Evaluation of lesion clustering in irradiated plasmid DNA

Purpose: To measure the yield of DNA strand breaks and clustered lesions in plasmid DNA irradiated with protons, helium nuclei, and gamma- rays. Materials and methods: Plasmid DNA was irradiated with 1.03, 19.3 and 249 MeV protons ( linear energy transfer = 25.5, 2.7, and 0.39 keV mu m(-1) respectively), 26 MeV helium nuclei ( 25.5 keV mu m) and gamma-rays ( Cs-137 or Co-60) in phosphate buffer containing 2 mM or 200 mM glycerol. Single- and double- strand breaks ( SSB and DSB) were measured by gel electrophoresis, and clustered lesions containing base lesions were quantified by converting them into irreparable DSB in transformed bacteria. Results: For protons, SSB yield decreased with increasing LET ( linear energy transfer). The yield of DSB and all clustered lesions seemed to reach a minimum around 3 keV mu m(-1). There was a higher yield of SSB, DSB and total clustered lesions for protons compared to helium nuclei at 25.5 keV mu m(-1). A difference in the yields between Cs-137 and Co-60 gamma-rays was also observed, especially for SSB. Conclusion: In this work we have demonstrated the complex LET dependence of clustered- lesion yields, governed by interplay of the radical recombination and change in track structure. As expected, there was also a significant difference in clustered lesion yields between various radiation fields, having the same or similar LET values, but differing in nanometric track structure.