Replication bypass model of sister chromatid exchanges and implications for Bloom's syndrome and Fanconi's anemia

Telomeres and NextGen CO-FISH: Directional Genomic Hybridization (Telo-dGH™)

The cytogenomics-based methodology of Directional Genomic Hybridization (dGH™) emerged from the concept of strand-specific hybridization, first made possible by Chromosome Orientation FISH (CO-FISH), the utility of which was demonstrated in a variety of early applications, often involving telomeres. Similar to standard whole chromosome painting (FISH), dGH™ is capable of identifying inter-chromosomal rearrangements (translocations between chromosomes), but its distinctive strength stems from its ability to detect intra-chromosomal rearrangements (inversions within chromosomes), and to do so at higher resolution than previously possible. dGH™ brings together the strand specificity and directionality of CO-FISH with sophisticated bioinformatics-based oligonucleotide probe design to unique sequences. dGH™ serves not only as a powerful discovery tool-capable of interrogating the entire genome at the megabase level-it can also be used for high-resolution targeted detection of known inversions, a valuable attribute in both research and clinical settings. Detection of chromosomal inversions, particularly small ones, poses a formidable challenge for more traditional cytogenetic approaches, especially when they occur near the ends or telomeric regions. Here, we describe Telo-dGH™, a strand-specific scheme that utilizes dGH™ in combination with telomere CO-FISH to differentiate between terminal exchange events, specifically terminal inversions, and an altogether different form of genetic recombination that often occurs near the telomere, namely sister chromatid exchange (SCE).

Spontaneous sister chromatid exchange in mitotic chromosomes of the chinchilla (Chinchilla lanigera)

Kuchta-Gładysz, M., Wójcik, E., Szeleszczuk, O., Niedbała, P. and Tyblewska, K. 2015. Spontaneous sister chromatid exchange in mitotic chromosomes of the chinchilla (Chinchilla lanigera). Can. J. Anim. Sci. 95: 543–550. The sister chromatid exchange (SCE) test is a cytogenetic tool with applications as a short-term screen. It is used to assess the influence of physical and chemical factors with potential mutagenic and genotoxic properties on the animal organism. The test results make it possible to eliminate mutagens, as well as helping to predict possible genetic consequences in animal cells and assess animal resistance. The mitotic chromosomes were obtained from an in vitro culture of peripheral blood lymphocytes with added bromodeoxyuridine (BrdU), at five different concentrations: 0.25, 0.5, 1.0, 2.5, and 5.0 µg mL−1. The chromosomes were stained according to the FPG method. Our analyses revealed the spontaneous SCE level in the chinchilla at the concentration of 0.5 µg mL−1. Higher concentrations of this substance have a genotoxic effect and cause damage to the DNA structure of the chromosomes by inducing additional SCEs in the chromosomes of this species. The mean SCE/cell incidence in the chinchilla population was 4.34±1.28. We investigated the effects of age on the incidence of SCE and found it significantly affected this phenomenon in both sexes.

Frequency of sister chromatid exchanges and micronuclei monitored over time in patients with early-stage breast cancer: results of an observational study

Spontaneous chromosomal instability correlates with a high risk of cancer. The frequency of spontaneous sister chromatid exchanges (SCE) and micronuclei (MN) in peripheral blood lymphocytes was used for evaluation of spontaneous chromosomal instability in early-stage breast cancer patients to determine whether SCE and MN frequencies are biomarkers of damage from chemotherapy and radiotherapy. In 20 stage I-II breast cancer patients, SCE and MN were measured before surgery and at 4 weeks after. In patients who received adjuvant chemotherapy (CTx), they were also determined before starting radiotherapy (RTx). Other assessments were done 2, 6, and 12 months after RTx in almost all patients and at 18 months in 4 patients. Generalized estimating equations (GEE) were used to estimate population averaged effects at the different treatment and follow-up time points. Moreover, SCE and MN baseline values in patients were compared with those of a control group of 12 healthy women. A significant difference emerged between patients and healthy controls (P<0.0001 for SCE and P<0.0003 for MN; Mann-Whitney test); SCE increased significantly after CTx and MN increased significantly after RTx. In the GEE model, the smoking habit was associated with increased SCE in patients treated with CTx; age significantly affected MN frequencies. Both MN and SCE frequencies are increased in breast cancer patients and are indicators of CTx and RTx damage, respectively. The increased SCE levels in patients treated with CTx may be due to a synergic effect of smoking and chemotherapy.

Acquired chromosome instability in the elderly--the effect of diepoxybutane

Aging and Alzheimer's disease (AD) have been the subject of many studies. It has been suggested that chromosomal alterations may be involved in the etiology and/or pathogenesis of ageing and AD. The purpose of the present study was to examine the effect of diepoxybutane (DEB) on lymphocyte chromosomal instability in the elderly. We examined lymphocytes cytogenetically with, as well as, without DEB treatment, in a group of 12 elderly (range of age 72-96 years), nine of them suffering from AD type. Without DEB treatment six of the donors expressed chromosomal instability in at least 6% of the analyzed cells. After treatment with DEB, lymphocytes showed an increase in the chromosomal instability in up to 20% of the analyzed in eight donors. The sex chromosomes were the main chromosomes involved in the acquired chromosomal abnormalities. It is not clear from this study whether this chromosomal instability is related to the AD. The significance of the involvement of sex chromosomes either in ageing or in AD, as well as, the question whether the chromosomal instability is the cause of or part of ageing processes, has to be addressed.

