Sister-chromatid exchanges and gene mutations are induced by the replication of 5-bromo- and 5-chloro-deoxyuridine substituted DNA

Evaluating the Genotoxic and Cytotoxic Effects of Thymidine Analogs, 5-Ethynyl-2'-Deoxyuridine and 5-Bromo-2'-Deoxyurdine to Mammalian Cells

BrdU (bromodeoxyuridine) and EdU (ethynyldeoxyuridine) have been largely utilized as the means of monitoring DNA replication and cellular division. Although BrdU induces gene and chromosomal mutations and induces sensitization to photons, EdU's effects have not been extensively studied yet. Therefore, we investigated EdU's potential cytotoxic and mutagenic effects and its related underlying mechanisms when administered to Chinese hamster ovary (CHO) wild type and DNA repair-deficient cells. EdU treatment displayed a higher cytotoxicity and genotoxicity than BrdU treatment. Cells with defective homologous recombination repair displayed a greater growth delay and severe inhibition of clonogenicity with EdU compared to wild type and other DNA repair-deficient cells. Inductions of sister chromatid exchange and hypoxanthine phosphorybosyl transferase (HPRT) mutation were observed in EdU-incorporated cells as well. Interestingly, on the other hand, EdU did not induce sensitization to photons to the same degree as BrdU. Our results demonstrate that elevated concentrations (similar to manufacturers suggested concentration; >5-10 μM) of EdU treatment were toxic to the cell cultures, particularly in cells with a defect in homologous recombination repair. Therefore, EdU should be administered with additional precautions.

The effect of water-mediated catalysis on the intramolecular proton-transfer reactions of the isomers of 5-chlorouracil: a theoretical study

The geometrical structures and thermal energies (E), enthalpies (H) and Gibbs free energies (G) of 13 isomers of 5-chlorouracil (5ClU) in the gas and water phases were investigated using the density functional theory (DFT) method at the M06-2X/6-311++g(3df,3pd) level. The isomers of 5ClU can be microhydrated at different molecular target sites. The mono- and dihydrated forms are the most stable in both the gas and water phases, and, because of the intermolecular interactions, the hydrations lead to a degree of change in the stability trend. Two types of isomerizations were considered: the internal H-O bond rotations in which the H atom rotates 180° around the C-O bond and the intramolecular proton-transfer reactions in which an H atom is transferred between an O atom and a neighbouring N atom. The forward and backward energy barriers for isomerizations of nonhydrated 5ClU were calculated. In addition, 16 optimized transition-state structures for water-mediated catalysis on isomerizations of 5ClU were investigated. The forward and backward proton-transfer energy barriers of water-mediated catalysis on isomerizations of 5ClU were obtained. The results indicate that the catalytic effect of two H₂O molecules is much greater than that of one H₂O molecule in isomerizations of 5ClU.

Bromodeoxyuridine does not contribute to sister chromatid exchange events in normal or Bloom syndrome cells

Sister chromatid exchanges (SCEs) are considered sensitive indicators of genome instability. Detection of SCEs typically requires cells to incorporate bromodeoxyuridine (BrdU) during two rounds of DNA synthesis. Previous studies have suggested that SCEs are induced by DNA replication over BrdU-substituted DNA and that BrdU incorporation alone could be responsible for the high number of SCE events observed in cells from patients with Bloom syndrome (BS), a rare genetic disorder characterized by marked genome instability and high SCE frequency. Here we show using Strand-seq, a single cell DNA template strand sequencing technique, that the presence of variable BrdU concentrations in the cell culture medium and in DNA template strands has no effect on SCE frequency in either normal or BS cells. We conclude that BrdU does not induce SCEs and that SCEs detected in either normal or BS cells reflect DNA repair events that occur spontaneously.

Enzymatic synthesis of nucleoside analogues using immobilized 2'-deoxyribosyltransferase from Lactobacillus reuteri

Covalent attachment of recombinant Lactobacillus reuteri 2'-deoxyribosyltransferase to Sepabeads EC-EP303 leads to the immobilized biocatalyst SLrNDT4, which displayed an enzymatic activity of 65.4 IU/g of wet biocatalyst in 2'-deoxyadenosine synthesis from 2'-deoxyuridine and adenine at 40°C and pH 6.5. Response surface methodology was employed for the optimization of SLrNDT4 activity. Optimal conditions for SLrNDT4 highest activity were observed at 40°C and pH 6.5. Immobilized biocatalyst retained 50% of its maximal activity after 17.9 h at 60°C, whereas 96% activity was observed after storage at 40°C for 110 h. This novel immobilized biocatalyst has been successfully employed in the enzymatic synthesis of different natural and therapeutic nucleosides effective against cancer and viral diseases. Among these last products, enzymatic synthesis of therapeutic nucleosides such as 5-ethyl-2'-deoxyuridine and 5-trifluorothymidine has been carried out for the first time. Importantly for its potential application, SLrNDT4 could be recycled for 26 consecutive batch reactions in the synthesis of 2,6-diaminopurine-2'-deoxyriboside with negligible loss of catalytic activity.

Excision of 5-halogenated uracils by human thymine DNA glycosylase. Robust activity for DNA contexts other than CpG

Thymine DNA glycosylase (TDG) excises thymine from G.T mispairs and removes a variety of damaged bases (X) with a preference for lesions in a CpG.X context. We recently reported that human TDG rapidly excises 5-halogenated uracils, exhibiting much greater activity for CpG.FU, CpG.ClU, and CpG.BrU than for CpG.T. Here we examine the effects of altering the CpG context on the excision activity for U, T, FU, ClU, and BrU. We show that the maximal activity (k(max)) for G.X substrates depends significantly on the 5' base pair. For example, k(max) decreases by 6-, 11-, and 82-fold for TpG.ClU, GpG.ClU, and ApG.ClU, respectively, as compared with CpG.ClU. For the other G.X substrates, the 5'-neighbor effects have a similar trend but vary in magnitude. The activity for G.FU, G.ClU, and G.BrU, with any 5'-flanking pair, meets and in most cases significantly exceeds the CpG.T activity. Strikingly, human TDG activity is reduced 10(2.3)-10(4.3)-fold for A.X relative to G.X pairs and reduced further for A.X pairs with a 5' pair other than C.G. The effect of altering the 5' pair and/or the opposing base (G.X versus A.X) is greater for substrates that are larger (bromodeoxyuridine, dT) or have a more stable N-glycosidic bond (such as dT). The largest CpG context effects are observed for the excision of thymine. The potential role played by human TDG in the cytotoxic effects of ClU and BrU incorporation into DNA, which can occur under inflammatory conditions and in the cytotoxicity of FU, a widely used anticancer agent, are discussed.

