Evidence for a broader role of the tumor suppressor BRCA1 at specific genomic sites during their replication
BRCA1 mutations are involved in a substantial portion of familial breast and ovarian cancers. Following induced DNA damage, BRCA1 foci form at sites of DNA repair, supporting a role for BRCA1 in DNA repair. In contrast, it is not known why and where BRCA1 foci form in non-irradiated S-phase nuclei. While speculated to be storage sites, these accumulations of BRCA1 may reflect an unrecognized function of BRCA1 at specific genomic sites. It was reported (Cell 2002 Vol. 111(3):393-405) that BRCA1 co-localizes with XIST RNA and supports its localization across the inactive X (Xi) chromosome. However, we investigated several key findings of this study and were unable to confirm that BRCA1 has a role in localization of XIST RNA. Although BRCA1 foci abut Xi in a subfraction of cells, we did not find substantial colocalization of BRCA1 with XIST RNA. This incomplete association led us to question whether BRCA1 foci interact with some part of chromosomes more generally. We began by investigating the relationship of BRCA1 to late-replicating DNA using BrdU. Although we confirm earlier studies that the BRCA1 and BrdU patterns are largely not coincident, closer scrutiny indicated a significant though incomplete relationship between BRCA1 and some sites of mid to late S-phase DNA replication, including the Xi. Moreover, we demonstrate that BRCA1 shows a preferential relationship with heterochromatic versus euchromatic nuclear compartments, with the vast majority of BRCA1 foci in heterochromatic sites. Further analyses revealed a structural link between many BRCA1 foci and the interphase centromere/kinetochore in human and mouse cells. In mouse nuclei, BRCA1 showed a striking relationship to pericentric heterochromatin (PCH) and associated centromeric DNA, especially at or near the time of replication. Finally, BRCA1 associates with centromeric regions of both the active and inactive human X chromosomes, which may account for the previous finding that BRCA1 associates with Xi. The finding that BRCA1 foci form at or near centric or PCH, near the time of their replication, suggests a functional role of BRCA1 with structures that are key to mitotic chromosome segregation and maintenance of genomic stability.