Switching on sex: transcriptional regulation of the testis-determining gene Sry

Abstract

Mammalian sex determination hinges on the development of ovaries or testes, with testis fate being triggered by the expression of the transcription factor sex-determining region Y (Sry). Reduced or delayed Sry expression impairs testis development, highlighting the importance of its accurate spatiotemporal regulation and implying a potential role for SRY dysregulation in human intersex disorders. Several epigenetic modifiers, transcription factors and kinases are implicated in regulating Sry transcription, but it remains unclear whether or how this farrago of factors acts co-ordinately. Here we review our current understanding of Sry regulation and provide a model that assembles all known regulators into three modules, each converging on a single transcription factor that binds to the Sry promoter. We also discuss potential future avenues for discovering the cis-elements and trans-factors required for Sry regulation.

Keywords: Disorders of sex development; Sex determination; Sry; Testis development; Transcriptional regulation.

Fig. 1.

Function and expression of Sry. (A) Testis fate is determined in most mammals by the presence of an Sry-containing Y chromosome. In individuals lacking a Y chromosome, genital ridges instead develop as ovaries. (B) Sry engages a sequence of regulatory steps that simultaneously promotes testis development and suppresses ovary development. (C) Sry is expressed in the supporting cells of the genital ridges between 10.5 and 12.5 dpc, with peak expression occurring at 11.5 dpc. Beneath is illustrated the centre-to-pole wave of Sry expression (blue) in the developing testis. (D) Immunostaining for SRY (green) showing expression in pre-Sertoli cells throughout the XY mouse genital ridge (outlined) at 11.5 dpc. Nuclei are counterstained with DAPI (blue); anti-E-cadherin (red) marks mesonephric tubules and (weakly) germ cells.

Fig. 2.

A model of Sry regulation in mouse. The many factors implicated in regulating Sry can be partitioned into three modules. In Module 1, the insulin (INSR/IGF1R) pathway induces Gadd45g expression, which activates a MAP kinase cascade involving MAP3K4, an as yet unknown member of the MAP2K family, and p38α/β, leading to phosphorylation (P) of GATA4. Simultaneously, SIX1 and SIX4 upregulate FOG2, which partners with GATA4. The resulting complex is able to bind to two specific regions of the Sry promoter. In Module 2, CITED2 cooperates with WT1 to upregulate NR5A1, a process that also involves transcription factors including CBX2, LHX9 and SIX1/4. NR5A1 then binds to an unknown position on the Sry locus where it contributes to Sry upregulation. Module 3 involves the transcription factor WT1, which is also able to bind directly to DNA, with in vitro evidence suggesting a specific binding site in mouse Sry between −56 and −47, near the TSS (+1, blue arrow). Possible regulators of WT1 remain unknown.

Fig. 3.

Sry promoter constructs reveal putative Sry regulatory regions. Promoter constructs have been generated using the regulatory regions of mouse (A-F), human (H,I), pig (J-N) and goat (O,P) Sry. These constructs regulate the expression of either the endogenous Sry ORF or a fluorescent reporter protein, typically in transgenic mice or a transfected cell line. The TSS is illustrated for the first three of these species; the Sry TSS has not been characterised in goat. The region included in each construct is indicated in green or red and the locations of putative cis-regulatory regions suggested by these studies are marked (orange). Also included is the result of Yokouchi et al. (2003) (G), which does not involve a promoter construct but assays DNase hypersensitivity of the mouse Sry promoter region.

Fig. 4.

Deletions in human SRY flanking regions associated with XY disorders of sexual development (DSD). A large deletion 5′ of SRY has been reported in a patient (NV) and leads to streak gonads and external female genitalia (McElreavey et al., 1992). This deletion may remove almost the entire region between SRY and RPS4Y1. A second deletion, 3′ of SRY, has also been reported (patient SC) and is associated with dysgenetic testes and female external genitalia (McElreavey et al., 1996). This deletion crosses the pseudoautosomal boundary (PAB) and extends some way into the pseudoautosomal region.

Fig. 5.

Pericentric inversions of the Y chromosome associated with XY DSD. (A) In one reported case of XY DSD, SRY is translocated via a pericentric inversion (indicated by dashed lines) to the long arm of the Y chromosome. (B) In a second case, which is also associated with XY DSD, a different pericentric inversion leaves SRY in place but modifies the sequence some 350 kb 5′ of SRY (Mitsuhashi et al., 2010). Chromosomes are not shown to scale.

Fig. 6.

Summary of the regulatory regions and sequence motifs that lie 5′ of Sry. Sry transcripts in mouse (top) and human (bottom) are shown in dark blue. Regions implicated in the binding of GATA4 (purple) and WT1 (red) are illustrated, along with five CpG sites that exhibit gonad-specific demethylation (black arrowheads) and a regulatory region inferred by Ito et al. (2005) (orange; see Fig. 3E,F). Two regions of moderate homology with human are present in the proximal 2 kb of the mouse Sry 5′ region (Hacker et al., 1995) (light blue shading). The inverted repeat sequence flanking mouse Sry is indicated by chevrons. Additionally, a number of short motifs within the Sry 5′ region are conserved between human and various other species [green rectangles; the relevant studies are indicated (Ross et al., 2008; Pilon et al., 2003; Veitia et al., 1997b)]. The most proximal (Veitia C) and distal (Ross, Pilon) of these regions each overlap one of the two regions conserved between human and mouse.