Myeloperoxidase generates 5-chlorouracil in human atherosclerotic tissue: a potential pathway for somatic mutagenesis by macrophages

Somatic mutations induced by oxidative damage of DNA might play important roles in atherogenesis. However, the underlying mechanisms remain poorly understood. Myeloperoxidase, a heme protein expressed by select populations of artery wall macrophages, initiates one potentially mutagenic pathway by generating hypochlorous acid. This potent chlorinating agent reacts rapidly with primary amines to yield long-lived, selectively reactive N-chloramines. In the current studies, we demonstrate that myeloperoxidase produced by human macrophages differentiated in the presence of granulocyte macrophage colony-stimulating factor generates 5-chlorouracil, a mutagenic thymine analog. The primary amine taurine fails to block the reaction, suggesting that N-haloamines produced by macrophages might oxidize uracil. Model system studies demonstrated that N-chloramines convert uracil to 5-chlorouracil. Interestingly, the tertiary amine nicotine dramatically enhances uracil chlorination, suggesting that cigarette smoke might promote nucleobase oxidation by N-chloramines. To look for evidence that myeloperoxidase promotes uracil oxidation in vivo, we measured 5-chlorouracil levels in human aortic tissue, using isotope dilution gas chromatography-mass spectrometry. The level of 5-chlorouracil was 10-fold higher in atherosclerotic aortic tissue obtained during vascular surgery than in normal aortic tissue, suggesting that halogenated nucleobases produced by macrophages might contribute to atherogenesis. Because 5-chlorouracil can be incorporated into nuclear DNA, our observations raise the possibility that halogenation reactions initiated by phagocytes provide one pathway for mutagenesis, phenotypic modulation, and cytotoxicity during atherogenesis.

The relationship between BCL6 bodies and nuclear sites of normal and halogenated DNA and RNA synthesis

BCL6 is a POZ/BTB and zinc finger transcription factor that self-interacts and accumulates into discrete nuclear "bodies" of unknown function. We recently reported that BCL6 bodies associate with bromodeoxyuridine (BrdU)-substituted DNA, suggesting their implication in replication. To examine this possibility, we examine here by electron and confocal microscopy the relation between BCL6 bodies and replication foci (RF) using incorporation of various halogenated nucleotides (BrdU, chlorodeoxyuridine, CldU, and iododeoxyuridine, IdU) or PCNA (proliferating cell nuclear antigen) staining. We show that BCL6 bodies are found associated with RF, as revealed by PCNA staining. However, such association is markedly prolonged upon BrdU or CldU incorporation, but less, or not at all, upon IdU incorporation. Pulse-chase and double-labeling experiments indicate that IdU-substituted DNA leaves BCL6 bodies after a few tenths of minutes while BrdU- or CldU-substituted DNA stalls in their vicinity for several hours, thereby giving the characteristic "crowns" of DNA entirely surrounding BCL6 bodies. In all cases, however, the halogenated DNA ends up undergoing a movement from BCL6 bodies toward nucleoplasm and nuclear periphery to reach euchromatin and heterochromatin, respectively. We propose that replicating DNA is prone to be bound by BCL6, while BrdU/CldU incorporation increases this propensity possibly because these two events have synergistic effects on the structure and chromatinisation of the newly synthesized DNA. Finally, despite the known proximity between nuclear sites of transcription and replication, we show via several approaches that BCL6 bodies do not appear to be involved either in RNA synthesis or storage.

Identification and quantification of mutagenic halogenated cytosines by gas chromatography, fast atom bombardment, and electrospray ionization tandem mass spectrometry

Oxidative modification of nucleic acids has been implicated in carcinogenesis. One potential mechanism involves halogenation by the myeloperoxidase and eosinophil peroxidase systems of phagocytes. In the current studies, three mass spectrometric methods for the in vitro and in vivo analysis of halogenated cytosines and deoxycytidines were compared: gas chromatography-electron ionization-mass spectrometry (GC-EI-MS) with a quadrupole instrument, fast atom bombardment or electrospray ionization (ESI) tandem MS with a four-sector magnetic instrument, and liquid chromatography ESI tandem MS (HPLC-ESI-MS/MS) with an ion-trap instrument. GC-EI-MS with selected ion monitoring of dimethyl-tert-butylsilyl derivatives of nucleobases was the most sensitive method. High-energy collisionally induced dissociation MS/MS analysis with a four-sector magnetic instrument yielded detailed structural information about halogenated nucleoside adducts but required relatively large amounts of material. The most sensitive analysis of intact halogenated deoxycytidine was achieved with extracted ion chromatograms using HPLC-ESI-MS/MS with an ion-trap instrument. Our results indicate that GC-EI-MS is the methodology of choice for ultrasensitive analysis of halogenated cytosines. HPLC-ESI-MS/MS provides greater structural detail for these compounds and may rival GC-EI-MS in sensitivity with more advanced liquid chromatography applications. The mass spectrometric methods we have developed should be useful for evaluating the role of phagocyte-derived oxidants in halogenating nucleobases, nucleosides, and DNA at sites of inflammation.

As to the clastogenic-, sister-chromatid exchange inducing-and cytotoxic activity of inosine triphosphate in cultures of human peripheral lymphocytes

The influence of commercial inosine triphosphate (ITP) on the chromosome aberration rate, the mitotic rate, sister-chromatid exchange (SCE) frequency, and the proportion of first (X1), second (X2) and third (X3) division metaphases was investigated in 72h cultures of human peripheral lymphocytes. The blood donors had mild inactive arthrosis and a normal health check-up. All cultures of each volunteer were set-up simultaneously. In contrast to a previous report [Arch. Biochem. Biophys. 278 (1990) 238-244], it was demonstrated in two preliminary studies (number of subjects, n=5 each) that ITP at a final concentration of 100 microM does not induce chromosomal aberrations and, furthermore, that not ITP concentrations higher than 100 microM but ITP doses higher than 3.8mM prohibit culture growth. Based on these results, cultures with a final ITP concentration of 3.6mM (max.) and 1.8mM (max./2) were compared with control cultures (number of subjects n=10; three males and seven females, mean age x=57.6 years). Whereas no increase in the chromosomal breakage rate was observed in cultures with an ITP concentration of 1.8mM and only a marginally significant one (P=0.048) for 3.6mM ITP cultures, a highly significant induction of SCEs, not only at an ITP concentration of 3.6mM (P<0.0001) but also at 1.8mM (P<0.0001) was seen. The increase in the SCE frequency was not linear, but steeper from 0 to 1.8mM than from 1.8 to 3.6mM. Nevertheless, the difference between 1.8 and 3.6mM cultures was significant (P=0.027). The distribution of the number of SCEs per metaphase as well as the distribution of SCEs per chromosome correspond to the expected Poisson values. The investigation of the cytotoxic effect of the studied ITP concentrations revealed a highly significant reduction of the mitotic rate from 0 to 1.8mM as well as from 1.8 to 3.6mM in the aberration studies (all P values are equal to smallest possible one for a sample size of 10, namely, 0.002), and in the SCE studies there is a significant decrease in the X3 frequency when ITP is increased (0-1.8mM: P=0.0061 and 1.8-3.6mM: P<0.0001). The proportion of X1 within all X1 and X2 metaphases changes significantly only at the second dose step (0-1.8mM ITP: P=0.22 and 1.8-3.6mM ITP: P<0.0001). The results are discussed.

