Hydrophilic Material for the Selective Enrichment of 5-Hydroxymethylcytosine and Its Liquid Chromatography–Tandem Mass Spectrometry Detection

5-Methylcytosine (5-mC), an important epigenetic modification involved in development, can be converted enzymatically to 5-hydroxymethylcytosine (5-hmC). 5-hmC is considered an intermediate of active DNA cytosine demethylation and makes itself serve as an epigenetic mark. 5-hmC content in most mammalian cells is low and the quantification of 5-hmC by liquid chromatography–mass spectrometry (LC–MS) frequently suffers from ion suppression by the presence of unmodified nucleosides. To circumvent this problem, we developed a method to selectively transfer a glucosyl group to the hydroxymethyl moiety of 5-hmC and form a more hydrophilic residue (β-glucosyl-5-hydroxymethyl-2′-deoxycytidine, 5-gmdC) by using T4 β-glucosyltransferase. The more hydrophilic 5-gmdC can be selectively enriched by using NH2-silica via hydrophilic interaction prior to liquid chromatography–tandem mass spectrometry (LC–MS/MS) analysis, which eliminates the ion suppression and significantly improves the detection sensitivity and accuracy. Using this method, we successfully quantified 5-hmC content in genomic DNA of three human cell lines and seven yeast strains. To the best of our knowledge, this is the first report about the existence of 5-hmC in the model organism of yeast. In addition, the contents of 5-hmC in two yeast strains of Schizosaccharomyces pombe are even higher than those of 5-mC, indicating that 5-hmC may play important roles on the physiological functions of yeast.