 (29%) seizures; none had cardiomyopathy, and one patient with hypertrophic cardiomyopathy was described outside the series [54]. Life expectancy in type I is not defined: one patient died at 3 weeks with necrotising enterocolitis and three were aged 5, 7 and 12 years; in monozygotic twin girls with the same genotype, one had multiple congenital anomalies and severe delay and the other normal development with macrocephaly [54, 94]. In a Palestinian family, two homozygous siblings were asymptomatic; in another, the patient had only controlled absences, reading difficulty and hyperactivity [94]. Saudubray's textbook describes type I as less severe and more variable, without rapid progression when there is no early epileptic encephalopathy [57].

In type II, onset occurred by 2 years in all 19 patients; all had developmental delay (11 severe), 17 (89%) hypotonia, 15 (79%) seizures and 9 (47%) cardiomyopathy (7 dilated, 1 hypertrophic, 1 of unspecified type); the course was mainly progressive, with death of 9 patients between 4 months and 22 years (mean 6.5 years) and 10 alive between 2.8 and 19 years [54]. Saudubray's textbook estimates death in childhood in about 50% and describes, in severe patients, neonatal onset with epileptic encephalopathy, cortical blindness and cardiomyopathy [57]. Other manifestations recorded before the types were distinguished include early vomiting that led to pyloromyotomy, macrocephaly with subdural collections, involuntary movements (chorea, dystonia, opisthotonus), facial dysmorphism (flat face, broad nasal bridge), stridor and apnoea, cerebral infarcts and agenesis of the corpus callosum [94]. In a patient with p.(Arg140Gln), prolonged QTc (up to 546 ms) and cyclical vomiting were described, and in two others skin lesions (violaceous papules with sarcoid granulomas; nodules with histiocytic infiltrate) that disappeared under IDH2 inhibition [121, 122]. Neuroimaging in severe patients shows delayed gyration, myelination and operculisation, ventriculomegaly with occipital predominance, subependymal cysts over the head of the caudate nucleus and enlargement of the frontal subarachnoid spaces [54, 57].

L-2-hydroxyglutaric aciduria has a homogeneous phenotype with insidious onset. In the series of 106 patients, onset occurred between 0 and 7 years in 97%; at diagnosis, 52% had developmental delay, 42% epilepsy and 20% cerebellar ataxia; over the course of the disease, 93% had psychomotor delay, 82% cerebellar ataxia, 72% epilepsy, 48% macrocephaly and 38% extrapyramidal signs (tremor, dystonia); hypotonia predominated in the early stages and spasticity in the late ones, and a quarter of patients lost skills such as independent walking [54, 55]. The first year generally passes without abnormalities, and presentation is through delayed walking and speech, gait disturbance, hypotonia or febrile seizures; by adolescence, many patients are bedridden, with an intelligence quotient of 40–50 [57, 58]. Mild forms may remain undiagnosed until adulthood: two Japanese siblings began to deteriorate after 25 years of age, and there are adults who present with tremor, dystonia or parkinsonism [58, 123]. Exceptionally, the disease is neonatal and fatal: a newborn girl with apnoea and a burst-suppression pattern on the electroencephalogram died at 28 days, with small brainstem and cerebellum and loss of Purkinje cells [58].

Magnetic resonance imaging (MRI) has a pattern considered unique: slightly swollen subcortical white matter, with effacement of the sulci and progressive loss of the U fibres, relative sparing of the periventricular white matter, T2 hyperintensity of the dentate nuclei, globi pallidi, putamen and caudate and progressive cerebellar atrophy; the systematic series of 56 MRI scans confirmed this pattern, and as the disease progresses the changes become diffuse and atrophy appears [54, 57, 58].

L-2-hydroxyglutaric aciduria predisposes to central nervous system (CNS) tumours. A 2012 meta-analysis identified 14 patients with a brain neoplasm among 295 with the disease, a prevalence of about 5%, which the authors consider probably underestimated [56]. The tumours described include astrocytoma, glioblastoma, primitive neuroectodermal tumour, medulloblastoma, oligodendroglioma and ependymoma, supratentorial in all cases reviewed in the ATLAS; in the Portuguese series of 21 patients, two children and one adult had astrocytomas [54, 57, 58]. Rapid deterioration, rare in the usual course, was the sign of a tumour in nine cases; tumours outside the CNS (calvaria, Wilms tumour) have also been described, without a causal relationship being demonstrated [54]. Acute metabolic decompensations do not occur in this disease [54].

4. Clinical Recognition and Warning Signs

4.1 When to suspect

In the neonatal period, suspicion arises from a newborn with encephalopathy, hypotonia, poor feeding and lactic acidosis without hypoglycaemia, which Saudubray's textbook classifies as an “energy deficiency” presentation: polypnoea, hypotonia, acidosis of variable intensity, ketonuria present or absent, raised lactate and normal or increased ammonia; this category includes defects of pyruvate oxidation and of the cycle, respiratory chain diseases and lipoylation defects [59]. Hyperammonaemia with raised lactate and increased citrulline points to MAS defects, pyruvate carboxylase deficiency or TMEM70 [59]. In a child in coma, hyperlactataemia above 4 mmol/L with normal glucose includes multiple carboxylase deficiency, pyruvate carboxylase deficiency, cycle defects, the respiratory chain and pyruvate dehydrogenase [59]. In neonatal and infantile seizures, raised blood lactate and glycerol-3-phosphate with cycle intermediates in the urine point to mitochondrial, cycle and MAS diseases, a group that Saudubray's textbook flags as potentially responsive to pyridoxine and serine [59]. The presence of dysmorphism or brain malformations (polymicrogyria, agenesis of the corpus callosum, ventriculomegaly) in a metabolic encephalopathy suggests fumarase deficiency, listed by Saudubray's textbook among the inborn errors with malformations [59, 63].

In infants and children, the warning signs are early epileptic encephalopathy with progressive microcephaly and hypomyelination (AGC1, GOT2, MDH2), epilepsy with low serine and high ammonia or citrulline (GOT2), dystonia or regression triggered by infection, surgery or fasting (complex II, KDHC), progressive optic and cerebellar atrophy with normal tests (ACO2) and fatigable weakness with ptosis and mild intellectual disability (CMS23) [11, 69, 73, 75, 78, 109]. In adolescents and adults, progressive dystonia-parkinsonism with fumaric aciduria [110], ataxia with neuropathy and optic atrophy [79, 80] and isolated optic atrophy [16] extend suspicion beyond paediatrics. The adult approach textbook includes Krebs cycle deficiencies among the mitochondrial defects with acute manifestations triggered by infection, such as Leigh syndrome and acute optic neuropathy [124].

In the 2-hydroxyglutaric acidurias, the warning signs are macrocephaly with subdural collections and ventriculomegaly in an infant with developmental delay (D form, to be distinguished from child abuse and glutaric aciduria type I), dilated cardiomyopathy with encephalopathy (type II) and, in children, adolescents or adults, slowly progressive cerebellar ataxia with epilepsy, macrocephaly and subcortical leukoencephalopathy with involvement of the dentate nuclei (L form) [54, 55, 94]. In a patient with the L form, rapid deterioration or focal signs should prompt a search for a brain tumour [54, 56].

4.2 Emergency signs

Emergency signs are severe and persistent lactic acidosis, hyperammonaemia, hypoglycaemia during intercurrent illness, apnoeas and respiratory failure (D2L2AD, KDHC), status epilepticus and acute neurological deterioration after infection, anaesthesia or fasting [11, 36, 71, 75, 116]. In a patient with MDH2 deficiency already on a ketogenic diet, fatal decompensation manifested with vomiting, cough, tachypnoea and altered consciousness [11, 71]. In an infant with congenital heart disease and heart failure, transient excretion of fumaric acid should not be mistaken for fumarase deficiency [4].

4.3 Newborn screening

None of the sources consulted describes newborn screening targeting Krebs cycle or neuronal MAS defects. Citrin deficiency can be detected by raised citrulline on newborn screening, as happened in two Austrian patients [11], and its management is that of urea cycle disorders.

The 2-hydroxyglutaric acidurias are also not included in the newborn screening programmes described in the sources consulted; in a Japanese review of 13 published cases, almost half of the diagnoses were made only in adulthood [123].

5. Diagnosis: Criteria, Tests and Algorithms

5.1 Criteria

There are no published consensus clinical criteria for any entity of the class; the GeneReviews on fumarase deficiency defines suggestive findings and establishes the diagnosis by identification of biallelic pathogenic or likely pathogenic variants in FH or, when molecular testing is inconclusive, by reduced enzyme activity in fibroblasts or leucocytes [4]. The GeneReviews on SUCLA2 likewise recognises no consensus clinical criteria [5]. In the other entities, the diagnosis rests on a compatible phenotype, a supporting biochemical profile and biallelic variants classified according to the rules of the American College of Medical Genetics and Genomics and the Association for Molecular Pathology (ACMG/AMP), with functional study when the variant is novel [4, 11].

5.2 Initial biochemical investigation

First-line assessment of a patient with suspected mitochondrial disease includes full blood count, creatine kinase, transaminases, albumin, lactate and pyruvate, plasma amino acids and acylcarnitines and urinary organic acids; postprandial lactate is more sensitive than fasting lactate, but small elevations should not be overinterpreted [125]. The lactate/pyruvate (L/P) ratio and the 3-hydroxybutyrate/acetoacetate ratio, measured after an overnight fast, reflect the cytosolic and mitochondrial redox state: an increased L/P with a normal or decreased 3-hydroxybutyrate/acetoacetate suggests pyruvate carboxylase or 2-ketoglutarate dehydrogenase deficiency; both ratios increased with persistent hyperlactataemia suggest respiratory chain disease; a normal or low L/P with normal 3-hydroxybutyrate/acetoacetate suggests pyruvate dehydrogenase or pyruvate carrier deficiency [126]. The ATLAS algorithm for congenital lactic acidaemia separates gluconeogenesis defects (hypoglycaemia, fasting intolerance) from oxidation defects; for the latter, it proposes fibroblast culture with assays of the pyruvate dehydrogenase complex, pyruvate oxidation, 2-oxoglutarate dehydrogenase and E3 [127].

Magnetic resonance spectroscopy (MRS) adds markers: a succinate peak in the white matter in SDH deficiencies (mainly SDHA and SDHAF1), very low NAA with high myo-inositol in AGC1 deficiency and a lactate peak in several defects [11, 101, 117]. Urinary excretion of cycle intermediates (succinic, malic, fumaric, 2-oxoglutaric and citric acids) often accompanies oxidative phosphorylation disorders and is not specific [101]. In SDH deficiency, lactic acidosis and CSF lactate are inconstant and there is not always a characteristic organic aciduria [3].

5.3 Limitations of biochemical markers

The markers frequently fail. In fumarase deficiency, urinary fumarate was increased in seven of ten patients and was inconsistent on repeat testing [104]; the patient described by Ryder and colleagues had normal lactate and no other cycle intermediate in the urine [89], and the elevation of fumaric acid does not correlate with severity [89]. In D2L2AD, in one of the analyses of a sibling neither enantiomer was raised, so a single analysis may miss the diagnosis; the laboratory should separate the enantiomers rather than report total 2-HG [10, 94]. In MDH1 deficiency and in some patients with OGDH variants, routine tests were normal [72, 74]. In GOT2 deficiency, low serine and high citrulline were present in only one of the four original patients, and a patient with 27% residual activity had normal lactate, ammonia, plasma amino acids and urinary organic acids; a normal metabolic screen does not exclude this diagnosis [73, 108]. Mildly to moderately increased urinary 2-ketoglutarate is a common and nonspecific finding [3]. In CMS23 urinary 2-HG is normal [109].

