Hypersensitive character of Bloom syndrome B-lymphoblastoid cell lines usable for sensitive carcinogen detection

Various carcinogens were tested with regard to the induction of sister-chromatid exchanges (SCEs) and chromosome aberrations using 3 types of Bloom syndrome (BS) B-lymphoblastoid cell lines (LCLs) (type I with normal frequency of SCEs and normal karyotype; type II with high frequency of SCEs and normal karyotype; type III with high frequency of SCEs and abnormal karyotypes) in the presence and absence of S9 mix. Three types of BS B-LCLs and normal cells showed different responses to the various carcinogens in the level of SCE induction. BS type I cells had the same SCE response as normal cells to carcinogens. Some carcinogens that require metabolic activation (S9 mix) had little effect on type II cells without S9 mix but had high SCE levels with S9 mix. BS type III cells were highly susceptible to both direct and indirect carcinogens with respect to high SCE increase without S9 mix (ca. 140 SCEs/cell), though some carcinogens produced SCEs rated in the medium (ca. 120 SCEs/cell) range, and had a high rate (more than 10%) of centromere spreading (CS), in addition to quadriradials. Therefore BS type III is a unique cell line which can be used to detect carcinogens.     

Analysis of sister-chromatid exchanges, cell kinetics and mitotic index in lymphocytes of smoking pesticide sprayers

Whole blood of 50 smokers who were exposed to pesticides was set up in RPMI 1640 medium, and observed for sister-chromatid exchanges (SCEs), cell kinetics (CK) and mitotic index (MI). As controls, blood samples were collected from 20 non-smokers (control I) and 27 smokers (control II) who were not exposed to pesticides. A significant increase in SCEs was observed as the duration of exposure increased. The frequency of M1 metaphases increased significantly whereas M2 and M3+ metaphases decreased in the exposed group. The mitotic index increased in control II and in the exposed population while it showed a decrease at 11-25 years' exposure.     

Spontaneous and induced sister chromatid exchanges in the blood lymphocytes of healthy persons and of xeroderma pigmentosum patients exposed to the inhibitors of DNA repair and replication caffeine, 3-methoxybenzamide and novobiocin

The frequency of sister chromatid exchanges (SCEs), both spontaneous and induced by UV-light, X-rays, mitomycin C and ethylmetansulphonate (EMS), has been investigated in cultured human peripheral blood lymphocytes. Besides, frequency of spontaneous and induced SCEs was studied under the action of the inhibitors of topoisomerase II, polymerase poly(ADP-ribose), and DNA repair, i. e. novobiocin, 3-metoxybenzamide, and caffeine, respectively. It is shown that the base-line SCEs in lymphocytes of the patient with xeroderma pigmentosum II (XP2LE) is dramatically higher compared to that in normal and pigmented xerodermoid cells (XP3LE). The above inhibitors of DNA synthesis and repair enhance the rate of spontaneous SCEs in normal, XP2LE and XP3LE cells. UV-, X-ray and chemical mutagens induced an increased frequency of SCEs in these cells. Simultaneous treatment with mutagenes and inhibitors of DNA synthesis and DNA repair enhanced the rate of SCEs in lymphocytes of healthy donors and in the XP3LE patient. The frequency of the XP2LE cells. Novobiocin, 3-MBA and caffeine significantly decreased the frequency of SCEs in mitomycin C- and EMS-treated XP2LE lymphocyte, which nevertheless was much higher than that in normal cells treated with the same agents.     

Chromosomal aberrations and sister-chromatid exchanges in swine

Frequencies of chromosomal aberrations and sister-chromatid exchanges (SCEs) in peripheral blood lymphocytes were investigated in 56 swine from 3 herds (I, breeding sows; II and III, fattening pigs). The mean frequencies of aberrant cells (AB.C.) were 3.58 +/- 1.59%, 2.10 +/- 1.52% and 6.20 +/- 3.21%, respectively. The mean numbers of SCEs per cell were 7.73 +/- 0.86, 6.51 +/- 0.89 and 7.06 +/- 1.47, respectively. A significant difference was found between the herds under study with regard to the number of aberrant cells but not the SCE frequency. In a parallel study, the presence of aflatoxin B1, polychlorinated biphenyls (PCB), dichlorodiphenyltrichloromethylmethane (DDT), lindane, mercury, lead and cadmium in the environment of fattening pigs was investigated. The total exposure to mutagens of pigs from herd III with a mean frequency of 6.2% AB.C., was markedly higher than that of herd II with the mean frequency of 2.1% AB.C.     

