We have previously reported that CpG island methylation of both DAPK and CDH1, in non-neoplastic gastric mucosa, corresponded to an increased risk of gastric cancer . In the present study, the frequency of methylation in both genes was significantly higher in the subjects with GC than without GC. However, the frequency of CDH1 methylation in cancer lesion was significantly lower than that in non cancerous mucosa in the subjects with GC, although the frequency of DAPK methylation was not. From these findings, CDH1 methylation seems to take little part in the development of gastric cancer. In our studied subjects, gene methylation may not reflect the expression of protein or mRNA, although some previous studies demonstrate that these gene methylations contribute to decreasing levels of protein and mRNA [27–30]. Another possibility is that tumor may develop through E-cadherin independent pathways for carcinogenesis. The mechanism of carcinogenesis via gene methylation is still unclear. However, we pay attention to the fact that accumulation of gene methylation actually shows an increased risk of carcinogenesis as many studies have indicated [3, 4, 6, 7, 11–14, 31, 32], even if methylation of CDH1 may not directly affect the tumor development.
There are few reports that demonstrate an influence of polymorphisms of HRH2 on the risk for human disorders, but those there are report no association between rs2607474 and neurological or psychological disorders, such as schizophrenia or Parkinson’s disease [19, 20, 33]. We are unaware of any previous reports on the association between this polymorphism and gastric disorders.In the present study, we investigated the association between rs2607474 and the developments of CpG island methylation of both DAPK and CDH1 in non-cancerous gastric mucosa. Our current study reveals that rs2607474 GG genotype was positively associated with the development of CpG island methylation of both genes. In addition, we also reveal that gastric mucosal atrophy is more severe in comparatively older GG homozygote than GA + AA genotype, and that gastric mucosal atrophy progresses with age only in GG homozygote.
One of the most important factors causing gene methylation in the stomach is H. pylori infection . Infection with H. pylori first induces chronic superficial gastritis, which can progress to chronic atrophic gastritis, intestinal metaplasia, and dysplasia that can lead gastric carcinoma . The factors promoting H. pylori-mediated gastric atrophy are somewhat more controversial. H. pylori infection results in an elevation of serum gastrin level, in the early stage of infection, and proceeds to the development of atrophic gastritis. The majority of clinical studies, however, have accepted that proton pump inhibitors (PPIs), which induce achlorhydria and hypergastrinemia, accelerate the onset of atrophic gastritis in H. pylori positive patients [35–37]. Therefore, hypergastrinemia seems to promote the gastric mucosal atrophy that is influenced by H. pylori infection. Interstingly, long-term treatment of rats and mice with loxtidine, a potent H2 receptor antagonists inducing ECL cell hyperplasia, (as does omeprazole), did not result in loss of parietal cells, but instead appeared to result in increased parietal cells [38, 39]. In addition, HDC knockout mice kept on a low-histamine diet showed an expanded parietal cell pool despite exhibiting marked hypergastrinemia . These results suggest that it is the up-regulation of histamine, not achlorhydria nor hypergastrinemia, that contributes to the gradual down-regulation of parietal cell number and gastric atrophy. In our present study, gastric mucosal atrophy progressed with age in rs2607474 GG homozygote cohort, whilst in the GA + AA genotype it did not. In addition, the degree of gastric mucosal atrophy was higher in older subjects who were the GG homozygote than in those that were the GA + AA genotype. In our previous study, a correlation between intestinal metaplasia and gene methylation of various genes has been confirmed . We therefore suggest that gene methylation progresses more rapidly in parallel with gastric mucosal atrophy in the GG homozygote than in the GA + AA genotype. Furthermore, it is possible that the action of histamine is up-regulated in GG homozygote. However, there have been no studies for the effects of rs2607474 on function or expression of H2 receptors and our genetic statistical study did not reveal them. This is one of the limitations in our study. Further study using other methods and other techniques are necessary to evaluate the function of rs2607474.
It has been recently reported that GC patients show higher frequencies of aberrant CpG island methylation in non neoplastic mucosa than non-GC subjects . In our present study, the frequencies of DAPK and CDH1 methylation were also significantly higher in subjects with GC than without GC. On the other hand, rs2607474 GG homozygote was associated with a significantly increased risk for both DAPK and CDH1 methylation in subjects without GC, whereas it wasn’t in subjects with GC. The methylation frequency of DAPK and CDH1 was higher in GC patients than in non-GC cases, and there were few GC patients without methylation of DAPK and CDH1 (20/115 and 19/115, respectively). That is, almost all GC patients have non-cancerous gastric mucosa in which some genes are already methylated. In addition, the cohort composed of only GC patient may have a certain kind of bias. Therefore, the influence of HRH2 genotype on methylation status of non-cancerous mucosa in patients who have already developed gastric cancer may be indistinct.
A significantly increased risk of the GG homozygote for DAPK methylation was seen regardless of the age or H. pylori infection status, whilst that for CDH1 was not seen in the comparatively younger subjects or H. pylori positive subjects. Why such a difference was seen is unknown. The mechanisms which ageing or inflammation influences methylation are unclear. We demonstrated in this study that, in the GG homozygote, gastric mucosal atrophy was correlatively progressed with age and was more severe in the comparatively older subjects. As for one possibility, it might be hard for CDH1 methylation to undertake the influence of ageing or inflammation than DAPK methylation.