INCREASED CYTOSINE DNA-METHYLTRANSFERASE ACTIVITY DURING COLON-CANCERPROGRESSION

Background: Molecular changes during progressive stages of colon cance r and other human tumors commonly involve altered regulation of DNA me thylation. These changes include overall genomic hypomethylation, regi onal hypermethylation, and increased levels of messenger RNA (mRNA) fo r cytosine DNA-methyltransferase (DNA-MTase), the enzyme that catalyze s DNA methylation at CpG (cytosine-phosphoguanine) sites. This increas e in DNA-MTase transcripts (mRNA), if accompanied by increased DNA-MTa se activity, could play a role in the abnormal DNA methylation pattern s that appear early in colon tumor progression. Purpose: We sought to determine whether increased DNA-MTase mRNA levels during colon cancer progression are associated with increased cellular DNA-MTase enzymatic activity. Methods: We adapted a microassay for DNA-MTase and used it to measure activity in human colon carcinoma and in colon mucosa of no rmal control subjects and of patients with colon cancer or with famili al adenomatous polyposis (FAP), which is a risk factor for colon cance r. Steady-state DNA-MTase gene transcripts were measured by a reverse transcriptase polymerase chain reaction assay. To compare DNA-MTase ac tivity with mRNA levels, we determined both variables simultaneously f or one colon cancer specimen, its adjacent mucosa, and the colon mucos a of a control patient and compared the values. Results: Compared with DNA-MTase activity in mucosa from normal control subjects, activity w as elevated 1.4-fold in FAP mucosa, 1.6-fold in the uninvolved mucosa of patients with cancer, and 5.4-fold in the cancer specimens. All the se differences were statistically significant. Fourteen of 15 cancer s amples and 47% of the uninvolved adjacent mucosa samples had values th at were higher than the highest value in normal mucosa. In one patient who had both a benign adenomatous polyp and a malignant adenocarcinom a, increasing DNA-MTase activity was observed at each stage of tumor p rogression. Conclusion: These results demonstrate that an increased DN A methylation capacity accompanies the increase in DNA-MTase transcrip ts observed during progressive stages of colon cancer. Implication: Fu rther studies are needed to determine whether this abnormal methylatio n capacity plays a role in establishing the abnormal DNA methylation p atterns seen in human malignancies.