Enzymatic restriction of mammalian cell DNA using Pvu II and Bam H1: evidence for the double-strand break origin of chromosomal aberrations

Induction of oxidative single- and double-strand breaks in DNA by ferric citrate

The relative risk of primary hepatocellular carcinoma in genetic hemochromatosis (GH) is estimated at over 200 times as that of control populations. Recently, ferric ion chelated to citrate (Fe-citrate) was identified as the major non-transferrin-bound iron in the serum of GH patients. We investigated whether low concentration of Fe-citrate plus reductant could damage supercoiled plasmid DNA under physiological pH and ionic strength. Incubation of Fe-citrate with either H2O2, L-ascorbate, or L-cysteine induced single- and double-strand breaks in supercoiled plasmid pZ189 in a concentration- and time-dependent fashion. DNA strand breaks produced by Fe-citrate plus H2O2 increased at reduced pH (< or = 6.9). Catalase and free radical scavengers inhibited the DNA breakage produced by Fe-citrate in combination with each reductant, suggesting that H2O2 and finally .OH are responsible DNA damaging species. The catalytic ability of Fe-citrate to induce DNA strand breaks, particularly double-strand breaks (DSBs), may contribute to the carcinogenic processes observed in GH.

Alteration of gene expression by restriction enzymes electroporated into plant cells

The alteration in the expression of a beta-glucuronidase (GUS) reporter gene was used to monitor the effect of restriction endonucleases electroporated into the tobacco (Nicotiana tabacum L.) protoplasts. Restriction enzyme (RE) Hind III which does not have a recognition site within the gene cassette, had little effect on enzyme activity. In contrast restriction endonucleases Hae III and Sau3A1 which possess 8 and 16 recognition sites in the GUS cassette, were found to reduce the enzyme activity by 89% and 94% respectively when compared to control electroporations. Restriction-site mutation analysis (RSM) and Southern blot analysis indicated the enzymatic degradation of GUS coding sequence by the REs Hae III and Sau3A1. Results of this study suggest that on electroporation, REs can enter into plant cells and alter the expression of the GUS gene. The alteration of gene expression is thus correlated with the digestion of GUS template DNA. Future applications of this technique could include addressing fundamental questions with regard to DNA repair, site-specific recombination, identifying mutations, insertional mutagenesis, enhancement of stable transformation and gene tagging in plants.

Chromatid-type aberrations induced by AluI in Chinese hamster ovary cells

Treatment of CHO cells with AluI in the S-phase leads to chromatid-type aberrations whose frequencies are linearly correlated with dose. Treatment in the S-phase leads to fewer aberrations than treatment in the G1-phase, which is comparable to chromosomal aberration induction following X-irradiation in the G1- and S-phases. Treatment in the G1-phase leads to few chromatid-type interchanges, some of these may originate from DNA single-strand gaps induced by AluI in canonical structures of DNA and in DNA.RNA hybrids.

Ionizing radiation induced DNA lesions which lead to chromosomal aberrations

Several radiosensitive mutant cell lines of CHO and V79 cells have been studied to explore a possible correlation between radiation induced DNA lesions and chromosomal aberrations. In the xrs mutants which are deficient in DNA double strand break (DSB) repair, there is a correlation between the extent of the defect in repair and the frequencies of radiation induced chromosomal aberrations. In another type of radiosensitive mutant (V-C4), which has no detectable defect in DNA DSB repair, the frequencies of X-ray induced aberrations are high in comparison to wild type V79 cells. However, following treatment with restriction endonucleases or fission neutrons, the frequencies of aberrations are similar to those in V79, indicating that V-C4 cells are defective in repair of X-ray induced lesions other than DSBs. Though DSBs are the most important lesions leading to chromosomal aberrations, in repair deficient mutants, radiation induced lesions other than DSBs can lead to chromosomal aberrations.

Restriction-endonuclease-induced DNA double-strand breaks and chromosomal aberrations in mammalian cells

Restriction endonucleases (RE) can be used to mimic and model the clastogenic effects of ionising radiation. With the development of improved techniques for cell poration: electroporation and recently streptolysin O (SLO), it has become possible more confidently to study the relationships between DNA double-strand breaks (dsb) of various types (e.g. blunt or cohesive-ended) and the frequencies of induced metaphase chromosomal aberrations or micronuclei in cytokinesis-blocked cells. Although RE-induced dsb do not mimic the chemical end-structure of radiation-induced dsb (i.e. the 'dirty' ends of radiation-induced dsb), it has become clear that cohesive-ended dsb, which are thought to be the major type of dsb induced by radiation, are much less clastogenic than blunt-ended dsb. It has also been possible, with the aid of electroporation or SLO to measure the kinetics of dsb in cells as a function of time after treatment. These experiments have shown that some RE (e.g. Pvu II) are extremely stable inside CHO cells and at high concentrations persist and induce dsb over a period of many hours following treatment. Cutting of DNA by RE is thought to be at specific recognition sequences (as in free DNA) although the frequencies of sites in native chromatin available to RE is not yet known. DNA condensation and methylation are both factors limiting the numbers of available cutting sites. Relatively little is known about the kinetics of incision or repair of RE-induced dsb in cells. Direct ligation may be a method used by cells to rejoin the bulk of RE-induced dsb, since inhibitors such as araA, araC and aphidicolin appear not prevent rejoining, although these inhibitors have been found to lead to enhanced frequencies of chromosomal aberrations. 3-Aminobenzimide, the poly-ADP ribose polymerase inhibitor is the only agent that has so far been shown to inhibit rejoining of RE-induced dsb. Data from the radiosensitive xrs5 cell line, where chromosomal aberration frequencies are higher after RE treatments than in their normal parental CHO line, indicates that the xrs dsb repair pathway is involved in the repair of these dsb. We found that cells treated simultaneous with Pvu II and T4 ligase yielded lower levels of chromosomal damage than in the WT parental line indicating that Pvu II induced dsb retain their ability to be blunt-end ligated inside the cell.

