We evaluated the significance of aberrant DNA methyltransferase 1 (DNMT1) 
protein expression during gastric carcinogenesis. The protein expression of 
DNMT1, Muc2, human gastric mucin, E-cadherin, and proliferating cell nuclear 
antigen was examined immunohistochemically in gastric cancers and corresponding 
noncancerous mucosae from 134 patients. The DNA methylation status of the CpG 
islands of the p16, human MutL homologue 1 (hMLH1), E-cadherin, and 
thrombospondin-1 (THBS-1) genes and the methylated in tumor (MINT)-1, -2, -12, 
and -31 clones was examined by methylation-specific polymerase chain reaction 
and combined bisulfite restriction enzyme analysis. Epstein-Barr virus (EBV) 
infection was detected by in situ hybridization. Nuclear immunoreactivity for 
DNMT1 was not detected in any of the noncancerous epithelia, except in 
proliferative zones (positive internal control), but was found in 97 (72%) of 
the gastric cancers. DNMT1 overexpression correlated significantly with poorer 
tumor differentiation (P < 0.001), but not with the phenotype (gastric type 
versus intestinal type) of the cancer cells. It also correlated significantly 
with DNA hypermethylation of the CpG islands of the hMLH1 (P = 0.024) and THBS-1 
genes (P = 0.043), and with the CpG island methylator phenotype in the gastric 
cancers (P = 0.007). Reduced E-cadherin expression correlated significantly with 
poorer tumor differentiation (P = 0.002), DNA hypermethylation of the E-cadherin 
gene (P < 0.001) and DNMT1 overexpression (P = 0.014). DNMT1 overexpression was 
also associated with EBV infection (a potential etiological factor in gastric 
carcinogenesis) but not with the proliferative activity of the cancer cells as 
indicated by the proliferating cell nuclear antigen-labeling index. These 
results suggest that DNMT1 overexpression may not be just a secondary effect of 
increased cancer cell proliferative activity, but may be associated with EBV 
infection and other etiological factors during gastric carcinogenesis. 
Furthermore, DNMT1 may play a significant role in the development of poorly 
differentiated gastric cancers by inducing frequent DNA hypermethylation of 
multiple CpG islands.