Factors influencing the frequency of mitomycin C-induced sister-chromatid exchanges in 5-bromodeoxyuridine-substituted human lymphocytes in culture

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

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

Persistence during G1 of gamma ray- or mitomycin C-induced lesions eliciting SCE in murine salivary gland cells in vivo

The efficiency of mitomycin C or gamma rays to induce SCE in early or late G1 was determined in synchronized murine salivary gland cells in vivo, as a measure of the capacity of this tissue to repair the lesions involved in SCE formation before S. The SCE frequencies induced by MMC in the first division (before BrdU incorporation) were significantly lower in the early G1 compared to the late G1, indicating some repair of SCE-inducing lesions. In the second division (after BrdU incorporation), there was no difference between SCE induced in early and late G1, indicating that MMC-induced lesions in such conditions are very persistent and not repairable during G1. The radio induced SCE frequency at early G1 was significantly lower than that observed in late G1, in cells irradiated after BrdU incorporation, suggesting that half of gamma ray-induced DNA lesions that elicit SCEs were repaired.

Persistence of chromosomal lesions induced in mouse bone marrow cells by mitomycin C, as evaluated by SCE analysis

The frequency of sister-chromatid exchanges (SCE) was evaluated in mouse bone marrow cells at different time intervals (from 19 h to 10 days) after treatment i.p. with mitomycin C (MMC; 1 and 2 mg/kg body weight). Significantly higher frequencies of SCE were found during the first week after treatment, at both doses tested. This result confirms that chromosomal lesions induced by MMC in the mouse may persist in bone marrow cells, in agreement with previous evidence based on chromosomal aberration analysis in the same cell population. In addition, the observation of a unimodal distribution of SCE/cell frequencies at each time tested indicates that the bone marrow cell population on the whole is affected by increased SCE frequency, i.e., that persistent chromosomal lesions may be transmitted along with cell proliferation.

Mitomycin C, 4-nitroquinoline-1-oxide and ethyl methanesulfonate induce long-lived lesions in DNA which result in SCEs during successive cell cycles in human lymphocytes

The present study was carried out in order to analyze how persistent the lesions in DNA are which elicit sister-chromatid exchanges (SCEs), induced by three different chemical agents, mitomycin C (MMC), 4-nitroquinoline-1-oxide (4NQO) and ethyl methanesulfonate (EMS), in proliferating human lymphocytes. Cells were exposed to the mutagens for 1 h just before starting bromodeoxyuridine substitution and SCEs were examined in third-cycle metaphases showing three-way-differential staining, by means of our previously standardized method. The results show that, in spite of the fact that these three compounds have different modes of action, the lesions induced by all of them seem to be capable of persisting in DNA and eliciting SCEs for at least three successive cell cycles.

Persistence of chromosomal lesions induced in actively proliferating bone marrow cells of the mouse

The persistence of chromosomal lesions induced in vivo by mitomycin C (MMC) was evaluated by cytogenetic analysis of mouse bone marrow cells. Chromosome aberration (CA) and micronucleus (MN) frequencies were estimated at different times after treatment, up to 42 days. The frequency of CA per cell decreased in the first 3 days after treatment, but a secondary peak appeared on the 4th day, followed by a stabilization around 0.03 CA per cell (significantly different from the control value), which persisted up to 17 days. At the next time intervals tested (28 and 42 days), the CA frequency returned to the control level. In disagreement with these data obtained directly on metaphases, the MN frequency, as evaluated in polychromatic erythrocytes, decreased quickly after treatment, reaching the control value on the 5th day. We attempted to enhance the sensitivity of the MN test by using CREST antibodies and indirect immunofluorescence. However, higher proportions of CREST- MN in treated than in control animals were observed only at short time intervals, confirming the results obtained with the conventional MN assay.

Cytogenetic characterization of the ionizing radiation-sensitive Chinese hamster mutant irs1

The X-ray-sensitive mutant V79 cell line irs1 was characterized with respect to chromosomal aberrations induced by 137Cs, mitomycin C (MMC), and decarbamoyl mitomycin C (DCMMC). To measure chromosome damage induced at different cell cycle stages, irs1 and the parental V79-4 cell lines were pulse-labeled with bromodeoxyuridine (BrdUrd) at the time of exposure and harvested at various intervals corresponding to exposure in G1, S, and G2 phases of the cell cycle. Metaphase spreads were stained with an anti-BrdUrd antibody, followed by a fluorescein-conjugated second antibody. With propidium iodide as a counter stain, cells were scored for aberrations. Compared to the parental V79 cells, irs1 cells had: (1) greatly increased sensitivity to all 3 agents; (2) a high frequency of chromatid exchanges after exposure in each phase of the cell cycle; and (3) more sensitivity to the agent causing crosslinks (MMC) than its monofunctional analog (DCMMC). The finding of chromatid-type damage in cells exposed to ionizing radiation during G1 is atypical of normal cells, but is similar to observations made in several mutant rodent cell lines and in ataxia telangiectasia cells. Our results suggest that the defect in irs1 cells can manifest itself as misrepair or misreplication during all phases of the cell cycle and leads to a high incidence of chromatid exchanges and deletions.

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.

