Dissecting and targeting canonical and non-canonical oncogenic functions of EZH2 in cancer

PROJECT SUMMARY/ABSTRACT Leukemia patients bearing MLL1 rearrangement generally display very poor prognosis, demanding new treatment strategies. Multiple independent studies have identified EZH2, a histone methyltransferase and catalytic subunit of Polycomb Repressive Complex 2 (PRC2), as an attractive drug target in MLL1-rearranged leukemias. However, the current catalytic inhibitors of EZH2 have limited anti-tumor effect. Our preliminary studies show that EZH2 also binds c-MYC and has a non-canonical function in activation of oncogenes (such as Cyclin E1) in MLL1-rearranged leukemias, which differs from the well-known PRC2:EZH2-driven canonical function related to gene repression. In order to target both canonical and non-canonical oncogenic actions by EZH2, we used the Proteolysis Targeting Chimera (PROTAC) technology to generate novel EZH2 small- molecule degraders including MS177. Our extensive preliminary studies have demonstrated that MS177 effectively degrades both PRC2:EZH2 and non-PRC2 partners of EZH2 (e.g., c-MYC, which binds EZH2 to activate oncogenes), thus suppressing both canonical and non-canonical oncogenic activities of EZH2 in tumor. Importantly, our preliminary results also show that MS177 is superior to all of enzymatic inhibitors of EZH2 in treating MLL1-rearranged leukemias. Thus, we hypothesize that: (1) EZH2 has a less-studied, non- canonical oncogenic function, which acts in parallel with the PRC2:EZH2-dependent one to produce more aggressive tumor phenotypes seen in MLL1-rearranged leukemias; and (2) targeting both canonical and non- canonical activities of EZH2 by PROTACs represents a novel and superior therapeutic strategy to inhibition of EZH2’s enzymatic activity alone. Dissection of the mechanisms underlying EZH2-mediated oncogenesis and development of an optimal EZH2 PROTAC as a drug candidate will have significant impact on improving treatments for MLL1-rearranged leukemia patients. Towards this goal, we will further characterize such a new non-canonical oncogenic role of EZH2 in MLL1-rearranged leukemias (Aim 1a) and define effects of EZH2 PROTACs on suppressing both canonical and non-canonical oncogenic activities of EZH2 (Aim 1b). We will also determine in vitro and vivo therapeutic effects of EZH2 PROTACs by employing multiple independent MLL1-rearranged leukemia models including human/murine cancer cells and patient-derived xenografts (PDX) (Aim 2). Lastly, we will optimize our EZH2 PROTAC leads into a drug candidate (Aim 3). Completion of the proposed research will not only provide novel mechanistic understanding of how MLL1-rearranged leukemias develop, but will also validate an innovative therapeutic strategy and deliver a promising therapeutic candidate for the treatment of affected cancer patients.