Double-strand breaks in plasmid DNA and the induction of deletions

Double-strand breaks (dsbs) have been produced in plasmid DNA by various restriction endonucleases and the survival and the deletion mutation incidence have been measured in E. coli. The deletion formation is known to depend upon the occurrence of short direct repeats within the DNA molecule. In order to study the role of these repeats we constructed plasmid molecules with repeats of various lengths or with a 10-base pair repeat at different distances from each other. Furthermore the influence of the location and the structure of the dsb was studied. Repair and deletion frequencies of the linearized plasmids were measured after transformation of E. coli. The yield of the specific deletion mutation (the one which occurs between the introduced repeats) increases nearly linearly with the square of the length of the repeat, while the yield of the correctly repaired DNA and the yield of all other deletion mutants remained constant. The slope of the linear increase of the yield of the specific deletion depends on the location and the structure of the dsb. The yield of the specific deletion mutation decreases with increasing distance between the repeats. A proposal for the rate-determining step of the deletion formation is made.

Restriction endonucleases do induce sister-chromatid exchanges in Chinese hamster ovary cells

Restriction endonucleases (REs) AluI, EcoRI, MspI, PvuII and SmaI were introduced into exponentially growing Chinese hamster ovary (CHO) cells permeabilised by the bacterial cytotoxin, streptolysin O (SLO). Sister-chromatid exchanges (SCEs) were scored in posttreatment metaphases following recovery times of 20 and 24 h. At both fixation times, all five REs tested were found to increase the frequencies of SCEs compared with that of SLO-treated controls. Among the REs used, AluI and PvuII were found to be most effective, inducing on average a 2-3-fold increase, with a proportion of cells exhibiting more than 80 SCEs. These results confirm the earlier findings of Natarajan et al. (1985) and tend to suggest that double-strand breaks (DSBs) induced by REs are involved in the formation of SCEs.

Noncomplementary DNA double-strand-break rejoining in bacterial and human cells

We examined the rejoining of noncomplementary restriction enzyme-produced DNA double-strand breaks in Escherichia coli and in cultured human cells. The enzymes used in this study, ClaI, BamHI and SalI, produce double-strand breaks with 5 protruding single strands. The joining of a ClaI-produced DNA end to a BamHI-produced end or to a SalI-produced end was examined at the DNA sequence level. End rejoining in E.coli was studied by transforming cultures with linear plasmid DNA that was gel purified from restriction digests, and end rejoining in cultured human cells was studied by introducing enzymes into the cells by electroporation. The human cells used contain an Epstein-Barr virus (EBV)-based shuttle vector, pHAZE, that was recovered and introduced into E.coli for further analysis. The major products of DNA end-joining processes observed in linear plasmid-transformed E.coli and in the human cells exposed to restriction enzymes were identical. Furthermore, the deletions observed in both systems and in the spontaneous mutant plasmids in untreated human cells had a common underlying feature: short stretches of directly repeated DNA at the junction sites.

Chromosome damage induced by nanomolar concentrations of bleomycin in porated mammalian cells

We have examined chromosome damage caused by a wide range of bleomycin (BLM) concentrations in Chinese hamster ovary (CHO-K1) cells reversibly porated by the bacterial cytotoxin streptolysin-O (SLO). Chromosome damage was measured using the micronucleus cytokinesis block technique (employing cytochalasin-B). Treatment of exponentially growing cells with 0.045 IU/mL SLO for 5 min resulted in up to a thousand-fold and a million-fold increase in biological effectiveness, compared to treatment in the absence of SLO for 24 hr and 5 min, respectively. Increases in micronuclei of 4-5 times background level were observed after only 5 min exposure to the drug in the presence of SLO at doses as low as 100 pg/mL (approximately 70 pmol/L). These results indicate that the use of SLO may facilitate the treatment of cells with BLM for periods of time resembling acute exposure to ionizing radiations.

Higher G2 sensitivity to the induction of chromosomal damage in the CHO mutant EM9 than in its parental line AA8 by camptothecin, an inhibitor of DNA topoisomerase I

The induction of chromosomal alterations by camptothecin (CPT), an inhibitor of DNA topoisomerase I, in the G2 stage was studied in a CHO mutant cell line EM9, which has an elevated baseline frequency of SCEs, and in its parental cell line AA8. EM9 cells were found to be more sensitive to CPT than the parental cell line. It is proposed that the effect of camptothecin in this phase of the cell cycle is caused by a 'residual' DNA synthesis which is higher in EM9 than in AA8 cells.

Mutagenic effects of restriction enzymes in Chinese hamster cells: evidence for high mutagenicity of Sau3AI at the hprt locus

CHO cells were exposed to seven different restriction endonucleases by electroporation and their cytotoxicity and mutagenicity measured. Cell killing as determined by a colony formation assay occurred in a concentration-dependent manner for each enzyme. The D0 of the survival curves were: MspI = 24U; AluI = 31U; Sau3AI = 106U; HaeIII = 46U; HinfI = 30U; PvuII = 35U; BamHI = 163U. BamHI and Sau3AI were particularly ineffective in cell killing. For the 6-base recognition sequence enzymes, PvuII (a blunt-ended cutter) was much more cytotoxic per unit electroporated than BamHI (a sticky-ended cutter). Among the 4-base cutters, Sau3AI and HaeIII were generally less cytotoxic than HinfI or PvuII. Cell killing appeared to depend on the nature of the recognition sequence and cutting sites rather than on the cutting frequency. The mutagenic effects of these restriction endonucleases were investigated by measuring the induced frequencies of hprt gene mutations. The mutagenicity of Sau3AI was dramatically higher than the other enzymes, increasing linearly with dose up to 35U. When normalized for survival, the mutagenicity of Sau3AI relative to the other enzymes was even greater. The mutagenic effect of BamHI, which has the same 5' protruding site as Sau3AI, was much lower at similar dose and survival levels. MspI, BamHI, and PvuII which have no recognition sites within the hprt coding sequence were marginally- or non-mutagenic. Based on these results and the distribution of cutting sites within the hprt cDNA for the enzymes studied, the hypothesis is discussed that a region in exon 4 is highly sensitive to the induction of mutants by DNA double-strand breaks.

