BRCA1 foci in normal S-phase nuclei are linked to interphase centromeres and replication of pericentric heterochromatin.
Breast cancer-associated protein 1 (BRCA1) forms foci at sites of induced DNA damage, but any significance of these normal S-phase foci is unknown. BRCA1 distribution does not simply mirror or overlap that of replicating DNA; however, BRCA1 foci frequently abut sites of BrdU incorporation, mostly at mid-to-late S phase. Although BRCA1 does not overlap XIST RNA across the inactive X chromosome, BRCA1 foci position overwhelmingly in heterochromatic regions, particularly the nucleolar periphery where many centromeres reside. In humans and mice, including early embryonic cells, BRCA1 commonly associates with interphase centromere-kinetochore complexes, including pericentric heterochromatin. Proliferating cell nuclear antigen or BrdU labeling demonstrates that BRCA1 localizes adjacent to, or "paints," major satellite blocks as chromocenters replicate, where topoisomerase is also enriched. BRCA1 loss is often associated with proliferative defects, including postmitotic bridges enriched with satellite DNA. These findings implicate BRCA1 in replication-linked maintenance of centric/pericentric heterochromatin and suggest a novel means whereby BRCA1 loss may contribute to genomic instability and cancer.