Evaluation of radiation risk at the low dose rates that generally apply in occupational, environmental and medical diagnostic exposures requires extrapolation of the risks know from high-dose acute exposures, mainly from follow-up of A-bomb survivors. The proposal aims to provide data, which will improve understanding of dose-versus-risk relationships to input into mechanistic models of radiation carcinogenesis for these extrapolations. We propose to obtain data on cellular mechanisms of radiation effects which are believed to lead to cancer: DNA damage and its cellular processing into stable genomic damage as chromosomal aberrations and mutations. A novel feature is to use counted charged particle micro-beam techniques to mimic, using cells in vitro, the single-particle effects that dominate protection-level exposures. The studies include low-dose effects that dominate protection-level exposures. The studies include low-dose effects of radiation quality (LET) which relate to radiation weighting factors.
The main achievements from the RADNA project are summarised below. Workpackage 1 - Development of microbeam probes of radiation effectiveness1) Increase of European competence with technology transfer between partners, two new microbeams (and other outside contract) Increase range of radiations (low- (.e.g., fast protons, focused x-rays) to high-LET), accuracy and approaches. 2) Importance of cellular signalling at individual cell level, identify source and receptor of signal. Subcellular precision (e.g., cytoplasmic versus nuclear targeting). Window on how organised tissues respond. Low dose risk of single tracks and individual electrons. 3) Therapy relevance : Potential to test for modulation of radiation effects and signal transduction pathways in tumors and normal tissues by modifiers of radiation effects. 4) Development of ideas of relationships of bystander and adaptive responses. Workpackage 2 - DNA DSB induction and distributions 1) Ability to measure DNA damage at lowest possible dose (low LET) and at the level of single particle traversals 2) Developed and refined assays for dsb studied with respect to radiation quality (evolving from earlier contracts) 3) Improvements in measurements of yields of dsb with respect to improvements in PFGE analysis (heat labile and background). Understanding of track-correlated breaks. Development of analytical models to simulate break distributions and allow for background breaks. Quantitative description of clustering process. Extrapolation to low doses (?H2AX).

Measurement of breaks at ~1 electron track per cell and with <1 dsb (average) per cell. Workpackage 3 - Rejoining and repair fidelity of DNA 1) Evolution of activities from previous contracts 2) Importance of repair and influence of complexity of reparability for different radiations 3) Differences between correct rejoining and misrepair 4) Differences between low-dose rate and acute exposure versus radiation quality - evidence for intra-track misrepair, increasing with LET 5) HR and NHEJ both contribute considerably to the repair of IR-induced DSBs in G2, whereas HR appears to play no major role in situations where no sister chromatid is available. Number of breaks determines mis-rejoining frequencies for low LET radiation (spatial proximity) 6) Importance of clustered damage 7) Role of different repair pathways, Spontaneously induced breaks (S-phase) repaired exclusively by HR (associated with replication forks), radiation uses NHEJ 8) NHEJ important for correct rejoining of complex breaks 9) Improved measure of kinetics of repair (Heat labile) less repair in NHEJ mutants Workpackage 4 - Mechanisms of chromosome aberration formation 1) Evidence for lesion non-lesion interaction 2) Evidence for a single-hit formation of chromatid breaks 3) Development of a model of fast local chromatin denaturation 4) Previously unknown complexity of chromosome aberrations using developments of FISH technologies potential future signatures of exposure 5) Experimental data from RGL validating Cremer hypothesis for formation of exchanges, Workpackage 5 - Mutation Induction 1) Non-contiguous deletions, consistent with chromosome level (small fragments) LET dependence 2) Deletion patterns reflect cell type? Future study 3) Little difference in lesion spectrum in repair inhibited cells (although differences in mutation yields) AEKI 1) Heterogeneous distribution of damage at lung branch points and modelling of activity distribution. 2) Alpha and neutron induced radio adaptation have been measured also in bystandar cells.