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

Abstract

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.

Keywords:

• ultrasoft X-rays
• double-strand breaks
• core events
• cellular radiobiology
• DNA damage
• DNA DSB repair
• ionising radiation
• radiation physics

Acknowledgements

We wish to thank E. Balanzat for his help in the preparation of this experiment and M. Sacchi for his kind support on the SU7 beamline at the LURE synchroton.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

Note

1. The energy of the absorption edge of the phosphorus L2/L3-shell is 140 eV. It is 284, 410, 543 and 2150 eV for carbon, nitrogen, oxygen and phosphorus K-shells, respectively (Henke et al. 1993).

2. Phosphorus LMM ∼120 eV; carbon KLL ∼262 eV; nitrogen KLL ∼371 eV; oxygen KLL ∼498 eV (Watanabe et al. 2004).

3. Irradiation energies above the carbon-K threshold were chosen to fulfil the isoattenuation criteria with 250 eV photons, slightly different in cells and plasmids (Fayard et al. 2002). Earlier studies in cells were performed at 340 eV. In the present study, 350 eV photons were chosen as the isoattenuation criterion is better fulfilled.

4. The average mass of a base pair is equal to 650 Da.