Abstract
BackgroundLittle is known about the cellular effects of exposure to mixed beams of high and low linear energy transfer radiation. So far, the effects of combined exposures have mainly been assessed with clonogenic survival or cytogenetic methods, and the results are contradictory. The gamma-H2AX assay has up to now not been applied in this context, and it is a promising tool for investigating the early cellular response to mixed beam irradiation.PurposeTo determine the dose response and repair kinetics of gamma-H2AX ionizing radiation-induced foci in VH10 human fibroblasts exposed to mixed beams of 241Am alpha particles and X-rays.ResultsVH10 human fibroblasts were irradiated with each radiation type individually or both in combination at 37°C. Foci were scored for repair kinetics 0.5, 1, 3 and 24 h after irradiation (one dose per irradiation type), and for dose response at the 1 h time point. The dose response effect of mixed beam was additive, and the relative biological effectiveness for alpha particles (as compared to X-rays) was of 0.76 ± 0.52 for the total number of foci, and 2.54 ± 1.11 for large foci. The repair kinetics for total number of foci in cells exposed to mixed beam irradiation was intermediate to that of cells exposed to alpha particles and X-rays. However, for mixed beam-irradiated cells the frequency and area of large foci were initially lower than predicted and increased during the first 3 hours of repair (while the predicted number and area did not).ConclusionsThe repair kinetics of large foci after mixed beam exposure was significantly different from predicted based on the effect of the single dose components. The formation of large foci was delayed and they did not reach their maximum area until 1 h after irradiation. We hypothesize that the presence of low X-ray-induced damage engages the DNA repair machinery leading to a delayed DNA damage response to the more complex DNA damage induced by alpha particles.
Highlights
Little is known about the cellular effects of exposure to mixed beams of high and low linear energy transfer radiation
The repair kinetics for total number of foci in cells exposed to mixed beam irradiation was intermediate to that of cells exposed to alpha particles and X-rays
The repair kinetics of large foci after mixed beam exposure was significantly different from predicted based on the effect of the single dose components
Summary
Little is known about the cellular effects of exposure to mixed beams of high and low linear energy transfer radiation. Purpose: To determine the dose response and repair kinetics of gamma-H2AX ionizing radiation-induced foci in VH10 human fibroblasts exposed to mixed beams of 241Am alpha particles and X-rays. Cells can be irradiated with sparsely ionizing X or gamma rays (referred to as low linear energy transfer (LET) radiation) or with densely ionizing alpha particles or heavy ions (referred to as high LET radiation). In the case of combined action of two radiation types there is no reason to assume that the level of initial DNA damage differs from additivity, because the level of damage is directly proportional to the amount of energy absorbed by the cell. It is possible that the simultaneous action of the two radiation types leads to a change of damage quality (an increased damage complexity within chromosome domains) or that the damage induced by one radiation engages the DNA repair machinery to such an extent that the damage induced by the second radiation is not repaired properly
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