Parallel distribution of sister chromatid exchanges and chromosome aberrations

7,12-DMBA-induced rat leukemia: a review with insights into future research

7,12-Dimethylbenz[a]anthracene (DMBA) elicits leukemia in Long-Evans rats (LE). This leukemia is mostly erythroblastic and 30% of leukemias have total and partial trisomy of #2 chromosome and the rest have diploid karyotype. The common duplication site is in 2q26-q34 and N-ras gene is located in 2q34. 7,8,12-Trimethylbenz[a]anthracene (TMBA) also induces similar leukemias. These leukemias reveal a highly specific mutation of N-ras gene as in human leukemias. N-ras mutation is induced 48h after DMBA treatment. Wild type N-ras allele is frequently lost in diploid leukemias but not in trisomy type. Therefore, a gene dosage problem related to the mutant N-ras gene is involved in development of leukemia. Some secondary genetic rearrangements involving abl and H-ras are also observed in cultured leukemia cells. DMBA-induced chromosome aberrations as well as leukemia are enhanced by erythropoietin and blocked by Sudan III given prior to DMBA treatment. This leukemia will provide an important tool for chemical carcinogenesis and leukemia studies.

N-ras mutation in 7,12-dimethylbenz[a]anthracene (DMBA)-induced erythroleukemia in Long-Evans rats

Intravenous injections of 7,12-dimethylbenz[a]anthracene (DMBA) induce erythroblastic leukemia (erythroleukemia) with No.2 trisomy in Long-Evans rats. Activation of some oncogenes such as abl and Ha-ras has been reported to occur in relation to the secondary chromosomal translocations. In the present studies, a consistent type of mutation, A to T transversion in codon 61 of N-ras gene, was found in all of 6 cultured leukemia cell lines and 5 primary leukemias induced by DMBA. The N-ras mutation was also found in bone marrow cells of 2 out of 8 preleukemias. On the contrary, no mutation was observed in Ha- and Ki-ras genes in all leukemias and preleukemias. The consistent occurrence of above N-ras mutation in leukemias indicates that it plays an important role in DMBA-leukemogenesis.

Sister-chromatid exchanges and chromosomal aberrations in lymphocytes of male albino rats treated with an alkylating agent, apholate (NSC 26, 812; ENT 26, 316)

A lymphocyte culture system was used to study the frequency of chromosomal aberrations and SCEs at various time intervals after an intraperitoneal injection of apholate (0.5 mg/kg b.w.) in male albino rats. Treatment with apholate increased the frequency of chromosomal aberrations significantly (P less than 0.001). The majority of chromosomal aberrations were single chromatid breaks, although other aberrations were also present. The distribution of chromatid breaks was non-random and the large chromosomes were affected to a greater extent. SCEs frequency in lymphocytes of apholate-treated rats was more than 5-fold higher in comparison to controls. The highest number of chromosomal aberrations and SCEs/rat was recorded at 48 h post-treatment, after which there was a gradual decline in both SCE and chromosomal aberration frequency. The results indicate that apholate is a mutagenic compound and it may also be carcinogenic.

Stage-specific induction of sister-chromatid exchanges in utero

In order to correlate the induction of sister-chromatid exchanges (SCE) to biological endpoints, and elucidate aspects of this relationships, 7,12-dimethylbenz[a]anthracene (DMBA) and methyl methanesulfonate (MMS), chemicals with different biological actions at different stages in development, have been evaluated for their ability to induce SCE at different gestational ages in the Sprague-Dawley rat. Transplacental exposure to these agents was accomplished by a recently developed intraperitoneal infusion technique to replenish metabolically degraded 5-bromo-2'-deoxyuridine used for SCE visualization. Maternal bone marrow and whole fetal tissue, fetal liver and fetal brain were compared. Day-9 embryo was found to be very sensitive to the effects of both agents, with the ability to induce SCE declining in development in whole fetus and fetal organs. The embryotoxic effects of the agents seem to be ones best correlated with the capacity of the agents to induce SCE. Also, fetal liver is more sensitive than fetal brain to the effects of DMBA compared with MMS, suggesting fetal metabolic activation may have occurred. Measurement of the amount of radiolabelled DMBA reaching the fetal tissue used to estimate SCE indicates that the amount of chemical reaching the fetus does not account for the increased sensitivity, especially at Day 9. Some factor(s) in development, such as differentiation stage, rather than the fetal accessibility to chemical, seem to be important in the induction of SCE in utero.

