DOT1L inhibition reprograms innate immunity to potentiate immunomodulatory drug responses in multiple myeloma

Immunomodulatory drugs (IMiDs), such as lenalidomide, are a cornerstone of multiple myeloma (MM) treatment; however, their efficacy remains suboptimal. We previously reported that inhibition of DOT1L, a histone H3 lysine 79 methyltransferase, upregulates interferon (IFN)-regulated genes (IRGs) and exerts anti-MM effects. In this study, we aimed to further elucidate the epigenetic dependency of DOT1L in MM and the mechanism by which its inhibition induces innate immune signaling. Analysis of DepMap portal data revealed that MM cells are preferentially dependent on DOT1L, among epigenetic regulators, for survival. DOT1L inhibition activated type I IFN responses and increased expression of human leukocyte antigen (HLA) class II genes in MM cells. Notably, DOT1L inhibition was associated with induction of DNA damage responses. CRISPR/Cas9-mediated knockout of STING1 attenuated IRG induction and diminished the anti-proliferative effects of DOT1L inhibition, suggesting that activation of STING signaling contributes to its anti-MM activity. Furthermore, DOT1L inhibition downregulated IKZF1/3 and IRF4, which was also associated with IRG induction. Finally, DOT1L inhibition enhanced the anti-MM efficacy of lenalidomide by further upregulating IRGs and suppressing IRF4-MYC signaling. These findings suggest that DOT1L is a preferential epigenetic therapeutic target in MM. Its inhibition not only activates innate immune signaling but also enhances the efficacy of lenalidomide.