Different behavior of SCE-eliciting lesions induced by low and high doses of busulfan

Our previous studies suggested a dose-dependent transition in the types of DNA lesions induced by busulfan, as measured using the comet assay and by micronuclei analyses. The aim of the present study was to investigate the dose-dependent induction of different sister-chromatid exchange-eliciting lesions; lesions were distinguished by their efficiency in producing sister-chromatid exchange (SCE), and by their reparability during G1. Synchronously dividing murine salivary gland cells were assayed in vivo. Groups of mice were intraperitoneally injected with either 30 or 80 micromol busulfan/kg body weight solution at early or late G1. The rate of SCE/micromol busulfan/kg body weight obtained by exposure at late G1 with the high dose was twice that of the low dose. SCE induction during early G1 was higher than at late G1 with both doses; only the low-dose response was statistically significant. The frequency distribution of SCEs per cell demonstrated that cells exposed at the late G1 phase showed typical profiles that closely fit a Gaussian curve. However, an irregular profile was obtained for cells treated during early G1, which showed some cells with high-SCE frequency. Cells treated in early G1 have more time to repair lesions before DNA synthesis; therefore, the results suggest that instead of repair, secondary SCE-eliciting lesions during G1 were produced, especially at the lower dose. The results obtained in this study indicate that there are dose-dependent differences in the types of SCE-eliciting lesions induced by busulfan.

Elevated sister chromatid exchange frequencies in dividing human peripheral blood lymphocytes exposed to 50 Hz magnetic fields

The in vitro cytomolecular technique, sister chromatid exchange (SCE), was applied to test the clastogenic potentiality of extremely low frequency (ELF) electromagnetic fields (EMFs) on human peripheral blood lymphocytes (HPBLs). SCE frequencies were scored in dividing peripheral blood lymphocytes (PBLs) from six healthy male blood donors in two rounds of experiments, R1 and R2, to determine reproducibility. Lymphocyte cultures in the eight experiments conducted in each round were exposed to 50 Hz sinusoidal (continuous or pulsed) or square (continuous or pulsed) MFs at field strengths of 1 microT or 1 mT for 72 h. A significant increase in the number of SCEs/cell in the grouped experimental conditions compared to the controls was observed in both rounds. The highest SCE frequency in R1 was 10.03 for a square continuous field, and 10.39 for a square continuous field was the second highest frequency in R2. DNA crosslinking at the replication fork is proposed as a model which could explain the mechanistic link between ELF EMF exposure and increased SCE frequency.

DNA interstrand crosslinks are induced in cells prelabelled with 5-bromo-2'-deoxyuridine and exposed to UVC radiation

It has been observed previously that 5-bromo-2'-deoxyuridine (BrdU) potentiates the effect of UVC radiation on the level of sister chromatid exchanges. It is not known which type of DNA damage is responsible for this enhancing effect and we have proposed this to be the DNA interstrand crosslink (ICL) which, theoretically, may arise in cells that are labelled with BrdU for one round of replication and exposed to UVC radiation. The aim of the present investigation was to verify if ICLs are indeed formed during this irradiation scenario. CHO-K1 cells were prelabelled with BrdU and exposed to UVC. ICLs were detected by a modified version of the comet assay that relies on the reduction of induced DNA migration in the agarose gel. Carboplatin was used as a positive control. We found that BrdU+UVC treatment indeed results in a reduction of the damage induced by gamma-radiation. Furthermore, we observed that CL-V4B cells exposed to BrdU+UVC, but not to UVC alone, showed a very high level of chromosomal damage. These cells have a deficient Rad51C paralog that renders them extremely sensitive towards ICLs. Interestingly, the cytogenetic results did not correlate with cell survival, where it was found that the CL-V4B cells tolerate BrdU+UVC better than the wild type cells. The possible reasons are discussed. Taken together our results indicate that ICLs are formed in DNA that was prelabelled with BrdU and exposed to UVC radiation.

Rad51C-deficient CL-V4B cells exhibit normal levels of mitomycin C-induced SCEs but reduced levels of UVC-induced SCEs

The mechanisms of sister chromatid exchanges (SCEs) are not known. One hypothesis is that SCE is a manifestation of Rad51-dependent homologous recombination repair. In order to test this hypothesis, we have compared the frequencies of SCEs induced by mitomycin C (MMC) and 254nm ultraviolet radiation (UVC) in wt V79B and the Rad51C-deficient CL-V4B cells. SCEs were analysed in the first (M1) and second (M2) post-treatment mitoses. In M1 MMC induced the same frequencies of SCEs in CL-V4B and V79B cells, while the UVC-induced SCE frequencies were lower in CL-V4B than V79B cells. In CL-V4B cells, MMC-induced SCEs were higher in M2 than in M1, suggesting that interstrand cross-links (ICL) are either not removed completely or are transformed into another form of DNA damage that persists until the next cell cycle. We suggest that SCEs may represent a mechanism to bypass MMC-induced ICL without their removal.

