Expression of the CTCF-paralogous cancer-testis gene, brother of the regulator of imprinted sites (BORIS), is regulated by three alternative promoters modulated by CpG methylation and by CTCF and p53 transcription factors

Abstract

BORIS, like other members of the ‘cancer/testis antigen’ family, is normally expressed in testicular germ cells and repressed in somatic cells, but is aberrantly activated in cancers. To understand regulatory mechanisms governing human BORIS expression, we characterized its 5′-flanking region. Using 5′ RACE, we identified three promoters, designated A, B and C, corresponding to transcription start sites at −1447, −899 and −658 bp upstream of the first ATG. Alternative promoter usage generated at least five alternatively spliced BORIS mRNAs with different half-lives determined by varying 5′-UTRs. In normal testis, BORIS is transcribed from all three promoters, but 84% of the 30 cancer cell lines tested used only promoter(s) A and/or C while the others utilized primarily promoters B and C. The differences in promoter usage between normal and cancer cells suggested that they were subject to differential regulation. We found that DNA methylation and functional p53 contributes to the negative regulation of each promoter. Moreover, reduction of CTCF in normally BORIS-negative human fibroblasts resulted in derepression of BORIS promoters. These results provide a mechanistic basis for understanding cancer-related associations between haploinsufficiency of CTCF and BORIS derepression, and between the lack of functional p53 and aberrant activation of BORIS.

BORIS, a paralogue of the transcription factor, CTCF, is aberrantly expressed in breast tumours

“…The availability of the component(s) in the growth media that may be important for BORIS activation could be a factor; access to such component(s) may be restricted in the microenvironment of some tumours. Given the complex nature of BORIS regulation at the transcriptional level it may be possible that different promoters are involved in the control of BORIS expression in cultured cells and tumours; thus, providing different molecular mechanisms of BORIS activation in vivo and in vitro (Renaud et al, 2007) (see below).…”

“…Recent reports reveal that both genetic and epigenetic mechanisms are likely to be implicated in this process. Thus, DNA methylation, functional p53 and CTCF play an important role in the negative regulation of the promoters of the BORIS gene (Renaud et al, 2007). Demethylation of DNA, knockout of CTCF and absence of functional p53 can lead to strong activation of BORIS.…”

“…Demethylation of DNA, knockout of CTCF and absence of functional p53 can lead to strong activation of BORIS. Selective utilisation of different promoters (Renaud et al, 2007) and alternative splicing (Pugacheva et al, manuscript in preparation) are also likely to contribute to regulation of BORIS expression in different cell types. The research efforts of several laboratories are currently focused on uncovering the details of the molecular mechanisms of activation and regulation of BORIS.…”

DNA damage-induced upregulation of miR-709 in the germline downregulates BORIS to counteract aberrant DNA hypomethylation

“…BORIS is an important regulator of DNA methylation and imprinting, and it itself is regulated by DNA methylation. 9,11,12 Having found that significant radiation-induced changes in miR-709 levels were associated with the altered levels of BORIS and DNA methylation, we proceeded to deduce regulation of miR-709 expression per se. The location of a particular miRNA gene is an important factor in determining its expression pattern.…”

“…It is well-established that in the normal unexposed germline, genome-wide erasure of methylation overlaps extensively with BORIS upregulation, and remethylation of DNA is associated with BORIS silencing. 11,12 Furthermore, DNA hypomethylation, also with CTCF is a proven positive regulator of BORIS expression. 11,12 Interestingly, CTCF levels were unchanged after irradiation (data not shown).…”