title: The genetic and functional basis of isolated 17,20-lyase deficiency.
abstract: 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.
authors.affiliation: Department of Pediatrics, University of California-San Francisco 94143-0978, USA.
authors.forename: D H
authors.initials: DH
authors.lastname: Geller
authors.forename: R J
authors.initials: RJ
authors.lastname: Auchus
authors.forename: B B
authors.initials: BB
authors.lastname: Mendonça
authors.forename: W L
authors.initials: WL
authors.lastname: Miller
mesh_terms: 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
chemical_list: D017931:DNA Primers; D013254:Steroid 17-alpha-Hydroxylase