The genetic and functional basis of isolated 17,20-lyase deficiency.
Human male sexual differentiation requires production of fetal testicular testosterone, whose biosynthesis requires steroid 17,20-lyase activity. Patients with putative isolated 17,20-lyase deficiency have been reported. The existence of true isolated 17,20-lyase deficiency, however, has been questioned because 17 alpha-hydroxylase and 17,20-lyase activities are catalyzed by a single enzyme, microsomal cytochrome P450c17, and because the index case of apparent isolated 17,20-lyase deficiency had combined deficiencies of both activities. We studied two patients with clinical and hormonal findings suggestive of isolated 17,20-lyase deficiency. We found two patients homozygous for substitution mutations in CYP17, the gene encoding P450c17. When 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 substrate-binding capacity, but co-transfection of cells with P450 oxidoreductase, the electron donor used by P450c17, indicated that the mutants had a diminished ability to interact with redox partners. Computer-graphic modelling of P450c17 suggests that both mutations lie in or near the redox-partner binding site, on the opposite side of the haem from the substrate-binding pocket. These mutations alter electrostatic charge distribution in the redox-partner binding site, so that electron transfer for the 17,20-lyase reaction is selectively lost or diverted to uncoupling reactions. These are the first proven cases of isolated 17,20-lyase deficiency, and they demonstrate a novel mechanism for loss of enzymatic activity.
Geller
D H
DH
Department of Pediatrics, University of California-San Francisco 94143-0978, USA.
Auchus
R J
RJ
Mendonça
B B
BB
Miller
W L
WL
D000293:Adolescent; D000312:Adrenal Hyperplasia, Congenital; D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D019556:COS Cells; D017931:DNA Primers / Q000235:genetics; D006720:Homozygote; D006801:Humans; D007223:Infant; D008297:Male; D008958:Models, Molecular; D009154:Mutation; D010084:Oxidation-Reduction; D016133:Polymerase Chain Reaction; D011487:Protein Conformation; D013254:Steroid 17-alpha-Hydroxylase / Q000235:genetics* / Q000378:metabolism; D014162:Transfection
D017931:DNA Primers; D013254:Steroid 17-alpha-Hydroxylase