Application of the biotin-dUTP chromosome labelling technique to study the role of 5-bromo-2'-deoxyuridine in the formation of UV-induced sister chromatid exchanges in CHO cells

The role of 5-bromo-2'-deoxyuridine (BrdU) in the formation of sister chromatid exchanges (SCEs) in cells exposed to UV radiation was studied. Cells were unifilarily labelled (labelling of one strand of chromosomal DNA) with BrdU or biotin-16-2'-deoxyuridine (biotin-dU) and irradiated in G(1) phase of the cell cycle either with 254 nm, which is absorbed by all nucleobases including bromouracil (BrU) or with 313 nm radiation, which is predominantly absorbed by the BrU moiety. Elevated SCE frequencies were observed in cells irradiated at 254 nm (1.2 and 3.0 J m(-2)) which were pre-labelled with BrdU or biotin-dU. Following irradiation at 313 nm (38 and 96 J m(-2)) a statistically elevated SCE frequency was observed in cells pre-labelled with BrdU but not with biotin-dU. In cells pre-labelled with BrdU, UV-radiation at 254 nm was 50-80 times more effective in inducing SCEs than that at 313 nm. This result can be accounted for by the fact that in BrdU-DNA the cross-section for uracilyl radical and bromine atom formation is approximately 100-fold higher at 254 nm than that at 313 nm. Upon irradiation at 254 nm, BrdU had a strong sensitising effect on SCE induction: the SCE frequencies observed in cells pre-labelled with BrdU are approximately 6 times higher than in cells pre-labelled with biotin-dU. From this it follows that BrdU-induced damage is responsible for more than 80% of the SCEs formed in UV irradiated cells unifilarily labelled with BrdU. Based on photochemical considerations and the fact that chemical agents which form DNA interstrand cross-links are among the most potent inducers of SCEs, we propose that an interstrand cross-link may be the major lesion leading to SCEs in BrdU-labelled cells.

Analysis of inversions and sister chromatid exchanges in chromosome 3 of human lymphocytes exposed to X-rays

It has been shown repeatedly that exposure of G(1) cells unifilarily labelled with 5-bromodeoxyuridine (BrdU) to X-rays leads to sister chromatid exchanges (SCE) when the cells are allowed to grow for one further cycle in the absence of BrdU. It has been suggested that damage induced by ionizing radiation does not lead to 'true' SCE and that the observed SCE are 'false', resulting from structural chromosomal aberrations, especially interstitial inversions. We used a painting probe for the p14 region of human chromosome 3 and anti-BrdU antibodies to analyse the frequency of radiation-induced SCE in that chromosome. This method allowed us to discriminate between para- and pericentric 'true' and 'false' SCE. Our results indicate that most radiation-induced SCE do not result from inversions.

X-irradiation of G1 CHO cells induces SCE which are both true and false in BrdU-substituted cells but only false in biotin-dUTP-substituted cells

The SCE-test is widely used in genetic toxicology and therefore knowledge of the contribution of BrdU to the formation of spontaneous and induced SCE is of great importance. The present study was undertaken to analyse the role of BrdU in X-ray-induced SCE. If SCE resulted from inversions, rings and double minutes (RDM) would be the asymmetrical counterparts of SCE and should therefore have the same frequencies. Dose-effect relationships of SCE and RDM show that the frequencies of SCE are much higher than those of RDM. We conclude that only a few SCE may represent inversions. In a second set of experiments, endoreduplications were induced in cells irradiated either before or after labelling with BrdU. Analysis of SCE in endoreduplicated chromosomes allows the discrimination of the cell cycle in which they originated. The results show that SCE are only induced in the first cell cycle following irradiation of BrdU-substituted cells, indicating that labelling with BrdU is a necessary prerequisite for the formation of SCE. In order to test this directly, radiation-induced SCE frequencies were studied in cells prelabelled with BrdU or biotin-dUTP in a third set of experiments. The structure of biotin-dUTP suggests that, in contrast to BrdU, it does not give rise to radicals during irradiation. Significantly lower frequencies of SCE were observed in biotin-dUTP-substituted cells than in BrdU-labelled cells. Calculations show that nearly all SCE induced in biotin-dUTP-labelled chromosomes can be explained by chromosomal aberrations (false SCE). In contrast to this, most SCE induced by X-rays in BrdU-labelled cells are not due to chromosomal aberrations, but result from S-dependent lesions (true SCE). This clearly points towards radiation damage in BrdU-moieties as the source of DNA lesions leading to SCE.

Segregation of DNA polynucleotide strands into sister chromatids and the use of endoreduplicated cells to track sister chromatid exchanges induced by crosslinks, alkylations, or x-ray damage

The method of Matsumoto and Ohta [Matsumoto, K. & Ohta, T. (1992) Chromosoma 102, 60-65; Matsumoto, K. & Ohta, T. (1995) Mutat. Res. 326, 93-98] to induce large numbers of endoreduplicated Chinese hamster ovary cells has now been coupled with the fluorescence-plus-Giemsa method of Perry and Wolff [Perry, P. & Wolff, S. (1974) Nature (London) 251, 156-158] to produce harlequin endoreduplicated chromosomes that after the third round of DNA replication are composed of a chromosome with a light chromatid and a dark chromatid in close apposition to its sister chromosome containing two light chromatids. Unless the pattern is disrupted by sister chromatid exchange (SCE), the dark chromatid is always in the center, so that the order of the chromatids is light-dark light-light. The advent of this method, which permits the observation of SCEs in endoreduplicated cells, makes it possible to determine with great ease in which cell cycle an SCE occurred. This now allows us to approach several vexing questions about the induction of SCEs (genetic damage and its repair) after exposure to various types of mutagenic carcinogens. The present experiments have allowed us to observe how many cell cycles various types of lesions that are induced in DNA by a crosslinking agent, an alkylating agent, or ionizing radiation, and that are responsible for the induction of SCEs, persist before being repaired and thus lose their ability to inflict genetic damage. Other experiments with various types of mutagenic carcinogens and various types of cell lines that have defects in different DNA repair processes, such as mismatch repair, excision repair, crosslink repair, and DNA-strand-break repair, can now be carried out to determine the role of these types of repair in removing specific types of lesions.

