Abstract 2113: Elucidating mechanisms of regulation of homologous recombination utilizing a PALB2 fusion protein that contains the BRCT repeats of BRCA1

Abstract The products of the breast cancer susceptibility genes, BRCA1 and PALB2, directly interact through coiled-coil domains present in each protein. How PALB2 is recruited to sites of DNA damage and how the BRCA1-PALB2 interaction regulates the repair of DNA double-strand breaks (DSBs) by homologous recombination (HR) have remained unclear, however. Here, the BRCT repeats of BRCA1, which are required for its localization, were fused to a PALB2 mutant incapable of binding BRCA1. We introduced this fusion protein into HCC1937 breast cancer cells that contain a truncated BRCA1 or into cells in which full-length BRCA1 was depleted using a siRNA. The fusion protein corrected the deficiency for PALB2 foci in both systems, either with or without exposure to ionizing radiation. Importantly, expression of the fusion protein also corrected a defect in PALB2-dependent assembly of foci by the RAD51 recombinase. Further, we also introduced the fusion protein into PALB2/FANCN-deficient cells from a Fanconi anemia patient. The fusion protein restored both PALB2 and RAD51 foci in these cells without binding to BRCA1. Strikingly, the fusion protein also conferred resistance to mitomycin C (MMC) and corrected a defect in DSB-initiated HR in PALB2-deficient cells. Our data convincingly demonstrate that BRCA1 localizes PALB2 to sites of DNA damage, and that this is necessary for recruitment of RAD51 and for HR. Contrary to a published report, since the fusion protein is incapable of binding to PALB2, our results suggest that PALB2 functions, such as HR, are mediated by the BRCA1-PALB2 interaction rather than the PALB2-PALB2 dimer. Given that BRCA1 and PALB2 participate in a pathway of DNA repair by HR, our results suggest that mutations which disrupt the BRCA1-PALB2 interaction may lead to the genetic instability that underlies the development of cancer. Additionally, the BRCA1-PALB2 interaction may be a potential therapeutic target for sensitization of cancer cells to DNA damaging agents. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2113. doi:1538-7445.AM2012-2113