Methylation of the alpha-fetoprotein gene in productive and nonproductive rat hepatocellular carcinomas

The extent of methylation of Hpall-Mspl and Hhal sites in DNAs isolated from normal rat livers and from the transplantable hepatocellular carcinomas (THC) THC 7777 and THC 252 was compared. It was found that the overall level of methylation of the internal cytosine in CCGG sequences was lower in the THC DNAs than in the normal liver DNAs. This difference could also be detected in the extent of methylation of CCGG sites flanking a 400-base pair repetitive sequence. Examination of methylation of specific sites within the alpha-fetoprotein gene revealed differences between the DNAs that appear to reflect both the level of activity of the gene and the overall level of methylation of cellular DNA. This gene, which is repressed in normal adult liver and the nonproductive THC 252 and highly active in the productive THC 7777 (S. Sell et al., Cell Biol. Int. Rep., 4: 235-254, 1980), contains several CCGG sites that are methylated in both normal liver and THC 252 DNA but not in THC 7777 DNA. However, Hhal (GCGC) sites in the alpha-fetoprotein gene were less methylated in both hepatoma DNAs than in liver DNA, which the exception of one site in the productive tumor found to be no longer methylated.