Complementation studies in murine/human hybrids suggest multiple etiology for increased rate of sister chromatid exchange in mammalian cells

Two mutational changes which occurred in culture and are associated with a high rate of sister chromatid exchange (SCE) phenotype have been identified in the L-A9 murine cell genome by means of complementation studies with somatic cell hybrids. Preliminary cytogenetical evidence suggests that the retention of human autosome 6 (namely the region comprised between Xq12 and Xqter) or human autosome 19 is required in the hybrid metaphases for complementation to occur, independently of their being derived from normal human or Bloom syndrome (BS) cells. These data and other complementation studies previously reported by our group and by other investigators suggest that mammalian cells may possess several independent systems involved in the control of SCEs during chromatid replication. Thus, the high rate of SCE can be regarded as the common phenotype resulting from a variety of qualitative or quantitative changes affecting the mammalian cell genome. Bloom syndrome is evidently an example of homozygosity for a recessive mutation occurring in nature. The high SCE mutants found among rodent cells (as those seen in unstable rodent-human hybrid cells) are more likely the result of chromosomal loss or rearrangement occurring in culture at one or more of the genetic systems hypothesized above. The occurrence of complementation within or between the species barrier, following cell hybridization or cocultivation, indicates the recessive nature of the corresponding mutations and the possible homology of the relevant genetic systems in different mammalian species. The isolation of rodent clonal cell lines with a stable high rate of SCEs and the production of somatic cell hybrids between them and BS cells offer a promising experimental tool for studying the biology of SCEs in general and the genetics of BS in particular.

The replication of unsubstituted and 5-bromodeoxyuridine- or 5-chlorodeoxyuridine-substituted DNA regulates the rate of induction of sister chromatid exchanges

The thymidine (dThd) analog 5-bromodeoxyuridine (BrdUrd) is widely used in studies of the induction of sister chromatid exchanges (SCEs), since growth in the presence of BrdUrd allows the subsequent differential staining of the chromosomes through the fluorescence-plus-Giemsa (FPG) method. However, the analog itself induces SCEs, an aspect of its use which is often not considered. We have studied the induction of SCE by BrdUrd and a second dThd analog 5-chlorodeoxyuridine (CldUrd). Growth of Chinese hamster ovary (CHO) cells for 2 rounds of replication in the presence of different concentrations of either analog results in increasing results in increasing SCE frequencies which are linearly proportional to the degree of analog substitution for dThd in the DNA. However, CldUrd causes 3 to 5 times the number of SCEs found with BrdUrd, at equivalent substitution for dThd. With both analogs the increase in SCE frequency is due to the replication of the analog-substituted DNA and not to the incorporation of analog into nascent DNA. This induction of SCE can be considered at the level of a single strand of DNA since the replication of bifilarly substituted DNA results in twice the number of SCEs that are induced by the replication of unifilarly substituted DNA.

The mechanism of 3-aminobenzamide-mediated increases in spontaneous and induced SCEs

Cells exposed to the poly-(ADP-ribose)-polymerase inhibitor, 3-aminobenzamide (3AMB), have increased levels of sister chromatid exchanges (SCEs). In addition, cells exposed to certain monofunctional alkylating agents in combination with 3AMB have frequencies of SCEs much greater than expected if the 2 agents acted independently. Because poly-(ADP-ribose)-polymerase is stimulated by the production of DNA strand breaks, and 3AMB is said to inhibit strand-break rejoining, we determined the relationship between DNA strand breakage and SCE formation. Alkylating agents and the incorporation of bromodeoxyuridine (BrdUrd) into DNA led to the production of DNA strand breaks and the most potent SCE-inducing agents were also the most efficient DNA strand-breaking agents. In increasing SCE frequency, 3AMB interacted most strongly with those agents that produced the greatest number of DNA strand breaks. 3AMB also increased the frequency of strand breaks by delaying strand-break rejoining and introducing new strand breaks. The protease inhibitor, antipain, inhibited the formation of 3AMB-induced strand breaks in BrdUrd-containing cells and reduced by half the 3AMB-mediated increase in SCE frequency, suggesting that one mechanism for SCE induction involves the introduction of breaks into DNA possibly in response to changes in chromatin structure. We conclude that the primary lesion responsible for SCE induction after exposure to alkylating agents or incorporation of BrdUrd into DNA is either a DNA strand break or a lesion that can result in a strand break. 3AMB increases SCE frequency in combination with these agents by introducing new breaks and possibly delaying strand-break rejoining.

