Dna damage in peripheral blood nuclear cells assessed by comet assay from individuals submitted to scintigraphic examinations

Stannous chloride (SnCl2) is employed as a reducing agent to obtain Technetium-99m-labelled radiophamaceuticals in nuclear medicine kits, being injected endovenously in humans. Toxic effects of these kits were not studied, thus making it important to evaluate their impact in humans. In this study, the toxic effects were evaluated from peripheral blood nuclear cells (PBNC) from patients who received radiopharmaceuticals obtained using such kits. The analyses included results performed by comet assay. DNA damage was visualized in PBNC samples collected within a time up to 2 hr, and 24 hr after radiopharmaceutical injection in the patients. Initially we observed an increase of comet signals, which subsequently were reduced to zero after 24 hr. The diminishing of comet amounts probably is associated with DNA repair of damaged cells or with the elimination by apoptosis of cells whose DNA are not repaired.

Genotoxic effects of stannous chloride (SnCl2) in K562 cell line

The toxic effects of SnCl2 in K562 cells were analyzed in this study. This cell line is resistant to reactive oxygen species (ROS) making it suitable to evaluate the impact of SnCl2 in culture either through ROS or by direct toxicity using Trypan blue dye exclusion, comet and flow cytometry assays. An important loss of viability induced by SnCl2 in a dose-response manner was observed in cells treated in Tris-buffered saline (TBS). This necrotic cell death was further confirmed by flow cytometry. On the other hand, there was no loss of viability when cells were treated in rich medium (RPMI). DNA damage was visualized in SnCl2-treated K562 cells in both tested conditions. The data indicate that SnCl2 induces DNA damage and reduces K562 viability. Both actions seem to be correlated with ROS formation and direct linkage to DNA.

Damage induced by stannous chloride in plasmid DNA

Stannous chloride (SnCl(2)) is widely used in daily human life, for example, to conserve soft drinks, in food manufacturing and biocidal preparations. In nuclear medicine, stannous chloride is used as a reducing agent of Technetium-99m, a radionuclide used to label different cells and molecules. In spite of this, stannous chloride is able to generate reactive oxygen species (ROS) which can damage DNA. In this work, plasmid DNA (pUC 9.1) was incubated with SnCl(2) under different conditions and the results analyzed through DNA migration in agarose gel electrophoresis. Our data reinforce the powerful damaging effect induced by stannous ion and suggest that this salt can play a direct role in inducing DNA lesions.

Copper ions mediate the lethality induced by hydrogen peroxide in low iron conditions in Escherichia coli

Iron ions mediate the formation of lethal DNA damage by hydrogen peroxide. However, when cells are depleted of iron ions by the treatment with iron chelators, DNA damage can still be detected. Here we show that the formation of such damage in low iron conditions is due to the participation of copper ions. Copper chelators can inhibit cell inactivation, DNA strand breakage and mutagenesis induced by hydrogen peroxide in cells pre-treated with iron chelators. The Fpg and UvrA proteins play an important role in the repair of DNA lesions formed in these conditions, as suggested by the great sensitivity of the uvrA and fpg mutant strains to the treatment when compared to the wild type strain.

Repair of DNA lesions induced by hydrogen peroxide in the presence of iron chelators in Escherichia coli: participation of endonuclease IV and Fpg

In Escherichia coli, the repair of lethal DNA damage induced by H(2)O(2) requires exonuclease III, the xthA gene product. Here, we report that both endonuclease IV (the nfo gene product) and exonuclease III can mediate the repair of lesions induced by H(2)O(2) under low-iron conditions. Neither the xthA nor the nfo mutants was sensitive to H(2)O(2) in the presence of iron chelators, while the xthA nfo double mutant was significantly sensitive to this treatment, suggesting that both exonuclease III and endonuclease IV can mediate the repair of DNA lesions formed under such conditions. Sedimentation studies in alkaline sucrose gradients also demonstrated that both xthA and nfo mutants, but not the xthA nfo double mutant, can carry out complete repair of DNA strand breaks and alkali-labile bonds generated by H(2)O(2) under low-iron conditions. We also found indications that the formation of substrates for exonuclease III and endonuclease IV is mediated by the Fpg DNA glycosylase, as suggested by experiments in which the fpg mutation increased the level of cell survival, as well as repair of DNA strand breaks, in an AP endonuclease-null background.

