Elucidating the Importance of DOT1L Recruitment in MLL-AF9 Leukemia and Hematopoiesis

SIMPLE SUMMARY: MLL-rearranged leukemia, driven by MLL-fusion proteins, is an aggressive, therapy-resistant leukemia found in >60% of infant leukemia and ~10% of adult leukemia. Studies have shown that inhibiting DOT1L enzymatic activity blocks leukemogenesis. However, DOT1L is critical for various normal cellular functions, including hematopoiesis. This study aimed to show that targeting the interaction between the MLL-AF9 fusion and DOT1L would inhibit leukemogenesis while sparing non-leukemic hematopoiesis. We found that disrupting the AF9-DOT1L interaction with a single point mutation was sufficient to impair leukemogenesis. We also demonstrate that genetic interventions that result in loss of DOT1L enzymatic activity in non-leukemic cells rapidly depletes hematopoietic stem and progenitor cells within 7–10 days; however, hematopoiesis was preserved when the AF9-DOT1L interaction was disrupted, leaving the enzymatic function intact. These studies are a proof of concept demonstrating the potential therapeutic advantage of inhibiting the AF9-DOT1L interaction and disrupting the integrity of the MLL-fusion complex. ABSTRACT: MLL1 (KMT2a) gene rearrangements underlie the pathogenesis of aggressive MLL-driven acute leukemia. AF9, one of the most common MLL-fusion partners, recruits the histone H3K79 methyltransferase DOT1L to MLL target genes, constitutively activating transcription of pro-leukemic targets. DOT1L has emerged as a therapeutic target in patients with MLL-driven leukemia. However, global DOT1L enzymatic inhibition may lead to off-target toxicities in non-leukemic cells that could decrease the therapeutic index of DOT1L inhibitors. To bypass this problem, we developed a novel approach targeting specific protein-protein interactions (PPIs) that mediate DOT1L recruitment to MLL target genes, and compared the effects of enzymatic and PPIs inhibition on leukemic and non-leukemic hematopoiesis. MLL-AF9 cell lines were engineered to carry mutant DOT1L constructs with a defective AF9 interaction site or lacking enzymatic activity. In cell lines expressing a DOT1L mutant with defective AF9 binding, we observed complete disruption of DOT1L recruitment to critical target genes and inhibition of leukemic cell growth. To evaluate the overall impact of DOT1L loss in non-leukemic hematopoiesis, we first assessed the impact of acute Dot1l inactivation in adult mouse bone marrow. We observed a rapid reduction in myeloid progenitor cell numbers within 7 days, followed by a loss of long-term hematopoietic stem cells. Furthermore, WT and PPI-deficient DOT1L mutants but not an enzymatically inactive DOT1L mutant were able to rescue sustained hematopoiesis. These data show that the AF9-DOT1L interaction is dispensable in non-leukemic hematopoiesis. Our findings support targeting of the MLL-AF9–DOT1L interaction as a promising therapeutic strategy that is selectively toxic to MLL-driven leukemic cells.