The frequency and distribution of isolabelling in Chinese hamster chromosomes after exposure to x-rays

2 experiments were carried out, following the same scheme. Chinese hamster cells (line CHEF-125) were cultivated for 4 h in the presence of tritiated thymidine ([³H]TdR) (Expt. 1, 0.5 μCi/ml; Expt. 2, 0.1 μCi/ml). After removal of the isotope, some of the cultures were X-irradiated in stage S-G₂ preceding M₁ (series S-G₂ I), others in stage S-G₂ preceding M₂ (series S-G₂ II).Analysis of the colchicine-C-anaphase of M₂ cells showed that in both experiments the X-rays produced an increase in the isolabelling regions only if given in the S-G₂ stage preceding M₁. On the other hand, the X-rays produced an increase in sister chromatid exchanges (SCEs) in both series of the second experiment. In the first experiment, where the controls had a high frequency of SCEs, no increase in SCEs was recorded in either of the two series treated with X-rays. The frequency of the isolabelling regions, in relation to the total labelling (Expt. 2), was 1.4% in the control series, 2.5% in series S-G₂ I and 1.3% in series S-G₂ II.Analysis of the distribution of the isolabelled regions along the chromosome showed that the regions furthest from the centromere were those most involved in this phenomenon and that, for the most part, the isolabelling was not directly associated with an exchange.The results have been interpreted, in accordance with the theory put forward in a previous work, to mean that SCEs and isolabelling regions are due to different phenomena, the former resulting from an exchange involving the whole chromatid, the latter from subchromatid exchanges probably involving a single polynucleotide strand.     

Sister-chromatid exchanges in association with occupational exposure to ethylene oxide

Sister-chromatid exchange (SCE) frequencies in employees potentially exposed to ethylene oxide (ETO) were compared with those in unexposed control groups. Three worksites where the previous environmental control of ETO was known to have differed were chosen. Within these worksites, subjects were categorized into high potential exposed, low potential exposed and control groups. An additional community control group was obtained. Blood samples for chromosome studies of peripheral lymphocytes were drawn at several time points over a period of 24 months. The effects on SCE of age, sex, smoking habits and reader variation were considered. Worksites I, II and III, respectively, represented increasing levels of exposure. At Worksite III large differences among groups persisted over 24 months. At Worksite II, the SCEs in the high potential exposed workers were higher than those in the other groups. At no time was the low potential exposed group at Worksite II statistically significantly higher in mean SCE than the worksite controls. No consistent differences among groups were noted in Worksite I.     

cis-diamminedichloroplatinum(II)-induced sister-chromatid exchanges and chromosome aberration formation in cultured human lymphocytes and their inhibition by sodium thiosulfate

cis-Diamminedichloroplatinum(II) (cis-DDP)-induced sister-chromatid exchanges (SCEs) and chromosome aberration formation were studied in human lymphocytes. The mitotic index decreased abruptly at 2 X 10(-6) M cis-DDP and the frequency of SCEs was dose-related; a marked increase was recorded at 10(-6) M cis-DDP. A dose-dependent effect was also found for chromosome aberration formation at concentrations between 10(-11) and 4 X 10(-6) M. The aberrations observed were primarily chromatid breaks and gaps. We also examined the inhibition of these genotoxicities by treating the cells with sodium thiosulfate (STS). Simultaneous treatment with 10(-4)-10(-3) M STS (100-1000-fold molar ratio to cis-DDP) significantly reduced the frequency of SCEs induced by 10(-6) M cis-DDP. Furthermore, a 3-h delay in treating with STS significantly reduced cis-DDP-induced SCEs, but not chromosome aberration formation.     

