Combined Inhibition of DNMT and HDAC Blocks the Tumorigenicity of Cancer Stem-like Cells and Attenuates Mammary Tumor Growth

Recently, impressive technical advancements have been made in the isolation and validation of mammary stem cells and cancer stem cells (CSC), but the signaling pathways that regulate stem cell self-renewal are largely unknown. Furthermore, CSCs are believed to contribute to chemo- and radioresistance. In this study, we used the MMTV-Neu-Tg mouse mammary tumor model to identify potential new strategies for eliminating CSCs. We found that both luminal progenitor and basal stem cells are susceptible to genetic and epigenetic modifications, which facilitate oncogenic transformation and tumorigenic potential. A combination of the DNMT inhibitor 5-azacytidine and the HDAC inhibitor butyrate markedly reduced CSC abundance and increased the overall survival in this mouse model. RNA-seq analysis of CSCs treated with 5-azacytidine plus butyrate provided evidence that inhibition of chromatin modifiers blocks growth-promoting signaling molecules such as RAD51AP1 and SPC25, which play key roles in DNA damage repair and kinetochore assembly. Moreover, RAD51AP1 and SPC25 were significantly overexpressed in human breast tumor tissues and were associated with reduced overall patient survival. In conclusion, our studies suggest that breast CSCs are intrinsically sensitive to genetic and epigenetic modifications and can therefore be significantly affected by epigenetic-based therapies, warranting further investigation of combined DNMT and HDAC inhibition in refractory or drug-resistant breast cancer. Cancer Res; 76(11); 3224-35. ©2016 AACR.