Induction and repair of DNA strand breaks and 1-beta-D-arabinofuranosylcytosine-detectable sites in 40-75 kVp X-irradiated compared to 60Co gamma-irradiated human cell lines

Abstract

We have compared the initial yield and subsequent enzymatic repair of DNA strand breaks and 1-beta-D-arabinofuranosylcytosine (araC)-detectable sites in cultured human cell lines after irradiation with soft (40-75 kVp) X rays and hard (60Co) gamma rays, two forms of low-LET radiation whose average ionization densities differ by 15-fold (i.e., approximately 4.5 keV/mu and approximately 0.3 keV/mu, respectively). Incubation of X- or gamma-irradiated cell cultures in the presence of araC resulted in the accumulation of DNA single-strand interruptions, as measured by conventional velocity sedimentation analysis in alkaline sucrose gradients. Since the drug is a selective inhibitor of DNA polymerase alpha, this observation implicates polymerase alpha in the repair of radiogenic damage. Specifically, DNA repair analysis in X-ray-treated cells suggested that polymerase alpha is a key participant in the repair of a distinct, albeit structurally undefined, class of radioproducts [presumably a combination of double-strand breaks and alkali-stable lesions (e.g., modified base/sugar residues and DNA crosslinks)], but takes little, if any, part in the restitution of single-strand breaks (whether frank scissions or alkali-labile bonds) induced by ionizing radiation. Accordingly, the drug did not significantly inhibit the repair of DNA damage in cultures exposed to less than or equal to 100 Gy of gamma radiation. A remarkable difference was evident with respect to the absolute yields of these two major classes of DNA injury--that is, araC-detectable sites and single-strand breaks--depending on the type of radiation administered. For the same dose, the level of araC-detectable sites found in cells treated with gamma rays (less than or equal to 150 Gy) was as much as 3.4 times lower than that observed in X-ray-exposed cells (i.e., one site per 1,155 and 335 eV, respectively). In contrast, the number of strand breaks induced by gamma rays was approximately 2.3-fold higher than that produced by X rays (one break per 27 and 61 eV, respectively). Assuming that, among other conditions, the relative biological effectiveness of the comparatively densely ionizing soft X rays is roughly twice that of the sparsely ionizing hard gamma rays, our data strongly imply that those radioproducts which are manifested as araC-detectable sites may be approximately 45 times more deleterious, on a per lesion basis, than those observed as single-strand breaks.