A search of a 35-kilobase region of the human Y chromosome necessary for male sex determination has resulted in the identification of a new gene. This gene is conserved and Y-specific among a wide range of mammals, and encodes a testis-specific transcript. It shares homology with the mating-type protein, Mc, from the fission yeast Schizosaccharomyces pombe and a conserved DNA-binding motif present in the nuclear high-mobility-group proteins HMG1 and HMG2. This gene has been termed SRY (for sex-determining region Y) and proposed to be a candidate for the elusive testis-determining gene, TDF.

A gene from the human sex-determining region encodes a protein with homology to a conserved DNA-binding motif

Each cell lineage specified in the preimplantation mammalian embryo depends on intrinsic factors for its development, but there is also mutual interdependence between them. OCT4 is required for the ICM/epiblast lineage, and at transient high levels for extraembryonic endoderm, but also indirectly through its role in regulating Fgf4 expression, for the establishment and proliferation of extraembryonic ectoderm from polar trophectoderm. The transcription factor SOX2 has also been implicated in the regulation of Fgf4 expression. We have used gene targeting to inactivate Sox2, examining the phenotypic consequences in mutant embryos and in chimeras in which the epiblast is rescued with wild-type ES cells. We find a cell-autonomous requirement for the gene in both epiblast and extraembryonic ectoderm, the multipotent precursors of all embryonic and trophoblast cell types, respectively. However, an earlier role within the ICM may be masked by the persistence of maternal protein, whereas the lack of SOX2 only becomes critical in the chorion after 7.5 days postcoitum. Our data suggest that maternal components could be involved in establishing early cell fate decisions and that a combinatorial code, requiring SOX2 and OCT4, specifies the first three lineages present at implantation.

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Multipotent cell lineages in early mouse development depend on SOX2 function

Rapid improvements in sequencing and array-based platforms are resulting in a flood of diverse genome-wide data, including data from exome and whole-genome sequencing, epigenetic surveys, expression profiling of coding and noncoding RNAs, single nucleotide polymorphism (SNP) and copy number profiling, and functional assays. Analysis of these large, diverse data sets holds the promise of a more comprehensive understanding of the genome and its relation to human disease. Experienced and knowledgeable human review is an essential component of this process, complementing computational approaches. This calls for efficient and intuitive visualization tools able to scale to very large data sets and to flexibly integrate multiple data types, including clinical data. However, the sheer volume and scope of data pose a significant challenge to the development of such tools.

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The initiation of male development in mammals requires one or more genes on the Y chromosome. A recently isolated gene, termed SRY in humans and Sry in mouse, has many of the genetic and biological properties expected of a Y-located testis-determining gene. It is now shown that Sry on a 14-kilobase genomic DNA fragment is sufficient to induce testis differentiation and subsequent male development when introduced into chromosomally female mouse embryos.

Male development of chromosomally female mice transgenic for Sry

A gene mapping to the sex-determining region of the mouse Y chromosome is deleted in a line of XY female mice mutant for Tdy, and is expressed at a stage during male gonadal development consistent with its having a role in testis determination. This gene is a member of a new family of at least five mouse genes, related by an amino-acid motif showing homology to other known or putative DNA-binding domains.

A gene mapping to the sex-determining region of the mouse Y chromosome is a member of a novel family of embryonically expressed genes

The Sox gene family consists of a large number of embryonically expressed genes related via the possession of a 79-amino-acid DNA-binding domain known as the HMG box Partial clones for the first three Sox genes (al-a3) were isolated by homology to the HMG box of the testis-determining gene Sry and are now termed Sox-1, Sox-2 and Sox-3, Sox-3 is highly conserved amongst mammalian species and is located on the X chromosome This has led to the proposal that Sry evolved from Sox-3 We present the cloning and sequencing of Sox-1, Sox-2 and Sox-3 from the mouse and show that Sox-3 is most closely relate to Sry We also confirm that mouse Sox-3 is located on the X chromosome between Hprt and Dmd Analysis of the distribution of Sox-3 RNA shows that its main site of expression is in the developing central nervous system, suggesting a role for Sox-3 in neural development Moreover, we demonstrate that Sox-3, as well as Sox-1 and Sox-2, are expressed in the urogenital ridge and that their protein products are able to bind the same DNA sequence motif as Sry in vitro, but with different affinities These observations prompt discussion of an evolutionary link between the genes and support the model that Sry has evolved from Sox-3 However our findings imply that if this is true, then Sry has undergone concomitant changes resulting in loss of CNS expression and altered DNA-binding properties