Erythrocytes modulate the baseline frequency of sister-chromatid exchanges and the kinetics of lymphocyte division in culture

The baseline sister-chromatid exchange (SCE) frequencies of human plasma lymphocyte cultures (PLC), but not pig PLC, were nearly twice as high as those of whole-blood cultures (WBC). Addition of human red blood cells (RBCs) to human PLC decreased the SCE frequency in proportion to the RBC-leukocyte co-incubation interval. When the period of RBC-leukocyte co-incubation was equivalent to the total length of the culture period (72 h), the SCE frequency was similar to that observed in WBC. Shorter co-incubation periods yielded SCE frequencies intermediate between those of PLC and WBC. Regardless of the species, cell proliferation was slower in PLC than in WBC. Experiments where RBCs were added to PLC showed that the time sequence of RBC incorporation also affects the cell-cycle progression of human and pig lymphocytes. When either human or pig RBCs were added immediately after PLC stimulation, the cell-cycle kinetics was similar to that of WBC. Shorter co-incubation periods made cell-cycle progression intermediate between PLC and WBC values. Thus, PBCs modulate the baseline frequency of SCEs in human PLC and the cell-cycle progression of both human and pig lymphocytes in a time-dependent manner. Two possible hypotheses for the heightened frequency of SCEs of human lymphocytes in RBC-free cultures were assessed. The loss of RBC-to-lymphocyte cellular contact in PLC did not influence the SCE frequencies of lymphocytes. Finally, the increase of SCEs in human PLC could not be related to differences in the generation time of lymphocytes in culture.

Influence of treatment to sacrifice time and the presence of BrdUrd on chemically-induced aberration rates in mouse marrow cells

4 chemicals, with various modes of clastogenic action were used to evaluate induced chromosomal aberrations in mouse bone marrow at different times after intraperitoneal injection. Aberration frequencies induced by mitomycin C, cyclophosphamide and dimethylbenz[a]anthracene increased with increasing time between treatment and sampling until those time points (approximately 18 h) when significant proportions of second-division metaphases were among the cells being scored; this increase was not obvious following treatment with 4-nitroquinoline 1-oxide. When BrdUrd tablets were implanted prior to treatment and scoring was restricted to first-division metaphases, aberration rates continued to increase for as long as 24 h post-treatment. The presence of BrdUrd did not affect significantly the rate of aberration induction by the chemicals. Our data indicate that the sensitivity of the in vivo mouse marrow assay for clastogenic chemicals can be greatly increased by utilizing BrdUrd to insure the scoring of only first-division metaphases at post-treatment times of approx. 18 h.

Repair of sister-chromatid exchange-inducing lesions in mutagen-treated cultures of human whole blood and purified fresh or frozen lymphocytes

Repair of mutagen-induced lesions that result in sister-chromatid exchanges was evaluated in 10 normal individuals. The mutagens used were mitomycin C (MMC), 4-nitroquinoline 1-oxide (4NQO), and N-methyl-N'nitro-N-nitrosoguanidine (MNNG). Cultures of whole blood, freshly purified lymphocytes, or purified lymphocytes cryopreserved for 6 months were analyzed after the mutagen treatments. All 3 mutagens induced reparable damage as evaluated by comparison of sister-chromatid exchanges between cultures that were given time to repair induced damage before 5-bromo-2'-deoxyuridine (BrdUrd) was added to the culture medium with those where BrdUrd was added immediately after the administration of the mutagens (MMC or 4NQO) or at culture initiation (MNNG). Repair of mutagen-induced DNA damage was detected in all 3 culture types; thus cryopreservation did not appear to alter the capacity of lymphocytes to repair mutagen-induced lesions. Quantitative differences in apparent repair capabilities were observed among individuals. Variability also existed among the different culture types within an individual, suggesting that caution should be exercised in interpreting these apparent differences.

Enhancement of cytogenetic damage by chlorpromazine in human lymphocytes treated with alkylating antineoplastics and caffeine

In cultured human lymphocytes chlorpromazine (CPZ) was found to induce cell division delays and to have no effect on sister-chromatid exchanges (SCEs) or on mitotic indices (MIs). CPZ induces cytotoxic effects in combination with caffeine (CAF) and alkylating agents. In combination with CAF it induced cell division delays and suppression of MIs. In combination with melphalan (MEL) and CAF, CPZ synergistically induced SCEs, caused cell division delay and suppressed MIs. In combination with chlorambucil (CBC) and CAF, CPZ produced synergism on induction of SCEs, enhanced cell division delays and reduced MIs.

Persistence of SCE-inducing lesions in lymphocytes of mice exposed to diaziquone

Male C57B1/6 mice were injected i.p. with either 1.25 or 5.0 mg/kg diaziquone (AZQ) and killed at various time intervals from 1 to 99 days post treatment for examination of sister chromatid exchange (SCE) persistence in the peripheral blood lymphocytes (PBLs) and splenocytes. SCE frequencies were found to decay steeply during the first week after exposure in both PBLs and splenocytes. This pattern was followed by a slower decline to baseline over the next week. However, high-frequency cell (HFC) analysis indicates that significant numbers of HFCs persist in the PBLs through day 28 and splenocytes at day 99 post exposure. Mathematical modeling of the time-response curves indicates that the average life span of the majority of AZQ-induced SCE-producing lesions in murine PBLs and splenocytes responsive to phytohemagglutinin is between 3 and 5 days.

