PROJECT SUMMARY African American (AA) women are disproportionately affected by triple negative breast cancer (TNBC), a highly heterogeneous and aggressive subtype of breast cancer. We found that AA patients with TNBC have a higher risk of death from the disease than their European American (EA) counterparts, which is independent of their sociodemographic and clinical factors. Notably, this TNBC disparity is more obvious in patients receiving chemotherapy. However, the molecular mechanisms driving differential tumor response to chemotherapy and the consequent prognostic disparities in AA vs. EA TNBCs remains unknown. Most chemotherapy agents exert their cytotoxic effects through the induction of deadly DNA double-strand breaks (DSBs) that are repaired by two major mechanisms: error-free homologous recombination (HR) and error-prone non-homologous end- joining (NHEJ). Genomic stability and survival of tumor cells upon genotoxic treatments are known to depend on HR. Our pilot study showed that AA TNBCs tend to have higher expression of HR-related proteins and lower expression of NHEJ-related proteins compared with EA TNBCs. The AAA+ ATPase VCP has been suggested to influence the choice of DSB repair pathways in favor of HR as opposed to NHEJ. We recently reported that Ser784 phosphorylation of VCP is required for DNA damage repair and intra-tumor pSer784-VCP levels predict poor survival in TNBC patients, particularly those receiving chemotherapy. Our pilot data also showed that both total VCP protein and DNA damage-induced pSer784-VCP levels are higher in AA vs. EA TNBC samples. Based on these data, we hypothesize that AA TNBCs, relative to EA TNBCs, are intrinsically more capable of repairing chemotherapy-induced DSBs by HR as opposed to NHEJ due to differential expression levels and functionality of DSB repair factors including pSer784-VCP, which underlies their worse clinical outcomes. We propose three specific aims. First, we will confirm racial differences in protein expression of HR and NHEJ factors in tumor tissues obtained from two large cohorts of TNBC patients and examine their contribution to survival disparities. Second, we will experimentally compare HR and NHEJ efficiencies between AA and EA TNBC cell line and patient-derived mouse xenograft (PDX) models, and correlate the racial differences in DSB repair pathway choice and genotoxic chemotherapy effects. Third, we will examine the ability of pSer784-VCP to regulate DSB repair pathway choice and genotoxic chemotherapy effects in AA TNBC models. The proposed study will greatly improve our understanding of the molecular mechanisms underlying racial disparities in TNBC treatment response and outcomes. The results will also help create new disparity intervention strategies by establishing pSer784-VCP as a novel predictive biomarker and sensitizing target for genotoxic chemotherapy treatments in AA TNBCs. .