Global DNA Methylation: a Common Early Event in Oral Cancer Cases with Exposure to Environmental Carcinogens or Viral Agents

Abstract

Introduction: Two separate molecular pathways have been proposed for the early carcinogenic events observed in the oral cavity and pharynx: one is associated with chemical etiological factors such as smoking and drinking, and the other one is associated with HPV insertion. Objective: A proof-of-principle study was performed to ascertain if global DNA methylation could be used to distinguish between the early molecular changes in premalignant oral lesions. Methods: Personal histories of tobacco and alcohol use were obtained by questionnaire. HPV insertion in tumor tissue was detected by polymerase chain reaction (PCR). Global DNA methylation levels were obtained using HPLC for fraction separation and mass spectrometry for quantification. Predictive simulations were performed to explore potential associations between different etiological factors and the global DNA methylation index. Significance of results was ascertained using Pearson’s Chi-squared test. Results: The global methylation index was found to be 4.28 (95% CI, 4.1, 4.4) in an oral cancer case series. Pearson’s chi squared test showed no statistically significant difference between cases that had smoking (p=0.21), drinking (p=0.31) or HPV insertion (p=0.34) as etiologic risk factors, when compared to cases that did not. An inverse significant association between smoking and DNA methylation was observed. As the smoking effect increases, the global methylation index decreases. In addition, no associations between the probability of DNA methylation and drinking, or DNA methylation and HPV insertion were observed in simulations. Conclusions: The global DNA methylation index was shown to vary for oral cancer cases with different etiologies. Smoking was inversely correlated with DNA methylation levels when generalized linear model simulations were performed. Future studies should look at global DNA methylation alterations associated to the progression from normal to premalignant oral epithelium tissue in a cohort of smokers and nonsmokers.