The phenomenon of premature chromosome condensation: its relevance to basic and applied research

ASAR15, A cis-acting locus that controls chromosome-wide replication timing and stability of human chromosome 15

DNA replication initiates at multiple sites along each mammalian chromosome at different times during each S phase, following a temporal replication program. We have used a Cre/loxP-based strategy to identify cis-acting elements that control this replication-timing program on individual human chromosomes. In this report, we show that rearrangements at a complex locus at chromosome 15q24.3 result in delayed replication and structural instability of human chromosome 15. Characterization of this locus identified long, RNA transcripts that are retained in the nucleus and form a “cloud” on one homolog of chromosome 15. We also found that this locus displays asynchronous replication that is coordinated with other random monoallelic genes on chromosome 15. We have named this locus ASynchronous replication and Autosomal RNA on chromosome 15, or ASAR15. Previously, we found that disruption of the ASAR6 lincRNA gene results in delayed replication, delayed mitotic condensation and structural instability of human chromosome 6. Previous studies in the mouse found that deletion of the Xist gene, from the X chromosome in adult somatic cells, results in a delayed replication and instability phenotype that is indistinguishable from the phenotype caused by disruption of either ASAR6 or ASAR15. In addition, delayed replication and chromosome instability were detected following structural rearrangement of many different human or mouse chromosomes. These observations suggest that all mammalian chromosomes contain similar cis-acting loci. Thus, under this scenario, all mammalian chromosomes contain four distinct types of essential cis-acting elements: origins, telomeres, centromeres and “inactivation/stability centers”, all functioning to promote proper replication, segregation and structural stability of each chromosome.

Mammalian cells replicate their DNA along each chromosome during a precise temporal replication program. In this report, we used a novel “chromosome-engineering” strategy to identify a DNA element that controls this replication-timing program of human chromosome 15. Characterization of this element indicated that it encodes large non-protein-coding RNAs that are retained in the nucleus and form a “cloud” on one copy of chromosome 15. Previously, we found that structural rearrangements of a similar element on human chromosome 6 causes delayed replication and structural instability of chromosome 6. Mammalian chromosomes are known to contain three distinct types of essential DNA elements that promote proper chromosome function. Thus, every chromosome contains: 1) origins of replication, which are responsible for proper initiation of DNA synthesis; 2) centromeres, which are responsible for proper chromosome separation during cell division; and 3) telomeres, which are responsible for replication and protection of the ends of linear chromosomes. Our work supports a model in which all mammalian chromosomes contain a fourth type of essential DNA element, the “inactivation/stability center”, which is responsible for proper DNA replication timing and structural stability of each chromosome.

Chromoanagenesis and cancer: mechanisms and consequences of localized, complex chromosomal rearrangements

Next-generation sequencing of DNA from human tumors or individuals with developmental abnormalities has led to the discovery of a process we term chromoanagenesis, in which large numbers of complex rearrangements occur at one or a few chromosomal loci in a single catastrophic event. Two mechanisms underlie these rearrangements, both of which can be facilitated by a mitotic chromosome segregation error to produce a micronucleus containing the chromosome to undergo rearrangement. In the first, chromosome shattering (chromothripsis) is produced by mitotic entry before completion of DNA replication within the micronucleus, with a failure to disassemble the micronuclear envelope encapsulating the chromosomal fragments for random reassembly in the subsequent interphase. Alternatively, locally defective DNA replication initiates serial, microhomology-mediated template switching (chromoanasynthesis) that produces local rearrangements with altered gene copy numbers. Complex rearrangements are present in a broad spectrum of tumors and in individuals with congenital or developmental defects, highlighting the impact of chromoanagenesis on human disease.

Mammalian chromosomes contain cis-acting elements that control replication timing, mitotic condensation, and stability of entire chromosomes

Recent studies indicate that mammalian chromosomes contain discrete cis-acting loci that control replication timing, mitotic condensation, and stability of entire chromosomes. Disruption of the large non-coding RNA gene ASAR6 results in late replication, an under-condensed appearance during mitosis, and structural instability of human chromosome 6. Similarly, disruption of the mouse Xist gene in adult somatic cells results in a late replication and instability phenotype on the X chromosome. ASAR6 shares many characteristics with Xist, including random mono-allelic expression and asynchronous replication timing. Additional "chromosome engineering" studies indicate that certain chromosome rearrangements affecting many different chromosomes display this abnormal replication and instability phenotype. These observations suggest that all mammalian chromosomes contain "inactivation/stability centers" that control proper replication, condensation, and stability of individual chromosomes. Therefore, mammalian chromosomes contain four types of cis-acting elements, origins, telomeres, centromeres, and "inactivation/stability centers", all functioning to ensure proper replication, condensation, segregation, and stability of individual chromosomes.