In vivo effect of gamma ray pretreatment on sister-chromatid exchange induction by mitomycin C in murine bone marrow cells

The effect of gamma ray pretreatment on sister-chromatid exchange induction by mitomycin C (MMC) was determined in murine bone marrow cells in vivo. A 30% reduction in the expected SCE frequency was observed assuming an additive effect. These results support the prediction of the replicative model for SCE formation with regard to the interactions between mutagens, and confirm previous results focused on the adaptive response.

Evidence that SCEs induced by mutagens do not occur at the same locus in successive cell cycles: lack of cancellation in three-way stained CHO chromosomes

An approach based on the synchronization of CHO cells after a first cell cycle incorporating a relatively low amount of bromodeoxyuridine (BrdUrd) into DNA, followed by mutagenic treatment and subsequent culture for second and third generations of BrdUrd incorporation for the scoring of sister chromatid exchanges (SCEs) per cell cycle in three-way differentially (TWD) stained chromosomes, has been used to investigate the possible cancellation of SCEs. Cancellation is expected to occur if two mutagen-induced SCEs occur at exactly the same site in subsequent rounds of replication. Lesions in DNA seem to persist and are able to induce SCE throughout two cell cycles after treatment with the three mutagens tested--mitomycin C (MMC), ethyl methanesulfonate (EMS) and ultraviolet (UV) light--though this latter agent was shown as only moderately persistent. Our results seem to indicate that SCEs induced by these mutagens do not take place at the same locus in successive cell generations, as assessed by a lack of SCE cancellation.

Sister-chromatid exchange: second report of the Gene-Tox Program

This paper reviews the ability of a number of chemicals to induce sister-chromatid exchanges (SCEs). The SCE data for animal cells in vivo and in vitro, and human cells in vitro are presented in 6 tables according to their relative effectiveness. A seventh table summarizes what is known about the effects of specific chemicals on SCEs for humans exposed in vivo. The data support the concept that SCEs provide a useful indication of exposure, although the mechanism and biological significance of SCE formation still remain to be elucidated.

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.

In vitro approach to fertility research: genotoxicity tests on primordial germ cells and embryonic stem cells

In vitro screening tests for reproductive toxicology are required by the 7th amendment of the directive 67/548 EEC and the OECD-programme on existing chemicals. Unfortunately, appropriate methods for testing developmental toxicity or impairment of fertility are not at hand. Therefore, we have tried to design test method based on mutagenic effects in germ cells that may be used as a test of fertility impairment as well. Embryonic stem cells (ESC) derived from mouse blastocysts can be kept in culture routinely. Establishment of ESC and improvement of their culture conditions are described and special properties of ESC are discussed in relation to germ cells. Because some properties of germ cells and pluripotent embryonic stem cells (ESC) are found to be comparable, permanent lines of ESC hold promise to be used as an in vitro test of impairment of fertility.

Molecular dosimetry for sister-chromatid exchange induction and cytotoxicity by monofunctional and bifunctional alkylating agents

The induction of sister-chromatid exchanges (SCEs) and cytotoxicity in 9L cells treated with monofunctional and bifunctional alkylating agents has been investigated. Three classes of monofunctional and bifunctional agents were studied: nitrosoureas, mustards and epoxides. Independent of class the bifunctional agents were 55-630-fold more effective at inducing SCEs and 300-2400-fold more effective at inducing cellular cytotoxicity than the corresponding monofunctional agents. Comparing the induction of SCEs and cytotoxicity by these agents showed that these two cellular responses to DNA damage are highly correlated. The extent of DNA alkylation in cells treated with 1-ethyl-1-nitrosourea (ENU) or 1-(2-chloro-ethyl)-1-nitrosourea (CNU) was similar indicating that the increased effectiveness of CNU to induce SCEs and cytotoxicity was not due to increased DNA alkylation. Molecular dosimetry calculations indicate that for CNU and ENU treatment of 9L cells there are 116 and 8500 alkylations per SCE induced and 2.6 x 10(4) and 4.6 x 10(6) alkylations at the dose required to reduce survival of 9L cells by 90%. Comparison of the DNA alkylation products produced by CNU and ENU treatment of 9L cells suggests that the formation of the intrastrand crosslink N7-bis(guanyl)ethane and the interstrand crosslink 1-(3-deoxycytidyl)-2-(1-deoxyguanosinyl)ethane by CNU is responsible for the increased effectiveness of CNU treatment at both induction of SCEs and cytotoxicity.

