Characterization of two Sp1 binding sites of the human sex determining SRY promoter

Mutation of the Sp1 binding site in the 5' flanking region of SRY causes sex reversal in rabbits

Sex-determining region Y is a crucial gene that initiates male sex determination in mammals. Mutations of the Sp1-binding site in the 5' flanking region of SRY are associated with clinical male-to-female sex reversal syndrome, although such occurrences are rare and, until now, have not been reported in animal models. In this study, we mutated Sp1-binding sites in the 5' flanking region of the rabbit SRY gene using the CRISPR/Cas9 system. As expected, the SRY-Sp1 knockout rabbits had female external and internal genitalia and exhibited normal female copulatory behaviors, but they were infertile, and the adults displayed reduced follicles. Interestingly, we successfully obtained offspring from sex-reversed SRY-Sp1 knockout rabbits using embryo transfer. In summary, our study demonstrates that Sp1 is a major regulator in SRY gene transcription, and mutations of the Sp1 binding sites (Sp1-B and Sp1-C) in the 5' flanking region of SRY induce sex reversal in rabbits, which can be used as targets for clinical research of male-to-female sex reversal syndrome. Additionally, we provide the first evidence that sex reversal syndrome patients have the potential to become pregnant with the use of embryo transfer.

Molecular mechanism of male differentiation is conserved in the SRY-absent mammal, Tokudaia osimensis

The sex-determining gene SRY induces SOX9 expression in the testes of eutherian mammals via two pathways. SRY binds to testis-specific enhancer of Sox9 (TESCO) with SF1 to activate SOX9 transcription. SRY also up-regulates ER71 expression, and ER71 activates Sox9 transcription. After the initiation of testis differentiation, SOX9 enhances Amh expression by binding to its promoter with SF1. SOX8, SOX9 and SOX10, members of the SOXE gene family, also enhance the activities of the Amh promoter and TESCO. In this study, we investigated the regulation of these sexual differentiation genes in Tokudaia osimensis, which lacks a Y chromosome and the SRY gene. The activity of the AMH promoter was stimulated by SOXE genes and SF1. Mutant AMH promoters, with mutations in its SOX and SF1 binding sites, did not show significant activity by SOX9 and SF1. These results indicate that AMH expression was regulated by the binding of SOX9 and SF1. By contrast, SOXE genes could not enhance TESCO activity. These results indicate that TESCO enhancer activity was lost in this species. Furthermore, the activity of the SOX9 promoter was enhanced by ER71, indicating that ER71 may play an important role in the testis-specific expression of SOX9.

Conservation analysis of sequences flanking the testis-determining gene Sry in 17 mammalian species

BACKGROUND

Sex determination in mammals requires expression of the Y-linked gene Sry in the bipotential genital ridges of the XY embryo. Even minor delay of the onset of Sry expression can result in XY sex reversal, highlighting the need for accurate gene regulation during sex determination. However, the location of critical regulatory elements remains unknown. Here, we analysed Sry flanking sequences across many species, using newly available genome sequences and computational tools, to better understand Sry's genomic context and to identify conserved regions predictive of functional roles.

METHODS

Flanking sequences from 17 species were analysed using both global and local sequence alignment methods. Multiple motif searches were employed to characterise common motifs in otherwise unconserved sequence.

RESULTS

We identified position-specific conservation of binding motifs for multiple transcription factor families, including GATA binding factors and Oct/Sox dimers. In contrast with the landscape of extremely low sequence conservation around the Sry coding region, our analysis highlighted a strongly conserved interval of ~106 bp within the Sry promoter (which we term the Sry Proximal Conserved Interval, SPCI). We further report that inverted repeats flanking murine Sry are much larger than previously recognised.

CONCLUSIONS

The unusually fast pace of sequence drift on the Y chromosome sharpens the likely functional significance of both the SPCI and the identified binding motifs, providing a basis for future studies of the role(s) of these elements in Sry regulation.

