Mammalian DNA contains two modified cytosine bases; 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC). Both of these have been known for decades but have received very different levels of attention in the scientific literature. 5mC has been studied extensively, and its role as an epigenetic modification involved in gene regulation, X-chromosome inactivation, genomic imprinting, long-term silencing of transposons and cancer development is well described. 5hmC, on the other hand, has only recently entered center stage when it was shown that the Ten-Eleven-Translocation (TET) family of oxygenases catalyzes the conversion of 5mC to 5hmC, and that one of these enzymes, TET2, is frequently mutated in myeloid neoplasms. The formation of 5hmC can lead to demethylation of DNA, which may contribute to the dynamics of DNA methylation. 5hmC has been found in many cell types and tissues, with particularly high levels in the brain, and TET1 has been shown to be important for self-renewal and maintenance of embryonic stem cells. Future challenges include better understanding the normal molecular, cellular and physiological roles of 5hmC and TET proteins, understanding the exact roles of TET proteins in cancer development, and developing sequencing methodologies that can accurately distinguish among cytosine, 5mC and 5hmC at single-base-pair resolution.
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