THE GENETIC AND FUNCTIONAL BASIS OF ISOLATED 17,20-LYASE DEFICIENCY

Human male sexual differentiation requires production of fetal testicu lar testosterone, whose biosynthesis requires steroid 17,20-lyase acti vity(1,2). Patients with putative isolated 17,20-lyase deficiency have been reported(3,4). The existence of true isolated 17,20-lyase defici ency, however, has been questioned because 17 alpha-hydroxylase and 17 ,20-lyase activities are catalyzed by a single enzyme(5-8), microsomal cytochrome P450c17, and because the index case of apparent isolated 1 7,20-lyase deficiency had combined deficiencies of both activities(9,1 0). We studied two patients with clinical and hormonal findings sugges tive of isolated 17,20-lyase deficiency. We found two patients homozyg ous for substitution mutations in CYP17, the gene encoding P450c17. Wh en expressed in COS-1 cells, the mutants retained 17 alpha-hydroxylase activity but had minimal 17,20-lyase activity. Substrate competition experiments suggested that the mutations did not alter the enzyme's su bstrate-binding capacity, but co-transfection of cells with P450 oxido reductase, the electron donor used by P450c17, indicated that the muta nts had a diminished ability to interact with redox partners. Computer -graphic modelling of P450c17 suggests that both mutations lie in or n ear the redox-partner binding site, on the opposite side of the haem f rom the substrate-binding pocket. These mutations alter electrostatic charge distribution in the redox-partner binding site, so that electro n transfer for the 17,20-lyase reaction is selectively lost or diverte d to uncoupling reactions. These are the first proven cases of isolate d 17,20-lyase deficiency, and they demonstrate a novel mechanism for l oss of enzymatic activity.