Comparison of mitotic cell death by chromosome fragmentation to premature chromosome condensation

Mitotic cell death is an important form of cell death, particularly in cancer. Chromosome fragmentation is a major form of mitotic cell death which is identifiable during common cytogenetic analysis by its unique phenotype of progressively degraded chromosomes. This morphology however, can appear similar to the morphology of premature chromosome condensation (PCC) and thus, PCC has been at times confused with chromosome fragmentation. In this analysis the phenomena of chromosome fragmentation and PCC are reviewed and their similarities and differences are discussed in order to facilitate differentiation of the similar morphologies. Furthermore, chromosome pulverization, which has been used almost synonymously with PCC, is re-examined. Interestingly, many past reports of chromosome pulverization are identified here as chromosome fragmentation and not PCC. These reports describe broad ranging mechanisms of pulverization induction and agree with recent evidence showing chromosome fragmentation is a cellular response to stress. Finally, biological aspects of chromosome fragmentation are discussed, including its application as one form of non-clonal chromosome aberration (NCCA), the driving force of cancer evolution.

Chromosome shattering: a mitotic catastrophe due to chromosome condensation failure

Chromosome shattering has been described as a special form of mitotic catastrophe, which occurs in cells with unrepaired DNA damage. The shattered chromosome phenotype was detected after application of a methanol/acetic acid (MAA) fixation protocol routinely used for the preparation of metaphase spreads. The corresponding phenotype in the living cell and the mechanism leading to this mitotic catastrophe have remained speculative so far. In the present study, we used V79 Chinese hamster cells, stably transfected with histone H2BmRFP for live-cell observations, and induced generalized chromosome shattering (GCS) by the synergistic effect of UV irradiation and caffeine posttreatment. We demonstrate that GCS can be derived from abnormal mitotic cells with a parachute-like chromatin configuration (PALCC) consisting of a bulky chromatin mass and extended chromatin fibers that tether centromeres at a remote, yet normally shaped spindle apparatus. This result hints at a chromosome condensation failure, yielding a "shattered" chromosome complement after MAA fixation. Live mitotic cells with PALCCs proceeded to interphase within a period similar to normal mitotic cells but did not divide. Instead they formed cells with highly abnormal nuclear configurations subject to apoptosis after several hours. We propose a factor depletion model where a limited pool of proteins is involved both in DNA repair and chromatin condensation. Chromosome condensation failure occurs when this pool becomes depleted.

Drug-induced premature chromosome condensation (PCC) protocols: cytogenetic approaches in mitotic chromosome and interphase chromatin

Chromosome analysis is a fundamental technique for cytogenetic studies. Chromosomes are conventionally prepared from mitotic cells arrested by colcemid block protocol. However, obtaining the mitotic chromosomes is often hampered under several circumstances. As a result, cytogenetic analysis will be sometimes difficult or even impossible in such cases. Premature chromosome condensation (PCC) is an alternative method that has proved to be a unique and useful way in chromosome analysis. Usually, PCC has been achieved following cell fusion mediated either by fusogenic viruses or by polyethylene glycol (cell-fusion PCC), but the cell-fusion PCC has several drawbacks. The novel drug-induced PCC using protein phosphatase inhibitors was introduced about 10 years ago. This method is much simple and easy even than the conventional mitotic chromosome preparation using colcemid block protocol and obtained PCC index (equivalent to mitotic index for metaphase chromosome) is much higher. Furthermore, this method allows the interphase chromatin to be condensed and visualized like mitotic chromosomes, thus opened the way for chromosome analysis not only in metaphase chromosomes but also in interphase chromatin. The drug-induced PCC has therefore proven the usefulness in cytogenetics and other cell biology fields.

Visualizing the dynamics of chromosome structure formation coupled with DNA replication

A basic question of cell biology is how DNA folds to chromosome. Numbers of examples have suggested the involvement of DNA replication in chromosome structure formation. To visualize and identify the dynamics of chromosome structure formation and to elucidate the involvement of DNA replication in chromosome construction, Cy3-2'-deoxyuridine-5'-triphosphate direct-labeled active replicating DNA was observed in prematurely condensed chromosomes (PCCs) under a confocal scanning microscope utilized with drug-induced premature chromosome condensation (PCC) technique that facilitates the visualization of interphase chromatin as condensed chromosome form. S-phase PCCs revealed clearly the drastic dynamics of chromosome formation that transits during S-phase from a 'cloudy nebula' to numerous numbers of 'beads on a string' and finally to 'striped arrays of banding structured chromosome' along with the progress of DNA replication. The number, distribution, and shape of replication foci were also measured in individual subphases of S-phase more precisely than reported previously; maximally, approximately 1,400 foci of 0.35 microm average radius size were scored at the beginning of the S-phase, and the number reduced to approximately 100 at the end of the S-phase. Drug-induced PCC clearly provided the new insight that eukaryote DNA replication is tightly coupled with the chromosome condensation/compaction for the construction of the higher-ordered structure of the eukaryote chromosome.

