TISSUE AND CELL-SPECIFIC METHYLATION, REPAIR AND SYNTHESIS OF DNA IN THE UPPER GASTROINTESTINAL-TRACT OF WISTAR RATS TREATED WITH SINGLE DOSES OF N-METHYL-N'-NITRO-N-NITROSOGUANIDINE

Several potential cancer risk factors have been monitored concurrently in the upper gastrointestinal tract of young adult male Wistar rats g iven single (i.g.) doses of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG ) which readily induces forestomach tumours under these conditions. Ra dioimmunoassay was used to determine the formation of O6-methyl-2'-deo xyguanosine (O6-MedG) in DNA after MNNG doses of 1, 5, 25 or 50 mg/kg and was found to be highest in the pylorus, with progressively lower l evels in the corpus, forestomach, duodenum, oesophagus and jejunum. Im munohistochemical procedures showed that cells with nuclei containing O6-MedG were heterogeneously distributed in these tissues. O6-Alkylgua nine-DNA alkyltransferase activity in untreated animals was highest in the mucosae of the corpus, lower and relatively similar in that of th e pylorus, duodenum and jejunum and lowest in the tissues of oesophagu s and forestomach. Estimates of DNA synthesis and cell proliferation i ndicated a 5-fold increase in the DNA labelling index in the forestoma ch whereas perturbations of DNA synthetic activity in the other tissue s of the upper gastrointestinal tract were much less marked. As a resu lt of these changes, cells with nuclei that contained O6-MedG and were also undergoing DNA synthesis (determined by sequential immunohistoch emical analysis and autoradiography) were found most commonly in the f orestomach and to a lesser extent in the pylorus. This distribution of replicating damaged cells corresponds with the relative tumour yields in these upper gastrointestinal tract tissues and such cells are the probable targets in this single dose carcinogenesis regime. Thus, whil st the highest concentration of O6-MedG did not correlate tumour incid ence, the overall risk for tumour induction did correlate with a signi ficant level of DNA damage, a lower capacity for DNA repair and a mark ed increase in DNA synthesis over the constitutive level in the target cells. Carcinogenic risk in this system is therefore more readily det ermined by studying several risk factors simultaneously.