Lesch-Nyhan Syndrome and the HPRT Deficiency Spectrum

Hypoxanthine-guanine phosphoribosyltransferase (HGprt) deficiency: Lesch-Nyhan disease, HPRT1-related neurological dysfunction and HPRT1-related hyperuricaemia (Kelley-Seegmiller syndrome).

Literature and regulatory information updated: 6 October 2026.

0. Initial Reference Table

1. Introduction

Hypoxanthine-guanine phosphoribosyltransferase (HGprt, also abbreviated HPRT or HGPRT) deficiency is the classic model of a disease in which a single step in purine salvage produces two different kinds of clinical problem. The first is metabolic and treatable: uric acid overproduction leads to crystalluria, stones, nephropathy, tophi and gouty arthritis. The second is neurobehavioural and, to date, has no disease-modifying treatment: severe dystonia, cognitive impairment and compulsive self-injurious behaviour. Separation of these two axes guides the entire article, because controlling serum urate prevents renal failure but does not alter the neurological disease [1, 3, 7].

In 1964, Michael Lesch and William Nyhan described in the American Journal of Medicine a familial disease in two brothers combining hyperuricaemia, uricosuria and altered central nervous system function, with athetoid cerebral palsy and self-injury [7, 14]. The Atlas, written by Nyhan himself, emphasises that purine production from labelled glycine was approximately 20 times normal, whereas in adults with gout the highest values did not exceed twice normal; the distinctive behavioural feature was tissue loss through biting [3]. Pedigree studies rapidly located the gene on the X chromosome, and in 1967 Seegmiller, Rosenbloom and Kelley demonstrated loss of HGprt activity, the first example of a relationship between a specific enzyme defect and compulsive behaviour [3, 15]. In the same year, Kelley and colleagues identified partial deficiencies of the same enzyme in adults with gout and uric acid overproduction, without evident neurological involvement, the form subsequently called Kelley-Seegmiller syndrome [7, 16].

The reference patient for the intermediate form had been described before the enzyme was identified: in 1959, Catel and Schmidt reported a child with familial gouty diathesis and cerebral and renal symptoms, subsequently recognised as having a neurological variant with normal intelligence and behaviour [3, 7]. The gene was cloned in 1982–1983 by Jolly, Friedmann and colleagues, enabling characterisation of hundreds of variants; Fu and colleagues' 2014 review assembled 615 mutations and their relationship with enzyme activity and phenotype [3, 17].

Nomenclature evolved with understanding of the spectrum. The binary classification (Lesch-Nyhan versus Kelley-Seegmiller) was replaced by three categories (LND, HND and HRH), and the Spanish La Paz group proposed four grades based on functional independence, activity in haemolysate and intact erythrocytes, and predicted protein size [7, 18]. GeneReviews prefers “disease” to “syndrome”, because the cause is known, and emphasises that the categories are points on a continuum [1]. The searches performed identified no formal international scientific society consensus specific to this disease; the French-Italian series of 101 patients explicitly states that optimal management recommendations do not exist [19]. The recommendations in this article therefore rest on GeneReviews, supplemented by reference books, Torres and Puig's review, official labels and, for adults with gout, the American College of Rheumatology (ACR) and European Alliance of Associations for Rheumatology (EULAR) gout guidelines, applied by extrapolation [1, 7, 20, 21].

2. Molecular Genetics and Pathophysiology

2.1. The catalysed reaction and uric acid overproduction

HGprt (EC 2.4.2.8) transfers the 5′-phosphoribosyl group from PRPP to hypoxanthine and guanine, forming inosine monophosphate (IMP, inosinic acid) and guanosine monophosphate (GMP, guanylic acid), respectively. The enzyme exists in all cells, with particularly high activity in the basal ganglia and testis [3, 5]. When salvage fails, the bases are oxidised by xanthine oxidase to uric acid, and unused PRPP accumulates. Since PRPP amidotransferase, the rate-limiting enzyme in the de novo pathway, is not saturated by its substrate, increased PRPP and reduced nucleotide-mediated feedback inhibition accelerate purine synthesis [3, 5, 7].

The degree of overproduction is similar in LND and its variants: purine production appears fixed at a maximal level regardless of residual activity, explaining why renal and joint disease are common throughout the spectrum [3, 7]. Madeo's series confirms that enzyme activity and genotype correlate with neurological severity, but not with nephropathy or gout [19].

2.2. Neurobehavioural pathophysiology

Neurological pathogenesis remains unexplained. Post mortem studies show no consistent degenerative lesion, but reveal a 60–90% reduction in basal ganglia dopamine, depressed dopaminergic function in the caudate, putamen, nucleus accumbens and external pallidum, and increased putaminal D1 and D2 receptors, consistent with dopaminergic hypersensitivity [3, 5, 22]. On positron emission tomography (PET), dopamine transporter binding was reduced by 73% in the putamen and 56% in the caudate, and another study showed more than 60% lower fluorodopa uptake; CSF homovanillic acid is low [3]. In five brains, Göttle and colleagues documented reduced melanisation and tyrosine hydroxylase immunoreactivity in substantia nigra neurons, without degeneration, meaning loss of the dopaminergic neurochemical phenotype without neuronal loss [23].

Magnetic resonance imaging (MRI) volumetry in 21 LND patients showed a 20% reduction in intracranial volume compared with healthy adults (17% in grey matter and 26% in white matter), predominantly in the basal ganglia and frontotemporal and limbic regions, with parieto-occipital preservation; patients with variants were consistently intermediate between controls and LND [24]. An earlier study recorded a 34% reduction in caudate volume, interpreted as a developmental abnormality rather than atrophy [3]. These findings support a static neurodevelopmental lesion, consistent with the absence of neurodegenerative progression reported by GeneReviews [1].