Engineering translocations with delayed replication: evidence for cis control of chromosome replication timing

Certain chromosome rearrangements, found in cancer cells or in cells exposed to ionizing radiation, exhibit a chromosome-wide delay in replication timing (DRT) that is associated with a delay in mitotic chromosome condensation (DMC). We have developed a chromosome engineering strategy that allows the generation of chromosomes with this DRT/DMC phenotype. We found that approximately 10% of inter-chromosomal translocations induced by two distinct mechanisms, site-specific recombination mediated by Cre or non-homologous end joining of DNA double-strand breaks induced by I-Sce1, result in DRT/DMC. Furthermore, on certain balanced translocations only one of the derivative chromosomes displays the phenotype. Finally, we show that the engineered DRT/DMC chromosomes acquire gross chromosomal rearrangements at an increased rate when compared with non-DRT/DMC chromosomes. These results indicate that the DRT/DMC phenotype is not the result of a stochastic process that could occur at any translocation breakpoint or as an epigenetic response to chromosome damage. Instead, our data indicate that the replication timing of certain derivative chromosomes is regulated by a cis-acting mechanism that delays both initiation and completion of DNA synthesis along the entire length of the chromosome. Because chromosomes with DRT/DMC are common in tumor cells and in cells exposed to ionizing radiation, we propose that DRT/DMC represents a common mechanism responsible for the genomic instability found in cancer cells and for the persistent chromosomal instability associated with cells exposed to ionizing radiation.

Ionizing Radiation Induces Frequent Translocations with Delayed Replication and Condensation

Certain chromosome rearrangements display a significant delay in replication timing that is associated with a delay in mitotic chromosome condensation. Chromosomes with delay in replication timing/delay in mitotic chromosome condensation participate in frequent secondary rearrangements, indicating that cells with delay in replication timing/delay in mitotic chromosome condensation display chromosomal instability. In this report, we show that exposing cell lines or primary blood lymphocytes to ionizing radiation results in chromosomes with the delay in replication timing/delay in mitotic chromosome condensation phenotype, and that the delay in replication timing/delay in mitotic chromosome condensation phenotype occurs predominantly on chromosome translocations. In addition, exposing mice to ionizing radiation also induces cells with delay in replication timing/delay in mitotic chromosome condensation chromosomes that persist for as long as 2 years. Cells containing delay in replication timing/delay in mitotic chromosome condensation chromosomes frequently display hyperdiploid karyotypes, indicating that delay in replication timing/delay in mitotic chromosome condensation is associated with aneuploidy. Finally, using a chromosome engineering strategy, we show that only a subset of chromosome translocations displays delay in replication timing/delay in mitotic chromosome condensation. Our results indicate that specific chromosome rearrangements result in the generation of the delay in replication timing/delay in mitotic chromosome condensation phenotype and that this phenotype occurs frequently in cells exposed to ionizing radiation both in vitro and in vivo.

Premature chromosome condensation in humans associated with microcephaly and mental retardation: a novel autosomal recessive condition

We report a novel autosomal recessive disorder characterized by premature chromosome condensation in the early G2 phase. It was observed in two siblings, from consanguineous parents, affected with microcephaly, growth retardation, and severe mental retardation. Chromosome analysis showed a high frequency of prophase-like cells (>10%) in lymphocytes, fibroblasts, and lymphoblast cell lines with an otherwise normal karyotype. (3)H-thymidine-pulse labeling and autoradiography showed that, 2 h after the pulse, 28%-35% of the prophases were labeled, compared with 9%-11% in healthy control subjects, indicating that the phenomenon is due to premature chromosome condensation. Flow cytometry studies demonstrate that the entire cell cycle is not prolonged, compared with that in healthy control subjects, and compartment sizes did not differ from those in healthy control subjects. No increased reaction of the cells to X-irradiation or treatments with the clastogens bleomycin and mitomycin C was observed, in contrast to results in the cell-cycle mutants ataxia telangiectasia and Fanconi anemia. The rates of sister chromatid exchanges and the mitotic nondisjunction rates were inconspicuous. Premature entry of cells into mitosis suggests that a gene involved in cell-cycle regulation is mutated in these siblings.

