A model relating cell survival to DNA fragment loss and unrepaired double-strand breaks

The model of radiation action that is presented relates the surviving fraction of irradiated cells to unrepaired DNA double-strand breaks (DSBs). The following assumptions are made in the model: (i) A DNA fragment created by the induced DSBs may move out of its chromosome (become lost), and the probability of that process depends on the fragment size. (ii) An irradiated cell will lose its proliferative capacity if it has an unrepaired DSB (including DNA fragments) at certain points in the cell cycle. Mathematical expressions of the model yield the dose and time dependencies of the surviving fraction, the number of unrepaired DSBs, and the number of prematurely condensed chromosome fragments. Radiobiological phenomena described include effects of low dose rate, delayed plating, hypertonic solution, araA, and high-LET radiation. The calculated dose dependence of the residual number of unrepaired DSBs for ataxia telangiectasia and normal fibroblast cells is very close to the experimentally obtained [M. N. Cornforth and J. S. Bedford, Radiat. Res. 111, 385-405 (1987)] total number of chromosomal aberrations. This leads to the conclusion that each unrepaired DSB becomes a chromosomal aberration. Analysis in terms of the model shows that the radiosensitivity of various cell lines is predominantly due to the different amounts of time available for DSB repair in these cells.