Comprehensive analysis of DNA methylation and gene expression of rat liver in a 2-stage hepatocarcinogenesis model

Recent studies have shown that epigenetic alterations correlate with carcinogenesis in various tissues. Identification of these alterations might help characterize the early stages of carcinogenesis. We comprehensively analyzed DNA methylation and gene expression in livers obtained from rats exposed to nitrosodiethylamine (DEN) followed by a promoter of hepatic carcinogenesis, phenobarbital (PB). The combination of DEN and PB induced marked increases in number and area of glutathione S-transferase-placental form (GST-P)-positive foci in the liver. In the liver of rats that received 30 mg/kg of DEN, pathway analysis revealed alterations of common genes in terms of gene expression and DNA methylation, and that these alterations were related to immune responses. Hierarchical clustering analysis of the expression of common genes from public data obtained through the Toxicogenomics Project-Genomics Assisted Toxicity Evaluation system (TG-GATEs) showed that carcinogenic compounds clustered together. MBD-seq and GeneChip analysis indicated that major histocompatibility complex class Ib gene RT1-CE5, which has an important role in antigen presentation, was hypomethylated around the promoter region and specifically induced in the livers of DEN-treated rats. Further, immunohistochemical analysis indicated that the co-localization of GST-P and protein homologous to RT1-CE5 was present at the foci of some regions. These results suggest that common genes were altered in terms of both DNA methylation and expression in livers, with preneoplastic foci indicating carcinogenic potential, and that immune responses are involved in early carcinogenesis. In conclusion, the present study identified a specific profile of DNA methylation and gene expression in livers with preneoplastic foci. Early epigenetic perturbations of immune responses might correlate with the early stages of hepatocarcinogenesis.