Uncovering the role of the BRCA1-RAP80 complex in cancer susceptibility and DNA repair

DNA lesions such as double-strand breaks (DSB) and interstrand crosslinks (ICL) result in extensive chromatin modification by phosphorylation, ubiquitination, SUMOylation and others. These modifications serve to increase the accessibility of chromatin to DNA repair factors and to provide platforms for binding and recruitment of DNA repair proteins. BRCA1 is a known tumor suppressor that is recruited to sites of DNA damage. It is a member of four distinct protein complexes, each of which contributes to DNA repair. One specific BRCA1 complex recruited to lysine 63 conjugated ubiquitin structures surrounding the DSB is the BRCA1-RAP80 complex. While it is known that the BRCA1-RAP80 complex is needed for resistance to ionizing radiation and the G2-M checkpoint, its function at DSBs has not been well elucidated. Here we describe the role of the BRCA1-RAP80 complex in cancer susceptibility, along with its role in both DSB and ICL repair. We have discovered a mutation in RAP80 in BRCA1/2 mutation negative breast cancer families that abrogates its ability to bind ubiquitin and translocate to sites of DNA damage, resulting in genome instability. Additionally, we describe the molecular role for the BRCA1-RAP80 complex at DSBs. The complex is particularly important in regulating the choice between the DSB repair pathways of homologous recombination (HR) and non-homologous end joining (NHEJ). Loss of BRCA1-RAP80 results in increased HR and decreased NHEJ. This occurs through an increase in the process of DSB end resection, which favors repair by HR. Finally, we have uncovered a novel role for the BRCA1-RAP80 complex in ICL repair. We have found that loss of BRCA1-RAP80 causes extreme sensitivity to DNA crosslinking agents and that this sensitivity is not epistatic with FANCD2, a protein known to be important for ICL repair. Thus, the BRCA1-RAP80 complex plays a critical role in maintaining genome stability and preventing tumorigenesis through the repair of both DSBs and ICLs.