Distinct roles of DNMT1-dependent and –independent methylation patterns in the genome of mouse ES cells

We generated base-resolution DNA methylomes of a series of DNMT knockout (KO) ES cells with improved coverage at highly repetitive elements. We find that DNMT1- and DNMT3a/3b-dependent activities actually work complementarily and simultaneously to establish symmetric CG methylation and CHH (H=A, T or C) methylation, and unexpectedly have “division of labor” to suppress retrotransposon long terminal repeats (LTRs) and long interspersed elements (LINEs), respectively. Our data also reveal CG density number of 30 seems like a 'threshold' to predetermine the level of methylation in wild type cells and the magnitude of methylation reduction in KO cells. Only genes with low CG number are either induced or surprisingly suppressed in hypomethylated genome. Our data unveil the concerted action of DNMT enzymes in the establishment/maintenance of methylation patterns. Genomic DNA from four different cell lines were treated with bisulfite and sequenced with the Illumina HiSeq platform using paired end reads.