Interaction between DNA Polymerase β and BRCA1

The breast cancer 1 (BRCA1) protein is a tumor suppressor playing roles in DNA repair and cell cycle regulation. Studies of DNA repair functions of BRCA1 have focused on double-strand break (DSB) repair pathways and have recently included base excision repair (BER). However, the function of BRCA1 in BER is not well defined. Here, we examined a BRCA1 role in BER, first in relation to alkylating agent (MMS) treatment of cells and the BER enzyme DNA polymerase beta (pol beta). MMS treatment of BRCA1 negative human ovarian and chicken DT40 cells revealed hypersensitivity, and the combined gene deletion of BRCA1 and pol beta in DT40 cells was consistent with these factors acting in the same repair pathway, possibly BER. Using cell extracts and purified proteins, BRCA1 and pol beta were found to interact in immunoprecipitation assays, yet in vivo and in vitro assays for a BER role of BRCA1 were negative. An alternate approach with the human cells of immunofluorescence imaging and laser-induced DNA damage revealed negligible BRCA1 recruitment during the first 60 s after irradiation, the period typical of recruitment of pol beta and other BER factors. Instead, 15 min after irradiation, BRCA1 recruitment was strong and there was gamma-H2AX co-localization, consistent with DSBs and repair. The rapid recruitment of pol beta was similar in BRCA1 positive and negative cells. However, a fraction of pol beta initially recruited remained associated with damage sites much longer in BRCA1 positive than negative cells. Interestingly, pol beta expression was required for BRCA1 recruitment, suggesting a partnership between these repair factors in DSB repair.