Abstract LB-131: Targeting expression of EZH2 with increased H3K27 trimethylation activity drives and synergizes with oncogene to promote mammary tumorigenesis in genetically engineered mouse models

EZH2, a core component of the PRC2 complexes, functions as a transcription repressor by catalyzing histone H3K27 trimethylation. EZH2 mutation frequently occurs in hematologic cancers, and overexpression is found in several solid tumor types such as breast, prostate, melanoma and bladder cancer. Moreover, overexpression of EZH2 is associated with highly proliferative and aggressive types of breast and prostate tumors. Particularly, EZH2 was more frequently overexpressed in the triple-negative breast cancers (TNBCs), which do not express the genes for estrogen receptor, progesterone receptor or HER2, indicating that EZH2 may be a tumor driver and contribute to basal-like lineage choice of tumor cells. However, transgenic expression of wild type of EZH2 (EZH2WT) in mammary and prostate gland epithelial cells failed to develop tumors. These results indicate that overexpression of EZH2 in mammary and prostate tissues alone is not sufficient to drive tumorigensis, and activation of EZH2 may be required for initiating tumor development. To test this hypothesis, we generated a new transgenic mouse expressing an EZH2 mutant (EZH2T487A) which has increased enzymatic activity (Wei YK et al, 2011) in mammary glands. Our data showed that expression of the activating mutant EZH2T487A but not EZH2WT in mammary glands developed heterogeneous tumors with basal or/and basal-luminal bilineage phenotypes in more than 50% of offsprings. In addition, crossing the EZH2T487A transgenic mice with the well-established luminal mammary tumor mouse model Tg-MMTV-HER2/Neu mice developed luminal type of mammary tumors with shorter latency as compared to each single transgenic mouse, suggesting that EZH2T487A expression, leading to global increase of H3K27Me3 is insufficient to reprogram the HER2/Neu-driven tumor cell lineage commitment. TCGA database analysis uncovered that EZH2 overexpression in TNBCs often coincides with p53 mutation. Accordingly, we examined whether p53 inactivation synergizes EZH2 to drive tumorigenesis with basal-like tumors. By crossing MMTV-EZH2T487A mice with MMTV-Cre/Trp53flox/flox mice we generated mice with EZH2T487A;p53+/- genotype. These mice exclusively developed multiple tumors in all offsprings with early onset and poor survival as compared to their parental mice and the resultant tumors displayed TNBC characteristics. Our findings indicate that expression of the activating EZH2 alone could drive tumorigenesis, and in cooperation with oncogenic HER2 and p53, EZH2 activation accelerated tumor development but barely affected lineage choice of tumors induced by the predominant drivers. Importantly, the developed tumors in these mouse models exhibited the phenotypes of some subtypes of human breast cancers, in particular, the TNBCs. Therefore, such spontaneous breast tumors with both EZH2 overexpression and p53 mutation could be more faithfully to recapitulate human TNBC development, natural tumor microenvironment and immune response, which can be used for assessing breast tumor immunity and evaluating targeted and/or immunotherapies for TNBCs. Citation Format: Lei Nie, Yongkun Wei, Hirohito Yamaguchi, Longfei Huo, Weiya Xia, Rong Deng, Jun Tang, Cihui Zhu, Baozhen Ke, Mien-Chie Hung. Targeting expression of EZH2 with increased H3K27 trimethylation activity drives and synergizes with oncogene to promote mammary tumorigenesis in genetically engineered mouse models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr LB-131. doi:10.1158/1538-7445.AM2017-LB-131