title: CYP17 mutation E305G causes isolated 17,20-lyase deficiency by selectively altering substrate binding.
abstract: Cytochrome p450c17 (CYP17) converts the C21 steroids pregnenolone and progesterone to the C19 androgen precursors dehydroepiandrosterone (DHEA) and androstenedione, respectively, via sequential 17alpha-hydroxylase and 17,20-lyase reactions. Disabling mutations in CYP17 cause combined 17alpha-hydroxylase/17,20-lyase deficiency, but rare missense mutations cause isolated loss of 17,20-lyase activity by disrupting interactions of redox partner proteins with CYP17. We studied an adolescent male with clinical and biochemical features of isolated 17,20-lyase deficiency, including micropenis, hypospadias, and gynecomastia, who is homozygous for CYP17 mutation E305G, which lies in the active site. When expressed in HEK-293 cells or Saccharomyces cerevisiae, mutation E305G retains 17alpha-hydroxylase activities, converting pregnenolone and progesterone to 17alpha-hydroxysteroids. However, mutation E305G lacks 17,20-lyase activity for the conversion of 17alpha-hydroxypregnenolone to DHEA, which is the dominant pathway to C19 steroids catalyzed by human CYP17 (the delta5-steroid pathway). In contrast, mutation E305G exhibits 11-fold greater catalytic efficiency (kcat/Km) for the cleavage of 17alpha-hydroxyprogesterone to androstenedione compared with wild-type CYP17. We conclude that mutation E305G selectively impairs 17,20-lyase activity for DHEA synthesis despite an increased capacity to form androstenedione. Mutation E305G provides genetic evidence that androstenedione formation from 17alpha-hydroxyprogesterone via the minor delta4-steroid pathway alone is not sufficient for complete formation of the male phenotype in humans.
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002460:Cell Line; D017931:DNA Primers; D006801:Humans; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D020125:Mutation, Missense; D017386:Sequence Homology, Amino Acid; D013254:Steroid 17-alpha-Hydroxylase / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D013379:Substrate Specificity
chemical_list: D017931:DNA Primers; D013254:Steroid 17-alpha-Hydroxylase