Differential response of constitutive and facultative heterochromatin in the manifestation of mitomycin induced chromosome aberrations in Chinese hamster cells in vitro

Evaluation of human embryonic stem cells and their differentiated fibroblastic progenies as cellular models for in vitro genotoxicity screening

This study evaluated human embryonic stem cells (hESC) and their differentiated fibroblastic progenies as cellular models for genotoxicity screening. The DNA damage response of hESCs and their differentiated fibroblastic progenies were compared to a fibroblastic cell line (HEPM, CRL1486) and primary cultures of peripheral blood lymphocytes (PBL), upon exposure to Mitomycin C, gamma irradiation and H2O2. It was demonstrated that hESC-derived fibroblastic progenies (H1F) displayed significantly higher chromosomal aberrations, micronuclei formation and double strand break (DSB) formation, as compared to undifferentiated hESC upon exposure to genotoxic stress. Nevertheless, H1F cell types displayed comparable sensitivities to genotoxic challenge as HEPM and PBL, both of which are representative of somatic cell types commonly used for genotoxicity screening. Subsequently, transcriptomic and pathways analysis identified differential expression of critical genes involved in cell death and DNA damage response upon exposure to gamma irradiation. The results thus demonstrate that hESC-derived fibroblastic progenies are as sensitive as commonly-used somatic cell types for genotoxicity screening. Moreover, hESCs have additional advantages, such as their genetic normality compared to immortalized cell lines, as well as their amenability to scale-up for producing large, standardized quantities of cells for genotoxicity screening on an industrial scale, something which can never be achieved with primary cell cultures.

Chromosome banding patterns and the origin of the B genome in wheat

The distribution of heterochromatic regions in the chromosomes of diploid, tetraploid and hexaploid wheat shows that the B genome possesses characteristic large blocks. Though analyses of probable B genome donors indicate thatAegilops speltoideshas a pattern of distribution of heterochromatin nearest to the B genome chromosomes, a polyphyletic origin of tetraploid wheat seems more plausible.

Human embryonic stem cells may display higher resistance to genotoxic stress as compared to primary explanted somatic cells

The use of human embryonic stem (hES) cells in genotoxicity screening can potentially overcome the deficiencies associated with using immortalized cell lines, primary explanted somatic cells, and live animal models. Hence this study sought to compare the responses of hES cells and primary explanted somatic cells (IMR-90 cells, human fetal lung fibroblasts) to genotoxic stress, to evaluate whether hES cells can accurately reflect the normal physiology of human somatic cells. The effects of mitomycin C (MMC) on the chromosomal stability of hESC and IMR-90 was assayed and compared by fluorescence in situ hybridization (FISH) with telomere-specific peptide nucleic acid and multicolor (m) FISH techniques. The results showed that, the percentage of aberrant cells increased from 6% in the untreated control to 57.5% at the higher dose of 0.06 microg/ml MMC (9.6-fold increase) group in the case of IMR-90 cells, whereas hES cells displayed a corresponding increase from 6% to 28% (4.6-fold increase). Telomere FISH ascertained that the main types of damage induced by MMC are chromosomal breaks and the loss of telomeric signals. No fusions were observed in all samples analyzed. This was further confirmed by mFISH, which showed that fusions and translocations were not the type of aberration induced by MMC, with no such aberrations being observed in all samples analyzed. Hence, hES cells of the H1 line are apparently more resistant to MMC-induced DNA damage, as compared to the IMR-90 cells. These results highlight possible intrinsic differences in response to damaging agents between hES cells and normal somatic cells.

Formation of chromosome aberrations: insights from FISH

Most of the mutagenic and carcinogenic agents induce chromosome aberrations in vivo and in vitro. Conventional solid staining (such as Giemsa) has been employed to evaluate the frequencies and types of spontaneous and induced chromosomal aberrations. Recently, molecular cytogenetic techniques such as fluorescence in situ hybridization (FISH) using chromosome specific or chromosome region-specific DNA libraries have become available, which have increased the resolution of the detection of aberrations. This has lead to a better understanding on the mechanisms of formation of chromosome aberrations, especially following treatment with ionizing radiation. The present paper reviews briefly the results obtained using FISH technique both from basic and applied studies.

