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CpG island promoter methylation of tumor suppressor genes is one of the most characteristic abnormalities in Helicobacter pylori (HP)-associated gastric carcinoma (GC).Aberrant promoter methylation and expression loss of O6-methylguanine-DNA methyltransferase (MGMT) were evaluated in GC tissues using methylation-specific PCR and immunohistochernistry, respectively, and showed that both abnormalities occurred concurrently in HP-associated GC.MGMT abnormalities were recapitulated in GC cell lines, SGC-7901 and MKN-45, infected with HP, and DNA methyltransferase-1 (DNMT1) was commonly overexpressed in both cell lines.Stable and transient transfection systems were used to similarly show that CagA upregulated DNMT1 in SGC-7901 cells, leading to increased.methylation of the MGMT promoter.Importantly, the level of P-AKT increased in the CagA-expressing GC cells and knockdown of AKT counteracted CagA-mediated DNMT1 overexpression.Immunohistochemical analysis of both MGMT and DNMT 1 showed a negative relationship between them in both the nuclei and cytoplasm of cancer cells in HP-associated GC tissues.The above results were also validated in vivo in C57BL/6 mice infected with HP.Taken together, CagA induced the phosphorylation of AKT, which activated DNMT1 transcription, causing MGMT expression loss through CpG island methylation of the MGMT promoter in HP-associated GC.Thus, CagA plays an essential role in the epigenetic abnormalities seen in host stomach cells and in the development and maintenance of HP-associated cancer.