Phosphorus and osmium as elemental tags for the determination of global DNA methylation—A novel application of high performance liquid chromatography inductively coupled plasma mass spectrometry in epigenetic studies

The hyphenation of high performance liquid chromatography with inductively coupled plasma mass spectrometry (HPLC–ICP-MS) is proposed in this work as a novel approach for the evaluation of DNA methylation, defined as the ratio between methylated cytosine and total cytosine bases in DNA. In the first part, reversed phase separation of 5-methyl-2′-deoxycytidine monophosphate (5mdCMP) and four deoxynucleotides with specific ICP-MS detection on 31P had been explored. In further development, selective labeling of 5-methylcytosine in ssDNA was carried out using potassium osmate (K2OsO4) in the presence of strong oxidant (K3Fe(CN)6) and N,N,N′,N′-tetramethylethylenediamine (TEMED). The sample was then cleaned up and introduced to size exclusion chromatography–ICP-MS for specific detection at 31P and 189Os and for evaluation of the molar ratio between Os and P eluted in DNA molecular mass fraction. The quantification of the two elemental tags was achieved by external calibration with phosphoric acid and Os(VI)-TEMED, respectively. The amount of Os in DNA fraction corresponded to methylated cytosines, while P signal was directly proportional to the total amount of DNA and could be recalculated to the amount of cytosine bases. The two procedures were tested by analyzing salmon testes DNA and a commercial oligonucleotide of known composition. For comparative purposes, these same samples were digested to deoxynucleosides and analyzed by reversed phase HPLC with spectrophotometric detection (DAD) at 280 nm. The results obtained using two procedures based on ICP-MS detection were in good agreement and also in agreement with the results obtained by HPLC–DAD procedure. In conclusion, ICP-MS specific detection at internal or external element tags seems to be an interesting alternative for the evaluation of global DNA in epigenetic studies.