Production of brominating intermediates by myeloperoxidase. A transhalogenation pathway for generating mutagenic nucleobases during inflammation

The existence of interhalogen compounds was proposed more than a century ago, but no biological roles have been attributed to these highly oxidizing intermediates. In this study, we determined whether the peroxidases of white blood cells can generate the interhalogen gas bromine chloride (BrCl). Myeloperoxidase, the heme enzyme secreted by activated neutrophils and monocytes, uses H2O2 and Cl(-) to produce HOCl, a chlorinating intermediate. In contrast, eosinophil peroxidase preferentially converts Br(-) to HOBr. Remarkably, both myeloperoxidase and eosinophil peroxidase were able to brominate deoxycytidine, a nucleoside, and uracil, a nucleobase, at plasma concentrations of Br(-) (100 microM) and Cl(-) (100 mM). The two enzymes used different reaction pathways, however. When HOCl brominated deoxycytidine, the reaction required Br(-) and was inhibited by taurine. In contrast, bromination by HOBr was independent of Br(-) and unaffected by taurine. Moreover, taurine inhibited 5-bromodeoxycytidine production by the myeloperoxidase-H2O2-Cl(-)- Br(-) system but not by the eosinophil peroxidase-H2O2-Cl(-)-Br(-) system, indicating that bromination by myeloperoxidase involves the initial production of HOCl. Both HOCl-Br(-) and the myeloperoxidase-H2O2-Cl(-)-Br(-) system generated a gas that converted cyclohexene into 1-bromo-2-chlorocyclohexane, implicating BrCl in the reaction. Moreover, human neutrophils used myeloperoxidase, H2O2, and Br(-) to brominate deoxycytidine by a taurine-sensitive pathway, suggesting that transhalogenation reactions may be physiologically relevant. 5-Bromouracil incorporated into nuclear DNA is a well known mutagen. Our observations therefore raise the possibility that transhalogenation reactions initiated by phagocytes provide one pathway for mutagenesis and cytotoxicity at sites of inflammation.

Molecular chlorine generated by the myeloperoxidase-hydrogen peroxide-chloride system of phagocytes produces 5-chlorocytosine in bacterial RNA

Myeloperoxidase, a heme enzyme secreted by activated phagocytes, uses H(2)O(2) and Cl(-) to generate the chlorinating intermediate hypochlorous acid (HOCl). This potent cytotoxic oxidant plays a critical role in host defenses against invading pathogens. In this study, we explore the possibility that myeloperoxidase-derived HOCl might oxidize nucleic acids. When we exposed 2'-deoxycytidine to the myeloperoxidase-H(2)O(2)-Cl(-) system, we obtained a single major product that was identified as 5-chloro-2'-deoxycytidine using mass spectrometry, high performance liquid chromatography, UV-visible spectroscopy, and NMR spectroscopy. 5-Chloro-2'-deoxycytidine production by myeloperoxidase required H(2)O(2) and Cl(-), suggesting that HOCl is an intermediate in the reaction. However, reagent HOCl failed to generate 5-chloro-2'-deoxycytidine in the absence of Cl(-). Moreover, chlorination of 2'-deoxycytidine was optimal under acidic conditions in the presence of Cl(-). These results implicate molecular chlorine (Cl(2)), which is in equilibrium with HOCl through a reaction requiring Cl(-) and H(+), in the generation of 5-chloro-2'-deoxycytidine. Activated human neutrophils were able to generate 5-chloro-2'-deoxycytidine. Cellular chlorination was blocked by catalase and heme poisons, consistent with a myeloperoxidase-catalyzed reaction. The myeloperoxidase-H(2)O(2)-Cl(-) system generated similar levels of 5-chlorocytosine in RNA and DNA in vitro. In striking contrast, only cell-associated RNA acquired detectable levels of 5-chlorocytosine when intact Escherichia coli was exposed to the myeloperoxidase system. This observation suggests that oxidizing intermediates generated by myeloperoxidase selectively target intracellular RNA for chlorination. Collectively, these results indicate that Cl(2) derived from HOCl generates 5-chloro-2'-deoxycytidine during the myeloperoxidase-catalyzed oxidation of 2'-deoxycytidine. Phagocytic generation of Cl(2) therefore may constitute one mechanism for oxidizing nucleic acids at sites of inflammation.

DNA strand methylation and sister chromatid exchanges in mammalian cells in vitro

Among other targets, DNA demethylating agents are known to affect the sister chromatid exchange (SCE) frequency in mammalian cells in vitro. The SCE increase appears to be maintained for many (10-16) cell cycles after the end of the pulse in a given cell population, unlike SCEs induced by DNA damaging agents. Yet, epigenetic changes (such as demethylation) would not be expected to affect SCE at all. In the present report we challenge the working hypothesis of a relation between SCEs and demethylation by comparing SCE induction during different rounds of replication when the parental strands were normally methylated or demethylated. Azacytidine (AZA), ethionine (ETH), mitomycin-C (MMC), UV-irradiation (UV) and hydrogen peroxide (H(2)O(2)) were tested for SCE induction in a Chinese hamster ovary cell line after a single pulse, one or two cell cycles before fixation. Whereas MMC, UV and H(2)O(2) induce SCE in both protocols, AZA and ETH show an effect on SCEs only if administered two cycles before fixation. Because two cell cycles are needed in order to achieve demethylation of the parental DNA strand, the data reported here support our working hypothesis that demethylation in the parental DNA strand, at the level of the replication fork (i.e., the region where SCEs are formed), is responsible for an increase in mistaken ligations of processed damage, eventually yielding an increase in SCEs.

