Targeting MYC-Driven B-Cell Lymphoma By Inhibition of the Histone Methyltransferase DOT1L

Aberrant activation of the MYC proto-oncogene is a recurrent feature in human B-cell lymphomas of diverse sub-types, correlating with adverse prognosis and therapy resistance. Direct pharmacological MYC-targeting has proved difficult, but recent studies have shown that targeting chromatin regulators critical for MYC-driven oncogenesis may provide alternative avenues for therapeutic intervention. Recently, it has been demonstrated that MYC-driven oncogenesis in certain solid tumors is dependent on the histone 3 lysine 79 (H3K79) methyltransferase DOT1L. We hypothesized that B-cell lymphomas with hyperactive MYC-signaling might be responsive to DOT1L inhibition. In order to test this hypothesis, we tested the effect of the DOT1L inhibitor Pinometostat (EPZ-5676) on a panel of human B-cell lymphoma cell lines featuring elevated MYC. Pinometostat treatment reduced global H3K79 methylation levels, accompanied by a time and dose-dependent decrease in proliferation of several Burkitt's lymphoma cell lines including P493-6, Daudi and Raji. We observed that key MYC-target genes including CDK4, PPAT and NPM1 were significantly downregulated upon Pinometostat treatment, suggesting that DOT1L is required for the transcriptional activation of MYC-target genes in these cells. Pinometostat-treated B-lymphoma cells showed a significant decrease of cells in S-phase compared to controls as assessed by BrdU-labeling assays. Similar results were also obtained in a panel of B-cell lymphoma cell lines with MYC-rearrangements including mantle cell lymphoma (MCL) cell lines Jeko-1, JVM2, Mino-1 and Maver-1 and the diffused large B-cell lymphoma (DLBCL) cell line Karpas 422.

Next, we sought to investigate whether the DOT1L-dependence of MYC-driven B-cell lymphoma could be reproduced in a well-defined model of MYC-driven B-cell lymphoma. Towards this end, we utilized a mouse model in which expression of the Cre recombinase from a B cell specific promoter leads to ectopic expression of a transgenic human MYC allele and concomitant deletion of the tumor suppressor Pten in B cells. Similar to our in vitro studies, Pinometostat treatment led to a significant reduction in proliferation of B-cell lymphoma cells from these mice with an IC₅₀ of 0.5 µM.

Furthermore, we sought to ascertain whether these findings reflected on-target effects related to DOT1L inhibition. Therefore, we deleted DOT1L using CRISPR/Cas9 in B-cell lymphoma cell lines and assessed the effect on proliferation using competitive-proliferation assays. We observed that DOT1L-deletion progressively diminished the relative growth of anti-DOT1L sgRNA-expressing P493-6 and Jeko1 cells compared to non-targeted cells invitro. In order to test the requirement for DOT1L in lymphoma propagation in vivo, we performed intravenous injections of equal number of Jeko-1 cells with either anti-DOT1L or anti-Renilla control sgRNAs into sub-lethally irradiated non-obese diabetic/severe combined immunodeficiency mice (NOD/SCID) mice. Mice injected with control anti-Renilla sgRNAs succumbed to disease with a median latency of 34 days while the latency of disease in the anti-DOT1L sgRNA cohort was 45 days. In summary, DOT1L depletion significantly delayed disease latency in this invivo disseminated model of B-cell lymphoma (P=0.02).

We then performed transcriptomic analyses of Pinometostat-treated B-cell lymphoma cell lines compared to DMSO-treated counterparts using RNA-seq. Gene-set enrichment analysis (GSEA) of RNA-seq data from three different B-cell lymphoma cell lines demonstrated that Pinometostat treatment significantly decreased the expression of MYC-target genes. In order to investigate the intriguing role of DOT1L in regulating MYC-target gene expression, we used ChIP-seq to assess the genome-wide occupancy of MYC following DOT1L inhibitor treatment. Strikingly, our studies demonstrated that DOT1L inhibition significantly reduced the chromatin occupancy of MYC.

Taken together, our experiments demonstrate the role of DOT1L in MYC-driven B-cell lymphoma pathogenesis invitro and invivo. Furthermore, our genome-wide studies demonstrate the importance of DOT1L for genomic MYC occupancy. Based on these findings, we propose that therapeutic DOT1L targeting may be a viable strategy in MYC-driven B-cell lymphoma.