Abstract DNA methyltransferase (DNMT) inhibitors are FDA-approved for various hematological malignancies but have limited efficacy in solid tumors. DNA hypomethylation with these drugs is associated with elevated lysine 27 tri-methylation on histone H3 (H3K27me3). We hypothesized that this EZH2-dependent repressive mark limits the full potential of DNMT inhibition. Here, we show in cell line and tumoroid models of colorectal cancer, that low-dose DNMT inhibition sensitizes cells to selective EZH2 inhibitors that have limited single agent toxicity, and that EZH2 inhibition enhances DNMT inhibitor-driven molecular and therapeutic effects. Through integrative epigenomic analyses, we reveal that DNMT inhibition induces H3K27me3 accumulation at genomic regions poised with EZH2. Unexpectedly, combined treatment alters the epigenome landscape to promote transcriptional upregulation of the calcium-calcineurin-NFAT signaling pathway. Blocking this pathway limits the transcriptional activating effects of the drug combination, including expression of transposable elements and innate immune response genes within a viral defense pathway. Consistently, we demonstrate positive correlations between DNMT inhibitor- and innate immune response-associated transcription profiles and calcium signal activation in primary human colon cancer specimens. Collectively, our study demonstrates that compensatory EZH2 activity following DNA hypomethylation presents a barrier to the therapeutic action of DNMT inhibition in colon cancer, reveals a new application of EZH2 inhibitors beyond cancers associated with PRC2 hyperactivity, and links calcium-calcineurin-NFAT signaling to epigenetic therapy-induced viral mimicry. Highlights Select EZH2 inhibitors enhance the transcriptional activating and antiproliferative effects of DNA hypomethylating agents in colon cancer cells. The mechanism involves blockade of H3K27me3 accumulation in regions of the genome poised for PRC2 activity. DNMT inhibitor + EZH2 inhibitor treatment transcriptionally upregulates calcium-calcineurin- NFAT signaling, and this pathway is necessary for complete induction of viral mimicry and innate immune response pathways. The therapeutic utility of EZH2 inhibitors may be extended beyond cancers with PRC2 hyperactivity in combination regimens with DNMT inhibitors.

Select EZH2 inhibitors enhance the viral mimicry effects of DNMT inhibition through a mechanism involving calcium-calcineurin-NFAT signaling