Abstract
DOT1L maintains MLL fusion target gene expression by antagonizing SIRT1-mediated silencing.
Major finding: DOT1L maintains MLL fusion target gene expression by antagonizing SIRT1-mediated silencing.
Mechanism: SIRT1- and SUV39H1-mediated modulation of H3K9 blocks access to and suppresses leukemic genes.
Impact: Combination therapy with DOT1L inhibitors and SIRT1 activators may have clinical significance.
The initiation and maintenance of leukemias harboring rearrangements of the mixed-lineage leukemia (MLL) gene are dependent on the histone H3 lysine 79 methyltransferase DOT1L. Small-molecule DOT1L inhibitors are currently in clinical trials for MLL-rearranged leukemia; however, the mechanism by which DOT1L maintains MLL fusion–driven expression of leukemic genes remains unclear. Chen and colleagues conducted a genome-scale RNAi screen in mouse MLL–AF9 leukemia cells before and after inactivation of Dot1l in order to identify negative regulators of DOT1L. Analysis of shRNAs enriched after Dot1l deletion highlighted the histone deacetylase gene sirtuin 1 (Sirt1) as necessary for suppression of MLL–AF9-induced leukemic gene expression upon DOT1L inhibition. Suppression of SIRT1 alone did not alter the proliferation of MLL–AF9 cells, but blocked the antiproliferative effects of a DOT1L inhibitor. The ability of SIRT1 to silence gene expression in MLL cells was mediated by epigenetic modulation of H3 lysine 9 (H3K9); SIRT1 was recruited to actively transcribed genes in response to DOT1L inhibition and preferentially promoted H3K9 deacetylation at the promoters of MLL–AF9 target genes, which exhibited a unique H3K9 epigenomic signature. In addition, SIRT1 expression was required for the localization of the H3K9 methyltransferase suppressor of variegation 3-9 homolog 1 (SUV39H1) to MLL–AF9-driven leukemic genes following DOT1L inhibition, resulting in H3K9 dimethylation and decreased chromatin accessibility at these loci. Similar to the effects of SIRT1 suppression, depletion of SUV39H1 sustained leukemic gene expression and desensitized leukemic cells to DOT1L inhibition. Importantly, combined treatment with a DOT1L inhibitor and a SIRT1 agonist decreased expression of MLL–AF9 target genes and impaired colony-forming potential in vitro, and resulted in delayed leukemia onset and prolonged survival in mice. These findings identify the mechanism underlying the dependence of MLL-rearranged leukemia on DOT1L and suggest a therapeutic strategy to enhance the efficacy of DOT1L inhibitors.