The role of SWI/SNF chromatin remodelers in homologous recombination and genome stability - Multiple subunits of the SWI/SNF chromatin remodeling complexes have been identified as a novel class of tumor suppressors mutated in up to 20% of human cancers, but their mechanism of tumor suppression is still poorly understood. These complexes are composed of a mutually exclusive ATPase subunit (either BRG1 or BRM), a set of core factors, and a group of accessory subunits. SWI/SNF complexes were first identified as transcriptional activators and therefore most of the research efforts have been focused on the role of these ATPases in the transcriptional regulation of a variety of cellular pathways involved in carcinogenesis. More recent work has linked BRG1 to DNA repair. We identified a complex containing TopBP1-E2F1-RB responsible for the recruitment of BRG1 to DNA double strand breaks (DSBs). Moreover, we also identified a function for BRG1 stimulating DNA end resection and homologous recombination (HR) by reducing nucleosome density at DSBs and promoting the recruitment of the CtIP nuclease. However, the role that the other ATPase (BRM) and other non-catalytic subunits of the SWI/SNF complex may play in this function is still unknown. The goal of this proposal is to further define the mechanism by which BRG1 and BRM stimulate DNA end resection and HR, to determine to what extent SWI/SNF subunits ARID1A and BRD7 are required for this function, and to test whether we can sensitize breast cancer cells to chemotherapeutic agents by inactivating these SWI/SNF subunits. Our hypothesis is that BRG1 reduces nucleosome density at DSBs, thus allowing the recruitment of the DNA end resection nuclease CtIP and stimulating DNA end resection and HR, and that ARID1A and BRD7 are important for anchoring these complexes at DSBs. The uncovering of a function in DSB repair for SWI/SNF accessory subunits could help explain their tumor suppressor activities. Moreover, identifying multiple SWI/SNF subunits required for HR, would also identify a DNA repair vulnerability that could be exploited therapeutically against SWI/SNF-mutated cancers. In order to test this hypothesis, we will, first, determine whether a BRG1- or BRM- ATPase-dead mutant would be able to stimulate DNA end resection and promote the recruitment of the CtIP nuclease. Second, we will define the role non-catalytic SWI/SNF subunits ARID1A and BRD7 play in the recruitment of BRG1, chromatin remodeling at DSBs, DNA end resection, and HR, by analyzing the generation of single-stranded DNA and ATR signaling after DNA damage. Third, we will determine whether we can sensitize breast cancer cells to chemotherapeutic agents, including Poly (ADP-ribose) polymerase (PARP) inhibitors, by inactivating BRG1, BRM, ARID1A, and BRD7 subunits. This systematic study of the function of the SWI/SNF subunits in DNA end resection and HR will further our understanding of the biology of these complexes, their functions, and the required composition of these complexes in order to perform this function in HR. Moreover, we could potentially identify DNA repair vulnerabilities in SWI/SNF-mutated cancers that could be exploited through specific chemotherapeutic regimens designed for HR-deficient malignancies.
