Epigenetic regulation and downstream targets of the Rhox5 homeobox gene.

The discovery of the Rhox homeobox gene cluster on the X chromosome opens up new vistas in the regulation of reproductive processes in mammals. In mice, this cluster comprises more than 30 genes that are selectively expressed in reproductive tissues. A subset of Rhox genes are androgen and AR regulated in postnatal and adult Sertoli cells, making them candidates to mediate androgen-dependent steps during spermatogenesis. The best characterized of these androgen/AR-regulated genes is Rhox5 (Pem), the founding member of the Rhox gene cluster. Targeted deletion of Rhox5 in mice causes male subfertility marked by increased germ-cell apoptosis and decreased sperm count and motility. Microarray analyses identified a wide variety of genes regulated by Rhox5 in Sertoli cells. One of them is the tumour suppressor UNC5C, a pro-apoptotic molecule previously only known to be involved in brain development. Targeted deletion of Unc5c causes decreased germ-cell apoptosis in postnatal and adult testes, indicating that it also has a role in spermatogenesis and supporting a model in which Rhox5 promotes germ-cell survival by downregulating Unc5c. Rhox5 has two independently regulated promoters that have distinct expression patterns. The unique tissue-specific and developmentally regulated transcription pattern of these two promoters appear to be controlled by DNA methylation. Both promoters are methylated in tissues in which they are not expressed, suggesting that DNA methylation serves to repress Rhox5 expression in inappropriate cell types and tissues. In summary, the Rhox gene cluster is an epigenetically regulated set of genes encoding a large number of transcription factors that are strong candidates to regulate gametogenesis and other aspects of reproduction.