Abstract MIP-049: ATR–DEPENDENT REGULATION OF THE OVARIAN CANCER PROTEIN PALB2 AT DNA DOUBLE–STRAND BREAKS (2017)

Abstract Cancer is a constant threat to humans since one out of three individuals will develop cancer during their lifetime. It has become increasingly clear that tumor formation can be triggered by mutations in enzymes involved in the surveillance of genome integrity, such as BRCA1, BRCA2 and PALB2. BRCA1, BRCA2 and PALB2 are essential players in double–strand break repair by homologous recombination and have been associated with a heightened lifetime risk for ovarian cancer development. Cancer cells with BRCA1/2 and PALB2 deficiency are extremely sensitive to inhibitors of the DNA repair protein PARP, which have recently emerged as promising anti–cancer drugs. However, mutations in BRCA1/2 and PALB2 account only for around 15–20 % of ovarian cancers overall. Developing new strategies to specifically target ovarian cancer is still presenting a major challenge. During homologous recombination, PALB2 links BRCA1 and BRCA2 to promote RAD51 filament formation at DNA double–strand breaks repair. PALB2 interacts directly with BRCA1 via its N–terminal coiled–coil domain and with BRCA2 via its C–terminal WD40 domain. After DNA damage, PALB2–BRCA1 interaction is enhanced to promote PALB2, BRCA2 and RAD51 localization to DNA double–strand breaks. However, how DNA damage promotes the interaction between PALB2 and BRCA1 is still not understood. In this study, we found that the phosphatidylinositol 3–kinase–like proteins kinase ATR is essential to promote PALB2 and RAD51 to DNA double–strand breaks by enhancing the interaction between PALB2 and BRCA1. We identified two functions of ATR important to enhance PALB2–BRCA1 interaction. First, ATR directly phosphorylates PALB2 on serine 59 after DNA damage. Secondly after DNA double–strand breaks resection, ATR down–regulates CDKs activity leading to the decrease of serine 64 phosphorylation on PALB2. Together, these dual events lead to a direct enhancement of the interaction between PALB2 and BRCA1. Furthermore, we generated PALB2 mutants mimicking the active and inactive state of PALB2. We showed that PALB2 phospho–mutants that recapitulated the active state of PALB2 are able to bypass the absence of ATR activity in cells while mutants that mimic the inactive state of PALB2 showed a defect even in presence of active ATR. These results explain for the first time why ATR is essential to promote DNA double–strand break repair by homologous recombination. My results suggest that that ovarian cancer cells that do not carry BRCA1/2 or PALB2 mutations may be rendered “BRCA–like” by treatment with ATR inhibitor, making them susceptible to treatments with PARPi and others DNA damaging drugs. Citation Format: Rémi Buisson, Niraj Joshi, Chu Kwen Ho, Amélie Rodrigue, Tzeh Keong Foo, Emilie Hardy, Wilhelm Haas, Bing Xia, Jean–Yves Masson and Lee Zou. ATR–DEPENDENT REGULATION OF THE OVARIAN CANCER PROTEIN PALB2 AT DNA DOUBLE–STRAND BREAKS [abstract]. In: Proceedings of the 11th Biennial Ovarian Cancer Research Symposium; Sep 12-13, 2016; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(11 Suppl):Abstract nr MIP-049.