Epigenetic Elimination of Mixed-Lineage Leukemia Leukemic Stem Cells by a Potent Small Molecule PRC2 Inhibitor.

Abstract 2456

Leukemia carrying a chromosomal translocation involving the mixed-lineage leukemia (MLL) gene is particular resistant to current therapy, and is associated with frequent relapse. Current evidence points towards self-renewing leukemic stem cells (LSC) as the driver of relapse. The eradication of LSC is thought to be critical for successful anti-leukemia therapy. Recent studies suggest that the multi-protein complex polycomb repressive complex 2 (PRC2) is required for MLL-rearranged AML. PRC2 contains the core proteins EZH2, EED and SUZ12, which have intrinsic histone methyltransferase activity specific to trimethylation of histone H3 on lysine 27. It is involved in the epigenetic silencing of tumor suppressor genes (e.g., p16 and p19). In this study we determined the effect of a PRC2-associated inhibitor (hereafter named PAI) on the PRC2 components in LSC isolated from bone marrow cells of mice with established leukemia induced by the MLL-AF9 oncogene. Treatment with PAI (0.1 to 0.5 μM) for 24 hours in culture media depleted protein levels of EZH2 and EED in LSC. Following treatment, quantitative real-time PCR and immunoblotting analysis demonstrated increased mRNA and protein levels of p16 and p19, as well as inhibition of cell cycle progression. PAI treatment also induced a 10-fold increase of apoptosis in LSC. As a consequence, colony formation assays showed that LSC treated for 24 hours with 0.1 μM PAI completely lost their ability to form compact colonies in methylcellulose. We are currently assessing the effects of PAI in in vivo models of MLL-rearranged leukemia where preliminary data indicate that we are able to achieve pharmacologic inhibition of the PRC2 activity. Collectively, these findings indicate that PAI depletes the levels of PRC2 components, and eliminates LSC, thus representing a potentially promising novel treatment for AML.