38. Dose-dependent Increase of Spi- Mutations in the Spleen and Liver of Atm-disrupted Mice Following X-ray Irradiation
Ikuko Furuno-Fukushi, Yuko Noda,
Takeshi Furuse, Yuko Hoki, Masahiko Takahagi,
Ken-ichi Masumura1, Hiroshi Suzuki2,
Takehiko Nohmi1 and Kouichi Tatsumi
(1Nat. Inst. Health. Sci.; 2Chugai
Pharmaceutical Co. Ltd.)
Keywords: Atm, Spi-, mutation, in vivo, X-rays
We have studied X-ray-induced mutations in Atm deficient (-/-) mice. The gpt-delta transgenic mice with Atm deficiency were generated for in vivo mutation assay. After X-irradiation, the transgenic mice were sacrificed and genomic DNA containing an inserted lambda shuttle vector was isolated from the liver and spleen. The DNA was packaged in vitro, and the phage was subjected to Spi- (sensitive to P2 interference) selection. When both Red and Gam functions were inactivated, deletion mutants were positively selected as Spi- plaques. Both the spontaneous mutant frequencies of the liver and spleen in Atm homozygously disrupted (-/-) mice were the same for those in Atm wild-type (+/+) mice. A linear increase in mutant frequency was observed as the dose of X-rays increased in Atm (-/-) mice and Atm (+/+) mice. The induced mutant frequency in the liver of Atm (-/-) mice exhibited no significant difference from that in Atm (+/+) mice. The induced mutant frequency in the spleen of Atm (-/-) mice was significantly higher than that in the spleen of Atm (+/+) mice. PCR analysis of the Spi- mutants obtained from irradiated mice liver and spleen revealed that deletion mutants prevailed. However, no difference was observed in the proportion of mutants with deletions between the Atm (-/-) mice and Atm (+/+) mice. These results indicate that effects of Atm deficiency on mutation induction in the spleen are different from those in the liver. The liver cells do not divide regularly and are relatively resistant to radiation. The lymphoid tissues such as spleen are very sensitive and depleted of cells by rather small radiation doses. Atm-mediated repair mechanisms of mutagenic damage after irradiation may be different depending on the organ.