Abnormally high incidence of SCE in three successive cell cycles in the CHO mutant EM9 as detected by a three-way immunoperoxidase differential staining

A high resolution three-way immunoperoxidase method for the detection of SCE per cell generation in chromosomes with very low levels of BrdU in DNA has been recently standardized in our laboratory. We have made use of this methodology to assess whether the CHO mutant EM9 is solely hypersensitive to BrdU incorporated into template DNA as regards the extraordinarily high SCE frequency observed as compared with its parental line, AA8, or also shows an elevated spontaneous frequency of exchanges. Our results confirm the importance of BrdU incorporated for SCE in EM9 but also show that the estimated yield of spontaneous SCE is about 10 times higher in this mutant than in the parental line, AA8.

The effect of simultaneous exposure to bromodeoxyuridine and methyl methanesulphonate on sister-chromatid exchange frequency in cultured human lymphocytes

Assessment of genotoxicity in cultured cells or in experimental animals through the measurement of sister-chromatid exchanges (SCEs) commonly requires their simultaneous exposure to both the test agent and bromodeoxyuridine (BrdUrd). This dual exposure could lead to modified responses because of either synergistic or antagonistic interactions. Differences in protocol may also have their effect. There is, for example, time for DNA repair to take place in protocols in which there is separate exposure to the test agent and BrdUrd, such as human genetic monitoring studies. In this study, human lymphocyte cultures have been used to investigate the effect of the duration of simultaneous exposure to the mutagen methyl methanesulphonate (MMS) and to BrdUrd on SCE incidence. There was a direct relationship between SCE frequency and the time of simultaneous exposure to MMS and BrdUrd that was not dependent on either the total culture time or the total time of exposure to BrdUrd. This suggestion of an interaction between MMS and BrdUrd in inducing SCEs has important implications for the interpretation of SCE data in both experimental and human monitoring studies.

The genetic toxicology of 5-fluoropyrimidines and 5-chlorouracil

The halogenated pyrimidines were synthesized in the 1950s as potential anti-tumor agents after the discovery that certain tumors preferentially incorporated uracil rather than thymine into the DNA. The fluorinated derivatives are widely recognized today as effective treatment modalities, especially with tumors of the head, neck and breast. Mechanistically, efficacy of the fluorinated pyrimidines results from the ability of these compounds to incorporate into RNA and inhibit its maturation to those forms necessary for cellular metabolism and from the inhibition of the enzyme, thymidylate synthetase, which controls the biosynthesis of thymine and DNA synthesis. The 5-fluoropyrimidines can incorporate into DNA, but the contribution of this phenomenon to the overall efficacy of this class of chemotherapeutic agents is not totally resolved. Evidence exists that this class of compounds possesses the properties to induce genotoxic effects, both in bacterial and eukaryotic cells. Most notably, these effects include the induction of cellular toxicity and the induction of chromosome aberrations. The biology and chemistry of the chlorinated pyrimidines were first explored as a possible means of sensitizing the DNA to ionizing radiation in a manner similar to the sensitization observed when DNA incorporates bromodeoxyuridine. This approach was not utilized clinically. The genetic toxicology of this compound became important with the discovery of the ribonucleoside in the effluents of sewage treatment plants. Evidence is now available that the chlorinated pyrimidines, upon conversion to deoxyribonucleosides, are effective mutagens, clastogens and toxicants, as well as extremely effective inducers of sister-chromatid exchanges.

Three-way differentiation of Chinese hamster ovary chromosomes by immunoperoxidase technique using a monoclonal anti-bromodeoxyuridine antibody

A new permanent staining procedure for sister chromatid differentiation (SCD) in cultured Chinese hamster ovary cells has been established by combining the three-way differentiation in third mitosis (M3) chromosomes and the immunoperoxidase reaction developed with 3,3'-diaminobenzidine using a monoclonal anti-bromodeoxyuridine (BrdU) antibody. This procedure allows SCD at very low BrdU concentrations, and the evaluation of the sister chromatid exchange frequencies on a per cell-cycle basis.

Influence of low doses of BrdU and estimation of spontaneous SCE in CHO chromosomes: three-way differential staining and an immunoperoxidase method

The influence of low doses of 5-bromodeoxyuridine (BrdU) on the occurrence of sister chromatid exchanges (SCEs) during the first cell cycle, when unsubstituted DNA templates replicate in the presence of the halogenated nucleoside (SCE1) has been assessed in third mitosis (M3) Chinese hamster ovary (CHO) cells showing three-way differential (TWD) staining. In addition, lower concentrations of BrdU, not detectable by Giemsa staining, have been tested by a high resolution immunoperoxidase method (anti-BrdU monoclonal antibody) and SCEs were scored in second mitosis (M2) cells. Our findings was a dose-response curve for SCE1 that allows an estimated mean spontaneous yield of 1.32/cell per cell cycle by extrapolation to zero concentration of BrdU. On the other hand, when the total SCE frequency corresponding to the first and second rounds of replication (SCE1 + SCE2) found in M3 chromosomes was compared with the yield of SCEs scored in M2 cells grown in BrdU at doses lower than 1 microM no further reduction was achieved. This seems to indicate that SCEs can occur spontaneously in this cell line, though the estimated frequency is higher than that reported in vivo.

Effect of bromodeoxyuridine on the proliferation and growth of ethyl methanesulfonate-exposed P3 cells: relationship to the induction of sister-chromatid exchanges

Although sister-chromatid exchange (SCE) analysis is recognized as an indicator of exposure to DNA-damaging agents, the results of these analyses have been confounded by the use of bromodeoxyuridine (BrdUrd) to differentially label the sister chromatids. Not only does BrdUrd itself induce SCE, it also modulates the frequency of SCE induced by certain DNA-damaging agents. In order to examine this effect of BrdUrd on SCE frequency, an indirect method which lends itself to measurements both with and without BrdUrd was employed. Human teratocarcinoma-derived (P3) cells were exposed to ethyl methanesulfonate (EMS) and cultured with increasing concentrations of BrdUrd for lengths of time corresponding to one, two, and three generations of cell growth. At each time point, the distribution of nuclei among the phases of the cell-cycle and cell growth were evaluated for each concentration and chemical. A statistical model was employed which tested both for the main effects of chemicals and culture times and for interactions between these factors. Both EMS and BrdUrd significantly affected the percentages of nuclei within the cell-cycle. Exposure to EMS resulted in decreases in the percentages of nuclei in G0 + G1 and increases in the G2 + M compartment. Exposure to BrdUrd affected the size of the G0 + G1 compartment as well as the percentage of S-phase nuclei. Cell growth was reduced as a consequence of increasing EMS concentration and as a function of BrdUrd concentration; the effects of these chemicals were more readily apparent at the later time points. Most importantly, for both the cell-cycle kinetics data and the cell growth data, no evidence of an interaction between the effects of EMS and the effects of BrdUrd was detected statistically. These results may be interpreted to mean that while both EMS and BrdUrd affect the induction of SCE, under the conditions of this experiment, the effects are additive rather than interactive.