The effect of agent treatment time on the induction of sister-chromatid exchanges in mouse bone marrow cells in vivo

The effect of time of agent administration, via intraperitoneal injection, on the yield of SCEs in bone marrow cells of male B6C3F1 mice was determined for cyclophosphamide (CP), 7,12-dimethylbenz[a]anthracene (DMBA) and mitomycin C (MMC). Animals were treated with several doses of each carcinogen/mutagen at 3 different treatment times: -1, +1 and +8 h in relation to the onset of BrdUrd administration. The results of these studies indicate that the optimal treatment time for inducing a maximal SCE response is agent-specific. For CP, the slope of the SCE response was greatest at the +8 h treatment time while the maximal response for DMBA occurred at the -1 h treatment time. For MMC, the slope of the SCE response was independent of treatment time and of the method of bromodeoxyuridine administration (intravenous infusion vs. tablet implantation) but dependent on the laboratory conducting the study (Brookhaven National Laboratory vs. Oak Ridge Associated Universities). Based on the results of these studies, the +1 h acute treatment time is considered optimal for the in vivo cytogenetic evaluation of suspect chemicals for genotoxic activity when bone marrow is used as the target cell population.

The kinetics of in vivo sister chromatid exchange induction in mouse bone marrow cells by ethylnitrosourea and methylnitrosourea

Administration of ethylnitrosourea (ENU) (20, 25, 40, 50, and 60 mg/kg body weight) or methylnitrosourea (MNU) (25, 40, 50, 60, 75, and 80 mg/kg body weight) to male CD-1 mice 2 hr after sc implantation of a 5-bromo-2'-deoxyuridine (BrdUrd) pellet (55 mg) resulted in a dose-dependent increase in sister chromatid exchanges (SCE) in bone marrow cells. Treatment with ENU (50 mg/kg body weight) at several time points prior to BrdUrd implantation resulted in a multiphasic curve of SCE induction indicating at least two events that result in SCEs. Treatment with ENU at the time of BrdUrd implantation and post-BrdUrd reflected a similar mechanism apparent in the pre-BrdUrd curve. Treatment with MNU (50 mg/kg body weight) pre- and post-BrdUrd resulted in a linear monophasic curve of SCE induction in both the pre- and post-BrdUrd time periods. The overall MNU time-course curve resembled an inverted V function suggesting the mechanisms of SCE induction for ENU and MNU are different. These observations suggest that at least one explanation for the differences in the time courses for ENU and MNU SCE induction may result from a more persistent lesion being induced by ENU. In addition, these results indicate that in vivo SCE protocols which utilize a single acute chemical exposure at or near the time of BrdUrd labeling may not be useful for judging the relative activities of genotoxic agents.

Chemical induction of sister-chromatid exchanges in human lymphocytes treated in G0 prior to stimulation by different mitogens and revealed 72 h later in second division cells

Frequencies of sister-chromatid exchanges (SCE) were determined in second-division metaphases of human lymphocytes, exposed for 1 h during the G0 phase to mitomycin C (MMC) alone or to cyclophosphamide (CP) in the presence of S9 mix. The cells were then cultured for 72 h in the presence of phytohemagglutinin (PHA), concanavalin A (Con A), Wistaria floribunda (WFA) or Lens culinaris (LcH-A) extracts. Large differences in mitotic indices (MI) and cell-cycle kinetics were observed among cells subjected to the various treatments. However, in the controls as well as in the cultures submitted to a G0 mutagenic exposure, the yield of SCE was not influenced by the mitogenic agent and was, therefore, independent of the proliferation properties of the cultured lymphocyte population.

Effects of temperature on chemically induced sister-chromatid exchange in human lymphocytes

Lymphocytes from healthy adults were studied for sister-chromatid exchanges (SCEs) when pulse-treated in G0 with mitomycin C (MMC), ethyl methanesulfonate (EMS), or 4-nitroquinoline N-oxide (4NQO) at various temperatures ranging from 0 degrees C to 41 degrees C and then cultured in medium containing 5-bromodeoxyuridine at 37 degrees C. The results showed that the frequencies of SCEs induced by MMC or EMS varied according to the treatment temperature. In MMC- or EMS-exposed cultures, the SCE frequency increased continuously with increasing treatment temperature; treatment at 37 degrees C resulted in a 3-4 times greater induction of SCEs than did that at room temperature (25 degrees C). On the other hand, SCE frequencies in cells exposed to 4NQO remained within normal deviation, showing no temperature-dependent changes. Baseline SCE frequencies remained almost constant within the temperature range tested. These data indicate that treatment temperature is a very critical factor in determining the sensitivity of cells to the chemical induction of SCEs.

Induction and persistence of SCE-inducing damage in human lymphocytes exposed to vinyl acetate and acetaldehyde in vitro

The frequency of sister-chromatid exchange (SCE) was studied in cultures of human lymphocytes exposed to vinyl acetate (VA) or acetaldehyde (AA) for various time periods and in different phases of the cell cycle. Equimolar concentrations (0.1-2.4 mM) of VA and AA were found to induce very similar, dose-dependent increases of SCE. The SCE frequency in cells treated with VA was found to increase linearly with exposure times up to 24 h. Cells exposed to VA or AA in the late G1-phase of the cell cycle showed a 2-fold higher SCE frequency than cells exposed in early G1. Cultures treated with VA in the first G1-phase showed a significant increase of SCE during 3 subsequent cell cycles. These results indicate that (1) AA is likely to be responsible for the SCE induction observed in VA-treated cells, (2) the SCE-inducing activity of AA persists for several cell cycles in vitro, and (3) removal of SCE-inducing AA-damage occurs during G1. Taken together, the data suggest that AA has a slow turn-over in human lymphocytes in vitro, and may accumulate in the cells, possibly by forming reversible Schiff bases, and when released gives rise to SCE-inducing DNA cross-links.

