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  • Title: Genesis by meiotic unequal crossover of a de novo deletion that contributes to steroid 21-hydroxylase deficiency.
    Author: Sinnott P, Collier S, Costigan C, Dyer PA, Harris R, Strachan T.
    Journal: Proc Natl Acad Sci U S A; 1990 Mar; 87(6):2107-11. PubMed ID: 2315306.
    Abstract:
    The HLA-linked human steroid 21-hydroxylase gene CYP21B and its closely homologous pseudogene CYP21A are each normally located centromeric to a fourth component of complement (C4) gene, C4B and C4A, respectively, in an organization suggesting tandem duplication of a ca. 30-kilobase DNA unit containing a CYP21 gene and a C4 gene. Such an organization has been considered to facilitate gene deletion and addition events by unequal crossover between the tandem repeats. We have identified a steroid 21-hydroxylase [steroid, hydrogen-donor:oxygen oxidoreductase (21-hydroxylating), EC 1.14.99.10] deficiency patient who has a maternally inherited disease haplotype that carries a de novo deletion of a ca. 30-kilobase repeat unit including the CYP21B gene and associated C4B gene. This disease haplotype appears to have been generated as a result of meiotic unequal crossover between maternal homologous chromosomes. One of the maternal haplotypes is the frequently occurring HLA-DR3, B8, A1 haplotype that normally carries a deletion of a ca. 30-kilobase unit including the CYP21A gene and C4A gene. Haplotypes of this type may possibly act as premutations, increasing the susceptibility of developing a 21-hydroxylase deficiency mutation by facilitating unequal chromosome pairing.
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