Abstract A15: Using genetic and pharmacological methods to explore role of the methyltransferase Ezh2 in mouse mammary tumorigenesis

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Abstract ErbB2 (HER2/Neu) is a receptor tyrosine kinase over expressed in 20-30% of primary human breast cancer cancer, and correlated with poor clinical outcome. However, the mechanisms through which ErbB2 contributes to tumorigenesis and poor patients prognosis is unclear. Over the last decade, several lines of evidence have demonstrated that the transformation of a normal epithelial cells to a malignant phenotype requires the accumulation of multiple genetics and epigenetic events, including post translational modifications of histones. Enhancer of Zeste 2 (Ezh2) is a polycomb group protein that, as a part of the PRC2 complex mediates gene silencing through tri-methylation of lysine 27 on histone 3 (H3K27me3). Interestingly, several clinical studies have reported EZH2 transcript and protein levels to be elevated in aggressive human breast carcinomas. To investigate the requirement of Ezh2 in mouse mammary epithelial tumour initiation, we have crossed an Ezh2 conditional knock out mouse strain to a tetracycline inducible mammary epithelium specific Polyoma virus Middle T (PyVmT) transgenic mouse strain. The PyVmT model provides insight into how active receptor tyrosine kinase signalling can direct cellular transformation and its benefits include rapid tumour onset and well characterized disease progression, while the tetracycline-inducible systems allows for bypassing any developmental defects in the mammary gland due to Ezh2 ablation. Tumour onset in Ezh2 deficient mice are delayed(t50=135 days, n=15) compared to the wild type cohort (t50=63, n=28), and less multifocal in comparison to the wild type cohorts. End point tumours in Ezh2 deficient mice upregulate transcriptional signatures related to fibrosis and matrix remodelling, lipid biogenesis and cell cycle regulation. To compliment our genetic studies, we have also acquired a potent Ezh2 inhibitor, GSK126, to target the methyl transferase activity of Ezh2 in vivo and in vitro. Preliminary results indicate that the effect of global H3K27me3 loss is more pronounced during tumour initiation than tumour maintenance, and that in vivo effects of GSK-126 administration are likely mediated by the immune system. Genetic studies will allow for us to dissect how Ezh2 (through H3K27me3) controls transcriptional programs during tumour initiation and maintenance. In combination with the development of a highly selective and potent Ezh2 inhibitor, GSK126, combined pharmaceutical studies and the use of genetic models will provide insight into potential therapeutic windows during disease progression. Citation Format: Alison Hirukawa, Harvey Smith, William Muller. Using genetic and pharmacological methods to explore role of the methyltransferase Ezh2 in mouse mammary tumorigenesis. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr A15.