A comparison of the properties of Sox-3 with Sry and two related genes, Sox-1 and Sox-2

Activation of the first lens-specific gene of the chicken, delta 1-crystallin, is dependent on a group of lens nuclear factors, deltaEF2, interacting with the delta1-crystallin minimal enhancer, DC5. One of the deltaEF2 factors was previously identified as SOX2. We show that two related SOX proteins, SOX1 and SOX3, account for the remaining members of deltaEF2. Activation of the DC5 enhancer is dependent on their C-terminal domains. Expression of Sox1-3 in the eye region during lens induction was studied in comparison with Pax6 and delta1-crystallin. Pax6, known to be required for the inductive response of the ectoderm, is broadly expressed in the lateral head ectoderm from before lens induction. After tight association of the optic vesicle (around stage 10-11, 40 hours after egg incubation), expression of Sox2 and Sox3 is activated in the vesicle-facing ectoderm at stage 12 (44 hours). These cells, expressing together Pax6 and Sox2/3, subsequently give rise to the lens, beginning with formation of the lens placode and expression of delta-crystallin at stage 13 (48 hours). Sox1 then starts to be expessed in the lens-forming cells at stage 14. When the prospective retina area of the neural plate was unilaterally ablated at stage 7, expression of Sox2/3 was lost in the side of lateral head ectoderm lacking the optic cup, implying that an inductive signal from the optic cup activates Sox2/3 expression. In the mouse embryonic lens, this subfamily of Sox genes is expressed in an analogous fashion, although Sox3 transcripts have not been detected and Sox2 expression is down-regulated when Sox1 is activated. In ectodermal tissues of the chicken embryo, delta -crystallin expression occurs in a few ectopic sites. These are always characterized by overlapping expression of Sox2/3 and Pax6. Thus, an essential molecular event in lens induction is the 'turning on' of the transcriptional regulators SOX2/3 in the Pax6-expressing ectoderm and these SOX proteins activate crystallin gene expression. Continued activity, especially of SOX1, is then essential for further development of the lens.

Involvement of Sox1, 2 and 3 in the early and subsequent molecular events of lens induction.

The mammalian genome contains a family of genes that are related to SRY, the mammalian sex determining gene. The homology is restricted to the region of SRY that encodes a DNA binding motif of the HMG-box class. These genes have been named SOX genes (SRY-related HMG-box genes). We have cloned and characterised SOX3, a member of the human SOX gene family. SOX3 maps to the X chromosome in the region Xq26-27. A mentally retarded male patient with haemophilia B is deleted for both the Factor IX gene and SOX3. This suggests that SOX3 is not essential for testis formation. The phenotype of the patient and the expression of SOX3 gene in neuronal tissues raises the possibility that this gene is a candidate gene for Borjeson-Forssman-Lehmann, an X-linked mental retardation syndrome.

SOX3 is an X-linked gene related to SRY.

The Y chromosome determines maleness in mammals. A Y chromosome-linked gene diverts the indifferent embryonic gonad from the default ovarian pathway in favour of testis differentiation, initiating male development. Study of this basic developmental switch requires the isolation of the testis-determining gene, termed TDF in humans and Tdy in mice. ZFY, a candidate gene for TDF, potentially encodes a zinc-finger protein, and has two Y-linked homologues, Zfy-1 and Zfy-2, in mice. Although ZFY, Zfy-1 and Zfy-2 seem to map to the sex-determining regions of the human and mouse Y chromosomes, there is no direct evidence that these genes are involved in testis determination. We report here that Zfy-1 but not Zfy-2 is expressed in differentiating embryonic mouse testes. Neither gene, however, is expressed in We/We mutant embryonic testes which lack germ cells. These observations exclude both Zfy-1 and Zfy-2 as candidates for the mouse testis-determining gene.

Jérôme Collignon

Papers

A gene mapping to the sex-determining region of the mouse Y chromosome is a member of a novel family of embryonically expressed genes

A comparison of the properties of Sox-3 with Sry and two related genes, Sox-1 and Sox-2

Involvement of Sox1, 2 and 3 in the early and subsequent molecular events of lens induction.

SOX3 is an X-linked gene related to SRY.

Zfy gene expression patterns are not compatible with a primary role in mouse sex determination.