DNA methylation in esophageal diseases including cancer: special reference to hMLH1 gene promoter status

Chronic inflammation leading to malignancy in the esophagus may be due to errors in mismatch repair (MMR) genes such as hMLH1. Promoter hypermethylation has been suggested as the main cause of hMLH1 silencing. In this study we assessed hMLH1 promoter hypermethylation in a range of esophageal diseases. Further, we evaluated the role of factors affecting the methylation cycle: (1) methylenetetrahydrofolate reductase (MTHFR) C677T mutation and (2) serum homocysteine levels.

We endoscopically and histologically categorized 124 paired tissue and blood samples from patients into cancer, precancer, reflux esophagitis, other inflammatory esophagitis and controls (endoscopically normal). Restriction enzyme-based methylation analysis was carried out to assess hMLH1 promoter hypermethylation.

hMLH1 promoter hypermethylation in tissue was seen in 63.5% of patients with cancer and 53.8% of those with precancer, which was significantly increased when compared with controls (P < 0.001). There appears to be an increasing degree of hMLH1 hypermethylation with disease progression. Patients with gastroesophageal reflux disease (GERD) showed a high degree of hMLH1 hypermethylation (88.8%), indicating that local environment due to reflux may be promoting hypermethylation. We suggest that GERD is a progressive condition with an increased risk for developing into cancer. Only 14.5% of cases exhibited hypermethylation both in tissue and blood. Hence, we conclude that hMLH1 promoter hypermethylation is a tissue-specific change in the esophagus and blood testing cannot be used as a noninvasive tool to assess it. DNA methylation is dependent on the methylation cycle; MTHFR is a major enzyme in this pathway. MTHFR mutations did not correlate with hypermethylation or clinical pathology (P > 0.5). Elevated homocysteine levels, independent of MTHFR mutation, correlated significantly with hMLH1 hypermethylation in tissue (P < 0.005). Our study shows that hMLH1 hypermethylation in tissue may be the primary event caused by endogenous/exogenous factors in esophageal diseases, aiding disease progression.