Abstract
We have studies combined effect of X-rays and ENU on the development of thymic lymphoma (TL) using B6C3F1 mice. Our previous results indicated that the induction of TL by ENU was suppressed by prior irradiation of low dose X-rays (0.2Gy or 0.4Gy for 4 consecutive weeks), while high dose irradiation showed synergistic effect. This result may imply that the mechanism of TL development after combined exposures was different between low and high dose irradiation. To provide a molecular clue of combined effect of X-rays and ENU, we examined mutation rate of thymic cells after combined exposure using gpt-delta transgenic mice with B6C3F1 background. [Material and method] . Four-weeks-old gpt delta mice were exposed weekly to whole body X irradiation at 0.2 or 1Gy for 4 consecutive weeks. After irradiation, mice were given ENU at concentration of 200ppm by free-choice drinking for 4 weeks. After 4 weeks of the end of ENU treatment, genome DNA was prepared from thymus of mice for gpt assay. Lambda EG 10 was recovered from the genome using in vitro packaging followed by transfection into E.coli. Mutated gpt genes were selected in the E.coli colonies recovered from 6-TG plates and were analyzed for DNA sequencing. [Results] .Mutant frequency of gpt gene in the thymic cells from ENU treated mice was significantly increased. Preliminary data indicated that the frequency was approximately 10-4. Mutations were predominantly G:C to A:T and A:T to T:A. Further analysis in mutation of thymic cells from combined exposure is currently undertaken.