Several complementary hypotheses have been proposed: local guanosine triphosphate (GTP) deficiency could limit synthesis of tetrahydrobiopterin (BH4), a tyrosine hydroxylase cofactor; HPRT-deficient mice have reduced striatal dopamine and BH4, but BH4 administration did not correct dopamine; adenosinergic systems and one-carbon metabolism are altered [3, 25]. The mouse knockout model does not reproduce self-injury, unlike Breese's pharmacological model (neonatal rats lesioned with 6-hydroxydopamine and exposed to dopaminergic agonists), which supported investigation of D1 antagonists [7, 26]. Recent research extends these observations to plasma proteomics (29 patients) and the relationship between purine metabolism and dystonia, without immediate clinical consequences [27, 28].

2.3. Gene, variant spectrum and nomenclature

HPRT1 spans more than 44 kb at Xq26, with nine exons and a 654-nucleotide coding region; the functional protein forms tetramers [3, 13]. In historical literature, the protein is sometimes described as having 217 amino acids (numbering without the initial methionine), whereas the current reference sequence has 218; variants in this article use HGVS (Human Genome Variation Society) numbering on NM_000194.3/NP_000185.1, the MANE Select transcript [3, 4]. Most variants are private; only a few recur in unrelated families, at hypermutable CpG dinucleotides. Benign polymorphisms are rare in the gene, and splicing variants account for approximately 12.5% of the known total, with pleiotropic effects on several ribonucleic acid (RNA) products [1, 3].

The table reproduces variants highlighted by the clinical sources; correspondence of the two HPRTSalamanca variants with HGVS numbering was inferred from the Atlas nucleotide description and codons 43 and 44 of the reference protein, without checking a ClinVar entry. Older literature mixes numbering systems: the Atlas reports CpG hotspots at arginines 51 and 170, which match current numbering, but also a nonsense variant “at the arginine 169 CpG” described in 1992, probably corresponding to Arg170 counted without the initial methionine. Normalisation should therefore use the reference transcript and nucleotide change rather than the amino acid number alone [3, 29].

2.4. Genotype–activity–phenotype correlation

Severity depends mainly on residual enzyme activity. Loss-of-function variants (nonsense, frameshifts caused by small or large insertions or deletions), missense variants with absent or near-absent activity (through altered kinetics, dimerisation or stability) and most splicing variants with a major coding-region impact are associated with LND. HND is associated mainly with missense or splicing variants leaving a small fraction of correct transcript (usually 2–8% activity), and rarely duplications with variable somatic reversion. HRH is associated with missense variants with greater residual activity [1, 17, 31]. In the Atlas summary, major gene disruptions and protein truncations produce LND; in variants, changes tend to cluster at the amino terminus, and the same position can produce different phenotypes depending on the substitution (leucine→valine at codon 78: variant; leucine→glutamine: LND) [3].

There are clinically relevant exceptions. Patients with the same variant may have different phenotypes within one family because of differences in splicing fidelity or protein stability; Hladnik and colleagues described three phenotypes in a single family, and Sarafoglou documented a severely affected grandfather and two grandsons with mild delay, with messenger RNA expression proportional to enzyme activity [1, 3]. A Turkish family with c.481G>T (p.Ala161Ser), published in 2026, illustrated this variability in patients with HPRT1-related gout: males had earlier onset, universal tophi and more severe structural damage, including amputations in older members, and one female carrier developed clinical gout [32]. In fibroblasts from 36 patients, Fu and colleagues showed a strong correlation between severity and hypoxanthine or guanine salvage, with compensatorily increased de novo synthesis (purinosomes) that did not correlate with severity [31].

3. Clinical Spectrum and Manifestations by System

3.1. Overview of the spectrum

GeneReviews describes uric acid overproduction as present from birth throughout the spectrum, with variable degrees of neurological and behavioural involvement [1]. The following categories are useful conventions for prognosis and counselling, but their boundaries are not fixed: a young child classified as HND may develop self-injury and be reclassified as LND [1].

3.2. Lesch-Nyhan disease: natural history

Pregnancy and the perinatal period are generally normal for boys and apparently normal development during the first six to eight months. The first sign is often orange, sand-like crystals in the nappy; this information is regularly obtained from parents and could enable diagnosis before neurological manifestations, but its significance frequently goes unnoticed for years [1, 3]. In the French-Italian cohort of 101 patients, initial manifestations were non-specific and orange nappy staining was reported in 22% [19]. Haematuria, crystalluria and stones may appear in the first months [3].

Motor delay becomes evident in the second half of the first year: hypotonia, poor head control and failure to achieve sitting. In Madeo's series, median age at onset of involuntary movements was 1.0 year and of self-injury 3 years [19]. Jinnah and colleagues' prospective multicentre study of 44 patients aged 2 to 38 years defined the motor presentation as severe action dystonia superimposed on baseline hypotonia; choreoathetosis, ballism, opisthotonus and pyramidal signs may coexist but are always less prominent than dystonia [36]. GeneReviews reports corticospinal signs in approximately one-third [1]. The Atlas adds opisthotonic spasms and retrocollis persisting into adulthood, sometimes incorporated into behaviour, scissoring adduction of the lower limbs, bilateral hip dislocation in most patients and flexion contractures; reflexes may be difficult to elicit because of examination-induced excitement [3]. Classic patients do not walk and require a wheelchair with thoracic and abdominal supports; most need assistance with feeding and personal care [1, 3].

Oculomotor function is altered: 15 patients with severe deficiency had oculomotor apraxia with fixation interrupted by unwanted saccades, inability to fixate a central target for one minute and initiation of voluntary saccades with head movement or blinking; four had blepharospasm, whereas seven neurologically normal patients with partial deficiency had normal motility [3]. Seizures occur in approximately one-third, and electroencephalography (EEG) frequently shows non-specific slowing [1]. Dysarthria makes speech difficult to understand, although all learn to speak [3].

Cognitive assessment is hampered by dystonia, a short attention span and behaviour. In the Atlas experience, many patients obtained an intelligence quotient (IQ) close to 50, but the authors consider that cognitive abilities exceed the level of motor disability; two patients followed the standard secondary school curriculum [3]. GeneReviews and Torres and Puig place the impairment, on adapted tests, in the mild to moderate range, with attention deficits and often preserved non-verbal intelligence [1, 7]. In Christie's historical series of 19 patients, all had hyperuricaemia, self-injury and the motor syndrome, and all but one had below-normal intelligence [37].