5.4 Enzymatic and functional studies

Fumarase activity can be measured in mononuclear leucocytes or fibroblasts by spectrophotometry; in severe patients it is often below 10% of the control mean, but some patients have 11–35%, and overlap with heterozygotes precludes use of the assay to detect carriers [3, 4]. In KDHC deficiency, activity in fibroblasts was historically the only means of diagnosis [3]. SDH deficiency is confirmed by complex II activity in the respiratory chain, preferably in muscle [3, 39]. MPC deficiency is demonstrated by comparing the oxidation of [2-14C]pyruvate in digitonin-permeabilised fibroblasts and in disrupted fibroblasts [3, 64]. Aconitase activity can be measured in lymphoblasts [69]. Respiratory chain assays in muscle may be normal in AGC1 deficiency and in succinyl-CoA ligase deficiency [14, 107].

5.5 Genetic tests

Because of its high sensitivity, FH sequencing has become the preferred confirmatory test for fumarase deficiency: it detects more than 99% of the known pathogenic variants, with targeted deletion/duplication analysis when one or no variant is found; a whole-gene deletion has been described in a patient with fumarase deficiency [4]. A multigene panel or exome/genome sequencing are the options when the phenotype is nonspecific; the ACMG and the American Academy of Pediatrics recommend exome or genome sequencing as a first- or second-tier test in children with developmental delay, intellectual disability or multiple congenital anomalies [4]. Most entities of the class were identified by exome sequencing in consanguineous families [11, 12, 75].

The technical limitations have practical consequences. Sequencing may miss single- or multi-exon deletions; cryptic FH splicing variants have been described in FH tumour syndrome and would probably escape exome sequencing [4]. Apparently missense variants may act through splicing (OGDH NM_002541.4:c.935G>A; SLC25A1 NM_005984.5:c.821C>T), and synonymous variants may be pathogenic (SLC25A12 NM_003705.5:c.1747C>A p.(Arg583=)) [11, 13, 75]. Apparent homozygosity may correspond to an undetected deletion on the other allele or to uniparental isodisomy, described for chromosome 1 in fumarase deficiency [4]. In SLC25A1, a 22q11.2 deletion may unmask a recessive allele on the remaining chromosome [10]. The report should state the gene, the versioned transcript, the HGVS nomenclature, the zygosity and the ACMG/AMP class; two VUS, or one pathogenic variant with one VUS, neither establish nor exclude the diagnosis, and parental segregation and functional studies help with reclassification [4, 13, 106]. In a patient with an FH variant found by exome sequencing, interpretation must separate the recessive diagnosis from heterozygous tumour predisposition [77, 104].

5.6 Proposed algorithm

5.7 Prenatal and preimplantation diagnosis

Once the familial variants have been identified, prenatal diagnosis and preimplantation genetic testing are possible [4, 5]. In fumarase deficiency, fumaric acid in amniotic fluid at 15–18 weeks is not diagnostic and should be interpreted with extreme caution; fumarase activity in chorionic villi at 10–12 weeks or in cultured amniocytes is possible, but some affected fetuses have considerable residual activity; ultrasound may show ventriculomegaly, agenesis of the corpus callosum or a Dandy-Walker cyst [4]. In MPC deficiency, metabolic demonstration of the defect in an at-risk pregnancy was made in the first report, and in one family the metabolic diagnosis led to termination of the pregnancy [11, 64].

5.8 2-Hydroxyglutaric acidurias: diagnosis

Gas chromatography–mass spectrometry (GC-MS) of urinary organic acids detects 2-HG but does not distinguish the enantiomers; chiral separation, by derivatisation with a chiral reagent (O-acetyl-di-2-butyl esters on GC-MS or diacetyltartaric anhydride on liquid chromatography–tandem mass spectrometry, LC-MS/MS) or by a chiral stationary phase, is mandatory for the differential diagnosis [54, 57, 94]. An isolated increase of the D enantiomer leads to sequencing of IDH2 (codon 140) and D2HGDH, and to D-2-HGDH activity in fibroblasts or lymphoblasts when the genetics is inconclusive; an isolated increase of the L enantiomer leads to L2HGDH with deletion analysis (MLPA); both enantiomers increased, with D > L, low urinary citrate and cycle intermediates, lead to SLC25A1 [54, 55, 85, 94]. D-2-HG concentrations separate types I and II with little overlap in plasma (mean about five times higher in type II), but do not correlate with severity within each type, and one severely affected patient had intermittently normal excretion [54, 94]. In the L form, the CSF/plasma L-2-HG ratio is greater than 1; for the D form, the 2012 review indicates a ratio below 1 in both types, whereas the ATLAS, with data from older series without distinction of types, describes CSF higher than plasma in most patients [54, 94].

D-2-HG is also raised in multiple acyl-CoA dehydrogenase deficiency (glutaric aciduria type II), in succinic semialdehyde dehydrogenase deficiency (via HOT) and, rarely, in glutaric aciduria type I; the profile of the other organic acids distinguishes them [54, 57]. Undifferentiated 2-HG is also raised in dihydrolipoamide dehydrogenase deficiency and DOOR syndrome, and it has been suggested that poorly stored urine samples (non-enzymatic conversion of 2-oxoglutarate or bacterial or fungal growth) might give false positives, which the reference laboratory has not observed [54]. L-2-HGDH activity can be measured in fibroblasts, lymphoblasts and lymphocytes with labelled substrate and LC-MS/MS [54]. In three of the 18 Portuguese families and in one of the 83 families in the 2010 series the molecular cause was not identified, so negative sequencing does not exclude the disease when L-2-HG is raised [55, 112]. Prenatal diagnosis is made by the familial variant or by measurement of the enantiomer in amniotic fluid by stable isotope dilution GC-MS; in fetuses affected by D-2-hydroxyglutaric aciduria, D-2-HG was about tenfold increased [58, 94].

6. Differential Diagnosis

The most frequent distinction in practice is between pyruvate carrier deficiency and pyruvate dehydrogenase deficiency, which share lactic acidosis with a normal L/P ratio: normal PDH activity and reduced pyruvate oxidation only in fibroblasts with intact mitochondrial membranes separate them [3, 64]. Another decisive distinction is that from pyruvate carboxylase deficiency, because the ketogenic diet, useful in MAS and MPC defects, is an absolute contraindication in pyruvate carboxylase deficiency [128]. The pattern of isolated urinary 2-ketoglutarate extends to defects of thiamine metabolism and of E3, and low KDHC activity in fibroblasts does not identify the gene [9, 126].

Among the epileptic encephalopathies with low serine, GOT2 deficiency resembles the serine synthesis deficiencies in its microcephaly, intellectual disability and epilepsy, but is distinguished by high lactate, increased ammonia and citrulline and low aspartate [12, 73]. In the 2-hydroxyglutaric acidurias, identification of both enantiomers, with D predominance, and of low urinary citrate points to SLC25A1; detection of only one enantiomer leads to D2HGDH, IDH2 or L2HGDH [94, 116]. Isolated and persistent fumaric aciduria with brain malformations points to FH; with other cycle intermediates and lactate, it points instead to respiratory chain disease, Leigh syndrome or pyruvate dehydrogenase deficiency [4, 126].

In the 2-hydroxyglutaric acidurias, chiral separation resolves most of the differentials: secondary increases of D-2-HG in multiple acyl-CoA dehydrogenase deficiency and in succinic semialdehyde dehydrogenase deficiency are accompanied by glutaric and ethylmalonic acids or by γ-hydroxybutyrate, and the MRI pattern of the L form (subcortical white matter, dentate nuclei and pallidum, cerebellar atrophy) distinguishes it from the leukodystrophies with periventricular predominance [54, 57, 58].

7. Treatments and Risk Management Strategies

7.1 General principles

There are no published clinical practice guidelines for any Krebs cycle or MAS defect; the GeneReviews recommendations for fumarase deficiency are based on the authors' experience [4], and those for SUCLA2 are likewise supportive [5]. Management combines three layers: mechanism-directed therapies, with evidence limited to case series (ketogenic diet in MAS and MPC defects, serine and pyridoxine in GOT2 deficiency, citrate in citrate carrier deficiency, acetylcholinesterase inhibitors in CMS23); metabolic protection and drug safety measures common to primary mitochondrial diseases, supported by consensus statements of the Mitochondrial Medicine Society (MMS) and by an international Delphi consensus; and multidisciplinary supportive care [11, 73, 109, 116, 125, 130, 131]. Patients should be followed in centres with experience in metabolic diseases, such as those of MetabERN [2, 11].

7.2 Dietary therapy

Rationale. Ketogenic diets (KD) are isocaloric diets low in carbohydrate and high in fat that reduce glycolysis and increase β-oxidation and ketone body production and utilisation [11]. In MAS defects, reduced glycolysis lowers the cytosolic NADH/NAD+ ratio and ketone bodies and fatty acids enter the cycle through the mitochondrion, bypassing the shuttle; in MPC deficiency, they bypass pyruvate entry, as in pyruvate dehydrogenase deficiency, where KD is the treatment of first choice [11, 128]. In the enzymatic defects of the cycle itself, the reasoning is reversed: Saudubray's textbook considers KD “usually contraindicated” in the epilepsy of fumarase deficiency and of the other enzymatic defects of the cycle, because reduced cycle activity may compromise the condensation of acetyl-CoA from β-oxidation with oxaloacetate, and the GeneReviews on fumarase deficiency repeats the contraindication [3, 4].

Evidence in MAS and MPC defects. In the 2022 retrospective series, 13 patients with neurological MAS or MPC defects received classic KD; 11 had at least temporary benefit and 10 (77%) a substantial reduction in seizures, above the 40–50% probability of a reduction of at least half of seizures expected with KD in general [11, 128]. The results by entity are shown in the table.

The 2026 review on AGC1 deficiency confirms the reproducible reduction in seizures and, in some patients, improvement in myelination and NAA, but stresses that the response is heterogeneous and depends on how early, how long and how stable the ketosis is, and calls for prospective studies with standardised outcomes, MRI with spectroscopy and measurement of ketosis [133]. The limitations of the 2022 series are those of a small retrospective study, with non-uniform diets and no standardised BHB target; in some patients, increasing the ketogenic ratio from 3:1 to 4:1 restored seizure control [11].

Evidence in enzymatic cycle defects. In fumarase deficiency, a girl with 9% enzyme activity and a genotype predicted to be severe started at 14 months a high-fat, low-carbohydrate diet (60% of energy from fat, 30% carbohydrate, 10% protein; not a classic KD) and progressed with mild to moderate intellectual disability and controlled seizures; the authors cannot attribute the course to the diet [4, 89]. In a Portuguese case with the dystonic form, KD had limited success on seizures and no effect on dystonia and myoclonus [110]. Low-protein diets showed no clinical or biochemical benefit (0.71 g/kg/day for 3 days; 0.8 g/kg/day in a 45-day-old infant; unpublished data from the GeneReviews authors) [4, 89]. There are no data on KD in OGDH, ACO2 or SDH deficiencies in the sources consulted.

Divergence between sources. The 2018 International Ketogenic Diet Study Group consensus lists as absolute contraindications to KD deficiencies of carnitine, carnitine palmitoyltransferase I and II and carnitine translocase, β-oxidation defects, pyruvate carboxylase deficiency and porphyria, and includes Krebs cycle defects neither among the contraindications nor among the indications [128]. Saudubray's textbook and GeneReviews consider KD contraindicated in fumarase deficiency [3, 4], whereas the ATLAS algorithm for congenital lactic acidaemia proposes starting a high-fat or ketogenic diet in the oxidation defect branch, without distinguishing pyruvate dehydrogenase deficiency from cycle defects [127]. In the absence of comparative studies, the practice that follows from the sources is to use KD in MAS and MPC defects, where there is documented benefit, and to reserve high-fat diets in enzymatic cycle defects for individual decision in a specialised centre, with monitoring of lactate, ketosis and clinical status.

