Ultrastructure of mammalian chromosome aberrations

Chromosomal aberration analysis in workers exposed to chemical and biological hazards in research laboratories

Cytogenetic monitoring of individuals occupationally exposed to chemical and biological hazards has found increased frequencies of cells with chromosomal aberrations. During the present study we analyzed the frequency of chromosomal aberrations in cells from individuals working in various chemistry and biology research laboratories of the University of Brasilia, Brazil. When evaluated jointly and compared to a control group there was no significant increase in the frequency of chromosomal aberrations in the laboratory personnel. However, a group constituted of individuals of the Genetics Laboratory showed a statistically significant increase in the frequency of structural chromosomal aberrations of the chromatid gap type.

Radiation response of cell organelles

The cellular responses to various form of radiation, including ionizing- and UV-irradiation or exposure to electromagnetic fields is manifested as irreversible and reversible structural and functional changes to cells and cell organelles. Moreover, beside the morphological signs related to cell death, there are several reversible alterations in the structure of different cell organelles. The radiation-induced changes in the supramolecular organization of the membranes, including plasma membrane, and different cell organelle membranes, play a significant role in the development of acute radiation injury. These signs of radiation-induced reversible perturbation biological membranes reflect changes in the organization and/or composition of the glycocalix, modified activity and/or distribution of different membrane domains, including enzymes and binding sites. The observed changes of the cell surface micromorphology and the alteration of intercellular connections are closely related to the reorganization of the cytoskeletal elements in the irradiated cells. The mitochondria, endoplasmic reticulum, Golgi-complex, the lysosomal system have long been considered to be direct intracellular targets of irradiation. The listed morphological alterations of nuclear chromatin (e.g. changes of fine structure, altered number of nucleolar organizing regions and micronuclei, development of chromosome aberrations) may originate from the radiation-induced damage to the supramolecular organization of DNA and/or nucleus specific proteins. These endpoints of radiation effects resulted as direct consequence(s) of absorbed radiation energy, and indirectly altered intra-, intercellular communication or modified signal transduction. Some complementary data suggest that all these effects are not strictly specific to radiation and may be best considered as general stress responses, similar to those observed after application of various injurious agents and treatments to cells. Moreover, they may be equally responsible for direct degradation of supramolecular component of cells, altered signal transduction, or changes in the amount or ratio of any extracellular mediators upon irradiation. Nevertheless, qualitative and/or quantitative evaluation of any changes of chromosomes by different techniques (morphological analysis of metaphase chromosomes, fluorescent in situ hybridization, development of micronuclei etc.) are useful biological indicators as well as "biological dosimeters" of radiation injury. It is suggested, that some modern methods such as immunohistochemical detection of different proteins, specific markers of cell organelles and cytoskeleton, inspection of distribution of cell surface charged sites and different membrane domains and application of tracer substances may all be included into protocols for evaluation of cell alterations induced by different types and intensities of radiation.

The evolution of chromosomal instability in Chinese hamster cells: a changing picture?

PURPOSE

To investigate the kinetics of chromosomal instability induced in clones of Chinese hamster cells following X-irradiation.

MATERIALS AND METHODS

X-irradiated clones of GM10115, human-hamster hybrid cells containing a single human chromosome 4 (HC4), have been previously established. These clones were defined as unstable if they contained > or = three subpopulations of cells with unique rearrangements of HC4 as detected by FISH. Stable and unstable clones were analysed by FISH and Giemsa staining at various times post-irradiation.

RESULTS

While most of the stable clones continued to show chromosomal stability of HC4 over time, one became marginally unstable at approximately 45 population doublings post-irradiation. Clones exhibiting chromosomal instability had one of several fates. Many of the unstable clones were showed similar levels of instability over time. However, one unstable clone became stable with time in culture, while another became even more unstable over time. Cytogenetic analyses of all clones after Giemsa staining indicated that in some clones the hamster chromosomes were rearranged independent of HC4, demonstrating increased frequencies of chromatid breaks and dicentric chromosomes. The majority of the unstable clones also had higher yields of chromatid gaps.