Stress factors affecting expression of O6-methylguanine-DNA methyltransferase mRNA in rat hepatoma cells

O6-Methylguanine-DNA methyltransferase (MGMT) is decisively involved in protecting mammalian cells against genotoxic effects of alkylating carcinogens. We analysed regulation of MGMT expression after exposing rat hepatoma H4IIE cells to various 'stress' factors. Treatments that damage DNA such as alkylation, hydrogen peroxide, ultraviolet or X-ray exposure, as well as restriction enzymes introduced into cells by electroporation or arrest of replication by hydroxyurea significantly induced MGMT mRNA (2.5 to 5-fold). Slight induction (up to 2.5-fold) was observed after heat shock or cadmium/zinc treatment. No or only a very weak induction (less than 1.5-fold) was observed after treatment with 6-thioguanine, 5-azacytidine, transfection of methylated DNA, depletion of MGMT by feeding with O6-methylguanine or O6-benzylguanine, serum starvation and feeding of starved cells, cAMP, TPA and dexamethasone treatment. Inhibitors of protein kinases, H8 and H9, induced MGMT mRNA. On the other hand, an inhibitor of phosphatases (sodium vanadate) prevented induction of MGMT by N-methyl-N'-nitro-N-nitrosoguanidine. The data indicate that DNA breaks are an ultimate signal for MGMT mRNA induction and that protein phosphorylation is involved in regulating MGMT expression.

Molecular mechanisms of the formation of DNA double-strand breaks and induction of genomic rearrangements

The probability that damage occurs in closely opposed sites on complementary DNA strands increases when DNA is heavily modified with mutagenic agents. Enzymatic excision of the opposite lesions produces DNA double-strand breaks which give rise to genomic rearrangements (deletions, insertions, etc.). Plasmid systems were developed for studying chemical lesions leading to double-strand breaks and the fate of broken plasmid molecules within bacterial cells. Deletions result from the base-pairing of fortuitously located direct repeats flanking the DNA broken ends; as a consequence, the latter are joined, while the DNA fragment between the direct repeats is deleted. Genomic rearrangements arise during the repair of the DNA double-strand breaks, and both events are due to similar repair enzymes which maintain the integrity of the DNA primary structure when conditions are not stressful. A number of genomic rearrangements and point mutations seem to be predetermined by the DNA primary structure.

Residual DNA damage: what is left over and how does this determine cell fate?

Radiation-induced DNA damage may remain unrepaired for a number of reasons: it may be too severe, it may be in inaccessible parts of the genome, it may be induced at critical points in the cell cycle or it may be converted into large DNA deletions. This residual damage is likely to be responsible for cell death either by physical restriction of replication or transcription or by metabolic disruption due to loss of function of critical genes. Although residual damage is important, cells may differ in their ability to tolerate it, which may be a factor that determines the relative radiosensitivity of a given cell population.

Elevated levels of DNA double-strand breaks (dsb) in restriction endonuclease-treated xrs5 cells correlate with the reduced capacity to repair dsb

Recently we have reported the kinetics of DNA double-strand breaks (dsb) induced in electroporated mammalian (CHO) cells that had been treated with the restriction endonuclease PvuII, as measured by the filter elution assay at the non-denaturing pH of 9.6. A gradual accumulation of dsb was observed over a 24-h incubation period following the restriction endonuclease (RE) treatment and this was attributed to a competition between incision of the DNA by PvuII and dsb repair. In order to test this 'competition' hypothesis we have carried out similar experiments in the radiosensitive xrs5 mutant cell line, which has been shown to be deficient in dsb repair. The levels of dsb monitored by the non-denaturing filter elution assay in the xrs5 cell line treated with PvuII was found to be 3-4 times higher than that found for the wild-type CHO K1 cell line. Levels of dsb were also significantly raised in xrs5 cells treated with BamHI, as compared with the background levels observed in the CHO line. These data lend strong support to the competition hypothesis of simultaneous incision and repair of RE-induced dsb.

The genetic toxicology of 5-bromodeoxyuridine in mammalian cells

The thymidine analog, BrdUrd, induces many biological responses which are of importance to the field of genetic toxicology and related disciplines. These include the induction of SCE, specific-locus mutations, and toxicity, inhibition of cell proliferation, and the expression of fragile sites in the human genome. In early models which addressed the mechanisms of the biological effects of BrdUrd exposure, two pathways were proposed to account for the induction of the biological responses. Incorporation of the enol form of BrdUrd into the nascent DNA strand after pairing with deoxyguanosine was proposed as one pathway, whereas the incorporation of BrdUrd opposite adenosine in place of thymidine was proposed as the second pathway. Many novel and sophisticated techniques have been applied to the study of the mechanism of the induction of biological effects by BrdUrd leading to a substantial increase in our understanding of these mechanisms. However, the experimental evidence clearly supports the contention that BrdUrd exerts its effects on eukaryotic cells through mechanisms similar to those originally proposed to explain the genotoxicity of BrdUrd.

Molecular assays of radiation-induced DNA damage

There is a need for assays of DNA damage in many areas of laboratory research applied to radiation therapy, in order to understand the molecular processes involved in cell killing by ionising radiation and to predict in vivo response. Assays exist which measure many types of DNA damage following ionising radiation. From studies of the dose-response relationships for different types of damage, the double-strand break (dsb) has been shown to be the most significant lesion. Assays for DNA dsb have been of low sensitivity, such that supralethal doses of radiation had to be used in order to study dsb induction or repair. New assays, such as pulsed-field gel electrophoresis, are sensitive to dsb in a dose range relevant to cell survival. In addition, these assays can assess the distribution of dsb in different parts of the genome and determine heterogeneity of damage induction and repair. Assays which measure the effects of strand breaks on DNA complexed with nuclear matrix can reveal features of chromatin organisation and their influence on cellular radiosensitivity.

Lack of interference of DNA single-strand breaks with the measurement of double-strand breaks in mammalian cells using the neutral filter elution assay

In this study, the effect of DNA single strand breaks (ssb) on the neutral (pH 9.6) filter elution of DNA from Chinese hamster ovary (CHO K1) cells containing DNA double strand breaks (dsb) was investigated. Protein associated ssb were induced by the inhibition of DNA topoisomerase I with camptothecin (cpt). Protein associated dsb were introduced by treating cells with the DNA topoisomerase II poison; etoposide (VP-16). Protein associated ssb and dsb were converted to ssb and dsb by proteinase K present in the lysis solution. In some experiments dsb were generated by the restriction endonuclease Pvu II. It was found that elution of DNA in the presence and absence of ssb was similar under neutral conditions. This finding is consistent with the view that the fast component of the bi-phasic repair kinetics observed in irradiated mammalian cells with the neutral filter elution technique is not attributable to the interference of ssb with the measurement of dsb, and thus suggests that the two components of repair observed with the neutral filter elution elution technique may represent two different types of dsb or modes of repair of dsb.