Implementation. Classic KD is calculated as a ratio of grams of fat to grams of protein and carbohydrate (4:1 corresponds to 90% of calories from fat); the alternatives are the medium-chain triglyceride (MCT) diet (60% of energy from MCT in the traditional form) and the modified Atkins diet (ratio of about 1:1–1.5:1) [128]. Before starting, a fatty acid transport or oxidation disorder should be excluded when suspected, and renal stones, swallowing difficulties, reflux, constipation, cardiomyopathy and nutritional status should be assessed [128]. Universal supplementation includes a multivitamin with minerals (including selenium) and calcium and vitamin D at the recommended daily allowance; empirical oral citrates reduced renal stones from 6.7% to 0.9% in one study; selenium falls at 6–12 months and there are reports of associated cardiomyopathy; metabolic acidosis may worsen with acetazolamide, topiramate or zonisamide [128]. Reassessment at 3 months is suggested and, if there is benefit, the duration should be considered at 2 years, prolonging it in disorders of energy metabolism in which the mechanism is bypassing of the block [128].

7.3 Entity-specific targeted therapies

The 2025 GOT2 deficiency cohort points out that seven patients controlled their seizures with antiseizure medication alone and that the response to supplements was assessed on regimens with several drugs simultaneously; the authors consider supplementation beneficial in most, but not decisive on its own [12]. The optimal dose is not defined: in the published patients, serine ranged between 200 and 500 mg/kg/day and pyridoxine between 20 mg/kg/day and 100–200 mg/day, with adjustments dictated by recurrence of seizures and by shortages of the products; in individual 4, seizures returned during a period without serine and disappeared when it was resumed [73, 108]. The 2026 letter cites the 2025 cohort as having shown improvement in 6 of 13 treated patients, but the original article reports 7 supplemented patients among 13 with data, 6 of them with partial or complete seizure control [12, 108]. Aspartate supplementation is not useful, because cells take up little aspartate from the blood [12, 73]. Pyruvate corrects serine synthesis in GOT2-deficient cells and a low-carbohydrate, high-fat (mainly MCT) diet with ketone bodies has been proposed, but none has been tested in patients [73]. In AGC1 deficiency, serine and glycine supplementation normalised CSF serine in one patient without clinical benefit, which only appeared with KD [11].

Other drugs used empirically in mitochondrial diseases have weak evidence. Saudubray's textbook indicates dichloroacetate 25 mg/kg/day in two doses in primary lactic acidosis, with a risk of polyneuropathy on prolonged use, and ubiquinone 30 mg/kg/day (up to 3 g/day in adults), with unproven benefit outside primary coenzyme Q10 deficiencies [134]; the MMS concludes that a Cochrane review found no evidence for any vitamin or cofactor supplement [125].

7.4 Supportive care

In fumarase deficiency, GeneReviews recommends assessment by a paediatric neurologist (with MRI and electroencephalogram), feeding assessment with aspiration risk and possible gastrostomy, feeding therapy, physiotherapy and occupational therapy for contractures, assistive devices, orthopaedic management of scoliosis, early intervention and special education, augmentative and alternative communication, social support, palliative care and a transition plan in adolescence; seizures are treated with antiseizure medication by an experienced paediatric neurologist and are often difficult to control [4]. Spasticity and dystonia may require baclofen, tizanidine, botulinum toxin, antiparkinsonian drugs or orthopaedic surgery [4]. In SUCLA2-related forms, hearing aids or cochlear implants, blepharoplasty for ptosis and respiratory physiotherapy with early antibiotic treatment are added [5]. The GOT2 cohort stresses the need for nutritional, behavioural and infection control care [12]. In a Portuguese case, levodopa up to 400 mg/day did not modify the dystonia-parkinsonism, and levetiracetam 700 mg/day improved seizures and myoclonus [110].

7.5 Acute illness, fasting and the perioperative period

Intercurrent illness, fasting and surgery trigger decompensation in several entities: acidotic decompensations with hyperammonaemia and hypoglycaemia during infections in OGDH deficiency, regression after acute illness in SDH deficiency, fatal metabolic decompensation in MDH2 deficiency and sudden death after general anaesthesia in a Portuguese patient with 2-ketoglutarate dehydrogenase deficiency [3, 36, 71, 75]. The MMS recommends a written emergency plan and a medical alert bracelet, avoidance of prolonged fasting and administration of intravenous fluids with dextrose before, during and after procedures and in decompensation, continuing usual therapies; dextrose should be limited or omitted in disorders of pyruvate metabolism, in patients on KD and in those who have had an adverse reaction to high glucose loads; lipids may be used [125]. In a patient with OGDH deficiency, glucose-containing fluids produced rapid clinical improvement [75]. In the case of fumarase deficiency treated with a high-fat diet, the home sick-day plan used a 50:50 ratio of lipids to carbohydrates with a target of 2200 kcal/day, and the hospital plan for illness or surgery provided for 0.45% NaCl with 5% dextrose and 20% lipid emulsion at 2–3 g/kg/day [89]. These measures apply by extrapolation from primary mitochondrial diseases and the individual report; they have not been evaluated in studies in cycle defects.

For anaesthesia, the 2015 MMS statement advises caution with volatile anaesthetics, avoidance of propofol or its limitation to short procedures (because of the risk of propofol infusion syndrome) and considers local anaesthetics well tolerated [125]. The 2020 Delphi consensus considers volatile anaesthetics safe, propofol for induction and, in refractory mitochondrial status epilepticus, propofol infusion for up to 48 hours, and recommends additional caution with neuromuscular blockade in myopathic phenotypes [131]. The 2018 KD consensus considers concomitant use of propofol a relative contraindication [128].

7.6 Drugs to use with caution

Sources: [125, 131]. Both positions apply to primary mitochondrial disease in general; for cycle defects, the most recent source is the 2020 consensus. In patients on KD, acidosis worsens with carbonic anhydrase inhibitors (acetazolamide, topiramate, zonisamide) [128].

7.7 Regulatory status

No medicine has an approved indication for any Krebs cycle or MAS defect; all targeted therapies are off-label or in compassionate use.

7.8 Studies with results

None of these studies is controlled. The figures come from small, often retrospective series, with non-uniform doses and regimens, co-interventions (several antiseizure drugs, diet) and clinician-reported outcomes; the effects cannot be compared between studies and the absence of confidence intervals reflects the descriptive design. An uncontrolled series also does not exclude that part of the improvement results from the natural history, as the authors of the fumarase deficiency case themselves acknowledge [89]. Searches of ClinicalTrials.gov on 31 October 2026 identified no trials for fumarase, OGDH, MDH2, GOT2, AGC1, MPC1 or ACO2 deficiencies, which does not prove that none exist [147].

7.9 Developments in the last 12 months

Between November 2025 and October 2026, the developments were mainly in characterisation. At the level of a published series: the English cohort of ten patients with fumarase deficiency, which showed inconsistent fumarate excretion, a predominant c.410C>T allele and renal lesions in two heterozygous parents (April 2026) [104]; the Franco-British series of 55 patients with dominant optic atrophy due to ACO2 (2026) [119]. At the level of case reports and an animal model: the description of monoallelic OGDH variants in late-onset neuropathy with ataxia and optic atrophy (June 2026) [80]. At the level of a review: the synthesis on ketogenic therapy in AGC1 deficiency (May 2026) [133] and the review on OGDHL (December 2025) [98]. At the level of a registered trial: the single-patient study of sodium phenylbutyrate in D2L2AD, started in July 2025 [147]. At the preclinical level: ketone bodies and glutamine restored aerobic energy production in AGC1-deficient neuronal progenitors [107]. The multicentre cohort of 16 patients with GOT2 deficiency was published in December 2025; a letter of October 2026 added a patient with the p.(His369Pro) variant, 27% residual activity, a normal metabolic screen and a reported response to pyridoxine alone [12, 108]. There were no regulatory authorisations or opinions for this class in the period.

In the 2-hydroxyglutaric acidurias, at the level of case reports: the absence of clinical and biochemical response to riboflavin in a patient with L-2-hydroxyglutaric aciduria and a null genotype, with a review of treated cases (May 2026), and functional recovery with riboflavin and levocarnitine in a Japanese adult diagnosed at 37 years, with undetermined subtype (August 2026) [114, 123]. Inhibition of mutant IDH2 with enasidenib in D2HGA2 remains limited to individual reports published between 2023 and 2025, with no registered trial for this indication identified on ClinicalTrials.gov [121, 122, 143].

7.10 Pregnancy, lactation and transition

The sources consulted do not describe pregnancies in women with neurological defects of the class; in fumarase deficiency no affected individuals with offspring have been reported [4]. A pregnancy with a fetus affected by fumarase deficiency has complications in 23% and preterm delivery in 30% [4]. The Benadon® SmPC contraindicates high-dose pyridoxine in pregnancy and breastfeeding, and the pyridostigmine SmPC states that safety in pregnancy and breastfeeding has not been established, that the drug crosses the placenta (avoid excessive doses and monitor the newborn) and that only negligible amounts pass into breast milk [137, 139]. For survivors, GeneReviews recommends a plan for transition to adult care from adolescence [4]; late-onset forms (optic atrophy due to ACO2, neuropathy due to monoallelic OGDH, dystonia-parkinsonism due to FH) present directly to adult services [16, 80, 110].

7.11 Management of tumour risk in carriers

Heterozygous relatives of children with fumarase deficiency may be at increased risk of HLRCC: there are mothers with cutaneous or uterine leiomyomas, one maternal death from renal cell carcinoma and other cases of renal cancer, and two parents with renal lesions in the 2026 series [4, 104]. GeneReviews considers lifelong renal surveillance of heterozygotes and molecular testing of at-risk relatives appropriate [4]. According to GeneGuidance, renal surveillance in HLRCC is performed with annual MRI with 1–3 mm slices from 10 years of age (8 years in the summary of the GeneReviews on the tumour syndrome) and ultrasound should not be the primary modality [77]; in the patient described by Ryder, annual abdominal MRI from adolescence and the same screening for the parents were recommended [89]. In MDH2, the authors of the original description recommended surveillance of heterozygous parents, but the association with paragangliomas has not been validated (contribution below 1% in 830 selected patients) and there is no established surveillance protocol [71, 82, 148]. In SDHA, the GeneReviews on paraganglioma syndrome does not recommend surveillance for an incidental finding without a personal or family history of tumour, because of the low penetrance (10% at 70 years in non-probands) [78]; carriers with such a history follow the surveillance protocol for hereditary paragangliomas.

7.12 2-Hydroxyglutaric acidurias

There is no therapy with demonstrated efficacy in D-2-hydroxyglutaric aciduria type I; riboflavin and L-carnitine have shown no benefit, and addition of FAD to the purified enzyme did not increase activity [54, 57]. Treatment is supportive, with antiseizure medication, rehabilitation and nutritional support, and seizures may be difficult to control [57].

In type II, the strategy is to block the source of the metabolite. In a murine model, an inhibitor of mutant IDH2 corrected the cardiomyopathy and improved survival [57]. Enasidenib, a selective inhibitor of mutant IDH2 approved in the USA for adult acute myeloid leukaemia, has been used in two children (50 and 60 mg/day), with normalisation of D-2-HG and functional and cardiac improvement, and in a child aged 3 years and 11 months (50 mg/day) followed for 36 months [121, 143]. In the latter, urinary 2-HG became undetectable within 2 weeks; ALT rose to 118 IU/L and CK to 334 U/L at 18 months, normalising 12 weeks after the frequency was reduced to three doses a week, whereas two doses a week allowed D-2-HG to reappear in the trough sample [121]. The risk of differentiation syndrome, the subject of a boxed warning in the leukaemia label, was considered very low in the absence of an immature myeloid population, but the label requires a full blood count and biochemistry at least every 2 weeks for the first 3 months; the drug is embryotoxic in animals and the label advises against breastfeeding [121, 144]. The paediatric dose, the duration and the effect on neuroimaging are not defined, and treatment remains off-label and based on reports.

