Cbx2, a Polycomb Group Gene, Is Required for Sry Gene Expression in Mice

Abstract

Mice lacking the function of the polycomb group protein CBX2 (chromobox homolog 2; also known as M33) show defects in gonadal, adrenal, and splenic development. In particular, XY knockout (KO) mice develop ovaries but not testes, and the gonads are hypoplastic in both sexes. However, how CBX2 regulates development of these tissues remains largely unknown. In the present study, we used microarray, RT-PCR, and immunohistochemical analyses to show that the expression of Sry, Sox9, Lhx9, Ad4BP/SF-1, Dax-1, Gata4, Arx, and Dmrt1, genes encoding transcription factors essential for gonadal development, is affected in Cbx2 KO gonads. Male-to-female sex reversal in Cbx2 KO mice was rescued by crossing them with transgenic mice displaying forced expression of Sry or Sox9. However, testes remained hypoplastic in these mice, indicating that the size and the sex of the gonad are determined by different sets of genes. Our study implicates Cbx2 in testis differentiation through regulating Sry gene expression.

Molecular mechanisms involved in mammalian primary sex determination

“…In humans, the loss-of-function mutations of CBX2 result in maleto-female sex reversal, suggesting that CBX2 may function upstream of SRY to repress the ovarian differentiation (Biason-Lauber et al 2009). The expression levels of several transcription factors involved in gonadal development, such as Sox9, Lhx9, Sf1, Dax1, Dmrt1, are decreased by approximately twofold or more in the Cbx2-KO mouse gonads (Katoh-Fukui et al 2012).…”

“…Microassay, immunohistochemical, and ChIP analyses of mice deficient in chromobox homolog2 (Cbx2; also known as M33) implicates that disruption of Cbx2 dramatically decreased Sry gene expression and Cbx2 may directly or indirectly regulate the expression of Sry through binding to the promoter of Sf1 or other unknown mechanisms (Katoh-Fukui et al 2012). In humans, the loss-of-function mutations of CBX2 result in maleto-female sex reversal, suggesting that CBX2 may function upstream of SRY to repress the ovarian differentiation (Biason-Lauber et al 2009).…”

Switching on sex: transcriptional regulation of the testis-determining geneSry

“…XY sex reversal occurs in humans carrying CBX2 mutations (Biason-Lauber et al, 2009) and in mice lacking Cbx2 function (Katoh-Fukui et al, 2012). In these mice, Sry is expressed at negligible levels and far fewer cells express Nr5a1 compared with wild-type gonads, suggesting that, rather than directly regulating Sry, CBX2 might mediate its activities through the regulation of Nr5a1 (Katoh-Fukui et al, 2012) ( Fig.…”

“…A number of mutations in candidate Sry-regulating genes result in decreased Sry expression (Bradford et al, 2009;Fujimoto et al, 2013;Katoh-Fukui et al, 2012). In these situations, it is necessary to determine whether reduced expression is a result of lower Sry expression in each cell, and hence supportive of a defect in Sry regulation, or simply reflects a reduced population of Sry-expressing cells.…”

The role of non-coding RNAs in male sex determination and differentiation

“…Mutation analysis of a number of genes revealed insights into the regulation of Sry expression (for review see Svingen & Koopman (2013)). Genes that have been implicated in the regulation of Sry expression include chromobox homolog 2 (Cbx2) (Katoh-Fukui et al 2012), the CKST isoform of WT1 (Hammes et al 2001), steroidogenic factor 1 (SF1), encoded by the gene Nr5a1 (Pilon et al 2003), as well as the transcriptional co-factor CITED2 in combination with WT1 and SF1 (Buaas et al 2009), GATA-binding protein 4 (Gata4) and its cofactor friend of GATA 2 (Fog2) (Tevosian et al 2002), combined loss-of-function mutations in the insulin receptor genes, Ir and Igf1r (Nef et al 2003, Pitetti et al 2013, as well as mutations in the MAPK pathway, including Map3k4, its activator Gadd45g and p38MAPKs (Bogani et al 2009, Gierl et al 2012, Warr et al 2012. Interestingly, in addition to its expression in the fetal testis, where Sry is both necessary and sufficient to induce male development, an unusual circular Sry transcript has been detected in adult mouse testis (Capel et al 1993).…”