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.

Abnormal sister-chromatid exchange induction by 3-aminobenzamide in an SV40-transformed Indian muntjac cell line: relationships with DNA maturation and DNA-strand breakage

In SVM cells, an SV40-transformed line of Indian muntjac fibroblasts, levels of sister-chromatid exchanges are known to be abnormally high after UV-irradiation or alkylation. The SVM line is also known to have a defect in the processing of DNA-strand breaks. Sister-chromatid exchange in other cells is known to be stimulated by the poly(ADP-ribose) transferase inhibitor, 3-aminobenzamide, which also retards DNA-break sealing. Sister-chromatid exchanges in SVM cells are found to be hypersensitive to 3-aminobenzamide, or to nicotinamide deprivation which similarly inhibits poly(ADP-ribosyl)ation; DNA-strand breaks are likewise induced by 3-aminobenzamide. Bromodeoxyuridine, needed to detect sister-chromatid exchanges, is more toxic to SVM cells and itself induces sister-chromatid exchanges to a greater extent than it does in normal muntjac cells. However, in contrast to the situation reported for other cell types prone to sister-chromatid exchange (the Chinese hamster ovary mutant EM9 and human Bloom's Syndrome cells), SVM cells do not show an abnormal delay in DNA maturation when, under the influence of bromodeoxyuridine and 3-aminobenzamide, they show a high level of sister-chromatid exchange. The mechanism by which BrdU exerts its effects can largely be explained in terms of familiar effects on deoxyribonucleotide pools and DNA integrity. 3-Aminobenzamide, however, induces sister-chromatid exchanges in SVM cells by another mechanism.

Scoring of SCE frequency per cell cycle in CHO chromosomes by means of a standardized 3-way-differential staining method

We have developed 2 alternative protocols to obtain 3-way differentiation (TWD) in Chinese hamster ovary (CHO) cells, in order to analyze the sister-chromatid exchange (SCE) frequency on a per-generation basis. In protocol A, 5-bromodeoxyuridine (BrdU) substitution into DNA was low during the first S period (S1) and high during the next 2 (S2 and S3). In protocol B, on the other hand, BrdU substitution was high during S1 and low during S2 and S3. The main advantage of our procedures is the high efficiency and reproducibility reached by controlling the relative incorporation of BrdU and deoxythymidine (dT) into replicating DNA in the presence of 5-fluorodeoxyuridine (FdU) throughout 3 consecutive S periods. The comparison of the results obtained for both protocols allowed us to evaluate the role of BrdU in the induction of SCEs in each cell cycle. Our results from M3 chromosomes seem to indicate that, under our experimental conditions, neither the concentration of BrdU in the medium nor the rate of BrdU incorporation into nascent DNA is the basic feature responsible for SCE formation. Indeed, the experiments reported here seem to demonstrate that the nature of parental DNA is crucial for the formation of the BrdU-induced SCEs.

A pulse BrdU method for SCE

Using 'reverse' harlequin staining (bromouracil-substituted chromatin staining dark), it is possible to detect at metaphase a pulse of bromodeoxyuridine (BrdU) incorporated during S-phase. Provided that this pulse is of reasonable duration, fairly uniform staining along the chromatids of some S-cells is achieved, and no difficulty is encountered in observing and scoring SCE in such cells at second division. Thus, it is possible to define within an asynchronous population a narrow cohort of target cells and recover these for SCE scoring at second division irrespective of treatment induced perturbation. This serves to reduce the heterogeneity found in the usual terminal BrdU SCE protocols for such populations and should lead to more reliable and repeatable quantitative results. The method is illustrated for mitomycin C given to dividing human blood lymphocytes using both simultaneous and delayed pulse modes.

Molecular analysis of spontaneous mutations at the gpt locus in Chinese hamster ovary (AS52) cells

AS52 cells are Chinese hamster ovary (CHO) cells that carry a single functional copy of the bacterial gpt gene and allow the isolation of 6-thioguanine-resistant (6TGr)mutants arising from mutation at the chromosally integrated gpt locus. The gpt locus in AS52 cells is extremely stable, giving rise to 6TGr mutants at frequencies comparable to the endogenous CHO hprt locus. In this study, we describe the spectrum of spontaneous mutations observed in AS52 cells by Southern blot and DNA sequence analyses. Using the polymerase chain reaction (PCR) and the Thermus aquaticus (Taq) polymerase, we have enzymatically amplified 6TGr mutant gpt sequences in vitro. The PCR product was then sequenced without further cloning manipulations to directly identify gpt structural gene mutations. Deletions predominant among the 62 spontaneous 6TGr-AS52 mutant clones analyzed in this study. Of these, 79% (49/62) of the mutations were identified as deletions either by Southern blotting, PCR amplification or DNA sequence analysis. Among these deletions is a predominant 3-base deletion that was observed in 31% (19/62) of the mutants. These data provide a basis for future comparisons of induced point mutational spectra derived in the AS52 cell line, and demonstrate the utility of PCR in the generation of DNA sequence spectra derived from chromosomally integrated mammalian loci.

A review and analysis of the Chinese hamster ovary/hypoxanthine guanine phosphoribosyl transferase assay to determine the mutagenicity of chemical agents. A report of phase III of the U.S. Environmental Protection Agency Gene-Tox Program

Published literature on the Chinese hamster ovary cell/hypoxanthine guanine phosphoribosyl transferase (CHO/HGPRT) assay from mid-1979 through June 1986 was reviewed and evaluated. Data from the papers considered acceptable include test results on 121 chemicals belonging to 25 chemical classes. A total of 87 chemicals were evaluated positive, 3 negative, and 31 inconclusive. Mutagenicity data on 49 of the 121 chemicals evaluated could also be compared with in vivo animal carcinogenicity data. 40 of the 43 reported animal carcinogens were considered mutagenic. Caprolactam, the only definitive noncarcinogen in the group of 49, was not mutagenic. The CHO/HGPRT assay was concluded to be an appropriate assay system for use in the screening of chemicals for genotoxicity.

