Identification of a TSPY co-expression network associated with DNA hypomethylation and tumor gene expression in somatic cancers

Testis specific protein Y-encoded（TSPY） is a Y-located proto-oncogene predominantly expressed in normal male germ cells and various types of germ cell tumor. Significantly, TSPY is frequently expressed in somatic cancers including liver cancer but not in adjacent normal tissues, suggesting that ectopic TSPY expression could be associated with oncogenesis in non-germ cell cancers. Various studies demonstrated that TSPY expression promotes growth and proliferation in cancer cells; however, its relationship to other oncogenic events in TSPY-positive cancers remains unknown. The present study seeks to correlate TSPY expression with other molecular features in clinical cancer samples, by analyses of RNA-seq transcriptome and DNA methylation data in the Cancer Genome Atlas（TCGA） database. A total of 53 genes,including oncogenic lineage protein 28 homolog B（LIN28B） gene and RNA-binding motif protein Y-linked（RBMY） gene, are identified to be consistently co-expressed with TSPY, and have been collectively designated as the TSPY co-expression network（TCN）. TCN genes were simultaneously activated in subsets of liver hepatocellular carcinoma（30%） and lung adenocarcinoma（10%） regardless of pathological stage, but only minimally in other cancer types. Further analysis revealed that the DNA methylation level was globally lower in the TCN-active than TCN-silent cancers. The specific expression and methylation patterns of TCN genes suggest that they could be useful as biomarkers for the diagnosis,prognosis and clinical management of cancers, especially those for liver and lung cancers, associated with TSPY co-expression network genes.

Battle of the sexes: contrasting roles of testis-specific protein Y-encoded (TSPY) and TSPX in human oncogenesis

The Y-located testis-specific protein Y-encoded (TSPY) and its X-homologue TSPX originated from the same ancestral gene, but act as a proto-oncogene and a tumor suppressor gene, respectively. TSPY has specialized in male-specific functions, while TSPX has assumed the functions of the ancestral gene. Both TSPY and TSPX harbor a conserved SET/NAP domain, but are divergent at flanking structures. Specifically, TSPX contains a C-terminal acidic domain, absent in TSPY. They possess contrasting properties, in which TSPY and TSPX, respectively, accelerate and arrest cell proliferation, stimulate and inhibit cyclin B-CDK1 phosphorylation activities, have no effect and promote proteosomal degradation of the viral HBx oncoprotein, and exacerbate and repress androgen receptor (AR) and constitutively active AR variant, such as AR-V7, gene transactivation. The inhibitory domain has been mapped to the carboxyl acidic domain in TSPX, truncation of which results in an abbreviated TSPX exerting positive actions as TSPY. Transposition of the acidic domain to the C-terminus of TSPY results in an inhibitory protein as intact TSPX. Hence, genomic mutations/aberrant splicing events could generate TSPX proteins with truncated acidic domain and oncogenic properties as those for TSPY. Further, TSPY is upregulated by AR and AR-V7 in ligand-dependent and ligand-independent manners, respectively, suggesting the existence of a positive feedback loop between a Y-located proto-oncogene and male sex hormone/receptors, thereby amplifying the respective male oncogenic actions in human cancers and diseases. TSPX counteracts such positive feedback loop. Hence, TSPY and TSPX are homologues on the sex chromosomes that function at the two extremes of the human oncogenic spectrum.

Roles of the Y chromosome genes in human cancers

Male and female differ genetically by their respective sex chromosome composition, that is, XY as male and XX as female. Although both X and Y chromosomes evolved from the same ancestor pair of autosomes, the Y chromosome harbors male-specific genes, which play pivotal roles in male sex determination, germ cell differentiation, and masculinization of various tissues. Deletions or translocation of the sex-determining gene, SRY, from the Y chromosome causes disorders of sex development （previously termed as an intersex condition） with dysgenic gonads. Failure of gonadal development results not only in infertility, but also in increased risks of germ cell tumor （GCT）, such as gonadoblastoma and various types of testicular GCT. Recent studies demonstrate that either loss of Y chromosome or ectopic expression of Y chromosome genes is closely associated with various male-biased diseases, including selected somatic cancers. These observations suggest that the Y-linked genes are involved in male health and diseases in more frequently than expected. Although only a small number of protein-coding genes are present in the male-specific region of Y chromosome, the impacts of Y chromosome genes on human diseases are still largely unknown, due to lack of in vivo models and differences between the Y chromosomes of human and rodents. In this review, we highlight the involvement of selected Y chromosome genes in cancer development in men.

The human and mouse sex-determining SRY genes repress the Rspo1/β-catenin signaling

The sex-determining region Y （SRY） is the gene on the Y chromosome responsible for switching on male sex determination during mammalian embryogenesis. In its absence, ovaries develop in the embryo. Hence, ovarian determination and differentiation is considered to be a default, or passive, developmental pathway. Recently this classical paradigm of sex determination has been challenged with the discovery of the R-spondin 1 （RSPO1） as an active ovarian determinant. Mutations of RSPO1 cause a female-to-male sex reversal. RSPO1 synergizes with WNT4 in activating an ovarian development in the bipotential gonad via the canonical Wnt signaling. Early studies showed that SRY represses such Wnt signaling, but also generated discrepancies on whether only mouse Sry is capable of inhibiting such Wnt signaling and whether both human and mouse SRY proteins are able to interact with β-catenin, the intracellular messenger responsible for executing the Wnt signals. Our studies show that both human SRY and mouse Sry are capable of repressing the Rspol/Wnt/β-catenin signaling. However, the repression activities vary among different SRY/Sry proteins and paradoxically related to the presence and/or size of an acidic/glutamine-rich domain. The HMG box of human SRY could bind directly to β-catenin while the mouse Sry binds to β-catenin via its HMG box and glutamine-rich domain. The results clarify some of the initial discrepancies, and raise the possibility that SRY interacts with β-catenin in the nucleus and represses the transcriptional activation of the Rspol/Wnt target genes involved in ovarian determination, thereby switching on testis determination.

Expression of the human TSPY gene in the brains of transgenic mice suggests a potential role of this Y chromosome gene in neural functions

The testis specific protein Y-encoded （TSPY） is a member of TSPY/SET/NAP1 superfamily, encoded within the gonadoblastoma locus on the Y chromosome. TSPY shares a highly conserved SET/NAP-domain responsible for protein--protein interaction among TSPY/SET/NAP 1 proteins. Accumulating data, so far, support the role of TSPYas the gonadoblastoma gene, involved in germ cell tumorigenesis. The X-chromosome homolog of TSPY, TSPX is expressed in various tissues at both fetal and adult stages, including the brain, and is capable of interacting with the multi-domain adapter protein CASK, thereby influencing the synaptic and transcriptional functions and developmental regulation of CASK in the brain and other neural tissues. Similar to TSPX, we demonstrated that TSPY could interact with CASK at its SET/NAP-domain in cultured cells. Transgenic mice harboring a human TSPYgene and flanking sequences showed specific expression of the human TSPYtransgene in both testis and brain. The neural expression pattern of the human TSPYgene overlapped with those of the endogenous mouse Cask and Tspx gene. Similarly with TSPX, TSPY was co-localized with CASK in neuronal axon fibers in the brain, suggesting a potential role（s） of TSPY in development and/or physiology of the nervous system.