Regulation of male sex determination: genital ridge formation and Sry activation in mice

Sex determination is essential for the sexual reproduction to generate the next generation by the formation of functional male or female gametes. In mammals, primary sex determination is commenced by the presence or absence of the Y chromosome, which controls the fate of the gonadal primordium. The somatic precursor of gonads, the genital ridge is formed at the mid-gestation stage and gives rise to one of two organs, a testis or an ovary. The fate of the genital ridge, which is governed by the differentiation of somatic cells into Sertoli cells in the testes or granulosa cells in the ovaries, further determines the sex of an individual and their germ cells. Mutation studies in human patients with disorders of sex development and mouse models have revealed factors that are involved in mammalian sex determination. In most of mammals, a single genetic trigger, the Y-linked gene Sry (sex determination region on Y chromosome), regulates testicular differentiation. Despite identification of Sry in 1990, precise mechanisms underlying the sex determination of bipotential genital ridges are still largely unknown. Here, we review the recent progress that has provided new insights into the mechanisms underlying genital ridge formation as well as the regulation of Sry expression and its functions in male sex determination of mice.

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

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.

Transcription factors ER71/ETV2 and SOX9 participate in a positive feedback loop in fetal and adult mouse testis

ER71, also known as ETV2, is an ETS transcription factor that is expressed during embryogenesis and in adult testes. We show that Er71 transcription can be up-regulated by SRY, the key determinant of male differentiation. Accordingly, SRY bound to and activated the Er71 promoter, and mutation of a putative SRY binding site abolished this promoter activation. In turn, ER71 was able to bind to the promoter of Sox9, the primary target of SRY and a critical transcription factor for maintenance of the Sertoli cell phenotype. Mutation of the ER71 binding site in the Sox9 promoter suppressed ER71-dependent up-regulation of Sox9 transcription, and a dominant-negative ER71 molecule severely reduced Sox9 transcription in a Sertoli cell line. Conversely, SOX9 bound the Er71 promoter in vivo and Sox9 down-regulation reduced Er71 transcript levels. Together, these data suggest a mechanism by which SRY induces Sox9 and Er71 transcription early in testis differentiation, whereas ER71 and SOX9 participate in an autoregulatory loop to sustain each other's expression after Sry expression has subsided in mice. Thereby, ER71 and SOX9 may affect late testis development as well as the function of the adult male gonad.

New insights into SRY regulation through identification of 5' conserved sequences

BACKGROUND

SRY is the pivotal gene initiating male sex determination in most mammals, but how its expression is regulated is still not understood. In this study we derived novel SRY 5' flanking genomic sequence data from bovine and caprine genomic BAC clones.

RESULTS

We identified four intervals of high homology upstream of SRY by comparison of human, bovine, pig, goat and mouse genomic sequences. These conserved regions contain putative binding sites for a large number of known transcription factor families, including several that have been implicated previously in sex determination and early gonadal development.

CONCLUSION

Our results reveal potentially important SRY regulatory elements, mutations in which might underlie cases of idiopathic human XY sex reversal.

Sry and the hesitant beginnings of male development

In mammals, Sry (sex-determining region Y gene) is the master regulator of male sex determination. The discovery of Sry in 1990 was expected to provide the key to unravelling the network of gene regulation underlying testis development. Intriguingly, no target gene of SRY protein has yet been discovered, and the mechanisms by which it mediates its developmental functions are still elusive. What is clear is that instead of the robust gene one might expect as the pillar of male sexual development, Sry function hangs by a thin thread, a situation that has profound biological, medical and evolutionary implications.

Human and pigSRY 5′ flanking sequences can direct reporter transgene expression to the genital ridge and to migrating neural crest cells

