Positive display of methylated sites: a novel method for the detection of promoter methylation.

Promoter methylation represents an important mechanism for silencing gene expression in higher eukaryotes. To study methylation of the promoter of the tumor suppressor p16INK4a, a fast and simple method was developed that, in contrast to previous studies, relies on the positive display of methylated sites (PDM). The method is based on bisulfite treatment of DNA, polymerase chain reaction (PCR)-amplification of the modified DNA and restriction digest of de novo created restriction sites to positively display DNA methylation in a background of unmethylated DNA. Since methylated as well as unmethylated DNA is amplified, information on the proportion of both is provided. Using this approach, 33 ductal invasive carcinomas, 4 normal mammary tissues, and 4 cell lines were analyzed for methylation. Methylation in the p16INK4a promoter was detected in 1 of 33 carcinomas (3%) and in 0 of 4 normal tissues. The conclusion is that PDM provides a useful tool in determining the degree and pattern of promoter methylation and is suitable to screen large series of tissue samples.