Methyl DNA Phosphate Adduct Formation in Lung Tumor Tissue and Adjacent Normal Tissue of Lung Cancer Patients.

The formation of methyl DNA adducts is a critical step in carcinogenesis initiated by exposure to methylating carcinogens. Methyl DNA phosphate adducts, formed by methylation of the oxygen atoms of the DNA phosphate backbone, have been detected in animals treated with methylating carcinogens. However, detection of these adducts in human tissues has not been reported. We developed an ultrasensitive liquid chromatography-nanoelectrospray ionization-high resolution tandem mass spectrometry method for detecting methyl DNA phosphate adducts. Using 50 micrograms of human lung DNA, a limit of quantitation of 2 adducts/1010 nucleobases was achieved. Twenty-two structurally unique methyl DNA phosphate adducts were detected in human lung DNA. The adduct levels were measured in both tumor and adjacent normal tissues from 30 lung cancer patients, including 13 current smokers and 17 current nonsmokers, as confirmed by measurements of urinary cotinine and NNAL. Levels of total methyl DNA phosphate adducts in normal lung tissues were higher in smokers than nonsmokers, with an average of 13 and 8 adducts/109 nucleobases, respectively. Methyl DNA phosphate adducts were also detected in lung tissues from untreated rats with steady state levels of 5-7 adducts/109 nucleobases over a period of 70 weeks. This is the first study to report detection of methyl DNA phosphate adducts in human lung tissues. The results provide new insights towards using these DNA adducts as potential biomarkers to study human exposure to environmental methylating carcinogens.