Chromosome aberrations: past, present and future

Spontaneous and induced chromosome aberrations have been studied over more than a century. The resolution of detection of aberrations has depended on the improvement of available techniques. An overview on the major high lights in this area of research, from the time of solid staining to fluorescence in situ hybridization technique is presented in this review.

Application of mFISH for the analysis of chemically-induced chromosomal aberrations: a model for the formation of triradial chromosomes

Using a human lymphoblastoid cell line WTK-1, we applied multicolor fluorescence in situ hybridization (mFISH) technique to analyze mitomycin C (MMC)-induced chromatid exchanges, focusing especially on the triradial chromosomes. It was found that the triradial chromosomes were formed with a specific rearrangement, "recipient and donor" relationship. The exchange sites of the recipient chromosomes were on single chromatid breaks and distributed randomly throughout the interstitial, pericentromeric, and terminal regions. In counterpart, donor chromosomes exchanged on isochromatid breaks of their telomeric and/or subtelomeric regions with the single chromatid breaks of recipient chromosomes. More than 80% of the scored triradial chromosomes were formed with such rearrangements, and few acentric chromosome fragments derived from the donor chromosomes could be detected in the metaphases observed. We therefore suggest that biological mechanisms of breakages between the recipient and donor chromosomes are different: the former due to direct DNA-damage by MMC, but the latter due to indirect DNA-damage depending on telomeric specific structure/function.

Induction, processing and persistence of radiation-induced chromosomal aberrations involving hamster euchromatin and heterochromatin

Euchromatic and heterochromatic regions are easily distinguished in Chinese hamster sex chromosomes, hence offering the possibility of studying the role of chromatin structure in the induction, processing and persistence of radiation-induced chromosome damage. X-ray (4 Gy)-induced breaks in the euchromatic Xp and in the heterochromatic Xq were analysed immediately and 4h after irradiation by premature chromosome condensation (PCC) in combination with either FISH using chromosome arm-specific probes or Giemsa staining. The study, performed with female Chinese hamster splenocytes, was extended to a 34 h recovery followed by arm-specific FISH in metaphase. A significant over-involvement of the heterochromatic Xq in radiation-induced breakage was observed at all sampling times (p<0.001). However, the heterochromatic state had little effect on the processing of the induced lesions. In a second experiment, the persistence of radiation-induced chromosome aberrations (CAs) involving Xp, Xq and Y chromosome was studied with cultured Chinese hamster male splenocytes sampled 30, 56 and 96 h after irradiation (4 Gy). A higher involvement of the heterochromatic regions (Xq and Y) in radiation-induced CAs was again observed in the first sampling time (p<0.001), suggesting that Chinese hamster heterochromatin could be more radiosensitive than euchromatin. Cells with CAs involving heterochromatin were apparently less persistent than those with lesions involving euchromatin. This observation could be attributable to either the distribution of CA per cell or to the fraction of potentially stable exchanges.

Biological effects of a bifunctional DNA crosslinker. I. Generation of triradial and quadriradial chromosomes

Interduplex crosslinks by a bifunctional anthramycin DNA crosslinker produced triradial and quadriradial chromosomes. The crosslinker alkylates guanine at N-2. Bovine chromosomes contain GC-rich density satellite DNAs at the centromeric heterochromatin and is the basis for the formation of triradial and quadriradial chromosomes at the centromeres. The in situ crosslinking of interphase chromosomes indicates that the distance between centromeres is 17.5 A. We conclude that the nuclear matrix associated DNA in the centromeric heterochromatin of interphase chromosomes are positioned close enough for crosslinking to occur. We propose a model for the generation of triradial and quadriradial chromosomes based upon the number of interduplex crosslinks between two chromosomes.

Recent developments in the assessment of chromosomal damage

Ionizing radiation and restriction endonucleases are very efficient in inducing chromosomal aberrations (CAs). These aberrations are mainly consequences of misrepair of DNA double-strand breaks (DSBs). The fast repairing component of DSBs induced by ionizing radiation seems to be responsible for exchange aberration. Use of premature chromosome condensation technique in combination with DNA repair inhibitors such as ara A has given valuable information on the assessment of the frequencies of initial chromosome breaks and the kinetics of their repair following low LET radiation. The recently developed 'chromosome painting' technique using chromosome-specific libraries has also increased considerably the resolution of identifying and scoring of CAs. After low LET radiation, stable chromosome exchanges (translocations) are induced more frequently than unstable chromosome exchanges (dicentrics). Fluorescence in situ hybridization employing telomeric probe has made it possible to score efficiently exchange aberrations involving the acrocentric chromosomes of mouse. Chinese hamster cells have several intercalary telomeric sequences present in most of the chromosomes. These telomeric blocks have been found to be associated with chromosomal aberrations induced by restriction endonucleases and short wave UV and evidence has been obtained for apparent amplification of telomeric sequences at the break points.

