Dependence of the Yield of DNA Double-Strand Breaks in Chinese Hamster V79-4 Cells on the Photon Energy of Ultrasoft X Rays

Abstract

Induction of DNA DSBs by low-LET radiations reflects clustered damage produced predominantly by low-energy, secondary electron "track ends". Cell inactivation and induction of DSBs and their rejoining, assayed using pulsed-field gel electrophoresis, were determined in Chinese hamster V79-4 cells irradiated as a monolayer with characteristic carbon K-shell (CK) (0.28 keV), aluminum K-shell (AlK) (1.49 keV), and titanium K-shell (TiK) (4.55 keV) ultrasoft X rays under aerobic and anaerobic conditions. Relative to (60)Co gamma rays, the relative biological effectiveness (RBE) for cell inactivation at 10% survival and for induction of DSBs increases as the photon energy of the ultrasoft X rays decreases. The RBE values for cell inactivation and for induction of DSBs by CK ultrasoft X rays are 2.8 +/- 0.3 and 2.7 +/- 0.3, respectively, and by TiK ultrasoft X rays are 1.5 +/- 0.1 and 1.4 +/- 0.1, respectively. Oxygen enhancement ratios (OERs) of approximately 2 for cell inactivation and induction of DSBs by ultrasoft X rays are independent of the photon energy. The time scale for rejoining of DNA DSBs is similar for both ultrasoft X rays and 60Co gamma rays. From the size distribution of small DNA fragments down to 0.48 kbp, we concluded that DSBs are induced randomly by CK and AlK ultrasoft X rays. Therefore, ultrasoft X rays are more efficient per unit dose than gamma radiation at inducing DNA DSBs, the yield of which increases with decreasing photon energy.

Strand breaks induced in plasmid DNA by ultrasoft X-rays: Influence of hydration and packing

“…In mammalian cells irradiated with 278 and 960 eV ultrasoft X-rays (de Lara et al 2001), DSB yields are at least three times higher than those measured in the present paper. This difference could be attributed to a difference in DNA conformation, packing and hydration.…”

Genome-based, mechanism-driven computational modeling of risks of ionizing radiation: The next frontier in genetic risk estimation?

“…The activity elapsed time of sequentially independent steps of repair performed by NHEJ proteins was calculated for separate DSBs. The kinetics of repair of DSBs were computed and compared with published data on repair kinetics determined using the PFGE method in Chinese hamster V79-4 cells [118,119]. There was good agreement between the model predictions and empirical observations for ultrasoft X-rays.…”

Fifth Warren K. Sinclair Keynote Address: Issues in Quantifying the Effects of Low-Level Radiation

“…It has also been widely demonstrated over the past 30 y that irradiation exclusively with low-energy electrons, produced by ultrasoft x-rays, is highly effective at producing subcellular and cellular damage. High effectiveness has been shown for effects ranging from DSBs to mutations, chromosome aberrations, cell transformation and cell inactivation (Goodhead et al 1979;Goodhead 1994b;de Lara et al 2001;Hill et al 2001). …”