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.     

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.     

Cytological characterization of repair-deficient CHO cell line 43-3B. I. Induction of chromosomal aberrations and sister-chromatid exchanges by UV and its modulation with 3-aminobenzamide

The induction of chromosomal aberrations and sister-chromatid exchanges (SCEs) by short-wave ultraviolet (UV) and X-irradiation was studied in Chinese hamster ovary (CHO) wild-type (WT) cells and one of its UV-hypersensitive mutants, 43-3B. The results indicate that CHO 43-3B show high levels of spontaneously occurring chromosomal aberrations and SCEs; these levels are, respectively, approximately 4 and 1.7 times those found in WT CHO. Treatment with UV produced a considerable delay in the cell-cycle progression of the mutant cells compared to the WT cells. Doses of UV that had no effect on WT cells, significantly induced chromosomal alterations in the mutant in a dose-dependent manner. An approximately 5-fold increase in the induced frequencies of SCEs was obtained in 43-3B cells after UV treatment. No synergistic effect was observed with UV irradiation and the inhibitor of poly(ADP-ribose) synthetase, 3-aminobenzamide (3AB), in either cell type. The frequency of SCEs in the mutant cell lines was lower than would be expected if the effects of UV and the inhibitor were additive. X-Ray alone in G1 and in combination with 3AB in G2 did not induce increased frequencies of chromosomal aberrations in mutant cells in comparison to the WT cells.     

Effects of mitomycin C on sister chromatid exchange in normal and Bloom's syndrome cells

Bloom's syndrome lymphocytes, which are characterized by a high incidence of sister chromatid exchanges (SCE: 80.6 per cell), were treated with mitomycin C (MMC) and the effect of the chemical on SCE frequency compared with that in normal cells. Raising the concentration of MMC from 1 X 10(-9) to 1 X 10(-7) g/ml led to about 10-fold increase (61.7 SCE per cell) in the SCE frequency over the base line in normal lymphocytes (6.4 SCE per cell), though chromosome aberrations remained at a relatively low frequency. MMC caused about a two-fold rise in SCE in cells of Bloom's syndrome (128.8 SCE at 10(-9) g/ml; 139.3 SCE at 10(-8) g/ml). The frequency of chromosome aberrations in Bloom's syndrome cells at concentrations of MMC of 1 X 10(-9) and 1 X 10(-8) g/ml was 0.350 and 0.825 per cell, respectively, and low when compared to the increased number of SCE. The increased frequency of SCE in normal and Bloom's syndrome cells is in contrast to the reported findings with cells from Fanconi's anemia and xeroderma pigmentosum. The distribution of SCE in MMC-treated normal cell correlates with that of spontaneous SCE in cells of Bloom's syndrome.     

Increased sister chromatid exchange in bone marrow and blood cells from Bloom's syndrome.

Bone-marrow cells from a patient with Bloom's syndrome cultured for 48 h in the presence of BudR exhibited a striking increase in the number of sister chromatid exchanges (SCEs) in comparison to that in the marrow cells of a patient with treated polycythemia vera (PV). Thus, it appears that an increased incidence of SCE in Bloom's syndrome occurs in various differentiated types of cells, not just blood lymphocytes, and constitutes the syndrome's most characteristic cytogenetic feature. In contrast, the incidence of SCE was not increased in marrow cells and lymphocytes of the particular PV patient studied here, whose cells did exhibit increased numbers of chromatid and chromosome gaps and breaks, presumably as result of the patient's earlier treatment. An increased frequency of SCE was demonstrated in Bloom's syndrome lymphocytes using both a technique based on BudR incorporation and one based on labeling with tritated deoxycytidine. This observation constitutes evidence against the increase of SCE being due to an unusual reaction to BudR. By conventional cytogenetic techniques, chromosome instability, including chromatid and chromosome breaks, but no homologous chromatid interchanges were also recognized in Bloom's syndrome bone-marrow cells incubated in vitro (without BudR) for either 1.k or 16 h. This observation points to the existence of chromosome instability in vivo.     

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.     