3.3. Self-injurious behaviour and other behavioural abnormalities

Self-injurious behaviour is the phenotypic hallmark of LND. It usually develops between 2 and 4 years, but may appear in the first year or only in late adolescence; Torres and Puig describe onset between 2 and 16 years and worsening with psychological stress [1, 7]. It mainly involves biting the fingers, hands, lips and cheeks, with tissue loss and partial amputations; in a survey of 64 families, the commonest methods after biting were head-banging, extending the arms when passing through doorways, overturning the wheelchair, poking the eyes and placing fingers in wheelchair spokes [3, 38]. Patients have no sensory impairment: they feel pain, scream when they bite themselves, fear episodes in advance and feel relieved when restrained, distinguishing LND from sensory neuropathies [3, 7]. GeneGuidance emphasises the involuntary nature of the behaviour, which should be treated as a neurological symptom rather than indiscipline [1, 38].

Frequency depends on how the population is defined. The Atlas and Christie's series report self-injury in 100% of classic patients, although documented exceptions exist, such as two Spanish patients aged 6 and 21 years without self-injury despite relatives with the complete phenotype [3, 37]. In an Italian survey published in 2026, 21 of 24 LND patients (87.5%) reported self-injurious behaviour, with fingers (80%) and lips (70%) the commonest sites and multiple sites involved in 80% [39]. In Schretlen and colleagues' standardised assessment (22 LND patients, 11 with variants and 11 controls), LND was associated with severe self-injury, aggression, anxious and depressive symptoms, distractibility and motor stereotypies; variants were intermediate between controls and LND, but attention problems were as severe as in LND [34]. Other difficult behaviours include impulsivity, opposition, recurrent vomiting, spitting and coprolalia [1, 3].

A 2026 Japanese series described periodic limb movements during sleep preceding self-injury onset in three children aged 2 to 7 years, with polysomnographic indices of 6.9 and 8.8 events per hour; the authors propose altered spinal sensory control with a dopaminergic basis, a hypothesis remaining speculative in so small a series [40].

3.4. Kidney, urate and joints

Overproduction manifests as hyperuricaemia (usually 9–12 mg/dL in the Atlas, up to 18 mg/dL or 1 mmol/L in Saudubray), urinary excretion of 600–1,000 mg/24 h in patients weighing more than 15 kg (40–70 mg/kg) and a urinary uric acid/creatinine ratio of 3–4 mg/mg [3, 5]. Before puberty, serum urate may be normal or high-normal because renal urate clearance is high [5, 7]. Without treatment, haematuria, crystalluria with abdominal pain, stones (radiolucent if composed of uric acid or xanthine), urinary infections, nephropathy due to sodium urate deposition in the parenchyma and renal failure develop; before allopurinol, death from renal failure before 10 years was the expected outcome [3]. An intercurrent illness with vomiting or dehydration can precipitate bilateral ureteric obstruction by crystalline sludge, requiring surgery and irrigation; this has also been observed with oxipurinol crystals during treatment [3]. Presentation with acute renal failure and bilateral ureteric stones continues to be reported in 2026 [41]. In Madeo's series, renal manifestations affected 66.3% of patients, at any age (median 1.1 years), and gout 25.7%, later (median 18 years) and more frequently in attenuated variants than in LND [19].

According to the Atlas, gout requires approximately 20 years of hyperuricaemia and therefore occurs mainly in adults with variants; exceptions exist, such as a patient with attacks from 1 year of age [3]. Tophi and chronic tophaceous gout were seen in untreated patients surviving beyond 10 years [3]. A less familiar presentation is recurrent painless haematuria mimicking haemorrhagic cystitis or glomerulonephritis, as in a 7-year-old boy with 3% erythrocyte activity [3].

3.5. Attenuated variants (HND and HRH)

Jinnah and colleagues' international prospective study included 46 patients from 34 families, aged 3 to 65 years: all had uric acid overproduction, 91% had motor abnormalities (from subtle coordination difficulties to disabling generalised dystonia), 67% had cognitive abnormalities that were never severe, none had self-injury and only three had no neurological dysfunction [6]. The classic neurological variant has an examination identical to LND, normal or near-normal intelligence and normal behaviour; Catel and Schmidt's index patient completed university and spoke English and German [3]. Puig described an intellectually normal patient unable to walk because of dystonia, and the Atlas adds motor deterioration in adults from a family with the same variant, with loss of walking [3]. In HRH, the commonest presentation is gout in a young adult, a uric acid stone or incidentally discovered hyperuricaemia; attention deficits and mild coordination difficulties are evident only on formal assessment [1, 3, 33].

3.6. Other manifestations

Macrocytic anaemia is a common feature: in a study of 257 blood counts from 65 patients over 23 years, macrocytic erythrocytes were present in 81–92% of patients with LND or neurological variants and 97% after excluding iron deficiency; anaemia was usually mild and rarely severe [35]. Its absence should prompt investigation for iron deficiency, and it does not respond to folate or vitamin B12 [1, 35]. Testicular atrophy with growth and pubertal delay is very frequent, and most patients have below-expected weight and height, with dysphagia, vomiting and urinary loss of nitrogen and purines [1, 3]. Approximately one-third require gastrostomy for dysphagia or recurrent aspiration [1]. Unexplained apnoea, recurrent vomiting and dystonic crises with autonomic abnormalities are rare but serious, and unexplained sudden death also occurs in well-monitored patients [1, 42]. Torres and Puig also describe microcytic anaemia associated with hiatus hernia [7]. Routine neuroimaging is frequently normal or shows non-specific atrophy; diffuse T2 white matter hyperintensity was described in a 2-month-old infant [5, 30].

3.7. Heterozygous females

Heterozygous females are almost always clinically normal, although they may have slightly increased urate production and develop gout later in life; in a study of 35 women from 16 Spanish families, the 22 heterozygotes had significantly higher plasma urate and hypoxanthine, xanthine and uric acid excretion than the 13 non-carriers [1, 43]. Rarely, a woman has the LND phenotype because of non-random inactivation of the X carrying the normal allele; De Gregorio and colleagues described discordant monozygotic twins carrying a splicing variant (c.609+4A>G), with LND only in the twin with skewed inactivation in fibroblasts [1, 44]. Females with biallelic variants have also been described [1].