Induction of sister-chromatid exchanges and cell-cycle delays in human lymphocytes by vitamin A alone or in combination with melphalan and caffeine

In cultured human lymphocytes vitamin A was found to increase SCE rates, to reduce the mitotic index and to have no effect on cell kinetics. Vitamin A induces cytotoxic effects: in combination with melphalan (MELPH), as can be deduced from the resulted synergism on induction of SCEs, the produced cell division delay and the suppressed mitotic index; in combination with caffeine (CAF), producing synergism on induction of SCEs and suppressing the mitotic index; and in combination with MELPH and CAF, producing cell-cycle delays and reducing the mitotic index.

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.

Dependency of the yield of sister-chromatid exchanges induced by alkylating agents on fixation time. Possible involvement of secondary lesions in sister-chromatid exchange induction

Chinese hamster V79 cells were pulse-treated (for 60 min) with various mutagens three, two or one cell cycles before fixation (treatment variants A, B and C, respectively) and the frequencies of induced SCEs were analysed and compared. The degree of increase in frequency of SCEs with dose in the treatment variants depended on the mutagen used. For the methylating agents MNU, MNNG and DMPNU, high yields of SCEs were obtained in the treatment variants A and B, and there was no difference in the efficiency with which these agents induced SCEs in these treatment variants. In the treatment variant C, however, no SCEs were induced with mutagen doses yielding a linear increase in SCE frequency in treatment variants A and B. A slight increase in SCE frequency in treatment variant C was observed only when relatively high doses of MNU or MNNG were applied. Like the above agents, EMS, ENU and MMS induced more SCEs in treatment variants A and B than in C, but for these agents treatment variant B was most effective and SCEs were induced over the entire dose range, also in treatment variant C. As opposed to the methylating and ethylating agents, MMC induced SCEs with high efficiency when treatment occurred one or two generations prior to fixation. There was no difference in SCE frequency between these treatment variants. MMC was completely ineffective for the induction of SCEs when treatment occurred three generations before fixation. The unexpectedly low SCE frequencies induced by the methylating and ethylating agents when treatment occurred one generation before fixation were not due to the exposure of cells to BrdU prior to mutagen treatment. From the results obtained, it is concluded that DNA methylation and ethylation lesions give rise to SCEs only with very low probability during the replication cycle after the lesion's induction, and that subsequent lesions produced during or after replication of the methylated or ethylated template (secondary lesions) are of prime importance for SCE formation after alkylation. For MMC, however, primary lesions seem to be most important for SCE induction.

Effect of 5-bromodeoxyuridine substitution on sister chromatid exchange induction by chemicals

The fluorescence-plus-Giemsa (FPG) technique for analysis of sister chromatid exchange (SCE) is widely used as an assay for mutagenic carcinogens. There is very little information, however, on whether incorporation of the bromodeoxyuridine (BrdU) necessary for visualization of SCEs affects the sensitivity of the SCE test system to different chemical agents. We have investigated the effect of BrdU incorporation on SCE induction by labeling cells with BrdU for either the first cell cycle or the first and second cell cycles. The cells were then treated with bleomycin, which produces DNA strand breakage; proflavine, which intercalates into DNA; mitomycin C, which produces monoadducts and DNA crosslinks; or aphidicolin, which inhibits DNA polymerase alpha. Chemicals were added before BrdU exposure or during the first, second, or both cell cycles. Only mitomycin C, which induces long-lived lesions, elevated the SCE frequency when cells were treated before BrdU labeling. When bleomycin, proflavine, or mitomycin C was present concurrently with BrdU, the frequency of SCEs was increased independently of the BrdU labeling protocol. Aphidicolin, on the other hand, induced more SCEs when present for the second cell cycle, when DNA replicates on a template DNA strand containing BrdU. We also examined the induction of SCEs in the first cell cycle (twins) and in the second cell cycle (singles) after continuous treatment of cells with BrdU and the test chemicals. Only aphidicolin increased SCE frequency in the second cell cycle. These results indicate that aphidicolin, but not bleomycin, proflavine, or mitomycin C, affects BrdU-substituted DNA and unsubstituted DNA differently. This type of interaction should be taken into consideration when the SCE test is used as an assay system.

