Genome-scale DNA methylation maps of pluripotent and differentiated cells

Abstract

DNA methylation, an important mechanism of epigenetic modification that produces different patterns of gene expression from a single DNA sequence, is vital to normal development and its malfunction can cause cancer and other abnormalities. A map of DNA methylation in embryonic stem cells, and in various cell types derived from them, has now been produced at nucleotide resolution using high-throughput bisulphite sequencing combined with single molecule-based sequencing. The map reveals specific sites in the genome where methylation changes as cells develop, for instance when embryonic stem cells mature into nerve cells. More generally, the methodology will be of value for the epigenetic profiling of cell populations relevant to developmental biology, cancer and regenerative medicine. Genome-scale DNA methylation profiles have been generated at nucleotide resolution in mammalian cells using a combination of high-throughput bisulphite sequencing and single molecule-based sequencing. The DNA methylation maps cover the vast majority of CpG islands, as well as several other genomic regions, in murine embryonic stem cells, cells derived from ES cells and various primary cells. DNA methylation is essential for normal development1,2,3 and has been implicated in many pathologies including cancer4,5. Our knowledge about the genome-wide distribution of DNA methylation, how it changes during cellular differentiation and how it relates to histone methylation and other chromatin modifications in mammals remains limited. Here we report the generation and analysis of genome-scale DNA methylation profiles at nucleotide resolution in mammalian cells. Using high-throughput reduced representation bisulphite sequencing6 and single-molecule-based sequencing, we generated DNA methylation maps covering most CpG islands, and a representative sampling of conserved non-coding elements, transposons and other genomic features, for mouse embryonic stem cells, embryonic-stem-cell-derived and primary neural cells, and eight other primary tissues. Several key findings emerge from the data. First, DNA methylation patterns are better correlated with histone methylation patterns than with the underlying genome sequence context. Second, methylation of CpGs are dynamic epigenetic marks that undergo extensive changes during cellular differentiation, particularly in regulatory regions outside of core promoters. Third, analysis of embryonic-stem-cell-derived and primary cells reveals that ‘weak’ CpG islands associated with a specific set of developmentally regulated genes undergo aberrant hypermethylation during extended proliferation in vitro, in a pattern reminiscent of that reported in some primary tumours. More generally, the results establish reduced representation bisulphite sequencing as a powerful technology for epigenetic profiling of cell populations relevant to developmental biology, cancer and regenerative medicine.