On the origin of chromosomal aberrations in human peripheral lymphocytes in vitro. I. Experiments with Neurospora endonuclease and polyethylene glycol

Post-treatment of mutagen-treated human peripheral lymphocytes with a single-strand specific endonuclease from Neurospora crassa leads to a significant elevation of the rate of structural chromosomal aberrations. Our results indicate that DNA double-strand breaks (DSB) are ultimate lesions for the formation of chromosomal aberrations in the G1 and G2 phase of the cell cycle and probably also in the S-phase. Post-treatment of X-irradiated G2 cells with polyethylene glycol (PEG) leads to an elevation of the frequencies of chromatid type aberrations. This result is taken as an indication that nucleases from PEG-damaged lysosomes transform lesions in X-ray damaged chromosomes to DSB. With respect to the origin of chromosomal aberrations, our results are in favour of the breakage and reunion hypothesis of K. Sax , and not of Revell 's exchange hypothesis.

The exchange hypothesis for the formation of chromatid aberrations: an experimental test using bleomycin

The association between bleomycin-induced chromatid aberrations and BUdR-label exchange between sister chromatids was investigated in order to evaluate Revell's exchange hypothesis for the formation of chromatid aberrations. The results of this study indicate that a larger than expected proportion of chromatid breaks can be accounted for by the exchange hypothesis though not all breaks are the result of incomplete exchange.

Evidence that sister chromatid exchanges and chromatid breaks are two independent events

The relative frequencies of sister chromatid exchanges (SCE) and chromatid breaks in BrdU (5-bromodeoxyuridine) - sensitive site (lq22 leads to lq23) in Chinese hamster cells after BrdU incorporation were studied. The results show that chromatid breaks do not follow the "exchange hypothesis" and provide evidence that chromatid breaks and SCEs are two independent events despite some common features.

Relationship of carcinogen-induced sister chromatid exchange and neoplastic cell transformation

Sister chromatid exchange (SCE) and morphological transformation induced by five chemical carcinogens, N-acetoxy-2-fluorenyl-acetamide (AcAAF), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), methyl methanesulfonate (MMS), benzo[a]-pyrene (BP), and cisplatinum(II) diamine dichloride (PT) as well as by X-irradiation were quantified in parallel experiments with cultures of Syrian hamster embryo cells (HEC). Incubation of HEC with 5-bromodeoxyuridine (10(-5)M) for two rounds of replication (24 h) required for SCE visualization neither caused morphological transformation nor altered the transformation frequency induced by carcinogen alone. All chemical carcinogens, but not X-irradiation, produced a dose-dependent increase in SCE and transformation frequency, demonstrating the sensitivity of both assays to carcinogens. The ratios of induced SCE relative to transformation frequency, however, varied with the carcinogen. BP, MMNG, and AcAAF were similarly efficient in inducing SCE compared to transformation but were considerably less effective than MMS and PT. X-irradiation at doses of 200, 300, and 500 R did not cause transformation and induced a low frequency of SCE. On a molar basis, MMS and PT were the most effective in SCE induction relative to transformation, MNNG and AcAAF were less effective, and BP was the least effective carcinogen. The positive linear correlation between chemical carcinogen-induced SCE and transformation suggests a relationship between the two cellular responses.

Localisation of 7-12-dimethylbenz(a)anthracene induced chromatid breaks and sister chromatid exchanges in chromosomes 1 and 2 of bone marrow cells of rat in vivo

The frequency of chromatid breaks associated with sister chromatid exchanges at the break point was determined in rat bone marrow cells treated in vivo with 7-12 DMBA, during the late S phase of the cell cycle. The chromosomal aberrations and SCEs were scored in the same cells. Under the experimental conditions employed, more than 40% of the chromatid breaks were found to be associated with an SCE, a frequency expected according to Revell's hypothesis for the formation of chromatid breaks.

