Contributions of DNA methylation aberrancies in shaping the cancer epigenome

DNA methylation is an important gene expression regulator, and serves as a guiding force for X chromosome inactivation, cellular differentiation and development, genomic imprinting and the suppression of repetitive elements. In mammalian cells, DNA methylation is mainly restricted to the C-5 position of cytosine in the 5'-CG-3', or CpG, sequence context, and are stable with successive rounds of cell division [reviewed in (1,2)]. DNA methylation in non-CpG regions, CpA, CpC or CpT (generically labeled as CpH), is present in embryonic stem (ES) cells (3). DNA methylation profiles are erased during in embryonic development, and then are re-established as cells develop towards the differentiated, somatic state [reviewed in (4)]