Abstract 681: Targeted MLL-AF9 degradation is phenocopied by combined DOT1L and Menin inhibition

Abstract MLL1 (KMT2A) translocations lead to aberrant expression of stem cell associated gene programs in hematopoietic cells producing a particularly aggressive subtype of leukemia, namely MLL-rearranged acute myeloid and lymphoblastic leukemia (AML and ALL, respectively). In pediatric leukemia, the most common rearrangement is the t(9; 11) (p22; q23) reciprocal translocation, which results in the expression of the MLL-AF9 fusion gene. Since the MLL-AF9 rearrangement does not lead to direct activation of an enzyme, small molecule drug discovery remains challenging and despite decades of research on MLL-rearranged leukemia, we have been limited by the inability of acutely assessing the consequences of direct MLL-AF9 inactivation in relevant model systems. By coupling rapid PROTAC-mediated degradation of the oncogenic MLL-AF9 fusion protein with genome-wide gene expression and chromatin analyses we have now been able to identify the immediate transcriptional and chromatin state consequences of MLL-AF9 degradation. Specifically, we have established a core set of MLL-AF9 target genes whose expression changes within minutes of MLL-AF9 degradation. These rapid expression changes are mediated by changes in productive RNA Polymerase II elongation and increased RNA Polymerase II pausing. At later timepoints, degradation of the fusion protein also induces loss of an active chromatin landscape at MLL-AF9 target genes, characterized by loss of MLL-AF9 associated proteins and activating histone modifications. These insights improve our mechanistic understanding of how MLL-AF9 mediates chromatin remodeling and leukemogenesis, while also helping us evaluate small molecule inhibitors of epigenetic mechanisms that target the MLL-AF9 protein complex. Specifically, we investigated how degradation of MLL-AF9 compares to enzymatic inhibition of DOT1L or inhibition of the MENIN-MLL interaction. Interestingly, both DOT1L and MENIN inhibitors fell short when comparing their effects to MLL-AF9 degradation and only combined DOT1L/MENIN inhibition induced global MLL-AF9 destabilization on chromatin, thus disrupting the full MLL-AF9 directed epigenetic and transcriptional program. Ultimately, we were able to confirm these findings in a patient-derived xenograft model in vivo, where only the combination was able to significantly prolong leukemia free survival. In summary, our studies establish the cellular and molecular consequences of targeted degradation of the MLL-AF9 fusion oncoprotein and help improve our understanding of small molecule inhibitors of epigenetic mechanisms as cancer therapies. Citation Format: Sarah Naomi Olsen, Laura Godfrey, James P. Healy, Charles Hatton, Scott A. Armstrong. Targeted MLL-AF9 degradation is phenocopied by combined DOT1L and Menin inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 681.