Expression of DNA methyltransferases in multistep hepatocarcinogenesis
Hypermethylation of cell cycle regulators and increased DNA methyltransferase 1 (Dnmt1) mRNA level have been reported in hepatocarcinogenesis. However, the expression of Dnmts has not yet been examined in hepatocellular carcinomas (HCCs). We examined 13 cases of HCCs in dysplastic nodules (DNs) and 28 cases of advanced HCCs for Dnmt1 and Dnmt3a, and compared the results with those of 9 cases of low-grade DNs, 24 cases of high-grade DNs, and 59 cases of nonneoplastic liver tissues from 59 cases of surgically resected livers by immunohistochemical staining. Nuclear expression of Dnmt1 was increased significantly in all HCCs in DNs and advanced HCCs compared with those of nonneoplastic livers, low-grade DNs, and high-grade DNs (P <0.05). Nuclear expression of Dnmt3a was not detectable in nonneoplastic liver and low-grade DN, whereas it was observed in high-grade DNs (7 of 24, 29.2%), HCCs in DNs (7 of 13, 53.8%), and advanced HCCs (11 of 28, 39.3%). Different from Dnmt1 immunostaining, cytoplasmic immunoreactivity for Dmnt3a was significantly decreased or absent in 13 of 24 cases of high-grade DNs (54.1%), 12 of 13 cases of HCCs in DNs (92.3%), and 22 of 28 cases of advanced HCCs (78.6%), compared with nonneoplastic livers and low-grade DNs (P <0.05). Our data suggest that Dnmt1 and Dnmt3a play a role in the early stage of hepatocarcinogenesis and that dysregulation of Dnmt3a may be involved in the progression of HCC. Furthermore, the significantly decreased cytoplasmic immunoreactivity for Dnmt3a in high-grade DNs and HCCs can be used as a diagnostic adjunct.