Congenital Adrenal Hyperplasia, Antley-Bixler Syndrome and Mutant P450 Oxidoreductase

Patients with biochemical evidence of apparent combined deficiencies of 17a- and 21-hydroxylase have been recognized for almost 2 decades. The clinical features include infant females born with mildly to moderately virilized external genitalia in whom virilization does not progress post partum, to adult women with menstrual irregularities (amenorrhea). Affected males have normal external genitalia, cryptorchidism, or hypospadias. Serum and urine glucocorticoid and androgen values are normal or low, while levels of precursors (progesterone, 17-hydroxyprogesterone, pregnenediol, pregnanediol, pregnanetriol) are elevated. Recent analysis of the genes encoding the enzyme proteins (CYP17A1, OMIM 202110, chromosome 10q24.3; CYP21B, OMIM 201910, chromosome 6p21.3) did not disclose any mutations. The Antley-Bixler syndrome (ABS, OMIM 207410) is characterized by facial (midface hypoplasia with proptosis, choanal atresia, frontal bossing, dysplastic ears) and skeletal abnormalities (cranial, humero-radial and radio-ulnar synostoses, femoral and ulnar bowing, camptodactyly, joint contractures), and in some patients by genitourinary abnormalities (renal agenesis, vaginal atresia, virilization of female external genitalia, undermasculination of male genitalia). Heretofore, ABS has been primarily attributed to mutations in FGFR2.

Steroidogenesis and its impairment in patients affected by apparent combined P450C17 and P450C21 deficiency Serum steroids reported in increased amounts are shown in red, and pathologically altered urinary steroid metabolites are shown in blue boxes if raised and gray boxes if reduced, linked to the steroid from which they are derived. Crosses indicate impairment of enzymatic activity.
(Reprinted with permission from: Arlt W, Walker EA, Draper N, et al. Lancet.2004;363:2128-35.Copyright ©2004 Elsevier.)

Fluck et al and Arlt et al reasoned that perhaps the primary problem in patients with apparent combined deficiencies of 17a- and 21-hydroxylase might be due to decreased levels of a common co-factor (Figure). Both enzymes require transfer of electrons to achieve the activated state. The electron donor is cytochrome P450 oxidoreductase (POR, OMIM 124015, chromosome 7q11.2), a flavoprotein that contributes electrons to all microsomal P450 enzymes. It does so by binding to NADPH through its flavin adenine dinucleotide (FAD) domain to which NADPH contributes 2 electrons; these electrons are then transferred to the flavin mononucleotide (FMN) domain of POR that, in turn, donates them to the target P450 enzyme. Mutations in CYP17A1 that involve its binding to POR lead to decreased 17a-hydroxylase activity. Accordingly, these investigators analyzed POR in 7 patients with combined deficiencies of 17a- and 21-hydroxylase deficiency, some of whom had clinical and skeletal anomalies consistent with ABS. They found compound heterozygous or homozygous loss-of-function mutations in all patients including: 531TÞG: Tyr178Asp; 731+1GÞA: donor splice site intron 6; ^*859GÞC: Ala287Pro; ^*1370GÞA: Arg457His; 1475TÞA: Val492Glu; ^*1706GÞA: Cys569Tyr; 1822GÞT: Val608Phe. The Ala287, Arg457, and Val492 mutations were in the FAD domain that binds NADPH and, predictably, changed steric conformation or charge leading to greatly reduced 17a-hydroxylase and 17-20-lyase activities. The Cys569 and Val608 mutations were in the region that binds NADP⁺ and resulted in less loss of enzyme activity. Patients with ABS tended to have the more severe mutations in POR, while those with the less severe defects only had disordered steroidogenesis. In no patient studied was a mutation in FGFR2 identified.

Arlt W, Walker EA, Draper N, et al. Congenital adrenal hyperplasia caused by mutant P450 oxidoreductase and human androgen synthesis: analytical study. Lancet. 2004;363:2128-35.

Fluck CE, Tajima T, Pandey AV, et al. Mutant P450 oxidoreductase causes disordered steroidogenesis with and without Antley-Bixler syndrome. Nat Genet. 2004;36:228-230.

Editor’s Comment: Pregnant women who are heterozygous carriers of a loss-of-function mutation in POR may manifest gestational hyperandrogenism (acne, hirsutism) possibly due to the effects of both fetal hyperandrogenemia and impaired endogenous steroidogenesis. This resembles the hyperandrogenism seen in patients with a luteoma of pregnancy or placental aromatase deficiency. Arlt et al suggest that in the fetus with loss of POR activity, an alternate pathway of androgen synthesis is pursued: 17a-hydroxyprogesterone is converted to 5a-pregnane-3, 17a-diol-20-one and the latter to androsterone by sequential actions of 5a-reductase type I, 3a-hydroxysteroid dehydrogenase, and low levels of 17a-hydroxylase. Since this pathway disappears in early infancy, virilization does not progress. These data suggest that ABS is genetically heterogeneous; one type is due to loss of FGFR2, and is not associated with genital malformation; the second type is due to loss of POR. POR is required for activity of both adrenal and hepatic microsomal P450 enzymes. Indeed, in infants of mothers treated with the antifungal agent fluconazole, that inhibits ergosterol synthesis by interfering with lanosterol 14a-demethylase activity, skeletal deformities resembling those in neonates with ABS have been observed. The skeletal deformities observed in children with deficiency of POR may reflect an error in this pathway that affects skeletal embryogenesis.

Allen W. Root, MD