Enhanced assays detect increased chromosome damage and sister-chromatid exchanges in heroin addicts

To refine previous studies of chromosome damage (CD) and sister-chromatid exchanges (SCE) in heroin addicts, we applied new methods developed in our laboratory to enhance detection of the cytogenetic effects of low-level radiation exposure in hospital workers. For CD analysis, we applied our thymidine-fluorodeoxyuridine-caffeine (TFC) enhancement procedure in which cells at setup receive 1 x 10(-7) M fluorodeoxyuridine to inhibit thymidylate synthetase and 4 X 10(-5) M thymidine to satisfy the induced requirement, and then in G2 receive 2.2 mM caffeine to modulate DNA repair. For SCE enhancement, caffeine treatment was initiated in G1 at 19 h before harvest. Using both standard and enhanced procedures for CD and SCE analysis, blood samples were evaluated from 20 street heroin addicts and 22 controls. Standard 2-day CD and 3-day SCE assays showed small, insignificant genotoxic increases in addicts while the enhanced CD and SCE assays showed highly significant increases. Most CD events were in the form of chromatid and chromosome breaks. There were no rings and only a few dicentrics were observed in the TFC-enhanced cultures. Although quadriradials are rare, 10 were found in addict TFC-cultures and 3 in control TFC-cultures. With the standard CD assay, the mean number of chromosome breaks per 100 cells was 0.727 for controls and 1.056 for addicts (not significant). With the TFC-enhanced assay, the same measure showed 1.483 chromosome breaks for controls and 5.143 for addicts (highly significant, ANOVA: p less than 0.0001). A highly significant difference was also observed for chromatid-type damage with the TFC-enhanced assay (chromatid breaks per 100 cells: 16.793 for controls; 48.191 for addicts). The SCE data also showed significant differences with the enhanced assay. Scoring 25 cells/condition, standard SCE cultures showed 10.892 SCE/cell for controls and 11.732 SCE/cell for addicts (not significant). With CAF enhancement there were 13.08 SCE/cell for controls and 17.05 SCE/cell for addicts (ANOVA: p less than 0.008). These findings indicate that detection of CD and SCE effects can be significantly enhanced by the use of these new procedures. The finding of greatly increased chromatid damage in the addicts with the TFC procedure suggests that at least part of the CD detected occurred in vitro and is not a product of prior in vivo damage. Therefore exposure to this drug and perhaps other environmental agents may not only leave a residue of DNA or chromosome damage but may also induce a sensitivity to further genotoxic damage that is revealed by using the enhanced procedures.

Fate of DNA lesions that elicit sister-chromatid exchanges

Using 3-way differential staining (TWD) of sister chromatids, the fate of DNA lesions involved in sister-chromatid exchange (SCE) formation was determined in murine bone marrow cells in vivo, after treatment with either mitomycin C (MMC) or cyclophosphamide (CP). Both MMC (2.6 mg/kg b.w.) and CP (7 mg/kg b.w.) induced an SCE frequency near the expected in the 2 subsequent cell divisions, but the frequency of SCE occurring at the same locus in successive cell divisions was substantially lower than expected. The results are compared with previous data obtained after exposure to gamma-rays. A model of SCE induction is proposed.

Sister chromatid exchange and proliferation pattern in lymphocytes from newborns, elderly subjects and in premature aging syndromes

Sister chromatid exchange (SCE) frequency and cell proliferation were examined in lymphocyte cultures from a group of newborns, a group of elderly subjects and from patients with syndromes who exhibit progeriform characteristics (progeria, Cockayne syndrome, Rothmund-Thomson syndrome and Christ-Siemens-Touraine syndrome) by using the bromodeoxyuridine incorporation differential staining technique. We observed a significantly increase in basal SCE frequency and a less intensive cell proliferation in cultures from elderly subjects than from newborns, as shown by the significant increase in percentage of cells in first generation simultaneous with a reduction of cells in more advanced generations. Lymphocyte cultures from each one of the patients studied also showed a decreased cell proliferation in relation to their respective control and to newborn cultures. Each of these syndromes showed higher baseline SCE levels than the control and than the newborn and elderly groups. Only the patient with progeria showed values similar to those for the elderly group. Thus, in addition to showing clinical characteristics similar to those observed during the normal aging process, these progeriform syndromes also show cytogenetic characteristics similar to those of older individuals.

Induction of sister chromatid exchanges with hypotonic treatment

Hypotonic NaCl solutions and diluted culture medium were tested for induction of sister chromatid exchanges (SCEs) and chromosomal aberrations (CA). Hypotonic treatment conditions were found to induce high frequencies of CA and SCEs. However, an increase in SCEs was observed only in cells that also had CA. Possible lesions and mechanisms leading to SCEs and CA are discussed.

Biogenetic effects of opiates

Our laboratories have shown that opiate addicts have higher chromosome damage and sister chromatid exchange frequencies, and that these effects are more pronounced in the addicts when we employ enhanced culturing assays developed in our laboratory. We have also demonstrated that opiates diminish DNA repair capacity and reduce immunoresponsiveness as measured by T-cell E-rosetting and other assays. These interactions of opiates with T-lymphocytes may regulate cell metabolism and could thereby be responsible for the sensitivity of cells from opiate addicts to both genotoxic damage and immunological effects.

DNA topoisomerases and models of sister-chromatid exchange

Pommier et al. (1985) suggested that sister-chromatid exchange (SCE) results from exchange of topoisomerase II subunits. "Homologous displacement", an alternative mechanism, is proposed in which strand switching occurs during removal of parental helical turns by topoisomerases. The steps in the SCE model proposed by Ishii and Bender (1980) for SCE occurring at a blocked replication fork could occur by this mechanism and would require the action of both topoisomerases I and II. Homologous displacement involving topoisomerase II alone provides a mechanism for the strand switching required in the models of Kato (1977) and Cleaver (1981) in which SCE occur between replicated double strands. These mechanisms and models are discussed in relation to current knowledge of the locations and functions of topoisomerases during DNA replication.

