The Role of BRCT-Containing Proteins BRCA1 and PAXIP1 in Cancer

Modular domains of proteins are important in cellular signaling processes. Eukaryotic cells are constantly undergoing DNA damage due to exogenous and endogenous sources of damage. The DNA damage response (DDR) involves a complex network of signaling events mediated by modular domains such as the BRCT (BRCA1 C-terminal) domains. Therefore, proteins containing BRCT domains are important for DNA damage detection and signaling. In this dissertation, we focus on two BRCT-containing proteins BRCA1 and PAXIP1. BRCA1 is a gene that is known to be associated with increased risk of hereditary breast and ovarian cancer. Germline variants of BRCA1 are assessed to determine lifetime risk of developing breast and ovarian cancer. This is performed by genetic testing of the BRCA1 sequence and the variants can be classified as pathogenic, non-pathogenic or variants of unknown significance (VUS). Using family history, segregation analysis, co-occurrence and tumor pathology, certain variants have been classified as either pathogenic or non-pathogenic. However, a large majority of the variants are classified as VUS. Functional assays are critical in providing insight in the case of VUS results. We have a developed a visualization resource to aid in functional analysis of BRCA1 missense variants that occur due to single amino acid changes. This tool is known as BRCA1 Circos (http://research.nhgri.nih.gov/bic/circos/) and it aggregates, harmonizes and allows interpretation of data from all published studies on functional analysis of BRCA1 missense variants. Therefore, this is an important tool that will aid in the meta-analysis of functional data needed to better assess VUS. Functional studies of BRCA1 also demonstrate that majority of the variants that have a functional impact on the protein lie in the BRCT region of the protein. This indicates that the BRCT region is important in cancer development. To further analyze the function of BRCT-containing proteins, a study was previously undertaken to evaluate the role of BRCT-containing proteins and their interaction partners in the DNA damage response and consequently, cancer. BRCT domains of seven BRCT-containing proteins were used as baits and their binding partners were demonstrated to be highly enriched in the DDR process. We hypothesized that members of this BRCT-centric protein-protein interaction network could constitute targets for sensitization to DNA damaging chemotherapy agents in lung cancer. Therefore, we probed this established dataset containing the protein-protein interaction network (PPIN) of seven BRCT-containing proteins to identify seventeen kinases. A systematic pharmacological screen was performed to evaluate these kinases as targets to enhance platinum-based chemotherapy in lung cancer and this revealed WEE1, a mitotic kinase, as a potential target. Of the seventeen kinases, inhibition of mitotic kinase, WEE1, was found to have the most effective response in combination with platinum-based compounds in lung cancer cell lines. In the PPIN, WEE1 was shown to interact with PAXIP1 (PTIP), a BRCT-containing protein involved in transcription and in the cellular response to DNA damage. PAXIP1 has been shown to bind DDR proteins, such as 53BP1 and γH2AX, and also shown to be an important part of immune development. In this dissertation, we observe that WEE1 binds to PAXIP1 and PAXIP1 regulates the WEE1-mediated phosphorylation of its main substrate, CDK1. We also demonstrate that ectopic expression of PAXIP1 combined with WEE1 inhibitor, AZD1775, leads to an increase in the mitotic index at the G2/M checkpoint. Overexpression of PAXIP1 combined with AZD1775 treatment in cells with prior DNA damage causes high levels of caspase-3 mediated apoptosis as compared to AZD1775 treatment alone. In summary, we identify the role of PAXIP1 in sensitizing lung cancer cells to the WEE1 inhibitor, AZD1775, in combination with platinum-based therapy and propose the use of WEE1 and PAXIP1 levels as mechanism-based biomarkers. Overall, these studies indicate that BRCT-containing proteins through their role in the DDR and the cell cycle are crucial for both cancer prevention and therapy