Hypomethylation of DNA
DNA hypomethylation refers to the loss of the methyl group in the 5-methylcytosine nucleotide. Methylation is a natural modification of DNA and mainly affects the cytosine base (C) when it is followed by a guanosine (G) in mammals (methylation). The term hypomethylation can be applied to describe the unmethylated state of most CpG sites in a specific sequence that is normally methylated, or as a general phenomenon affecting the bulk of the genome; this is a decrease in the proportion of methylated versus unmethylated cytosines.
In human, DNA methylation mainly occurs at CpG sites. Up to 80 % of all CpG sites in human DNA are methylated. However, this methylation occurs primarily in areas where CpG density is low or at repeat DNA sites, such as Alu elements. CpG islands are regions where CpG density is high and most of them are unmethylated. Patterns of DNA methylation have been linked to control of gene...