DYNAMIC CHANGES OF BRCA1 SUBNUCLEAR LOCATION AND PHOSPHORYLATION STATE ARE INITIATED BY DNA-DAMAGE

BRCA1 localizes to discrete nuclear foci (dots) during S phase. Hydrox yurea-mediated DNA synthesis arrest of S phase MCF7 cells led to a los s of BRCA1 from these structures. Ultraviolet light, mitomycin C, or g amma irradiation produced a similar effect but with no concurrent arre st of DNA synthesis. BARD1 and Rad51, two proteins associated with the BRCA1 dots, behaved similarly. Loss of the BRCA1 foci was accompanied by a specific, dose-dependent change(s) in the state of BRCA1 phospho rylation. Three distinct DNA damaging agents preferentially induced th is change in S phase. The S phase BRCA1 phosphorylation response to DN A damage occurred in cells lacking, respectively, two DNA damage-sensi ng protein kinases, DNA-PK and Atm, implying that neither plays a prim e role in this process. Finally, after BRCA1 dot dispersal, BRCA1, BAR D1, and Rad51 accumulated, focally, on PCNA(+) replication structures, implying an interaction of BRCA1/BARD1/Rad51 containing complexes wit h damaged, replicating DNA. Taken together, the data imply that the BR CA1 S phase foci are dynamic physiological elements, responsive to DNA damage, and that BRCA1-containing multiprotein complexes participate in a replication checkpoint response.