Effect of exogenous thymidine on sister-chromatid exchange frequency in Chinese hamster ovary cells with bromodeoxyuridine- and chlorodeoxyuridine-substituted chromosomes

There are conflicting reports on the effect of exogenous thymidine (dThd) on the frequency of sister-chromatid exchanges (SCEs) in Chinese hamster ovary (CHO) cells. Thymidine has been reported either to increase or to have no effect on SCE frequency under similar experimental conditions. To resolve this controversy, we have carried out a series of experiments to examine the effect of dThd on CHO cells cultured with 5-bromodeoxyuridine (BrdUrd). In addition, we have examined the effect of dThd on CHO cells cultured with 5-chlorodeoxyuridine (CldUrd), a much more potent inducer of SCEs than BrdUrd. The addition of 100 microM dThd to the culture medium caused a consistent decrease in the yield of SCEs in cells grown in BrdUrd for two cell cycles. The decrease was even greater when cells were grown in dThd and CldUrd. Analysis of twin and single SCEs indicated that dThd must be present during the first cell cycle to reduce the frequency of SCEs. Because excess dThd is thought to have an effect when DNA replicates on a template substituted with a halogenated nucleoside, dThd at concentrations from 100 microM to 9 mM was added to cultures for the second cell cycle after a first cell cycle in BrdUrd. In this experiment, the presence of dThd increased SCE frequency in a dose-dependent manner. The results suggest that if dThd competes with halogenated nucleosides and thus decreases their incorporation into DNA, SCEs are suppressed in the subsequent cell cycle, whereas if excess dThd creates a dNTP pool imbalance, SCEs can be increased.

Incorporated bromodeoxyuridine enhances the sister-chromatid exchange and chromosomal aberration frequencies in an EMS-sensitive Chinese hamster cell line

The mutant Chinese hamster cell line, EM9, is characterized by a high baseline sister-chromatid exchange (SCE) frequency, increased sensitivity to cell killing, and a defect in DNA strand-break repair. The molecular basis for this pleotrophic phenotype is not known. We examined, at the chromosomal level, the increased sensitivity of this mutant to incorporated BrdUrd. By varying the amount of BrdUrd in template DNA and measuring the frequency of SCEs and chromosomal aberrations, we demonstrated the enhanced sensitivity of EM9 to BrdUrd present in the template strand of DNA. Our results show that a 6-fold increase in SCEs occurs due to DNA replication over a BrdUrd-substituted template relative to a dThd-substituted template. With regard to aberration production in EM9, there is a significant enhancement of aberrations and a specific bias toward damage for the chromatid with Brdurd in the template strand. While these cells share some phenotypic properties with cells from patients with Bloom's syndrome, the genotypic similarities have not yet been established.

Detection of bromodeoxyuridine incorporation in mammalian chromosomes by a bromodeoxyuridine antibody. II. Demonstration of sister chromatid exchanges

A commercially available bromodeoxyuridine (BrdUrd) antibody was used to demonstrate sister chromatid differentiation (SCD) and to evaluate sister chromatid exchanges (SCEs) in V79 Chinese hamster cells. V79 cells were cultivated for one cell cycle in the presence of BrdUrd, followed by a second cell cycle in the absence of BrdUrd. Chromosome preparations were stained by a common immunologic staining technique. The staining pattern observed is similar to that after FPG (fluorescent plus Giemsa) staining, though with reverse staining specificity. The sensitivity of BrdUrd detection is enhanced by a factor of 20 compared to the FPG technique and thus allows the evaluation of SCEs at very low BrdUrd concentrations. The application of the antibody technique gives information about the origin and localization of SCEs and produces further evidence for the spontaneous occurrence of SCEs.

Quantitative and molecular analyses of ethyl methanesulfonate- and ICR 191-induced mutation in AS52 cells

A pSV2gpt-transformed Chinese hamster ovary (CHO) cell line has been used to study mutation at the molecular level. This cell line, designated AS52, was constructed from a hypoxanthine-guanine phosphoribosyl transferase (HPRT)-deficient CHO cell line, and has been previously shown to contain a single, functional copy of the E. coli xanthine-guanine phosphoribosyl transferase (XPRT) gene (gpt) stably integrated into the Chinese hamster genome. In this study, conditions for its use in the study of mammalian cell mutagenesis have been stringently defined. The spontaneous mutation rate (2 X 10(-6)/cell division) and phenotypic expression time (7 days) of the gpt locus compare favorably with those of the hprt locus in wild-type CHO-K1-BH4 cells. While both cell lines exhibit similar cytotoxic responses to ethyl methanesulfonate (EMSO and ICR 191, significant differences in mutation induction were observed. Ratios of XPRT to HPRT mutants induced per unit dose of EMS and ICR 191 are 0.70 and 1.6, respectively. Southern blot hybridization analyses revealed that most XPRT mutant cell lines which arose following treatment with EMS (20/22) or ICR 191 (20/24) exhibited no alterations of the gpt locus detectable by this technique. Similar observations were made for the hprt locus in EMS-(21/21) and ICR 191-induced (22/22) HPRT mutants. In contrast, most spontaneous gpt mutants (14/23) contained deletions, while most spontaneous hprt mutants (18/23) exhibited no detectable alterations. Results of this study indicate that the AS52 cell line promises to be useful for future study of mutation in mammalian cells at the DNA sequence level.

Effects of 3-aminobenzamide on Chinese hamster cells treated with thymidine analogues and DNA-damaging agents. Chromosomal aberrations, mutations and cell-cycle progression

The nuclear enzyme, poly(ADP-ribose) synthetase is involved in the repair of damaged DNA. We report here the results obtained with 3-aminobenzamide (3AB), an inhibitor of this enzyme, on induced biological effects. 3AB increases the frequency of chromosomal aberrations induced by DMS, EMS, ENU, bleomycin and CldUrd. The magnitude of the effect is dependent on the type of chemical used, the combinations with DMS and EMS being the most potent ones. No potentiation was observed after treatment of cells with MMC. Mutation frequencies were determined on the HPRT locus and showed that 3AB did not increase the frequency of gene mutations induced by EMS, ENU and CldUrd. Cell-cycle progression is affected when cells are grown in medium containing CldUrd and 3AB, primarily when the inhibitor is present during the second cell cycle when substituted DNA becomes replicated. The extent of the effect depends on the amount of analogue incorporated and is independent of the presence of the analogue in the medium during the second cell cycle. Analysis of chromosomal aberrations in delayed G2 cells with the aid of the premature chromosome-condensation technique revealed numerous aberrations after incorporation of CldUrd and treatment with 3AB.