CONCLUSIONS

These data demonstrate the dynamic nature of chromosomal instability as measured by two different cytogenetic assays.

Insect sex chromosomes, XI. 3H-TdR induces random aberrations in the X chromosome(s) of Gryllotalpa fossor (Orthoptera)

The pattern of titrated thymidine (3H-TdR), a direct precursor of DNA, induced aberrations on the X chromosome of Gryllotalpa fossor was examined. 3H-TdR produced aberrations randomly distributed over the entire length of the X chromosome; breaks were observed in both the eu- and the heterochromatic arms of the X chromosome in both the sexes. Since the eu- and the heterochromatic arms cannot be distinguished cytologically in this insect, the presence of aberrations on both arms of the same X chromosome in the male and damage to both X chromosomes in the female indicate that both euchromatic and heterochromatic regions (facultative or constitutive) are equally liable to aberrations induced by H-TdR. This is in contrast to the non-random induction of aberrations by 3H-UdR, which causes chromosome damage due to the proximity of the labeled RNA to the DNA template during transcription.

Induction of micronuclei and anaphase aberrations by cytochalasin B in human lymphocyte cultures

The frequency of micronucleated cells in isolated 72-h human lymphocyte cultures treated with cytochalasin B (Cyt-B; 1.5-6 micrograms/ml for the last 28 h) was 9-21 times higher (mean 14.6 times) among multinucleate than binucleate cells. At 3 micrograms/ml, the concentration of Cyt-B originally recommended for the human lymphocyte micronucleus assay, the frequency of micronucleated multinucleate cells was 8.5%, while 0.7% of the binucleate cells had a micronucleus. Although no dose-dependent induction of micronuclei could be observed for either of the cell types, increase in the concentration of Cyt-B was associated with a decrease in the ratio of multinucleate to binucleate cells. Treatment with Cyt-B (1.5-12 micrograms/ml) increased the frequency of anaphase cells with aberrations, especially lagging chromatids. This finding was explained by a dose-dependent increase in multipolar (greater than or equal to 3 poles) divisions which had a high frequency of anaphase aberrations (39-53%), irrespective of the concentration of Cyt-B. Bipolar anaphases did not show a significant increase in aberrant cells, although a suggestive dependence on the concentration of Cyt-B was observed. The findings indicate that the high frequency of micronuclei in multinucleate lymphocytes produced by Cyt-B is due to mitotic errors arising when bi- (and multi-) nuclear cells divide. To avoid possible artifactually high micronucleus frequencies due to inclusion of cells that have divided greater than or equal to 2 times in the presence of Cyt-B, it is recommended that, in the human lymphocyte micronucleus assay using the cytokinesis-block method, the cell culture time is reduced to minimize the frequency of such cells and that only good preparations and regularly shaped binucleates are included in the analysis.

Scanning electron microscope analysis of structural changes and aberrations in human chromosomes associated with the inhibition and reversal of inhibition of ultraviolet light induced DNA repair

Metaphase chromosomes appear decondensed in preparations from mitotic cells that have been irradiated with ultraviolet light (UV) and incubated with inhibitors of DNA synthesis; under these conditions DNA repair is inhibited and both single and double strand DNA breaks accumulate. After reversal of the inhibition chromosomes are condensed, but are often damaged. In this paper we show by scanning electron microscopy (SEM) that decondensed HeLa chromosomes are composed of fibre clusters similar to those previously described for the large chromosomes of the Indian muntjac. This suggests that the clusters may be a universal higher order packing unit in mammalian metaphase chromosomes. We also examine by SEM the nature of the aberrations that appear after the reversal of periods of inhibited repair; these include gaps, breaks, deletions and telomeric packing abnormalities. SEM analysis allows an extension and reconsideration of conclusions about chromatid continuity based on the study of conventional light microscope (LM) preparations.