Mechanisms for sex determination vary greatly between animal groups, and include chromosome dosage and haploid-diploid mechanisms as seen in insects, temperature and environmental cues as seen in fish and reptiles, and gene-based mechanisms as seen in birds and mammals. In eutherian mammals, sex determination is genetic, and SRY is the Y chromosome located gene representing the dominant testes determining factor. How SRY took over this function from ancestral mechanisms is not known, nor is it known what those ancestral mechanisms were. What is known is that SRY is haploid and thus poorly protected from mutations, and consequently is poorly conserved between mammalian species. To functionally compare SRY promoter sequences, we have generated transgenic mice with fluorescent reporter genes under the control of various lengths of human and pig SRY 5' flanking sequences. Human SRY 5' flanking sequences (5 Kb) supported reporter transgene expression within the genital ridge of male embryos at the time of sex determination and also supported expression within migrating truncal neural crest cells of both male and female embryos. The 4.6 Kb of pig SRY 5' flanking sequences supported reporter transgene expression within the male genital ridge but not within the neural crest; however, 2.6 Kb and 1.6 Kb of pig SRY 5' flanking sequences retained male genital ridge expression and now supported extensive expression within cells of the neural crest in embryos of both sexes. When 2 Kb of mouse SRY 5' flanking sequences (-3 to -1 Kb) were placed in front of the 1.6 Kb of pig SRY 5' flanking sequences and this transgene was introduced into mice, reporter transgene expression within the male genital ridge was retained but neural crest expression was lost. These observations suggest that SRY 5' flanking sequences from at least two mammalian species contain elements that can support transgene expression within cells of the migrating neural crest and that additional SRY 5' flanking sequences can extinguish this expression.

A naturally occurring deletion in the SRY promoter region affecting the Sp1 binding site is associated with sex reversal

Male to female sex reversal results from failure of testis development. Mutations in the SRY gene or in other genes involved in the sexual differentiation pathway are considered to cause XY gonadal dysgenesis. The majority of the mutations in the SRY described so far are located within the SRY coding region, mainly in the HMG-box conserved domain. Comparison of 5' flanking SRY gene sequences among different species indicated the presence of several putative conserved consensus sequences for different transcription regulators. In this study, we investigated a 360 bp sequence encompassing the SRY putative core promoter, in 17 patients with variable degrees of 46,XY sex reversal, which have been previously shown not to bear mutations in the SRYcoding region. Sequencing analysis of the SRYpromoter in one patient with complete XY gonadal dysgenesis revealed a three base pair deletion in one of the Sp1 binding sites. The deletion abolished Sp1 binding in vitro. This is the first report on a naturally occurring mutation affecting the Sp1 regulatory element in the SRY promoter region, which is associated with sex reversal. Additionally, upon familial investigation the father, who had 18 genital surgeries due to severe hypospadia without cryptorchidism, was found to bear the same deletion and several relatives were referred to have sexual ambiguity.

The molecular action and regulation of the testis-determining factors, SRY (sex-determining region on the Y chromosome) and SOX9 [SRY-related high-mobility group (HMG) box 9]

Despite 12 yr since the discovery of SRY, little is known at the molecular level about how SRY and the SRY-related protein, SOX9 [SRY-related high-mobility group (HMG) box 9], initiate the program of gene expression required to commit the bipotential embryonic gonad to develop into a testis rather than an ovary. Analysis of SRY and SOX9 clinical mutant proteins and XX mice transgenic for testis-determining genes have provided some insight into their normal functions. SRY and SOX9 contain an HMG domain, a DNA-binding motif. The HMG domain plays a central role, being highly conserved between species and the site of nearly all missense mutations causing XY gonadal dysgenesis. SRY and SOX9 are architectural transcription factors; their HMG domain is capable of directing nuclear import and DNA bending. Whether SRY and SOX9 activate testis-forming genes, repress ovary-forming genes, or both remains speculative until downstream DNA target genes are identified. However, factors that control SRY and SOX9 gene expression have been identified, as have a dozen sex-determining genes, allowing some of the pieces in this molecular genetic puzzle to be connected. Many genes, however, remain unidentified, because in the majority of cases of XY females and in all cases of XX males lacking SRY, the mutated gene is unknown.

