Epigenetic Mechanisms and Leukemia Stem Cell Development

Leukemias often develop as a result of activation of stem cell-associated gene expression programs. We have been studying the mechanisms by which this gene expression is regulated with the hope of using this information to develop new therapeutic approaches Most work has focused on leukemias harboring mixed lineage leukemia (MLL) gene abnormalities which are associated with poor clinical outcomes. Rearrangement of the MLL gene generates chimeric proteins that fuse the NH3-terminus of MLL to the COOH-terminus of its translocation partners. These MLL-fusion oncoproteins drive the expression of homeobox genes such as HOXA cluster genes and MEIS1, which are known to induce leukemic transformation of hematopoietic progenitors. Genome-wide histone methylation studies have revealed that the abnormal expression of MLL-fusion target genes is associated with high levels of H3K79 methylation at these gene loci. The only known enzyme that catalyzes methylation of H3K79 is disruptor of telomeric-silencing 1-like (DOT1L). Loss-of-function mouse models as well as small molecular inhibitors of DOT1L demonstrate that leukemias driven by MLL-translocations are dependent on DOT1L enzymatic activity for proliferation and for the maintenance of HOXA gene expression. Furthermore, DOT1L also appears to be important for HOXA gene expression in other settings including leukemias with select genetic abnormalities. These discoveries have established a foundation for disease-specific therapies that target chromatin modifications in highly malignant leukemias harboring specific genetic abnormalities. In order to further understand the mechanistic basis for DOT1L dependency in MLL fusion leukemias we recently conducted a genome-scale RNAi screen and found that the histone deacetylase SIRT1 is required for the establishment of a heterochromatin-like state around MLL fusion target genes after DOT1L inhibition. DOT1L inhibits chromatin localization of a repressive complex composed of SIRT1 and the H3K9 methyltransferase SUV39H1, thereby maintaining an open chromatin state with elevated H3K9 acetylation and minimal H3K9 methylation at MLL fusion target genes. Furthermore, the combination of SIRT1 activators and DOT1L inhibitors shows enhanced antiproliferative activity against MLL-rearranged leukemia cells. These results indicate that the dynamic interplay between chromatin regulators controlling the activation and repression of gene expression could provide novel opportunities for combination therapy. I will discuss our most recent studies on these mechanisms and how they work to induce inappropriate gene expression in hematopoietic stem and progenitor cells.