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

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

TAGCNA: a method to identify significant consensus events of copy number alterations in cancer

Somatic copy number alteration (CNA) is a common phenomenon in cancer genome. Distinguishing significant consensus events (SCEs) from random background CNAs in a set of subjects has been proven to be a valuable tool to study cancer. In order to identify SCEs with an acceptable type I error rate, better computational approaches should be developed based on reasonable statistics and null distributions. In this article, we propose a new approach named TAGCNA for identifying SCEs in somatic CNAs that may encompass cancer driver genes. TAGCNA employs a peel-off permutation scheme to generate a reasonable null distribution based on a prior step of selecting tag CNA markers from the genome being considered. We demonstrate the statistical power of TAGCNA on simulated ground truth data, and validate its applicability using two publicly available cancer datasets: lung and prostate adenocarcinoma. TAGCNA identifies SCEs that are known to be involved with proto-oncogenes (e.g. EGFR, CDK4) and tumor suppressor genes (e.g. CDKN2A, CDKN2B), and provides many additional SCEs with potential biological relevance in these data. TAGCNA can be used to analyze the significance of CNAs in various cancers. It is implemented in R and is freely available at http://tagcna.sourceforge.net/.     

Response of V79 cells to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) treatment: inhibition of poly(ADP-ribose) and topoisomerase activity.

MNNG-induced killing of V79 cells has been found to be enhanced on inhibition of topoisomerase II activity by nalidixic acid and poly(ADP-ribose) polymerase synthesis by benzamide. Using these 2 inhibitors in conjunction after MNNG treatment, some overlap in the functions of these 2 enzymes was observed. Nalidixic acid and benzamide were found to suppress the yields of mutations and SCEs induced by MNNG. Benzamide was more effective in suppressing the mutation yield whereas nalidixic acid was more effective in suppressing SCEs. A model based on the relative requirement of topoisomerase and poly(ADP-ribose) for the repair of different types of damage has been proposed to explain the results.     

Enhancement of UV-induced mutagenesis and sister-chromatid exchanges by nickel ions in V79 cells: evidence for inhibition of DNA repair

With regard to contradictory results concerning the mutagenicity of nickel compounds in short-term assays, especially in bacterial test systems, Chinese hamster V79 cells were used to measure mutagenicity, comutagenicity and the induction of sister-chromatid exchanges (SCEs) by NiCl2. We confirmed the induction of mutations at the HGPRT locus as well as SCEs. In addition, NiCl2 shows a pronounced comutagenic effect towards UV. When using confluent cultures or resting cells due to serum deprivation, where more time is given for repair processes, the comutagenic effect is higher compared to logarithmically growing cells (10 and 4 times, respectively, compared to twice). Hence, we attribute this enhancement in mutagenicity to inhibition of DNA repair. Also the increase in induced SCEs after combined treatment with UV and NiCl2 supports this thesis. Furthermore, NiCl2 enhances the cyto-toxicity of cis-DDP about 12-fold. Since no comutagenic effect is observed in combination with MMS, we suggest that the inhibition of DNA repair by Ni(II) applies to all DNA changes that are repaired by the 'long-patch' excision repair system. This inhibition may occur via replacement of other divalent metal ions essential in repair and regulation processes.     

SCE frequencies in lymphocytes of systemic lupus erythematosus patients

The frequency of sister-chromatid exchanges (SCEs) was investigated in peripheral lymphocytes of lupus erythematosus patients and compared with values obtained for healthy controls. Irrespective of the kind of medical treatment, an increased level of spontaneously occurring SCEs could be demonstrated in lupus patients. In addition to spontaneously occurring SCEs, mitomycin C (MMC)-induced SCEs were evaluated. No difference between patients and controls was found with respect to MMC-induced SCEs.     