Chromosomal aberrations in muntjac cells resulting from exposure to interferon

Cells of the Indian deer (Muntiacus muntjac) are sensitive to the antiviral and antiproliferative action of human beta-interferon (beta-IFN). Because of their low diploid chromosome number and readily identifiable chromosomes, they provide a convenient model system in which to test for the ability of IFN treatment to result in chromosome abnormalities. Increases in the frequencies of chromosome gaps and breaks have been observed after 72 h of treatment with IFN at a concentration of 100 U/ml. At IFN concentrations of 10-100 U/ml, there is a higher proportion of aberrations in the X chromosome than would be expected in a random distribution. At 1,000-1,700 U/ml IFN, there is an increase in the proportion of cells with multiple abnormalities over that observed at 0-100 U/ml IFN, and the distribution of aberrations appears to be random.

The kinetochore of mammalian chromosomes: structure and function in normal mitosis and aneuploidy

The kinetochore is a structurally differentiated site on mitotic chromosomes to which spindle microtubules (MTs) are attached. In mammalian cells, the kinetochore is organized into a trilamellar plate and is morphologically distinct from the centromere. Although kinetochores and centromeres are morphologically and biochemically distinct regions, they are functionally linked and necessary for normal chromosome movement and segregation. Recent biochemical and immunocytochemical studies suggest that the kinetochore is composed of several polypeptides, DNA, and possibly RNA. The kinetochore plates are composed of tubulin and two antigens of 17 Kd and 80 Kd, as detected by scleroderma CREST antiserum. Colcemid, a MT inhibitor, also causes reversible rearrangements of kinetochore structure. Mitomycin C binds to heterochromatin and causes the trilamellar plates to become detached from the chromosome. Diethylstilbestrol (DES), a synthetic estrogen, inhibits mitosis in mammalian cells and causes chromosome lagging or malorientation during recovery. Electron microscopy indicates that DES causes disruption of the mitotic spindle, centriole elongation, and unusual chromosome associations due to interkinetochore microtubules. No apparent damage to kinetochores was noted in lagging or maloriented chromosomes.

Anaphase-telophase analysis of chromosomal damage induced by chemicals

Three main aspects involved in the chemical induction of anaphase-telophase aberrations in the first mitosis after treatment were analyzed: 1) the relationship between the frequency of anaphase-telophase aberrations and the time of fixation after treatment; 2) the dose-response relationships; and 3) the proliferative rate of cells exposed to chemicals which interact with DNA by different mechanisms. Experiments were carried out using Chinese hamster ovary (CHO) cells. The compounds examined were adriamycin (ADR) and mitomycin C (MMC). The frequency of cells with chromatin bridges or with lagging chromosomes as well as the mitotic index was determined in each experiment. The results obtained showed that 1) chromatin bridges and lagging chromosomes are apparently induced during the S period of the previous interphase; 2) the increase in the cytotoxicity index (inferred from the mitotic index) and the frequency of cells with chromatin bridges and lagging chromosomes were proportional to the treatment lapse and to the dose employed; and 3) the effect of ADR on cell growth differs from the effect of MMC. While ADR decreased the mitotic activity of cells in logarithmic growth phase, MMC induced mitotic delay. In accordance with these results, the occurrence of chromatin bridges in anaphase-telophase could be explained by the induction of chromosome stickiness and, to a lesser extent, by the induction of exchange-type aberrations. On the other hand, lagging chromosomes seem to be the result of chromatid or chromosome breaks because the lagging chromosomes observed were primarily, if not all, fragments and not whole chromosomes. Our evaluation of the anaphase-telophase test indicates that it is very sensitive method for the detection of chemical clastogens, but other factors, such as mitotic depression, must be taken into account to avoid false-negative results.

