No dose-dependence of DNA double-strand break misrejoining following alpha-particle irradiation

Purpose: To investigate whether an explanation for the high effectiveness o f densely ionizing radiation with regard to complex biological endpoints ca n be derived from measurements of radiation-induced double-strand break (DS B) misrejoining. Materials and methods: Misrejoining of radiation-induced DSB in normal huma n fibroblasts was determined by comparing hybridization analysis of large r estriction fragments as a measure for correct rejoining, with results from a conventional pulsed-field gel electrophoresis technique (FAR) that measur es total DSB rejoining. In order to investigate DSB misrejoining at doses f or which chromosome aberration data are available, a dose fractionation pro tocol was applied so that the number of DSB at any given timepoint was low but the cumulative amount of misrejoined DSB sufficient for detection and p recise quantitation. Results and conclusion: After an acute 80 Gy alpha-particle exposure and a repair incubation period of 24 h, 50% of all initially induced DSB were mis rejoined, in agreement with data obtained for X-rays. X-irrradiation with 1 6 x 5 Gy, 8 x 10 Gy, 4 x 20 Gy, or 2 x 40 Gy and repair incubation of 24 h following each individual dose fraction was recently reported to yield misr ejoining frequencies that strongly decrease with increasing fractionation ( Lobrich et al. 2000; Genes, Chromosomes and Cancer, 27, 59-68). In the pres ent study, constant misrejoining frequencies of 50% were observed after alp ha-particle exposure with the same fractionation protocol. This difference between alpha-particles and X-rays is in accordance with the high biologica l effectiveness of densely ionizing radiation and provides a direct link be tween misrejoining of DSB grid cytologically visible exchange aberrations. Further evidence suggests that if the same dose range is compared, the numb er of misrejoined DSB exceeds the number of microscopically visible aberrat ions by an order of magnitude for both radiation types, probably reflecting the high resolution of the hybridization approach compared with cytologica l techniques.