Protracted NP95 binding to hemimethylated DNA disrupts SETDB1-mediated proviral silencing [ChIP-seq]. Mus musculus

Genetic ablation of the maintenance methyltransferase Dnmt1 induces widespread demethylation and transcriptional activation of CpG-rich IAP (intracisternal A particle) proviruses. Here, we report that this phenomenon is not simply a consequence of loss of DNA methylation. By exploiting conditional deletions of Dnmt1 and Np95, each of which is essential for maintenance methylation, we find that while IAPs are indeed de-repressed in Dnmt1-ablated embryos and embryonic stem cells (ESCs), these proviruses remain silenced in Np95-ablated cells, despite similar kinetics of passive demethylation. Paradoxically, transient IAP activation in Dnmt1-ablated ESCs requires the presence of NP95. We subsequently show that in the absence of NP95, the H3K9 methyltransferase SETDB1 maintains IAP silencing; while in the absence of DNMT1, prolonged binding of NP95 to hemimethylated DNA perturbs SETDB1-dependent H3K9me3 deposition. Taken together, these observations reveal that following acute loss of Dnmt1, H3K9 methylation-dependent IAP silencing is disrupted by aberrant NP95 binding to hemimethylated DNA. Overall design: RNA-seq for Np95, Dnmt1 and Setdb1 wt, single conditional KO (cKO) and double cKO ES cells; RRBS-seq for Dnmt1 and Np95 single and double cKO ESCs; Myc-tagged NP95, DNMT1 ChIP-seq; and wt and Np95wt and cKO H3K9me3 ChIP-seq.