Effects of a tumor promoter and an anti-promoter on spontaneous and UV-induced 6-thioguanine-resistant mutations and sister-chromatid exchanges in V79 Chinese hamster cells

The effects of a tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) and/or an anti-promoter antipain (protease inhibitor) on spontaneous and ultraviolet-induced sister-chromatid exchanges (SCEs) and 6-thioguanine-resistant (6TGr) recessive mutations were examined in V79 Chinese hamster cells in culture. TPA and/or antipain neither significantly altered base-line and UV-induced immediate SCE frequencies, nor decreased the level of delayed SCEs which persisted 6-7 days after irradiation. TPA and/or antipain appeared to enhance the recovery of UV-induced 6TGr colonies at the plateau expression phase despite non-mutagenicity by themselves and unaltered metabolic cooperation. Thus, the results conceivably imply that the 6TGr-recessive mutation expression, but not fixation, can be modulated at the cell level by TPA and/or antipain. Our results, together with the recent results of Loveday and Latt, may argue against the notion that TPA enhances the antipain-suppressible SCEs as an index of mitotic recombination in relevance with a tumor-promotion mechanism.

Analysis of a BrdU-sensitive site in the cactus mouse (Peromyscus eremicus): chromosomal breakage and sister-chromatid exchange

When the thymidine analog BrdU was incorporated into the DNA of a fibroblast cell line derived from the cactus mouse Peromyscus eremicus, a chromosome region with an increased frequency of gaps and breaks was observed. Nearly a third of the chromatid aberrations found at this site were associated with a sister-chromatid exchange (SCE) although this chromosome region showed no increase in sister-chromatid exchange in the absence of a gap or break. SCEs were significantly decreased in the remainder of the chromosome arm when it contained an aberration at the unstable site. This BrdU-sensitive region, unlike others reported, was found not to be late-replicating.--In this chromosome complement, the frequency of sister-chromatid exchange in C-band positive regions was significantly lower than that in C-band negative regions.

Reduced N-methyl-N'-nitro-N-nitrosoguanidine sister chromatid exchange induction in Chinese hamster V79 cells pre-exposed to 5-bromodeoxyuridine

The sequence in which N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and 5-bromodeoxyuridine (BrdU) are added to cell cultures affects the number of sister chromatid exchanges (SCE) induced by MNNG. When V79 Chinese hamster cell monolayer cultures were treated with MNNG for 2 h prior to addition of BrdUrd, approximately a 4-5-fold increase in SCE was observed at the second division metaphases compared to controls exposed to BrdU alone. This effect was independent of whether one or three DNA strands had been substituted as a result of incubating the cells through one or two DNA synthesis periods in the presence of BrdU. This increase in SCE also occurred after MNNG exposure and BrdU incubation was extended for three division cycles. In contrast, when BrdU incorporation preceded MNNG treatment, the average number of SCE/metaphase was reduced 70-80% at the second division cycle and 60% relative to the total number found in three division cycles. SCE induction by MNNG does not involve a caffeine sensitive step since caffeine had not effect on the SCE frequency regardless of the treatment protocol. The conditions in which BrdU preceded MNNG exposure may be responsible for either reducing the number of DNA sites available for interaction with MNNG or preventing the expression of SCE.

Chromosome aberration formation and sister chromatid exchange in relation to DNA repair in human cells

Apparent association between the ability to induce chromosome aberrations and sister chromatid exchanges and mutagenic-carcinogenic potential found in a variety of physical and chemical agents has led us to speculate that these cytogenetic changes might be reflection of DNA damage and repair and might provide induces of mutagenic changes. However, the mechanisms of their formation and their relation to DNA repair as well as the mechanism of their linking to mutation are by no means well understood. Studies in some human genetic mutant cells defective in their ability to repair DNA damage indicate, as a testable proposition, that sister chromatid exchanges and chromosome aberrations are cytological manifestations of replication-mediated dual-step repair pathways that are in operation to tolerate DNA damage when damage-bearing DNA enters and passes through semiconservative replication. The observations are also in line with idea that the majority of sister chromatid exchanges can arise when damage DNA attempts replication possibly by a process relating with the replicative bypass repair mechanisms such as those proposed by Fujiwara and Tatsumi [34] and Higgins et al. [54], while chromosome aberration formation and some fraction of sister chromatid exchanges are related with the post-replication repair processes which attempt to rescue damaged template post-replicationally by de novo synthesis or recombination type repair systems. The former sister chromatid exchange-relating process seems to link mutation to less extent, if any, than the latter process, which is caffeine sensitive and likely to be error-prone.

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.