A model for chromosome structure during the mitotic and meiotic cell cycles

The chromosome scaffold model in which loops of chromatin are attached to a central, coiled chromosome core (scaffold) is the current paradigm for chromosome structure. Here we present a modified version of the chromosome scaffold model to describe chromosome structure and behavior through the mitotic and meiotic cell cycles. We suggest that a salient feature of chromosome structure is established during DNA replication when sister loops of DNA extend in opposite directions from replication sites on nuclear matrix strands. This orientation is maintained into prophase when the nuclear matrix strand is converted into two closely associated sister chromatid cores with sister DNA loops extending in opposite directions. We propose that chromatid cores are contractile and show, using a physical model, that contraction of cores during late prophase can result in coiled chromatids. Coiling accounts for the majority of chromosome shortening that is needed to separate sister chromatids within the confines of a cell. In early prophase I of meiosis, the orientation of sister DNA loops in opposite directions from axial elements assures that DNA loops interact preferentially with homologous DNA loops rather than with sister DNA loops. In this context, we propose a bar code model for homologous presynaptic chromosome alignment that involves weak paranemic interactions of homologous DNA loops. Opposite orientation of sister loops also suppresses crossing over between sister chromatids in favor of crossing over between homologous non-sister chromatids. After crossing over is completed in pachytene and the synaptonemal complex breaks down in early diplotene (= diffuse stage), new contractile cores are laid down along each chromatid. These chromatid cores are comparable to the chromatid cores in mitotic prophase chromosomes. As an aside, we propose that leptotene through early diplotene represent the 'missing' G2 period of the premeiotic interphase. The new chromosome cores, along with sister chromatid cohesion, stabilize chiasmata. Contraction of cores in late diplotene causes chromosomes to coil in a configuration that encourages subsequent syntelic orientation of sister kinetochores and amphitelic orientation of homologous kinetochore pairs on the spindle at metaphase I.

The role of DNA damage in cellular aging: is it time for a reassessment?

There is now evidence that the immediate cause of the loss of proliferative capacity in senescent cells is mediated by a specific inhibitor. If this tentative interpretation is correct, the next hurdle will be to determine mechanism(s) that regulate this putative senescence cell inhibitor that would, in effect, be the determinant of proliferative life span. One previously proposed hypothesis predicts that the decline of replicative activity is analogous to a checkpoint response to accumulated chromosomal damage (Rosenberger et al., 1991). Advances in our basic understanding of the nature of DNA damage, DNA repair mechanisms, and the response of eukaryotic cells to accumulated DNA damage provide a solid rationale for a reassessment of the causal role of the accumulation of chromosomal damage in cell senescence in vitro.

Mapping and chromosome analysis: the potential of fluorescence in situ hybridization

Fluorescence in situ hybridization (FISH) is a method widely used for the delineation of chromosomal DNA. FISH is applied in many areas of basic research as well as in clinical cytogenetics. In this review important technical improvements as well as the various applications of this method are summarized. In the first part different labeling and detection procedures are described and the potential of various kinds of probes are discussed. Recent developments in optical instrumentation and digital imaging procedures are outlined in the second part. The following important applications of FISH are discussed: (a) new strategies for high resolution mapping of DNA sequences; (b) detection of chromosomal aberrations in clinical material; (c) techniques allowing the simultaneous detection of numerous probes by multiple color FISH; and (d) the new approach of comparative genomic hybridization, allowing a rapid and comprehensive analysis of chromosomal imbalances in cell populations, which is particularly useful for the cytogenetic analysis of tumor samples.

Checkpoint controls that couple mitosis to completion of DNA replication

Cells treated with inhibitors of DNA synthesis do not normally enter mitosis. Incompletely replicated DNA apparently activates a regulatory mechanism that prevents activation of the mitotic inducer M-phase kinase by controlling the dephosphorylation of a critical tyrosine residue in the active site of the kinase. The control system may also target a second mitotic inducer, possibly the NIMA protein kinase. Unreplicated DNA may be detected and signalled by a complex of RCC1, a DNA-binding protein, and Ran, a Ras-related protein. This article reviews these recent developments and discusses the possibility that the control system also operates in the normal cell cycle, to ensure that mitosis strictly follows S phase.

A model of chromatin-dependent DNA replication sequences based on the decondensation units hypothesis

A model of chromatin-dependent DNA replication sequences was developed on the previously reported "decondensation units" hypothesis and its kinetic properties were examined by way of calculating various numerical indices using a Monte Carlo procedure. The model has much in common with the previous one but a fundamental difference is that the unit is assumed to consist of linearly arranged H-, D-, A- and S-zones each containing genes of different functional categories which are called H-, D-, A- and S-genes, respectively. The units are decondensed by the action of D-factors, i.e. decondensation factors, from H-zone to the end of S-zone and the genes in decondensed regions release signals to produce housekeeping enzymes, D-factors, A-factors and S-factors. These products are stored and at the same time degraded. A-factors activate replication origins in the decondensed regions and S-factors induce DNA synthesis at the activated origins. Replicated DNA is recondensed and gene activities are shut down in the recondensed chromatin. The factors are produced under the control of chromosome cycle and in turn affect chromosomes. Thus, dual control mechanism operates as Mazia and Prescott have argued. Biochemical and cytogenetic basis of this model was reviewed briefly and some results of simulation presented which include DNA synthesis rate vs. DNA content relationships. An outstanding characteristic of the model is the constancy of cellular state in A-subphase located in the late G1.

Transcriptional activity of constitutive heterochromatin in the mammal Microtus agrestis (Rodentia, Cricetidae)

Two cytological approaches were applied to the analysis of genetic activity in euchromatin and heterochromatin in the European field vole Microtus agrestis. The first is based on the transcriptional activity of prematurely condensed chromosomes, the second on in situ hybridization of labeled cellular RNA to metaphase chromosomes. The results show that in cultivated fibroblasts, the transcriptional activity of constitutive heterochromatin is in the same order of magnitude as that of euchromatin; its chromosomal pattern is nonrandom. It is concluded that--as has already been shown for insects and amphibians--also in mammals, constitutive heterochromatin is transcribed in its decondensed state but is genetically inactive when forming chromocenters at interphase.

