Abstract
We have studied combined effect of X-rays and N-ethyl-N-nitrosurea (ENU) on the development of thymic lymphoma (TL) using B6C3Fl mice. Our previous results indicated that the induction of TL by ENU was suppressed by prior irradiation of low dose X-rays (0.2 Gy for 4 consecutive weeks), while high dose irradiation (1 Gy for 4 consecutive weeks) showed synergistic effect. This was coincident with the reduced and enhanced frequency of ENU-induced gpt mutations by prior exposure to low and high dose radiation, respectively. To provide further molecular clue of combined effect, we examined deletion mutations (Spi-) of thymic cells.
[Material and methods]
Four-weeks-old gpt-delta mice were exposed weekly to whole body X irradiation at 0.2 or I Gy for 4 consecutive weeks. After irradiation, mice were given ENU at concentration of 200 ppm by free-choice drinking for 4 weeks. After 4 weeks of the end of ENU treatment, genome DNA was prepared from the thymus for Spi- assay, which can detect deletions of red/gam gene.
[Result]
We found that net mutant frequency varied from I to 7 x 10-6. Mutations occurring in gam gene were predominantly one base deletion in run sequence in control and X-irradiated groups. In ENU- and combined-exposure groups, however, base substitution mutations were also developed. There was little difference in occurrence of large deletion mutation among groups (< 2 folds). These results suggested that, unlike gpt mutation, occurrence of deletion mutations were not affected by pre-irradiation. Taken together, it is suggested that the development of TL by combined exposure was mainly caused by increase or decrease in base substitution, which is dose-dependent.
