Abstract
Pathological mutations in steroid 21-hydroxylase deficiency appear to derive exclusively from sequence exchange between the 21-hydroxylase gene (CYP21B) and a closely related pseudogene (CYP21A), located 30 kb telomeric to CYP21B and functionally inactive because of several mutations distributed along its length. Three basic classes of disease chromosome are seen: (a) deletion chromosomes where about 30 kb of DNA has been eliminated and a single 21-hydroxylase gene, often a CYP21A/CYP21B fusion gene, remains; (b) chromosomes which have 2 or more CYP21 genes, but without an apparent evidence for CYP21B-specifisce quences; (c) chromosomes which have 2 or more CYP21 genes, in which there is at least one gene with CYP21B-specific sequences, either a fusion gene or a mosaic, so-called “converted” gene. In the latter type of gene a central segment which may be very small or substantial shows CYP21A-specifics equences. Data will be presented on de novo mutations, showing how disease chromosomes can arise byunequal crossover and gene conversion-like events. The frequencies of different pathological mutations and genotype-phenotype correlations will also be discussed.
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