Modelling androgen synthesis and action during human sexual differentiation

Abstract

The sex of an individual is established by a complex interplay of genetic and hormonal factors during early development. Differentiation of the external genitalia concordant with genetic sex is essential, as abnormalities fundamentally compromise reproductive capacity. Much current understanding of this process comes from the study of pathological phenotypes, which provides some insight into the underlying genetics. Beyond this, because of a lack of relevant material, there is a general reliance on animal models and cancer cell lines to study the underlying pathways. Neither of these approaches is ideal. Using a very comprehensive collection of human first trimester material, this project aimed to study sexual differentiation of human external genitalia directly in the tissue of interest, and at the precise time when it occurs at 7-12 weeks post-conception.

Chapter 3 investigated the biosynthesis of testicular testosterone during the critical period of sexual differentiation, revealing that testosterone biosynthesis is unlikely to be regulated by the anterior pituitary or placental hCG during sexual differentiation; suggesting constitutive regulation occurs. In Chapter 4, characterisation of the expression of key genes in the developing human external genitalia illustrated escalating expression of nuclear androgen receptor (AR) during the first trimester in males. AR was also detected in female external genitalia; its nuclear location implied it was ligand-bound by androgen. To pursue this, in Chapter 5 an in vitro culture model of fetal external genitalia stromal cells was established. Male and female cells retained AR expression, which, in the absence of androgen, was diffusely cytoplasmic. Nuclear translocation occurred indistinguishably in male and female cells with thresholds of 100 pM DHT, 1 nM testosterone and 10 nM androstenedione. Androgen secretion in this range has been previously documented from the first trimester female adrenal gland. Conditioned media from the adrenal gland also elicited nuclear AR translocation in external genitalia cells. In Chapter 6, through the use of expression microarray analysis, the widespread genetic consequences of AR function during human sexual differentiation were determined.

This project provides significant insight into a critical period of human development and has identified that early human androgen secretion in males is likely to be predominantly constitutive. Strikingly, AR bioactivity appears part of normal female sexual differentiation, reinforcing a theory that in human females, sexual differentiation unfolds within a significant androgenic milieu, underlining its vulnerability to excessive androgen in conditions such as congenital adrenal hyperplasia. Furthermore, several putative candidate genes for inactivating mutations in syndromes of male under-virilisation, such as hypospadias, were identified.

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