Differences in accumulation of blunt- and cohesive-ended double-strand breaks generated by restriction endonucleases in electroporated CHO cells

Restriction endonucleases (RE) have been used in cytogenetic studies to mimic the DNA double-strand break (dsb)-inducing action of radiation. In the experiments presented here, we have treated electroporated CHO cells with RE and have measured the resulting dsb using the filter elution technique under non-denaturing conditions (pH 9.6). PvuII, which generates blunt-ended dsb, gave rise to a significant number of measurable dsb. The frequency of the dsb induced by PvuII is shown to increase over a 3-12-h post-treatment incubation period, which implies that the RE is active in the cell for a considerable length of time. We postulate that the accumulation of dsb reflects a competition between enzymatic incision and repair of the DNA. The presence of araA, a known inhibitor of DNA synthesis, did not affect the frequency of PvuII-induced breaks indicating a lack of an inhibitory effect of araA on the repair of RE-induced dsb. Two RE which cause cohesive-ended dsb, namely BamHI and EcoRI, were found to be ineffective in giving rise to measurable dsb. Our interpretation of this is that for cohesive-ended dsb (caused by BamHI and EcoRI) the rate at which these breaks are rejoined matches or exceeds the rate of enzymatic incision and hence no dsb were observed. In the case of PvuII, the possibly slower rate of repair of blunt-ended termini would on this hypothesis result in the observed net accumulation of dsb.

Induction of chromosomal aberrations by restriction endonucleases encapsulated in liposomes

We used liposomes to deliver the restriction endonucleases BamHI and SmaI into human heteroploid HEp-2 cells. With this method very low concentrations of enzymes (2 units/ml) were active in the production of chromosomal aberrations. SmaI, which produces blunt-ended double-strand breaks in the DNA molecule, induces chromosomal aberrations more effectively than BamHI, which produces cohesive ends. Our results indicate that liposomes are suitable vectors for introducing restriction endonucleases into cultured human cells.

Induction of chromosomal aberrations in the CHO mutant EM9 and its parental line AA8 by EcoRI restriction endonuclease: electroporation experiments

EcoRI restriction endonuclease (RE), which produces cohesive-ended double-strand breaks (dsb) in DNA, was tested in the ethyl methanesulfonate- and X-ray-sensitive CHO mutant EM9 and its parental cell strain AA8 for its chromosomal aberration-inducing effect. The RE was efficiently introduced by electroporation into AA8 cells, while the mutant cells showed a very high sensitivity to electroporation, which consistently resulted in cell death. Nevertheless, the incubation of EM9 cells in the presence of EcoRI, without electroporation, was sufficient to induce about three times the chromosome damage observed in the electroporated parental cell line AA8 for any given dose of the RE.

Adaptive response to chromosome damage in cultured human lymphocytes primed with low doses of X-rays

Human lymphocytes exposed to 0.02 Gy of X-rays in the G1 but not the G0 phase became less susceptible to the induction of chromosome aberrations of the chromosome type by subsequent exposure to 3 Gy of X-rays. The induction of chromatid-type aberrations was not affected by the pretreatment with the priming dose. The expression of this adaptive-type response was transitory, being maximum at 5 h, and disappeared at 9 h after the initial low-dose exposure. Cell-cycle analysis excluded the possibility of a spurious consequence of differential cell-cycle progression.

On the time course of the interactions between DNA breaks in the production of a radiation-induced chromosome exchange aberration

Quiescent human lymphocytes were X-irradiated and allowed to repair for various times at 37 degrees C before the cells were challenged with the DNA-repair inhibitor ara-C. The observed yield of chromosome exchange aberrations (dicentrics) was about twice the yield induced by X-rays alone, if ara-C was added immediately after irradiation. The yield as a function of the repair time between X-irradiation and ara-C treatment decreased with a half-life of 15-30 min and was almost down to the baseline yield for X-rays alone after 1 h. This shows that an exchange aberration can be formed from a short-lived DNA break. In contrast, previously published results from dose-split experiments demonstrate that the half-life of the interacting DNA breaks is of the order of several hours. A model is proposed which can account for the different estimates of the time course of the interactions involved in the process which leads to an exchange aberration.

Chromosome-type exchange aberrations are induced by inhibiting repair of UVC-induced DNA lesions in quiescent human lymphocytes

Human lymphocytes in the quiescent stage were UVC-irradiated and then incubated for 90 min in the presence of the DNA-repair inhibitor ara-C. The cells were then cultured and analyzed for chromosome aberrations. A single treatment with UVC or ara-C gives rise to a very low yield of dicentrics, whereas the combined treatment can induce a high frequency of these chromosome-type aberrations. The yield in the combined treatment is approximately proportional to the square of the UVC fluence in the range 1-3 J/m2. In addition, the experiments demonstrate that synergistic effects arise when cells are treated with UVC + ara-C and then exposed to X-rays. The results can be explained on the assumption that the UVC + ara-C treatment induces DNA double-strand breaks which, to the first approximation, are randomly distributed over the chromosomes. These breaks are able to interact with each other as well as with X-ray-induced DNA double-strand breaks to form a chromosome-type exchange aberration.

Enhanced mutability at the tk locus in the radiosensitive double-strand break repair mutant xrs5

The thymidine kinase locus (tk) has been utilised as the target locus to measure the induced mutation frequency following X-irradiation in the X-ray-sensitive xrs5 mutant and its parent CHO K1 line of Chinese hamster cells. Mutations to tk- cells were measured by plating cells in selective medium containing trifluorothymidine after a post-irradiation expression time of 4 days. Our results show that the mutation frequency was 3-4 times higher in the xrs5 mutant than in the CHO K1 cell line. This enhanced mutation frequency in xrs5 is though to result from the deficiency in DNA double-strand break repair in this cell line which also results in the enhanced cell killing and higher frequencies of chromosomal aberrations in response to X-irradiation. The findings of the present study suggest that DNA double-strand break is a critical lesion leading to mutations in irradiated cells.

