Identification of Epigenetic Targets in the Treatment of Recurrent Breast Cancer

Abstract

Breast cancer is the leading cause of cancer‐related death in women, with most deaths resulting from tumor relapses. Among the largest challenges in treating breast cancer is the presence of tumor cells that survive initial chemotherapy and persist in a quiescent‐like state. These residual tumor cells lay dormant for up to 20 years before reemerging. Studying the biology of dormant and recurrent tumors is hampered by the difficulty of obtaining patient samples and developing relevant mouse models.To address these issues, we utilize a genetically engineered mouse that emulates clinical breast cancer recurrence. Mice express a mammary‐specific, doxycycline‐inducible HER2 construct that generates primary tumors in mammary glands of mice. Upon withdrawal of doxycycline, tumors regress into a minimal, non‐proliferative disease that recurs spontaneously after a period of months, independent of HER2 signaling.With this model, we are employing unbiased genetic and epigenetic approaches to investigate pathways that contribute to the survival of residual tumor cells and initiation of recurrent disease. Identification of these targets is of interest in the development of novel therapies designed to treat or prevent recurrent tumors and improve breast cancer care.Evaluation of changes in global gene expression between primary and recurrent tumors showed that prostate apoptosis response‐4 (par‐4), a pro‐apoptotic protein, is downregulated during tumor recurrence. Furthermore, primary tumors lacking par‐4 develop recurrent tumors more quickly than controls, while par‐4 overexpression delays recurrence. Par‐4 is repressed in recurrent tumors by epigenetic deposition of bivalent histone marks H3K4me3 and H3K27me3 and low histone acetylation. Par‐4 repression is pharmacologically reversible with inhibition of histone deacetylases or enhancer of zeste homolog‐2 (EZH2). Reexpression of par‐4 sensitizes recurrent tumor cell lines to chemotherapies and decreases cell motility. Taken together, these findings suggest that epigenetic therapies designed to reexpress par‐4 may be of value in the treatment of recurrent breast cancer.Additionally, we have determined that recurrent tumors demonstrate two‐to threefold increased expression of G9a/GLP. Moreover, recurrent tumors exhibit 10‐fold higher sensitivity to pharmacologic inhibition of G9a/GLP and genetic knockdown results in deleterious effects on growth. Cell death induced by G9a inhibition is p53‐dependent, as increased expression of downstream p53 effectors is required for induction of apoptosis in recurrent tumor cell lines. This suggests that recurrent tumors may become dependent on G9a/GLP and provides a novel therapeutic target in these tumors.Together our findings suggest primary and recurrent tumors differentially regulate their epigenetic landscapes, and uncovering epigenetic vulnerabilities will provide valuable therapeutic targets in the treatment or prevention of recurrent breast cancer.Support or Funding InformationResearch reported in this poster was supported by the NCI under award number R01CA208042. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.