In human DNA, 5-methylcytosine is found in approximately 1.5% of genomic DNA. In somatic cells, 5-mC occurs almost exclusively in the context of paired symmetrical methylation of a CpG site. An exception to this is seen in embryonic stem (ES) cells, where a substantial amount of 5-mC is also observed in non-CpG contexts. In the bulk of genomic DNA, most CpG sites are heavily methylated while CpG islands (sites of CpG clusters) in germ-line tissues and located near promoters of normal somatic cells, remain unmethylated, thus allowing gene expression to occur. Furthermore, the position of methylation within a transcriptional region can influence the level of gene control (i.e, gene silencing, activation, chromatin stability, etc.). Thus, a full understanding of the function of DNA methylation requires methylation analysis of the entire genome (Jones, 2012). To explore epigenetic variation (of methylation patterns) in normal tissues, researchers at the University of California, San Diego Medical School and the Ludwig Institute for Cancer Research in California performed whole-genome bisulfite sequencing of 17 different tissues spanning all 3 germ layers and extraembryonic placenta derived from a single pregnant female mouse. By mapping base-resolution methylomes (the methylated genome) they were able to characterize the biological roles of tissue-specific differentially methylated regions (tsDMRs).
Their findings are summarized below:
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