4. Clinical Recognition and Warning Signs

LND diagnosis is traditionally delayed until self-injury appears, although urate overproduction is present from birth [1, 7]. Suspicion should arise when motor developmental delay is associated with evidence of uric acid overproduction [1]. The Oxford Desk Reference includes urate in baseline biochemistry for developmental regression, and Saudubray places LND among inborn errors with predominantly extrapyramidal symptoms, chorea with self-injury and oculomotor apraxia [45, 46]. In adults, severe generalised dystonia with cognitive impairment and self-injury, or isolated dystonia with mild cognitive or behavioural problems, and the association of gout, nephrolithiasis and tophi with a neurological presentation should raise the possibility [47].

Two laboratory pitfalls recur. General laboratory reference ranges are defined in adults: a serum urate of 5 mg/dL in a child is already elevated, and the Atlas cites a report that considered a value of 7.6 mg/dL normal in a 1-year-old infant [3]. Conversely, 24-hour urine collection at room temperature favours bacterial purine consumption, so a fresh sample analysed immediately is preferable for diagnosis [3].

5. Diagnosis: Criteria, Tests and Algorithms

5.1. Criteria and supporting findings

GeneReviews establishes diagnosis in a male proband with suggestive clinical and laboratory findings and a hemizygous pathogenic HPRT1 variant and/or low HGprt enzyme activity; a hemizygous variant of uncertain significance (VUS) neither establishes nor excludes diagnosis, and absence of family history does not exclude it [1].

5.2. Enzyme assay

HGprt activity can be measured in any tissue. Erythrocyte haemolysate is the most widely used assay, but mutant enzyme structures are often unstable and lose activity when the cell is lysed: patients with mild phenotypes may therefore have no activity in haemolysate, and lysate activity does not correlate with severity [1, 3]. The intact-cell assay (cultured fibroblasts incubated with labelled hypoxanthine, with products separated by high-performance liquid chromatography, or intact erythrocytes) correlates better with phenotype [3, 7, 31]. APRT activity is increased in haemolysate [7]. In an infant, distinguishing LND from a variant has major prognostic importance for parents [3].

Thresholds are not interchangeable between laboratories and methods, and GeneReviews presents slightly different values in two sections; this article keeps sources separate rather than imposing a single cut-off. In assays using low and saturating substrate concentrations, some variants are revealed only by altered kinetics [1, 3].

5.3. Molecular tests and their limitations

HPRT1 sequencing detects point variants, small insertions/deletions and splicing variants close to exons; if negative, gene-targeted deletion and duplication analysis should follow (quantitative or long polymerase chain reaction, PCR; MLPA — multiplex ligation-dependent probe amplification — or targeted microarray). Panels for developmental delay, dystonia, spasticity, intellectual impairment or epilepsy include HPRT1, and exome or genome sequencing enables diagnosis when the condition was not suspected [1, 49]. In GeneReviews Table 2, approximately 80% of probands with a detectable variant have a change identified by sequencing and approximately 20% by deletion/duplication analysis; these percentages describe the distribution of variant types, not clinical sensitivity [1].

The EuroGentest clinical utility gene card assigns molecular analysis approximately 95% analytical and clinical sensitivity and 100% specificity: in approximately 5% of patients, despite absent or reduced enzyme activity, the coding region is normal and messenger RNA expression is decreased for an unknown reason. In these cases, diagnosis rests on the enzyme assay, and messenger RNA quantification by real-time PCR or complementary deoxyribonucleic acid (DNA) sequencing may help; a complementary DNA finding should be confirmed in genomic DNA [13]. Negative molecular testing does not exclude HGprt deficiency in the presence of a convincing phenotype and biochemistry [1, 13].

The report should state the gene, transcript with version (NM_000194.3), HGVS c. and p. nomenclature, zygosity (hemizygosity in males, heterozygosity in females), five-class American College of Medical Genetics and Genomics/Association for Molecular Pathology (ACMG/AMP) classification, method coverage (copy number variations, CNV; introns, regulatory regions) and possibility of mosaicism. The importance of this precision is illustrated by a 2025 report describing the patient's variant as “HPRT1 NM_032578.4:c.880delG”; NM_032578.4 belongs to the myopalladin gene (MYPN), and c.880 exceeds the 654-nucleotide HPRT1 coding region, so the published designation cannot be normalised from the article [4, 50]. Familial segregation or a therapeutic response do not reclassify a VUS as pathogenic; functional confirmation uses the enzyme assay [1].

5.4. Diagnostic algorithm

The following algorithm was constructed from GeneReviews, Torres and Puig and the EuroGentest card; it is not an algorithm published by a society.

5.5. Newborn and familial screening

The consulted sources describe no population newborn screening programme for HGprt deficiency. GeneReviews recommends clarifying the status of at-risk boys immediately after birth to start a xanthine oxidase inhibitor early and anticipate needs [1]. A series of three Taiwanese patients and the case of a Sichuanese infant show that exome sequencing can establish diagnosis before self-injury [30, 49].

6. Differential Diagnosis

When all three elements — urate overproduction, neurological dysfunction, and cognitive and behavioural abnormalities — are present, there is no other known genetic cause of the complete phenotype. Difficulties arise in early stages and attenuated forms [1].

7. Treatments and Risk Management Strategies

7.1. Principles and initial assessment

Treatment has two distinct objectives. Controlling uric acid overproduction prevents nephropathy, stones, gout and tophi but does not improve dystonia, cognitive impairment or self-injury; GeneGuidance insists that this distinction be explained to families [1, 7, 52]. Neurological and behavioural management is symptomatic and multidisciplinary: medical genetics, neurology, behavioural management, developmental paediatrics, physical and rehabilitation medicine, physiotherapy, occupational therapy, speech and language therapy, dentistry and nephrology [1].