Hypersensitivity of Bloom's syndrome fibroblasts to N-ethyl-N-nitrosourea

Fibroblast cells from two Japanese patients with Bloom's syndrome (BS) and normal donors were studied for the inactivation of colony-forming ability and the induction of sister-chromatid exchanges (SCEs) after N-ethyl-N-nitrosourea (ENU) treatment. The reduction of ENU-induced SCEs as a function of post-treatment incubation time was also compared between BS and normal fibroblasts. BS cells were approximately 4 times more sensitive than normal cells to the lethal effect of ENU and remarkably hypersensitive to the SCE induction by ENU. The post-treatment incubation of ENU-treated normal cells in the fresh medium resulted in a time-dependent decrease of the SCE level until 6 h after which time the SCE level remained the plateau of about 50% of the initial level. In contrast, the ENU-induced SCEs in BS cells decreased much more slowly with post-treatment incubation time and its half life was 24 h. These results collectively support the view that BS cells may be defective in the rapid repair of certain type(s) of DNA damages induced by ENU.     

The effect of 1,10-phenanthroline on the chromosome damage and sister-chromatid exchanges induced by streptozotocin in mammalian and insect cells

The effect of the metal chelating agent 1,10-Phenanthroline (PNT) on the streptozotocin (STZ)-induced chromosomal aberrations (CAs) and sister-chromatid exchanges (SCEs) in Chinese hamster ovary (CHO) and mosquito (Aedes albopictus) cells was investigated. Treatment of CHO and mosquito cells with STZ produced a significant and dose-response increase in the yield of CAs as well as SCEs (p<0.05). The addition of PNT prevented the induction of CAs by STZ in both types of cells, causing a significant decrease in the frequency of STZ-induced CAs (46.5-72.5%) (p<0.05). This fact indicates that intracellular transition metals are implicated in STZ-induced CAs and that the Fenton reaction (Fe(2+)+H(2)O(2)-->OH degrees +OH(-)+ Fe(3+)) is partly responsible for the production of CAs by this compound. On the other hand, the addition of PNT to CHO and mosquito cell cultures did not prevent the induction of SCEs by STZ. Therefore, it is valid to assume that the induction of CAs and SCEs by STZ occurs by different mechanisms.     

Modifying effects of components of plant essence on the induction of sister-chromatid exchanges in cultured Chinese hamster ovary cells

The influence of 21 kinds of components of plant essence on sister-chromatid exchanges (SCEs) induced by mitomycin C was investigated in cultured Chinese hamster CHO K-1 cells. Posttreatment with scopoletin, jasmone, caffeic acid and ferulic acid significantly increased the frequency of SCEs. Further investigation of the SCE-enhancing effect of analogues of caffeic acid and ferulic acid revealed that an alpha,beta-unsaturated carbonyl group may be necessary for SCE-enhancing effects. The influence of caffeic acid and ferulic acid on X-ray- or UV-induced SCEs was also studied. The frequencies of SCEs induced by UV were increased by treatment with these compounds. This increasing effect was observed in the S phase of the cell cycle. On the contrary, X-ray-induced SCEs were reduced by the treatment with these compounds. The decreasing effect was observed in the G1 phase but not in the S or G2 phase. To explain these contradictory results, we assumed that caffeic acid and ferulic acid may modify the repair of DNA strand breaks.     

Presence of abnormally high incidences of sister chromatid exchanges in three successive cell cycles in Bloom's syndrome lymphocytes