Induction of sister-chromatid exchanges (SCEs) by 5-fluorodeoxyuridine. The role of 5-bromodeoxyuridine incorporated into parental DNA

The effect of thymidine (dT) and 5-fluorodeoxyuridine (FdU) on the frequency of sister-chromatid exchanges (SCEs) was examined in root-tip cells of Allium cepa. dT was unable to induce SCEs at all the concentrations tested. In contrast, FdU was efficient in the induction of SCEs. A nearly 3-fold increase in the frequency of SCEs was obtained with 5 X 10(-7) M FdU when it was given simultaneously with 5-bromodeoxyuridine (BrdU) for 2 consecutive cell cycles. Also, when FdU was present only for the first round of replication, the rate of SCEs in M2 chromosomes was notably higher than when it was given only during the second replication round. By analyzing M3 chromosomes treated during the second replication round with FdU the results also supported the hypothesis that the majority of the FdU-induced SCEs arise as a consequence of a higher rate of BrdU incorporation, and that most of them are formed when BrdU-containing strands are used as template for DNA synthesis.

Mutational and pseudomutational effects of 5-bromodeoxyuridine in human lymphoblasts

We have studied the effects of 5-bromodeoxyuridine (BrdUrd) at two genetic loci in diploid human lymphoblast cells. In thymidine kinase heterozygotes (tk +/-), a 2-h dose of BrdUrd induced a transient, non-heritable resistance to the thymidine analogue, trifluorothymidine (F3TdR). We have called this phenomenon pseudomutation and have shown that affected cells acquire the ability to survive in the presence of F3TdR and then, after degradation of F3TdR in the medium, return to an apparently normal wild-type state. Our data suggest that BrdUrd incorporation into DNA as a thymidine analogue is responsible for the effect, which we interpret as a temporary loss of thymidine kinase activity. This effect is not seen in tk +/+ homozygotes. In contrast, at the hypoxanthine-guanine phosphoribosyl transferase locus in tk +/- heterozygotes, BrdUrd did not induce a permanent, heritable resistance to 6-thioguanine (gene locus mutation). We detected such mutations only in the tk +/+ homozygote and only at external BrdUrd concentrations considerably higher than those which saturate the uptake of BrdUrd into DNA as a thymidine analogue. We postulate that the reduced TK enzyme levels (30%) in the heterozygote prevent the build-up of a sufficiently high intracellular BrdUrd triphosphate pool to promote the misincorporations as deoxycytidine triphosphate which may be responsible for gene locus mutation.

Persistence of DNA lesions and the cytological cancellation of sister chromatid exchanges

The ability of UV light, mitomycin C and ionizing radiation to induce the formation of sister chromatid exchanges (SCEs) at the same locus in successive cell generations was investigated in human lymphocytes. Cells were exposed to the DNA damaging agents after they had completed their first round of DNA replication, and SCEs were examined at the third division in chromosomes that had been differentially stained three ways. Although some of these treatments induced long-lived lesions that increased the frequency of SCEs in successive cell generations, none of the lesions led to the formation of consecutive SCEs at the same locus in successive cell generations. This observation seriously challenges the hypothesis that SCE cancellation results as a consequence of persistence of the lesions induced by these agents.

Fluorodeoxyuridine-induced sex differences in baseline sister-chromatid exchanges in C57BL/6 mice and Chinese hamsters

The baseline sister-chromatid exchanges (SCEs) and the percentage of first (M1), second (M2) and third or higher metaphase (M3+) chromosomes were analysed in bone-marrow cells of male and female C57BL/6 mice and Chinese hamsters following serial intraperitoneal injections of 40 micrograms/g body weight (b.w.) of 5-bromo-2'-deoxyuridine (BrdUrd) and 2 micrograms/g b.w. of 5-fluorodeoxyuridine (FdUrd) or 40 micrograms/g b.w. of BrdUrd and 10 micrograms/g b.w. of deoxycytidine (dC). Female animals receiving BrdUrd/FdUrd showed significantly higher (P less than 0.01) baseline SCEs compared to the other groups. No sex difference in the baseline SCEs was found in animals treated with BrdUrd/dC. The distribution patterns of M1, M2 and M3+ metaphases in BrdUrd/FdUrd-treated animals differ significantly from those in BrdUrd/dC-treated animals.

Strand breaks arising from the repair of the 5-bromodeoxyuridine-substituted template and methyl methanesulphonate-induced lesions can explain the formation of sister chromatid exchanges

5-Bromodeoxyuridine (BrdU)-induced sister chromatid exchanges (SCEs) are mainly determined during replication on a BrdU-substituted template. The BrdU, once incorporated, is rapidly excised as uracil (U), and the gap is repaired with the incorporation of BrdU from the medium, which leads to further repair. During the second S period in BrdU medium, this process continues as the strand acts as template. Experiments suggest that 3-aminobenzamide (3AB) delays the ligation of the gaps formed after U excision, resulting in enhanced SCE levels during the second cycle of BrdU incorporation. When normal templates of G1 cells are treated before BrdU introduction with methyl methanesulphonate (MMS), 3AB in the first cycle doubles the MMS-induced SCEs but has no effect on them during the second cycle. When the BrdU-substituted template is treated with MMS in G1 of the second cycle, 3AB again doubles the SCEs due to MMS and also enhances the SCEs resulting from delays in ligation of the gaps following U excision in the BrdU-substituted template. The repair processes of MMS lesions that are sensitive to 3AB and lead to SCEs take place rapidly, while the repair process of late repairing lesions that lead to SCEs appear to be insensitive to 3AB. A model for SCE induction is proposed involving a single-strand break or gap as the initial requirement for SCE initiation at the replicating fork. Subsequent events represent natural stages in the repair process of a lesion, ensuring replication without loss of genetic information.(ABSTRACT TRUNCATED AT 250 WORDS)

Bloom's syndrome and EM9 cells in BrdU-containing medium exhibit similarly elevated frequencies of sister chromatid exchange but dissimilar amounts of cellular proliferation and chromosome disruption

