A cell-autonomous role for WT1 in regulating Sry in vivo

Human patients with Frasier syndrome express reduced levels of the +KTS isoforms of the developmental regulator WT1 and exhibit complete XY gonadal dysgenesis and male-to-female sex reversal. Mice with a targeted mutation that blocks production of these isoforms show a reduction in Sry mRNA in the gonad, but the molecular and cellular basis of this reduction has not been established. Using immunofluorescence analysis, we found a significantly lower level of SRY protein per cell in XY Wt1(+KTS)-null mouse gonads. We also found a reduced number of SRY-expressing cells, correlating with a decrease in cell proliferation at and near the coelomic epithelium at 11.5 dpc. No reduction in somatic cell numbers was seen in XX Wt1(+KTS)-null gonads, indicating that the effect of WT1 on cell proliferation is mediated by Sry. Sertoli cell differentiation was blocked in XY Wt1(+KTS)-null mouse gonads, as indicated by the loss of SOX9 and Fgf9 expression, but the addition of recombinant FGF9 to ex vivo gonad cultures rescued the mutant phenotype, as indicated by the induction of the Sertoli-cell specific marker anti-Mullerian hormone. Our data suggest that WT1(+KTS) is involved in the cell-autonomous regulation of Sry expression, which in turn influences cell proliferation and Sertoli cell differentiation via FGF9. Thus, sex reversal in Wt1(+KTS)-null mice and Frasier syndrome patients results from a failure of Sertoli cells both to fully differentiate and to reach sufficient numbers to direct testis development.