Abstract P5-07-12: RAD51AP1 is a novel prognostic marker and therapeutic target for breast cancer

Abstract

Abstract Background: Ionizing radiation is one of the most effective therapeutic strategies for the treatment of breast cancer and is considered as a more appropriate therapy for patients with high-risk of recurrence. Despite substantial benefits are achievable with this treatment, especially for ductal carcinoma and early invasive cancer, the critical barrier in using this treatment strategy is that tumor cells develop radioresistance, which in turn progress into advanced invasive cancer. Breast cancer stem cells (BCSCs), a subpopulation of cells within the tumor with a characteristic feature of self-renewal, play a critical role radioresistance and treatment failure. BCSCs exhibit increased DNA repair activity by increasing RAD51AP1 for their prolonged survival and to evade from the radiation therapy. We explored the expression profile of RAD51AP1 in BCSCs, human normal and various subtypes of breast tumor tissues and cell lines and response to chemo- and radiation- therapy. Methods: Gene expression (RNA and protein) profile was assessed using semi-quantitative and real-time PCR (qPCR) and western blot analyses. RAD51AP1 expression and its prognostic value in large cohort of human samples were analyzed by TCGA, GOBO, and Kaplan-Meier plotter integrative bioinformatics interface analyses. Breast cancer stem cell (BCSC) status was analyzed by FACS using CD24 and CD49f cell surface marker. Cell death was analyzed by propidium iodide (PI) stained cell cycle analysis. Results: We found that tumor propagating CD49f+CD24+ cells activate RAD51AP1 more promptly than non-tumorigenic CD49f-CD24- cells and confer chemo- and radiation- therapy resistance. RAD51AP1 inactivation facilitates chemo- and radiation- therapy response by depleting CD49f+CD24+ cells with significant activation of apoptotic cell death signaling. RAD51AP1 expression was significantly higher in BC, especially in the basal triple-negative and HER2-positive BC subtype, than in normal mammary tissue. Further, RAD51AP1 expression is highest in grade III histological tumor types and negatively associated to overall disease-free survival. RAD51AP1 levels across different BC cell lines showed that triple-negative breast cancer (TNBC) cell lines expressed highest level of this gene than other sub types. Conclusion:Overall, our findings provide evidence that BCSCs utilize DNA repair signaling for their self-renewal and RAD51AP1 play a critical role in BCSC self-renewal and maintenance. Further, RAD51AP1 expression profile can be used as a prognostic marker to monitor disease progression and chemotherapy response. Citation Format: Thangaraju M, Kolhe RB, Pathania R. RAD51AP1 is a novel prognostic marker and therapeutic target for breast cancer [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P5-07-12.