Mammalian sex determination—insights from humans and mice

Abstract

Disorders of sex development (DSD) are congenital conditions in which the development of chromosomal, gonadal, or anatomical sex is atypical. Many of the genes required for gonad development have been identified by analysis of DSD patients. However, the use of knockout and transgenic mouse strains have contributed enormously to the study of gonad gene function and interactions within the development network. Although the genetic basis of mammalian sex determination and differentiation has advanced considerably in recent years, a majority of 46,XY gonadal dysgenesis patients still cannot be provided with an accurate diagnosis. Some of these unexplained DSD cases may be due to mutations in novel DSD genes or genomic rearrangements affecting regulatory regions that lead to atypical gene expression. Here, we review our current knowledge of mammalian sex determination drawing on insights from human DSD patients and mouse models.

Fig. 1

Overview of the key genes and regulatory networks leading from the bi-potential gonad to either testis or ovary development in mouse. a Genes that are essential for the development of the bi-potential gonad have been identified due to the total lack of gonads in the corresponding knockout mouse strain and they have been shown to be expressed in the bi-potential gonad. Functional studies revealed that specifically the WT1 −KTS isoform binds to and activates the Nr5a1 promoter in conjunction with LHX9. In the spleen and adrenal gland, CBX2 has been shown to regulate Nr5a1, leading to the hypothesis of a similar function in the bi-potential gonad. b Various genes have been implicated in the pathways leading to testis development in mouse. In XY mouse embryos, Sry is transiently expressed in the bi-potential gonad, whereas in human embryos, SRY expression is periodic. However, in both species, Sry expression initiates an increase of Sox9 expression, which then stimulates Fgf9 expression. Both FGF9 and SOX9 act in a positive feedback loop, which act to suppress the female specific genes, especially Wnt4 and subsequently lead to the manifestation of the testis-specific program. Numerous other genes and their gene products, such as Gata4, Fog2, Wt1 (+KTS isoform), Nr5a1, Pgsd, Fgfr1, Cbx2, Sox8, Amh, Dax1, and Dhh are necessary for the regulation (positive as well as negative) and maintenance of this crucial male determining pathway. Dmrt1 has recently been shown to be required for the maintenance of gonadal sex, especially to prevent female reprogramming in postnatal mouse testis. c In XX individuals, Sry is absent and ovary-specific genes, such as Rspo1, Wnt4, and Foxl2 are expressed. Rspo1 −/− ovaries show reduced levels of WNT4, suggesting that Rspo1 acts upstream of Wnt4. However, a synergistic action of WNT4 and RSPO1 to activate β-catenin has also been suggested. WNT4/β-catenin have been proposed to suppress the SOX9/FGF9 positive feedback loop, allowing the ovarian-specific pathway to progress. WNT4 and FOXL2 are also involved in the positive regulation of Bmp2. Together, FOXL2, RSPO1, and WNT4 activate Fst expression. Genes of the female pathway that have been shown or suggested to interact with the male pathway are shown in red, genes of the male pathway interacting with the female pathway are highlighted in blue. In this figure, solid lines do not necessarily imply direct interactions. Question marks indicate that the position of that gene and the interaction with other genes has been proposed. Many pathways shown in this figure are similar or even identical between mouse and human; however, for some of them, there might be differences between the two species

Fig. 2

Known genes and pathways of the different cell types of the developing testis in mouse (modified from Wainwright and Wilhelm 2010). a Postulated molecular pathways underlying Sertoli cell specification, including the regulation of Sry, induction of Sox9 expression, and the maintenance of Sox9 expression. b Differentiation of peritubular myoid (PM) cells is regulated by signaling from Sertoli cells via desert hedgehog (DHH) and its receptor patched 1 (PTCH1). In addition, Dax1 expression in Sertoli cells is required for PM cell differentiation, but the molecular mechanism still remains to be elucidated. The interaction between Sertoli cells and PM cells results in the secretion of extracellular matrix (ECM) molecules by both cell types, which finally leads to the formation of basement membrane (BM). Both cell types contribute distinct components to the ECM, with PM cells secreting fibronectin, collagens, and proteoglycans. c–f The regulation of Leydig cell development via Sertoli cell and Leydig cell interactions. The morphogen DHH is secreted by Sertoli cells and induces Leydig cell specification through its receptor PTCH1. Signaling via the receptor NOTCH3 and its effector HES1 is crucial for the maintenance of the progenitor population and the restriction of their differentiation to fetal Leydig cells. Platelet-derived growth factor A (PDGFA) signaling via the receptor PDGFRα plays a role in Leydig cell differentiation. In addition, Sertoli cell expressed DAX1/NR0B1 has been implicated in Leydig cell survival. As Leydig cells start to differentiate, they start expressing genes required for steroid synthesis, such as side chain cleavage (Scc). Differentiated Leydig cells synthesize testosterone using the four enzymes P450 side chain cleavage (SCC), 3-β-hydroxysteroid dehydrogenase/Delta-5-4 isomerase type 2 (HSD3B2), cytochrome P450 17-hydroxylase (CYP17), and 17β-hydroxysteroid dehydrogenase 3 (17βHSDIII). The biosynthesis of testosterone leads to the masculinization of the developing embryo. Furthermore, fetal Leydig cells express insulin-like factor 3 (INSL3), which regulates testis decent. Both INSL3 and SCC are regulated by SF1/NR5A1 at the transcriptional level. Gene products from the female pathways known to interact with the male-specific pathway are shown in red. Solid lines in this figure do not indicate if the interactions occur in a direct or indirect manner. Genes/gene products shown in gray are not mentioned in the text, but have been included to provide a more accurate summary of or current knowledge