Cooperation between EZH2, NSPc1-mediated histone H2A ubiquitination and Dnmt1 in HOX gene silencing

An intricate interplay between DNA methylation and polycomb-mediated gene silencing has been highlighted recently. Here we provided evidence that Nervous System Polycomb 1 (NSPc1), a BMI1 homologous polycomb protein, plays important roles in promoting H2A ubiquitination and cooperates with DNA methylation in HOX gene silencing. We showed that NSPc1 stimulates H2A ubiquitination in vivo and in vitro through direct interaction with both RING2 and H2A. RT-PCR analysis revealed that loss of NSPc1, EZH2 or DNA methyltransferase 1 (Dnmt1), or inhibition of DNA methylation in HeLa cells de-represses the expression of HOXA7. Chromatin immunoprecipitation (ChIP) assays demonstrated that NSPc1, EZH2 and Dnmt1 bind to the promoter of HOXA7, which is frequently hypermethylated in tumors. Knockdown of NSPc1 results in significant reduction of H2A ubiquitination and DNA demethylation as well as Dnmt1 dissociation in the HOXA7 promoter. Meanwhile Dnmt1 deficiency affects NSPc1 recruitment and H2A ubiquitination, whereas on both cases EZH2-mediated H3K27 trimethylation remains unaffected. When EZH2 was depleted, however, NSPc1 and Dnmt1 enrichment was abolished concomitant with local reduction of H3K27 trimethylation, H2A ubiquitination and DNA methylation. Taken together, our findings indicated that NSPc1-mediated H2A ubiquitination and DNA methylation, both being directed by EZH2, are interdependent in long-term target gene silencing within cancer cells.