Alterations in hepatic p53 gene methylation patterns during tumor progression with folate/methyl deficiency in the rat.

Chronic dietary methyl deficiency in F344 rats was used as an in vivo mammalian model in which to evaluate the gene-specific alterations in DNA methylation patterns during multistage hepatocarcinogenesis. Using bisulfite mapping, the site-specific methylation profile within exons 6-7 of the 53 gene was determined in control liver, preneoplastic nodules (after 36 weeks of folate/methyl deficiency) and in hepatocellular carcinoma (after 54 weeks of deficiency). A progressive loss of methyl groups was observed at most CpG sites on both coding and non-coding strands during the first 36 weeks of folate/methyl deficiency, with the greatest loss occurring on the coding strand. When the same sequence was evaluated in tumor DNA after 54 weeks of deficiency, the majority of cytosines were unexpectedly found to have become remethylated. CpG sites that had previously lost methyl groups on both strands during preneoplasia as well as CpG sites that had been constitutively non-methylated, had undergone de novo methylation in tumor DNA. Maintenance methyltransferase and de novo methyltransferase activity in nuclear extracts were assessed using hemimethylated and non-methylated DNA substrates, respectively. In tumor, de novo methyltransferase capacity was increased approximately 4-fold relative to control or preneoplastic liver and associated with a relative increase in both p53 and genome-wide methylation density. In the preneoplastic nodules, the level p53 mRNA was increased and associated with hypomethylation in the coding region of the gene, whereas in tumor tissue, p53 mRNA was decreased and associated with relative hypermethylation. Taken together, these results provide additional insights into the dysregulation and instability in DNA methylation that accompanies the transition to tumor.