Distribution of X-ray-induced G2 chromatid damage among Chinese hamster chromosomes: influence of chromatin conformation

Exponentially growing primary embryonic Chinese hamster cells, in which the pattern of distribution of heterochromatin is well characterized, were X-irradiated and fixed at 1, 2, 3, and 4 h following irradiation. In one set of cells repair of damage was completely blocked by ara A. The frequencies of chromatid breaks and exchanges were evaluated for individual chromosomes. An analysis of observed and estimated expected frequencies of chromosomal aberrations indicated that in general, (a) the initial damage was higher in euchromatic regions than the heterochromatic regions and (b) the repair of DNA lesions (as evaluated by the frequencies of chromatid gaps and breaks) was more efficient in euchromatic regions than heterochromatic regions.

Mechanism of chromosome aberration induction in the mouse egg fertilized with sperm recovered from postmeiotic germ cells treated with methyl methanesulfonate

Chromosome aberrations induced in spermatozoa to late spermatocytes following treatment with methyl methanesulfonate (MMS) were examined at the first-cleavage metaphase of fertilized eggs in an attempt to clarify the mechanism of chromosomal damage in postmeiotic germ cells. A high frequency of chromosome aberrations was induced in early spermatozoa to mid spermatids, while few chromosome aberrations were observed in early spermatids to late spermatocytes. 3-Aminobenzamide (3AB) potentiated the effect of MMS-induced chromosome aberrations in spermatozoa to mid spermatids indicating that a large amount of DNA lesions produced at these stages during spermatogenesis were not repaired prior to fertilization of the oocytes. Furthermore, from the cell-cycle analysis of the repair capacity in the fertilized egg it became clear that the lesions which remained in sperm until fertilization could be divided into 2 types: (1) DNA-strand breaks induced by stress in the chromatin structure produced by protamine alkylation; these lesions were converted to chromosome-type aberrations by 3AB treatment of the eggs during G1 phase; and (2) alkylated DNA which produces apurinic or apyrimidinic sites, of which there were a significant number mainly converted to chromatid exchanges by 3AB treatment of the eggs during S phase. This type of lesion appears to be constantly induced through all spermiogenic stages in contrast to the former type of lesion which is induced specifically during the stage of protamine maturation.

On the localization of mitomycin C-induced aberrations in normal human and Fanconi's anaemia cells

This paper summarizes the results of a series of experiments with primary cultures of normal human fibroblasts and lymphocytes designed to investigate chromatid aberration 'break-point' localization after a 1-h pulse of mitomycin C. For discontinuities and interchanges, 60-70% of the inferred 'break-points' were localized to defined paracentric heterochromatin and the centromeric regions (i.e. approximately 21% by length of the normal karyotype), irrespective of 'dose', aberration frequency, sample time or cycle sub-phase as determined by replication banding. Chromatid intrachanges are non-(or negatively) localized because of an inescapable scoring bias. SCE in fibroblasts show no such localization. Cells from a number of Fanconi's anaemia subjects were examined. In poorly growing cultures, localization was as high as in normal cells but in vigorous cultures localization was reduced to approximately 30%. It is suggested that the enhanced aberration sensitivity of this syndrome could arise because non-localized aberrations, usually eliminated before division in normal cells, are allowed to reach mitosis in FA cells.

