5-Hydroxymethylcytosine and disease.

Epigenetics is the study of inherited changes in phenotype or gene expression that do not alter DNA sequence. Recently, scientists have focused their attention on 5-hydroxymethylcytosine (5hmC), a newly discovered epigenetic marker, also known as sixth DNA base of the genome. In mammals, this novel epigenetic marker is derived from 5-methylcytosine (5mC) in a process catalyzed by ten-eleven translocation (TET) enzymes. Although 5hmC has only been subjected to study for a short while, a great deal of data has been accumulated regarding its generation, distribution, demethylation, function, and disease implications. All this information suggested that 5hmC acts not only as an intermediate in the DNA demethylation process but also as an independent epigenetic marker, playing an important role in the regulation of gene expression. This review focuses on recent progress in the study of the relationship between 5hmC and human diseases, such as cancer and Rett syndrome (RTT).