Sister chromatid exchange in lymphocytes from patients with malignant lymphoma

Sister chromatid exchange (SCE) frequencies were studied in differentially stained lymphocytes from 47 patients with malignant lymphoma. Thirteen patients were untreated when studied. The mean SCE frequency [+/- standard error (SE)] for these patients was 12.7 +/- 0.9 per mitosis. The mean score for 40 controls was 6.1 +/- 0.3. SCE mean scores were significantly higher in the untreated patients than in the controls (P less than 0.001). Seven patients were treated with radiotherapy alone. They demonstrated a mean SCE frequency (8.8 +/- 0.8) significantly lower (P less than 0.01) than that found in untreated patients. Eleven patients received cyclophosphamide within 4 weeks prior to study. They demonstrated a mean SCE frequency (14.3 +/- 1.3) significantly higher (P less than 0.05) than that found in patients who had received regimens that did not contain cyclophosphamide in the prior 4 weeks (11.1 +/- 1.3) or who had been off drugs for at least 8 weeks (10.1 +/- 0.8). Our data suggest that untreated patients with malignant lymphoma have elevated SCE frequencies, which may be further increased by certain chemotherapeutic agents.

The effects of busulphan on the chromosomes of normal human lymphocytes

In vitro exposure of human lymphocytes to busulphan (BUS) produced an increase in chromosome aberrations and in sister-chromatid exchange (SCE) frequency. The distribution of chromosome breaks throughout the karyotype was non-random and they occurred mainly in the G-negative bands. Certain bands had a marked susceptibility to BUS and comparisons with the human chromosome-break distributions reported for a number of drugs revealed that some of these bands were equally susceptible to other alkylating agents. Both the number of chromosome gaps and breaks and the SCE frequency increased with BUS concentration, but only the SCE dose--response was a clearly defined linear relationship. Therefore a standard SCE dose--response curve was constructed for future comparison with the results of similar investigations of patients on BUS therapy.

Psoralen/UVA treatment and chromosomes. I. Aberrations and sister chromatid exchange in human lymphocytes in vitro and synergism with caffeine

Treatment of human lymphocytes in vitro with trimethylpsoralen or 8-methoxypsoralen and UVA irradiation (PUVA) induced chromosome damage, mainly constrictions and gaps, but also breaks and exchanges, and increased the frequency of sister chromatid exchange (SCE). The localization of the chromosome aberrations was nonrandom. The coincidence of many PUVA hits with mercaptoenthanol hits suggest that PUVA may have other targets in the cell than the DNA, perhaps the folding proteins of the chromosomes and the nuclear membrane/chromatin attachment organelles. Caffeine increased in a synergistic way the chromosome aberration yield if added after PUVA treatment, but there was no effect when caffeine was present before and during PUVA treatment. The SCE frequency was increased in the presence of caffeine.

Sister chromatid exchanges in balanced translocation carriers and in patients with unbalanced karyotypes

Sister chromatid exchanges (SCEs) were studied in peripheral human leukocytes from 16 patients with balanced translocations or with unbalanced karyotypes, and from 4 controls. No difference was seen between these two groups of people in the mean number of SCEs per cell, or in the total number of SCEs observed for each pair of autosomes involved in the translocations studied. With this last number no difference from the expected number of SCEs, if one supposes that SCEs follow a random distribution, was seen.

Vulnerability of specific rat chromosomes to in vitro chemically induced damage

The treatment of rat embryo secondary cultures with DMBA or DMBA-3H for 5, 9, or 24 h resulted in chromosome damage consisting mainly of chromatid type aberrations. There was an increase in the percentage of labelled nuclei and metaphases with increasing length of exposure. In terms of incidence of chromatid lesions, the largest telocentric chromosome (No. 2) was the most susceptible of the autosomes. Banding pattern analysis demonstrated that the region associated with negative band 2q24 of the No. 2 chromosome had the highest number of lesions. An increased accumulation of DMBA-3H label occurred in approximately the same chromatid area of a small fraction of cells exposed for either 5 or 9 h prior to mitosis. The complete loss of DMBA-3H chromosomal labelling after DNAse treatment suggests that the visible grains represent carcinogen-bound DNA. After DMBA and BrdUrd, there was an increase in the number of sister chromatid exchanges compared to controls treated with BrdUrd only; the location of the exchange points on chromosome No. 2 was similar in samples treated with either DMBA and BrdUrd or BrdUrd alone. Additional experiments with thymidine-3H showed that the non-random chromatid lesions on chromosome No. 2 may result from endogenous radiation from the incorporated tritium. These studies demonstrate that a specific chromosome may be affected by diverse agents and that chromatid lesions frequently occur at the site of sister chromatid exchanges.