Perturbations of cell-cycle progression in gamma-irradiated ataxia telangiectasia and Huntington's disease cells detected by DNA flow cytometric analysis

The effects of ionizing radiation on cell-cycle progression in lymphoblastoid cell lines derived from ataxia telangiectasia (AT) and Huntington's disease (HD) patients, and from normal individuals, were studied using DNA flow cytometric analysis. A dose of 100 rad gamma irradiation blocked a proportion of normal and HD cells in G1. A higher radiation dose applied to normal cells increased the number of cells blocked in G1 and significantly delayed cells which were in S at the time of irradiation from reaching G2 DNA content. The reduced cumulative mitotic index in irradiated cultures of normal cells 2 h after irradiation suggests that cells in G2 at the time of irradiation are delayed before entering mitosis. After irradiation HD cells responded similarly to normal cells except that a greater proportion of HD cells were blocked in G1. AT cells do not show the normal delay in progression from G1 to S, or from S to G2 in the first cycle after irradiation. The cumulative mitotic index was reduced in irradiated cells, implying that they are delayed in G2. Thus AT cells did not recognize or respond to signals from damaged DNA which in normal and HD cells caused a proportional block in G1 and an S-phase delay. The only point of arrest in cell-cycle progression in irradiated AT cells was in G2.

Insect sex chromosomes. VI. A presumptive hyperactivation of the male X chromosome in Acheta domesticus (L.)

The functional status of the X chromosome in Acheta domesticus has been analysed at the whole chromosome level on the basis of (1) 3H-thymidine autoradiography, (2) 5-BrdU/AO fluorescence microscopy (3) in vivo 5-BrdU incorporation and (4) 3H-UdR induced aberrations. The rationale of these techniques in relation to the functional aspect of the X chromosome is that the inactive X chromosome would (1) show asynchrony in DNA synthesis, (2) show differential fluorescence, (3) respond differentially to in vivo 5-BrdU treatment and (4) the active X chromosome would show aberrations when treated with 3H-Uridine. From the results, it appears that the X chromosomes in both male (XO) and female (XX) somatic cells of Acheta are euchromatic (active). Further, the single X in the male is transcriptionally as active as the two X chromosomes in the female. In other words, the single X in the male is hyperactive when compared with the single X in the female. From this it is inferred that the male X chromosome is differentially regulated in order to bring about an equalization of it's gene product(s) to that produced by both Xs in the female. Drosophila melanogaster has a comparable system of dosage compensation. Thus, Acheta is yet another insect showing evidence for an X chromosome regulatory mechanism of dosage compensation. Additionally, it is surmised that sex determination in Acheta is based on an autosomes/X chromosome balance mechanism.

Effects of 1-beta-D-arabinofuranosylcytosine on chromosomes, depending upon the cell cycle stage at the time of exposure

1-beta-D-Arabinofuranosyl cytosine (ara-C) is a clinically important cytotoxic drug which is a potent inhibitor of DNA but which has a minimal effect on other cellular processes. The cytotoxic action of ara-C on mammalian cells has been suggested to be due to the chromosome aberrations induced by this compound. Using a marsupial cell line (JU56), the cells of which contain only 9 readily identified chromosomes, the different types of chromosome aberrations induced by a pulse of ara-C have been quantified, and the cell cycle dependence of the damage has been assessed. It was found that, for cells exposed in G2, both chromatid-type and chromosome-type lesions were produced. The frequency of these lesions was reduced by a chase of deoxycytidine, and there was some evidence that the initial lesions are gaps which may later be converted to true breaks. In early G2 and late S cells, lesions were produced chiefly at one chromosome locations; this location was not specifically late-replicating. At all stages of S, lesions were chiefly chromatid-type, and some exchanges occurred. The level of damage in S cells was not influenced by a deoxycytidine chase. There was negligible damage in cells exposed in G1. It is suggested that the reason previous investigators have obtained very different cell cycle dependence of chromosome damage is that the delaying effects of ara-C on cell cycle progression was not taken into account.

Is inosine the physiological energy source of pig erythrocytes?

Pig erythrocytes are unable to metabolize glucose and their physiological energy source is unknown. These cells have a high-capacity nucleoside transport system with similar properties to that responsible for nucleoside transport in other species. Nucleoside transport is sufficiently rapid to allow the possibility that inosine and/or adenosine may represent major energy substrates for pig erythrocytes in vivo. Normal and adenosine deaminase-deficient pig erythrocytes have similar ATP levels, suggesting that adenosine is not important in this respect. However, it was calculated that an extracellular inosine concentration of only 40 nM could support the cells' entire energy requirement, a value 40-fold lower than plasma levels of this nucleoside.