BCNU-induced sister chromatid exchanges are increased by X irradiation

We have studied the effect on sister chromatid exchange (SCE) induction in 9L rat brain tumor cells caused by combination treatment with BCNU and X rays. Over the dose and concentration ranges used in these experiments, BCNU induced relatively large numbers of SCEs, while X rays induced few SCEs. When cells were X irradiated immediately after BCNU treatment, the number of SCEs induced was greater than the number of SCEs expected by adding the number of SCEs induced by each agent alone; the number of SCEs induced as a result of this BCNU-X-ray interaction increased as the concentration of BCNU and/or dose of X rays increased. When the addition of bromodeoxyuridine was delayed from 0 to 16 hr after BCNU treatment, the number of SCEs induced declined to control levels by 16 hr. If X irradiation was delayed for up to 16 hr after BCNU treatment the same pattern of decrease was observed; the number of SCEs induced at each time point, however, was greater than that induced by BCNU and X rays alone. X irradiation from 0-16 hr before BCNU treatment produced the same number of SCEs as that produced by BCNU alone. Thus the SCE assay is capable of detecting a drug-X-ray interaction in mammalian cells and provides a sensitive means of studying the sequencing and timing that leads to the interaction.     

Sister-chromatid exchanges induced by ultraviolet light in Bloom's syndrome fibroblasts

The relationships between the cytotoxic effect of ultraviolet light and the UV-induced sister-chromatid exchanges (SCEs) were compared among fibroblast cell strains from two unrelated Bloom's syndrome (BS) patients, one xeroderma pigmentosum (XP) patient belonging to complementation group A and two unrelated normal controls. The "net" induced SCEs as a function of UV fluence, obtained by subtracting spontaneous SCEs from observed SCEs, were much higher in both BS cells and XP group A cells than in normal cells. The relative efficiency of induced SCE, defined as the "net" induced SCEs as a function of surviving fraction after UV irradiation, was higher in BS cells than in normal and XP cells, and there was essentially no difference between XP and normal cells. These results imply that in addition to the extremely high frequency of spontaneous SCEs, the increased efficiency in UV induction of SCEs may reflect the intrinsic defect(s) in BS cells.     

The relation between chemically induced sister-chromatid exchanges and chromatid breakage.

Studies of classical chromosome aberrations and sister-chromatid exchanges (SCES) suggest independent mechanisms for the two events despite some common features. Examination of chromosome breakage caused by X-rays, visible light, and viruses has shown that few chromatid breaks are accompanied by SCEs at the sites of breaks. No similar observations were available for chemically induced breaks, but it has been reported that rat chromosomes exposed to dimethylbenzanthracene (DMBA) contained a preponderance of both aberrations and SCEs in certain specific regions, implicating a common process in their formation. These conclusions were drawn from a comparison of breaks induced in vivo with SCEs induced in vitro. However, we used 7 chemical mutagens to induce both chromatid breaks and SCEs in "harlequin" chromosomes of cultured rat and Chinese hamster ovary (CHO) cells and found that 25% of the 914 breaks scored were associated with SCEs. The proportion of breaks accompanied by SCEs is related to the overall SCE frequency and falls into the range predicted on the basis that breaks and SCEs occur independently. The reported association between sites for SCEs and aberrations also reflects secondary factors, such as induction of SCEs and aberrations during DNA synthesis in late replicating regions of the chromosomes.     

Bimodal induction of sister-chromatid exchanges by luminol,an inhibitor of poly(ADP-ribose) synthetase,during the S-phase of the cell cycle

The cell cycle dependence of sister chromatid exchanges (SCEs) induced by luminol, a new potent inhibitor of poly(ADP-ribose) synthetase, was studied in Chinese hamster V79 cells. Continuous treatment with luminol during two whole cell cycles in the presence of 5-bromo-2-deoxyuridine (BrdUrd), or in the first or second cycle induced SCEs very efficiently in a linear dosedependent manner. However, no enhancement of SCE levels was observed after luminol treatment in a cycle preceding BrdUrd treatment, in contrast to results found with other strong SCE inducers such ascis-diammine-dichloroplatinum (II) (CDDP) and mitomycin C (MMC). Luminol was about ten times as effective in inducing SCEs as 3-aminobenzamide (3AB), an inhibitor of the NAD⁺ site of poly(ADP-ribose) synthetase. The induction of SCEs by luminol was restricted to the Sphase of the cell cycle with peaks at an early and a late stage, corresponding to the biphasic replication of DNA. The mechanism of SCE appears to be the same at the early and late stages of S-phase for luminol-induced SCE formation.     