7.2. Controlling urate overproduction: allopurinol

Allopurinol inhibits xanthine oxidase through its metabolite oxipurinol. In HGprt deficiency, unlike ordinary gout, hypoxanthine and xanthine accumulating upstream are not reutilised, so total purine production does not decrease: uric acid is replaced by hypoxanthine (approximately ten times more soluble) and xanthine (only slightly more soluble than urate) [3, 5, 51]. Treatment can therefore cause xanthine stones, and the optimal dose requires a balance between urate and oxypurines [3, 52]. Treatment should begin as soon as diagnosis is established [1, 7].

In Torres, Prior and Puig's series, all 19 patients (13 with LND and 6 with partial deficiency) treated at a mean dose of 6.4 mg/kg/day (3.7–9.7) and followed for a mean 7.6 years normalised serum urate, with mean reductions of 47% in serum urate and 74% in the urinary uric acid/creatinine ratio; hypoxanthine and xanthine excretion increased 5.4- and 9.5-fold. Renal function remained stable or improved, three patients had urolithiasis, two with xanthine stones requiring dose adjustment, there were no hypersensitivity reactions and the neurological presentation did not change [52]. In the United Kingdom, McCarthy's population study concluded that continuous dose monitoring and metabolic screening were necessary [8].

Sources differ on dose, reflecting the absence of comparative trials. The table distinguishes expert review, centre experience and label information (comparison); no dose was estimated outside these sources.

In the absence of a guideline, the recommended dose is that of the expert review by Torres and Puig: start at 5–10 mg/kg/day, titrated according to serum urate, the urinary uric acid/creatinine ratio and urinary oxypurines, with observed doses of 50–600 mg/day [7]. SmPCs with an explicit indication serve as comparison: in children <15 years, 10–20 mg/kg/day, maximum 400 mg/day, and in adults a maximum of 800 mg/day in Portugal (900 mg in the Irish SmPC) [10, 53]. The Atlas 15–20 mg/kg range exceeds the lower end of the SmPC dose for high turnover situations and contrasts with the Spanish cohort's mean dose of 6.4 mg/kg; the 5–10 mg/kg/day starting dose of Torres and Puig and the doses below 10 mg/kg/day in children in the Spanish cohort and in the section 10 case fall below the SmPC paediatric range, and doses of 600 mg/day before 15 years of age exceed the labelled maximum, situations outside the conditions of the label. In practice, dose should be individualised according to serum urate, the urinary ratio and, where available, urinary oxypurines, respecting the jurisdiction's label maximum. ACR gout guidelines (target <6 mg/dL, initial dose ≤100 mg/day, gradual increase) and EULAR guidelines (target <6 mg/dL and <5 mg/dL in severe gout) apply to adults with gout and only by extrapolation to HRH; the GeneReviews therapeutic target differs because of concern about excessive suppression [1, 20, 21]. In the section 10 report, the infant received 2.6 mg/kg/day; this was management of a single case, not a recommended dose [30].

Adverse effects and management. Rash is the commonest adverse effect; severe cutaneous reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis and drug reaction with eosinophilia and systemic symptoms, DRESS) occur in approximately 5 per 10,000 patients and require permanent withdrawal at the first sign. The human leucocyte antigen (HLA) allele HLA-B*58:01 increases risk and is more frequent in African, Asian (Han Chinese, Korean, Thai) and Pacific Island ancestries; the FDA label recommends considering genotyping in these populations, and the ACR conditionally recommends it in patients of Southeast Asian and African American ancestry [20, 54]. Other effects include nausea, diarrhoea, elevated liver enzymes, hepatotoxicity, myelosuppression and nephrotoxicity from xanthine stones; hydration sufficient for urine output of at least 2 L/day in adults (100 mL/m²/hour in paediatric oncology) and neutral or slightly alkaline urine are recommended in the label [54]. Allopurinol interacts with mercaptopurine and azathioprine (dose reduction), capecitabine and pegloticase, and rash risk increases with thiazides, ampicillin, amoxicillin and bendamustine [10, 54]. In pregnancy, human data are limited (approximately 50 published pregnancies, two with major malformations), and the label advises informing patients about potential risk [54]. Gout flare prophylaxis with colchicine or anti-inflammatories when starting treatment applies to adults with gout; the ACR recommends it for at least 3–6 months [7, 20, 54].

7.3. Febuxostat: differences between sources

GeneReviews and GeneGuidance accept febuxostat in allopurinol hypersensitivity, and Saudubray mentions allopurinol or febuxostat [1, 5]. However, both major labels specifically advise against it: the European Adenuric SmPC states that, because of lack of experience and xanthine deposition risk, use in Lesch-Nyhan syndrome is not recommended, and the US Uloric label does not recommend it in patients with markedly increased urate production, including Lesch-Nyhan syndrome [9, 55]. Neither has established safety below 18 years. The FDA label has a boxed warning about increased cardiovascular mortality in gout patients with established cardiovascular disease compared with allopurinol, and the ACR conditionally recommends switching febuxostat if cardiovascular disease is present [9, 20]. Febuxostat is contraindicated (USA) or not recommended (EU) with azathioprine or mercaptopurine [9, 55]. Its use in HGprt deficiency is therefore an exceptional individual decision, outside indications and contrary to a label warning, which should be documented and accompanied by oxypurine measurements.

7.4. Renal measures, nutrition and agents to avoid

Saudubray recommends high fluid intake, urinary alkalinisation with sodium bicarbonate, potassium citrate or citrate mixtures to a urine pH of 6.0–6.5 and a low-purine diet (excluding red meat and offal, dried pulses and fish such as anchovy, herring, mackerel, salmon, sardine and tuna) [5]. Conversely, the Atlas considers purine restriction ineffective because production is metabolically fixed, and recommends a protein- and calorie-rich diet given thinness, urinary nitrogen and purine loss, dysphagia and vomiting [3]. This difference is not resolved by trials; an approach consistent with both sources is to avoid excessive purines without compromising energy and protein intake in patients at nutritional risk. The Atlas suggests dietary or supplemental folate because purine synthesis consumes folate cofactors and megaloblastic anaemia occurs, but GeneReviews and Cakmakli's study indicate that folate and vitamin B12 do not correct macrocytosis and that it rarely requires treatment [1, 3, 35].

