Alteration of DNA methylation during hepatocarcinogenesis

Abstract

Human carcinogenesis has been widely considered to consist of multiple steps reflecting the accumulation of both genetic and epigenetic events. Alteration of DNA methylation is one of the most consistent epigenetic changes in human cancers. Regional DNA hypermethylation on chromosome 16, where loss of heterozygosity (LOH) has been detected in human hepatocellular carcinomas (HCCs), was demonstrated even in chronic hepatitis (CH) and liver cirrhosis (LC), which are precancerous conditions. Precancerous conditions with aberrant DNA methylation appear to rapidly generate HCCs, which are already at an advanced stage when diagnosed. CpG methylation around the promoter of the E-cadherin gene, which is located on 16q22.1, was correlated significantly with its reduced expression, resulting in destruction of tissue morphology in HCCs. DNA hypermethylation at the D17S5 locus was detected even in CH and LC, and expression of mRNA for the hypermethylated-in-cancer-1 gene, which was identified at this locus, was lower in CH and LC than in histologically normal liver. The tissue microdissection technique was then employed to dissect out individual regenerative nodules. The incidence of aberrant DNA hypermethylation in histologically normal liver from HCC patients was similar to that in CH and LC, although neither LOH nor microsatellite instability (MSI) was found in histologically normal liver. Absence of silencing of the hMLH1 gene by DNA hypermethylation is consistent with the low incidence of MSI in HCCs. DNA hypermethylation in particular, which precedes LOH, is an early event during hepatocarcinogenesis. mRNA levels of DNA methyltransferase (DNMT) 1 were higher in CH and LC than in histologically normal liver. The incidence of increased DNMT1 protein expression in HCCs correlated significantly with poor tumor differentiation and portal vein involvement and may be a biological predictor of both HCC recurrence and a poor prognosis in HCC patients. Overexpression of a splice variant of DNMT3b, DNMT3b4, which may lack DNA methyltransferase activity and compete with DNMT3b3, the major splice variant in normal liver tissues, for targeting to pericentromeric satellite regions, results in DNA hypomethylation on these regions, even in precancerous stages, and plays a critical role in human hepatocarcinogenesis by inducing chromosomal instability. Alteration of DNA methylation is associated with multistep carcinogenesis, and correction of DNA methylation status may offer a new strategy for preventing human cancers in patients with precancerous conditions.