Porcine SRY promoter is a target for steroidogenic factor 1

To study the process of mammalian sex determination and in particular to further understand the mechanisms of transcriptional regulation of the SRY gene, we have isolated a 4.5-kilobase (kb) pig SRY 5' flanking sequence. To facilitate the in vitro analysis of these sequences, we have generated a porcine genital ridge (PGR) cell line (9E11) that expresses SRY as well as SOX9, steroidogenic factor-1 (SF-1), and DAX1. Via primer extension analysis on RNA from this cell line, a transcription start site for porcine SRY was identified at -661 base pairs (bps) 5' from the translation initiation site. Deletion studies of the SRY 5' flanking sequences in PGR 9E11 cells demonstrated that -1.4 kb of 5' flanking sequences retained full transcriptional activity compared with the -4.5 kb fragment, but that transcriptional activity fell when further deletions were made. Sequences downstream of the transcriptional start site are important for promoter activity, because deleting transcribed but not translated sequences eliminated promoter activity. Sequence analysis of the -1.4 kb fragment identified two potential binding sites for SF-1, at -1369 and at -290 from the ATG. To address the role of SF-1 transactivation in SRY promoter activity, mutagenesis studies of the potential SF-1 binding sites were performed and revealed that these sites were indeed important for SRY promoter activity. Cotransfection studies in a heterologous cell system (mouse CV-1 cells) demonstrated that pig SF-1 was able to transactivate the pig SRY promoter. Gel shift assays confirmed that the upstream site was recognized by mouse SF-1 protein. We conclude that two sites for SF-1 transactivation exist within the pig SRY promoter, at -1369 bp and at -290 bp, and that the site at -1369 bp is quantitatively the most important.

In vitro Cre/loxP system in cells from developing gonads: investigation of the Sry promoter

There have been few studies on the regulatory elements of the Sry gene, mainly because no Sry-expressing cell lines have yet been established. This paper describes a useful tool for investigating the regulation and upstream region of Sry by means of the in vitro Cre/loxP system. Using plasmids containing the 9.9 kb mouse genomic Sry previously shown to induce testis development in XX transgenic mice, we constructed a Sry/Cre fusion gene plasmid in which Cre expression is controlled by the 5' and 3' untranslated regions of mouse Sry. To distinguish between male and female gonads of 11.5 days post-coitus (d.p.c.) fetuses, double transgenic fetuses carrying both the CAG (cytomegalovirus enhancer and beta-actin promoter)/loxP/lacZ transgene on the autosome and the green fluorescent protein transgene ubiquitously expressed on the Y chromosome were produced by crossing between two transgenic mouse lines. When Sry/Cre plasmids were transfected into the cells that had been prepared from the gonads, brains and livers of double transgenic fetuses, only a small number of X-gal-stained cells were detected among the primary cultured cells from male and female gonads, and none were detected among the cells from the other tissues. The X-gal-positive cells were negative for alkaline phosphatase, indicating that these cells were somatic cells expressing Sry. The Sry/Cre plasmids with a 0.4 kb upstream region of Sry yielded a large number of X-gal-positive cells in the cells from gonads, including various tissues of 11.5 d.p.c. fetuses, indicating the loss of the tissue-specific expression of Sry. The Sry/Cre with a 1.4 kb upstream region maintained tissue-specific activity of Sry. The results indicate that the present in vitro Cre/loxP system using transgenic mice is a simple and useful system for investigating the regulatory element of sex determination-related genes, including Sry.

The porcine SRY promoter is transactivated within a male genital ridge environment

In mammals the SRY gene functions as a dominant genetic switch for testis determination (Gubbay et al.: Nature 346:1128-1135, 1990; Koopman et al.: Nature 351:117-121, 1991; Sinclair et al.: Nature 346:240-244, 1990). To study SRY transcriptional regulation within an evolutionary context, we have generated transgenic mice that express green fluorescent protein (GFP) under the control of 4.5 kb of pig SRY 5' flanking sequences (pSRYp-GFP). Autofluorescence was observed in the genital ridges of e11.5 male embryos (18-21 tail somites), and by e12.5 (27 tail somites) autofluorescence was observed within the testes cords. The expression of the transgene did not display the abrupt termination characteristic of endogenous mouse SRY, but rather showed a gradual reduction in expression characteristic of human, pig and sheep SRY. Surprisingly, no autofluorescence was observed in normal XX genital ridges, although more sensitive RT-PCR analysis detected transgene transcription. When the transgene was bred into a constitutively male line of mice (Odsex; Bishop et al.: Nat Genet 26:490-494, 2000), autofluorescence was visible in genital ridges of XX animals, in the genetic absence of Sry protein. Via RT-PCR analysis, purified autofluorescent cells from e12.5 gonadal ridges expressed mouse SRY but not Oct4 transcripts, whereas autofluorescent cells from e14.5 gonadal ridges expressed MIS but not Oct4 transcripts, in each case consistent with a pre-Sertoli cell phenotype. In vitro expression studies performed in CV-1 cells demonstrated that pig SOX9 cDNA transactivated the pig SRY promoter but that pig SRY cDNA did not. When a SOX9 potential binding site identified at -205 of the pig SRY 5' flanking sequences was mutated, the SOX9 transactivation effect was reduced by 70%. This site is conserved in the 5' flanking sequences of bovine and human SRY genes but not in the mouse gene. Gel retardation assays using this binding site showed specific binding to SOX9-enriched nuclear extracts that was competed by excess unlabelled binding site but not by mutated binding site. We suggest that pig SRY gene is responsive to a testicular environment and propose a model of feedback amplification of pig SRY transcription by SOX9.

