BORIS is a cancer/testis (CT) gene with the properties of a candidate oncogene and/or a cell-immortalizing gene. It maps to a cancer-associated hot spot on human chromosome 20q13 that shows frequent gains in copy number and/or amplification in a wide variety of human tumors. Similar to other CT, BORIS is aberrantly expressed in the majority of primary tumors and related cancer cell lines. As a CTCF- paralog, BORIS shares with CTCF a nearly identical 11 Zn-finger (11ZF) DNA binding domain (DBD), but their flanking NH2- and COOH-terminal regions are divergent. The 11ZF region was previously identified in the lab as a multivalent DBD, which is able to recognize and bind extended (around 50bp) target sequences. By virtue of sharing the identical DBD, CTCF and BORIS can recognize the same DNA sequences, but likely have distinct properties and form different associations with protein co-factors. Furthermore, due to the tissue-specific expression of BORIS in male germ cells, it is likely involved in re-establishment of paternal-specific DNA methylation patterns at particular imprinted sites of the Igf2/H19 locus through specific loop formation, by utilizing novel CTCF/BORIS sites. Based on our studies we predicted that most ICR sequences would contain meCpG-sensitive CTCF/BORIS target sites, which was validated for several unrelated imprinted loci. In addition to its role in development, BORIS likely plays a key role in oncogenesis. We and others characterized BORIS expression in uterine cancers, breast cancers, osteosarcomas, lung cancers, and prostate cancers. However, as BORIS is itself a gene expression regulator, it was hypothesized that BORIS-mediated regulation of promoters is the regulatory network responsible for the expression of multiple CT genes. Using the Boris KO model, we demonstrated that BORIS directly regulates the testis-specific protease gene TSP50, which is in turn negatively regulated by p53. We found that aberrant expression of both BORIS and TSP50 genes in cancers often coincide, suggesting the role of BORIS in activation of CT genes as downstream targets. In another line of experiments, we discovered that DNA methylation plays dual role in the regulation of human telomerase gene, hTERT, one of the key cell immortalization factors. Methylation prevents binding of CTCF, which has repressor activity, but partial hypomethylation of the core promoter is necessary for hTERT expression. In lymphoid cells, however, telomerase appears to be activated through a methylation-independent mechanism. In our follow-up work we found that in B cells, some T cell lymphomas, and in non-neoplastic lymph nodes, the hTERT promoter is unmethylated. The B cell-specific transcription factor PAX5 can override the repressive function of CTCF and activate hTERT in telomerase-positive B cells by a methylation-independent mechanism. The sum of recent studies suggests that that methylation per se is not the chief mechanism inhibiting CTCF binding at hTERT. We tested a hypothesis that abnormal activation of BORIS in cancer cells prevents CTCF binding to some key sites, including the hTERT promoter. Using human cancer cell lines where abnormal expression of BORIS was documented, as well as cells with transient expression of BORIS-coding vectors, we showed that BORIS binds the hTERT gene within the first exon and facilitates its transcription. Downregulation of BORIS led to a decrease of hTERT transcription in transient transfection experiments. However, in testicular and ovarian cell lines BORIS downregulation did not affect endogenous hTERT transcription. Thus, BORIS may play the role of CTCF antagonist, enabling the expression of hTERT in cancer and immortalized cells, but it is not a simple binary system.
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