The risk of cancer due to exposure to ionizing radiation is higher in infants than in adults. In a previous study, the effect of adult-onset calorie restriction (CR) on carcinogenesis in mice after early-life exposure to X-rays was examined (Shang, Y, Kakinuma, S, Yamauchi, K, et al. Cancer prevention by adult-onset calorie restriction after infant exposure to ionizing radiation in B6C3F1 male mice. Int J Cancer. 2014; 135: 1038-47). The results showed that the tumor frequency was reduced in the CR group. However, the mechanism of tumor suppression by CR is not yet clear. In this study, we examined the effects of CR on radiation-induced mutations using gpt delta mice, which are useful to analyze mutations in various tissues throughout the whole body. Infant male mice (1-week old) were exposed to 3.8 Gy X-rays and fed a control (95 kcal/week/mouse) or CR (65 kcal/week/mouse) diet from adult stage (7-weeks old). Mice were sacrificed at the age of 7 weeks, 8 weeks and 100 days, and organs (spleen, liver, lung, thymus) were harvested. Mutations at the gpt gene in the DNA from the spleen were analyzed by using a gpt assay protocol that detects primarily point mutations in the gpt gene. The results showed that mutation frequencies were decreased in CR groups compared with non-CR groups. Sequence analysis of the gpt gene in mutants revealed a reduction in the G:C to T:A transversion in CR groups. Since it is known that 8-oxoguanine could result in this base substitution and that CR has an effect of reducing oxidative stress, these results indicate that the suppression of oxidative stress by CR is the cause of the reduction of this transversion.

DNA mutations play a crucial role in the origins of cancer, and the clonal expansion of mutant cells is one of the fundamental steps in multistage carcinogenesis. In this study, we correlated tumor incidence in B6C3F1 mice during the period after exposure to N-ethyl-N-nitrosourea (ENU) with the persistence of ENU-induced mutant clones in transgenic gpt delta B6C3F1 mice. The induced gpt mutations afforded no selective advantage in the mouse cells and could be distinguished by a mutational spectrum that is characteristic of ENU treatment. The gpt mutations were passengers of the mutant cell of origin and its daughter cells and thus could be used as neutral markers of clones that arose and persisted in the tissues. Female B6C3F1 mice exposed for 1 month to 200 ppm ENU in the drinking water developed early thymic lymphomas and late liver and lung tumors. To assay gpt mutations, we sampled the thymus, liver, lung, and small intestine of female gpt delta mice at 3 days, 4 weeks, and 8 weeks after the end of ENU exposure. Our results reveal that, in all four tissues, the ENU-induced gpt mutations persisted for weeks after the end of mutagen exposure. Clonal expansion of mutant cells was observed in the thymus and small intestine, with the thymus showing larger clone sizes. These results indicate that the clearance of mutant cells and the potential for clonal expansion during normal tissue growth depends on tissue type and that these factors may affect the sensitivity of different tissues to carcinogenesis. Environ. Mol. Mutagen. 58:592-606, 2017. © 2017 Wiley Periodicals, Inc.

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