The role of short-lived DNA lesions in the production of chromosome-exchange aberrations.

Human lymphocytes were treated with combined UVC radiation and X-rays or they were X-irradiated and incubated for 60-90 min in the presence of the DNA-repair inhibitor ara-C. The X-ray-induced chromosome exchange aberration yield was enhanced both by UVC and ara-C. approximately a factor of two in the linear (low dose) portion of the dose-response curve. The enhancement was small in the dose squared (high dose) portion where previous dose-fractionation experiments have shown that X-ray-induced lesions leading to aberrations exist for several hours. The yield of aberrations in lymphocytes incubated after irradiation in the presence of ara-C reaches a saturation level almost immediately after irradiation (5-15 min). These cytogenetic observations together with a previous finding (Holmberg and Strausmanis, 1983) give direct and indirect evidence that the enhanced aberration yield is due to short-lived DNA breaks formed immediately after X-irradiation. Measurements on the repair kinetics of the DNA breaks induced by X-irradiation show that ara-C strongly impairs the repair of short-lived X-ray-induced DNA breaks. It was also observed that the DNA breaks generated after UVC irradiation occur almost immediately after irradiation and the level of these transient DNA breaks reaches saturation even for short incubation times. Thus, the repair of these breaks can complete with the repair of short-lived X-ray-induced DNA-breaks in combined irradiation with UVC and X-rays. The experimental results can be explained on the assumption that X-ray-induced aberrations originate from exchange complexes formed in interactions between both short-lived and long-lived DNA breaks. The short-lived DNA breaks give rise to exchange complexes mainly within single ionization tracks where the DNA breaks are close together. The time between irradiation and exchange complex formation is of the order of 5-15 min within such a track, and short-lived breaks might be repaired before complexes have been formed. If the DNA repair of these breaks is delayed by UVC or ara-C treatment this results in a higher probability of exchange-complex formation. In contrast, interactions between breaks in different tracks originate from long-lived DNA breaks and the probability for complex formation from these breaks is not markedly affected by UVC or ara-C.