chromosomal inversion

**Definition:** A segment of a chromosome is excised, inverted 180°, and reinserted. Requires two breaks on the same chromosome. Net genetic material is unchanged, so it is a *balanced* rearrangement and carriers are usually phenotypically normal (unless a breakpoint disrupts a gene or regulatory region).

**Types:** - **Paracentric:** Both breaks are in the same arm (p or q); the centromere is NOT included in the inverted segment. - **Pericentric:** Breaks are on opposite sides of the centromere (one in p, one in q); the centromere IS included. This can change arm-length ratio and chromosome morphology.

**Pathophysiology at meiosis:** - Homologous chromosomes form an **inversion loop** to pair properly in meiosis I. A crossover within the loop produces recombinant gametes. - **Paracentric crossover:** Produces a **dicentric** chromosome (forms an anaphase bridge and breaks) and an **acentric** fragment (lost). Result: nonviable gametes or zygotes, so mainly reduced fertility and miscarriage, with few live-born unbalanced offspring. - **Pericentric crossover:** Produces chromosomes with **duplication of one region and deletion of another** (duplication/deficiency). These can result in viable but unbalanced offspring with congenital anomalies and intellectual disability. - Only the non-recombinant gametes (normal or balanced inverted) yield normal or carrier offspring.

**Clinical significance:** - Often discovered during workup for recurrent pregnancy loss, infertility, or after the birth of a child with congenital anomalies. Diagnosed by karyotype (G-banding); FISH or microarray can help. Note that microarray cannot detect balanced inversions. - Risk of an unbalanced liveborn child is higher for pericentric inversions, and depends on inversion size (larger inversions give a larger loop and a greater chance of a viable unbalanced recombinant product with smaller deleted/duplicated segments). - Genetic counseling is indicated for carriers; options include prenatal diagnosis or PGT.

**High-yield examples:** - **Inversion 9 (pericentric inv(9)):** Common normal variant, generally benign. - **Hemophilia A:** Intron 22 inversion of the F8 gene on the X chromosome is the most common severe cause (about 40-50% of severe cases). - **Inv(16) in AML (M4Eo):** CBFB-MYH11 fusion; favorable prognosis. Associated with acute myelomonocytic leukemia with eosinophilia. - **Inv(14) / T-cell neoplasms** and inv(3) in AML (poor prognosis). - **Hunter syndrome (MPS II):** Some cases from IDS gene inversion. - **Chromosome 8 inv dup del** (recombinant 8 syndrome) from parental pericentric inv(8) in Hispanic populations.

**Comparisons to remember:** - Inversion = balanced, no net loss of DNA (similar to balanced reciprocal and Robertsonian translocations, which are also generally phenotypically normal in carriers). - Contrast with deletions (e.g., cri-du-chat 5p-, DiGeorge 22q11), duplications, isochromosomes, ring chromosomes, and translocations. - Inversions suppress recombination in heterozygotes, a concept from genetics (e.g., preserving linked gene blocks).