• Alkyltransferase transgenic mice: probes of chemical carcinogenesis.

      Gerson, S L; Zaidi, N H; Dumenco, L L; Allay, E; Fan, Chun-Yang; Liu, L; O'Connor, Peter J; Department of Medicine, Case Western Reserve University School of Medicine, Cleveland, OH 44106-4937. (1994-06-01)
      Transgenic mice expressing DNA-repair genes are an instructive model with which to study the protective role of DNA-repair pathways in both spontaneous and chemical carcinogenesis. Of particular interest in chemical carcinogenesis is the DNA-repair protein O6-alkylguanine-DNA alkyltransferase (alkyltransferase) which repairs O6-alkylguanine-DNA adducts. Transgenic mice carrying expression constructs for the alkyltransferase gene--either the human MGMT cDNA or the bacterial ada gene--express increased levels of alkyltransferase and have increased capacity to remove O6-methylguanine-DNA adducts. Protection from the DNA damaging effects of N-nitroso compounds occurs specifically in the cells and tissues in which the alkyltransferase transgene is expressed. For instance, mice carrying the PEPCKada construct have increased alkyltransferase in the liver and more rapid removal of O6methylguanine-DNA adducts. The protective effect is noted in hepatocytes, which express PEPCK-linked genes, not in nonparenchymal cells of the liver, which do not. Other tissues that express the transgene in the various models include the thymus, spleen, testes, muscle, stomach and brain. Mice expressing the human alkyltransferase in the thymus have a reduced incidence of thymic lymphomas following exposure to methyl nitrosourea (MNU), evidence of a role for this DNA-repair protein in protection from carcinogenesis due to N-nitroso compounds. Protection has also been observed in the induction of hepatic tumors by N-nitroso-dimethylamine (NDMA). These models will be used to identify whether overexpression of a single DNA-repair gene can block the carcinogenic process of N-nitroso compounds in many different tissues.
    • Identification in rat stomach mucosae of a cell population characterized by a deficiency for the repair of O6-methyldeoxyguanosine from DNA.

      Zaidi, N H; O'Connor, Peter J; Cancer Research Campaign Department of Carcinogenesis, Paterson Institute for Cancer Research, Christie Hospital (NHS Trust), Manchester, UK. (1995-03)
      Immunohistochemical studies have been used to show the time course for the cell-specific methylation of DNA in the upper gastrointestinal tract of rats treated with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The doses used were 1, 5, 25 and 50 mg MNNG/kg (i.g.) and tissue samples were analysed at intervals of from 1 to 192 h after treatment. Relatively little reaction with nuclear DNA was observed in the forestomach and still less in the oesophagus. Reaction with DNA was most extensive in the corpus, pylorus and the duodenum, reaching a peak of staining intensity between 2 and 4 h and then declining progressively there-after. Staining for the presence of O6-methyldeoxyguanosine (O6-MedG) in DNA was highly selective and tended to occur in the nuclei of the basal cell of the oesophagus and forestomach and in the cells of the lumenal border of the glands and villi of the corpus, pylorus and duodenum. There were also areas, 5-15, glands apart where staining for O6-MedG in the corpus and pylorus extended as far down as the basal mucosa. From 12 h after MNNG treatment, in the corpus and pylorus, a band of strongly methylated cells became apparent about 3-6 cells deep from the lumen and remained identifiable up to 168 h after treatment with the higher doses. These cells, which apparently have a very low O6-MedG repair capacity, are stationary (i.e. not part of the escalator) and are located in the mesenchymal tissue elements as demonstrated by staining of serial sections with cytokeratin or vementin. The significance of this population of cells is unknown.
    • N-methyl-N1-nitro-N-nitrosoguanidine-induced carcinogenesis: differential pattern of upper gastrointestinal tract tumours in Wistar rats after single or chronic oral doses

      Zaidi, N H; O'Connor, Peter J; Butler, W H; Cancer Research Campaign Department of Carcinogenesis, Paterson Institute for Cancer Research, Christie Hospital NHS Trust, Manchester M20 9BX (1993)
    • Tissue and cell specific methylation, repair and synthesis of DNA in the upper gastrointestinal tract of Wistar rats treated with N-methyl-N'-nitro-N-nitrosoguanidine via the drinking water.

      Zaidi, N H; Potten, Christopher S; Margison, Geoffrey P; Cooper, Donald P; O'Connor, Peter J; Cancer Research Campaign Department of Carcinogenesis, Paterson Institute for Cancer Research, Christie Hospital, Manchester, UK. (1993-10)
      Several potential cancer risk factors have been monitored concurrently in the upper gastrointestinal tract of young male Wistar rats given N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) via the drinking water, a regimen that induces a high yield of tumours in the pylorus and to a lesser extent in the duodenum. Radioimmunoassay was used to determine the amounts of O6-methyl-2'-deoxyguanosine (O6-MedG) formed in the tissue DNA of rats given MNNG at doses of 40 or 80 micrograms/ml for periods of 3, 6 and 12 weeks. The highest adduct concentration was found in the pylorus with progressively lower concentrations in the corpus and duodenum, jejunum, forestomach and oesophagus. Between 3 and 12 weeks these adduct levels decreased in all tissues and there was no evidence of a dose dependent accumulation of O6-MedG. When analysed by immunohistochemistry the distribution of cells with nuclei containing O6-MedG was seen to be heterogeneous in the various tissues. O6-Alkylguanine-DNA alkyltransferase activity increased during the 12 weeks of MNNG treatment in oesophagus and forestomach, but decreased to approximately 50% of the initial value in the corpus, pylorus, duodenum and jejunum. The major changes in DNA synthesis and cell proliferation were the marked upward expansion (i.e. towards the lumen) of the zone of replicating cells in the glands of the pylorus and the greatly increased numbers of replicating damaged cells (i.e. cells that contained O6-MedG whilst undergoing DNA synthesis) as determined by sequential immunohistochemical analysis and autoradiography. Such cells are the probable target cells in this chronic dose carcinogenesis regime. Although similar changes also occurred in the glands of the corpus these were of lesser extent and the changes of labelling index in the oesophagus and forestomach were relatively minor. In the duodenum, MNNG treatment led to erosion of the upper part of the glands so that the zone of cells containing O6-MedG overlapped with the zone of proliferating cells resulting in the formation of many replicating damaged cells. Thus, as in the single dose study (see preceding paper) the distribution of replicating damaged cells coincides with the tumour yield in the tissues of the upper gastrointestinal tract. As in the case of single doses of MNNG the risk factors for carcinogenesis are, a significant level of DNA damage, a lower capacity for DNA repair and an increased DNA synthetic activity, again suggesting that carcinogenic risk cannot readily be determined by studying risk factors individually.