Disorders of sex development (DSDs) are a range of congenital conditions, including 46,XY gonadal dysgenesis (GD), where only 50% of cases receive a specific molecular diagnosis. Improving our understanding of the genetic causes of 46,XY GD is critical to improve clinical diagnosis and management of these conditions. Among the genes promoting male sex determination, is fibroblast growth factor (FGF) 9.
FGF9 is critical for the repression of pro-ovarian signalling pathways such as WNT4/RSPO1 and FOXL2. This ensures sufficient levels of SOX9 are expressed in the somatic cells of the embryonic gonad to drive Sertoli cell differentiation and ultimately, male testicular development. Loss of Fgf9 results in complete male to female sex reversal, while Fgf9 gene mutants causing impaired FGF9 homodimerization in humans and mice lead to skeletal defects such as synostosis.
Here we investigate the requirement of FGF9 dimer formation for testicular development. Using homodimer-compromised FGF9 mutants from both a mouse model and a human DSD patient.
The spontaneous mouse Fgf9 mutant; Elbow knee synostosis (Eks), amino acid substitution (N143T), which lies at the homodimerization interface results in impaired FGF9 homodimerization. Examination of XY Fgf9N143T/N143T gonads showed delayed testes cord development and ectopic expression of the female Granulosa cell marker FOXL2 at the gonadal poles, indicative of XY sex reversal.
We have identified a 46,XY GD DSD patient with an amino acid substitution (D195N), previous studies indicated that the D195 residue is critical for the homodimerization of FGF9. Purified recombinant FGF9D195N protein showed reduced affinity for heparin, a property required for homodimerization. In vitro analysis showed reduced ability to induce Sertoli cell proliferation, which is required for normal testis development.
In summary, this data suggests that homodimerization of FGF9 is critical for FGF-mediated male sex determination.