Why Boys Will Be Boys: Two Pathways of Fetal Testicular Androgen Biosynthesis Are Needed for Male Sexual Differentiation

Abstract

Human sexual determination is initiated by a cascade of genes that lead to the development of the fetal gonad. Whereas development of the female external genitalia does not require fetal ovarian hormones, male genital development requires the action of testicular testosterone and its more potent derivative dihydrotestosterone (DHT). The "classic" biosynthetic pathway from cholesterol to testosterone in the testis and the subsequent conversion of testosterone to DHT in genital skin is well established. Recently, an alternative pathway leading to DHT has been described in marsupials, but its potential importance to human development is unclear. AKR1C2 is an enzyme that participates in the alternative but not the classic pathway. Using a candidate gene approach, we identified AKR1C2 mutations with sex-limited recessive inheritance in four 46,XY individuals with disordered sexual development (DSD). Analysis of the inheritance of microsatellite markers excluded other candidate loci. Affected individuals had moderate to severe undervirilization at birth; when recreated by site-directed mutagenesis and expressed in bacteria, the mutant AKR1C2 had diminished but not absent catalytic activities. The 46,XY DSD individuals also carry a mutation causing aberrant splicing in AKR1C4, which encodes an enzyme with similar activity. This suggests a mode of inheritance where the severity of the developmental defect depends on the number of mutations in the two genes. An unrelated 46,XY DSD patient carried AKR1C2 mutations on both alleles, confirming the essential role of AKR1C2 and corroborating the hypothesis that both the classic and alternative pathways of testicular androgen biosynthesis are needed for normal human male sexual differentiation.

Regulation of Steroidogenesis

“…36 This remarkable pathway (discussed below), first discovered in studies of steroidogenesis in the marsupial fetal testis, 36,37 plays a central role in human male sexual differentiation. 29 In this pathway, 17OHP is converted to DHT without going through DHEA, androstenedione, or testosterone, and hence provides a mechanism by which 17OHP can contribute to the virilization of female fetuses with 21-hydroxylase deficiency. 16,38 The "backdoor pathway" is characterized by both reductive and oxidative 3αHSD activities; the reductive activity can be catalyzed by either AKR1C2 (3αHSD3) or AKR1C4 (3αHSD1), 29 but the nature of the oxidative enzyme(s) is uncertain.…”

“…29 In this pathway, 17OHP is converted to DHT without going through DHEA, androstenedione, or testosterone, and hence provides a mechanism by which 17OHP can contribute to the virilization of female fetuses with 21-hydroxylase deficiency. 16,38 The "backdoor pathway" is characterized by both reductive and oxidative 3αHSD activities; the reductive activity can be catalyzed by either AKR1C2 (3αHSD3) or AKR1C4 (3αHSD1), 29 but the nature of the oxidative enzyme(s) is uncertain. 2 AKR1C3 (3αHSD2), which converts androstenedione to testosterone in the adrenals, is also known as 17βHSD5.…”

“…The 5α-reductase genes exhibit distinct patterns of developmental regulation. The type-1 gene is not widely expressed in the fetus, but is expressed in the fetal testis, where it participates in the alternative "backdoor" pathway of androgen synthesis; 29 it is then briefly expressed in newborn skin, and then remains unexpressed again until after puberty. The type-2 gene is expressed in fetal genital skin, where it participates in normal male sexual differentiation, and is also expressed in adult prostate and prostatic adenocarcinoma cells.…”

Monogenic Disorders of Adrenal Steroidogenesis

“…Some cases of CYP17A1 and POR deficiency have also been described as isolated 17,20 lyase deficiency; however, biochemistry reveals impaired 17α-hydroxylase activity in both instances [102, 127, 128]. Three affected individuals from a Swiss pedigree with isolated 17,20 lyase deficiency reported in the 1970s [129] have been shown to have mutations in the AKR1C2 and AKR1C4 genes [130]. Both enzymes are thought to be involved in the proposed alternative “backdoor” pathway to DHT, exemplified by POR deficiency and described in the respective above section of this review.…”

“…CYP17A1 17,20 lyase activity supported by CYB5A is required in the early part of that pathway. These cases suggest the need for the alternative pathway to be active for normal male sexual differentiation [130]. …”