Double-strand break induction and repair in V79-4 hamster cells: the role of core ionisations, as probed by ultrasoft X-rays.

PURPOSE: To compare the induction of double-strand breaks (DSB) in cells irradiated by 250 and 350 eV ultrasoft X-rays and assess the residual yield of breaks 2 hours post irradiation 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 (284 eV) and the induction of core events in DNA atoms. MATERIALS AND METHODS: V79-4 hamster cells were irradiated with synchrotron ultrasoft X-rays at isoattenuating energies of 250 eV and 350 eV. DSB were quantified using pulse field gel electrophoresis. RESULTS: A significant increase in DSB induction was observed for 350 eV ultrasoft X-rays above the carbon-K threshold, compared to 250 eV below the threshold, per unit dose to the cell. The DSB induced by the 350 eV ultrasoft X-rays were less repaired 2 h after irradiation. CONCLUSION: The increased DSB induction at 350 eV is attributed to the increase in the relative proportion of photon interactions in DNA resulting in significant dose inhomogeneity across the cell with a local increase in dose to DNA. It results from an increase in carbon-K shell interactions and the short range of the electrons produced. Core ionisations in DNA, through core-hole relaxation in conjunction with localised effects of spatially correlated low-energy photo- and Auger-electrons lead to an increase in number and the complexity of DSB.

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

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.

Cellular inactivation and chromosomal aberrations: initial damage.

It has been proposed that unrepaired or misrepaired complex lesions of DNA are responsible for cell inactivation and chromosomal aberrations. The detailed features of the critical damage and the nature of initiating physical events are actively investigated. We studied the role of inner-shell (core) ionizations in DNA atoms is studied. Ultrasoft X-rays from LURE synchrotron radiation have been used to mimic core events induced by ionizing radiations. For biological matter, inner-shell photoionization is indeed the main interaction channel of these radiations. Moreover, by tuning the X-ray energy below and above the carbon K-threshold, it is possible to achieve a two-fold increase in the number of core-ionizations in DNA for a same dose. Cell survival and chromosome aberrations have thus been studied at three iso-attenuated energies: 250, 350, and 810 eV. Relative biological efficiencies (RBEs) for cell inactivation and chromosome aberrations were found to be strongly correlated with the yields of core events in DNA.