Repair studies on methylated DNA of CHO cells

DNA methylation pattern was investigated on Chinese hamster ovary (CHO) cells after treatment with N-(14C)-methyl-N-nitrosourea (14C-MNU). The main target was the N-7 position of guanine, exceeding the methylation in the O6 position of guanine by a factor of 8 and that in the N-3 position of guanine and adenine by a factor of 20. No DNA repair could be observed within 2 hours after methylation. Pretreatment of cells with gamma irradiation (7 rad) before application of MNU induced repair of N-7-methylguanine. This methylation product was decreased to about 50% within two hours, whereas the repair of the other methylated bases was not influenced. The analysis was carried out by high performance liquid chromatography after acid hydrolysis of isolated DNA. 14C-methylated products were determined by liquid scintillation counting.