Feicheng County has a high incidence of esophageal cancer compared to the rest of China. Worldwide mortality rates have decreased from 75.82 per 100,000 in 1970-1974 to 57.22 per 100,000 in 2000-2004 . In the present study, we demonstrate that specific allelic combination significantly increased the risk for esophageal cancer (by as much as 8-fold). While reproducibility of studies linking genotype to disease risk is often problematic, there are several strengths of this study. First, the subjects in the study were diagnosed by biopsy, so misclassification bias was very low. Our DNA collection method avoided biases that may arise from single-center or multi-center collection. Furthermore, several steps were taken to ensure high quality and repeatability of results. These included initial DNA sequencing of SNP regions to prove the reliability, blinding of the operator to the case-control status of samples to reduce observer bias, and simultaneous analysis of case and control samples to avoid differential misclassification. Moreover, the allele frequencies reported among normal controls in this study were similar to those reported in previous studies of Chinese subjects. In sum, these controls indicated that our findings have high validity and reliability.
In the present study, we found that the ALDH2 genotype was associated with BCH, ESCD, and ESCC, the main stages of carcinogenic transition in the esophagus. Acetaldehyde is formed by the oxidation of ethanol by alcohol dehydrogenase (ADH), and is eliminated by aldehyde dehydrogenase (ALDH). The ALDH2 gene carries two alleles, ALDH2*1 and ALDH2*2, with different kinetic properties and distinct distributions among ethnicities [23, 26]. The ALDH2*2 allele is found at a frequency of only 50% in Orientals, while the ALDH2*1 allele is more predominant in Caucasians . The ALDH2*2 allele codes for an inactive ALDH2, and is closely associated with alcohol related cancers in the upper aerodigestive tract [27–29]. The accumulation of acetaldehyde plays a protective role against excessive alcohol consumption as it causes unpleasant reactions, including "Oriental facial flushing" and other symptoms due to alcohol sensitivity, such as headache, nausea, vomiting, tachycardia, hypotension, and sleepiness . Essentially, a person harboring the ALDH2*2 allele may not become a heavy drinker. Indeed, genetic epidemiologic studies have indicated that the ALDH2*2 allele inhibits the development of alcoholism. However, many studies demonstrated that patients harboring ALDH2*2 allele who are heavy drinkers were at increased risk of ESCC. It is unknown why patients harboring the ALDH2*2 allele became heavy drinkers despite the unpleasant reaction to acetaldehyde [28–31].
In the present study, heavy drinkers with ALDH2*2/2*2, ALDH2*1/2*2, and ALDH2*1/2*1 genotypes comprised 3.9%, 25.5%, and 14.7% of the cancer group, and 2.5%, 20.0%, and 0.8% of the control group, respectively. It is clear that persons harboring the ALDH2*2/2*2 genotype are less likely to be heavy drinkers than those harboring the ALDH2*1/2*2 or ALDH2*1/2*1 genotypes. Aside from the increased sensitivity of alcohol-induced nausea, this may reflect a very low level of alcohol consumption in Feicheng County, where living standards are relatively low and the majority of farmers cannot afford alcoholic beverages [32, 33].
The TT genotype of the MTHFR gene had a significantly positive association with ESCC (OR = 1.85, 95% CI 1.02-3.34) but not with BCH or ESCD. There was also a significant association between esophageal cancer and the MTHFR TT genotype with which the patient was also a heavy smoker. Associational studies linking polymorphisms of the MTHFR C677T genotype with ESCC risk have yielded inconsistent result. A meta-analysis of studies examined the association of the MTHFR C667T polymorphism with risk of esophageal cancer .
The association between esophageal cancer and MTHFR enzyme activity is most likely related to the metabolism of folic acid, as risk of esophageal cancer depends on the status of folic acid intake. When folic acid intake is sufficient, individuals with the MTHFR CT or TT genotypes may actually have a decreased risk of esophageal lesions because the lower MTHFR activity of the 677TT allele may lead to an elevation in 5, 10-methylenetetrahydrofolate, facilitating DNA synthesis. In contrast, both impaired DNA methylation and DNA synthesis/repair may become the primary mechanisms of carcinogenesis in the presence of low folic acid intake [16, 35–37]. However, the TT genotype was not related to BCH or ESCD, suggesting a weaker or absent linkage.
We present evidence that two susceptibility genes, ALDH2*2 and MTHFR 677T, contribute to the process of esophageal carcinogenesis. The combination of the two high-risk genotypes 2*2/1*2 of ALDH 2 and TT/TC of MTHFR C677T resulted in a 4-fold higher risk of developing BCH, a 3.7-fold increased risk of ESCD, and a 8.72 times higher ESSC risk. The ALDH 2 and MTHFR C677T genes showed a significant association with ESCC in our population.
In contrast to ALDH 2 and MTHFR, polymorphisms of CYP2E1 G1259C, MPO G463A, and hOGG1 C326G genes were not associated with BCH, ESCD, or ESCC risk in this study. These negative results may be attributable to the fact that the study population came from the same community where residents share a similar life style and diet. This homogeneity may cause an over-match, such that the association of these two metabolic enzyme genes (CYP2E1, MPO) and one repair gene (hOGG1) with lesions of the esophagus cannot be demonstrated or is too low to estimate. In addition, because these alleles were associated with smaller ORs (<2.0) for risk of the diseases, this effects would not be detected due to an allele null frequency less than 0.10. The sample size, therefore, may not have been large enough to detect an association.