The induction and repair of DNA damage detected by the DNA precipitation assay in Chinese hamster ovary cells treated with acridine orange + visible light

The photodynamic effect of the dye acridine orange (AO) in combination with visible light (400-700 nm) was studied in Chinese hamster ovary (CHO) cells, the endpoints investigated being induction, as well as repair, of DNA strand breaks. Cells were treated for 20 min with AO (0.1-3.0 micrograms/ml), washed free of excess dye and subsequently exposed to low doses of visible light (2 x 40 W/8 W/m2) for 5-15 min. AO proved to be an efficient sensitizer for light-induced DNA strand breaks, detected with the DNA precipitation assay, and expressed as percentage of DNA precipitated. The induction of breaks was linear up to 0.5 micrograms/ml AO + 10 min of light, which corresponds to 55% precipitated DNA, and was dependent on the concentration of AO as well as on the dose of light delivered. As a comparison, 18 Gy of X-rays was required to yield an equivalent amount of induced DNA strand breaks. The rejoining of the light-induced DNA strand breaks was studied by incubating the AO-sensitized cells for 30-120 min at 37 degrees C directly after light exposure. A fast recover of 67-91% of the damage (compared to initial damage, recovery time = 0, and dependent on the concentration of AO) was observed during the first 30 min of incubation. However, a significant amount of DNA damage remained after 2 h of recovery. These remaining, long-lived lesions might be involved in the photoinduced and acridine-sensitized chromosomal aberrations and sister-chromatid exchanges (SCE). The significance of these observations is discussed in relation to AO-sensitized and photoinduced DNA damage and chromosomal alterations.

Effects of hyperthermia on the repair of radiation-induced DNA single- and double-strand breaks in DNA double-strand break repair-deficient and repair-proficient cell lines

The effect of heat on the induction and repair of DNA single (ssb) and double (dsb) strand breaks was studied in irradiated exponentially growing or plateau-phase CHO cells and their DNA dsb repair-deficient, radiation-sensitive counterpart, the xrs-5 cells. Induction and repair of DNA ssb was measured by the alkaline unwinding technique, whereas induction and repair of DNA dsb was measured by the non-unwinding filter elution technique. The results indicated that pre-exposure of cells to heat (45 x 5 degrees C) for 8-30 min did not affect the induction of DNA ssb or DNA dsb per Gy and dalton of DNA in CHO or xrs-5 cells, tested either in the exponential or in the plateau-phase of growth. On the other hand, pre-exposure to heat inhibited DNA repair processes and increased the fraction of unrepaired radiation-induced damage measured 2 h after irradiation. Repair of DNA dsb was more heat-sensitive than repair of DNA ssb in both cell lines. Repair of radiation-induced ssb or dsb was inhibited in xrs-5 cells to a larger extent than in CHO cells after a similar exposure to heat. These results complement those previously reported on heat-induced radiosensitization in these cell lines, and suggest that the reduction in heat-induced radiosensitization observed in xrs-5 cells is largely due to their deficiency in repairing DNA dsb, rather than to a reduction in the ability of heat to inhibit DNA repair processes in general. The data presented here provide further support to the hypothesis that DNA dsb repair proficiency is a prerequisite for heat-induced radiosensitization.

A DNA double-strand break repair-deficient mutant of CHO cells shows reduced radiosensitization after exposure to hyperthermic temperatures in the plateau phase of growth

Heat response and heat-induced radiosensitization were studied in plateau-phase cultures of CHO cells and their radiation-sensitive counterpart, the xrs-5 cells. The xrs-5 cells were more sensitive to heat alone than were CHO cells. A large enhancement in radiation-induced killing was observed in CHO cells pre-exposed to heat (43 degrees C), expressed as a reduction in the values of Do and Dq. Contrary to the results obtained with CHO cells, pre-exposure to heat of xrs-5 cells affected radiation sensitivity to a much lesser extent. D1, the radiation dose required to reduce cell survival to 1%, decreased in CHO cells from 8.7 Gy to 2.5 Gy with increasing heat damage (cell survival after exposure to heat alone), whereas it decreased from 1.4 Gy to 0.9 Gy in xrs-5 cells. These results suggest that heat-induced radiosensitization is compromised in plateau-phase xrs-5 cells. Since xrs-5 cells are deficient in DNA dsb repair, it is hypothesized that DNA dsb repair proficiency is a prerequisite for heat radiosensitization and that heat-induced inhibition of DNA dsb repair is likely to contribute to the radiosensitization observed in repair-proficient cell lines after exposure to high temperatures.

Interaction between X-ray- and restriction endonuclease-induced lesions in the formation of chromosomal aberrations

Experiments were performed to analyze the possible interaction between lesions induced by X-rays and restriction endonucleases in the production of chromosome-type exchanges. A stronger interaction was found between X-rays and the AluI-induced 'blunt termini' lesions than between X-rays and the BamHI-induced 'cohesive termini' lesions.

Biochemical and cytogenetical characterization of Chinese hamster ovary X-ray-sensitive mutant cells xrs 5 and xrs 6. V. The correlation of DNA strand breaks and base damage to chromosomal aberrations and sister-chromatid exchanges induced by X-irradiation

The X-ray-sensitive Chinese hamster ovary (CHO) mutant cell lines xrs 5 and xrs 6 were used to study the relation between X-ray-induced DNA lesions and biological effects. The frequencies of chromosomal aberrations and sister-chromatid exchanges (SCE) were determined in wild-type CHO-K1 as well as mutants xrs 5 and xrs 6 cells following X-irradiation under aerobic and anaerobic conditions. Furthermore, we used a newly developed immunochemical method (based on the binding of a monoclonal antibody to single-stranded DNA) to assay DNA single-strand breaks (SSBs) induced by gamma-rays in these CHO cells, after a repair time of up to 4 h. For all cell lines tested the frequency of X-ray-induced chromosomal aberrations was strongly increased after irradiation in air compared with hypoxic conditions. When compared to the wild-type line, the xrs mutants known to have a defect in repair of DNA double-strand breaks (DSBs) exhibited a markedly enhanced sensitivity to aerobic irradiation, and a high OER (oxygen enhancement ratio) of 2.8-3.5, compared with 1.8-2 in CHO-K1 cells. The induction of SCE by X-rays was relatively little affected in CHO-K1 irradiated in air compared with hypoxic conditions (OER = 0.8), and in xrs 5 (OER = 0.7). A dose-dependent increase in the frequency of SCEs was obtained in xrs 6 cells treated with X-rays in air, and a further increase by a factor of 2 was evident under hypoxic conditions (OER = 0.4). With the immunochemical assay of SSB following gamma-irradiation, no difference was found between wild-type and mutant strains in the number of SSBs induced. The observed rate of rejoining of SSBs was also the same for all cell lines studied.