Dose-response function for Painter's SCE-model

The replication model for sister chromatid exchange (SCE), when introduced in 1980 by Painter, was claimed to be consistent with the one hit property of SCE. However, the argument offered in favour of the one hit property was based on a defective dose-response function, as shown in this paper, since dose as the independent parameter of any dose-response function was not included in the considerations. This missing part of the model's dose-response function is added and, by using Bessel functions, a formula for the complete dose-response function is presented. A re-examination of the newly derived function shows that, in the model, linearity holds only under certain restricted circumstances.

Drug resistance and DNA repair

DNA repair confers resistance to anticancer drugs which kill cells by reacting with DNA. A review of our current information on the topic will be presented here. Our understanding of the molecular biology of repair of 0(6)-alkylguanine adducts in DNA has advanced as a result of the molecular cloning of the E. coli ada gene but the precise role of this lesion in the cytotoxic effects of alkylating agents in mammalian cells is not completely understood. Less progress has been made in understanding the enzymology and molecular biology of DNA cross-link repair even though such lesions are important for the cytotoxic effects of the widely used bifunctional alkylating agents and platinum compounds. It is evident that drug sensitive or resistant phenotypes are as highly complex as are the effects of DNA damage on cell metabolism and various aspects of these effects are discussed. Few clear correlations have been made between quantitative differences in DNA repair capacity and cellular sensitivity but assays which were developed to measure fidelity and intragenomic heterogeneity in DNA repair are beginning to be applied. Such studies may reveal subtle differences between sensitive and resistant cell lines. The molecular cloning of human DNA repair genes by transfection into drug sensitive rodent cells has been attempted. Some success has been achieved in this area but the functions of the cloned genes have yet to be identified.

Factors affecting the production of SCEs by maleic hydrazide in root-tip chromosomes of Allium cepa

We have investigated the influence of pH on the induction of chromatid-type aberrations and sister-chromatid exchanges (SCEs) by maleic hydrazide (MH) in root-tip cells of Allium cepa. For both cytogenetic endpoints, the lower the pH of the treatment solution, the higher were the frequencies of chromosome alterations detected at metaphase. We have further studied the persistence of lesions giving rise to SCEs during successive cell cycles, as well as the influence of BrdU concentration in the post-treatment medium on the yield of MH-induced SCEs. Our results suggest that the cytogenetic action of MH in many respects resembles that of bifunctional alkylating agents.

Processing of psoralen adducts in an active human gene: repair and replication of DNA containing monoadducts and interstrand cross-links

We have examined DNA repair in the dihydrofolate reductase (DHFR) gene in cultured human cells treated with 4'-hydroxymethyl-4,5',8-trimethylpsoralen (HMT) using a newly developed assay for interstrand DNA cross-linking in defined genomic sequences. Within 24 hr, 80% of the cross-links, but only 45% of the monoadducts, were removed from a 32 kb transcribed sequence, demonstrating that repair efficiency in an active human gene varies with the nature of the damage. HMT monoadducts were also detected in the replicated DHFR sequence at frequencies indicating little interference with replication. The existence of cross-linkable monoadduct sites in the replicated DNA implies strand continuity opposite those sites and a relatively error-free mechanism of bypass. Translesion replication could circumvent transcription blockage in a damaged gene. These findings have important implications for mechanisms of mutagenesis and DNA lesion tolerance in human cells.

Induction of sister-chromatid exchanges by restriction endonucleases

Restriction endonucleases Cfo 1, Pvu II, Sma I, Hpa II, Taq I and Hae III were tested for their ability to induce SCEs in CHO cells. The results indicate that the DNA double-strand breaks induced during S-phase by these enzymes lead to an increase in the frequencies of SCEs.

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.

The ability of bifunctional and monofunctional pyrrole compounds to induce sister-chromatid exchange (SCE) in human lymphocytes and mutations in Salmonella typhimurium

The ability to induce sister-chromatid exchange (SCE) in human lymphocytes and mutations in Salmonella typhimurium has been assessed for 4 pyrrole compounds. Three of the compounds, 2,3-bishydroxymethyl-1-methylpyrrole (BHMP), 2-hydroxymethyl-1-methylpyrrole (2HMP) and 3-hydroxymethyl-1-methylpyrrole (3HMP) are synthetic pyrrolic alcohols; the fourth compound, dehydroretronecine (DHR) is a metabolite of several naturally occurring pyrrolizidine alkaloids. The activity of these compounds was compared with that of mitomycin C (MMC) and decarbamoyl mitomycin C (DCMMC), chemicals related structurally to the pyrrole compounds. All 6 compounds caused an increase in the numbers of SCEs. Whereas the bifunctional pyrroles, DHR and BHMP, and the mitomycins, MMC and DCMMC, increased levels of SCEs by 8-12 times control levels, the monofunctional pyrroles gave increases of only 2 times. Three of the 4 pyrrole compounds (DHR, BHMP and 3HMP) induced mutations in the Salmonella typhimurium base substitution strain TA92, the fourth (2HMP) was not found to be mutagenic in any of the 8 strains used. The mitomycins induced mutations in the frameshift strain TA94 in addition to the base substitution strain TA92, with DCMMC always more mutagenic and less cytotoxic than MMC. All bifunctional compounds induced more mutations and were less cytotoxic in strains containing an efficient excision-repair system. With the pyrrole compounds numbers of SCEs and mutations were only increased when using chemical concentrations significantly higher than those required for the mitomycins: more than twice as high to produce significant numbers of SCEs and more than 100 times as high to produce equal numbers of mutations.

