CYP17 Mutation E305G Causes Isolated 17,20-Lyase Deficiency by Selectively Altering Substrate Binding

• Published in: The Journal of biological chemistry 2003-12, Vol.278 (49), p.48563-48569
• Main Authors: Sherbet, Daniel P., Tiosano, Dov, Kwist, Kerri M., Hochberg, Zeev, Auchus, Richard J.
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Animals; Base Sequence; Cell Line; CYP17 gene; DNA Primers; Humans; Molecular Sequence Data; Mutagenesis, Site-Directed; Mutation, Missense; pregnenolone; Saccharomyces cerevisiae; Sequence Homology, Amino Acid; steroid 17,20-lyase; Steroid 17-alpha-Hydroxylase - chemistry; Steroid 17-alpha-Hydroxylase - genetics; Steroid 17-alpha-Hydroxylase - metabolism; steroid 17^a-hydroxylase; Substrate Specificity
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M307586200

Cytochrome P450c17 (CYP17) converts the C21 steroids pregnenolone and progesterone to the C19 androgen precursors dehydroepiandrosterone (DHEA) and androstenedione, respectively, via sequential 17α-hydroxylase and 17,20-lyase reactions. Disabling mutations in CYP17 cause combined 17α-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 17α-hydroxylase activities, converting pregnenolone and progesterone to 17α-hydroxysteroids. However, mutation E305G lacks 17,20-lyase activity for the conversion of 17α-hydroxypregnenolone to DHEA, which is the dominant pathway to C19 steroids catalyzed by human CYP17 (the Δ5-steroid pathway). In contrast, mutation E305G exhibits 11-fold greater catalytic efficiency (kcat/Km) for the cleavage of 17α-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 17α-hydroxyprogesterone via the minor Δ4-steroid pathway alone is not sufficient for complete formation of the male phenotype in humans.