PWWP-ADD and N-terminal domains of DNMT3B1 confer specificity for developmentally regulated CpG island methylation

CpG methylation in mammalian genomes is established by the closely related de novo DNA methyltransferases DNMT3A and DNMT3B. Whilst both enzymes contribute to pervasive genome-wide CpG methylation, DNMT3B has a unique role in developmentally regulated CpG island (CGI) methylation both on the inactive X chromosome and at other sites in the genome. The mechanistic basis for this specificity is poorly understood. Here we have developed an in vitro embryonic stem cell model system to dissect critical determinants of DNMT3B specificity. Our model faithfully recapitulates developmentally regulated CGI methylation and additionally provides novel insights into CpG methylation at cis-regulatory elements. Using genetic complementation, we show that DNMT3B specificity is attributable solely to the catalytic isoform DNMT3B1. Domain swap experiments demonstrate an important role both for the PWWP-ADD chromatin binding and unstructured N-terminal domains. Together, these findings advance our mechanistic understanding of the unique roles of DNMT enzymes in establishing CpG methylation in development.