N-METHYL-N-NITROSOUREA MAKES REPLICATING DNA SUSCEPTIBLE TO S1 NUCLEASE

Recently we found hypermethylation of replicating hepatic DNA following N-methyl-N-nitrosourea (MNU) administration at the G1-S interface. In addition, numerous reports on DNA hypermethylation following ionizing radiation treatment and during tumoral progression further stimulated our interest to analyze the hypothesis of altered DNA methylation as a basis for altered gene expression in the process of cancer development. The present experiments were designed to single out the specific DNA sequence (s) involved in the MNU-induced hypermethylation phenomenon. Compensatory cell proliferation following partial hepatectomy (PH) was used as an experimental model and the hepatic DNA methylation patterns after MNU administration during the proliferative phase were studied by a number of restriction endonucleases. Contemporaneously, the [H-3]thymidine incorporation into DNA was monitored. The results of this study suggest that the hypermethylation effect produced by the N-nitroso compound might be due to the displacement and/or disruption of strands caused by the N-nitroso compound in replicating DNA.