Genetics; didn’t it seem so simple at school many years ago? Watson/Crick, Mendelian Inheritance, and the four nucleotides of the genetic code (adenine, thymine, guanine and cytosine). However, then came the human genome project, complex multi-gene/environment disorders such as cardiovascular disease and the emerging field of epigenetics.

Epigenetics is defined as the inheritance of genetic traits due to mechanisms other than changes in DNA sequence. One of the best described mechanisms is the methylation of cytosines to form 5-methylcytosine. Methylation typically occurs in CG nucleotide rich areas or “CpG” islands, which are found to occur in particular around the transcriptional start sites of many genes. Alteration in the methylation status of genes has been demonstrated to occur in a range of disease states, including numerous cancers. Studies of cancer related genes have revealed that gene hypermethylation has been associated with transcriptional inactivation in cancer. Therefore methylation status represents not only a potential diagnostic tool, but only a mechanism to target for therapeutic intervention.

In a recent online published article in Science magazine, two researchers from Rockefeller University have identified another form of methylated cytosine in mammals; 5-hydroxymethylcytosine

5-hydroxymethylcytosine has been previously identified in bacteriophage and shown to play a role in infectivity

The authors identified the nucleotide modification by chance when comparing different murine neuronal cell types. Using TLC, HPLC and MS, the authors found that around 40% of methylated cytosine in the large nucleated Purkinje cells were of the 5-hydroxymethylcytosine form. Anyone who has previously studied methylated DNA will be familiar with the method of bisulfite treatment which deaminates cytosine residues to uracil, whereas 5-methylcytosine remains unmodified. Subsequent analysis by PCR or sequencing can reveal these changes and hence the methylation status of a gene region. In particular this method is being used by the Human Epigenome Project, which is a large scale collaboration to identify and catalogue Methylation regions in the human genome. The significance of the current study is that bisulfite treatment is unable to differentiate between 5-methylcytosine or 5-hydroxymethylcytosine.

The authors speculate that this may too regulate gene expression, but perhaps in a different manner. The discovery of mammalian enzymes that convert 5-methylcytosine to 5-hydroxymethylcytosine, further suggests a regulatory role

So what is the biological role of 5-hydroxymethylcytosine? Does this have a neuronal function alone? What is its relevance in gene regulation? Does this exist in humans? Answers to these questions should provide further insight into the fascinating world of epigenetic regulation.