Cytomorphologic evaluation of the neoplastic potential of 28 cell culture lines by a panel of diagnostic cytopathologists

A panel of 7 diagnostic cytopathologists, i.e., physicians trained to diagnose the malignant potential of human cells in Papanicolaou-stained smears, was asked to evaluate two sets of microscope slides of stained coverslip preparations of 28 cell culture lines, 15 of which were neoplastic. Slide Set I consisted of 13 pairs of cell lines, one member of each pair being nontumorigenic and the other tumorigenic; the lines were of mouse (9 pairs), rat (3 pairs), and human (1 pair) origin. Slide Set II contained 4 human lines: one lung cancer, one melanoma, and two fibroblast lines. Of a total of 114 diagnostic decisions by the panel, 88 were correct (66/86, 77%) in choosing which member of a pair was neoplastic and 22 were correct (22/28, 79%) in choosing whether a given individual human line was or was not neoplastic. Two members of the panel were correct more frequently, with 16/17 (94%) correct diagnoses, each. Five nuclear morphologic criteria of malignancy used by cytopathologists were prominent in the tumorigenic lines: altered chromatin pattern characterized by increasing size of chromatin granules and chromatin clumping, sharp angularity of large nucleolar and/or chromocenter borders with spicule formation (pointed projection), irregular parachromatin clearing (increase in the clarity of the clear spaces between chromatin threads, granules and clumps), uneven thickness of chromatin at the nuclear border, and variability in nuclear size and shape from cell to cell. These markers of neoplastic transformation, when added to those previously reported, should increase overall accuracy in the diagnosis of neoplastic transformation of mammalian cells in culture.

Induction of chromosome aberrations by chemical mutagens in neural ganglia of Drosophila melanogaster

Chromatid aberrations induced by various concentrations of bleomycin, cyclophosphamide and mitomycin C were analyzed in neural ganglia of third-instar larvae of Drosophila melanogaster. A clear dose response was observed with increasing dose after treatment with bleomycin and mitomycin C, whereas no effect was observed after treatment with cyclophosphamide. A comparison with published data for the induction of sex-linked recessive lethals showed that, at least for the 3 drugs tested, the use of both tests eliminates false negatives and might comprise a useful procedure for testing mutagenicity in Drosophila.

Non-random distribution of aberrations and identification with C- and G-bandings of the position of breakage points on Muntjac chromosomes induced by mitomycin c, bromodeoxyuridine and hydroxylamine

The analysis of chromosomes from muntjac after treatment of its lymphocyte cultures with 3 chemical mutagens having different base-pair affinities and modes of action, namely mitomycin C (MC), 5-bromodeoxyuridine (BUdR) and hydroxylamine hydrochloride (HA), with G- and C-band staining displayed non-random distribution of chemically specific damage points on them. The randomness of the involvement of each site on the chromosomes were examined by assuming an expected value calculated on the basis of its relative mitotic length. The observation revealed that a large fraction of MC-induced aberrations was preferentially located in the C-band positive constitutive heterochromatin, especially in the long "neck-like" centromeric region of the X-chromosome. On the chromosomal arms, the light G-bands were involved in aberrations either in proportion to or higher than that expected. When the cells were treated with BUdR, the dark G-bands on all the chromosomes of the complement were the preferred sites, displaying statistically significant higher numbers of aberrations. A single "hot-spot" for induced damage on 1 mid-q was also recorded. HA induced a very high frequency of damage in the secondary constriction regions of the chromosome pairs 1, X and Y2, and the frequency was slightly lower than this in the centromeres of 1, 2 and X chromosomes. The observation of specific distribution of damage points induced by the 3 chemicals lead to the suggestion that, though the effect of a chemical on chromosome segments depends on several factors, each being partially responsible for the end result, it is perhaps primarily depended by the chemical's base-pair affinity and mode of action.

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.

Factors affecting the frequency and pattern of mitomycin C-induced chromatid exchanges

Data are reported on the frequency and pattern of Mitomycin C-induced exchanges in lymphocytes of a marsupial hybrid Macropus rufogriseus female x Wallabia bicolor male and in four individuals, two male and two female, of M. rufogriseus. In the hybrid, exchanges are found almost exclusively between the M. rufogriseus chromosomes and the breakpoints are in the C-banded pericentromeric regions. In the four M. rufogriseus animals the data show no evidence for preferential exchanges between homologues and so provide no justification for postulating somatic pairing between homologues. Comparisons of the frequency of exchanges between non-homologous chromosomes in the hybrid and in the four individuals of M. rufogriseus show that there is no difference in the frequency of exchange for any given chromosome but that the specific pattern of exchanges may differ between animals. The data are interpreted as providing evidence for different types of heterochromatin and possible bases for these differences are discussed.