Equilibrium binding assays reveal the elevated stoichiometry and salt dependence of the interaction between full-length human sex-determining region on the Y chromosome (SRY) and DNA

In an effort to better define the molecular mechanism of the functional specificity of human sex-determining region on the Y chromosome (SRY), we have carried out equilibrium binding assays to study the interaction of the full-length bacterial-expressed protein with a DNA response element derived from the CD3epsilon gene enhancer. These assays are based on the observation of the fluorescence anisotropy of a fluorescein moiety covalently bound to the target oligonucleotide. The low anisotropy value due to the fast tumbling of the free oligonucleotide in solution increases substantially upon binding the protein to the labeled target DNA. Our results indicate that the full-length human wild-type SRY (SRY(WT)) forms a complex of high stoichiometry with its target DNA. Moreover, we have demonstrated a strong salt dependence of both the affinity and specificity of the interaction. We have also addressed the DNA bending properties of full-length human SRY(WT) in solution by fluorescence resonance energy transfer and revealed that maximal bending is achieved with a protein to DNA ratio significantly higher than the classical 1:1. Oligomerization thus appears, at least in vitro, to be tightly coupled to SRY-DNA interactions. Alteration of protein-protein interactions observed for the mutant protein SRY(Y129N), identified in a patient presenting with 46,XY sex reversal, suggests that oligomerization may play an important role in vivo as well.

Steroidogenic factor-1 contributes to the cyclic-adenosine monophosphate down-regulation of human SRY gene expression

In mammals, male sex determination is initiated by SRY (sex-determining region of the Y chromosome) gene expression and followed by testicular development. This study describes specific down-regulation of the human SRY gene transcription by cAMP stimulation using reverse transcription-polymerase chain reaction experiments. Using transfection experiments, conserved nuclear hormone receptor (NHR1) and Sp1 consensus binding sites were identified as essential for this cAMP transcriptional response. Steroidogenic factor-1 (SF-1), a component of the sex-determination cascade, binds specifically to the NHR1 site and activates the SRY promoter. Activation of SF-1 was abolished by cAMP pretreatment of the cells, suggesting a possible effect of cAMP on the SF-1 protein itself. Indeed, human SF-1 protein contains at least two in vitro cAMP-dependent protein kinase (PKA) phosphorylation sites, leading after phosphorylation to a modification of both DNA-binding activity and interaction with general transcription factors such as Sp1. Taken together, these data suggest that cAMP responsiveness of human SRY promoter involves both SF-1 and Sp1 sites and could act via PKA phosphorylation of the SF-1 protein itself.

Mammalian sex reversal and intersexuality: deciphering the sex-determination cascade

The sex-determination cascade constitutes a model of the exquisite mechanisms of gene regulation that lead to the development of mammalian embryos. The discovery of the sex-determining region of the Y chromosome (SRY) in the early 1990s was the first crucial step towards a general understanding of sex determination. Since then, several genes that encode proteins with a role in this cascade, such as WT1, SF-1, SOX9, DAX-1 and WNT4, have been identified. Many of the interactions between these proteins have still to be elucidated, while, no-doubt, others are still to be identified. The study of mammalian intersexes forms a promising way towards the identification of the still-missing genes and a comprehensive view of mammalian sex determination. Intersexuality in the goat, studied for over a century, will, presumably, bring to light new genes involved in the female sex-determination pathway.