The frequencies of sister chromatid exchanges (SCEs) were examined in phytohaemagglutinin-stimulated blood lymphocytes of a normal individual, a Bloom's syndrome heterozygote (bl/+), and two Bloom's syndrome homozygotes (bl/bl). To determine the baseline SCE frequencies, lymphocytes were cultured with various concentrations of 5-bromodeoxyuridine (BrdUrd) for two cell cycles. The incidence of SCEs per two cell cycles inbl/bl lymphocytes levelled off at BrdUrd concentrations below 10 g/ml while that in normal andbl/+ lymphocytes stayed constant below 7.5 g/ml. The baseline SCE frequency in bl/bl cells was ten times higher than that in normal andbl/+ cells. At BrdUrd concentrations above 15 g/ml, SCEs inbl/bl cells were induced more frequently than in normal andbl/+ cells. These results indicate that at low concentrations BrdUrd has a minimal effect on the induction of SCEs in all individuals, while at higher concentrations the BrdUrd incorporated inbl/bl cells has a larger effect than that in normal andbl/+ cells. To elucidate the effect of BrdUrd incorporated into the daughter and parental DNA strands on SCE induction, SCEs occurring during each cell cycle were examined separately in three-way or two-way differentially stained, third-cycle metaphases. The incidence of SCEs detected in each cell cycle at 5 g/ml BrdUrd was constant in all individuals and the rates of SCEs in each cell cycle inbl/bl cells were remarkably higher than those observed in normal andbl/+ cells. These findings strongly indicate that most of the abnormally increased SCEs in thebl/bl cells used in our study occurred independently of any effect of BrdUrd incorporated into both the daughter and parental DNA strands. In addition, an abnormal response ofbl/bl cells to BrdUrd was not found for cell cycle progression or chromosomal aberration induction. Thus, the bl/bl cells did not exhibit an abnormal hypersensitivity to BrdUrd. From these results, it seems quite probable that the abnormally increased SCEs in thebl/bl lymphocytes used here were spontaneous.     

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.     

Effects of cell fusion and deoxynucleosides on sister-chromatid exchanges in B-lymphoblastoid cell lines from 5 Bloom syndrome patients

The effect of cell fusion and deoxynucleosides (deoxyadenosine, dA; deoxyguanosine, dG; deoxycytidine, dC; thymidine, T) on sister-chromatid exchanges (SCEs) in Bloom syndrome (BS) was studied in two types of BrdU (bromodeoxyuridine)-sensitive and BrdU-resistant B-lymphoblastoid cell lines (LCLs) with respect to cellular proliferation in BrdU-labeled culture conditions. Cell fusion between BrdU-sensitive and BrdU-resistant BS B-LCLs did not exhibit complementation, although when any of the BS B-LCLs (retaining high SCE character) labeled with BrdU were fused with non-labeled normal cells, the hybrid cells had a normal level of SCE at the first mitosis after fusion. Deoxycytidine addition showed no effect on SCEs in normal cells but decreased SCEs in BS cells from the baseline level of 70 SCEs/cell to about 60 SCE/cell. Purine deoxyribonucleosides (dG and dA) caused a significant concentration-dependent increase in SCE frequency both in normal and BS cells. Although T caused a 2-fold increase in normal SCEs, it highly decreased BS SCE from 70 SCEs/cell to 35 SCEs/cell. FrdU did not greatly affect BS SCE in the presence of BrdU and T. These observations indicate strongly that BS cells may have a low thymidine pool compared with normal cells, which could account for a more efficient BrdU substitution in the DNA thus potentiating the template effect on SCE.     

Influence of incorporated 5-bromodeoxyuridine on the frequencies of spontaneous and induced sister-chromatid exchanges, detected by immunological methods

We have utilized monoclonal antibody against BrdUrd to detect sister-chromatid exchanges in CHO cells. This technique allows detection of SCEs at very low levels of BrdUrd incorporation. At incorporation level of 0.5%, a frequency of about 2 SCEs/cell/cycle was found. In a UV-sensitive mutant (43-3B) which has an increased spontaneous frequency of SCEs, it is found that this increase is due to incorporated BrdUrd. In MMS- and MMC-treated cells, an influence of BrdUrd on the frequencies of induced SCEs was found only when high concentrations of mutagens were employed.     

The effect of glutathione depletion on sister-chromatid exchange induction by cytostatic drugs