Induction of DNA lesions, chromosomal aberrations, and G2 delay by bromo- and chlorodeoxyuridine

The presence of lesions in DNA of CHO cells, substituted with BrdUrd or CldUrd, has been investigated in a direct way by alkaline sucrose gradient and nucleoid sedimentation analysis and indirectly by screening for induced CAs. The influence of inhibitors of DNA repair (Caff and 3AMB) or DNA synthesis (HU) on the frequencies of such aberrations has been estimated. No randomly located DNA breaks could be detected under neutral conditions, but BrdUrd-substituted DNA was found to contain numerous alkali labile sites. At high concentrations, CldUrd causes G2 delay, similar to the action of known DNA-damaging agents. The extent of delay depends on the pattern of incorporation of the analog, i.e., incorporation for 2 cell cycles causes the longest delay, growth for 12 hr in CldUrd followed by 12 hr in dThd- containing medium gives less delay, and delay is not significant when the cells are incubated in the analog for only 12 hr prior to fixation. Numerous chromatid-type aberrations are present in cells incubated at the highest CldUrd concentration and their induction follows the pattern of induction of G2 delay, indicating the involvement of a common lesion. 3AMB, and HU increase the number of CAs when added 2 hr before fixation. The significance of these results is discussed.

The contribution of DNA single-strand breaks to the formation of chromosome aberrations and SCEs

The induction of sister chromatid exchanges (SCEs) and chromosomal aberrations (CAs) with bleomycin (BLM), hydrogen peroxide (H2O2), short-wave ultraviolet (UV)-irradiation, and long-wave UV-irradiation was investigated in V79 cells with BrdUrd-substituted DNA. The application of a Neurospora endonuclease (NE) which specifically cleaves single-stranded DNA after these treatments showed that DNA single-strand breaks (SSBs) are induced by these agents. The SSBs are converted to double-strand breaks (DSBs) by NE and become visible as CAs on metaphase chromosomes. H2O2 and both types of UV-irradiation also led to an induction of CAs and SCEs, whereas BLM only induced aberrations. Cysteine (Cys) reduced the frequency of the induced SSB-dependent CAs in all treatments, but had no influence on the SCE frequencies after BLM and H2O2 treatment and had only a slight effect on the UV-induced SCEs. The results confirm the opinion that directly induced SSBs can contribute to the induction of CAs in cells with BrdUrd-substituted DNA, but that these SSBs are not efficiently converted to SCEs. The more recent conceptions regarding the mechanism of SCE are in accordance with these findings and the conclusions derived therefrom.

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

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

Incidence of herpes simplex virus types 1 and 2 in penile lesions of college men

Herpes simplex virus (HSV) was isolated from penile lesions of 15 college men. Using (E)-5-(2-bromovinyl)-2'-deoxyuridine sensitivity, monoclonal antibody immunofluorescence, and restriction endonuclease EcoRI digestion of viral DNA, 4 of 15 (26%) isolates were found to be HSV-1, and 11 of 15 (74%) isolates were found to be HSV-2. It is likely that some of the genital HSV-1 infections are related to oral genital contact, but this fact could not be established for all cases, since the females in this group had previously been shown to have a high incidence of genital HSV-1.

The use of monoclonal antibodies to differentiate isolates of herpes simplex types 1 and 2 by neutralisation and reverse passive haemagglutination tests

Monoclonal antibodies specific for herpes simplex type 1 or type 2 were used in reverse passive haemagglutination tests or infectivity neutralisation tests to serotype 100 isolates of herpes simplex virus (HSV). All isolates were independently serotyped by measuring their sensitivity to bromovinyl deoxyuridine. Reverse passive haemagglutination tests with type-specific antibodies directed against the HSV glycoprotein D and major DNA binding protein gave results in perfect agreement with the results of drug-sensitivity measurement. A single isolate behaved anomalously in the neutralisation test with a type 1-specific antibody directed against glycoprotein A/B. Restriction-enzyme analysis of virus DNA suggests that this isolate contains a variant glycoprotein A/B. The two methods used for serotyping proved very sensitive, giving adequate results with samples containing as little as 100 plaque forming units (pfu) of HSV. The reverse passive haemagglutination test has the additional advantages of speed and simplicity.

[Decrease in the spontaneous frequency level of sister chromatid exchanges in cell division in culture]

The spontaneous level of sister chromatide exchanges (SCE) registered in human lymphocytes is shown to depend on the moment of BUdR introduction and the time of fixation. In early periods of BUdR introduction and fixation the general spontaneous level of SCE may be observed and in later periods only that part of SCE may be registered which is caused by internal conditions. The difference between the first and second results makes the part of SCE conditioned by the environmental effects.

