Genótipo de Helicobacter pytori na etiologia do câncer gástrico: correlação com metilação e expressão gênica

Abstract

Helicobacter pylori (H. pylori) is a relevant etiological factor associated with gastric cancer, although the mechanism by which it acts is still not well known, it is suggested a role of this microorganism in the promotion of methylation of DNA. In fact, gene silencing by metiiylation of promoter regions is highly frequent in gastric cancer. Thus, the present study aimed to evaluate the inactivation of the genes COX-2, CDKN2A and HMLH1 by methylation and its relationship with infection by H. pylori, in a series of gastric adenocarcinomas, in association with clinical and histopathological findmgs. This observational transversal study analyzed a sample of 82 gastric adenocarcinomas. The detection of H. pylori was done by amplification of the ureaseC gene, using the PCR technique, as well as the detection of the cagA, cagE, virBlï,flaA genes and the subtypes of the vacA gene. The pattern of expression and methylation of the COX-2, HMLH1 and CDKN2A genes was determined by immunohistochemistry and MS-PCR techniques, respectively. Statistically significant differences were evaluated by the chi-square test (^) or Fisher exact test and the results were considered statistically significant when the ^-values were less than 0.05. The most methylated gene was COX-2 (51.2%), followed by CDKN2A (46.3%) and HMLH1 (32.9%). The positivity ofCOX-2, HMLH1 and pl6INK4A was observed in 26%, 58.4% and 40.3% of the cases. A strong negative correlation was found between negativity of these markers and the presence of methylation in the promoters of their genes. In cárdia tumors, negativity of pl6IN was a significant finding (p=0.031), as well as the methylation in the promoter of COX-2 in intestinal tiunors of advanced stages (p=0.037). On the other hand, in noncardia tumors, the intestinal subtype lesions demonstrated a significant (p=0.006) inactivation of HMLH1 by methylation, while in the diffuse subtype tumors, a significant finding (p=0.022) was the negativity of CDKN2A by methylation. The infection by H. pylori was observed m 95.1% of the tumors. Considering the bacterial genotype, the genes vocA slml (70.5%) and flaA. (61.9%) were the most frequent. It was observed that the tumors with methylated COX-2 and HMLH1 genes were associated with the strains of H. pylori vacA sl (p= 0.028 and 0.047, respectively) and the non-methylated tumors were associated with the presence of the gene flaA (p=0.017). No correlation was found between the genes of H. pylori and absence of expression of the studied markers. These data suggest that the inactivation of the genes COX 2, HMLH1 and CDKN2A by methylation occur by distinct ways according to the histological subtype and tumor location and depends on the H. pylori genotype.