DNA hypomethylation promotes UHRF1- and SUV39H1/H2-dependent crosstalk between H3K18ub and H3K9me3 to reinforce heterochromatin states [RNA-Seq]

Mono-ubiquitination of lysine 18 on histone H3 (H3K18ub), catalyzed by UHRF1, is a DNMT1 docking site that facilitates replication-coupled DNA methylation maintenance. Its functions beyond this are unknown. Here, we genomically map simultaneous increases in UHRF1-dependent H3K18ub and SUV39H1/H2-dependent H3K9me3 following DNMT1 inhibition. Mechanistically, transient accumulation of hemi-methylated DNA at CpG islands facilitates UHRF1 recruitment and E3 ligase activity towards H3K18. Notably, H3K18ub enhances SUV39H1/H2 methyltransferase activity and, in colon cancer cells, nucleates new H3K9me3 domains at CpG island promoters of DNA methylation-silenced tumor suppressor genes. Disrupting UHRF1 enzyme activity prevents H3K9me3 accumulation while promoting PRC2-dependent H3K27me3 as a tertiary layer of gene repression in these regions. In contrast, disrupting H3K18ub-dependent SUV39H1/H2 activity enhances the transcriptional activating and antiproliferative effects of DNMT1 inhibition. Collectively, these findings reveal roles for UHRF1 and H3K18ub in regulating a hierarchy of repressive histone methylation signaling and rationalize a combination strategy for epigenetic cancer therapy. Overall design: RNA-seq in RKO cells with SUV39H2 KO and dox-inducible SUV39H1 shRNA KD. There are four treatment groups: cells were treated with 1) DMSO control, 2) 20ng/mL doxycycline (SUV39H1 KD), 3) 1uM GSK3484862(DNMT1 inhibition), and 4) doxycycline+GSK3484862 (SUV39H1 KD + DNMT1 inhibition) for 5 days before RNA collection.