CBX2 is required during male sex determination to repress female fate at bivalent loci
The epigenetic mechanisms that regulate the male vs. female cell-fate decision in the mammalian bipotential gonad are poorly understood. In this study, we developed a quantitative genome-wide profile of H3K4me3 and H3K27me3 in isolated XY and XX gonadal supporting cells before and after sex determination. We show that male and female sex-determining (SD) genes are bivalent before sex determination, providing insight into how the bipotential state of the gonad is established at the epigenetic level. After sex determination, many SD genes of the alternate pathway remain bivalent, possibly contributing to the ability of these cells to transdifferentiate even in adults. It was previously shown that loss of CBX2, the Polycomb group subunit that binds H3K27me3 and mediates silencing, leads to loss of Sry and male-to-female sex reversal. The finding that many genes in the Wnt signaling pathway were targeted for H3K27me3-mediated repression in Sertoli cells led us to test whether deletion of Wnt4 could rescue male development in Cbx2 mutants. We show that Sry expression and testis development were rescued in XY Cbx2-/-;Wnt4-/- mice. Furthermore, we show that CBX2 directly binds the downstream Wnt signaler Lef1, a female-promoting gene that remains bivalent in Sertoli cells. Our results suggest that stabilization of the male fate requires CBX2-mediated repression of bivalent female-determining genes, which would otherwise block Sry expression and male development.\n\nAuthor SummaryDuring development, the bipotential fetal gonad can commit to the male fate and develop as a testis, or to the female fate and develop as an ovary. Mutation of the epigenetic regulator CBX2 leads to ovary development in XY embryos, suggesting a critical role for chromatin remodeling during sex determination. However, the epigenetic mechanisms that regulate the male vs. female cell-fate decision in the mammalian bipotential gonad are poorly understood. In this study, we developed a genome-wide profile of two histone modifications that play critical roles during development: H3K27me3 (repressive) and H3K4me3 (active). We find that sex-determining genes that are initially co-expressed in XX and XY bipotential gonads are bivalent (marked by both H3K4me3 and H3K27me3) prior to sex determination, poised to engage either the male or female fate. Remarkably, after sex determination, repressed genes that promote the alternate fate remain bivalent. We show that stabilization of the male fate requires CBX2-mediated repression of bivalent female-determining genes, which would otherwise block testis development. Our study provides insight into how the bipotential state of the gonad is established at the epigenetic level, and how the male fate is stabilized by repression of the female fate during sex determination.