Relationship between sister-chromatid exchanges and heterochromatin or DNA replication in chromosomes of Crepis capillaris

The C-band patterns, DNA late replication patterns and distribution patterns of spontaneous and γ-ray-induced SCEs in Crepis capillaris chromosomes were studied. The fluorescence plus Giemsa (FPG) technique was used for detection of SCEs and late-replicating chromosome regions after unifilar incorporation of BrdU into DNA. An asynchronous replication of both euchromatic and heterochromatic chromosome regions was established. The frequency of SCEs is increased about 2-fold by 1.5 Gy γ-rays. The localization of the sites of SCEs was analyzed with special reference to eu- and heterochromatin and early- and late-replicating regions. The data obtained showed that SCEs were distributed nonrandomly along the chromosomes. Preferential occurrence of SCEs was observed in the following chromosome regions: at the junction between eu- and heterochromatic regions, the latter being rich in late-replicating DNA; at the junction between early- and late-replicating regions, the latter not being C-band positive. Certain heterochromatic regions were more rarely involved in SCEs than expected on the basis of their length. The lowest incidence of SCEs was found in the centromeric regions. Very similar distribution patterns of spontaneous and γ-ray-induced SCEs were observed. The possible role of the differences in the time of replication of the different chromosome regions in the formation of SCEs is discussed.     

Lack of Spontaneous Sister Chromatid Exchanges in Somatic Cells of DROSOPHILA MELANOGASTER

Neural ganglia of wild type third-instar larvae of Drosophila melanogaster were incubated for 13 hours at various concentrations of BUdR (1, 3, 9, 27 micrograms/ml). Metaphases were collected with colchicine, stained with Hoechst 33258, and scored under a fluorescence microscope. Metaphases in which the sister chromatids were clearly differentiated were scored for the presence of sister-chromatid exchanges (SCEs). At the lowest concentration of BUdR (1 microgram/ml), no SCEs were observed in either male or female neuroblasts. The SCEs were found at the higher concentrations of BUdR (3, 9, And 27 micrograms/ml) and with a greater frequency in females than in males. Therefore SCEs are not a spontaneous phenomenon in D. melanogaster, but are induced by BUdR incorporated in the DNA. A striking nonrandomness was found in the distribution of SCEs along the chromosomes. More than a third of the SCEs were clustered in the junctions between euchromatin and heterochromatin. The remaining SCEs were preferentially localized within the heterochromatic regions of the X chromosome and the autosomes and primarily on the entirely heterochromatic Y chromosome.--In order to find an alternative way of measuring the frequency of SCEs in the Drosophila neuroblasts, the occurrence of double dicentric rings was studied in two stocks carrying monocentric ring-X chromosomes. One ring chromosome, C(1)TR94--2, shows a rate of dicentric ring formation corresponding to the frequency of SCEs observed in the BUdR-labelled rod chromosomes. The other ring studied, R(1)2, exhibits a frequency of SCEs higher than that observed with both C(1) TR94--2 and rod chromosomes.     

Sister chromatid exchanges in Drosophila melanogaster cell lines in vitro

The frequency of sister chromatid exchanges (SCEs) in two cell lines of Drosophila melanogaster with different karyotypes (XX and XY) was determined, considering (1) the distribution of SCEs within each chromosome, with reference to eu- and heterochromatin and (2) the distribution of SCEs in different chromosomes. A comparison was made between chromosome pairs within each karyotype and between the two different karyotypes. The following results were obtained. The SCEs are not randomly distributed along chromosomes, since exchanges were never observed in heterochromatin. SCEs are more frequent in XY than in XX cells; moreover, in both cell types there exists a significantly higher frequency of SCEs in the X chromosome than in the autosomes. These findings are discussed in relation to chromosome aberrations and mitotic recombination.