Using sister-chromatid exchanges (SCEs) as an indicator for DNA damage, we investigated the role of glutathione (GSH) as a determinant of cellular sensitivity to the DNA-damaging effects of the cytostatic drugs adriamycin (AM) and cyclophosphamide (CP). Exposure of V79 cells to buthionine sulfoximine (BSO) resulted in a complete depletion of cellular GSH content without toxicity and without increasing the SCE frequency. Subsequent 3-h treatment of GSH-depleted cells with AM or S9-mix-activated CP caused a potentiation of SCE induction. In Chinese hamster ovary (CHO) cells, which showed a higher GSH level compared to V79 cells, BSO treatment led to a depletion of GSH to about 5% of the control and increased SCE induction by AM and CP. Compared to V79 cells, the effect of AM on SCE frequencies was less distinct in CHO cells, while CP exerted a similar effect in both cell lines. Pretreatment of V79 cells with GSH increased the cellular GSH content, but had no effect on the induction of SCEs by AM, and pretreatment with cysteine influenced neither GSH levels nor SCE induction by AM. The study shows that SCEs are a suitable indicator for testing the modulation of of drug genotoxicity by GSH. The importance of different GSH contents of cell lines for their response to mutagens is discussed.     

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.     

Effect of recombinant interferon-alpha on streptonigrin-induced chromosome aberrations and sister-chromatid exchanges in hamster cells

The effect of recombinant interferon-alpha-2a (rIFN-alpha-2a) on the induction of chromosomal aberrations (CAs) and sister-chromatid exchanges (SCEs) by the radiomimetic antibiotic streptonigrin (SN, 250 ng/ml, 20 min, 37 degrees C) in Chinese hamster ovary (CHO) cells was investigated. Recombinant IFN-alpha-2a (4500-45,000 IU/ml) was added to the cell cultures 30 min before SN and left in the culture medium until the end of SN treatment or until cell harvesting. A statistically significant increase in the frequency of CAs and SCEs was observed following treatment with SN (P < 0.05), whereas treatments with rIFN-alpha-2a alone did not produce any significant increase of CAs and SCEs over control values. Low rIFN-alpha-2a doses produced a reduction in the frequency of CAs and an increase in the yield of SCEs induced by SN, while high doses of the cytokine caused an increase in the yield of CAs and a reduction in the frequency of SCEs induced by the antibiotic. In addition, rIFN-alpha-2a caused a marked inhibition (around 50%) on the yield of SN-induced chromatid-type aberrations in the G(2) phase of the cell cycle. It is suggested that the inhibitory effect of rIFN-alpha-2a on the SN-induced chromosome damage is due to the stimulation of DNA synthesis and repair by the cytokine. On the other hand, our results give further support to our previous hypothesis that the induction of CAs and SCEs by SN is based on different mechanisms.     

Cytogenetical characterization of UV-sensitive repair-deficient CHO cell line 43-3B. II. Induction of cell killing, chromosomal aberrations and sister-chromatid exchanges by 4NQO, mono- and bi-functional alkylating agents

An established cell line of Chinese hamster ovary (CHO-9) cells and its UV-sensitive mutant 43-3B have been studied for the induction of cell killing, chromosomal aberrations and sister-chromatid exchanges (SCEs) after exposure to different types of DNA-damaging agents such as 4-nitroquinoline-1-oxide (4NQO), mitomycin C (MMC), diepoxybutane (DEB), methyl methanesulfonate (MMS), ethyl methanesulfonate (EMS) and ethyl nitrosourea (ENU). In comparison with the wild-type CHO cells, 43-3B cells showed very high sensitivity to the UV-mimetic agent 4NQO and the DNA cross-linking agents MMC and DEB. The 43-3B cells responded with higher sensitivity to the monofunctional alkylating agents (MMS, EMS and ENU). The increased cytotoxic effects of all these chemicals correlated well with the elevated increase in the frequency of chromosomal aberrations. In 43-3B cells exposed to 4NQO, MMC or DEB the increase in the frequency of chromosomal aberrations was much higher than the increase in the frequency of SCEs (4-10-fold) when compared to the wild-type CHO cells. This suggests that SCEs are results of fundamentally different cellular events. The responses of 43-3B cells to UV, 4NQO, MMC and DEB resemble those of 2 human syndromes, i.e., xeroderma pigmentosum and Fanconi's anemia. These data suggest that 43-3B cells are defective in excision repair as well as the other pathways involved in the repair of cross-links (MMC, DEB) and bulky DNA adducts (4NQO).     

