Targeting DOT1L Epigenetic Moonlighting in MLL-Rearranged Leukemia

KMT2A-rearranged (MLL-r) leukemias are highly aggressive hematological malignancies that require improved targeted therapies. DOT1L (histone H3K79 methyltransferase) functions as a critical oncogenic driver and represents an important therapeutic target in these high-risk leukemias. However, clinical responses to the first-in-class DOT1L inhibitor pinometostat (EPZ5676) have been modest, attributed to suboptimal pharmacokinetics and, more fundamentally, to the recognition that DOT1L possesses methyltransferase-independent functions that evade catalytic inhibition. This highlights the need for strategies that abrogate the full spectrum of DOT1L activity to effectively treat these high-risk leukemias. Proteolysis-targeting chimeras (PROTACs), which induce selective degradation of the DOT1L protein rather than inhibiting its catalytic activity, have therefore emerged as a promising approach. Notably, VHL-recruiting DOT1L PROTACs, such as DOT1L808, have demonstrated improved pharmacokinetic profiles and potent antileukemic activity in preclinical in vivo models. However, these findings remain preclinical, and significant challenges including oral bioavailability, potential toxicity, and lack of clinical validation must be addressed before clinical translation. In this review, we provide an overview of the evolving understanding of the biology of DOT1L, discuss existing MLL small molecule therapies, and evaluate current advances in therapeutically targeting DOT1L, with particular focus on the targeted degradation of DOT1L as a promising therapeutic strategy for high-risk KMT2A-r leukemia.