LINE-1 Hypomethylation in Noncancerous Esophageal Mucosae is Associated with Smoking History

Cancer cells exhibit two types of DNA methylation alterations: global DNA hypomethylation and site-specific CpG island promoter hypermethylation. Selected gene promoter hypermethylation in normal esophageal mucosae has attracted attention as a surrogate marker for the epigenetic field defect induced by smoking and/or drinking in esophageal squamous cell carcinoma (ESCC). However, the significance of global DNA hypomethylation for field cancerization remains unclear. By using histologically normal esophageal mucosa samples from 109 ESCC cases and 20 autopsy cases without ESCCs, we measured long interspersed nucleotide element 1 (LINE-1) methylation levels by pyrosequencing, which correlates with global DNA methylation level. LINE-1 methylation levels in normal esophageal mucosae of ESCC patients were significantly lower than those of autopsy individuals (P = 0.017). LINE-1 methylation of noncancerous mucosae ranged 68.3-93.0 on a 0-100 scale (mean 81.7, median 82.2, standard deviation 5.9). LINE-1 hypomethylation was significantly associated with smoking history (P = 0.014 for smoking duration; P = 0.0017 for number of cigarettes per day; P = 0.0002 for tobacco pack-year). LINE-1 methylation was not associated with alcohol drinking or age at diagnosis. A history of tobacco use was significantly associated with LINE-1 hypomethylation in noncancerous esophageal mucosae of ESCC patients. These results support the potential of LINE-1 methylation level as an indicator of epigenetic field for ESCC cancerization, particularly when caused by tobacco smoking.