Effect of tissue culture variables on sister chromatid exchange in a nontransformed rat cell line

The frequency of sister chromatid exchange (SCE) was determined in a nontransformed diploid rat cell line, FR3T3 , under several tissue culture variables such as cultivation temperature, growth conditions of cells, and concentrations of 5-bromo-2'-deoxyuridine (BrdU). The conclusions to be drawn from these experiments are: (a) The cell growth and mechanisms(s) of SCE formation in FR3T3 cells are largely temperature independent (or efficiently regulated) in the range between 33 and 40.5 degrees C. (b) The concentration limits for BrdU incorporation are 5 to 100 microM; baseline frequency is about 11 SCE/metaphase (constant up to 20 microM BrdU) and increases only moderately at higher BrdU concentrations. (c) Toxic levels of BrdU (150 microM) cause a decrease of SCE rates below that found at 100 microM, presumably due to selective cell death. (d) Keeping cells growth arrested over a long period causes substantial SCE induction after replating. (e) Induced increase of SCEs probably occurs in this manner during the first cell cycle after release from growth arrest. It is no longer detectable after the fourth consecutive cell division.

Sister chromatid exchange (SCE) induced by laser-UV-microirradiation: correlation between the distribution of photolesions and the distribution of SCEs

Small, medium, and large nuclear areas comprising approximately 5, 30, or 80% of the total area of the interphase nuclei of Chinese hamster cells (M3-1) cultivated in vitro were irradiated with a laser-UV-microbeam of wavelength 257 nm. The DNA of the cells was substituted with 5-bromodeoxyuridine (BrdUrd) for 1 cell cycle in one set of experiments. After microirradiation the cells were grown for a second cycle in medium without BrdUrd (protocol A). In a second set, cells with nonsubstituted DNA were microirradiated and grown for 2 additional cycles, the first in the presence, the second in the absence of BrdUrd (protocol B). In situ chromosome preparation and differential chromatid staining was subsequently performed. The induction of sister chromatid exchanges (SCEs) was found to be dependent on both the ultraviolet (UV) dose and the spatial distribution of the UV energy within the cell nucleus. Following both protocols the average number of chromosomes with SCEs was significantly higher after microirradiation of a large nuclear area as compared to microirradiation of a small nuclear area. In the latter case, multiple SCEs were noted on individual chromosome arms at the first postirradiation mitosis (protocol A). In other cells, especially at higher doses, protocol A resulted in shattering of a few closely neighbored chromosomes which were surrounded by intact ones with normal SCE levels. Microirradiation of medium-sized nuclear areas produced high levels of SCEs over a number of chromosomes which still appeared spatially related in a part of the metaphase spread. Finally, high SCE levels could be observed over most or all chromosomes when a large nuclear area (up to 100%) was exposed to the microbeam. Following protocol B the increase of SCEs was much less pronounced. Microirradiation of a small part of the cytoplasm in addition to the nuclei did not induce SCEs. Our results support the concept (i) that interphase chromosomes occupy distinct nuclear domains and indicate (ii) that the induction of SCEs by UV light is restricted to microirradiated chromatin.

DNA interstrand cross-linking, repair, and SCE mechanism in human cells in special reference to Fanconi anemia

The relation of DNA cross-linking and repair to sister chromatid exchange (SCE) formation was studied in normal human, Fanconi anemia (FA), and xeroderma pigmentosum (XP) cells. Despite a hypersensitive lethality response in FA cells, the SCE induction rates by mitomycin C (MMC), trimethylpsoralen (TMP)-light, cisplatin, and diepoxybutane were twice as high as in normal cells. For MMC, the induced SCE frequency in normal cells was reduced in a biphasic fashion with a repair incubation time (the first decline t1/2 = 2 hr; the second t1/2 = 14-18 hr) which corresponds exactly to the molecular kinetics of cross-link and monoadduct removal. However, FA cells lack the first half-excision process and exhibit a lack of the first rapid decline SCE component. The slow decline component is present, and a higher SCE frequency is observed 24 to 48 hr after treatment. By contrast, XP cells capable of the half-excision process reveal the first rapid decline component, followed by an extremely slow second-reduction component (t1/2 = 48 hr) due to defective monoadduct repair. The endoreduplication-SCE method revealed that rates of both twin (first cycle) and single (second cycle) SCE formations by MMC and TMP-light were higher in FA cells than in normal cells. These results indicate that cross-links remaining unrepaired induce SCEs as do monoadducts. The probabilistic SCE induction occurs at a rate of 1 SCE per 35 MMC cross-links in FA cells. Further, a non-SCE-forming tolerance mechanism also operates in hypersensitive FA cells. These molecular and cytogenetic results allow us to construct a new probabilistic model for cross-link-induced SCE into which the replication-fork model, random cross-link transfer to both chromatids, and chromatid breakage-reunion are incorporated.

Eukaryotic genome: model considerations

The paper presents a new model of chromosome structure based on the assumption that multiple circular subunits of DNA exist. The essential difference with previously described models is the circular DNA unit forms a central chromosome axis. Chromosome configurations during various phases of the cell cycle depend on the various conformations of this central integrating unit. The described model can be generalized for all haploid set of eukaryotic nucleus. Some aspects of the chromosome structure and their functions have been discussed.

