Our DNA is constantly exposed to damage due to environmental insults and normal cellular processes. In order to prevent the loss of important genetic information, cells have acquired mechanisms, which are dedicated to orchestrating the recognition and repair of damaged DNA. Homologous recombination (HR) represents one such mechanism that is responsible for facilitating the error-free repair of DNA double strand breaks. Importantly, inactivation of genes involved in the HR pathway, such as by mutation in the BRCA1 and BRCA2 genes, has been linked with increased genetic abnormalities and is frequently detected in ovarian tumors. Past work indicates that BRCA1/BRCA2-mutant ovarian cancer cells are sensitive to drugs that inhibit PARP1, an enzyme that is active in a parallel DNA repair pathway. Unfortunately, despite the early success of PARP1 inhibitors in phase 1 clinical trials, rapidly acquired resistance suggests that alternative therapies may be required for durable patient response.
To screen for new therapeutic targets in HR-deficient cancer cells, we took advantage of a large-scale study performed in collaboration with our laboratory, which aimed to examine how the loss of function for over 9,000 genes affected the growth of 100 human tumor-derived cell lines. By characterizing these cell lines for their HR repair capacity, we were able to identify 50 genes, which when inhibited, reduced the survival of HR-deficient cancer cells, but had little affect on cells with normal HR activity. While these findings are likely to highlight new genetic vulnerabilities in HR-deficient tumors, additional experiments are required to determine precisely how our candidate genes promote toxicity in cells that lack an intact HR pathway.
I hypothesize that a subset of our candidate genes may induce cell toxicity by stimulating a lethal accumulation of DNA double strand breaks in cells that are unable to activate HR repair. To test this, I will use a microscopy-based assay to monitor the level of DNA damage in cells with normal or impaired HR repair after candidate gene knockdown. Candidategenes shown to induce DNA damage selectively in cells with low HR will be prioritized and biochemical experiments will be used to validate and further dissect their relationship with HR-deficiency in vitro. In addition, I will rigorously test the role of our candidate genes in HR-deficient tumorigenesis using in vivo mouse studies and we will determine whether suppression of our candidate gene synergizes with carboplatin or PARP1 inhibitors to inhibit HR-deficient tumorigenesis. Ultimately, these studies will highlight new therapeutic targets for the growing population of patients with HR-deficient ovarian tumors.
This grant was made possible in part by a generous donation from the Ovarian Cancer Alliance of Greater Cincinnati, in memory of Jackie McCarren.