Synergistic antitumor effect of combined EZH2 and DOT1L inhibition in B-cell lymphoma

Despite the approval of several new treatments for patients with B-cell lymphoma, there is still a large unmet need. Diffuse large B-cell lymphoma (DLBCL) and Follicular lymphoma (FLs) are the two most common B-cell lymphoma subtypes, accounting for approximately 50% of all cases. EZH2 heterozygous gain-of-function somatic driver mutations are frequently found in Germinal Center B cell (GCB) DLBCLs and FLs. An EZH2 inhibitor has shown durable responses in patients with relapsed or refractory FL, however a considerable fraction of the patients did not show an objective response. To identify alternative therapeutic strategies, we performed CRISPR/Cas9 knockout screens in B-cell lymphoma cells treated with or without an EZH2 inhibitor. This led to the identification of the histone methyltransferase DOT1L as a potential therapeutic target. Specifically, we showed that an EZH2 inhibitor synergizes with a DOT1L inhibitor in a panel of B-cell lymphoma cell lines, regardless of EZH2 mutation status. Mechanistically, we demonstrated that the two inhibitors cooperatively suppress DOT1L-regulated cell cycle genes, up-regulate genes involved in interferon signaling including antigen presenting genes and ultimately drive B cell differentiation by de-repressing EZH2-regulated plasma cell signature genes. Furthermore, we demonstrated the effectiveness of this epigenetic combination strategy in a xenograft model, which resulted in significant abrogation of tumor growth. Together, our studies provide pre-clinical proof-of-concept for an epigenetic combination therapy to overcome resistance and improve durability of response for the treatment of B-cell lymphoma, warranting clinical investigation and illustrating an important convergent role of EZH2 and DOT1L in B cell lymphomagenesis.