Effect of bromodeoxyuridine on induced sister chromatid exchanges

Analysis of sister chromatid exchanges (SCEs) is widely used as an assay for mutagenic carcinogens. Visualization of SCEs generally requires that the cells be cultured for 2 cycles of replication with the thymidine analog bromodeoxyuridine (BrdUrd). To see if incorporation of BrdUrd into chromosomal DNA influences the SCE response after treatment with chemical compounds, we have studied the effect of BrdUrd incorporation on SCEs induced by 5 different chemicals: bleomycin (BLM), which causes DNA single- and double-strand breakage; proflavine (PF), which intercalates into DNA; mitomycin C (MMC), a polyfunctional alkylating agent that cross-links DNA and also forms monoadducts; and 2 chemicals that do not appear to interact with DNA directly, aphidicolin (APC), an inhibitor of DNA polymerase alpha; and 3-aminobenzamide (3AMB), an inhibitor of poly-(ADP-ribose)-polymerase. Chemical treatment was for the first, second, or both cell cycles, and BrdUrd was present for the first or both cell cycles. All treatments with BLM, PF, or MMC increased the SCE frequency independently of the BrdUrd labeling protocol. With APC and 3AMB, on the other hand, only small increases in SCE frequency were observed when treatment was for the first cell cycle, but there were far greater increases when the chemical was present for the second or for both successive cell cycles. To further determine at which cycle SCEs were formed after continuous treatment of cells with BrdUrd and a test chemical, we also examined the induction of SCEs in the first cell cycle (twins) and in the second cell cycle (singles) in tetraploid cells. Bleomycin, PF, and APC induced almost equal numbers of SCEs in both cell cycles, but MMC appeared to induce more SCEs in the second cycle than in the first. This is probably caused by long-lived lesions that induce SCEs. 3-Aminobenzamide, which does not form persisting lesions, also induced more single than twin SCEs, suggesting that this compound affects BrdUrd-substituted DNA differently than it does unsubstituted DNA. This type of interaction between a chemical and BrdUrdsubstituted DNA should be taken into consideration when SCE analysis is used as an assay system.

Persistence of SCE-inducing lesions after G0 exposure of human lymphocytes to differing classes of DNA-damaging chemicals

We conducted studies to determine whether cycling human lymphocytes are equally efficient in repairing sister chromatid exchange (SCE)-producing lesions induced by differing classes of DNA-damaging chemicals. Lymphocytes were pulse-treated during G0 with mitomycin C (MMC), N,N',N''-triethylenethiophosphoramide (ThioTEPA), ethylmethanesulfonate (EMS), or cis-diamminedichloroplatinum (cis-DDP). Bromodeoxyuridine (BrdUrd) was added to the 72 hr cultures at 0 hr or at 48 hr after phytohemmagglutinin stimulation. The concentrations of chemicals employed induced a greater than 2-fold increase in SCEs in second-division metaphases from lymphocytes cultured in the presence of BrdUrd for the entire 72 hr. The analysis of SCEs in uniformly harlequinized metaphases from G0-treated lymphocytes cultured in BrdUrd for the terminal 24 hr showed no increase above baseline after exposure to MMC, and intermediate increases above baseline after exposures to ThioTEPA and cis-DDP. However, after G0 treatment with EMS, the observed SCE frequency was consistent with that expected had all DNA lesions persisted and continued to give rise to SCEs during 3 cell cycles. These findings suggest that cycling human lymphocytes are not equally efficient in eliminating SCE-producing lesions after exposure to differing classes of DNA-damaging chemicals.

Removal and persistence of SCE-inducing damage in human lymphocytes in vitro

The SCE frequency was studied in PHA-stimulated human lymphocytes exposed to various SCE-inducing agents in different stages of the cell cycle. Melphalan, HN2, MMS, and UV light were found to induce a higher SCE frequency in late G1 (18-24 hr after PHA stimulation) than in early G1 (1-6 hr after PHA) or G0 (before PHA stimulation). In contrast, CCNU induced more SCEs in early G1 than in late G1, and the adriamycin-induced SCE frequency was about the same after treatment in early and late G1. These results suggest that SCE-inducing lesions are being removed at different rates in human G1 lymphocytes. The removal of SCE-inducing HN2 lesions was found to be about 10 times more rapid in late G1 than in early G1, indicating the activation of a cross-link repair mechanism prior to DNA replication in human lymphocytes. Cells treated with MMS in the second G1 (after cultivation for about 55 hr in the presence of PHA and BrdUrd) showed a higher SCE frequency than cells treated with the same dose of MMS in the first G1. This result indicates that some type of interaction occurs between MMS damage and BrdUrd lesions in the DNA during replication, which leads to an enhanced induction of SCE. Analysis of SCEs induced during the 2 first vs. the third cell cycle in third-generation metaphases showed that most of the SCE-inducing damage caused by treatment with HN2 and melphalan in G1 of the first cell cycle are removed before the S phase of the third cell cycle, whereas damage caused by MMS and adriamycin seem to be more persistent. These observations suggest that the rate by which different types of SCE-inducing damage are removed or modified in resting (G0) of PHA-stimulated human lymphocytes can have a great influence on the SCE frequency. This is of practical importance in studies using SCE analysis to evaluate human exposure to suspected genotoxic agents in the environment.

