Chromosome aberration frequencies and chromosome instability in mice after long-term exposure to low-dose-rate gamma-irradiation.

Chronological changes of chromosome aberration rates related to accumulated doses in chronically exposed humans and animals at a low-dose-rate have not been well studied. C3H female specific pathogen-free mice (8 weeks of age) were chronically irradiated. Chromosome aberration rate in mouse splenocytes after long-term exposure to low-dose-rate (LDR) gamma-rays was serially determined by conventional Giemsa method. Incidence of dicentrics and centric rings increased almost linearly up to 8000 mGy following irradiation for about 400 days at a LDR of 20 mGy/day. Clear dose-rate effects were observed in the chromosome aberration frequencies between dose rates of 20 mGy/day and 200 Gy/day. Furthermore, the frequencies of complex aberrations increased as accumulated doses increased in LDR irradiation. This trend was also observed for the incidences of micronuclei and trisomies of chromosomes 5, 13 and 18 in splenocytes, detected by micronucleus assay and metaphase fluorescence in situ hybridization (FISH) method, respectively. Incidences of 2-4 micronuclei and trisomy increased in mouse splenocytes after irradiation of 8000 mGy at a LDR of 20 mGy/day. These complex chromosome aberrations and numerical chromosome aberrations seem to be induced indirectly after radiation exposure and thus the results indicate that continuous gamma-ray irradiation for 400 days at LDR of 20 mGy/day induced chromosomal instability in mice. These results are important to evaluate the biological effects of long-term exposure to LDR radiation in humans.