Combinatorial DNA Damage Pairing Model Based on X-Ray-Induced Foci Predicts the Dose and LET Dependence of Cell Death in Human Breast Cells

Abstract

In contrast to the classic view of static DNA double strand breaks (DSBs) being repaired at the site of damage, we hypothesize that DSBs move and merge with each other over large distances (µm). As X-ray dose increases, the probability of having DSB clusters increases and so does the probability of misrepair and cell death. Experimental work characterizing the dose dependence of radiation-induced foci (RIF) from X-ray in nonmalignant human mammary epithelial cells (MCF10A) is used here to validate a DSB clustering model. We then use the principles of the local effect model (LEM) to predict the yield of DSB at the sub-micron level. Two mechanisms for DSB clustering are first compared: random coalescence of DSBs versus active movement of DSBs into repair domains. Simulations that best predict both RIF dose dependence and cell survival following X-ray favor the repair domain hypothesis, suggesting the nucleus is divided into an array of regularly spaced repair domains of ~1.55 µm sides. Applying the same approach to high-LET ion tracks, we can predict experimental RIF/µm along tracks with an overall relative error of 12%, for LET ranging between 30 and 350 keV/µm and for three different ions. Finally, cell death is predicted by assuming an exponential dependence on the total number of DSBs and of all possible combinations of paired DSBs within each simulated RIF. RBE predictions for cell survival of MCF10A exposed to high-LET show an LET dependence that matches previous experimental results for similar cell types. Overall, this work suggests that microdosimetric properties of ion tracks at the sub-micron level are sufficient to explain both RIF data and survival curves for any LET, similarly to the LEM assumption. On the other hand, high-LET death mechanism does not have to infer linear-quadratic dose formalism as done in the LEM. In addition, the size of repair domains derived in our model are based on experimental RIF and are three times larger than the hypothetical LEM voxel used to fit survival curves. Our model is therefore an alternative to previous approaches by providing a testable biological mechanism (i.e. RIF). More generally, DSB pairing will help develop more accurate alternatives to the simplistic linear cancer risk model (LNT) currently used for regulating exposure to very low levels of ionizing radiation.Vadhavkar et al.

Direct measurement of the 3-dimensional DNA lesion distribution induced by energetic charged particles in a mouse model tissue

“…S2F. This notion is in line with the observed saturation of foci numbers along tracks of high-and ultrahigh-LET particles (40)(41)(42). The slope of the solid curve, which reflects the average distance between observed foci, was calculated as 0.95 μm by a fit of the data between 0.4 and 3 μm.…”

Induction and Processing of the Radiation-Induced Gamma-H2AX Signal and Its Link to the Underlying Pattern of DSB: A Combined Experimental and Modelling Study

“…At the same time, the analysis of fragment size distribution after irradiation reported by different groups, revealed an excess of fragments in the 1–2 Mbp size region which could be associated to the underlying presence of a loop organization [59,60]. In the same direction go the results obtained by Neumaier et al [55] and by Vadhavkar et al [35]. DNA DSB complexity at different levels and the implications on lesion repair were considered in the recent review by Schipler and Iliakis [61].…”

“…One possibility would be that multiple DSB are simultaneously present in the same repair centre. The clustering of DSB at the micrometre scale within domains has been recently also exploited by Vadhavkar et al [35]. Based on this concept, they could interpret the induction of foci along high energy charged particle tracks.…”

“…The implications of the finite γ H2AX foci size in the scoring of DSB have been considered in the past, especially concerning high LET radiation, in experimental [32,33] and modelling [34,35] studies. Several mechanisms potentially leading to a reduced number of foci compared to the actual induced DSB have been proposed, as for instance the merging of close-by foci and/or the presence of multiple DSB in the same focus.…”

Simulating Space Radiation-Induced Breast Tumor Incidence Using Automata

“…In a second phase, the model is used to test new mechanisms of DNA misrepair and cell death from high-LET (7) to predict high-LET response and RBE for various cosmic radiation. This model is a first step for the growing demand of a deeper knowledge of biological processes underlying carcinogenesis and their disruption by heavy ions (1).…”