In studying the genetic basis of complex diseases, it is customary to think that the coding potential of the genome only lies within the arrangement of the four nucleotide bases (adenine, cytosine, guanine and thymine); however additional information that affects phenotype is encoded in the distribution of the modified base 5-methylcytosine. DNA methylation in mammalian cells occurs at the 5' position of cytosine within CpG dinucleotides. This form of information is flexible enough to be adapted for different somatic cell types, yet is stable enough to be retained during mitosis and/or meiosis. Indeed, DNA methylation plays a role in the protection against intragenomic parasites, genomic imprinting, X chromosome inactivation in females, and epigenetic human diseases including cancer.
Advances in experimental and computational technology have revolutionized the field of epigenetics. We are now in the position to investigate the epigenomics of complex human diseases in a systematic genome-wide fashion.