Research Fellow The University of Melbourne and The University of Newcastle Flemington, Victoria, Australia
Abstract Authors: Ben M. Lawrence1; Liza O’Donnell2; Anne-Louise Gannon1; Imogen Abbott1; Diane Rebourcet1; Lee B. Smith1, 2
1. College of Engineering, Science and Environment, The University of Newcastle, Callaghan, NSW, 2308, Australia
2. Office for Research, Griffith University, Southport, QLD, 4222, Australia
Abstract Text: Testosterone is essential for the regulation of androgen-dependent functions including male sexual development and spermatogenesis. In the adult testis, androgens are synthesized by highly specialized cells, known as the Leydig cells. Adult Leydig cells synthesise testosterone via the canonical androgen biosynthesis pathway, where the HSD17B3 enzyme catalyses the conversion of the androgen precursor androstenedione into testosterone. Consequently, loss of function mutations in the human HSD17B3 gene results in a disorder of sexual development. 46,XY HSD17B3-deficient individuals retain internal Wolffian structures, however the external genitalia is undermasculinised, appearing as female or ambiguous.
Two independent research groups have generated Hsd17b3-deficient mouse models. Surprisingly, and in contrast to human cases of HSD17B3-deficiency, male Hsd17b3 knockout mice are masculinized from birth and are fertile in adulthood. Although Hsd17b3 knockout mice exhibit high androstenedione/testosterone ratios (indicative of HSD17B3 dysfunction), intratesticular testosterone remains normal. This data suggests that mice have compensatory mechanisms/alternative enzymes which enable the continued production of testosterone in the absence of HSD17B3. We aimed to identify alternative hydroxysteroid dehydrogenase (HSD) enzymes that may be responsible for continued testosterone biosynthesis in Hsd17b3 knockout mice.
We have identified mouse HSD enzymes that can convert the precursor androstenedione into testosterone and validated this conversion in vitro. We have demonstrated that a key amino acid in a particular HSD allows it to synthesize testosterone, in contrast to the human enzyme which has a different amino acid and is unable to produce testosterone. To model human androgen production in mice, we developed a humanized transgenic mouse line expressing the HSD that is altered to express the human amino acid and is thus unable to produce testosterone. To determine if this mutated HSD enzyme is unable to compensate for the lack of Hsd17b3 we have cross bred this humanized HSD mouse line with Hsd17b3 knockout mice. The humanized mutation in mice has been validated by genomic sequencing and the phenotype has been characterized. Intratesticular and circulating steroid analysis is being performed and will indicate if this HSD plays an important functional role in testosterone biosynthesis in Hsd17b3 knockout mice.
In conclusion, we are generating mouse models that can be used to better understand human disorders of androgen biosynthesis and can be exploited to identify novel therapies for androgen deficiency.