Dehydration concentrates metabolites and precipitates stones and nephropathy; intercurrent illnesses with vomiting require active hydration [1, 3]. Stones forming despite treatment may require lithotripsy or surgery, and renal failure can occur despite adequate therapy [1].

The mycophenolate warning is relevant to renal or haematopoietic cell transplantation. In the 2025 umbilical cord blood transplantation report, mycophenolate mofetil was used for graft-versus-host disease prophylaxis; the article does not discuss the label warning, and this management should not be generalised [11, 50].

7.5. Motor, behavioural and dental management

Dental interventions are the clearest example of differing expert opinion and patient experience. GeneReviews considers mouthguards unreliable and extraction necessary in most LND patients [1]. Analysis of 170 social media comments from 46 caregivers assigned extraction a mean score of 4.86 on a scale of 1 to 5, an almost uniformly positive appraisal [57]. Two publications added data in 2026: in the Italian survey, 14 of 21 patients with self-injury used intraoral devices (78.6% reported greater protection, three had periodontal complications) and four had undergone extraction, all reporting relief but with self-injury persisting in 50% [39]; in an Israeli series of four patients followed for up to 14 years, a double-layer thermoplastic splint stopped oral biting in all, without extractions [56]. These are small, non-comparative studies that do not establish superiority of either strategy.

7.6. Experimental therapies and available evidence

Krajewski and colleagues' systematic review (PROSPERO CRD42023446513) included 34 articles, 22 with high risk of bias; individual case and trial results were often contradictory, failures were underreported, and S-adenosylmethionine (SAMe) and deep brain stimulation (DBS, deep brain stimulation) were the most studied interventions [58]. GeneReviews summarises that small studies reported benefit from dental devices, DBS, marrow transplantation, gabapentin, carbamazepine, SAMe, risperidone and others, without confirmation of sustained benefit in follow-up studies, often unpublished [1].

• SAMe: mixed results; in the Atlas, most patients had little benefit and some worsened, but some infants improved for a prolonged period; Saudubray and Harris report benefit in selected patients tolerating the drug [3, 5, 59].
• Levodopa and BH4: the systematic review suggests that timing of initiation may be decisive; with BH4 and precursors, one patient had unequivocal improvement lost at five weeks; the sapropterin trial (NCT00935753) was withdrawn without participants [3, 58, 60].
• Internal globus pallidus DBS: Deng and colleagues' review included 12 patients in 8 publications; dystonia improved variably and all had partial or complete self-injury control, but complications were frequent [61]. In the French-Italian series, DBS was associated with deterioration in patients' condition [19]. Evidence consists of cases, with publication bias, and does not support routine use.
• Haematopoietic cell transplantation: literature includes an early death without engraftment (1991), cord transplantation at 24 months with complete chimerism and no self-injury (2012), and, in 2025, 5/6 HLA cord transplantation at 14 months, with complete chimerism on day 32, increased HPRT1 protein by enzyme-linked immunosorbent assay (ELISA), neurological improvement and no self-injury at 36 months, at the cost of grade II acute graft-versus-host disease [50]. These are isolated cases; no established criteria exist, and mortality risk from myeloablative transplantation is unquantified in this disease.
• Gene therapy: retroviral vector transfer of HPRT1 into patient cells, with normal enzyme expression, was demonstrated in vitro in 1983; sustained in vivo expression remains a goal, and no registered gene therapy clinical trial was identified [3, 60].

Registered and published trials are as follows.

Interpretation is limited by very small samples, early termination, caregiver-assessed behavioural outcomes and absence of replication. Reductions observed in individual patients do not establish population efficacy. Ecopipam is not authorised for LND in any jurisdiction; on 19 August 2026, Teva announced that the FDA had accepted, with priority review and a target date at the end of the first quarter of 2027, the ecopipam authorisation application for paediatric Tourette syndrome, based on the phase 3 D1AMOND trial (relapse in 41.9% with ecopipam vs 68.1% with placebo among responders); this is a sponsor press release concerning another indication, constituting neither authorisation nor LND evidence [64].

7.7. Regulatory status

Product authorisation does not guarantee reimbursement or availability in each country; the consulted sources yielded no information on reimbursement status in Portugal (INFARMED) or Brazil (Agência Nacional de Vigilância Sanitária, ANVISA, and Comissão Nacional de Incorporação de Tecnologias no SUS, CONITEC) for paediatric formulations.

7.8. Special populations

Children. Urate treatment should start at diagnosis, including at-risk boys identified at birth [1]. The pharmaceutical form for infants is not defined in the US label for this indication; the FDA label suggests considering an alternative formulation when body surface area is below 0.5 m² in the oncology setting [54]. Extending gout or oncology label doses to LND is extrapolation and off-label use in the USA, but a label indication in some European SmPCs [10].

Perioperative care and intercurrent illness. Maintain hydration and avoid prolonged fasting because of the tendency towards ureteric obstruction by crystals; renal biopsy and cystourethrography precipitated oliguria and azotaemia in a patient with a variant [1, 3]. Sudden death probably has a respiratory basis (aspiration, laryngospasm, central apnoea, cyanotic apnoeic spells, high cervical injury), so assessment may include chest imaging, airway endoscopy, polysomnography, EEG and brain or spinal imaging [42].

Heterozygous females and pregnancy. Heterozygous females with gout may need urate-lowering treatment; allopurinol crosses the placenta and human pregnancy data are limited [1, 54]. Adults. Transition requires continuity of nephrological and rheumatological care; men with HRH may be followed according to gout guidelines, with the cautions about uricosurics and febuxostat described above [1, 20].

8. Reproductive Options and Family Counselling

HGprt deficiency is X-linked recessive. The father of an affected boy is not a carrier. When more than one family member is affected, the mother is probably heterozygous; if she is the first in the family to have an affected son, Haldane's rule predicts an approximately two-thirds probability of carrier status and one-third probability that the variant arose de novo [1]. Maternal studies from the biochemical era found a lower frequency of new mutations than the one-third expected for an X-linked lethal gene, with evidence of a paternal age effect and mutations arising in the carrier mother's generation [3].

