In the presence of the Y-chromosomal gene Sry, the bipotential mouse gonads develop as testes rather than as ovaries. The autosomal gene Sox9, a likely and possibly direct Sry target, can induce testis development in the absence of Sry. Sox9 is thus sufficient but not necessarily essential for testis induction. Mutational inactivation of one allele of SOX9/Sox9 causes sex reversal in humans but not in mice. Because Sox9(-/-) embryos die around Embryonic Day 11.5 (E11.5) at the onset of testicular morphogenesis, differentiation of the mutant XY gonad can be analyzed only ex vivo in organ culture. We have therefore conditionally inactivated both Sox9 alleles in the gonadal anlagen using the CRE/loxP recombination system, whereby CRE recombinase is under control of the cytokeratin 19 promoter. Analysis of resulting Sox9(-/-) XY gonads up to E15.5 reveals immediate, complete sex reversal, as shown by expression of the early ovary-specific markers Wnt4 and Foxl2 and by lack of testis cord and Leydig cell formation. Sry expression in mutant XY gonads indicates that downregulation of Wnt4 and Foxl2 is dependent on Sox9 rather than on Sry. Our results provide in vivo proof that, in contrast to the situation in humans, complete XY sex reversal in mice requires inactivation of both Sox9 alleles and that Sox9 is essential for testogenesis in mice.

SOX9 is an evolutionary conserved transcription factor that is expressed in a variety of tissues, with essential functions in cartilage, testis, heart, glial cell, inner ear and neural crest development. By comparing human and pufferfish genomic sequences, we previously identified eight highly conserved sequence elements between 290 kb 5' and 450 kb 3' to human SOX9. In this study, we assayed the regulatory potential of elements E1 to E7 in transgenic mice using a lacZ reporter gene driven by a 529 bp minimal mouse Sox9 promoter. We found that three of these elements and the Sox9 promoter control distinct subsets of the tissue-specific expression pattern of Sox9. E3, located 251 kb 5' to SOX9, directs lacZ expression to cranial neural crest cells and to the inner ear. E1 is located 28 kb 5' to SOX9 and controls expression in the node, notochord, gut, bronchial epithelium and pancreas. Transgene expression in the neuroectoderm is mediated by E7, located 95 kb 3' to SOX9, which regulates expression in the telencephalon and midbrain, and by the Sox9 minimal promoter which controls expression in the ventral spinal cord and hindbrain. We show that E3-directed reporter gene expression in neural crest cells of the first but not of the second and third pharyngeal arch is dependent on beta-catenin, revealing a complex regulation of Sox9 in cranial neural crest cells. Moreover, we identify and discuss highly conserved transcription factor binding sites within enhancer E3 that are in good agreement with current models for neural crest and inner ear development. Finally, we identify enhancer E1 as a cis-regulatory element conserved between vertebrates and invertebrates, indicating that some cis-regulatory sequences that control developmental genes in vertebrates might be phylogenetically ancient.

In mammalian embryonic gonads, SOX9 is required for the determination of Sertoli cells that orchestrate testis morphogenesis. To identify genetic networks directly regulated by SOX9, we combined analysis of SOX9-bound chromatin regions from murine and bovine foetal testes with sequencing of RNA samples from mouse testes lacking Sox9. We found that SOX9 controls a conserved genetic programme that involves most of the sex-determining genes. In foetal testes, SOX9 modulates both transcription and directly or indirectly sex-specific differential splicing of its target genes through binding to genomic regions with sequence motifs that are conserved among mammals and that we called ‘Sertoli Cell Signature’ (SCS). The SCS is characterized by a precise organization of binding motifs for the Sertoli cell reprogramming factors SOX9, GATA4 and DMRT1. As SOX9 biological role in mammalian gonads is to determine Sertoli cells, we correlated this genomic signature with the presence of SOX9 on chromatin in foetal testes, therefore equating this signature to a genomic bar code of the fate of foetal Sertoli cells. Starting from the hypothesis that nuclear factors that bind to genomic regions with SCS could functionally interact with SOX9, we identified TRIM28 as a new SOX9 partner in foetal testes.

In mammals, sex is determined in the bipotential embryonic gonad by a balanced network of gene actions which when altered causes disorders of sexual development (DSD, formerly known as intersex). In the XY gonad, presumptive Sertoli cells begin to differentiate when SRY up-regulates SOX9, which in turn activates FGF9 and PGDS to maintain its own expression. This study identifies a new and essential component of FGF signaling in sex determination. Fgfr2 mutant XY mice on a mixed 129/C57BL6 genetic background had either normal testes, or developed ovotestes, with predominantly testicular tissue. However, backcrossing to C57BL6 mice resulted in a wide range of gonadal phenotypes, from hypoplastic testes to ovotestes with predominantly ovarian tissue, similar to Fgf9 knockout mice. Since typical male-specific FGF9-binding to the coelomic epithelium was abolished in Fgfr2 mutant XY gonads, these results suggest that FGFR2 acts as the receptor for FGF9. Pgds and SOX9 remained expressed within the testicular portions of Fgfr2 mutant ovotestes, suggesting that the Prostaglandin pathway acts independently of FGFR2 to maintain SOX9 expression. We could further demonstrate that double-heterozygous Fgfr2/Sox9 knockout mice developed ovotestes, demonstrating that both Fgfr2 and Sox9 can act as modifier intersex genes in the heterozygous state. In summary, we provide evidence that FGFR2 is important for male sex determination in mice, thereby rendering human FGFR2 a candidate gene for unsolved DSD cases such as 10q26 deletions.