Cell Cycle of the BRCA1 Gene Product

This proposal is aimed at understanding the function of BRCA1 through analysis of its cell cycle behavior. We found that BRCA1 colocalizes in S phaseG2 foci with hRad51, a protein with key roles in homologous recombination. The proteins also colocalized in primary human spermatocytes, suggesting a role for BRCA1 in meiotic recombination. An endogenous BRCA1-Rad51 complex was detected in somatic cells. We found that BRCA1 underwent specific phosphorylation following DNA damage. in response to some DNA damaging agents, BRCA1 and Rad51 were recruited to replication areas of the S phase nucleus, suggesting an interaction with damaged, replicating DNA. A second major hereditary breast cancer gene product, BRCA2, can also interact with Rad51. We have developed reagents for working with hBRCA2, and identified a specific biochemical interaction between BRCA1 and BRCA2. BRCA2 colocalized with BRCA1 in nuclear foci during SG2 phases of the somatic cell cycle, both before and after DNA damage, and in meiotic cells. This suggests that BRCA1 and BRCA2 operate, at least in part, on a common pathway involved in homologous recombination. Further, the results indicate that defects in the proper regulation of homologous recombination may contribute to the etiology of early-onset breast cancer.