Carrier detection. Molecular testing targeted to the familial variant is the method of choice. Without a known variant or an available proband, sequencing is performed followed, if negative, by deletion/duplication analysis [1, 13]. Erythrocyte or leucocyte activity is usually normal in heterozygous females because of selection against deficient cells and cannot be used for detection; historical methods (analysis of at least 30 hair follicles, clonal selection in 6-thioguanine or azaguanine, urinary oxypurines after a low-purine diet for five days) are laborious and not infallible [3, 7, 43]. In one case, five heterozygous females showed no detectable mosaicism using follicles or fibroblast selection, an extreme example of selection against mutant cells [3]. Willers described simultaneous somatic and germline mosaicism in a mother, leading to incorrect risk assignment through intragenic haplotypes, and warned against exclusive use of indirect diagnosis [66].

Prenatal and preimplantation diagnosis. With the familial variant identified, prenatal testing is possible in chorionic villi (approximately 10–12 weeks) or amniocytes (approximately 15–18 weeks), as is preimplantation genetic testing [1, 7]. Without a known variant, an enzyme assay in amniocytes or villi is preferred; in villi, 5′-nucleotidase activity may degrade the IMP formed and simulate a false positive, so it should be inhibited in the assay [1, 3, 67]. The Atlas recommends offering prenatal diagnosis to a mother with an affected son even if she is not a blood carrier, because of gonadal mosaicism risk, and also assessing 46,XX foetuses, given the existence of affected females [3]. Perspectives on prenatal diagnosis vary among professionals and families, and the decision is personal [1].

Counselling is best undertaken before pregnancy and should be offered to affected, heterozygous or at-risk young adults; DNA banking is recommended for probands without a confirmed molecular diagnosis [1]. Testing at-risk boys at birth has direct benefit (starting the xanthine oxidase inhibitor), and identifying the individual in whom the variant arose de novo clarifies extended family risk [1].

9. Special Considerations: Population Diversity and Epidemiology

9.1. Frequency

The most frequently cited frequency, approximately 1 in 380,000 births, derives from an old Canadian estimate and applies to LND; GeneReviews and the Atlas use it as a reference value [1, 3, 7]. Milder forms (HND and HRH) are less studied and appear less frequent than LND, so no global frequency exists for the entire spectrum; GeneGuidance converts the LND value to 0.26 per 100,000 and emphasises that it should not be interpreted as the frequency of partial forms [1]. European and North American data available in the consulted sources are as follows.

The LND > HND > HRH distribution in centre series may also reflect underdiagnosis of mild forms presenting as gout or stones and rarely undergoing enzyme investigation; proportions in series should not be read as relative population frequencies [1, 19].

9.2. Diversity, founder populations and particular groups

GeneReviews states that HPRT1 disorders occur in all studied populations at relatively similar frequency, and the EuroGentest card identifies no ethnic group difference in prevalence [1, 13]. Founder variants in extended families are rare because men with LND do not reproduce and those with HND rarely do; multigenerational families mainly have HRH, such as the HPRTSalamanca family (four males in three generations, seasonal agricultural workers in southern France) or the Turkish p.Ala161Ser family [1, 3, 32]. Sources cite patients of Saudi, Chinese, Taiwanese and Japanese origin and Yamada's Asian variant review, without population frequency data [3, 30, 40, 49]. In the Chinese case in section 10, the parents were consanguineous but the variant was de novo, a reminder that consanguinity does not alter X-linked inheritance logic [30].

Ancestry has a direct therapeutic consequence: HLA-B*58:01, associated with severe allopurinol cutaneous reactions, is more frequent in African and Asian ancestries, supporting prior genotyping in these populations [20, 54]. The consulted base sources and main articles yielded no specific epidemiological data for Portugal or Brazil; this absence does not establish that publications about the disease in those countries do not exist.

9.3. European networks, registries and patient organisations

MetabERN is the European Reference Network (ERN, European Reference Network) for hereditary metabolic diseases; its seven subnetworks do not explicitly name purine diseases, and assignment of this disease to a subnetwork was not confirmed [68]. ERN-RND (rare neurological diseases) includes the “dystonias, neurodegeneration with brain iron accumulation (NBIA) and paroxysmal disorders” group, relevant to dystonia assessment and therapies such as DBS [69]. The Lesch-Nyhan Disease International Study Group maintains the variant database (lesch-nyhan.org), laboratory lists and patient organisation contacts [1, 13, 70]. Family organisations include Association Française Lesch-Nyhan Action (France, founded 1995), LND Famiglie Italiane (Italy) and the Lesch-Nyhan Syndrome Children's Research Foundation (United States) [70]. GeneReviews also lists National Institute of Neurological Disorders and Stroke (NINDS) and MedlinePlus resources, and notes that many families benefit from online communities sharing experiences that may inform decisions when trials are lacking [1, 57].

10. Illustrative Clinical Case

10.1. Renal failure and tophi as the presentation in infancy

In 2022, Yang and Guo described a boy of Tibetan (Zang) ethnicity from Sichuan province, the first child of healthy consanguineous parents, admitted at 2 months and 7 days because of feeding difficulties, vomiting and excessive night-time crying since the first month [30]. He was born at term without asphyxia, weighing 2,250 g (below the fifth percentile). Examination showed good responsiveness, normal strength and tone, but approximately 1 × 1 cm pressure-tender masses on the backs of the hands, the dorsum of the right foot and the left palm, and swelling of one finger; ultrasound identified them as tophi. Serum uric acid ranged from 1,192 to 1,535 µmol/L (approximately 20–26 mg/dL), creatinine from 386 to 1,046 µmol/L, with metabolic acidosis and respiratory alkalosis, mild hyponatraemia, glycosuria and ketonuria, urinary urate/creatinine ratio 4.84 and secondary acylcarnitine profile abnormalities attributed to tubulopathy and renal failure. Renal ultrasound showed enlarged kidneys with parenchymal hyperechogenicity and poor corticomedullary differentiation (Figure 1 of the article), and brain MRI showed diffuse T2 white matter hyperintensity (Figure 2) [30].