Cytogenetic findings in a case of anaplastic carcinoma of the pancreas

A cytogenetic study was performed on a short-term culture of a biopsy from a primary anaplastic carcinoma of the pancreas. The modal chromosome number was 60. Six numerical clonal anomalies involving chromosomes #2, #6, #7, #10, #15, and #16 were found, and marker chromosomes involving #1, #3, #5, #8, #11, #12, #13, #15, #16, #18, #20, #22, and X. Premature chromosome condensation (PCC) was observed with a high frequency. The results are discussed with reference to the scarce literature on chromosome changes in pancreatic cancer.

A model for the spatio-temporal organization of DNA replication in mammalian cells

The spatio-temporal organization of chromosomal DNA replication was analyzed using a model based on a "DNA unit" (or decondensation unit) hypothesis. The model is an extension of the fork movement theory of Huberman & Riggs (1968) and can account for a partially deterministic and partially stochastic order of DNA replication in chromosomes. It presumes that each chromosome is composed of DNA units that are arranged in sequence and that are replicated in parallel. A deterministic wave of chromatin decondensation propagates along the DNA unit continuously and progressively providing a field for the random activation of replication origin. Assignment of replication times to DNA compartments by a Monte Carlo method was programmed based on the model and the program was used to stimulate DNA synthesis rate curves that can be measured by the method of Dolbeare et al. (1983, 1985). The shape of the curve is shown to constrain possible parameter values of the model, which include the rate of fork movement, the fraction of chromatin that is decondensed at the start of S-phase, the initial number of origins activated, the rate at which new origins are activated, etc. The chromosomal organization that controls the molecular level of DNA replication is briefly reviewed and its relevance to the model is also discussed.

A comparison between short-term evolution of micronuclei induced by X-rays and colchicine in root tips of Vicia faba

The short-term evolution of micronuclei derived from acentric fragments and whole chromosomes was studied in root tips of Vicia faba. Micronuclei were induced by X-rays (30 cGy and 120 cGy) and colchicine (10(-5) M and 3 X 10(-4) M). Frequencies of chromosome breakage or loss of micronuclei in interphase and mitotic cells were studied. The DNA content of micronuclei in interphase cells was also measured. Micronuclei derived from whole chromosome showed a higher probability to survive and to undergo mitotic condensation in synchrony with main nuclei than micronuclei derived from an acentric fragment. PCC (Premature Chromosome Condensation) was not observed for both types of micronuclei in Vicia faba, in contrast to the ones reported in mammalian cells in culture.

Interactions between metaphase and interphase factors in heterokaryons produced by fusion of mouse oocytes and zygotes

The cytoplasmic factor responsible for chromosome condensation was introduced into mouse zygotes at different times after fertilization by fusion of the zygotes with metaphase I oocytes. In 72% of heterokaryons obtained after fusion of early zygotes (14-18 hr post-human chorionic gonadotrophin (HCG) with oocytes, the male and female pronuclei of the zygote decondensed. At the same time, the oocyte chromosomes became enclosed in a nuclear envelope and decondensed to an interphase state. However, in the rest of the heterokaryons, the chromatin of the pronuclei condensed to metaphase chromosomes, thus resulting in three sets of chromosomes. Fusion of zygotes that had begun DNA synthesis (20-22 hr post-HCG) with oocytes induced chromosome condensation of the pronuclei in 76% of the cases. In some heterokaryons, however, the oocyte chromosome decondensed to an interphase state similar to the zygote pronuclei. Fusion between late zygotes (27-29 hr post-HCG) with oocytes resulted in chromosome condensation of the pronuclei in all heterokaryons. On the basis of these results, the formation of the pronuclei and their progression toward mitosis in the zygote may be explained by changing levels of a metaphase factor in the cell, or by a balance between interphase and metaphase factors.

Premature chromosome condensation--studies on human metastatic carcinoma cells

Utilizing the phenomenon of premature chromosome condensation (PCC) studies were carried out on interphase chromatin of metastatic cells from 52 cancerous effusions obtained from 45 patients presenting with various solid carcinomas. A highly individual pattern of distribution of the various interphase stages was detected, reflecting the heterogeneity of human solid tumors in an advanced stage. Nevertheless a variety of clinical, biologic, and technical factors were examined for their possible influence on these PCC patterns. The duration in culture was one of the influencing factors, as were the time lapse between the first diagnosis and the sampling of the respective effusion, or the nature of cytostatic therapy. Cytogenetic equivalents of gene amplification, as represented by "double minutes", could be found in the prematurely interphase chromatin of 35 of the 52 effusions. G1-PCC proved to be most reliable with regard to screening of double minutes. In addition, an adequate quality of Giemsa banding was achieved in PCC of 21 out of 24 effusions yielding a sufficient number of well-spread PCC. In six of these 21 cases PCC was superior to metaphase analysis in obtaining karyotypes, while the same was true for 14 of the 52 effusions screened for double minutes. Thus the PCC technique was shown to be an indispensable additional source of cytogenetic information in cells of human solid tumors.

