Abstract
To determine whether mutations continually induced by low-level radiation persist and accumulate in somatic stem cells in vivo, mice heterozygous at the Dlb-1 locus (Dlb-1b/Dlb-1a) were irradiated at a dose rate of 0.04, 0.86 or 15.56mGy/day for 483 days from 8 to 78weeks of age to attain total doses of 0.02, 0.42 or 8.0Gy, respectively. Small intestines were sampled from control and irradiated mice 2weeks after the completion of irradiation and subjected to the Dlb-1 mutation assay in which evidence of mutations of Dlb-1b gene in the stem cells was detected as mutant clones on the surface of villi. About 10000 villi were observed for each of mice. Frequency of mutant clones(F) was defined as the number of clones detected per 10000 villi observed. F values in the control and the low, middle and high dose groups were 25.60±0.8, 25.62±0.8, 26.1±0.8, 35.2±1.0, respectively (N=10 for each group). Induced rate of mutant clones by unit dose of 1Gy is 1.38±0.33. Within experimental errors, this value is equal to induced rate of 1.20±0.08 that has been estimated based on F values determined for young heterozygous mice at ages of 8 to 13weeks after gamma irradiation with doses up to 8Gy in a dose-rate range of 0.3 to 3mGy/min. These results support the conclusions that (1) mutations continually induced by low-level radiation accumulate in somatic stem cells and persist for prolonged time; (2) susceptibility of stem cells to radiation mutagenesis does not change with age.
