Abstract 5527: Determining the apoptotic pathways required for the EZH2 inhibitor, GSK-126, in a mouse model of B cell lymphoma

Background: Approximately 40% of B-cell lymphomas are characterized by the presence of recurrent chromosomal translocation affecting the Myc/8q24 locus. Further, similar breakpoints may also occur as secondary events giving rise to “Double Hit” (DH) lymphomas. DH lymphomas frequently involve Myc with concurrent, Bcl-2 or Bcl6 translocations. These Myc associated B cell lymphomas are linked with poor survival, with a median overall survival of only 0.2-1.5 years, and novel therapeutic strategies are urgently required. EZH2 is the catalytic subunit of the polycomb repressive complex 2 (PRC2) and is shown to correlate with disease progression in many cancer types. EZH2 is involved in gene repression via tri-methylation of histone H3 lysine 27 (H3K27me3) and its expression is demonstrated to be increased by Myc, highlighting a potential therapeutic target towards Myc driven B cell lymphomas. In this study we determined the antitumor activity of a specific EZH2 inhibitor, GSK-126. Taking advantage of the well characterized Eμ-myc B cell lymphoma mouse model, we screened tumors with defined genetic alterations in apoptotic pathways to determine GSK-126 antitumor activity. Methods: The anti-tumor effects of GSK126 or ectopic expression of an EZH2 mutant deficient for histone methyltransferase activity (ΔSET-EZH2) was assessed towards Eµ-myc B cell lymphomas, and compound mutant B cell lymphomas (Eµ-myc/Bcl2, Eµ-myc/p53 -/- , Eµ-myc/p19 -/- , and Eµ-myc/apaf1 -/- ) by uptake of propidium iodide. Further apoptotic assessment was performed using annexin, cell cycle analysis, mitochondria damage and caspase activity by flow cytometry. GSK-126 inhibition of EZH2 activity was determined by H3K27me3 expression by immunoblot. In vivo anti-tumor activity was determined by transplanting Eµ-myc lymphomas and Eµ-myc compound mutants expressing control of ΔSET-EZH2 plasmids to wild type mice. Overall survival was determined by Kaplan-Meier analysis. Results: Preliminary data demonstrates that GSK-126 treatment of Eµ-myc lymphomas results in rapid apoptotic cell death in a dose dependent manner. GSK-126 mediated apoptosis is independent of p53, though requires mitochondria damage. Further experiments described above are currently being performed. Conclusion: Our current data demonstrate that EZH2 inhibition induces apoptosis in Eµ-myc B cell lymphomas independent of p53, but requires mitochondrial damage. Overall, this study will lead to a more comprehension of the apoptotic mechanisms underlying GSK-126 therapeutic effect and will provide translational tools to predict the therapeutic response in DH lymphomas overexpressing Bcl2 or Bcl6. Citation Format: Elena Lasorsa, Kevin Schaarschuch, Zafardjan Dalimov, Leigh Ellis. Determining the apoptotic pathways required for the EZH2 inhibitor, GSK-126, in a mouse model of B cell lymphoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5527. doi:10.1158/1538-7445.AM2014-5527