Differential spiralization along mammalian mitotic chromosomes. I. BUdR-revealed differentiation in Chinese hamster chromosomes

A historical overview of bromo-substituted DNA and sister chromatid differentiation

The thymidine analogue 5-bromo-2'-deoxyuridine (BrdU) has been widely used to make sister chromatid differentiation (SCD) evident in metaphase chromosomes of cells grown for two cycles in BrdU and, thus, containing varying amounts of the thymidine analogue. A direct consequence was the possibility of making sister chromatid exchange (SCE) evident without using autoradiographic procedures. The latter phenomenon was first discovered in 1953, and its frequency is considered a reliable marker of pathological cell situations, as well as an indicator of mutagenic compounds. Several experimental procedures were found which produced SCD, such as the use of fluorochromes like 33258 Hoechst or acridine orange, whose observation under fluorescence microscopy was directly recorded by photos or stained with Giemsa to make chromosome preparations permanent. Other treatments followed by Giemsa staining required the use of saline hot solutions, acid solutions, nuclease attack and specific monoclonal antibodies. Basically two molecular mechanisms were invoked to explain the different affinity of Giemsa stain for differential BrdU-substituted chromatid DNA. The first implied debromination of chromatid DNA, whose occurrence would be greater in chromatids containing an amount of BrdU greater than that present in sister chromatids. The second mechanism, although not denying the importance of DNA debromination, postulated that chromatin structural organization, in terms of DNA-protein and/or protein-protein DNA interaction, is responsible for SCD production.

[New technologies for genome analysis: Which use in prenatal diagnosis]

Array-CGH emergence allowed important diagnosis progress, and a better care of patients in postnatal. So, there is a great temptation to use it also in prenatal diagnosis. The point of view objective is to make a rapid overview of cytogenetic diagnosis evolution during the last 50 years, and to show all questions raised by the use of array-CGH, and problems that could arise in prenatal diagnosis. While array-CGH just comes in genetic laboratories, new diagnosis approaches emerged like whole genome sequencing or non-invasive prenatal diagnosis. The 2nd objective will be to review all these techniques for a probably close future.

Genotoxic assessment in peripheral blood lymphocytes of post-polio individuals using sister chromatid exchange analysis and micronucleus assay

Environmental pollution is a complex issue because of the diversity of anthropogenic agents, both chemical and physical, that have been detected and catalogued. The consequences to biota from exposure to genotoxic agents present an additional problem because of the potential for these agents to produce adverse change at the cellular and organism levels. Past studies in virus have focused on structural damage to the DNA of environmental species that may occur after exposure to genotoxic agents and the use of this information to document exposure and to monitor remediation. In an effort to predict effects at the population, community and ecosystem levels, in the present study, we attempt to characterize damage occurring through genotoxic agents like 5-bromo-2-deoxyuridine, BrdU, using sister chromatid exchange technique and the formation of micronuclei (MN) in the peripheral lymphocytes of the post-polio syndrome sequelae affected by poliovirus. Analysis of structural chromosomal aberrations (CAs) and involvement of the specific chromosome break were pursued in this study. They revealed a significantly higher incidence of CAs (chromatid and chromosome breaks) in patients compared with controls, where the specific chromosome break has emerged as specific. Also, the maximum numbers of breaks were found to be in chromosome 1 at the position 1p36.1. The results also suggest a correlation between CAs and content of MN.

Evaluation of the cytogenetic damage induced by the organophosphorous insecticide acephate

The organophosphorous insecticide acephate was tested for its ability to induce in vitro cytogenetic effect in human peripheral lymphocytes by using the chromosomal aberrations (CAs), sister chromatid exchange (SCE) and micronuclei (MN) assay. The level of nuclear DNA damage of acephate was evaluated by using the comet assay. Concentrations of 12.5, 25, 50, 100 and 200 mug mL(-1) of acephate were used. All concentrations of acephate induced significant increase in the frequency of CAs and in the formation of MN dose dependently (r = 0.92 at 24 h, r = 0.95 at 48 h for CAs, r = 0.87 for MN). A significant increase was observed in induction of SCE at 50, 100 and 200 mug mL(-1) concentrations during 24 h treatment and at all concentrations (except 12.5 mug mL(-1)) during 48 h treatment period in a dose-dependent manner (r = 0.84 at 24 h, r = 0.88 at 48 h). Acephate did not affect the replicative index and cytokinesis-block proliferation index (CBPI). However, it significantly decreased the mitotic index at all three highest concentrations (50, 100, 200 mug mL(-1)) for 24 h treatment and at all concentrations (except 12.5 mug mL(-1)) for 48 h treatment, dose-dependently (r = 0.94 at 24 h, r = 0.92 at 48 h). A significant increase in mean comet tail length was observed at 100 and 200 mug mL(-1) concentrations compared with negative control in a concentration-dependent manner (r = 0.94). The mean comet tail intensity was significantly increased at only 200 mug mL(-1) concentration. The present results indicate that acephate is a clastogenic, cytotoxic agent and it causes DNA damage at high concentrations in human lymphocytes in culture.