Bloom's syndrome (BS) and EM9 cells both display elevated frequencies of sister chromatid exchange (SCE) following growth for two rounds of DNA replication in bromodeoxyuridine (BrdU)-containing medium. To learn whether hyperresponsiveness to BrdU itself might play a role in causing the SCE elevation, the effects of BrdU on two other parameters, cellular proliferation and chromosome disruption, were examined, comparing the responses of BS and normal lymphoblastoid cells and of EM9 and CHO cells. BS and normal cells responded similarly with respect to growth for 4 days in BrdU-containing medium (0, 1, 3, and 5 micrograms/ml). Chromosome aberrations were increased only slightly in the BS and normal cells after 2 days in BrdU. CHO cells responded to growth in BrdU-containing medium like BS and normal cells; however, little growth of EM9 was detected at any of the BrdU concentrations employed. CHO and EM9 cells also exhibited strikingly different amounts of chromosome damage following growth in BrdU. After 2 days in 1, 3, and 5 micrograms/ml BrdU 21%, 46%, and 50%, respectively, of the CHO cells had chromosome aberrations in contrast to 92%, 96%, and 98% of the EM9 cells. Most of the aberrations in the BrdU-treated CHO cells consisted of what appeared to be polycentric and ring chromosomes or chromosomes exhibiting telemere association. Acentric fragments were absent from most cells with polycentric and ring chromosomes, indicating either that the abnormal chromosomes were formed during an earlier cell cycle or that the abnormal chromosomes represent a form of association in which the telomeres are apposed so tightly that the juncture between chromosomes cannot be identified microscopically.(ABSTRACT TRUNCATED AT 250 WORDS)

Biological consequences of photoproducts in mammalian cell DNA partially substituted with 5-bromouracil

The response of Chinese hamster ovary cells in which 10 per cent of the thymine of one DNA strand was substituted with bromouracil (BU) was compared with normal cells following u.v. irradiation. The bromouracil substitution resulted in a 21/2 fold enhancement of both u.v.-induced killing and mutation induction at the HGPRT locus. These BU-photoproducts do not, however, result in any further inhibition of DNA replication or inhibition of the repair of u.v.-induced DNA photoproducts identified as antibody-binding sites.

SCEs are induced by replication of BrdU-substituted DNA templates, but not by incorporation of BrdU into nascent DNA

After 3 rounds of DNA replication in the presence of BrdU, third-division metaphase cells can be scored for the frequencies of SCEs that occurred during cycles 1 and 2, and also for the frequency of SCE during cycle 3. This procedure was used to resolve the issue of SCE induction by replication of BrdU-substituted DNA templates versus induction by BrdU incorporation into nascent DNA. It was observed that third-cycle SCE frequencies in CHO are dependent upon the amount of BrdU that was present during cycles 1 and 2 and are independent of the BrdU concentration during the third cycle. It is therefore BrdU serving as a template, rather than BrdU being incorporated, that initiates the SCE event. A model is proposed that produces reasonable fits to the observed data. It also predicts a true background or spontaneous SCE frequency of 3 per cell per cycle as previously reported by Heartlein et al. (Mutation Res., 107 (1983) (103-109). The predicted single twin ratio is higher than that reported by Wolff and Perry (Exp. Cell Res., 93 (1975) 23-30), and possible explanations for this discrepancy are discussed.

In vivo SCE analysis using bromodeoxyuridine, iododeoxyuridine, and chlorodeoxyuridine

Sister-chromatid exchange (SCE) frequencies were determined for mouse and rabbit bone-marrow cells following incorporation of 3 different halogenated analogues of thymidine. For both species the SCE frequency was highest for chlorodeoxyuridine, lowest for iododeoxyuridine and intermediate for bromodeoxyuridine. The study demonstrates that halogenated pyrimidine analogues other than brominated compounds can be used for in vivo SCE analysis and that their effects on baseline SCE frequencies are qualitatively similar to results of previous in vitro studies.

The replication of unsubstituted and 5-bromodeoxyuridine- or 5-chlorodeoxyuridine-substituted DNA regulates the rate of induction of sister chromatid exchanges

The thymidine (dThd) analog 5-bromodeoxyuridine (BrdUrd) is widely used in studies of the induction of sister chromatid exchanges (SCEs), since growth in the presence of BrdUrd allows the subsequent differential staining of the chromosomes through the fluorescence-plus-Giemsa (FPG) method. However, the analog itself induces SCEs, an aspect of its use which is often not considered. We have studied the induction of SCE by BrdUrd and a second dThd analog 5-chlorodeoxyuridine (CldUrd). Growth of Chinese hamster ovary (CHO) cells for 2 rounds of replication in the presence of different concentrations of either analog results in increasing results in increasing SCE frequencies which are linearly proportional to the degree of analog substitution for dThd in the DNA. However, CldUrd causes 3 to 5 times the number of SCEs found with BrdUrd, at equivalent substitution for dThd. With both analogs the increase in SCE frequency is due to the replication of the analog-substituted DNA and not to the incorporation of analog into nascent DNA. This induction of SCE can be considered at the level of a single strand of DNA since the replication of bifilarly substituted DNA results in twice the number of SCEs that are induced by the replication of unifilarly substituted DNA.

Induction of DNA lesions, chromosomal aberrations, and G2 delay by bromo- and chlorodeoxyuridine

The presence of lesions in DNA of CHO cells, substituted with BrdUrd or CldUrd, has been investigated in a direct way by alkaline sucrose gradient and nucleoid sedimentation analysis and indirectly by screening for induced CAs. The influence of inhibitors of DNA repair (Caff and 3AMB) or DNA synthesis (HU) on the frequencies of such aberrations has been estimated. No randomly located DNA breaks could be detected under neutral conditions, but BrdUrd-substituted DNA was found to contain numerous alkali labile sites. At high concentrations, CldUrd causes G2 delay, similar to the action of known DNA-damaging agents. The extent of delay depends on the pattern of incorporation of the analog, i.e., incorporation for 2 cell cycles causes the longest delay, growth for 12 hr in CldUrd followed by 12 hr in dThd- containing medium gives less delay, and delay is not significant when the cells are incubated in the analog for only 12 hr prior to fixation. Numerous chromatid-type aberrations are present in cells incubated at the highest CldUrd concentration and their induction follows the pattern of induction of G2 delay, indicating the involvement of a common lesion. 3AMB, and HU increase the number of CAs when added 2 hr before fixation. The significance of these results is discussed.

Probing sister chromatid exchange formation with halogenated pyrimidines

This communication describes the use of 6 different halogenated pyrimidine analogues, bromodeoxyuridine (BrdUrd), chlorodeoxyuridine (CldUrd), iododeoxyuridine (IdUrd), bromodeoxycytidine (BrdCyd), chlorodeoxycytidine (CldCyd), and iododeoxycytidine (IdCyd), to achieve sister chromatid differentiation (SCD) and evaluate sister chromatid exchange (SCE) formation in mitogen-stimulated human lymphocytes. Also included are a description of an in vivo experiment with BrdUrd, CldUrd, and IdUrd; a discussion of pyrimidine metabolism effects on SCEs; and the presentation of an update on the "conformation hypothesis" for SCE formation. This hypothesis revision includes a model that centers on the idea that the sum of the conformational alterations of the DNA polymerase-DNA template complex at replication is the controlling factor in SCE formation.