Endocrine Abstracts

Congenital adrenal hyperplasia (CAH) caused by mutations in the electron donor enzyme P450 oxidoreductase (POR) is unique amongst all CAH variants in that it can be associated with ambiguous genitalia (disordered sex differentiation, DSD) both in 46,XX and 46,XY individuals. POR has a pivotal role in facilitating electron transfer from NADPH to microsomal P450 enzymes, including CYP17, which catalyses a key step in human androgen synthesis, the conversion of 17-hydroxypregnenolone (17Preg) to dehydroepiandrosterone (DHEA). Consequently, P450 oxidoreductase deficiency (ORD) results in low or subnormal circulating androgens. This contrasts with clear evidence of prenatal androgen excess in ORD, specifically virilisation in affected girls (46,XX DSD) and maternal virilisation during pregnancy. This apparent contradiction might be explained by the existence of an alternative pathway in human androgen biosynthesis, active in fetal life only and bypassing the conventional pathway via DHEA (Lancet 2004, 363:2128–2135). This alternative pathway still requires CYP17 activity to convert 5α-pregnanediolone (5-pdiolone) to androsterone (AT). Genotype-phenotype analysis in our ORD cohort (n=21) demonstrates that POR mutants have distinct effects with either 46,XX DSD or 46,XY DSD contained within a specific genotype. We therefore hypothesized that POR mutants might differentially affect the alternative pathway and tested this utilising yeast microsomes co-transformed with CYP17 and either wild-type or mutant POR. Enzyme kinetic analysis revealed that POR mutants similarly knocked down the catalytic efficiency (V_max/K_m) of the conversion of 17Preg to DHEA (12–21% of wild-type activity). By contrast, conversion of 5pdiolone to AT was differentially affected, with the lowest residual activity in H604P (30%) while A287P and R457H retained significantly higher activities for the alternative pathway reaction (63 and 46%, respectively). This is entirely consistent with phenotypic presentation, with A287P and R457H resulting in 46,XX DSD whilst H604P is associated with 46,XY DSD. These findings provide compelling evidence for the alternative androgen pathway hypothesis.