Sox10 gain-of-function causes XX sex reversal in mice: implications for human 22q-linked disorders of sex development.

Male development in mammals is normally initiated by the Y-linked gene Sry, which activates expression of Sox9, leading to a cascade of gene activity required for testis formation. Although defects in this genetic cascade lead to human disorders of sex development (DSD), only a dozen DSD genes have been identified, and causes of 46,XX DSD (XX maleness) other than SRY translocation are almost completely unknown. Here, we show that transgenic expression of Sox10, a close relative of Sox9, in gonads of XX mice resulted in development of testes and male physiology. The degree of sex reversal correlated with levels of Sox10 expression in different transgenic lines. Sox10 was expressed at low levels in primordial gonads of both sexes during normal mouse development, becoming male-specific during testis differentiation. SOX10 protein was able to activate transcriptional targets of SOX9, explaining at a mechanistic level its ability to direct male development. Because over-expression of SOX10 alone is able to mimic the XX DSD phenotypes associated with duplication of human chromosome 22q13, and given that human SOX10 maps to 22q13.1, our results functionally implicate SOX10 in the etiology of these DSDs.

Male-specific expression of Aldh1a1 in mouse and chicken fetal testes: implications for retinoid balance in gonad development.

Balanced production and degradation of retinoids is important in regulating development of several organ systems in the vertebrate embryo. Among these, it is known that retinoic acid (RA), and the retinoid-catabolyzing enzyme CYP26B1 together regulate the sex-specific behavior of germ cells in developing mouse gonads. We report here that the gene encoding a cytosolic class-1 aldehyde dehydrogenase, ALDH1A1, a weak catalyst of RA production, is strongly expressed in a male-specific manner in somatic cells of the developing mouse testis, beginning shortly after Sry expression is first detectable. This expression pattern is conserved in the developing male gonad of the chicken and is dependent on the testis-specific transcription factor SOX9. Our data suggest that low levels of RA may be required for early developmental events in the testis, or that Aldh1a1 expression in the fetus may prefigure a later requirement for ALDH1A1 in regulating spermatogenesis postnatally.

Retinoid signaling determines germ cell fate in mice.

Germ cells in the mouse embryo can develop as oocytes or spermatogonia, depending on molecular cues that have not been identified. We found that retinoic acid, produced by mesonephroi of both sexes, causes germ cells in the ovary to enter meiosis and initiate oogenesis. Meiosis is retarded in the fetal testis by the action of the retinoid-degrading enzyme CYP26B1, ultimately leading to spermatogenesis. In testes of Cyp26b1-knockout mouse embryos, germ cells enter meiosis precociously, as if in a normal ovary. Thus, precise regulation of retinoid levels during fetal gonad development provides the molecular control mechanism that specifies germ cell fate.