Targeting UHRF1-SUV39H1/H2 crosstalk enhances DNMT1 inhibitor efficacy in colon cancer

DNA hypomethylating agent (DNMTi) efficacy is associated with the re-expression of epigenetically silenced tumor suppressor genes (TSGs) and transposable elements (TEs) in preclinical cancer models, yet their clinical efficacy in solid tumors is limited. An emerging mechanism of resistance to DNMTi involves compensation through repressive histone post-translational modifications (PTMs). Here, we define a targetable chromatin-based mechanism through UHRF1-SUV39H1/H2 crosstalk that reinforces transcriptional silencing in colon cancer cells exposed to DNMTi's. Leveraging integrative epigenomic profiling and biochemical analyses, we discovered that transient DNA hypomethylation triggers UHRF1-dependent mono-ubiquitination of lysine 18 on histone H3 (H3K18ub), which in turn stimulates SUV39H1/H2 methyltransferases to deposit H3K9me3 at CpG island promoters of DNA methylation-silenced TSGs. This UHRF1-SUV39H1/H2 crosstalk establishes new heterochromatin domains that stabilize repression despite global DNA demethylation. Disrupting UHRF1 enzymatic function or an identified H3K18ub-recognition motif in SUV39H1 prevents H3K9me3 accumulation, increases TSG re-expression, and enhances DNMTi-induced antiproliferative effects in colon cancer cells. These findings reveal a compensatory heterochromatin signaling mechanism that limits DNMTi responses and identify the UHRF1-SUV39H1 pathway as a novel therapeutic target to improve epigenetic therapy efficacy in solid tumors.