Basal repression of BRCA1 by multiple E2Fs and pocket proteins at adjacent E2F sites

The BRCA1 gene is rarely mutated in sporadic tumors, although numerous studies suggest that the expression of this critical tumor suppressor gene is frequently downregulated in tumors derived from a wide range of tissues. However, little is known regarding the mechanisms by which BRCA1 is transcriptionally regulated in human cells in vivo. We have shown recently that the tumor microenvironmental stress of hypoxia induces an E2F-dependent repression of BRCA1 in cancer cells. Here, we report that multiple E2Fs and associated factors bind the BRCA1 promoter at two adjacent E2F sites in the proximal promoter region of the gene, as detected by quantitative chromatin immunoprecipitation (ChIP) analysis. Intriguingly, our data suggest that E2F1 and E2F4 bind the BRCA1 proximal promoter simultaneously at the two E2F sites, although E2F4 appears to be the predominant E2F bound in log-phase cells. In contrast to previous studies, we have also detected promoter occupancy by the pocket proteins p130 and p107, but not Rb, our data indicate that E2F4 and p130/p107 complexes basally repress the BRCA1 promoter. We find that binding by these factors requires both intact E2F sites, which suggests a novel interaction between adjacent promoter elements. Taken together, these findings provide an enhanced molecular portrait of BRCA1 transcriptional regulation by E2Fs and pocket proteins, they enhance our understanding of regulation by repressive E2F complexes at target genes in vivo. Furthermore, the identification of E2F4 and p130/ p107 complexes as basal repressors of BRCA1 expression may facilitate the development of strategies based on disruption of these interactions to rescue BRCA1 expression in human tumors.