Lung cancer: From single-gene methylation to methylome profiling
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DNA methylation as part of the epigenetic gene-silencing complex is a universal occurring change in lung cancer. Numerous studies investigated methylation of specific genes in primary tumors, in serum or plasma samples, and in specimens from the aerodigestive tract epithelium of lung cancer patients. In most studies, single genes or small numbers of genes were analyzed. Moreover, it has been observed that methylation of certain genes can already be detected in samples from the upper aerodigestive tract epithelium of cancer-free heavy smokers. These findings indicated that methylation of certain genes may be a useful biomarker for prognosis, disease recurrence, early detection, and lung cancer risk assessment. So far, several genes were identified which seem to be of worse prognostic relevance when they were found to be methylated. In addition, it has been shown that a panel of markers may be relevant to predict disease recurrence after surgery. In comparison to analysis of single or small numbers of genes, methods for genome-wide detection of methylation were developed recently. These approaches are focused on either pharmacological re-activation of methylated genes followed by expression microarray analysis or on microarray analysis of sodium bisulfite-treated or affinity-enriched methylated DNA sequences. With currently available methods for the simultaneous detection of methylation, up to 28,000 CpG islands can be analyzed. Overall, we are just at the beginning of translating these findings into the clinic and there is hope that future patients will benefit from these results.