Induction of endoreduplication by hydrazine in Chinese hamster V 79 cells and reduced incidence of sister chromatid exchanges in endoreduplicated mitoses

Hydrazine in high concentrations very effectively induces endoreduplication in Chinese hamster V79 cells. The addition of 5-bromodeoxyuridine (BrdU) for the duration of one cell cycle prior to the induction of endoreduplication produces diplochromosomes with sister chromatid differentiation (SCD) after differential chromatid staining. The fact that diplochromosomes with complete SCD are obtained shows that endoreduplication was induced in cells that were in G2-phase. The analysis of sister chromatid exchanges (SCEs) showed that hydrazine treatment rarely led to increased SCE frequencies in mitoses after endoreduplication, but that it caused a strong SCE induction in diploid second division metaphases in the same culture. Neither catalase nor cysteine had an effect on the induction of endoreduplication or the incidence of SCEs. Treatment of the cells with mitomycin C prior to addition of BrdU led to increased SCE frequencies. Compared with the normal mitoses from the same preparation, the mitoses after endoreduplication showed a significantly reduced induction of SCEs. In contrast to these findings, SCE induction was not reduced in the common tetraploid V 79 cells after colcemid-induced polyploidization.

Influence of various mitogens on the yield of sister-chromatid exchanges, induced by chemicals, in human lymphocytes

The fluorochrome-plus-Giemsa (FPG) method of Perry and Wolff was used to compare the frequencies of sister-chromatid exchanges (SCEs) induced by cyclophosphamide (CP) or mitomycin C (MMC) in human lymphocytes stimulated by phytohaemagglutinin (PHA), concanavalin A (Con A), Wistaria floribunda (WFA), or lentil lectin (LcH-A) extracts. These 4 mitogens, differing in lectin valency and/or sugar specificity, are considered as activating primarily thymus-derived (T) lymphocytes. Regardless of the mitogen used, control cultures displayed a mean yield of about 8 SCEs/cell. A contact, of 1 h, with mitomycin alone or with cyclophosphamide and enzymatic activation, resulted in a significant augmentation of SCEs dependent on the mitogen used. An approximately 2-fold, 4-fold, or 6-fold increase in SCEs was observed for the cultures stimulated by PHA, Con A, and WFA or LcH-A respectively. Furthermore, there were mitogen-dependent differences in mitotic indices and cell-cycle kinetics in human lymphocytes harvested 72 h after stimulation.

Comparative mammalian in vitro and in vivo studies on the mutagenic activity of rhodamine WT

Rhodamine WT, a xanthene dye used in the tracing of pollutants in water and in related studies, was tested for its mutagenicity in a battery of in vitro and in vivo mammalian assays. Using Chinese hamster ovary cells in the absence of metabolic activation mix, small dose-related increases in cytotoxicity, DNA damage (as detected by alkaline sucrose-gradient sedimentation) and sister-chromatid exchanges were detected, but an increase in the level of chromosomal damage was not seen. In the presence of metabolic activation a small, but statistically significant dose-related increase in sister-chromatid exchanges was evident, with no increase in cytotoxicity, DNA damage or chromosome aberrations. Furthermore, no increase in bone marrow micronuclei or sperm abnormalities was observed in male B6C3F1 mice. The data from all these mammalian assays, although involving different end-points, are in contrast to the mutagenic effects previously seen in Salmonella.

Sister-chromatid exchanges induced in swine lymphocytes by chronic oral doses of dimethylbenzanthracene

Sister-chromatid exchanges were scored at 3-week intervals in lymphocytes of female swine ingesting daily doses of 1.25 or 2.50 mg/kg of 7,12-dimethylbenzanthracene (DMBA) for 160 days. Exchanges increased with time for about 120 days then reached a plateau at approximately 2.5 times their pretreatment level. No increase in chromosome aberrations could be identified as resulting from ingestion of the chemical. Week-old progeny of animals that had ingested the chemical throughout pregnancy showed no increase of exchanges in their peripheral lymphocytes.

The kinetics of in vivo sister chromatid exchange induction in mouse bone marrow cells by alkylating agents: cyclophosphamide

Administration of cyclophosphamide (5, 10, 20 and 25 mg/kg body weight) to male CD-1 mice 2 hr after subcutaneous implantation of a 5-bromo-2'-deoxyuridine (BrdUrd) pellet (55 mg) resulted in a dose-dependent increase in sister chromatid exchanges (SCE) in bone marrow cells. Treatment with cyclophosphamide (15 mg/kg body weight) at the time of BrdUrd implantation and 2, 6.5, and 13 hr post-BrdUrd implantation resulted in the induction of approximately 19 SCE/cell indicating that the bone marrow SCE response was independent of the time of administration. Treatment with cyclophosphamide (15 mg/kg body weight) at 26, 19, 13, and 6 hr prior to BrdUrd implantation resulted in baseline SCE (3.3 SCE/cell) at 26 hr with an increasing number of SCE/cell with decreasing time prior to BrdUrd implantation. These results compare favorably with those obtained by Kram et al [1981] with mitomycin C (MMC) using a similar protocol. The time-dependent induction of SCE is qualitatively similar for CP and MMC, both of which are bifunctional alkylating agents metabolically activated by oxidation and reduction, respectively, and suggests that these two compounds may induce SCE by a similar mechanism.