In L-2-hydroxyglutaric aciduria, the therapeutic hypothesis is that FAD or riboflavin act as a chaperone for L-2-HGDH with residual activity. A 40-year-old woman, homozygous for NM_024884.3:c.845G>A p.(Arg282Gln), improved in tremor and dystonia with FAD 30 mg/day and levocarnitine chloride 900 mg/day, with normal gait and a 50% reduction in urinary L-2-HG at 6 months, maintained for more than 4 years; a 16-year-old boy improved with riboflavin 100 mg/day, with a 75% reduction in L-2-HG in 3 months and relapse on stopping [54, 114]. Conversely, a 9-year-old girl with a homozygous splicing variant did not respond to 200 mg/day for 2 months, and an adolescent girl with two null alleles did not respond to 100 mg/day for 6 months [54, 114]. The response appears limited to genotypes with residual activity and no improvement in the leukoencephalopathy is expected; Saudubray's textbook includes this disease among the indications for riboflavin, at 100–400 mg/day in 2–3 doses, with no maximum dose defined in the source [54, 134]. Epilepsy is generally controlled with the usual antiseizure medication [57, 58].

8. Reproductive Options and Family Counselling

8.1 Autosomal recessive forms

Almost all entities of the class are autosomal recessive. When both parents are heterozygous, each sibling of an affected individual has, at conception, a 25% probability of being affected, 50% of being heterozygous and 25% of inheriting neither variant [4, 5]. The proband's aunts and uncles have a 50% probability of being heterozygous [4]. Consanguinity is frequent in the families described with OGDH, GOT2, AGC1, MPC1 and MDH1 deficiencies [11, 12, 72, 75]. For the Faroese founder variant of SUCLA2, a carrier frequency of 1 in 33 justifies offering testing to the partner of a known carrier, particularly if they share the same origin [5, 100].

Parental testing should confirm that both parents are heterozygous. In fumarase deficiency two exceptions have been described: a father carrying neither of his son's variants, with confirmed paternity, in whom the most likely hypothesis was a de novo variant on the paternal allele (without excluding gonadal mosaicism), and a family with partial uniparental isodisomy of chromosome 1 [4]. When a proband appears homozygous, an undetected deletion of one or more exons on the other allele and uniparental isodisomy should be considered [4]. In SLC25A1, a 22q11.2 deletion in one parent may be transmitted together with a point variant on the other allele [10]. In GOT2 deficiency, each of the three original families lost a child in pregnancy, at birth or very early, which suggests that more severe genotypes may cause fetal or neonatal death [73].

8.2 Heterozygotes with their own risk

Counselling in a cycle defect includes the question of the carriers' risk. FH heterozygotes may have HLRCC; the 2026 series detected renal lesions in two parents and concludes that all variants from patients with fumarase deficiency should be assessed for tumour risk, and GeneReviews recommends offering testing to at-risk relatives and lifelong renal surveillance to heterozygotes [4, 104]. The doubt about variants such as p.(Pro174Arg) and p.(Ala308Gly), which might not confer tumour risk, has not been resolved [4]. SDHA heterozygotes have low penetrance for paragangliomas (10% at 70 years in non-probands), and surveillance is not recommended for an incidental finding without a history [78]. The association of heterozygous MDH2 with paragangliomas has not been validated [82, 148]. Carriers of these genes should be referred to a familial cancer risk clinic when there is a personal or family history of tumour.

8.3 Dominant and de novo forms

D-2-hydroxyglutaric aciduria type II results from de novo heterozygous IDH2 variants in most families; a family with three affected children of a mother with normal excretion suggested germline mosaicism [94]. Optic atrophy 9 is transmitted dominantly in most ACO2 cases identified in optic neuropathy cohorts, with a 50% risk for the offspring of a carrier, and recessively in the remainder [16, 32]. The monoallelic OGDH and SDHA variants associated with late-onset neurodegeneration also imply dominant transmission, but the evidence for OGDH is restricted to two patients [79, 80]. In ACO2, counselling must consider both modes of transmission [8].

In D-2-hydroxyglutaric aciduria type II, the IDH2 variants were not detected in the parents in eight of nine families; in a family with three affected pregnancies, diagnosed in amniocytes, the mother had somatic mosaicism in blood, which confirms germline mosaicism and justifies offering prenatal diagnosis even when the parents are apparently not carriers [54]. Each child of an affected individual would inherit the variant with a 50% probability, but the sources consulted describe no offspring of patients. In the recessive forms (type I and L form), the recurrence risk for siblings is 25%; in type I, counselling is complicated by the existence of asymptomatic homozygous siblings and by the discordance between monozygotic twin girls [54, 94]. Prenatal diagnosis is made by the familial variant or by the enantiomer in amniotic fluid (section 5.8) [58, 94].

8.4 Prenatal and preimplantation diagnosis and minors

Once the familial variants are known, reproductive planning should be discussed before pregnancy, with an offer of prenatal diagnosis or preimplantation genetic testing; in fumarase deficiency, prenatal biochemical methods (fumarate in amniotic fluid, enzyme activity in villi or amniocytes) are less reliable than molecular testing, and ultrasound may show cerebral signs [4]. DNA banking should be considered for probands without a molecular diagnosis [4]. Carrier testing and reproductive counselling are offered to young adults who are heterozygous or at risk [4]. For minors, the sources consulted direct counselling about recessive carrier status to young adults and set no age for this test; in families with FH variants, the question of testing at-risk minors is linked to the age at which renal surveillance for HLRCC begins (8 to 10 years) [4, 77].

9. Special Considerations: Population Diversity and Epidemiology

9.1 What the available figures measure

There are no population prevalence studies for any entity of the class; the figures are counts of published cases, Orphanet prevalence classes or carrier frequencies. Orphanet places fumaric aciduria, D,L-2-hydroxyglutaric aciduria and mitochondrial pyruvate carrier deficiency in the class of fewer than 1 case per 1,000,000, and oxoglutaric aciduria with unknown prevalence [7, 10, 19, 77]. The most recent counts are: fewer than 100 patients with fumarase deficiency [4]; fewer than ten with molecularly confirmed OGDH deficiency up to 2023 [9]; 16 with GOT2 deficiency [12]; 14 with AGC1 deficiency, four with MDH2 deficiency, two with MDH1 deficiency and eight with MPC1 deficiency up to 2022 [11]; 26 with D2L2AD up to 2018 [13]; about 100 with recessive ACO2 deficiency up to 2024 [17]; 61 with isolated complex II deficiency up to 2020 [15]; 71 with succinyl-CoA ligase deficiency up to 2016 [14]. These counts depend on ascertainment by exome sequencing in severe paediatric disease and provide no denominator [9].

The population frequency of heterozygotes for pathogenic FH variants has been estimated at between 1 in 2563 and 1 in 3247 (gnomAD and BRAVO) and between 1 in 901 and 1 in 1252 in another estimate (ExAC and 1000 Genomes); these values measure carriers, not disease, and the tumour penetrance is unknown [77]. In the population with congenital myasthenic syndrome, SLC25A1 accounts for less than 1% of cases [10, 50]. In European cohorts of unresolved hereditary optic neuropathy, ACO2 is the third autosomal cause [16].

9.2 Founder populations and geographical distribution

Fumarase deficiency has its highest known incidence in a religious community in the southwestern United States (on the border between Arizona and Utah), with the genetic defect traced back to one of the community's founding patriarchs and the first of his wives; Kerrigan's series of eight patients comes from a consanguineous family from this region [43, 63, 89]. SUCLA2 deficiency has an incidence of 1 in 1700 and a carrier frequency of 1 in 33 in the Faroe Islands, owing to a founder effect of the c.534+1G>A variant, and there are two further Scandinavian founder variants [14, 100]. The GOT2 variant p.(Arg262Gly) recurs in Egyptian families [12]. MPC1 deficiency has been described in consanguineous North African families and in European, Kuwaiti and Chinese cohorts [3, 90]. OGDH deficiency has been described in Iranian, Syrian, Tunisian and Ashkenazi Jewish families [60, 74, 75]. The SLC25A1 variant p.(Arg247Gln) recurs in British, Indian, Greek and Pakistani families, without a founder effect [109].

In the 2-hydroxyglutaric acidurias, the figures are patient counts from reference laboratories: up to 2012, 95 patients with the D form (26 type I, 24 type II, 43 unclassified and 2 without a variant in either gene), of various origins, with frequent consanguinity in type I and almost none in type II; Orphanet records about 80 cases of the D form and 140 of the L form, both in the prevalence class below 1 per 1,000,000 [29, 53, 54]. The L form is concentrated in populations with high consanguinity: the first series with the gene included 21 patients from 15 Turkish families, and in a Turkish cohort of 10 patients (five children and five adults, diagnosed at a mean age of 10.1 years) the variant c.905C>T p.(Pro302Leu) was present in five [54, 113]. In Japan, a 2026 review brought together 13 published cases of 2-hydroxyglutaric aciduria, nine of the L form [123]. The meta-analysis of 295 patients with the L form does not estimate population prevalence [56].

9.3 Portugal and Brazil

In Portugal, a case of fumarase deficiency followed in Coimbra was published in 2022, with a phenotype of progressive dystonia-parkinsonism and the genotype c.267+1_267+10del / p.(Lys477dup) [110]. OMIM also records two siblings born to consanguineous Portuguese parents with 2-ketoglutarate dehydrogenase deficiency (description by Guffon and colleagues, 1993), one of whom died suddenly after general anaesthesia [36]. In Brazil, the first case of fumaric aciduria was published in 2010, with colpocephaly, hepatic abnormalities, marked metabolic acidosis from birth and an atypical increase in plasma citrulline, tyrosine and methionine, accompanied by a review of 40 patients [149]. The sources consulted provide no carrier frequencies or national series for Portugal or Brazil.

For L-2-hydroxyglutaric aciduria, Portugal has one of the largest national series: 21 patients from 18 families studied at the Institute of Medical Genetics in Porto, with three frequent variants — c.529delC (normalised as NM_024884.3:c.530del p.(Pro177HisfsTer6)) in 12 of 30 mutant alleles (40%), c.208C>T p.(Arg70Ter) in 7 (23%) and c.293A>G p.(His98Arg) in 4 (13%) —, which suggests a common origin; three families remained without an identified variant, and three patients in the series had astrocytomas [54, 103, 112]. The sources consulted describe no Brazilian series of these diseases.

9.4 Special populations

Severe neonatal presentation and adult onset coexist within some entities, and mild forms are probably underdiagnosed: the severe form of D2L2AD is lethal early and the myasthenic form is mistaken for other myasthenic syndromes [10]; the dystonic and mild forms of fumarase deficiency reach diagnosis in adolescence [104, 110]. Patients on KD need nutritional and lipid monitoring, and children with complex needs benefit from care coordination and from social and palliative support [4, 128].

9.5 European network and patient associations

The relevant European reference network is MetabERN, through the PM-MD subnetwork (disorders of pyruvate metabolism, Krebs cycle defects, oxidative phosphorylation disorders and thiamine metabolism) [2]. The patient associations listed by GeneReviews are the United Mitochondrial Disease Foundation, Metabolic Support UK, the American Epilepsy Society and the Epilepsy Foundation [4, 150, 151]. For CMS23 and the optic neuropathies, the relevant networks are the neuromuscular and ophthalmological ones.