PCC technique reveals severe chromatin lesions and repair in G2-arrested cells after alpha irradiation

Exponentially growing Chinese hamster V79 cells were exposed to 4 MeV alpha particles emitted by an americium-241 source. The chromatin of cells arrested in G2 by alpha irradiation is severely damaged, though all cells were still capable to condensate their chromatin after fusion with mitotic cells. In addition to the common types of aberrations (breaks, gaps, dicentrics and exchanges) cells were found possessing one or more chromosomes with long stretches of undercondensed chromatin. Repair of these lesions was indicated by site-specific unscheduled DNA synthesis and by the observation that condensation of these regions improved during G2 arrest.

Isolation by a replica-plating technique of Chinese hamster temperature-sensitive cell cycle mutants

Isolation of a wide variety of temperature-sensitive (ts) cell cycle mutants in mammalian cells has previously proved to be a very difficult task. The various procedures used for the isolation of such mutants included a mutant enrichment step based on exposure of the cells to the restrictive temperatures in order to kill the growing wild-type cells with agents that kill DNA-synthesizing cells. Hence, these methods favored the isolation of ts mutants that do not lose viability rapidly at the restrictive temperatures. We have treated cells of the Chinese hamster established cell line E36 with the mutagen ethyl-methane-sulfonate (EMS) and used a replicating technique that we developed to screen the ts mutants for growth. This technique enabled us to recover all its mutants for growth including the ts cell cycle mutants. Screening of the ts cell cycle mutants among the ts mutants for growth was performed by the flow microfluorimetry technique and the premature chromosome condensation technique. Our results show that 1.3% of the survivors of the mutagenic treatment are ts mutants for growth. Six of 84 ts mutants analyzed were found to be ts cell cycle mutants. They include ts mutants arrested in phases G1, S, and G2. Many of the ts mutants for growth including the ts cell cycle mutants arrested in S and G2 lose viability very fast when incubated at the restrictive temperature. As a consequence they could not have been isolated by any method that includes a mutant enrichment step based on the exposure of the cells to the restrictive temperature.

DNA content of cells with generalized chromosome shattering induced by ultraviolet light plus caffeine

Asynchronously growing Chinese hamster cells (M3-1) were UV-irradiated (lambda = 254 nm) and then incubated with/without caffeine (2 mM) for 20 h. Microscopic evaluation of metaphase spreads revealed that after UV-irradiation alone (5.0 J/m2) appearing fragmented and/or pulverized ('GCS-like' cells; GCS, Generalized Chromosome Shattering) was very low while it was high following the combined treatment. Cytogenic and flow cytometric analysis of cells obtained by mechanical shaking cultures treated with UV and caffeine indicated that 'GCS-like' cells have the same DNA content as untreated cells in G2 phase and mitosis.

The central localization of the small and early replicating chromosomes in human diploid metaphase figures

Centromere-center distances are analyzed in 700 metaphase plates, which belong to four different samples. The descriptive analysis of the chromosome distribution shows that smaller, earlier replicating, genet-dense chromosomes are preferentially found near the metaphase plate center, surrounded by longer chromosomes which finish their replication rather late during S phase. This general pattern is highly constant in diploid metaphase samples and independent of sex, culture time, and number of individuals used in the sample. There is accumulating evidence that this overall distribution is not the result of technical artifacts. The metaphase plate data are complementary to the concept of an interphase nucleus structure in which late-replicating, genetically less active chromatin is accumulated at the periphery of the nucleus, while other, earlier replicating chromatin is connected with the intranuclear matrix. Although the currently available data should not be overinterpreted, an extension of the "bodyguard" hypothesis, which was suggested for C heterochromatin, provides a functional interpretation for these data: The peripherally localized, late-replicating genetic material protects the centrally localized euchromatin against mutagens, clastogens, and maybe also against viruses.

