Mechanistic simulation of radiation damage to DNA and its repair: on the track towards systems radiation biology modelling.

The biophysical simulation code PARTRAC enables, by combining track structure calculations with DNA models on diverse genomic scales, prediction of DNA damage yields and patterns for various radiation qualities. To extend its applicability to later endpoints such as mutagenesis or cell killing, a continuative model for repair of radiation-induced double-strand break (DSB) via non-homologous end-joining has complemented the PARTRAC code by about 12 orders of magnitude on a temporal scale. The repair model describes step-by-step by the Monte Carlo method the attachment and dissociation of involved repair enzymes and diffusion motion of DNA ends. The complexity of initial DNA lesion patterns influences the repair kinetics and outcome via additional cleaning steps required for dirty DNA ends. Model parameters have been taken from measured attachment kinetics of repair enzymes and adaptation to DSB rejoining kinetics after gamma irradiation. Application of the DNA repair model to damage patterns following nitrogen ion irradiation and comparison with experimental results reveal the need for further model refinements. Nevertheless, already the present model represents a promising step towards systems modelling of cellular response to radiation.