10. Illustrative Clinical Case

Ryder and colleagues described in 2018 an Australian girl with fumarase deficiency and a milder course than predicted, treated with a high-fat, low-carbohydrate diet [89]. The first child of non-consanguineous parents of Italian and Australian origin, she was born by emergency caesarean section at 35 weeks, because of maternal hypertension and abnormal liver function tests, with growth parameters above the 90th percentile and Apgar scores of 8 and 9. She required tube feeding for the first 4 days and phototherapy, and continued to have feeding difficulties; at 7 months her weight had fallen to the 3rd percentile and there was hypotonia and delay in early milestones. At 9 months, a urinary metabolic screen revealed increased fumaric acid. On examination she had relative macrocephaly (head circumference between the 75th and 90th percentiles), weight below the 3rd percentile, no dysmorphic features, eczema, reduced muscle bulk and generalised hypotonia, and was unable to sit unsupported. Blood and urine lactate were normal and there were no other cycle intermediates in the urine; amino acids and acylcarnitines were normal [89].

Fumarase activity in fibroblasts was 9% of normal (9 nmol/min/mg protein; reference 78–119), and molecular analysis showed compound heterozygosity for FH NM_000143.4:c.521C>G p.(Pro174Arg) and c.1204C>T p.(His402Tyr), both previously described in severe cases with early death; each parent was heterozygous for one of them [89, 103]. At 14 months she started a diet targeting 60% of energy from fat, 30% from carbohydrate and 10% from protein (starting from 36%, 48% and 16%), complicated by milk and soy allergy and by increased weight gain; she kept to the diet throughout life, with a minimum of 50% fat during periods of lower adherence, calcium supplementation and surveillance tests (full blood count, liver and renal function, lactate, amino acids, essential fatty acids, lipids and micronutrients) within normal limits. At 13 years the diet contained 58% fat, 25% carbohydrate and 14% protein (1.7 g/kg/day), without protein restriction. The home sick-day plan provided for a 50:50 ratio of lipids to carbohydrates, and the hospital plan for fluids with 0.45% NaCl and 5% dextrose and 20% lipid emulsion at 2–3 g/kg/day [89].

She sat at 11 months and walked at 18 months; at 4 years and 7 months she had mild to moderate intellectual disability and attended a special education unit. MRI at 1 year showed mild frontal atrophy, global reduction of white matter with prominent lateral ventricles and suspected frontal polymicrogyria; serial MRI confirmed asymmetric sulcal and gyral patterns consistent with polymicrogyria in the anterior frontal, posterior parietal and temporal regions, with ventriculomegaly due to white matter loss (figure 1 of the article: https://pmc.ncbi.nlm.nih.gov/articles/PMC6122040/figure/Fig1/) [89]. Seizures began at 22 months with status epilepticus and were relatively controlled with carbamazepine monotherapy; at 10 years a multifenestrated atrial septal defect was repaired. Urinary fumaric acid fell from values of 650–1262 µmol/mmol creatinine before the diet to 344–429 µmol/mmol creatinine in 2016–2017. At 14 years she had been seizure-free for more than a year, with weight on the 85th percentile, height on the 70th percentile and head circumference on the 98th percentile, took part in community activities and had had no metabolic decompensations [89]. In adolescence, annual abdominal MRI was recommended because of the HLRCC risk, with no lesions up to publication, and the same screening for her parents, who were referred to a familial cancer clinic [89].

The case illustrates the dissociation between predicted severity and course (9% activity and variants described in fatal cases), the possible normality of lactate and other intermediates, and the dual dimension of counselling, metabolic and oncological. The diet used is not a classic KD and runs counter to the contraindication of KD in fumarase deficiency expressed by Saudubray's textbook and GeneReviews; the authors cannot demonstrate that the favourable course is due to the diet, and the approach should not be presented as routine [3, 4, 89].

10.1 Complementary case: L-2-hydroxyglutaric aciduria

Lipiński and colleagues described a Polish girl, the second child of healthy parents, born at 38 weeks weighing 3650 g with a head circumference of 36 cm, with normal early development (she sat at 6 months, walked at 14 months and said her first words at 8 months) [114]. At 5 years she was admitted with first generalised tonic-clonic seizures; she had a head circumference of 51 cm (50th–75th percentile), mild intellectual disability and ataxic gait, with normal glucose, ammonia, lactate and blood gases and normal fundoscopy and echocardiogram. MRI showed leukodystrophic changes with T2 hyperintensity of the globi pallidi and dentate nuclei; the electroencephalogram showed generalised paroxysmal activity, and valproate controlled the seizures [114]. Urinary L-2-HG was 258 mmol/mol creatinine and serum lysine 299 µmol/L (reference 66–266).

Sequencing identified compound heterozygosity for NM_024884.3:c.829C>T p.(Arg277Ter), pathogenic and previously described in the Portuguese series and in the 2010 update, and for the novel splicing variant c.1196+1G>A, classified as likely pathogenic, both confirmed in the parents [103, 114]. Over 10 years of follow-up, the epilepsy remained controlled, the ataxia did not progress, and the intellectual disability became moderate, with autistic behaviour. At 14 years, 6 months of riboflavin 100 mg/day had no clinical or biochemical effect (L-2-HG 318 vs 251 mmol/mol creatinine), and subsequent MRI showed the same lesions, with more marked cortical atrophy and ventricular dilatation [114].

The case illustrates the characteristic MRI pattern, the slow course and the absence of response to riboflavin when both alleles are null, which is consistent with the chaperone hypothesis; a therapeutic trial of riboflavin is not a validated practice, and the decision weighs the genotype [54, 114].

11. Prognosis and Long-Term Follow-up

11.1 Prognosis by entity

The prognosis varies with the entity and, within it, with residual activity. In fumarase deficiency, many patients do not survive early childhood or die of secondary complications, such as respiratory failure, in the first decade; seizures become more frequent and refractory; there are long-term survivors with moderate intellectual disability and ambulant forms [4, 104, 110]. There is no clinically useful genotype–phenotype correlation, and enzyme activity below 10% is generally associated with a severe phenotype but does not reliably predict it [4, 89]. In D2L2AD, eight of 13 patients described up to 2014 died at a median age of 7.5 months; the combination of two alleles with less than 25% activity is associated with death in the first year or dependence on intensive care, and ventilation and gastrostomy were associated with longer survival, regardless of genotype [13, 116]. In CMS23 the weakness is not progressive [109].

In the MAS defects, AGC1 deficiency had a median follow-up of 5 years and 7 months (up to 11 years), with one death at 7 years; GOT2 deficiency has a progressive course with loss of skills, but with documented survival up to 30 years; one of the four patients with MDH2 deficiency died at 20 months from decompensation [11, 12, 71]. In MPC1 deficiency, severity ranges from death at 19 months to mild impairment with follow-up up to about 20 years [11, 64]. In OGDH deficiency, the descriptions include death at around 30 months in three siblings, death at 12 months during pneumonia and survival into adolescence with severe dependence [62, 74, 75]. In ICRD due to ACO2, psychomotor disability is profound, with documented survival up to 18 years [69]. In SUCLA2 deficiency, median survival is 20 years (21 years with missense variants and 15 with loss-of-function variants) and about 30% die in childhood; in SUCLG1, it is 20 months [5, 6, 14]. In complex II deficiency, the course ranges from fatal leukoencephalopathy in early childhood to indolent disease with survival into adulthood [78, 117].

In the 2-hydroxyglutaric acidurias, type II had deaths in 9 of 19 patients between 4 months and 22 years, and life expectancy in type I is not defined, with asymptomatic forms [54]. The L form progresses slowly; many patients are bedridden by adolescence, but there are diagnoses in adulthood, up to 57 years in the Japanese review, and the risk of brain tumour, about 5%, conditions the prognosis [56, 58, 123].

11.2 Surveillance

Systematic cardiological, endocrinological, audiological and ophthalmological surveillance derives from the standards of care for primary mitochondrial disease and applies by extrapolation to the entities with a multisystem phenotype; in fumarase deficiency and the MAS defects, GeneReviews and the published series focus surveillance on neurology, nutrition, development and the musculoskeletal system [4, 11, 130].

12. Summary and Final Considerations

Krebs cycle and malate-aspartate shuttle defects are ultra-rare diseases, almost all autosomal recessive, described mainly by exome sequencing in consanguineous families and known from counts of a few dozen patients. They share an early-onset encephalopathy with hypotonia, epilepsy, microcephaly, white matter abnormalities and raised lactate, but are distinguished by biochemical and imaging clues that guide the diagnosis: isolated fumaric acid and polymicrogyria in fumarase deficiency; urinary 2-ketoglutarate and high glutamine in OGDH deficiency; urinary malate and fumarate with a high lactate/pyruvate ratio in MDH2 deficiency; high ammonia, citrulline and lactate with low serine in GOT2 deficiency; very low brain NAA with hypomyelination in AGC1 deficiency; both 2-hydroxyglutarate enantiomers with low urinary citrate in citrate carrier deficiency; lactic acidosis with a normal lactate/pyruvate ratio and normal pyruvate dehydrogenase activity in MPC1 deficiency; a succinate peak on spectroscopy in complex II deficiency. The markers frequently fail, and exome or genome sequencing with copy number variant analysis, complemented by functional studies, is the route to confirmation.

Reading the zygosity is part of the diagnosis. The same FH variants that cause the recessive deficiency may confer on heterozygous parents and siblings a risk of leiomyomatosis and renal carcinoma, which requires oncological assessment of the family; ACO2 causes recessive disease and dominant optic atrophy; SDHA causes recessive mitochondrial disease and dominant tumour predisposition or neurodegeneration; and in 2026 OGDH also acquired a late-onset monoallelic form, still with limited evidence.

There are no guidelines or approved medicines for these diseases. The targeted therapies rest on case series: the ketogenic diet reduced seizures in about three quarters of treated patients with MAS or MPC1 defects, with a notable effect in AGC1 deficiency; the combination of L-serine and pyridoxine controlled seizures in most patients with GOT2 deficiency who received it; oral citrate stabilised some patients with citrate carrier deficiency; acetylcholinesterase inhibitors and 3,4-diaminopyridine help some patients with congenital myasthenic syndrome type 23. In the enzymatic defects of the cycle itself, the ketogenic diet is considered contraindicated by some textbooks but not by the 2018 international consensus, and the only favourable experience with a high-fat diet in fumarase deficiency is an isolated case. Prevention of catabolism, judicious choice of drugs and anaesthetics according to the mitochondrial disease consensus statements, multidisciplinary support and referral to MetabERN centres complete the approach.

The 2-hydroxyglutaric acidurias link the class to 2-oxoglutarate metabolism, and chiral separation of 2-HG points to D2HGDH, IDH2, L2HGDH or SLC25A1. D2HGA2, dominant and almost always de novo, has cardiomyopathy in about half of patients and is the first entity in the group with a mechanism-directed therapy, enasidenib, still limited to reports; the L form, recessive and slowly progressive, has a characteristic MRI pattern, a brain tumour risk of about 5% and a response to riboflavin restricted to some genotypes with residual activity [54, 56, 114, 121, 143].