Probing sister chromatid exchange formation with halogenated pyrimidines

This communication describes the use of 6 different halogenated pyrimidine analogues, bromodeoxyuridine (BrdUrd), chlorodeoxyuridine (CldUrd), iododeoxyuridine (IdUrd), bromodeoxycytidine (BrdCyd), chlorodeoxycytidine (CldCyd), and iododeoxycytidine (IdCyd), to achieve sister chromatid differentiation (SCD) and evaluate sister chromatid exchange (SCE) formation in mitogen-stimulated human lymphocytes. Also included are a description of an in vivo experiment with BrdUrd, CldUrd, and IdUrd; a discussion of pyrimidine metabolism effects on SCEs; and the presentation of an update on the "conformation hypothesis" for SCE formation. This hypothesis revision includes a model that centers on the idea that the sum of the conformational alterations of the DNA polymerase-DNA template complex at replication is the controlling factor in SCE formation.

Different properties in lymphoblastoid cell lines from patients with Bloom syndrome

In 3 lymphoblastoid cell lines derived from 3 patients with Bloom syndrome (BS), the baseline frequency of sister chromatid exchanges (SCEs), chromosomal breakage, sensitivity to ethylmethane-sulfonate (EMS), and bromodeoxyuridine (BrdUrd) were examined. The SCE frequency of 2 BS lines (EB-BS-NoKi-2 and EB-BS-AkSak) was about 2 to 2.5 times those of the normal cell lines, while that of another BS line (EB-BS-2KA) was about 10 to 11 times. The net increase in the number of SCEs in EB-BS-NoKi-2 and EB-BS-AkSak lines induced by EMS was similar to that of normal cell lines, but it was high in the BS-2KA line. Sensitivity to BrdUrd was examined using SCE induction at different concentrations of BrdUrd and by cell cycle analysis. EB-BS-NoKi-2 and EB-BS-AkSak lines were no more sensitive than normal cell lines, while the EB-BS-2KA line was more sensitive than controls. High frequency of chromosomal breakage was found only in the EB-BS-2KA line. These results suggest that 2 types of cells exist in the B-lymphoblastoid cells of BS.

An in vitro and in vivo study of a BrdU-sensitive fragile site in the Chinese hamster

The frequencies of chromosome aberrations and development of the bromodeoxyuridine (BrdU)-sensitive fragile site were studied in vitro in Chinese hamster kidney and bone marrow cells and in vivo in Chinese hamster bone marrow cells. Chromosome aberrations in these cell systems were measured in response to different concentrations of BrdU, fluorodeoxyuridine, or both. The fragile site was found in both homologues of chromosome 1 at 1q22. Treatment with BrdU in vitro but not in vivo produces significant chromosome aberrations. About 50% of chromosome aberrations found after treatment in vitro were at the BrdU-sensitive fragile site compared with 12.5% after treatment in vivo. These results show that BrdU is much more potent in vitro than in vivo in inducing both chromosome aberrations and the expression of the BrdU-sensitive site.

Development of rodent peripheral blood lymphocyte culture systems to detect cytogenetic damage in vivo

Peripheral blood lymphocytes (PBLs) offer many advantages for in vivo cytogenetic studies. They can be removed nonlethally from the animal allowing a subject to serve as its own control, permitting the analysis of cytogenetic damage over time. Furthermore, mature PBLs normally do not divide, and some populations are long-lived. Thus, they have the potential to accumulate DNA lesions during chronic exposures to genotoxicants. We have developed standard methodologies for the whole blood culture of rat and mouse PBLs to serve as models for determining the sensitivity of PBLs to cytogenetic damage. The cultures obtained with these protocols give reproducible results with high mitotic indices, stable baseline sister chromatid exchange (SCE) frequencies, and ample numbers of first-and second-division methaphases for scoring both chromosomal aberrations and SCEs. The methodologies have been especially useful for examining cytogenetic damage after inhalation exposures to toxicants such as ethylene oxide, formaldehyde, benzene, and nitrobenzene. Of these compounds, only benzene and ethylene oxide were found to induce significant dose-dependent increases in SCEs in PBLs. Also, dose-response curves have been obtained for several carcinogens administered by ip injection. These studies show that PBLs are sensitive indicators of the genotoxic effects of the carcinogens benzo (a)pyrene, 2-acetylaminofluorene, cyclophosphamide (CP), N-nitrosomorpholine, and ethylmethanesulfonate (EMS). In addition, because subpopulations of lymphocytes can be stimulated to divide using different mitogens, it has been possible to compare the sensitivity of murine B and T lymphocytes following in vitro and in vivo cyclophosphamide exposure. Once the sensitivity and selectivity of rodent lymphocyte cultures are determined, these assays should be valuable not only as a means for predicting which environmental agents could lead to increases in human cytogenetic damage, but also as a way to corroborate human cytogenetic studies.

