Empirical Risk in Chromosome Inversions

Why use it

Guides reproductive counseling for incidentally discovered or familial inversions. Provides differentiated risk estimates based on the critical variables: inversion type, segment size, ascertainment mode, and whether the inversion involves an acrocentric chromosome.

About

Evaluates the risk of live-born offspring with chromosomal imbalances originating from carriers of paracentric or pericentric inversions. For pericentric inversions, the risk depends on the size of the inverted segment relative to the whole chromosome: larger inversions produce smaller (more viable) terminal imbalances when recombination occurs. For paracentric inversions, recombination yields dicentric and acentric products that are nearly always lethal. The calculator uses empirical consensus data (5-15% range for pericentric, <1% for paracentric) and distinguishes ascertainment mode.

When to use

• Counseling a patient identified as carrying a chromosome inversion.
• Assessing recurrence risk after birth of a child with a recombinant chromosome.
• Deciding whether to offer prenatal diagnosis or PGT-SR for an inversion carrier.

Formula

Pearls & pitfalls

• Paracentric inversions: crossing over yields dicentric + acentric chromosomes → almost always lethal. Risk of viable unbalanced liveborn is <0.1-0.5%.
• Pericentric inversions: LARGER inversions = HIGHER risk. Large inverted segments produce small terminal dup/del that are more survivable.
• Consensus empirical risk for pericentric inversions ascertained through an abnormal child: 5-15% (Groupe de Cytogeneticiens Francais 1986, Sherman et al. 1986, Daniel et al. 1989).
• For pericentric inversions ascertained fortuitously (NOT through an abnormal child): overall risk is ~1%.
• The combined genetic content of the DISTAL (non-inverted) segments is the direct determinant of viability of the recombinant form.
• Acrocentric pericentric inversions: only q-arm imbalance matters; p-arm loss/gain is without consequence. Large inversions with distal q breakpoints may carry particularly high risk.
• Some inv(8)(p23.1q22.1) carriers can produce inv dup del(8p) recombinants via a special mechanism.
• Common variant inversions (inv(2)(p11.2q13), inv(1/9/16/Y) heterochromatin) are generally innocuous and excluded from risk calculations.
• Risk up to ~50% has been estimated for specific inversions like inv(13)(p11q22) where both recombinant forms are viable (Williamson et al. 1980).
• The inv(8)(p23.1) paracentric inversion is an important exception: despite being paracentric, it predisposes to inv dup 8p via NAHR.

Next steps

Discuss the option of prenatal diagnosis or PGT-SR if the risk is deemed significant. For paracentric inversions discovered fortuitously, a conservative counseling approach emphasizing the very low risk is appropriate.

Evidence

Gardner and Sutherland, Chapter 9. Ishii et al. (1997) review of 55 pericentric inversions with viable recombinants. Groupe de Cytogeneticiens Francais 1986b, Sherman et al. 1986, Daniel et al. 1989.

References

1. Gardner RJM, Sutherland GR, Shaffer LG. Chromosome Abnormalities and Genetic Counseling. Chapter 9: Inversions.
2. Ishii F, Fujita H, Nagai A, et al. Viable recombinants from pericentric inversions. Am J Med Genet. 1997;70:15-20.
3. Daniel A, Hook EB, Wulf G. Risks of unbalanced progeny at amniocentesis to carriers of chromosomal rearrangements. Am J Med Genet. 1989;33:14-53.