Co-transfer of restriction endonucleases and plasmid DNA into mammalian cells by electroporation: effects on stable transformation

We co-transferred cloned HSV thymidine kinase gene and restriction endonucleases, HindIII, XbaI, or XhoI, into thymidine kinase-deficient mouse LtK- cells by electroporation. Stimulation of the transformation efficiency was observed with HindIII or XbaI, while little effect was observed with XhoI. The stimulation was observed for various forms of exogenous DNA (linear or circular plasmid DNA and single-stranded phage DNA) and correlated with the number of strand breaks in the host-cell DNA. These results suggest that the presence of DNA double-strand breaks stimulates the integration of exogenous DNA into host cell genomes.

Effects of trypsin on X-ray-induced cell killing, chromosome abnormalities and kinetics of DNA repair in mammalian cells

When cells are trypsinized before irradiation a potentiation of X-ray damage may occur. This is known as the 'trypsin effect'. Potentiation of X-ray damage on cell killing was seen in V79 Chinese hamster cells but was marginal in Chinese hamster ovary (CHO K1) cells and not evident in murine Ehrlich ascites tumour (EAT) cells. Trypsinization did however increase the number of X-ray-induced chromosomal abnormalities in all 3 lines. To investigate the possibility that trypsin acts by digestion of proteins in chromatin, further experiments were performed to monitor DNA damage and repair. Induction of DNA breaks by X-rays was unaffected by trypsin but trypsinized EAT (suspension) cells repaired single-strand breaks (ssb) less rapidly than controls indicating an inhibitory effect of trypsin on ssb repair. However double-strand break (dsb) repair was unaffected by trypsin. It was also found that the EDTA solution in which the trypsin was dissolved also contributes to the inhibition of dsb repair. The results show that trypsinization can enhance X-ray-induced cell killing, chromosomal damage and DNA repair, the effect varying between cell lines.

Role of glutathione in the radiation response of mammalian cells in vitro and in vivo

Radiation interacts with biological systems to produce many types of molecular lesions. Much of the molecular damage is of little consequence with regard to cell killing. The lesions that are most likely to contribute to cell killing are DNA lesions produced by clusters of radicals. The formation of clusters of radicals is characteristic of ionizing radiation and accounts for its high efficiency as a cytotoxic agent. The mechanism by which these lesions kill cells is probably the formation of DNA double-strand breaks, ultimately resulting in chromosomal breaks. There is a possibility that some of the other types of molecular lesions produced by radiation may participate in more subtle mechanisms of cell damage. For instance, radiation induces a self-destructive process (apoptosis) in certain cell types, and the molecular lesions that initiate this process have not been identified. Glutathione (GSH) is a versatile protector. Several distinct mechanisms of radioprotection by GSH can be identified. These include radical scavenging, restoration of damaged molecules by hydrogen donation, reduction of peroxides and maintenance of protein thiols in the reduced state. Of these mechanisms, hydrogen donation to DNA radicals is probably the most important. Since competing reactions are very rapid, this mechanism requires a high concentration of GSH. Radioprotection by hydrogen donation to DNA radicals is not effective in oxygenated cells because the normal intracellular GSH concentration is not sufficient for effective competition with oxygen. Consequently, moderate depletion of GSH has no effect on the radiosensitivity of oxygenated cells. Under hypoxic conditions GSH becomes more competitive, and GSH depletion can markedly affect radiosensitivity. The radiosensitivity of hypoxic cells is most affected by GSH depletion in the presence of low concentrations of radiosensitizers. Since hypoxic cells are a characteristic feature of tumors, moderate depletion of GSH in combination with treatment with hypoxic cell radiosensitizers appears to be a promising strategy for selective tumor sensitization in radiation therapy. Oxidation of GSH can result in radiosensitization of both hypoxic and oxygenated cells. The mechanism of this effect appears to involve oxidation of protein thiols which are important for DNA repair. In principle, modification of DNA repair could have a greater impact on radiation therapy than modification of the number of lesions produced by radiation. However, a strategy for modification of GSH or protein thiol redox state in vivo has not yet been devised.

Oncogenic transformation of murine C3H 10T1/2 cells resulting from DNA double-strand breaks induced by a restriction endonuclease

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Decreased stable transfection frequencies of six X-ray-sensitive CHO strains, all members of the xrs complementation group

Six X-ray-sensitive strains (xrs) of the Chinese hamster ovary (CHO) cell line, all of which have a defect in double-strand break (dsb) rejoining, have been investigated for their proficiency in DNA transfection assays. All 6 strains and clonal isolates derived from them, show a decreased stable transfection frequency using the plasmids pSV2neo and pSV2gpt after transfection by either the CaPh method or the polybrene method. The magnitude of this effect is DNA concentration dependent and is more marked after transfection with higher DNA concentrations (5-20 micrograms DNA). A spontaneous X-ray-resistant reactivant (or revertant) of one xrs strain also acquired the elevated transfection frequency of the wild-type strain providing evidence for a causal relationship between the decreased transfection frequency and the xrs phenotype. In contrast, the strains show no defect when transfection is assayed using a transient transfection system. Since the transient transfection assay only depends on the uptake and transcriptional activity of foreign DNA, and does not necessitate DNA integration, this suggests that the xrs strains do not have a defect in the uptake of foreign DNA, but might have a defect in integration or the processing of DNA molecules prior to integration.

