Induction and repairability of DNA damage caused by ultrasoft X-rays: Role of core events

Abstract

Purpose: To investigate the severity of damage induced in plasmid DNA by ultrasoft X-rays at different energies, in order to unravel the correlation between the sharp increase in cell-killing efficiency of ultrasoft X-rays above versus below the carbon K-threshold and the induction of core events in DNA atoms.

Materials and methods: Bluescript (pBS, tight packing) and pSP189 (pSP, loose packing) plasmids were exposed to ultrasoft X-rays at 250, 380 and 760 eV energies, respectively, above phosphorus L-, carbon K- and oxygen K-thresholds. Complex DNA lesions were assayed by the repair protein Formamidopyrimidine DNA glycosylase (Fpg) and by in vitro repair assay using whole cell-free extracts.

Results: Clustered damage, as revealed by Fpg-induced double strand breaks, was observed at low level, but at similar rate at the three energies. Damage induced at 380 eV may be slightly less efficiently repaired by cell extracts than those produced at 250 eV. 760 eV photons which yield longer range electrons than 250 and 380 eV photons, induced more total damages which were more efficiently repaired, and thus likely more dispersed.

Conclusion: It is demonstrated that ultrasoft X-rays induce complex damage, which do not exhibit the same ability to be repaired, depending on the energy and on DNA packing.

Keywords:

• Ultrasoft X-rays
• core ionizations
• DNA damage
• DNA repair
• clustered lesions

Notes

Due to the strong attenuation of the photon beam through samples, irradiation energies were chosen to fulfill the isoattenuation criteria with 250 eV photons, slightly different in cells and in plasmids (Fayard et al. 2002).