SCE evaluations in human lymphocytes after G0 exposure to mitomycin C. Lack of expression of MMC-induced SCEs in cells that have undergone greater than two in vitro divisions

We conducted a series of experiments designed to determine whether DNA damage induced in G0 lymphocytes by mitomycin C (MMC) would be expressed as sister-chromatid exchanges during the second and third post-treatment cell cycles. Lymphocytes from normal donors were exposed to MMC for 2 h prior to culture in the presence of phytohemagglutinin. MMC-treated and control cells were subsequently exposed to bromodeoxyuridine (BrdUrd) for the entire culture period (i.e. 48 h or 72 h) or for the terminal 24 h of 72-h cultures. We observed a 3-4-fold increase in SCEs in MII metaphases from lymphocytes treated with MMC and cultured in the presence of BrdUrd for the entire culture period. In contrast, in replicate cultures of MMC-treated lymphocytes that were exposed to BrdUrd for the terminal 24 h only, the SCE frequency in uniformly harlequinized metaphases was not significantly different from that observed in control cultures. We interpret these data as providing evidence that MMC-induced lesions (or alterations) in the DNA of G0 lymphocytes are probably expressed as SCEs during the first period of mitogen-induced DNA synthesis, and that these lesions do not persist and give rise to SCEs in subsequent cell divisions.

The longevity of chemically induced sister chromatid exchanges in Chinese hamster ovary cells

The persistence of the lesions leading to sister chromatid exchanges (SCEs) following acute exposure of Chinese hamster ovary (CHO) cells to direct-acting chemical mutagens was measured. The results revealed that these lesions (and consequently SCEs) are rapidly eliminated from cells. SCE levels fell to near control values by the third or fourth day (six and eight cell cycles, respectively) following exposure of CHO cells to quinacrine mustard (QM) and mitomycin C (MMC). In contrast, cells exposed to methyl methanesulfonate (MMS) showed a small but significant increase in SCE level over control up to and including the fifth day following exposure (approximately ten cell cycles), suggesting that the behavior of these lesions in cells is influenced, at least in part, by the mechanism by which a specific agent interacts with DNA. The possibility that the decline in SCE level was due to the loss of cell populations with high numbers of exchanges was eliminated by the assessment of cloning efficiencies of treated and control cultures.

Sister chromatid exchange response of human diploid fibroblasts and Chinese hamster ovary cells to dimethylnitrosamine and benzo(a)pyrene

In the search for relevant assays for mutagenicity testing, considerable attention has been given to the use of mammalian cells in vitro and the incorporation of metabolic activation in the protocol. Chinese hamster ovary (CHO) cells are commonly chosen as the target cells for cytogenetic tests because of their excellent growth characteristics and long lifespan in culture. However, there may be cellular factors affecting the uptake, metabolism, and repair of damage which are not the same in all cell lines. The response of CHO cells and three human diploid fibroblast strains (IMR-90, WI-38, S-3299) to benzo(a) pyrene (BP) and dimethylnitrosamine (DMN) were compared using sister chromatid exchange (SCE) analysis as a measure of genetic damage. For both BP and DMN the human cells and the CHO cells showed dose-response slopes that were significantly different from zero, except CHO cells treated with BP for 1 hr and S-3299 cells treated with DMN. Whereas human and CHO cells showed similar dose-responses to BP and the three human cell strains had similar dose-responses to BP and DMN, the dose-response of the human cells to DMN was statistically less significant than that of CHO cells. Reducing the duration of chemical treatment in CHO cells had no effect on the slope of the dose-response curves for BP or DMN. The observed differences between human and CHO cells may reflect differences in the fate of metabolic intermediates of DMN.

Sister chromatid exchange (SCE) and structural chromosome aberration in mutagenicity testing

Data from previous studies published on the induction by mutagens of sister chromatid exchanges (SCEs) and structural chromosome damage were compared qualitatively and quantitatively. Although a good correlation between the incidence of both cytogenetic phenomena has been pointed out in many previous publications, about 30% of the agents for which comparable data were available yielded non-corresponding qualitative results concerning both indicator effects. However, even in groups with good qualitative agreement distinct quantitative differences indicated different molecular mechanisms of the formation of SCEs and breaks. Additional information supporting the importance of these differences for the validity of both indicator systems has been derived from the results obtained using strong clastogens exhibiting a low or no SCE-inducing activity and vice versa, from special observations on chromosomal breakage syndromes, and from studies on the action of known co- and anti-clastogens on SCE-induction by chemical mutagens. As a result, it has been suggested that the SCE-technique should be considered as a valuable additional method for cytogenetic mutagenicity testing, which, however, is not adequate to replace the classical methods of analysis of structural chromosome damage.

Sister-chromatid exchanges: a report of the GENE-TOX program

The effects of a number of chemicals on sister-chromatid exchange (SCE) frequencies in in vivo and in vitro systems are reviewed. Standardized protocols for future SCE testing in important systems, as well as for evaluation of test results, are presented. Data reported thus far suggest that SCE analysis may prove useful, especially at a secondary level, as a test of mutagenic carcinogens. Strengths and limitations of SCE analysis are summarized as a guide for future evaluation and use of this procedure.