Induction of chromosomal aberrations in CHO cells by restriction endonucleases: effects of blunt- and cohesive-ended double-strand breaks in cells treated by 'pellet' methods

In recent reports it has been suggested that restriction endonucleases (RE) producing cohesive-ended double-strand breaks (dsb), are of comparable effectiveness to those producing blunt-ended dsb in causing chromosomal aberrations (CA) in mammalian cells. In several of these reports, trypsinized cells or suspension cultures were treated as cell 'pellets' in small volumes containing RE and storage buffers. In this study we have examined this by comparing 2 'pellet' methods in which trypsinized Chinese hamster cells were treated with RE in small volumes, after cells were centrifuged to a pellet. In the first method, cells were treated with RE in storage buffer as previously reported (e.g. Obe et al., 1985). In the second method, cells were treated as pellets with Sendai virus and purified RE. For both methods we show that the frequency of chromosomal aberrations was higher in cells treated with RE causing blunt-ended dsb than those causing cohesive-ended dsb. The first method however was found to lead to substantial loss in cell viability. The results strengthen the conclusion drawn from our earlier work, using treatment of attached V79 or CHO-K1 cells with Sendai virus, that cohesive-ended dsb are less effective than blunt-ended dsb in causing chromosomal aberrations.

The distribution of MspI-induced breaks in human lymphocyte chromosomes and its relationship to common fragile sites

The restriction endonuclease MspI (cleavage site C/CGG) induces chromosomal breaks in human lymphocytes. The breakpoints are distributed non-randomly along the chromosomes and the pattern of MspI-induced breakage depends on the recovery time (20 h or 6 h). Chromosomal bands preferentially involved in breakage are likely to coincide with bands where common fragile sites are located.

Cytogenetical characterization of Chinese hamster ovary X-ray-sensitive mutant cells, xrs 5 and xrs 6. IV. Study of chromosomal aberrations and sister-chromatid exchanges by restriction endonucleases and inhibitors of DNA topoisomerase II

Induction of chromosomal aberrations and sister-chromatid exchanges (SCEs) was studied in wild-type Chinese hamster ovary (CHO-K1) cells and its 2 X-ray-sensitive mutants xrs 5 and xrs 6 (known to be deficient in repair of DNA double-strand breaks (DSBs] by restriction endonucleases (REs) and inhibitors of DNA topoisomerase II known to induce DNA strand breaks. Five different types of REs, namely CfoI, EcoRI, HpaII (which induce cohesive DSBs), HaeIII and AluI (which induce blunt DSBs) were employed. REs that induce blunt-end DNA DSBs were found to be more efficient in inducing chromosomal aberrations than those inducing cohesive breaks. xrs 5 and xrs 6 mutants responded with higher sensitivity (50-100% increase in the frequency of aberrations per aberrant cell) to these REs than wild-type CHO-K1 cells. All these REs were also tested for their ability to induce SCEs. The frequency of SCEs increased in wild-type as well as mutant CHO cells, the induced frequency being about 2-fold higher in xrs mutants than in the wild-type cells. We also studied the effect of inhibitors of DNA topoisomerase II, namely 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA) and etoposid (VP 16), at different stages of the cell cycle of these 3 types of cells. Both drugs increased the frequency of chromosomal aberrations in G2 cells. The mutants showed increased sensitivity to m-AMSA and VP 16, xrs 6 cells being 10- and 2-fold more sensitive than wild-type CHO-K1 cells respectively, and xrs 5 responding with 2-fold higher sensitivity than xrs 6 cells. G1 treatment of CHO cells with m-AMSA increased both chromosome- and chromatid-type aberrations, xrs mutants being about 3-fold more sensitive than CHO-K1 cells. The frequency of SCEs increased also after treatment of exponentially growing and S-phase CHO cells with m-AMSA and the higher sensitivity of xrs mutants (2-fold) was evident. The S-phase appeared to be a specific stage which is most prone for the induction of SCEs by m-AMSA. The results indicate that DNA DSBs induced by REs and inhibitors of DNA topoisomerase II correlate closely with induced chromosomal aberrations and SCEs in these cell lines, indicating that DSBs are responsible for the production of these 2 genetic endpoints.

Chromosomal aberrations and DNA repair ability of in vitro irradiated white blood cells of monkeys previously exposed to total body irradiation

Six monkeys (Macaca fascicularis) were total-body-irradiated with 60Co (fractionated irradiation of 8 or 10 Gy). Blood samples were collected at different times post total-body irradiation, then in vitro irradiated in order to test whether a prior in vivo irradiation could affect the radiosensitivity of their leukocytes. We suggested in a preliminary report that the enhanced chromosomal radiosensitivity of in vivo irradiated monkeys could be correlated with a DNA repair deficiency (Guedeney et al., 1986). Chromosomal aberrations, the rate of initial strand breaks and their rejoining estimated using a fluorescent assay for DNA unwinding were chosen as the endpoints in this more extensive study. We observed that the yield of dicentrics induced by a subsequent in vitro irradiation was lower than that scored in unirradiated monkeys in few cases (6/22) whereas the number of acentrics was found to be modified in 16 of the 22 samples. An altered DNA repair ability was observed in most but not all blood samples tested. Thus, in view of such intra-individual variability, the results of this more extensive study lead us to conclude that a previous total-body irradiation does not alter the gamma-induced chromosome aberrations and DNA repair ability in a reproducible manner.

Comparative studies on repair inhibition by araA, araC and aphidicolin of radiation induced DNA and chromosome damage in rodent cells: comparison with fixation of PLD

The effect of araA and araC, two specific inhibitors of DNA polymerases alpha and beta, and of aphidicolin, a specific inhibitor of DNA polymerase alpha, on repair of radiation induced damage was studied at the DNA, the chromosome and the cell level in plateau-phase CHO cells. For the experiments at the chromosome level, the premature chromosome condensation technique was used. Repair of total DNA breaks was measured by the unwinding technique and repair of DNA double strand breaks by the neutral filter elution technique. In agreement with the hypothesis that DNA polymerase activity is involved in cellular repair reactions, araA and aphidicolin strongly inhibited repair of radiation induced damage at the DNA and the chromosome level. Also, araC inhibited repair at these endpoints but only to a very limited extent. The relative inhibition of repair by these compounds was similar at the various endpoints studied. At the survival level, araA effectively fixed radiation induced PLD resulting in survival levels corresponding to an exponential survival curve. On the other hand, araC and aphidicolin had only a small effect on cell survival. DNA replication was effectively inhibited by aphidicolin, moderately by araC, and even less by araA. These observations demonstrate that the efficacy of a polymerase inhibitor to inhibit DNA and chromosome repair does not always coincide with its ability to fix radiation induced PLD, or with its ability to inhibit DNA replication. This finding indicates that partly different biochemical pathways may underlie PLD fixation, DNA repair inhibition, and DNA replication inhibition.