Induction of chromosome shattering by ultraviolet irradiation and caffeine: comparison of whole-cell and partial-cell irradiation

Synchronized and asynchronously growing cells of a V79 sub-line of the Chinese hamster were either whole-cell irradiated ( gamma, 254 nm) or laser-UV-microirradiated ( gamma, 257 nm). Post-incubation with caffeine (1-2 mM) often resulted in chromosome shattering, which was a rare event in the absence of this compound. In experiments with caffeine, the following results were obtained. Shattering of all the chromosomes of a cell (generalized chromosome shattering, GCS) was induced by whole-cell irradiation at the first post-irradiation mitosis when the UV fluence exceeded a "threshold" value in the sensitive phases of the cell cycle (G1 and S). GCS was also induced by laser-UV-microirradiation of a small part of the nucleus in G1 or S whereas microirradiation of cytoplasm beside the nucleus was not effective. An upper limit of the UV fluence in the non-irradiated nuclear part due to scattering of the microbeam was experimentally obtained. This UV fluence was significantly below the threshold fluence necessary to induce GCS in whole-cell irradiation experiments. In other cells, partial nuclear irradiation resulted in shattering of a few chromosomes only, while the majority remained intact (partial chromosome shattering, PCS). G1/early S was the most sensitive phase for induction of GCS by whole-cell and partial nuclear irradiation. The frequency of PCS was observed to increase when partial nuclear irradiation was performed either at lower incident doses or at later stages of S. We suggest that PCS and GCS indicate 2 levels of chromosome damage which can be produced by the synergistic action of UV irradiation and caffeine. PCS may be restricted to microirradiated chromatin whereas GCS involves both irradiated and unirradiated chromosomes in the microirradiated nucleus.

Induction of chromosome shattering and micronuclei by ultraviolet light and caffeine. I. Temporal relationship and antagonistic effects of the four deoxyribonucleosides

It is known that nucleosides may have antimutagenic and anticlastogenic effects. Here, we have investigated the influence of nucleosides on the induction of shattered chromosomes (fragmentation and/or pulverization of chromosomes of a mitotic cell) and of micronuclei by ultraviolet (UV) light and caffeine. Asynchronous cell cultures of a V79 subline of the Chinese hamster were irradiated at wavelength 254 nm using fluences up to 5.2 joules/m2. Following irradiation, the cells were postincubated either with 1.0 mM or 2.0 mM caffeine alone or with caffeine plus the four deoxyribonucleosides (dXs) (concentration 0.1 mM each). After different incubation times (three to 24 hours), chromosome preparations were performed. In other experiments, synchronized cells were used. The percentage of metaphase spreads with shattered chromosomes and the percentage of cells with micronuclei were determined. Post-treatment with caffeine alone resulted in shattered chromosomes in a high percentage of cells at the first post-irradiation mitosis as described previously. Formation of cells with micronuclei was observed only afer the appearance of mitotic cells with shattered chromosomes, the maximum percentage of cells with micronuclei being smaller than the maximum percentage of cells with shattered chromosomes. The strong potentiating effect of UV-light plus caffeine was significantly reduced, however, if the post-treatment was performed with caffeine plus nucleosides. A significant reduction was also observed in the percentage of micronuclei. An evaluation of the mitotic indices and of cell-cycle parameters indicates that the effect of nucleosides was not due to enhanced interphase death.

Trenimon: biochemical, physiological and genetic effects on cells and organisms

The trifunctional alkylating mutagen Trenimon interferes with the genetic material of a variety of organisms and test systems with respect to the induction of point and chromosomal mutations, sister-chromatid exchanges, recombination phenomena and phage induction. Beneath these mutagenic effects several biochemical and cell physiological aspects have been investigated. In this review we discuss chemical and cell physiological effects of Trenimon, aspects of cancer therapy with Trenimon and genetic effects induced by Trenimon. The available data on mutagenic effects of Trenimon are presented according to organisms or test systems. A short discussion on a possible genetic load by therapy with Trenimon in man concludes this review. DNA damage, especially the induction of cross-linkings, seems to represent the common reason for most of the described effects of Trenimon on cells and organisms.

Some comments regarding chromosome pulverization (premature chromosome condensation or PCC, prophasing)

Premature chromosome pulverization (PCC) or prophasing is a much misunderstood cytological entity. It must be separated from chromosome damage caused by a number of chemical, physical and biological agents. Prophasing is observed in fused cells in which one of the constituent cells must be in metaphase and another in interphase. The morphology of the "pulverized" interphase nucleus will depend on the phase of the cell cycle in which the interphase cell was in when exposed to a substance present in the cytoplasm of the metaphase cell leading to "prophasing". Prophasing is a normal cellular phenomenon occurring prematurely or under abnormal conditions (fusion of cells) and its demonstration in human cells or tumors may be indicative of the presence of a virus (or its products) which leads to cell fusion, but does not play a role in prophasing.

Analysis of banding patterns and mosaic configurations in a case of ring chromosome 15

Cytogenetic studies on lymphocytes from a 14-year-old mentally retarded girl with somatic anomalies suggestive of a chromosomal abnormality revealed a ring chromosome 15. The long arm of the defective chromosome is broken at band q24 or q25. The silver staining technique for nucleolus organizer regions showed that the ring had lost the achromatic stalk and the satellite. The chromosomal mosaicism resulting from the structural instability of the ring chromosome was analyzed and compared with 6 cases reported in the literature. It is proposed that the clinical manifestations in the different patients with ring chromosome 15 result from both the deficiency in the long arm and the mosaic configurations.