Trio exome sequencing identified the hemizygous HPRT1 variant c.508C>T (p.Arg170Ter), classified as pathogenic under ACMG criteria and absent in the mother, therefore de novo [30]. Treatment included oral sodium bicarbonate and allopurinol (2.6 mg/kg/day); after two weeks, acid-base balance normalised and urate and creatinine fell to 782 µmol/L and 82.5 µmol/L. One week after discharge at the parents' request, both were normal. Neurological development followed the LND pattern: poor head control and inability to reach objects at 4 months, inability to roll or hold objects with both hands at 6 months, and inability to sit unsupported at 8 months [30].

The case illustrates several points of this article. Urate overproduction can cause renal damage and tophi in the first months before any neurological sign, and urate measurement in infants with renal disease or imaging abnormalities can bring diagnosis forward [7, 30]. The p.Arg170Ter variant is a CpG hotspot associated with LND in GeneReviews, enabling phenotype prediction [1]. Absence of the variant in maternal leucocyte DNA reduces but does not eliminate recurrence risk because of possible germline mosaicism; the authors do not discuss this issue, which should be included in counselling [1, 66]. The allopurinol dose was low compared with section 7 sources and used in a renal failure setting; it is not a recommendation.

10.2. Neurological variant with normal intelligence

The Atlas describes the index neurological variant patient, initially published by Catel and Schmidt in 1959 and followed by Manzke: at 21 years, just after completing university, he spoke English and German and had no intellectual or behavioural abnormalities, but had a neurological presentation identical to LND, with wheelchair dependence (Atlas Figure 65.17). Haemolysate activity was absent; the intact-cell assay showed activity around 10% of control, sufficient for kinetic studies (9% conversion of labelled hypoxanthine and 27% of labelled guanine). A maternal uncle had a similar presentation [3]. The case demonstrates the haemolysate assay's prognostic limitation and the importance of intact-cell testing in a newly diagnosed infant [1, 3].

11. Prognosis and Long-Term Follow-up

11.1. Prognosis

Before allopurinol, death from renal failure before 10 years was the expected LND outcome [3]. With effective treatment, survival extends into the second to fourth decades; main causes of death are respiratory failure from pneumonia or aspiration, renal failure, sepsis and unexplained sudden death [1, 7]. Neychev and Jinnah's series described five sudden deaths between 2 and 45 years and three serious respiratory events, probably respiratory rather than cardiac in origin [42]. The disease is not neurodegenerative: neurological problems evolve in the first years, reflecting development on a static lesion, and some functional loss may occur with ageing, as in cerebral palsy; sudden deterioration should prompt investigation for a secondary cause [1]. However, the Atlas describes loss of walking in adulthood in patients with variants from the same family [3]. HND has longer life expectancy than LND, normal in mild cases, and HRH has normal life expectancy, conditioned by renal function and gout complications [1]. Renal disease may progress even with treatment [1, 8].

11.2. Surveillance

Other sources add urinary oxypurine monitoring to prevent xanthine stones, the urinary uric acid/creatinine ratio as a target, and blood counts, noting that absence of macrocytosis suggests iron deficiency [3, 7, 35]. Sudden behavioural deterioration occurs with pain (for example from a stone) or changes in the social environment and should prompt investigation for a somatic cause [1]. Research instruments include the Behavior Problems Inventory self-injury subscale (0–45), Child Behavior Checklist and American Association on Mental Retardation Adaptive Behavior Scale, which can document individual progression [34, 63].

11.3. Quality of life and transition

Dependence is total in LND and variable in HND, ranging from wheelchair use with normal cognition to independent living with a dystonic gait [7, 18]. LND patients are described as often affable and engaged when appropriately restrained, and behaviour is modulated by environmental factors, including education [7]. Transition to adult care should maintain renal and joint surveillance, dysphagia and airway management, caregiver support and access to special care dentistry [1, 39].

12. Summary and Final Considerations

HGprt deficiency, caused by HPRT1 variants with X-linked recessive inheritance, is a continuous spectrum defined by residual enzyme activity: Lesch-Nyhan disease with severe action dystonia, cognitive impairment and self-injury; HPRT1-related neurological dysfunction without self-injury; and HPRT1-related hyperuricaemia without evident neurological impairment. Uric acid overproduction is common to all forms and similar in intensity, whereas neurological severity correlates with enzyme activity and genotype [1, 17, 19].

Early recognition rests on simple signs — orange nappy crystals, hypotonia with hyperuricaemia, macrocytosis, stones or gout in a young man — and age-appropriate urate reference values. Diagnosis combines the urinary urate/creatinine ratio, molecular HPRT1 investigation (sequencing and deletion/duplication analysis, with approximately 5% of cases without a detectable defect) and enzyme assay, preferably in intact cells when prognosis depends on distinguishing LND from a variant [1, 3, 13].

Allopurinol prevents renal and joint disease but does not modify neurological disease; dose is individualised, with differing sources (5–10 mg/kg/day in Torres and Puig, 10–20 mg/kg/day up to 400 mg in the European SmPC, 15–20 mg/kg in the Atlas) and targets avoiding xanthine stones. Febuxostat is discouraged in Lesch-Nyhan syndrome by US and European labels. Uricosurics, methotrexate, mycophenolate and dehydration should be avoided [1, 3, 7, 9–11, 55]. Self-injury requires physical protection, behavioural extinction therapy and dental decisions shared with the family; ecopipam trials were stopped or terminated, and DBS and haematopoietic cell transplantation remain limited to cases [1, 26, 50, 61].

Counselling should consider Haldane's rule, germline mosaicism and prenatal or preimplantation diagnosis. Frequency estimates (1:235,000 to 1:380,000 for LND, incidence 0.18 per 100,000 in the United Kingdom) concern the classic form, and mild forms probably remain underdiagnosed [1, 7, 8].

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