DNA DSB induced in human cells by charged particles and gamma rays: Experimental results and theoretical approaches

Purpose: To quantify the role played by radiation track structure and background fragments in modulating DNA fragmentation in human cells exposed to gamma rays and light ions. Materials and methods: Human fibroblasts were exposed in vitro to different doses (in the range from 40 to 200 Gy) of 60Co gamma rays and 0.84 MeV protons (Linear Energy Transfer, LET, in tissue 28.5 keV/micrometre). The resulting DNA fragments were scored under two electrophoretic conditions, in order to optimize separation in the size ranges 0.023-1.0 Mbp and 1.0-5.7 Mbp. In parallel, DNA fragmentation was simulated both with a phenomenological approach based on the "generalized broken-stick” model, and with a mechanistic approach based on the PARTRAC (acronym of PARticle TRACk) Monte Carlo code (1.32 MeV photons were used for the simulation of 60Co gamma rays). Results: For both gamma rays and protons, the experimental dose response in the range 0.023-5.7 Mbp could be approximated as a straight line, the slope of which provided a yield of (5.3 ± 0.4)•10-9 Gy-1 bp-1 for gamma rays and (7.1 ± 0.6)•10-9 Gy-1 bp-1 for protons, leading to a Relative Biological Effectiveness (RBE) of 1.3 ± 0.2. From both theoretical analyses it appeared that, while gamma-ray data were consistent with double-strand breaks (DSB) random induction, protons at low doses showed significant deviation from randomness, implying enhanced production of small fragments in the low molecular weight part of the experimental range. The theoretical analysis of fragment production was then extended to ranges where data were not available, i.e. to fragments larger than 5.7 Mbp and smaller than 23 kbp. The main outcome was that small fragments (< 23 kbp) are produced almost exclusively via non random processes, since their number in considerably higher than that produced by a random insertion of DSB. Furthermore, for protons the number of these small fragments is a significant fraction (about 20 %) of the total number of fragments; these fragments remain undetected in these experiments. Calculations for 3.3 MeV alpha particle irradiation (for which no experimental data were available) were performed to further investigate the role of fragments smaller than 23 kbp; in this case, besides the non random character of their production, their number resulted to be at least as much as half of the total number of fragments. Conclusion: Comparison between experimental data and two different theoretical approaches provided further support to the hypothesis of an important role of track structure in modulating DNA damage. According to the theoretical approaches, non randomness of fragment production was found for proton irradiation for the smaller fragments in the experimental size range and, in a significantly larger extent, for fragments of size less than 23 kbp, both for protons and alpha particles.