Project 1: Recruitment of the BRCA1-associated Homologous Recombination Machinery

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PROJECT SUMMARY: “RECRUITMENT OF THE BRCA1-ASSOCIATED HOMOLOGOUS RECOMBINATION MACHINERY” A substantial proportion of tumors show patterns of mutation that arise because of problems with DNA repair. Deficiencies in key DNA repair genes, such as BRCA1, PALB2 and BRCA2, are associated with inefficient DNA repair by homologous recombination (HR). Each of these factors is required to load the key HR intermediate, RAD51, at DNA double-strand break sites. The exact mechanism by which BRCA1 is recruited to DNA break sites and becomes activated is not fully understood. Efforts to design effective therapies to counteract DNA repair deficiencies are undermined by this lack of understanding of BRCA1 function. The overall goal of this proposal is to produce an integrated model for how BRCA1 becomes localized and activated at DNA break sites. This goal will be realized by investigating three mechanisms that control the recruitment of the BRCA1-associated repair machinery. First, the contribution of BARD1 for BRCA1 recruitment will be evaluated. BARD1 is a RING domain protein that partners with BRCA1, and appears to control the recruitment of BRCA1 to DNA break sites in the early time points after DNA damage. Preliminary results with novel mouse models have demonstrated that the interaction of BARD1 with BRCA1 is essential for normal DNA repair. Work in this proposal will clarify the mechanism by which BARD1 controls BRCA1 repair activities. Secondly, the mechanism of TOPBP1-dependent recruitment of BRCA1 will be identified. TOPBP1 is a large scaffold protein that loads onto chromatin at DNA damage sites and recruits multiple DNA repair factors. Through the use of advanced proteomic methods to study the protein interaction networks of BRCA1 after induction of DNA breaks, a DNA damage-dependent association of BRCA1 with TOPBP1 has been revealed. These protein complexes of TOPBP1 and BRCA1 may be instrumental for driving productive HR, and will be evaluated in detail in this proposal. The third part of the proposal focuses on how post-translational modification by ubiquitination and SUMOylation of proteins at break sites affects BRCA1-mediated HR. In particular, RNF4, a ‘SUMO-targeted E3 ubiquitin ligase’, with a known role in DNA repair, will be the subject of further investigation. RNF4 mutations are found in breast cancer cells, often in association with mutations in the key cellular growth regulator, p53. Using a conditional knockout mouse model that was recently generated in this lab, in conjunction with mouse models of breast cancer and medulloblastoma, the importance of RNF4 for prevention of tumor formation will be measured. This research will deepen our understanding of the regulation of BRCA1 at DNA break sites, and identify potential targets and/or avenues to prevent mutations caused by defective DNA repair.