Structural analysis of the chimeric CYP21P/CYP21 gene in steroid 21-hydroxylase deficiency

Congenital adrenal hyperplasia (CAH) is a common autosomal recessive disorder mainly caused by defects in the steroid 21-hydroxylase (CYP21) gene. More than 90% of CAH cases are caused by mutations of the CYP21 gene. Approximately 75% of the defective CYP21 genes are generated through intergenic recombination, termed "apparent gene conversion," from the neighboring CYP21Ppseudogene. A chimeric CYP21P/CYP21gene with its 5' end corresponding to CYP21P and 3' end corresponding to CYP21 has been identified. This type of gene is nonfunctional because it produces a truncated protein. We found two distinct chimeric genes in CAH patients. Both genes had a sequence with -300 nucleotides of the 5' head as the CYP21P gene. The coding region consisted of a fusion molecule with the CYP21P gene in two different regions. One of the junctions was located in the chi-like sequence of GCTGGGC in the third intron and the other was in the minisatellite consensus TGGCAGGAGG of exon 5 of the CYP21P gene. In addition, analysis of restriction fragment length polymorphism for these two 3.3-kb chimeric molecules showed that these sequences arose as a consequence of unequal crossover between the CYP21Pand CYP21 genes. It is plausible that both consensus sequences are responsible for the gene conversion of these two chimeric genes.