Synergistic lethal effect between hydrogen peroxide and neocuproine (2,9-dimethyl 1,10-phenanthroline) in Escherichia coli

Despite 2,9-dimethyl 1,10-phenanthroline (NC) has been extensively used as a potential inhibitor of damage due to oxidative stress in biological systems, the incubation of E. coli cultures with the copper ion chelator NC prior to the challenge with hydrogen peroxide caused a lethal synergistic effect. The SOS response seems to be involved in the repair of the synergistic lesions through the recombination pathway. Furthermore, there is evidence for the UvrABC excinuclease participation in the repair of the synergistic lesions, and the base excision repair may also be required for bacterial survival to the synergistic effect mainly at high concentrations of H2O2, being the action of Fpg protein an important event. Incubation of lexA (Ind-) cultures with iron (II) ion chelator 2,2'-dipyridyl simultaneously with NC prevented the lethal synergistic effect. This result suggests an important role of the Fenton reaction on the phenomenon. NC treatment was able to increase the number of DNA strand breaks (DNAsb) induced by 10 mM of H2O2 in lexA (Ind-) strain and the simultaneous treatment with 2,2'-dipyridyl was able to block this effect.

Mutational potentiality of stannous chloride: an important reducing agent in the Tc-99m-radiopharmaceuticals

Stannous chloride (SnCl2) is frequently used in nuclear medicine as a reducing agent to label many radiopharmaceutical products with technetium-99m (99mTc). The aim of the present paper was to study the role of DNA repair genes in the repair of SnCl2-induced damage, using mutant strains of Escherichia coli lacking one or more DNA repair genes. Our results suggest that the product of the xthA gene, exonuclease III, is required for the repair of lesions induced by SnCl2. We further investigated the mutagenic properties of SnCl2 to a molecular level by using the supF tRNA gene as target in a forward mutational system. We have found that the survival of E. coli cells was strongly reduced with increasing concentrations of SnCl2. Moreover, when the shuttle vector pAC189 carrying the supF gene was treated with SnCl2, and then transfected to E. coli, we observed that its transformation efficiency dropped when compared to the non-treated control, with a parallel increase in mutation frequency after the damaged plasmids have replicated in bacterial cells. The mutation spectrum induced by SnCl2 reveals a high frequency of base substitutions, involving guanines. Sequence analysis of 41 independent supF mutant plasmids revealed that 39 mutants contained base substitutions, with 21 G:C to T:A and 17 G:C to C:G transversions. G to T transversions presumably resulted from 8-oxoG. However, the G to C one may be due to a yet unidentified lesion.

Implication of uracil in spontaneous mutagenesis on a single-stranded shuttle vector replicated in mammalian cells

Almost all spontaneous point mutations found on a single-stranded shuttle vector after its transfection and replication in monkey cells were located at cytosine residues. In order to understand this very specific type of targeting we have studied the possible implication of uracil residues in the induction of these spontaneous mutations. The single-stranded shuttle vector pCF3A carrying the supF tRNA gene as a mutagenesis target has been allowed to replicate in mammalian COS7 cells, mutations being screened in bacteria using the beta-galactosidase assay. Progenies from untreated DNA and DNA treated with the uracil-DNA glycosylase prior to transfection were analyzed to determine the amount and classes of mutations. While spontaneous mutation frequency was 9.7 x 10(-4) for control DNA, single-stranded vector treated with the E. coli uracil-DNA glycosylase exhibited a reduced mutation frequency of about 30%. The abolished mutations were mainly confined to the cytosine to thymine transitions for which a decrease by a factor of 5 was indeed observed. This finding fits well with the fact that it is usually admitted that uracil pairs with adenine, indicating therefore that approximately 30% of spontaneous mutations observed in our experimental conditions and 80% of C to T transitions may be due to the presence of uracil instead of cytosine.

