Leukemias harboring rearrangements of the MLL gene carry a poor prognosis. Over the past 6 years, it has become increasingly clear that fusions of MLL induce widespread epigenetic deregulation that may mediate much of their transforming activity. The histone methyltransferase DOT1L, which methylates histone 3 on lysine 79 (H3K79), has received particular attention. Genome-wide H3K79 methylation profiles in MLL-rearranged leukemias are abnormal, and can serve to distinguish MLL-rearranged from other types of leukemias. Loss of H3K79 methylation affects expression of MLL-target loci and is detrimental to the leukemogenic activity of MLL-rearranged cells, suggesting that a DOT1L dependent, aberrant epigenetic program drives transformation in these leukemias. Small molecule DOT1L inhibitors have been developed and selectively inhibit proliferation of MLL-rearranged leukemia cells. These molecules are now in clinical trials for patients with relapsed refractory leukemia. Also, the Polycomb Repressive Complex 2 (PRC2) has been implicated in self-renewal and cancer progression, and its components are overexpressed in many cancers, but its role in cancer development and progression remains unclear. We have used conditional alleles for the PRC2 components Enhancer of Zeste 2 (Ezh2) and Embryonic Ectoderm Development (Eed) to characterize the role of PRC2 function in leukemia development and progression. Compared to wildtype leukemia, Ezh2-null MLL-AF9-mediated acute myeloid leukemia (AML) show decreased organ infiltration and failed to accelerate upon secondary transplantation. However, Ezh2-null leukemias maintained self-renewal up to the third round of transplantation, indicating that Ezh2 is not strictly required for MLL-AF9 AML. In contrast, inactivation of the essential PRC2 gene, Eed led to complete ablation of PRC2-function, which is incompatible with leukemia growth. Gene expression array analyses indicated more profound gene expression changes in Eed-null compared to Ezh2-null leukemic cells, including downregulation of Myc target genes and upregulation of PRC2 targets. These data show that histone-modifying enzymes play critical roles in leukemia and may be relevant therapeutic targets in these diseases.

Citation Format: Scott A. Armstrong. Targeting histone methylation in leukemia. [abstract]. In: Proceedings of the AACR Special Conference on Chromatin and Epigenetics in Cancer; Jun 19-22, 2013; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2013;73(13 Suppl):Abstract nr IA08.