The repair of chromosome aberrations in human lymphocytes after combined irradiation with UV-radiation (254 nm) and X-rays.

Human lymphocytes were exposed to UV-radiation and X-rays. The previously reported synergistic effect on the frequency of chromosome aberrations (Holmberg and Jonasson, 1974) was measured as a function of the time between the 2 irradiations to study the effect of repair processes in cells in PBS at 20 degrees C. The synergistic effect was found to be rather constant as a function of time (in the interval up to 90 min) when the UV-radiation is delivered first. The synergistic effect decreases with a half-life of about 20 min when the cells are first X-irradiated and after various times are given a UV-treatment. This is not in accordance with findings from dose-fractionation experiments with X-rays, in which lesions interact with each other for several hours. It is proposed that the enhanced aberration frequency in the combined irradiations originates from interactions between short-lived, X-ray-induced DNA-lesions in close spatial proximity (mainly lesions in the same ionization track), and the repair of these lesions are affected by the UV-treatment. In contrast, the aberrations studied in dose-fractionation experiments, by definition, are due to interactions between (long-lived) lesions in different tracks. Further details of this model for aberration production are discussed.