Delay of DNA-adduct repair and severe toxicity in xeroderma pigmentosum group A gene (XPA) deficient mice treated with 2-amino-1-methyl-6-phenyl-imidazo [4,5–b] pyridine (PhIP)

Group-A xeroderma pigmentosum (XPA) gene-deficient mice are defective in nucleotide-excision repair and highly susceptible to ultraviolet-B-, and 9,10-dimethyl-1,2-benz[a]anthracene (DMBA)-induced skin carcinogenesis [1]. In this study, changes of 2-amino-1-methyl-6-phenylimidazo[4,5–b] pyridine (PhIP)-DNA adduct formations in the liver, colon and lung, as assessed by the 32P-postlabeling method and immunohistochemical analysis, and carcinogenic and/or toxic susceptibility of both sexes of XPA-deficient mice (XPA−/−) to PhIP, which is a carcinogenic heterocyclic amine, was examined. Levels of PhIP-DNA adduct formations in the liver, colon and lung, were almost twice as high in XPA−/− as in wild type mice (XPA+/+) mice, 7 days after a single i.g. administration of PhIP, and their delay in recovery was observed in XPA−/− mice. For the long-term experiment, XPA−/− and XPA+/+ type mice were treated with 80 ppm PhIP in the diet for the first 4 weeks followed by 40 ppm after a 2-week recovery period (long-term experiment I), or 40 ppm PhIP throughout the experiment (long-term experiment II). Severe toxicity, as evidenced by body weight retardation and poor survival, was observed in the PhIP treated XPA−/− mice of both sexes, but not in the XPA+/+. At week 40 the experiments were terminated and histopathological examinations were performed after complete autopsy. Only lymphomas/leukemias were observed as neoplastic lesions, but no significant differences were observed between the groups. As non-neoplastic lesions, degenerating changes, for example in the pancreatic acinar cells, were observed with XPA−/− mice tending to be more sensitive than XPA+/+ mice. The present study demonstrated that PhIP-DNA adduct formations in the liver, colon and lung of XPA−/− mice were demonstrated and their recovery rate was more delayed than XPA+/+ mice, and furthermore, more severe toxicity to PhIP in XPA-deficient mice was observed, but they were not susceptible to PhIP carcinogenicity under the conditions of the experiment.