Mutation R96W in cytochrome P450c17 gene causes combined 17 alpha-hydroxylase/17-20-lyase deficiency in two French Canadian patients

Congenital adrenal hyperplasia (CAH) is the most frequent cause of adrenal insufficiency and ambiguous genitalia in newborn children. In contrast to CAH caused by 2l alpha-hydroxylase and 11 beta-hydroxylase deficiencies, which impairs steroid formation in the adrenal exclusively, 17 alpha-hydroxylase/17,20-lyase deficiency impairs steroid biosynthesis in the adrenals and gonads. The sequence of CYP17 gene was determined by direct sequencing of asymmetric PCR products in two French-Canadian 46,XY pseudohermaphrodite siblings suffering from combined 17 alpha-hydroxylase/17,20-lyase deficiency. The two patients are homozygous for the novel missense mutation R96W caused by a C to T transition converting codon Arg(96) (CGG) into a Trp (TGG) in exon 1. The both parents are heterozygous for this missense mu tation. We assessed the effect of the R96W mutation on 17 alpha-hydroxylase/17,20-lyase activity by analysis of mutant enzyme, generated by site-directed mutagenesis, expressed in COS-1 cells. The presence of R96W substitution almost completely abolished the activity of the mutant protein. The present findings provide a molecular explanation for the signs and symptoms of combined 17 alpha-hydroxylase/17,20-lyase deficiency in these two patients and provide useful information on the structure-activity relationships of the P450c17 enzyme.