13. References

1. Orphanet. Tricarboxylic acid cycle disorder (ORPHA:254749). Available from: https://www.orpha.net/en/disease/detail/254749
2. MetabERN — European Reference Network for Hereditary Metabolic Disorders. Subnetworks (PM-MD). Available from: https://metab.ern-net.eu/
3. Brivet M, Gaignard P, Schiff M. Disorders of Pyruvate Metabolism and the Tricarboxylic Acid Cycle. In: Saudubray JM, Baumgartner MR, García-Cazorla Á, Walter J, editors. Inborn Metabolic Diseases: Diagnosis and Treatment. 7th ed. Berlin: Springer; 2022. Chapter 11, sections 11.5–11.13. doi:10.1007/978-3-662-63123-2_11
4. Coman D, Kranc KR, Christodoulou J. Fumarate Hydratase Deficiency. 2006 Jul 5 [updated 2026 Jun 9]. In: Adam MP, Feldman J, Mirzaa GM, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993–. Available from: https://www.ncbi.nlm.nih.gov/books/NBK1506/
5. El-Hattab AW, Scaglia F. SUCLA2-Related Mitochondrial DNA Depletion Syndrome, Encephalomyopathic Form with Methylmalonic Aciduria. 2009 May 26 [updated 2023 Sep 28]. In: Adam MP, Feldman J, Mirzaa GM, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993–. Available from: https://www.ncbi.nlm.nih.gov/books/NBK6803/
6. El-Hattab AW, Scaglia F. SUCLG1-Related Mitochondrial DNA Depletion Syndrome, Encephalomyopathic Form with Methylmalonic Aciduria. 2017 Mar 30. In: Adam MP, Feldman J, Mirzaa GM, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993–. Available from: https://www.ncbi.nlm.nih.gov/books/NBK425223/
7. GeneGuidance. MPC1 — gene and condition record (local database).
8. GeneGuidance. ACO2 — gene and condition record (local database).
9. GeneGuidance. OGDH — gene and condition record (local database).
10. GeneGuidance. SLC25A1 — gene and condition record (local database).
11. Bölsterli BK, Boltshauser E, Palmieri L, Spenger J, Brunner-Krainz M, Distelmaier F, et al. Ketogenic Diet Treatment of Defects in the Mitochondrial Malate Aspartate Shuttle and Pyruvate Carrier. Nutrients. 2022;14(17):3605. doi:10.3390/nu14173605
12. German HM, Zaki MS, Usmani MA, Karagoz I, Efthymiou S, Abdel-Hamid MS, et al. Comprehensive genotypic, phenotypic, and biochemical characterization of GOT2 deficiency: A progressive neurodevelopmental disorder with epilepsy and abnormal movements. Genet Med. 2025;27(12):101587. doi:10.1016/j.gim.2025.101587
13. Pop A, Williams M, Struys EA, Monné M, Jansen EEW, De Grassi A, et al. An overview of combined D-2- and L-2-hydroxyglutaric aciduria: functional analysis of CIC variants. J Inherit Metab Dis. 2018;41(2):169-180. doi:10.1007/s10545-017-0106-7
14. Carrozzo R, Verrigni D, Rasmussen M, de Coo R, Amartino H, Bianchi M, et al. Succinate-CoA ligase deficiency due to mutations in SUCLA2 and SUCLG1: phenotype and genotype correlations in 71 patients. J Inherit Metab Dis. 2016;39(2):243-252. doi:10.1007/s10545-015-9894-9
15. Fullerton M, McFarland R, Taylor RW, Alston CL. The genetic basis of isolated mitochondrial complex II deficiency. Mol Genet Metab. 2020;131(1-2):53-65. doi:10.1016/j.ymgme.2020.09.009
16. Charif M, Gueguen N, Ferré M, Elkarhat Z, Khiati S, LeMao M, et al. Dominant ACO2 mutations are a frequent cause of isolated optic atrophy. Brain Commun. 2021;3(2):fcab063. doi:10.1093/braincomms/fcab063
17. Penkl M, Mayr JA, Feichtinger RG, Reilmann R, Debus O, Fobker M, et al. Anaplerotic Therapy Using Triheptanoin in Two Brothers Suffering from Aconitase 2 Deficiency. Metabolites. 2024;14(4):238. doi:10.3390/metabo14040238
18. Orphanet. Fumaric aciduria (ORPHA:24). Available from: https://www.orpha.net/en/disease/detail/24
19. Orphanet. Oxoglutaric aciduria (ORPHA:31). Available from: https://www.orpha.net/en/disease/detail/31
20. Orphanet. Mitochondrial pyruvate carrier deficiency (ORPHA:447784). Available from: https://www.orpha.net/en/disease/detail/447784
21. Orphanet. D,L-2-hydroxyglutaric aciduria (ORPHA:356978). Available from: https://www.orpha.net/en/disease/detail/356978
22. Orphanet. Epileptic encephalopathy with global cerebral demyelination (ORPHA:353217). Available from: https://www.orpha.net/en/disease/detail/353217
23. Orphanet. Infantile cerebellar-retinal degeneration (ORPHA:313850). Available from: https://www.orpha.net/en/disease/detail/313850
24. Orphanet. Autosomal recessive isolated optic atrophy (ORPHA:98676). Available from: https://www.orpha.net/en/disease/detail/98676
25. Orphanet. Isolated succinate-CoQ reductase deficiency (ORPHA:3208). Available from: https://www.orpha.net/en/disease/detail/3208
26. Orphanet. Congenital myasthenic syndrome with mitochondrial defect (ORPHA:716917). Available from: https://www.orpha.net/en/disease/detail/716917
27. Orphanet. Mitochondrial DNA depletion syndrome, encephalomyopathic form with methylmalonic aciduria (ORPHA:1933). Available from: https://www.orpha.net/en/disease/detail/1933
28. Orphanet. Fatal infantile lactic acidosis with methylmalonic aciduria (ORPHA:17). Available from: https://www.orpha.net/en/disease/detail/17
29. Orphanet. D-2-hydroxyglutaric aciduria (ORPHA:79315). Available from: https://www.orpha.net/en/disease/detail/79315
30. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 614741 — MITOCHONDRIAL PYRUVATE CARRIER DEFICIENCY; MPYCD. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/614741
31. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 614559 — INFANTILE CEREBELLAR-RETINAL DEGENERATION; ICRD. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/614559
32. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 616289 — OPTIC ATROPHY 9; OPA9. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/616289
33. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 619007 — RETINITIS PIGMENTOSA 90; RP90. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/619007
34. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 612572 — RETINITIS PIGMENTOSA 46; RP46. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/612572
35. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 613657 — D-2-HYDROXYGLUTARIC ACIDURIA 2; D2HGA2. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/613657
36. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 203740 — OXOGLUTARATE DEHYDROGENASE DEFICIENCY; OGDHD. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/203740
37. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 612073 — MITOCHONDRIAL DNA DEPLETION SYNDROME 5 (ENCEPHALOMYOPATHIC WITH OR WITHOUT METHYLMALONIC ACIDURIA); MTDPS5. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/612073
38. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 245400 — MITOCHONDRIAL DNA DEPLETION SYNDROME 9 (ENCEPHALOMYOPATHIC TYPE WITH METHYLMALONIC ACIDURIA); MTDPS9. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/245400
39. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 252011 — MITOCHONDRIAL COMPLEX II DEFICIENCY, NUCLEAR TYPE 1; MC2DN1. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/252011
40. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 619166 — MITOCHONDRIAL COMPLEX II DEFICIENCY, NUCLEAR TYPE 2; MC2DN2. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/619166
41. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 619167 — MITOCHONDRIAL COMPLEX II DEFICIENCY, NUCLEAR TYPE 3; MC2DN3. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/619167
42. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 619224 — MITOCHONDRIAL COMPLEX II DEFICIENCY, NUCLEAR TYPE 4; MC2DN4. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/619224
43. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 606812 — FUMARASE DEFICIENCY; FMRD. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/606812
44. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 617339 — DEVELOPMENTAL AND EPILEPTIC ENCEPHALOPATHY 51; DEE51. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/617339
45. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 612949 — DEVELOPMENTAL AND EPILEPTIC ENCEPHALOPATHY 39 WITH LEUKODYSTROPHY; DEE39. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/612949
46. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 618959 — DEVELOPMENTAL AND EPILEPTIC ENCEPHALOPATHY 88; DEE88. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/618959
47. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 618721 — DEVELOPMENTAL AND EPILEPTIC ENCEPHALOPATHY 82; DEE82. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/618721
48. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 615182 — COMBINED D-2- AND L-2-HYDROXYGLUTARIC ACIDURIA; D2L2AD. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/615182
49. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 618197 — MYASTHENIC SYNDROME, CONGENITAL, 23, PRESYNAPTIC; CMS23. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/618197
50. Abicht A, Müller JS, Lochmüller H. Congenital Myasthenic Syndromes Overview. 2003 May 9 [updated 2021 Dec 23]. In: Adam MP, Feldman J, Mirzaa GM, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993–. Available from: https://www.ncbi.nlm.nih.gov/books/NBK1168/
51. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 600721 — D-2-HYDROXYGLUTARIC ACIDURIA 1; D2HGA1. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/600721
52. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 236792 — L-2-HYDROXYGLUTARIC ACIDURIA; L2HGA. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/236792
53. Orphanet. L-2-hydroxyglutaric aciduria (ORPHA:79314). Available from: https://www.orpha.net/en/disease/detail/79314
54. Kranendijk M, Struys EA, Salomons GS, Van der Knaap MS, Jakobs C. Progress in understanding 2-hydroxyglutaric acidurias. J Inherit Metab Dis. 2012;35(4):571-587. doi:10.1007/s10545-012-9462-5
55. Steenweg ME, Jakobs C, Errami A, van Dooren SJ, Adeva Bartolomé MT, Aerssens P, et al. An overview of L-2-hydroxyglutarate dehydrogenase gene (L2HGDH) variants: a genotype-phenotype study. Hum Mutat. 2010;31(4):380-390. doi:10.1002/humu.21197
56. Patay Z, Mills JC, Löbel U, Lambert A, Sablauer A, Ellison DW. Cerebral neoplasms in L-2 hydroxyglutaric aciduria: 3 new cases and meta-analysis of literature data. AJNR Am J Neuroradiol. 2012;33(5):940-943. doi:10.3174/ajnr.A2869
57. Kölker S, Hoffmann GF. Cerebral Organic Acid Disorders and Other Disorders of Lysine Catabolism. In: Saudubray JM, Baumgartner MR, García-Cazorla Á, Walter J, editors. Inborn Metabolic Diseases: Diagnosis and Treatment. 7th ed. Berlin: Springer; 2022. Chapter 22, sections 22.7, 22.8, 22.9 and 22.13. doi:10.1007/978-3-662-63123-2_22