Induction of SCE by DNA cross-links in human fibroblasts exposed to 8-MOP and UVA irradiation

To study the SCE-inducing effect of psoralen cross-links in the DNA of normal, human fibroblasts, cell cultures were exposed to PUVA (0.2-1 micrograms of 8-MOP per ml, followed by UVA irradiation at 0.04 J/cm2) and carefully washed to remove non-covalently bound psoralen. Some cell cultures were then given a second dose of UVA (1.1 J/cm2), either immediately after PUVA or 1-3 days later. By this type of treatment, cells with different proportions of DNA cross-links are obtained. The initial PUVA treatment will mainly give rise to psoralen monoadducts and only few cross-links in the DNA, and the second UVA irradiation will convert a number of the psoralen monoadducts into cross-links. SCE analysis was carried out on cells grown for 2 cell cycles in the presence of BrdUrd (10 mumoles/1). PUVA treatment alone did not induce an increase in the SCE frequency, whereas a clear increase of SCE was observed in cells treated with PUVA immediately followed by the second UVA dose. This PUVA + UVA-induced increase of SCE was also observed after incubation of the cells for 3 days at confluency, as well as when a period of 3 days at confluency was introduced between the PUVA exposure and the second irradiation with UVA. In contrast, the SCE frequency gradually returned to the normal level when PUVA + UVA-treated cells were allowed to proliferate for 1-2 days, or when a proliferation period of 2-4 days was introduced between the PUVA exposure and the second irradiation. Because the SCE frequency was not changed by the initial PUVA treatment but markedly increased by PUVA + UVA, it is concluded that psoralen cross-links are considerably more effective at inducing SCE than monoadducts. The results also indicate that SCE-inducing PUVA damage is removed very slowly if at all from the DNA of confluent cells. In contrast, repair functions that eliminate cross-links as well as monoadducts seem to become activated during cell proliferation.

The relationship between sister-chromatid exchange and perturbations in DNA replication in mutant EM9 and normal CHO cells

The majority of the high (12-fold elevated) baseline sister-chromatid exchanges (SCEs) that occur in the CHO mutant line EM9 appear to be a consequence of incorporated BrdUrd, and they arise during replication of DNA containing BrdUrd in a template strand. In normal CHO cells the alkaline elution patterns of DNA newly replicated on a BrdUrd-containing template are significantly altered compared with those seen during the replication on an unsubstituted template. The nascent DNA synthesized on such an altered template is delayed in reaching mature size, possibly because replication forks are temporarily blocked at sites occurring randomly along the template. Transient blockage of replication forks may be a prerequisite for SCE. The delay in replication on BrdUrd-substituted templates was greater in EM9 cells than in parental AA8 cells and was also greater in AA8 cells treated with benzamide, an inhibitor of poly(ADPR) polymerase, than in untreated AA8 cells. Under these conditions, treatment with benzamide also produced a 7-fold increase in SCEs in AA8. An EM9-derived revertant line that has a low baseline SCE frequency showed less delay in replication on BrdUrd-substituted templates than did EM9. However, under conditions where the template strand contained CldUrd, which was shown to produce 4-fold more SCEs than BrdUrd in AA8 cells, the replication delay in AA8 was not any greater in the CldUrd-substituted cells. Thus, other factors besides the delay appear to be involved in the production of SCEs by the template lesions resulting from incorporation of the halogen-substituted pyrimidine molecules.

Parallel increases in sister-chromatid exchanges at base level and with UV treatment in human opiate users

The SCE base level frequency and SCE levels induced by far-UV (254 nm) treatment of cells in early G1 and early S phases of the cell cycle were significantly higher in leukocytes from heroin addicts as compared to controls. The increased SCE levels in addicts was greatest at base level and smallest after UV irradiation of cells in S phase. These results corroborate and extend our previous findings of increased chromosome damage and reduced DNA-repair synthesis in heroin users. Since opiates do not directly damage DNA, the elevated cytogenetic effects associated with opiate use probably arise from secondary promotional effects related to opiate-mediated alterations in leukocyte metabolism.

Failure of diepoxybutane to enhance sister chromatid exchange levels in Fanconi's anemia patients and heterozygotes

Peripheral lymphocytes of three Fanconi's anemia (FA) patients and of five heterozygotes have been tested for their susceptibility to chromosomal breakage and sister chromatid exchange (SCE) induction by diepoxybutane (DEB). As previously shown, DEB dramatically increases the chromosomal breakage level in both homozygotes and heterozygotes. Slightly, but significantly, reduced spontaneous levels of SCE were found in cultures from FA patients and FA gene carriers as compared with controls. SCE rates were not enhanced in lymphocytes grown in the presence of DEB, irrespective of the time at which it had been added to the culture, and were comparable in cells of FA patients and FA heterozygotes. This was in contrast with SCE rates in control cells, which showed increases positively related to the length of DEB treatment.

Induction and rapid repair of sister-chromatid exchanges in multiple murine tissues in vivo by diepoxybutane

Sister-chromatid exchange (SCE) induction by the direct-acting bifunctional carcinogen, diepoxybutane (DEB), was investigated in multiple tissues in vivo. The log-log dose SCE response relationship was found to be parallel to that previously reported for DEB induction of lung adenomas. However, the SCE assay is approximately 20 times as sensitive in detecting genotoxic effects of DEB than indicated by the lung adenoma assay. Examination of second and third division cells following various treatment protocols revealed that regardless of the nature of initially induced lesions, they are rapidly repaired with no evidence of persistence beyond 1 cell cycle.