Laser UV microirradiation of interphase nuclei and post-treatment with caffeine. A new approach to establish the arrangement of interphase chromosomes

Laser UV microirradiation of Chinese hamster interphase cells combined with caffeine post-treatment produced different patterns of chromosome damage in mitosis following irradiation of a small area of the nucleus that may be classified in three categories: I)intact metaphase figures, II)chromosome damage confined to a small area of the metaphase spread, III)mitotic figures with damage on all chromosomes. Category III might be the consequence of a non-localized distortion of nuclear metabolism. By contrast, category II may reflect localized DNA damage induced by microirradiation, which could not be efficiently repaired due to the effect of caffeine. If this interpretation is right, in metaphase figures of category II chromosome damage should occur only at the irradiation site. The effect might then be used to investigate neighbourhood relationships of individual chromosomes in the interphase nucleus.

X-ray induced visible alterations in the giant chromosomes of Phryne cincta (Nematocera, Diptera): relation of radiation sensitivity to pronuclear chromosome structure

In order to induce chromosomal rearrangements, males were exposed to x-rays and then mated to non-irradiated females. The number of each type of structural alteration was determined by examination of the polytene chromosomes of the F1 progeny. -- A comparison of the results with similar studies made on Drosophila revealed a significantly greater sensitivity in Phryne. Parallel to that an extremely high frequency of small inversions was ascertained in Phryne, and the observed ratio of inversions to translocations was the inverse of that which would be expected from purely mathematical considerations based on the lengths of the different chromosomes. These facts allow the conclusion that the paternal pronuclear chromosomes in Phryne are highly spiralized. Besides, the kinetochore-to-translocation-breakpoint distance was measured in both of the chromosomes involved in each reciprocal translocation and the differences (kinetochore-break distance differences) were registered and from them the arrangement of the chromosomes in the pronucleus of Phryne deduced. The data obtained support the assumption of an ordered, polar-field type of orientation. In Drosophila, in contrast, the comparable data showed that the pronuclear chromosomes are not spiralized and are randomly arranged (Bauer, 1939). -- These results seem to indicate that a close correlation exists between the different radiation sensitivities of Drosophila and Phryne and the different states of spiralisation and arrangements of their chromosomes in the pronucleus stage. It is hypothesized that the influence of the maternal genome on the degree of spiralization of the paternal chromosomes could account for differences in the pronuclear chromosome structure of both species.

Premature chromosome condensation, structural chromosome aberrations, and micronuclei in early mouse embryos after treatment of paternal postmeiotic germ cells with triethylenemelamine: possible mechanisms for chemically induced dominant-lethal mutatiions

Cytogenetic effects in preimplantation 4-8-cell mouse embryos have been investigated after treating paternal postmeiotic germ cells with triethylenemelamine (TEM). Dose-levels of TEM which do not affect fertilization but yield high incidence of dominant-lethal mutations in sperm and spermatids were shown to produce relatively high frequencies of (a) premature chromosome condensation (PCC), (b) structural chromosome anomalies (breakage-reunion phenomena), and (c) micronuclei in these embryos. The results indicate that genetic death of embryos is mainly due to imbalance (i.e. loss) of genetic material, either from breaks leading to lagging fragments and micronuclei, or from the segregation of various types of exchange figures (dicentrics, rings etc.) resulting in mechanical disturbances of cleavage division. It is suggested that PCC, to some extent, is an expression of TEM-induced long-lived lesions which, transmitted into the egg, prevent the chromosomes in question from replicating and/or condensing normally. This phenomenon could well be associated with loss of chromosome material resulting in embryonic death.

The human leulocyte test system. VII. Further investigations concerning micronucleus-derived premature chromosome condensation

Premature chromosome condensation (PCC) from X-ray induced micronuclei shows a dose-effect relationship in human leukocytes in vitro. Preparations at different culture times without colcemide treatment reveal complex variations of the frequencies of micronuclei and PCC correlated with the fixation time. The positions of PCC patches in the metaphase plate and the frequencies of different PCC types (S and G2) ar independent on the X-ray dose. The latter indicates that the slowing down of the micronuclei in the cell cycle, which is the reason for the formation of PCC, may be an outcome rather of a regulatory phenomenon than of an unspecific physiological damage of the chromatin included in the micronuclei. This is especially evident from labeling experiments with tritiated thymidine, showing that the extent of asynchrony between main nuclei and micronuclei is independent on the X-ray dose. Labeling experiments with tritiated uridine reveal a X-ray dose dependent suppression of RNA synthesis in cells with main nuclei and micronuclei. THE S-phase nature of "pulverized" PCC patches could be verified by incorporation of tritiated thymidine in aound 50%. Staining of centromeric heterochromatin in micronuclei reveal a frequency of micronuclei with centromeric heterochromatin resembling the frequency of G2-phase PCC found in mitoses.

Premature chromosome condensation in a case of Fanconi's anemia

Evidence is presented for the occurrence of premature chromosome condensation (PCC) from micronuclei even in the first in vitro mitoses in a case of Fanconi's anemia.