Rad51 function is essential for viability of proliferating cells, such as cancer cells. BRCA2 is the major mediator protein for Rad51 function in mammalian cells, directly binding Rad51, which is required for normal Rad51 function in cells. Rad52 function appears to be dispensable unless the BRCA2 protein is inactivated, when Rad52 becomes vital for cell survival. We are characterizing the different roles of the Rad51 mediators and their synthetic lethal relationship.
BRCA1 has a number of functions in the DNA damage response in mammalian cells, including signaling to cell-cycle checkpoints, and recruiting proteins involved in homologous recombination. We have characterized a dissociation of function mutation of BRCA1 that preserves cell-cycle checkpoint function, but is highly defective in homologous recombination. This mutant BRCA1 provides significant insight into how BRCA1 engages the downstream components of homologous recombination. The MDC1 protein has been shown to help mediate Rad51 to the sites of DNA damage, whereas 53BP1 appears to prevent the processing of DNA double-strand breaks mediated by BRCA1 for homologous recombination. We are characterizing the relationship of the BRCA1 superfamily in the regulation of homologous recombination.
RPA is the major single-strand DNA binding protein required for DNA replication, DNA repair, and recombination. Phosphorylation and de-phosphorylation appear to be specifically required for its role in homologous recombination. We are investigating how phosphorylated RPA engages other proteins involved in homologous recombination. In addition, we are investigating how p53 regulates homologous recombination, through its interaction with RPA. A dissociation of function mutation of p53 has been discovered, which maintains normal checkpoint and cell death functions of p53, while failing to suppress homologous recombination. A knock-in mouse study is ongoing.
Inactivation of the BRCA1-BRCA2 pathway occurs in sporadic breast cancer as well as germline mutation carriers. The phenotype affects all conventional subtypes of breast cancer, although it is more prevalent in the triple-negative subgroup. The mechanism of inactivating the BRCA pathway in sporadic cancers is under investigation, but appears to be upstream of BRCA1 in the majority of cases studied so far. Why BRCA1 and BRCA2 germline mutations predispose to breast and ovarian cancer, rather than other tumor types, is under investigation. Drug screens for agents that specifically sensitize homologous recombination or homologous recombination-deficient cells are in progress.