Sister chromatid exchange and cell cycle in fibroblasts of Bloom's syndrome

Summary Sister chromatid exchange (SCE) has been studied in the fibroblasts of five Bloom's syndrome patients, one heterozygote, and two normal individuals. The high frequency of SCE already known in the lymphocytes of Bloom's syndrome was also found in the fibroblasts of all five patients. However, populations with low and high frequency of SCE were not found. In addition, chromosome aberrations appeared with a lower frequency.The cell cycle duration in the Bloom's fibroblasts appeared to be similar to that in the normal cell line, and the difference in the growth pattern appeared to be due to the variation in the mitotic index. The cell cycle lasted about 24 h in at least four of the Bloom's lines studied during the present experiments.     

Cytological characterization of Chinese hamster ovary X-ray-sensitive mutant cells, xrs 5 and xrs 6. II. Induction of sister-chromatid exchanges and chromosomal aberrations by X-rays and UV-irradiation and their modulation by inhibitors of poly(ADP-ribose) synthetase and alpha-polymerase

The cell killing and induction of sister-chromatid exchanges (SCEs) by X-rays and short-wave ultraviolet (UV) irradiation in combination with inhibitors of DNA repair, 3-aminobenzamide (3AB), cytosine arabinoside (ara-C) or aphidicolin (APC) were studied in wild-type CHO-K1 and two X-ray-sensitive mutants, xrs 5 and xrs 6 cells. The spontaneous frequency of SCEs was similar in the mutants and the wild-type CHO-K1 cells (8.4-10.3 SCEs/cell). Though X-rays are known to be poor inducers of SCEs, a dose-dependent increase in the frequency of SCEs in xrs 6 cells (doubling at 150 rad) was found in comparison to a small increase in xrs 5 and no increase in wild-type CHO-K1 cells. 3AB, an inhibitor of poly(ADP-ribose) synthetase increased the spontaneous frequency of SCEs in all the cell types. 3AB did not potentiate the X-ray-induced frequency of SCEs in any of the cell lines. Ara-C, an inhibitor of DNA polymerase alpha, increased the frequency of SCEs in all the cell lines. In combined treatment with X-rays, ara-C had no synergistic effect in xrs 5 and xrs 6 cells, but the frequency of SCEs increased in X-irradiated wild-type CHO-K1 cells post-treated with ara-C. For the induced frequency of SCEs, CHO-K1 cells treated with X-rays plus ara-C behaved like xrs 6 cells treated with X-rays alone, suggesting a possible defect in DNA base damage repair in xrs 6 cells, in addition to the known defective repair of DNA double-strand breaks (DSBs). Survival experiments revealed higher sensitivity of xrs 5 and xrs 6 mutant cells to the cell killing effect of X-rays in S-phase when compared to wild-type CHO-K1 cells. The mutants responded with lesser sensitivity to cell killing effect of ara-C and APC than CHO-K1 cells, the relative sensitivity to ara-C or APC being CHO-K1 greater than xrs 5 greater than xrs 6 cells. When X-irradiation was coupled with ara-C, the results obtained for survival were similar to those of the SCE test, i.e., unlike wild-type CHO-K1, no synergistic effect was observed in xrs 5 or xrs 6 cells. After UV-irradiation, the frequency of SCEs increased similarly in wild-type CHO-K1 and xrs 6 cells, but xrs 5 cells responded with lower frequency of SCEs.(ABSTRACT TRUNCATED AT 400 WORDS)     

Levels of sister-chromatid exchanges in hybrids between Bloom syndrome B-lymphoblastoid cells and various cell lines with lymphoid malignancy

The present study has been undertaken to examine the effect of cell hybridization of Bloom syndrome (BS) B-lymphoblastoid cell lines (LCLs) and various cell lines from lymphoid malignancies in order to clarify the relationship between sister-chromatoid exchange (SCE) and malignant conditions. Cell hybridization studies have shown that though BS high-SCE frequencies were completed by fusion with normal cells, fusion with various malignant cell lines did not result in complete normalization of BS SCEs, with 15-30 SCEs remaining per hybrid cell, demonstrating possibly common defects in DNA of BS and malignant cells. These findings strongly support the idea that the characteristic high SCE frequency in BS cells has some connection with the malignant condition, and that at least one step in carcinogenesis is either accompanied by the production of SCEs, or that SCEs themselves cause such a step to occur.