The use of integrating DNA vectors to analyse the molecular defects in ionising radiation-sensitive mutants of mammalian cells including ataxia telangiectasia

Integrating DNA vectors, encoding selectable recombinant genes, were used to assess rejoining and recombination in wild-type mammalian cells and their ionising radiation-sensitive mutants. To provide a simple model of an important radiation-induced lesion - the DNA double-strand break - the vectors were cut with restriction endonucleases at specific single sites. If these breaks were made in the coding sequence of a selectable gene, the fidelity of the rejoin/recombination process could be measured by survival of vector-transformed cells in selective medium. Rejoining was assessed using vectors without internal homologies, while recombination was measured using pairs of fragments or deletion vectors carrying homologous regions. Initial experiments were made with vectors carrying a single selectable gene but, to overcome potential artefacts, 2-gene vectors were then constructed where one gene acts as a linked marker and (unbroken) control for the other (broken) gene. Available data are reviewed to show that, compared to their respective wild-type counterparts: (1) an ataxia telangiectasia (A-T) cell line and the hamster irs1 mutant show a consistent reduction in the fidelity of rejoining double-strand breaks (while the hamster mutants irs2, irs3, xrs series, and EM9 show wild-type fidelity); (2) the hamster EM9 mutant shows a reduction in ability to recombine homologous vector fragments (while the A-T line and probably the xrs mutants show show wild-type abilities); and (3) the xrs mutants show a reduction in overall transformation frequency with vector DNA, whether broken or not, while the other mutants tested show approximately wild-type frequencies. A critical account of the techniques and data is given, together with speculations on the molecular nature of the processes which are defective in these mutants, leading to radiosensitivity.

A cell-cycle stage-related chromosomal X-ray hypersensitivity in larval neuroblasts of Drosophila mei-9 and mei-41 mutants suggesting defective DNA double-strand break repair

We have examined the chromosomal X-ray hypersensitivity in relation to the cell cycle in larval neuroblasts of the mutagen-sensitive and excision repair-defective mutant mei-9 and of the mutagen-sensitive and post-replication repair-defective mutant mei-41 of Drosophila melanogaster. When compared to wild-type cells, cells bearing the mei-9L1 allele produced unusually high levels in particular of chromatid deletions and to a lesser extent also of isochromatid deletions, but virtually no exchange aberrations. The chromosomal hypersensitivity is apparent at M1 when cells are irradiated in S or G2 but not when irradiated in G1. On the other hand, following irradiation cells bearing the mei-41D5 allele predominantly produce chromosome deletions. Also dicentric and chromatid exchange formation is enhanced with a moderate increase in chromatid deletions. The phases of major sensitivity are the S and G1. Mei-9 and mei-41 mutants have been classified to date as proficient in DNA double-strand break repair. The data presented in this paper revealed an S-independent clastogenic hypersensitivity of mei-9 and mei-41 cells. They are interpreted as indicative evidence for the presence of impaired DNA double-strand break repair. The cell-cycle-related difference in the ratio of chromatid- versus chromosome-type deletions in both mutants suggests repair defects at partially different phases of the cell cycle in mei-9 and mei-41 mutant cells.

Cell electroporation is a highly efficient method for introducing restriction endonucleases into cells

Restriction endonucleases that make either blunt- or cohesive-end DNA double-strand breaks can induce chromosome aberrations. We have used cell electroporation with great success to permeabilize Chinese hamster ovary cells for the introduction of restriction enzymes. The introduction of restriction enzymes by this method resulted in extremely high frequencies (greater than 90%) of aberrant metaphase cells and also a dramatic decrease in cell survival, as measured by subsequent colony formation. Cell electroporation by itself caused no increase in aberrant chromosomes and had only a slight effect on cell survival.

Induction of CAD gene amplification by restriction endonucleases in V79,B7 Chinese hamster cells

The restriction endonucleases PvuII, BamHI and EcoRI were tested for their ability to induce gene amplification in V79,B7 Chinese hamster cells. The results indicate that treatment with these enzymes efficiently increases the frequency of clones resistant to N-phosphonacetyl-L-aspartate, indicating induction of CAD gene amplification.

Inducible expression and cytogenetic effects of the EcoRI restriction endonuclease in Chinese hamster ovary cells

The cytogenetic endpoints sister chromatid exchange (SCE) and chromosome aberrations are widely used as indicators of DNA damage induced by mutagenic carcinogens. Chromosome aberrations appear to result directly from DNA double-strand breaks, but the lesion(s) giving rise to SCE formation remains unknown. Most compounds that induce SCEs induce a spectrum of lesions in DNA. To investigate the role of double-strand breakage in SCE formation, we constructed a plasmid that gives rise to one specific lesion, a staggered-end ("cohesive") DNA double-strand break. This plasmid, designated pMENs, contains a selectable marker, neo, which is a bacterial gene for neomycin resistance, and the coding sequence for the bacterial restriction endonuclease EcoRI attached to the mouse metallothionein gene promoter. EcoRI recognizes G decreases AATTC sequences in DNA and makes DNA double-strand breaks with four nucleotides overhanging as staggered ends. Cells transfected with pMENS were resistant to the antibiotic G418 and contained an integrated copy of the EcoRI gene, detectable by DNA filter hybridization. The addition of the heavy metal CdSO4 resulted in the intracellular production of EcoRI, as measured by an anti-EcoRI antibody. Cytogenetic analysis after the addition of CdSO4 indicated a dramatic increase in the frequency of chromosome aberrations but very little effect on SCE frequency. Although there was some intercellular heterogeneity, these results confirm that DNA double-strand breaks do result in chromosome aberrations but that these breaks are not sufficient to give rise to SCE formation.