Biomarkers of environmental exposure: genetic and epigenetic approaches

Abstract

Exposure assessment in cancer epidemiological studies relies on measurable intermediate molecular biomarkers with high sensitivity and specificity in order to prevent common problems due to misclassification of exposure. Studies on the early stages of carcinogenesis have helped to identify molecular changes that are detectable in pre-cancerous lesions and that are thought to occur as the result of specific exposures such as tobacco smoking. More recently, in vitro evidence started to support the potential cancer-protective role of various micronutrients acting through epigenetic and genetic mechanisms. Somatic mutations in “master” cancer genes and modifications of epigenetic patterns in the promoter region of specific genes involved in cell cycle, apoptosis or DNA repair may prove good candidates of carcinogenic and dietary exposure even if the evidence that these changes may be present and detectable in “normal” tissue are still scarce (due in part to the practical and ethical difficulty to conduct experimental prospective studies in healthy individuals). In this thesis, I have developed two projects exploring the application of TP53, KRAS, EGFR mutations and of DNA methylation changes as biomarkers of exposure to tobacco smoking, in experimental and observational study designs. Somatic mutations were analysed by dHPLC, ME-PCR, RFLP and sequencing and DNA-methylation analysis was performed by pyrosequencing. Moreover, somatic mutations were analysed in a prospective context of lung cancer recurrence; also the capacity of dietary polyphenols and isothiocyanates to modify methylation patterns in smokers was assessed in an intervention trial. The results show that somatic mutations are good markers of different forms of tobacco-related lung cancers but have limited short-term prognostic value, with the exception of KRAS mutations in adenocarcinoma. Methylation data suggested that a specific short-term dietary intervention may stabilize global epigenetic (LINE1 DNA methylation) patterns in peripheral white blood cells.