Inverse dose-rate effect for mutation induction by γ-rays in human lymphoblasts

In order to define further the effects of differences in recombinational proficiency on cell survival and mutation by ionizing radiation, we exposed the syngenic cell lines TK6 and WTK1 to continuous low dose-rate γ- irradiation. We previously demonstrated that acute X-ray exposure results in lower survival and lower mutation induction at both the thymidine kinase (tk) and the hypoxanthine-guanine phosphoribosyltransferase (hprt) loci in TK6 cells compared with WTK1 cells. These differences were attributed in part to reduced levels of recombination in the TK6 line relative to WTK1. Using a low dose rate ¹³⁷Cs irradiator, we exposed asynchronous growing populations of these cells to γ-rays at 14.3, 6.7 and 2.7 cGy/h. Both cell lines exhibited a dose-rate effect on survival. Compared with acute doses, the low dosed- rates also protected against mutation induction at the hrpt locus in WTK1, but protection was inversely related to dose-rate. There was also a slight inverse dose-rate effect in TK6, with mutation induction at the lowest dose- rate exceeding that at acute exposures.