Tissue-specific methylomes reveal epigenetic memory in adult mouse tissue

Cytosine methylation of DNA is an evolutionarily conserved mechanism from plants to animals with crucial roles in gene regulation. However, the variation between methylomes of normal tissues is largely unexplored. To better understand the epigenetic variation of a normal individual, we profiled DNA methylation using whole genome bisulfite sequencing in 17 tissues isolated from an individual mouse. We observed a unique distribution of CpG methylation for each tissue, which cluster based on cell lineage. Global analysis identified only one-eighth of the genome as tissue-specifically methylated. Remarkably, the vast majority of these regions exhibit hallmarks of cis-regulatory activity. Our results also reveal a novel class of dormant enhancers in adult tissues which retain an epigenetic memory of regulatory elements active during development. Together, these results expand the repertoire of regulatory information encoded within the methylome, and suggest mapping it as an alternative method to identify cell-type specific regulatory elements. Overall design: whole genome bisulfite sequencing of mouse adult tissue