Complex Regulation of the BRCA1 Gene

Abstract

We have analyzed the promoter region of the human BRCA1 gene in detail and demonstrate that the expression of the BRCA1 gene is under complex regulation. First, its transcription is under the control of two promoters generating two distinct transcripts ␣ and ␤, and second, promoter ␣ is shared with the adjacent NBR2 gene and is bi-directional. Both promoter ␣ and promoter ␤ are responsive to estrogen stimulation. We also discerned that there are striking differences in both the genomic organization and immediate cis-control elements of the BRCA1 gene between humans and mice.Since its isolation in 1994 (1), much effort has been devoted toward unraveling the biological function of the breast and ovarian cancer susceptibility gene, BRCA1. The fact that many germline mutations have been found has firmly established the involvement of the BRCA1 gene in familial breast and/or ovarian cancer (2). However, unlike the precedent of other tumor suppressor genes where mutant forms of the gene products are responsible for both the inherited and sporadic forms of the same type of cancer, no sporadic breast cancers and only a handful of sporadic ovarian cancers have been found to harbor BRCA1 mutations (3-5). It is thus postulated that either BRCA1 is involved only in the etiology of inherited breast cancer or that BRCA1 disruption in sporadic cancer occurs by mechanisms other than mutations in the coding region. A few lines of evidence are consistent with the latter hypothesis: in sporadic breast cancer the BRCA1 mRNA levels are decreased (6), while in familial breast cancer inferred regulatory mutations are present (2). Therefore, studies aimed at determining the regulation of the BRCA1 gene will be critical in helping to resolve this enigma as well as to understand the normal function of the gene product.We have previously established a comprehensive map of approximately 50-kilobase pair genomic DNA encompassing not only the 5Ј end of the BRCA1 gene but also the nearby NBR1 gene (previously named 1A1-3B) (7), which was isolated as a candidate for the ovarian cancer marker CA125 (8). The 5Ј ends of both genes are duplicated with a partial copy of the BRCA1 gene lying head to head with the NBR1 gene and a partial copy of the NBR1 gene lying head to head with the BRCA1 gene ( Fig. 1) (7). Further analysis of the expression of the partial copies of both genes showed that the partial copy of the NBR1 5Ј exons is in fact part of a new gene, NBR2 (9). The fact that the transcription start sites of the BRCA1 and NBR2 genes are only 218 bp 1 apart suggests that the intergenic region may function as a bi-directional promoter. In addition, we have previously identified two distinct BRCA1 transcripts differing by the alternative use of the first exons, predicting that the BRCA1 gene is regulated by two promoters (10). This study is therefore aimed at unravelling the regulation of the BRCA1 gene by functional analysis of the BRCA1 promoters. EXPERIMENTAL PROCEDURESConstruction of Plasmids-Plasmid p3ba containing the 5Ј region of both BR...

Down-regulation of BRCA1 in human sporadic breast cancer; analysis of DNA methylation patterns of the putative promoter region

“…However, although not statistically signi®cant, estrogen receptor rich (ER+) invasive breast carcinomas NOS exhibit a higher level of BRCA1 mRNA than that observed in the estrogen receptor negative samples (ER7). In line with this observation, in vitro studies indicate a potential relationship between estrogen receptors and the control of BRCA1 expression (Gudas et al, 1995;Xu et al, 1997).…”

MTA1-mediated transcriptional repression of BRCA1 tumor suppressor gene

“…Previously, MTA1 has been shown to inhibit the expression of estrogen-responsive genes by repressing the transactivating functions of ERa (Mazumdar et al, 2001). In addition, BRCA1 is itself an estrogenresponsive gene whose promoter contains an atypical ERE (Figure 2a; Xu et al, 1997). These observations point to the possibility that MTA1 might be recruited to (Figures 2b and c).…”

“…The notion that MTA1 might be a corepressor was merely speculative until the recent discovery that MTA1 associates with histone deacetylases (HDACs) and thereby helps to repress estrogen receptor-a (ERa) transactivation in breast cancer cells (Mazumdar et al, 2001;Talukder et al, 2003). Since the BRCA1 promoter contains an atypical estrogen-responsive element (ERE) (Xu et al, 1997), we hypothesized that MTA1 might play a role in BRCA1 repression. To test this, we addressed the mechanism for BRCA1 repression by MTA1 and further evaluated the mechanistic link, if any, between MTA1-mediated BRCA1 repression and centrosome number amplification in breast cancer cells.…”

Gene replacement with the human BRCA1 locus: tissue specific expression and rescue of embryonic lethality in mice

“…Interestingly, human BRCA1 transcripts were expressed in a 1 : 1 ratio with mouse Brca1 in male tissues (Figure 4c, lanes 6 ± 7), while the human transcript was always more elevated in female tissues; approximately 1.8 : 1 in mammary gland (Figure 4c, lane 2). While it was beyond the scope of this study to pursue the potential role of sex steroids in the regulation of BRCA1, it is worth noting that the human BRCA1 gene contains a putative estrogen response element (Xu et al, 1997) which appears to be absent from the mouse promoter. TgN´BRCA1-B transgenic animals could therefore play a role in the study of similarities and di erences in the regulation of mouse and human BRCA1 genes.…”