Mutagenicity of a unique apurinic/apyrimidinic site in mammalian cells

Abasic sites are common DNA lesions produced either spontaneously or as a consequence of the action of some genotoxic agent. The mutagenic properties of a unique abasic site replicated in mammalian cells have been studied using a shuttle vector. A plasmid, able to replicate both in mammalian cells and in bacteria, carrying a unique abasic site chemically synthesized has been constructed. After replication in mammalian cells, plasmid DNA was recovered and used to transform bacteria. Mutants were screened without selection pressure by differential hybridization with a labelled oligonucleotide and their DNA was sequenced. A mutation frequency ranging from 1% to 3% was found, depending on the base originally inserted during the vector construction, opposite the abasic site. All the sequenced mutants correspond to single base-pair substitutions targeted at the abasic site. We observed a deficit in guanine incorporation opposite the abasic site, while the three other bases were incorporated with a similar efficiency. The mutational potency of abasic sites was observed without any voluntary preconditioning treatment of mammalian cells in order to induce "SOS" like conditions.

Spontaneous and ultraviolet-induced mutations on a single-stranded shuttle vector transfected into monkey cells

The shuttle vector plasmid PCF3A, carrying the supF target gene, can be transfected into monkey COS7 cells as single-stranded or double-stranded DNA. Single strand-derived plasmid progeny exhibited a 10-fold higher spontaneous mutation frequency than double strand-derived progeny. The location of spontaneous mutations obtained after transfection of the single-stranded vector shared similarities with that for double-stranded vectors. However, the nature of base changes was very different. Single-stranded PCF3A DNA was used to study ultraviolet-induced mutagenesis. An earlier report (Madzak and Sarasin, J. Mol. Biol., 218 (1991) 667-673) showed that single-stranded DNA exhibited a lower survival and a higher mutation frequency than double-stranded DNA after ultraviolet irradiation. In the present report, sequence analysis of mutant plasmids is presented. The use of a single-stranded vector allowed us to show the targeting of mutations at putative lesion sites and to determine the exact nature of the base implicated in each mutation. Frameshift mutations were more frequent after transfection of control or irradiated plasmid as single-stranded DNA than as double-stranded DNA. Multiple mutations, observed at a high frequency in the spontaneous and ultraviolet-induced mutation spectra following single-stranded DNA transfection, could be due to an error-prone polymerisation step acting on a single-stranded template.

Mutagenic properties of a unique abasic site in mammalian cells

The mutagenic properties of a true unique abasic site located opposite a guanine residue were studied. An oligonucleotide containing a chemically-produced abasic site was inserted into a shuttle vector able to replicate both in simian cells and in bacteria. Plasmid DNA was rescued from simian cells and screened in bacteria by differential hybridization with a labelled oligonucleotide probe. Mutations were easily detected and sequenced. Results showed that opposite a guanine the abasic site was error free repaired or replicated by mammalian cells with an efficiency of 99%. Point mutations occurred at a frequency of approximately 1% in control host cells and at more than 3% in UV-pre-irradiated host cells. Adenine, cytosine or thymine were found to have been inserted opposite the abasic site. No preferential insertion for a particular base was observed in contrast to that reported in bacteria.

Damages induced in lambda phage DNA by enzyme-generated triplet acetone

Exposure of lambda phage to triplet acetone, generated via the oxidation of isobutanal by peroxidase, leads to genome lesions. The majority of these lesions are detected as DNA single-strand breaks only under alkaline conditions, and so true breaks do not occur. Also, no sites sensitive to UV-endonuclease from Micrococcus luteus were found in DNA from treated phage. The participation of triplet acetone in the generation of such DNA damage is discussed.