Physical, chemical, and biological factors affecting sister-chromatid exchange induction in human lymphocytes exposed to mitomycin C prior to culture

In experiments to assess the effects of several biological, chemical, and physical variables on sister-chromatid exchange (SCE) induction in cultured lymphocytes exposed to mitomycin C (MMC) before PHA stimulation we observed: (1) high SCE frequencies in female cells, and normal SCE frequencies in Y-bearing metaphases in mixed cultures containing equal numbers of MMC-treated female lymphocytes and untreated male lymphocytes; (2) small, but statistically significant, decreases in SCEs with increasing pH after G0 exposure in the pH range 6.6-7.6; (3) pronounced reductions in MMC-induced SCEs in lymphocytes exposed at 4 degrees C vs. 37 degrees C. In other studies, SCE induction was evaluated in cultures exposed during G0 to MMC concentrations ranging from 0.25 to 2.5 microgram/ml for varying time intervals ranging from 5 min to 24 h. For all concentrations tested SCE induction varied as a linear function of G0 exposure time. To compare SCE induction between cultures, we calculated the mean frequencies of SCEs induced per metaphase/unit dose MMC/unit G0 exposure time (SCE/microgram/h). A mean frequency of 20.7 +/- 4.8 SCE/microgram/h was observed for 41 lymphocyte cultures suggesting that a single term adequately describes the rate of SCE induction following G0 exposure to a 10-fold range in concentration of MMC for time intervals of 30 min to 24 h.

Analysis of mutagen-induced chromosome damage in a primate species (Saguinus oedipus oedipus) at risk for spontaneous adenocarcinoma of the colon

A high incidence of adenocarcinoma of the colon (greater than 16%) has been observed at necropsy in the South American primate, Saguinus oedipus oedipus (S. oedipus), while the disease has not been found in tamarins of the closely related species, Saguinus fuscicollis spp, housed in the same research colony. Cytogenetic analyses in cultured lymphocytes from 10 S. oedipus and 10 S. fuscicollis illigeri (S. fuscicollis) demonstrated no differences in the average frequencies of spontaneous or mitomycin C (MMC)-induced sister chromatid exchanges (SCEs) between animals of the two species. However, highly significant variability in MMC-induced chromosome lesions was observed between the individual S. oedipus, with one animal exhibiting increased sensitivity for both SCEs and chromosome breakages. At present we do not know the relationship, if any, between increased sensitivity to mutagen-induced cytogenetic lesions in specific S. oedipus tamarins and the increased risk for colon cancer that has been documented in this primate species. However, our cytogenetic findings in this one S. oedipus are similar to data obtained in evaluations of persons with several autosomal recessive conditions in which there is a genetic predisposition for developing malignancies.

Sister-chromatid exchange analyses in rodent maternal, embryonic and extra-embryonic tissues: transplacental and direct mutagen exposures

Sister-chromatid exchange (SCE) analyses were conducted in maternal, embryonic and extraembryonic tissues of pregnant rats and mice. The various tissues were substituted in vivo with 5-bromodeoxyuridine (BrdU) by implantation of a BrdU tablet in pregnant animals at mid-gestation. Following maternal exposure to 5-20 mg/kg cyclophosphamide, embryonic liver cells demonstrated dose-dependent SCE increases up to 10-fold that of control. Rat embryos revealed little intralitter variability for this transplacental effect. Maternal marrow and yolk sac cells examined in the rat also underwent significant increases in SCE, although to different extents. While marrow SCE frequencies were similar to those of embryo liver, yolk sac SCE frequencies were generally much lower. SCE analyses were also conducted in rat yolk sac cells substituted in vivo with BrdU and subsequently explanted to whole-embryo culture. In vitro exposure to cyclophosphamide at concentrations up to 100 microgram/ml had no SCE-inducing effect. However, similar exposures to phosphoramide mustard, a presumed metabolite of cyclophosphamide, caused dose-dependent increases in SCE up to 8-fold higher than control at 2 microgram/ml. Thus, cyclophosphamide appears to require maternal metabolic activation in order to cause an increased SCE frequency in yolk sac cells. The system described permits versatile SCE analyses which can help to define relative maternal and embryo tissue-specific sensitivities to chemical-induced genetic damage.

Effects of acute and chronic administration of mitomycin C on the induction of sister chromatid exchanges in vivo

Frequencies of sister chromatid exchanges (SCE) were analyzed in bone marrow cells of mice injected with mitomycin C (MMC) both before and during infusion with bromodeoxyuridine (BrdU). Administration of MMC at 1, 6.5, and 13 hours after the onset of BrdU infusion resulted in the induction of approximately 45 SCE/cell, independent of time of administration. When MMC was injected 26 hours prior to BrdU infusion, only baseline levels of SCE were noted. The effects of multiple doses of MMC (chronic administration) were examined in mice treated with 1--5 mg/kg on a weekly or bimonthly basis. SCE analysis was performed one week after the final injection. At all doses and with all treatment regimes, SCE frequencies did not differ from control levels. The results indicate that most or all MMC-induced DNA damage that results in SCE formation is removed in a single cell cycle after its administration.

Synergistic action of cysteamine and BrdU-substituted DNA in the induction of sister chromatid exchanges

The effect of different BrdU-concentrations on the cysteamine-induced SCE-rate was investigated in V-79 Chinese hamster cell monolayer cultures. Both cysteamine and its auto-oxidation product cystamine act synergistically with BrdU in the induction of SCEs. A given concentration of these substances produces a low SCE-frequency at low BrdU-concentrations -- but the incidence of SCEs is significantly increased at increased BrdU-concentrations. -- Using one-cycle substitution experiments and the determination of the relative level of substitution by means of 3HBrdU-incorporation, this synergism was shown to depend on the BrdU incorporated in the DNA and on the extent to which this incorporation takes place.