Exposure to chronic, low-dose rate gamma-radiation at Chornobyl does not induce point mutations in Big Blue mice

Empirical genetic effects resulting from low-dose rate irradiation and chronic, cumulative exposure are poorly characterized. Expected effects are based on epidemiological studies and downward, linear extrapolations from nonthreshold models derived from acute, high-dose exposures. These extrapolations and their associated risk coefficients have no experimental support, and because of their inherent uncertainty they are the subject of considerable debate. The expectation of deleterious genetic effects resulting from low-dose rate irradiation and chronic exposure is in need of empirical assessment because this type of exposure is typical of those encountered in occupational, residential, and environmental settings. Recent acute low-dose (<10 cGy) studies using cytogenetic and point mutation endpoints indicate that observed effects range from those lower than spontaneous to an increase in the frequency of point mutations. Using the Big Blue assay, we examined the ability of chronic, continuous gamma-irradiation (2.3 x 10(-3) cGy/min) in the Chornobyl environment to induce point mutations. This system has demonstrated a significant point mutation sensitivity (4.5-fold increase) to acute, high-dose (1-3 Gy) gamma-radiation. Mutant frequencies and the mutation spectra were examined in exposed and reference samples of Big Blue mice following 90 days exposure (cumulative absorbed dose = 3 Gy) to the Chornobyl environment. No significant increase in the mutant frequency or bias in the mutational spectrum was observed in exposed individuals. This finding suggests that low-dose rate gamma-irradiation at Chornobyl does not induce point mutations and that cumulative, chronically absorbed doses do not induce the same genetic effects as acute doses of the same magnitude.