Single-site methylation within the p53 promoter region reduces gene expression in a reporter gene construct : Possible in vivo relevance during tumorigenesis

It is not known whether transcriptional suppression by de novo methylation occurs within the promoter region of the p53 gene during multistage tumorigenesis. To address this question, in vivo alterations in the CpG methylation within the rat p53 promoter region were evaluated in control, preneoplastic, and tumor tissue during tumor progression using the folate/methyl-deficient model of hepatocarcinogenesis. Alterations in CpG methylation were found to be site-specific and to vary depending on the stage of carcinogenesis. To further explore the effect of site-specific methylation on p53 promoter activity, reporter gene constructs were prepared containing specifically methylated sites within the p53 promoter region, and the transcriptional activity in cultured mammalian cells was determined in a transient transfection assay. Relative to the unmethylated construct as a positive control, single-site methylation at nucleotide (nt) -450, which occurs 216 nt upstream from the 85-nt minimal promoter region, suppressed promoter activity by 85%. In contrast, single-site methylation at nt -179, which occurs within the minimal essential promoter region, suppressed activity by only 20%. The p53 promoter constructs containing the singly methylated CpG site at nt -450 were then reevaluated for processive changes in methylation status 48 h after transfection, during maximum suppression of promoter activity. Restriction analysis with methylation-sensitive enzymes revealed that de novo methylation had occurred after transfection at previously unmethylated sites. These findings suggest that nt -450 may constitute a critical site for initiation of de novo methylation and processive spreading of methylation associated with transcriptional inactivation of the p53 gene. Furthermore, the results suggest a possible alternative mechanism for the silencing of the p53 gene in tumors that do not have p53 mutations.