58. Nyhan WL, Hoffmann GF, Al-Aqeel AI, Barshop BA. L-2-hydroxyglutaric aciduria. In: Atlas of Inherited Metabolic Diseases. 4th ed. Boca Raton: CRC Press; 2020. p. 95–99.
59. Saudubray JM, García-Cazorla Á. Clinical Approach to Inborn Errors of Metabolism in Paediatrics. In: Saudubray JM, Baumgartner MR, García-Cazorla Á, Walter J, editors. Inborn Metabolic Diseases: Diagnosis and Treatment. 7th ed. Berlin: Springer; 2022. Chapter 1, sections 1.1.3.2, 1.3–1.5 (tables). doi:10.1007/978-3-662-63123-2_1
60. Kohlschütter A, Behbehani A, Langenbeck U, Albani M, Heidemann P, Hoffmann G, et al. A familial progressive neurodegenerative disease with 2-oxoglutaric aciduria. Eur J Pediatr. 1982;138(1):32-37. doi:10.1007/bf00442325
61. Zinn AB, Kerr DS, Hoppel CL. Fumarase deficiency: a new cause of mitochondrial encephalomyopathy. N Engl J Med. 1986;315(8):469-475. doi:10.1056/nejm198608213150801
62. Bonnefont JP, Chretien D, Rustin P, Robinson B, Vassault A, Aupetit J, et al. Alpha-ketoglutarate dehydrogenase deficiency presenting as congenital lactic acidosis. J Pediatr. 1992;121(2):255-258. doi:10.1016/s0022-3476(05)81199-0
63. Kerrigan JF, Aleck KA, Tarby TJ, Bird CR, Heidenreich RA. Fumaric aciduria: clinical and imaging features. Ann Neurol. 2000;47(5):583-588. doi:10.1002/1531-8249(200005)47:5<583::aid-ana5>3.0.co;2-y
64. Brivet M, Garcia-Cazorla A, Lyonnet S, Dumez Y, Nassogne MC, Slama A, et al. Impaired mitochondrial pyruvate importation in a patient and a fetus at risk. Mol Genet Metab. 2003;78(3):186-192. doi:10.1016/s1096-7192(03)00016-7
65. Bricker DK, Taylor EB, Schell JC, Orsak T, Boutron A, Chen YC, et al. A mitochondrial pyruvate carrier required for pyruvate uptake in yeast, Drosophila, and humans. Science. 2012;337(6090):96-100. doi:10.1126/science.1218099
66. Hartong DT, Dange M, McGee TL, Berson EL, Dryja TP, Colman RF. Insights from retinitis pigmentosa into the roles of isocitrate dehydrogenases in the Krebs cycle. Nat Genet. 2008;40(10):1230-1234. doi:10.1038/ng.223
67. Wibom R, Lasorsa FM, Töhönen V, Barbaro M, Sterky FH, Kucinski T, et al. AGC1 deficiency associated with global cerebral hypomyelination. N Engl J Med. 2009;361(5):489-495. doi:10.1056/nejmoa0900591
68. Ghezzi D, Goffrini P, Uziel G, Horvath R, Klopstock T, Lochmüller H, et al. SDHAF1, encoding a LYR complex-II specific assembly factor, is mutated in SDH-defective infantile leukoencephalopathy. Nat Genet. 2009;41(6):654-656. doi:10.1038/ng.378
69. Spiegel R, Pines O, Ta-Shma A, Burak E, Shaag A, Halvardson J, et al. Infantile cerebellar-retinal degeneration associated with a mutation in mitochondrial aconitase, ACO2. Am J Hum Genet. 2012;90(3):518-523. doi:10.1016/j.ajhg.2012.01.009
70. Nota B, Struys EA, Pop A, Jansen EE, Fernandez Ojeda MR, Kanhai WA, et al. Deficiency in SLC25A1, encoding the mitochondrial citrate carrier, causes combined D-2- and L-2-hydroxyglutaric aciduria. Am J Hum Genet. 2013;92(4):627-631. doi:10.1016/j.ajhg.2013.03.009
71. Ait-El-Mkadem S, Dayem-Quere M, Gusic M, Chaussenot A, Bannwarth S, François B, et al. Mutations in MDH2, Encoding a Krebs Cycle Enzyme, Cause Early-Onset Severe Encephalopathy. Am J Hum Genet. 2017;100(1):151-159. doi:10.1016/j.ajhg.2016.11.014
72. Broeks MH, Shamseldin HE, Alhashem A, Hashem M, Abdulwahab F, Alshedi T, et al. MDH1 deficiency is a metabolic disorder of the malate-aspartate shuttle associated with early onset severe encephalopathy. Hum Genet. 2019;138(11-12):1247-1257. doi:10.1007/s00439-019-02063-z
73. van Karnebeek CDM, Ramos RJ, Wen XY, Tarailo-Graovac M, Gleeson JG, Skrypnyk C, et al. Bi-allelic GOT2 Mutations Cause a Treatable Malate-Aspartate Shuttle-Related Encephalopathy. Am J Hum Genet. 2019;105(3):534-548. doi:10.1016/j.ajhg.2019.07.015
74. Yap ZY, Strucinska K, Matsuzaki S, Lee S, Si Y, Humphries K, et al. A biallelic pathogenic variant in the OGDH gene results in a neurological disorder with features of a mitochondrial disease. J Inherit Metab Dis. 2021;44(2):388-400. doi:10.1002/jimd.12248
75. Whittle EF, Chilian M, Karimiani EG, Progri H, Buhas D, Kose M, et al. Biallelic variants in OGDH encoding oxoglutarate dehydrogenase lead to a neurodevelopmental disorder characterized by global developmental delay, movement disorder, and metabolic abnormalities. Genet Med. 2023;25(2):100332. doi:10.1016/j.gim.2022.11.001
76. Chaouch A, Porcelli V, Cox D, Edvardson S, Scarcia P, De Grassi A, et al. Mutations in the Mitochondrial Citrate Carrier SLC25A1 are Associated with Impaired Neuromuscular Transmission. J Neuromuscul Dis. 2014;1(1):75-90. doi:10.3233/jnd-140021
77. GeneGuidance. FH — gene and condition record (local database).
78. GeneGuidance. SDHA — gene and condition record (local database).
79. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 619259 — NEURODEGENERATION WITH ATAXIA AND LATE-ONSET OPTIC ATROPHY; NDAXOA. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/619259
80. Van de Vondel L, Lee GS, De Winter J, Matsuzaki S, Sandoval A, Ramirez JF, et al. Heterozygous OGDH Variants Are Involved in Peripheral Neuropathy With Ataxia and Optical Atrophy. JIMD Rep. 2026;67(4):e70103. doi:10.1002/jmd2.70103
81. Cascón A, Comino-Méndez I, Currás-Freixes M, de Cubas AA, Contreras L, Richter S, et al. Whole-exome sequencing identifies MDH2 as a new familial paraganglioma gene. J Natl Cancer Inst. 2015;107(5):djv053. doi:10.1093/jnci/djv053
82. GeneGuidance. MDH2 — gene and condition record (local database).
83. GeneGuidance. DLST — gene and condition record (local database).
84. GeneGuidance. SLC25A11 — gene and condition record (local database).
85. Kranendijk M, Struys EA, Gibson KM, Wickenhagen WV, Abdenur JE, Buechner J, et al. Evidence for genetic heterogeneity in D-2-hydroxyglutaric aciduria. Hum Mutat. 2010;31(3):279-283. doi:10.1002/humu.21186
86. Kranendijk M, Struys EA, van Schaftingen E, Gibson KM, Kanhai WA, van der Knaap MS, et al. IDH2 mutations in patients with D-2-hydroxyglutaric aciduria. Science. 2010;330(6002):336. doi:10.1126/science.1192632
87. Orphanet. 2-hydroxyglutaric aciduria (ORPHA:19). Available from: https://www.orpha.net/en/disease/detail/19
88. Orphanet. Metaphyseal chondromatosis with D-2-hydroxyglutaric aciduria (ORPHA:99646). Available from: https://www.orpha.net/en/disease/detail/99646
89. Ryder B, Moore F, Mitchell A, Thompson S, Christodoulou J, Balasubramaniam S. Fumarase Deficiency: A Safe and Potentially Disease Modifying Effect of High Fat/Low Carbohydrate Diet. JIMD Rep. 2018;40:77-83. doi:10.1007/8904_2017_65
90. Jiang H, Alahmad A, Fu S, Fu X, Liu Z, Han X, et al. Identification and characterization of novel MPC1 gene variants causing mitochondrial pyruvate carrier deficiency. J Inherit Metab Dis. 2022;45(2):264-277. doi:10.1002/jimd.12462
91. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 614737 — MITOCHONDRIAL PYRUVATE CARRIER 2; MPC2. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/614737
92. GeneGuidance. IDH3B — gene and condition record (local database).
93. Pierrache LHM, Kimchi A, Ratnapriya R, Roberts L, Astuti GDN, Obolensky A, et al. Whole-Exome Sequencing Identifies Biallelic IDH3A Variants as a Cause of Retinitis Pigmentosa Accompanied by Pseudocoloboma. Ophthalmology. 2017;124(7):992-1003. doi:10.1016/j.ophtha.2017.03.010
94. Nyhan WL, Hoffmann GF, Al-Aqeel AI, Barshop BA. D-2-hydroxyglutaric (DL-2-hydroxyglutaric) aciduria. In: Atlas of Inherited Metabolic Diseases. 4th ed. Boca Raton: CRC Press; 2020. p. 87–94.
95. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 126063 — DIHYDROLIPOAMIDE S-SUCCINYLTRANSFERASE; DLST. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/126063
96. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 617513 — OXOGLUTARATE DEHYDROGENASE-LIKE PROTEIN; OGDHL. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/617513
97. Online Mendelian Inheritance in Man, OMIM®. MIM Number: 619701 — YOON-BELLEN NEURODEVELOPMENTAL SYNDROME; YOBELN. Johns Hopkins University, Baltimore, MD. Available from: https://omim.org/entry/619701
98. Liu X, Zhang G, Yu D, Han J. An Emerging Role for OGDHL: From Mitochondrial Energy Metabolism to Neurodevelopmental Disorders. Biology (Basel). 2025;14(12):1777. doi:10.3390/biology14121777
99. Jiang H, Xu C, Liu Z, Duan R, Yao X, Fu X, et al. Case Report: Biallelic variants in MRPS36, encoding a component of the 2-oxoglutarate dehydrogenase complex, cause leigh syndrome. Front Pediatr. 2025;13:1608840. doi:10.3389/fped.2025.1608840
100. Ostergaard E, Hansen FJ, Sorensen N, Duno M, Vissing J, Larsen PL, et al. Mitochondrial encephalomyopathy with elevated methylmalonic acid is caused by SUCLA2 mutations. Brain. 2007;130(Pt 3):853-861. doi:10.1093/brain/awl383
101. Rahman S, Mayr JA. Disorders of Oxidative Phosphorylation. In: Saudubray JM, Baumgartner MR, García-Cazorla Á, Walter J, editors. Inborn Metabolic Diseases: Diagnosis and Treatment. 7th ed. Berlin: Springer; 2022. Chapter 10. doi:10.1007/978-3-662-63123-2_10
102. Orphanet. Hereditary myopathy with lactic acidosis due to ISCU deficiency (ORPHA:43115). Available from: https://www.orpha.net/en/disease/detail/43115
103. Freeman PJ, Hart RK, Gretton LJ, Brookes AJ, Dalgleish R. VariantValidator: Accurate validation, mapping, and formatting of sequence variation descriptions. Hum Mutat. 2018;39(1):61-68. doi:10.1002/humu.23348
104. Ting SL, Bassett J, Meyer J, Anderson M, Bowron A, Weber A, et al. A Diagnostic Conundrum in Fumarase Deficiency: Expanding the Clinical and Genetic Spectrum in a Cohort of Pediatric and Adult Patients. Am J Med Genet A. 2026;200(9):2000-2009. doi:10.1002/ajmg.a.70160
105. Huck CJ, Devaprasad I, Wilde BR. Biochemical characterization of the FH variant p.Lys414Glu reveals loss of enzymatic function and disrupted multimerization. Mol Genet Metab Rep. 2026;46:101287. doi:10.1016/j.ymgmr.2025.101287
106. Ferris S, Menendez Perdomo IM, Tarailo-Graovac M. Functional analysis of FH variants of uncertain significance using Caenorhabditis elegans model. Genet Med Open. 2025;3:103469. doi:10.1016/j.gimo.2025.103469
107. GeneGuidance. SLC25A12 — gene and condition record (local database).