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.

Aberration induction by mitomycin C in early primary spermatocytes of mice

MC is well known to induce dominant lethal mutations in mouse spermatocytes. Tests were done to determine whether chromosomal aberrations could be identified in spermatocytes as being responsible for the dominant lethal effects. Male mice were treated with single doses of MC during DNA synthesis preceding meiosis and during early prophase of meiosis. Simultaneous labeling was performed to identify cells that were in S-phase during the time of treatment. Diakineses-metaphases I were analyzed for the occurrence of univalents, gaps, fragments and rearrangements. The frequencies of cells with aberrations increased with dose and time after treatment. Maximal values were obtained after 12 days, indicating that MC was most effective in cells undergoing DNA replication. 95% of these cells were labeled. The majority of aberrant cells contained one or more fragments. These cells will lead to dominant lethality of the zygotes after fertilization. Cells with rearrangements occurred 11 and 12 days after treatment. These cells can develop into sperm carrying a reciprocal translocation which would then give rise to semi-sterile progeny after fertilization. Further investigations are needed to study the transmission of rearrangements observed in primary spermatocytes.

Increased sister chromatid exchange events in the human late replicating X

Human female blood cultures were labeled with BrdU for detecting sister chromatid exchanges (SCEs) by the Hoechst 33258 fluorescence technique. Late labeling with 3H-thymidine and autoradiography allowed the identification of the late replicating X. The mean number of SCEs in the cells was 13. The isopycnotic X showed an exchange frequency according to its relative length in the karyotype; in the late replicating X a doubled number of SCE events was observed.

Giemsa banding and heterochromatin distribution in Ornithogalum

In Ornithogalum virens, following Giemsa staining, the mitotic chromosomes revealed distinct C-banding pattern. Interphase nucleus also showed 12 chromocentres corresponding to the number of C-bands. Based on the measurements of C-bands, about 13.5% of the genome is heterochromatic.

Heterochromatin, satellite DNA, and cell function. Structural DNA of eucaryotes may support and protect genes and aid in speciation

With the assumption that a portion that comprises some 10 percent of the genomes in higher organisms cannot be without a raison d'être, an extensive review led us to conclude that a certain amount of constitutive heterochromatin is essential in multicellular organisms at two levels of organization, chromosomal and nuclear. At the chromosomal level, constitutive heterochromatin is present around vital areas within the chromosomes. Around the centromeres, for example, heterochromatin is believed to confer protection and strength to the centromeric chromatin. Around secondary constrictions, heterochromatic blocks may ensure against evolutionary change of ribosomal cistrons by decreasing the frequency of crossing-over in these cistrons in meiosis and absorbing the effects of mutagenic agents. During meiosis heterochromatin may aid in the initial alignment of chromosomes prior to synapsis and may facilitate speciation by allowing chromosomal rearrangement and providing, through the species specificity of its DNA, barriers against cross-fertilization. At the nuclear level of organization, constitutive heterochromatin may help maintain the proper spatial relationships necessary for the efficient operation of the cell through the stages of mitosis and meiosis. In the unicellular procaryotes, the presence of a small amount of genetic information in one chromosome obviates the need for constitutive heterochromatin and a nuclear membrane. At higher levels of organization, with an increase in the size of the genome and with evolution of cellular and sexual differentiation, the need for compartmentalization and structural components in the nucleus became imminent. The portion of the genome that was concerned with synthesis of ribosomal RNA was enlarged and localized in specific chromosomes, and the centromere became part of each chromosome when the mitotic spindle was developed in evolution. Concomitant with these changes in the genome, repetitive sequences in the form of constitutive heterochromatin appeared, probably as a result of large-scale duplication. The repetitive DNA's were kept through natural selection because of their importance in preserving these vital regions and in maintaining the structural and functional integrity of the nucleus. The association of satellite (or highly repetitive) DNA with constitutive heterochromatin is understandable, since it stresses the importance of the structural rather than transcriptional roles of these entities. Nuclear satellite DNA's have one property in common despite their species specificity, namely heterochromatization. In this sense the apparent species specificity of satellite DNA may be the result of natural selection for duplicated short polynucleotide segments that are nontranscriptional and can be utilized in specific structural roles.