Abstract A02: The HER4 BH3-only protein and tumor suppressor is epigenetically silenced in endocrine refractory and triple negative breast cancers

Breast cancer is the most commonly diagnosed cancer in North American women and remains the second most common cause of cancer related deaths in women. In contrast to the mitogenic activities of the EGFR-family members EGFR itself, HER2, and HER3; HER4, the fourth and final member of the EGFR-family to be discovered, suppresses breast tumor cell growth. HER4 is also unique because it is the only member of the EGFR family to undergo proteolytic cleavage at the cell surface to generate an independently signaling intracellular domain (4ICD). Clinically, HER4 expression is extinguished during breast tumor progression and patients with tumors lacking HER4 expression have significantly worse prognosis. We investigated the apparent tumor suppressor activity of HER4 and mechanisms of HER4 suppression during breast tumor progression. We found that 4ICD harbors a pro-apoptotic BH3-only domain required for its tumor cell killing activity. To evade the potent tumor cell killing activity of 4ICD, tumor cells acquire mechanisms to suppress HER4 expression. Importantly, we identified a CpG island within the HER4 promoter and show by pyrosequencing of bisulfite treated DNA an inverse correlation between HER4 expression and the extent of promoter methylation. Clinically, increased levels of HER4 promoter methylation were significantly associated with the most aggressive breast cancer subtypes (hormone refractory, HER2 positive, and triple negative) and significantly worse patient prognosis. Consistent with these clinical findings, the HER4 promoter is hypermethylated and HER4 expression is suppressed in multiple endocrine resistant and triple negative breast cancer cell lines. Reestablishing, by ectopic expression, or reactivating, through 5-aza-2-deoxycytidine treatment, HER4 expression results in dramatic BH3-dependent apoptosis of both endocrine resistant and triple negative breast cancer cell lines. To identify alterations in epigenetic regulators common to multiple breast tumor cell lines with suppressed HER4 expression we used Qiagen RT-Profiler PCR Arrays to analyze expression of 168 key chromatin modifying enzymes and remodeling factors. The most dramatic alterations common to two endocrine resistant and three triple negative breast tumor cell lines were suppression of the chromatin remodeling protein CHD5 and suppression of the TET2 deoxygenase - which initiates a DNA demethylation pathway. We are currently investigating the impact of modulated CHD5 and TET2 expression on the reactivation of HER4 cell killing activity in endocrine resistant and triple negative breast tumors. In conclusion, our data supports a pro-apoptotic tumor suppressor function for HER4 which is epigenetically suppressed in endocrine refractory and triple negative breast tumors through promoter hypermethylation. Reactivation of epigenetically silenced HER4 results in dramatic tumor cell apoptosis, providing a therapeutic strategy for the treatment of the most aggressive forms of breast cancer. Note: This abstract was not presented at the conference. Citation Format: Wen Han, Eleanor Semmes, Frank E. Jones. The HER4 BH3-only protein and tumor suppressor is epigenetically silenced in endocrine refractory and triple negative breast cancers. [abstract]. In: Proceedings of the AACR Special Conference on Chromatin and Epigenetics in Cancer; Sep 24-27, 2015; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2016;76(2 Suppl):Abstract nr A02.