108. Croney R, Shakeel H, Hamilton MJ, Ansari M, Ferdinandusse S, Forbes K, et al. Correspondence on "Comprehensive genotypic, phenotypic, and biochemical characterization of GOT2 deficiency: A progressive neurodevelopmental disorder with epilepsy and abnormal movements" by German et al. Genet Med. 2026;28(10):102681. doi:10.1016/j.gim.2026.102681
109. Balaraju S, Töpf A, McMacken G, Kumar VP, Pechmann A, Roper H, et al. Congenital myasthenic syndrome with mild intellectual disability caused by a recurrent SLC25A1 variant. Eur J Hum Genet. 2020;28(3):373-377. doi:10.1038/s41431-019-0506-2
110. Reis-Carneiro D, Robalo C, Laço M, Vidailhet M, Diogo L. Progressive Generalized Dystonia-Parkinsonism in a Child with Fumaric Aciduria. Mov Disord Clin Pract. 2022;9(5):707-709. doi:10.1002/mdc3.13450
111. Falk MJ, Li D, Gai X, McCormick E, Place E, Lasorsa FM, et al. AGC1 Deficiency Causes Infantile Epilepsy, Abnormal Myelination, and Reduced N-Acetylaspartate. JIMD Rep. 2014;14:77-85. doi:10.1007/8904_2013_287
112. Vilarinho L, Cardoso ML, Gaspar P, Barbot C, Azevedo L, Diogo L, et al. Novel L2HGDH mutations in 21 patients with L-2-hydroxyglutaric aciduria of Portuguese origin. Hum Mutat. 2005;26(4):395-396. doi:10.1002/humu.9373
113. Engin Erdal A, Özbey SZ, Civelek Ürey GB, Küçükçongar Yavaş A, Bilginer Gürbüz B, Gündüz M, et al. Genetic, neuroimaging, and clinical characteristics of a cohort of individuals with L-2-hydroxyglutaric aciduria from Türkiye. J Pediatr Endocrinol Metab. 2025;38(9):956-963. doi:10.1515/jpem-2025-0021
114. Lipiński P, Ciara E, Bogdańska A, Jurkiewicz E, Tylki-Szymańska A. Riboflavin treatment in L-2-hydroxyglutaric aciduria: report on a pediatric patient and literature review. J Appl Genet. 2026;67(2):443-449. doi:10.1007/s13353-025-00974-4
115. Remacha L, Pirman D, Mahoney CE, Coloma J, Calsina B, Currás-Freixes M, et al. Recurrent Germline DLST Mutations in Individuals with Multiple Pheochromocytomas and Paragangliomas. Am J Hum Genet. 2019;104(4):651-664. doi:10.1016/j.ajhg.2019.02.017
116. Mühlhausen C, Salomons GS, Lukacs Z, Struys EA, van der Knaap MS, Ullrich K, et al. Combined D2-/L2-hydroxyglutaric aciduria (SLC25A1 deficiency): clinical course and effects of citrate treatment. J Inherit Metab Dis. 2014;37(5):775-781. doi:10.1007/s10545-014-9702-y
117. Ohlenbusch A, Edvardson S, Skorpen J, Bjornstad A, Saada A, Elpeleg O, et al. Leukoencephalopathy with accumulated succinate is indicative of SDHAF1 related complex II deficiency. Orphanet J Rare Dis. 2012;7:69. doi:10.1186/1750-1172-7-69
118. Buhl E, Garg S, Monaghan M, Preston A, Likeman M, Dare J, et al. Infantile Cerebellar-Retinal Degeneration Associated With Novel ACO2 Variants: Clinical Features and Insights From a Drosophila Model. Clin Genet. 2025;108(3):266-278. doi:10.1111/cge.14745
119. Beaulieu C, Bouzidi A, Desquiret-Dumas V, Dieu X, Makam R, Jurkute N, et al. Clinical and Genetic Spectrum of ACO2-Linked Dominant Optic Atrophy. JAMA Ophthalmol. 2026;144(5):452-462. doi:10.1001/jamaophthalmol.2026.0634
120. GeneGuidance. SUCLA2 — gene and condition record (local database).
121. Gold JI, Stefanatos AK, Fraser JL, Vanderver A, Cuddapah S. Enasidenib-induced hepatitis in an individual with Type II D2-hydroxyglutaric aciduria. JIMD Rep. 2024;65(3):156-162. doi:10.1002/jmd2.12421
122. Roux J, Brody G, Metz B, Gao J, Xiao C, Chang RC. Cutaneous manifestations in D-2-hydroxyglutaric aciduria type 2 and response to enasidenib therapy. JAAD Case Rep. 2025;56:11-13. doi:10.1016/j.jdcr.2024.10.020
123. Matsuzono K, Koide R, Fujimoto S. Successful treatment of 2-hydroxyglutaric aciduria diagnosed in adulthood, three decades after the onset: a case report and literature review. Intern Med. 2026;65(15):2118-2123. doi:10.2169/internalmedicine.6360-25
124. Mochel F, Sedel F. Inborn Errors of Metabolism in Adults: A Diagnostic Approach to Neurological and Psychiatric Presentations. In: Saudubray JM, Baumgartner MR, García-Cazorla Á, Walter J, editors. Inborn Metabolic Diseases: Diagnosis and Treatment. 7th ed. Berlin: Springer; 2022. Chapter 2 (mitochondrial defects). doi:10.1007/978-3-662-63123-2_2
125. Parikh S, Goldstein A, Koenig MK, Scaglia F, Enns GM, Saneto R, et al. Diagnosis and management of mitochondrial disease: a consensus statement from the Mitochondrial Medicine Society. Genet Med. 2015;17(9):689-701. doi:10.1038/gim.2014.177
126. Touati G, Mochel F, Artuch R. Diagnostic Procedures. In: Saudubray JM, Baumgartner MR, García-Cazorla Á, Walter J, editors. Inborn Metabolic Diseases: Diagnosis and Treatment. 7th ed. Berlin: Springer; 2022. Chapter 3, section 3.1 (Tables 3.1–3.3). doi:10.1007/978-3-662-63123-2_3
127. Nyhan WL, Hoffmann GF, Al-Aqeel AI, Barshop BA. Introduction to lactic acidemias. In: Atlas of Inherited Metabolic Diseases. 4th ed. Boca Raton: CRC Press; 2020. p. 354–361.
128. Kossoff EH, Zupec-Kania BA, Auvin S, Ballaban-Gil KR, Christina Bergqvist AG, Blackford R, et al. Optimal clinical management of children receiving dietary therapies for epilepsy: Updated recommendations of the International Ketogenic Diet Study Group. Epilepsia Open. 2018;3(2):175-192. doi:10.1002/epi4.12225
129. van der Crabben SN, de Koning TJ. Serine Deficiency Disorders. 2023 Jun 22. In: Adam MP, Feldman J, Mirzaa GM, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993–. Available from: https://www.ncbi.nlm.nih.gov/books/NBK592681/
130. Parikh S, Goldstein A, Karaa A, Koenig MK, Anselm I, Brunel-Guitton C, et al. Patient care standards for primary mitochondrial disease: a consensus statement from the Mitochondrial Medicine Society. Genet Med. 2017;19(12):1380-1397. doi:10.1038/gim.2017.107
131. De Vries MC, Brown DA, Allen ME, Bindoff L, Gorman GS, Karaa A, et al. Safety of drug use in patients with a primary mitochondrial disease: An international Delphi-based consensus. J Inherit Metab Dis. 2020;43(4):800-818. doi:10.1002/jimd.12196
132. GeneGuidance. MDH1 — gene and condition record (local database).
133. Murano M, Iaconisi GN, Monné M, Ahmed A, Fiermonte G, Capobianco L, et al. Aspartate-Glutamate Carrier 1 (SLC25A12) Deficiency: Malate-Aspartate Shuttle Failure, Neurodevelopmental Epileptic Encephalopathy, and Ketone-Based Metabolic Therapy. Int J Mol Sci. 2026;27(10):4455. doi:10.3390/ijms27104455
134. Morris AAM, Jones S. Medications Used in the Treatment of Inborn Errors of Metabolism. In: Saudubray JM, Baumgartner MR, García-Cazorla Á, Walter J, editors. Inborn Metabolic Diseases: Diagnosis and Treatment. 7th ed. Berlin: Springer; 2022. Chapter 45 (table: pyridoxine, L-serine, riboflavin, dichloroacetate, triheptanoin). doi:10.1007/978-3-662-63123-2_45
135. Çapan ÖY, Türkdoğan D, Atalay S, Çağlayan HS. Developmental and epileptic encephalopathy 82 (DEE82) with novel compound heterozygous mutations of GOT2 gene. Seizure. 2024;116:126-132. doi:10.1016/j.seizure.2023.11.003
136. INFARMED. Piridoxina Labesfal 40 mg tablets — Summary of Product Characteristics. Approved on 01-04-2022. Infomed (med_id 38068). Available from: https://extranet.infarmed.pt/INFOMED-fo/download-ficheiro.xhtml?med_id=38068&tipo_doc=rcm
137. INFARMED. Benadon 300 mg film-coated tablets (pyridoxine hydrochloride) — Summary of Product Characteristics. Approved on 05-06-2024. Infomed (med_id 897). Available from: https://extranet.infarmed.pt/INFOMED-fo/download-ficheiro.xhtml?med_id=897&tipo_doc=rcm
138. INFARMED. Uralyt-U granules (citric acid + potassium citrate + sodium citrate) — Summary of Product Characteristics. Approved on 28-06-2023. Infomed (med_id 8905). Available from: https://extranet.infarmed.pt/INFOMED-fo/download-ficheiro.xhtml?med_id=8905&tipo_doc=rcm
139. INFARMED. Mestinon 60 mg coated tablets (pyridostigmine bromide) — Summary of Product Characteristics. Approved on 08-07-2020. Infomed. Available from: https://extranet.infarmed.pt/INFOMED-fo/
140. U.S. Food and Drug Administration. FIRDAPSE (amifampridine) tablets — prescribing information (NDA 208078; Catalyst Pharmaceuticals). Revised 04/2026. Available from: https://www.accessdata.fda.gov/scripts/cder/daf/index.cfm?event=overview.process&ApplNo=208078
141. European Medicines Agency. Medicines output — medicines report. Accessed 31 Oct 2026. Available from: https://www.ema.europa.eu/en/documents/report/medicines-output-medicines_json-report_en.json
142. U.S. Food and Drug Administration. DOJOLVI (triheptanoin) oral liquid — prescribing information (NDA 213687; Ultragenyx Pharmaceutical). Revised 06/2025. Drugs@FDA/openFDA. Available from: https://www.accessdata.fda.gov/scripts/cder/daf/index.cfm?event=overview.process&ApplNo=213687
143. Geoerger B, Schiff M, Penard-Lacronique V, Darin N, Saad SM, Duchon C, et al. Enasidenib treatment in two individuals with D-2-hydroxyglutaric aciduria carrying a germline IDH2 mutation. Nat Med. 2023;29(6):1358-1363. doi:10.1038/s41591-023-02382-9
144. U.S. Food and Drug Administration. IDHIFA (enasidenib) tablets — prescribing information (NDA 209606; Bristol Myers Squibb). Initial approval 01-08-2017; version of 16-01-2025. Available from: https://www.accessdata.fda.gov/scripts/cder/daf/index.cfm?event=overview.process&ApplNo=209606
145. U.S. Food and Drug Administration. Drugs@FDA: MESTINON (pyridostigmine bromide), NDA 015193 (Bausch). Original approval 1965-01-25. Available from: https://www.accessdata.fda.gov/scripts/cder/daf/index.cfm?event=overview.process&ApplNo=015193
146. European Medicines Agency. Idhifa (enasidenib): withdrawal of the marketing authorisation application (6 December 2019). Available from: https://www.ema.europa.eu/en/medicines/human/withdrawn-applications/idhifa
147. University of Pittsburgh. An Individual Patient, Open Label Study to Use ACER-001 to Treat Combined D,L-2-hydroxyglutaric Aciduria (C-2HGA). ClinicalTrials.gov identifier: NCT07125066. First posted 2025-08-15; last update 2026-06-08. Available from: https://clinicaltrials.gov/study/NCT07125066
148. Calsina B, Currás-Freixes M, Buffet A, Pons T, Contreras L, Letón R, et al. Role of MDH2 pathogenic variant in pheochromocytoma and paraganglioma patients. Genet Med. 2018;20(12):1652-1662. doi:10.1038/s41436-018-0068-7
149. Allegri G, Fernandes MJ, Scalco FB, Correia P, Simoni RE, Llerena JC, et al. Fumaric aciduria: an overview and the first Brazilian case report. J Inherit Metab Dis. 2010;33(4):411-419. doi:10.1007/s10545-010-9134-2
150. United Mitochondrial Disease Foundation. Available from: https://www.umdf.org/
151. Metabolic Support UK. Available from: https://www.metabolicsupportuk.org/