Cytosine methylation at carbon-5 (5mC) in DNA plays crucial roles in epigenetic transcriptional regulation during metazoan development. The iron (II), 2-oxoglutarate-dependent Ten-Eleven Translocation (TET)-family dioxygenases initiate active demethylation of 5mC. TET2 oxidizes 5mC in nucleic acids into 5-hydroxymethylcytosine, 5-formylcytosine, and 5-carboxylcytosine by iterative oxidation. Mutations in the TET2 gene are frequently detected in myeloid malignancies. Despite the established and emerging roles of TET oxygenases in health and diseases, in vitro characterization of these enzymes and their mutants is still in rudimentary stages. Here, we describe an improved positive/negative ion-switching-based liquid chromatography-tandem mass spectrometry (LC–MS/MS) method that can separate and quantify modified cytosine bases produced by TET-family 5-methylcytosine dioxygenases. This method will help in further elucidate the function of epigenetically important cytosine modifications. To the best of our knowledge, this is the first study reporting ion-switching-based LC–MS/MS method to analyse cytosine variants produced in TET catalysed reactions.

Efficient Purification And Lc-ms/ms-based Assay Development For Ten-eleven Translocation-2 5-methylcytosine Dioxygenase. JoVE publishes peer-reviewed scientific video protocols to accelerate biological, medical, chemical and physical research. Watch our scientific video articles. JoVE is the world-leading producer and provider of science videos with the mission to improve scientific research, scientific journals, and education. Millions of scientists, educators and students at thousands of universities, colleges, hospitals and biopharmaceutical companies worldwide use JoVE for their research, teaching and learning.

In the mammalian genome, cytosine methylation predominantly occurs at CpG sites. In addition, a number of recent studies have uncovered extensive C5 cytosine methylation (5mC) at non-CpG (5mCpH, where H = A/C/T) sites. Little is known about the enzyme responsible for active demethylation of 5mCpH sites. Using a very sensitive and quantitative LC–MS/MS method, we demonstrate that the human TET2, an iron (II)- and 2OG-dependent dioxygenase, which is a frequently mutated gene in several myeloid malignancies, as well as in a number of other types of cancers, can oxidize 5mCpH sites in double-stranded DNA in vitro. Similar to oxidation of 5mCpG, oxidation of 5mC at CpH sites produces 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC), and 5-carboxycytosine (5caC) bases in DNA. After 5mCpG, which is the most preferred substrate, TET2 prefers 5mCpC as a substrate, followed by 5mCpA and then 5mCpT. Since the TDG/BER pathway and deformylation or decarboxylation of 5fC or 5caC, respectively, can convert 5fCpH and 5caCpH to an unmodified cytosine base in DNA, our results suggest a novel demethylation pathway of 5mCpH sites initiated by TET2 dioxygenase.

Three forms of methylated cytosines are present in the eukaryotic genome: 3-methylcytosine, 4-methylcytosine and 5-methylcytosine. 3-methylcytosines create methyl lesions, which impair local DNA function and flexibility, resulting in replication and transcription error. On the other hand, 5-methylcytosine is usually present at the gene promoter which blocks transcription and translation. Fe(II)/2OG-dependent nucleic acid-modifying enzymes are the class of enzymes responsible for the demethylation of these modified cytosines. ALKBH2 and 3 remove 3-methylcytosine via a one-step direct demethylation process. On the other hand, active demethylation of 5mC is initiated by Ten-Eleven Translocation (TET)-family dioxygenases. Via oxidative demethylation, TET1-3 converts 5mC into 5-hydroxymethylcytosine, 5-formylcytosine and 5-carboxylcytosine. Remarkably, recent findings demonstrate that ALKBH2,3 possess oxidative demethylation properties, along with direct demethylation. On the other hand, the TET family of enzymes possess direct demethylation properties along with oxidative demethylation. Here we review the importance of methylated cytosines in human DNA, their origin, function and removal. In addition, we discuss the recent findings of extraordinary flexibility of Fe(II)/2OG-dependent nucleic acid-modifying enzymes ALKBH2,3 and TET family of enzymes in cytosine demethylation, as well as their impact on epigenetics.