Scanning-electron microscopy of chromosome aberrations

This study is the first report of scanning-electron microscopy of isolated and purified metaphase chromosomes containing drug-induced aberrations. The technique reported allows high resolution topological examination of chromosomal aberrations which may pass undetected with conventional techniques.

Nucleoside and glucose transport in erythrocytes from new-born lambs

1. Glucose and inosine transport by erythrocytes from new-born lambs and adult sheep were compared. Uptake of both permeants was considerably faster in the new-born. Inosine uptake by erythrocytes from nucleoside-permeable and impermeable lambs were not significantly different at birth. The difference between the two phenotypes was first apparent 30 days after birth.2. The post-natal changes in glucose and inosine transport activity closely paralleled the progressive decrease in the percentage of fetal erythrocytes (i.e. cells containing fetal haemoglobin) in the circulation. Cell fractionation studies confirmed that the permeability changes were directly related to changes in the relative proportions of fetal and adult haemoglobin containing erythrocytes.3. The results demonstrate that fetal cells are highly permeable to both glucose and inosine. These cells are replaced by erythrocytes which contain adult haemoglobin and which have a much lower, but still significant, glucose permeability and either low or negligible inosine transport activity depending on the genotype of the animal.4. Inosine transport by fetal erythrocytes from both nucleoside-permeable and impermeable animals was mediated by a nucleoside transport system which had similar properties to that responsible for nucleoside transport in adult nucleoside-permeable cells. Glucose transport in both fetal and adult cells was highly stereo-specific, indicating the presence of a selective transport system.5. It is suggested that the regulatory mechanism responsible for initiating the switch from fetal to adult haemoglobin synthesis may also be responsible for the changes in glucose and nucleoside transport activity.

Genetic control of nucleoside transport in sheep erythrocytes

Nucleoside transport in sheep erythrocytes is under the genetic control of two allelomorphic genes (NuI and Nui), where NuI codes for the functional absence of a high-affinity nucleoside transport system and is dominant to the gene (Nui) coding for the presence of the transport system. Kinetic and inhibitor experiments show that the high-affinity transport system is not present in heterozygous erythrocytes, demonstrating that the NuI gene is completely dominant over the Nui gene. It is suggested that the Nu locus may not represent the structural gene locus of the nucleoside transport system. Instead, it may be a regulator gene locus.

Nucleoside transport in sheep erythrocytes: genetically controlled transport variation and its influence on erythrocyte ATP concentrations

1. The permeability of sheep erythrocytes to purine and pyrimidine nucleosides was investigated. Erythrocytes from most sheep (nucleoside-impermeable) were almost completely impermeable to 5 mM inosine whereas cells from approximately 5% of the animals studied (nucleoside-permeable) showed a rapid inosine uptake. Cells from both types of animal were permeable to 5 mM adenosine, although transport was slower in nucleoside-impermeable erythrocytes. 2. Two distinct nucleoside transport routes were present in nucleoside-permeable erythrocytes; a high affinity (apparent Km congruent to 0.2 mM) facilitated diffusion system which transported both purine and pyrimidine nucleosides, and a non-saturable uptake route selective for adenosine. The high affinity system was the major route of adenosine transport at physiological concentrations. 3. Transport by the high affinity system was completely inhibited by micromolar concentrations of dipyridamole and nitrobenzylthioinosine. Dipyridamole had no effect on the non-saturable component of adenosine uptake. 4. The transport differences between nucleoside-permeable and impermeable erythrocytes were due to the absence of the high affinity system from nucleoside-impermeable cells. 5. Nucleoside-permeable cells had a higher intracellular ATP concentration than nucleoside-impermeable erythrocytes, suggesting that the high affinity transport system participates in the energy metabolism of the cell.