Reduction of sister chromatid exchange frequency with time after mutagen exposure in Chinese hamster ovary cells in the presence of 3-aminobenzamide

3-Aminobenzamide (3AB) is a potent inhibitor of poly (ADP-ribose) synthesis and has been reported to inhibit DNA repair. Inhibition of poly(ADP-ribose) synthesis by 3AB results in a potentiation of the cytotoxic effects of alkylating agents and synergistically increases the frequency of sister chromatid exchanges (SCEs) induced in Chinese hamster ovary cells by methyl methanesulfonate (MMS). Experiments were carried out to determine if this synergism was related to an inhibition of the repair or the removal of SCE-forming lesions by 3AB. Cells were treated with MMS or ethyl nitrosourea (ENU) and either held in a nonproliferative state or maintained in exponential growth for up to 48 hr before SCE frequencies were measured. During this time, the SCE frequency decreased greatly. 3AB did not affect this decrease in either cycling or noncycling cells. Furthermore, 3AB appears to exert its effect only in cycling cells when bromodeoxyuridine is present. Therefore, the synergism between 3AB and alkylating agents in the induction of SCEs is unrelated to effects of 3AB on the repair or removal of SCE-forming lesions.

Modification by anisotonic treatment of repair and fixation of radiation damage in cell killing and transformation

Anisotonic treatment of V79 cells after irradiation resulted in extensive fixation of potentially lethal damage (PLD). Fixation of PLD occurred more rapidly when treated with 1.5 mol l-1 NaCl than with 0.05 mol l-1 NaCl. These effects were further enhanced when cells were held at 0 degree C instead of 37 degrees C during irradiation. When the X-ray (10.0 Gy) and salt treatments were separated by incubation at 37 degrees C, survival increased rapidly by a factor of 2000, while repair of PLD in cells held in a plateau phase after X-rays (10.0 Gy) caused survival to increase four-fold. The data indicate that these mammalian cells sustain and repair a much larger amount of PLD than is measured by the conventional method of holding cells in a non-progressing state after irradiation. Radiation sensitivity in cells was modified by BrdUrd incorporation into cellular DNA or exposure to DMSO during irradiation. For these cells, fixation of damage by post-irradiation anisotonic treatment proceeded at the same rate for BrdUrd-treated cells, and at a diminished rate for DMSO-treated cells, compared with cells irradiated in the absence of these compounds. In C3H-10T 1/2 cells, post-irradiation anisotonic treatment (with 0.05 or 1.5 mol l-1 NaCl) also resulted in fixation of PLD and a rapid recovery when the X-ray and salt treatments were separated by incubation at 37 degrees C. Treatment with 0.05 or 1.5 mol l-1 NaCl solutions immediately after irradiation also resulted in elevated transformation frequencies, which increased with exposure time to anisotonic solutions.(ABSTRACT TRUNCATED AT 250 WORDS)

[Combined action of gamma irradiation and 5-bromodeoxyuridine on human chromosomes]

A combined effect has been studied of 5-bromo-deoxyuridine (BDU) and gamma-radiation on human lymphocytes at the G0 stage. BDU is shown to induce chromatid aberrations irrespective of its radiation. The frequency of chromatid aberrations rises with the BDU concentration. BDU modifies the effect of gamma-radiation on human lymphocytes at the G0 stage, increasing the number of chromosome aberrations. BDU slows down the lymphocyte cell cycle.

A statistical analysis of Neanthes arenaceodentata, sister chromatid exchange data

This paper presents a statistical evaluation of the sister chromatid exchange (SCE) assay using Neanthes arenaceodentata. Specific issues discussed are distribution of baseline SCE frequencies, transformations of SCE data, statistical sensitivity of the assay, repeatability of test results, and sources of variability of SCE data. Probabilities of false positives and false negatives are considered and recommendations are made for appropriate transformation of data, experimental design, and test conditions.

Heritable fragile sites on human chromosomes. XI. Factors affecting expression of fragile sites at 10q25, 16q22, and 17p12

The fragile sites at 10q25, 16q22, and 17p12 can all be induced in lymphocyte culture by BrdU or BrdC added 6-12 hrs prior to harvest. Without induction, fra(10)(q25) is rarely expressed spontaneously, whereas fra(16)(q22) is frequently expressed spontaneously. Fra(17)(p12) is frequently expressed spontaneously but is probably expressed only after induction in some individuals. Distamycin A, netropsin, and Hoechst 33258 induced high levels of expression of fra(16)(q22) and fra(17)(p12) but did not enhance expression of fra(10)(q25). The mechanisms of induction of fra(16)(q22) by BrdU and distamycin A appear to be different, since the time of induction by BrdU reaches a maximum about 12 hrs prior to harvest whereas induction by distamycin A requires much longer exposure. The fragile sites at 10q25 and 16q22 were both induced in fibroblast culture by BrdU. Fra(17)(p12) is accepted as a fragile site because preliminary studies show that it behaves similarly in lymphocyte culture to fra(16)(q22); however, there is only limited evidence for fragility at 17p12.