Analysis of sister-chromatid exchanges

Two requirements for the cytogenetic analysis of sister-chromatid exchanges (SCEs) in somatic cells are (1) a population of actively proliferating cells that will provide an adequate number of metaphases and (2) sister chromatids that in some way are differentially labeled or stained in the metaphases. SCEs can be recognized as abrupt discontinuities in the staining patterns of the two chromatids of a metaphase chromosome at what appear to be identical sites, with reciprocal switching from one chromatid to its sister. This protocol uses phytohemagglutinin (PHA)-stimulated cultures of blood lymphocytes as a source of proliferating cells. The cells are incubated with the thymidine analog BrdU. Slides prepared from fixed cells with BrdU-substituted chromosomes are treated with Hoechst 33258, exposed to light and heat, and then Giemsa-stained to produce differentially stained chromosomes. The chromatids with bifilar substitution exhibit a lighter purple stain than their unifilarly substituted sister chromatids.

High-resolution R-banding at the 1250-band level. III. Comparative analysis of morphologic and dynamic R-band patterns (RHG and RBG)

High-resolution human chromosomes were obtained from lymphocytes after thymidine synchronization. The block was released either with thymidine to produce GTG (G-bands by trypsin using Giemsa) and RHG (R-bands by heating using Giemsa) banding or with BrdU (5-bromo-2'-deoxyuridine) for RBG (R-bands by BrdU using Giemsa) banding. RHG and RBG band patterns are only 75 to 85% congruent. The dissimilarities increase with the band number per genome and vary from one chromosome region to another. After high-resolution RBG banding, the BrdU-substituted bands show an unequal condensation delay, which can be, according to the bands involved, very important, minimal, or even absent. The bands showing the highest degree of condensation delay are the bands replicating the latest. The GTG- and RHG-band patterns show complementary matching for about 90% of the bands. It was found that two third of the chromosome surface appears positively stained after R-banding. This suggests that more DNA is replicated during early S-phase than during late S-phase. To obtain a fully developed RBG-band pattern in 90 to 95% of harvested mitoses, a period of 4.5 hours after the removal of the blocking agent is optimal. Such a brief release period also implies that late S-phase is much shorter than early S-phase.

Mechanism of in vivo sister-chromatid exchange induction by 5-azacytidine

The aim of the present study was to explore the in vivo mechanism of sister-chromatid exchange (SCE) induction by 5-azacytidine (5-azaC) in murine bone marrow cells. Experiments were performed to examine SCE induction in response to different doses of 5-azaC as well as several exposures. Additionally, we examined the persistence of SCE induction and the effect of bromodeoxiuridine (BrdU) incorporation. Sister-chromatid differentiation was obtained by injecting mice intraperitoneally with BrdU absorbed to activated charcoal. Before BrdU injection, different doses of 5-azaC were administered intraperitoneally either singly or in multiples. Colchicine in an aqueous solution was administered subcutaneously 22 h after BrdU injection. Two hours later, animals were sacrificed by cervical dislocation and both femurs were dissected. Bone marrow cells were processed to obtain chromosome preparations, which were stained by the fluorescence plus Giemsa method. Results indicate that 5-azaC caused SCE, albeit to a limited extent. In order to discern whether the limitation was due to cytotoxicity or to partial 5-azaC incorporation, we administered multiple sub-toxic doses of 5-azaC. This treatment increased 5-azaC incorporation and reduced cytotoxicity, but did not raise SCE frequency, indicating that the limitation was not due to either of the two factors mentioned above. SCE frequency induced by 5-azaC persisted for at least eight cell divisions, confirming that this agent had caused inhibition of DNA methyltransferase and subsequently the reduction of DNA re-methylation, which in turn induced the expression of a number of SCE-prone sites. Finally, SCE induction in response to 5-azaC was completely dependent on BrdU incorporation. The data allow us to conclude that 5-azaC causes SCE to a limited extent; limited SCE induction was not due to the direct effect of incorporation or cytotoxicity of 5-azaC, but rather the generation of a number of SCE-prone sites, the expression of which persists for at least eight cell divisions and is dependent on BrdU incorporation.

The DNA demethylating 5-azaC induces endoreduplication in cultured Chinese hamster cells

We have investigated the possible influence of 5-azacytidine (5-azaC) substitution for cytidine into DNA on topoisomerase II (topo II) function in chromosome segregation. The endpoint chosen has been the induction of endoreduplicated cells at mitosis showing diplochromosomes. Experiments were performed in the presence and absence of the cytidine analogue to assess the degree of 5-azaC-induced DNA hypomethylation, using differential cutting by restriction endonucleases Hpa II and Msp I. Using the pulsed-field gel electrophoresis (PFGE) technique, we have also observed a protective effect provided by 5-azaC treatment against DNA breakage induced by the topo II poison m-AMSA. Concentrations of 5-azaC shown as able to induce extensive DNA hypomethylation and capable to protect DNA from double-strand breaks induced by m-AMSA were used for our cytogenetic experiments to analyze chromosome segregation. Our results seem to indicate that the presence of 5-azaC in DNA induces a dose-dependent increase in the yield of endoreduplicated cells that parallels the levels of hypomethylation observed.

Quantitative investigation of reproduction of gonosomal condensed chromatin during trophoblast cell polyploidization and endoreduplication in the East-European field vole Microtus rossiaemeridionalis

Simultaneous determinations of DNA content in cell nuclei and condensed chromatin bodies formed by heterochromatized regions of sex chromosomes (gonosomal chromatin bodies, GCB) have been performed in two trophoblast cell populations of the East-European field vole Microtus rossiaemeridionalis: in the proliferative population of trophoblast cells of the junctional zone of placenta and in the secondary giant trophoblast cells. One or two GCBs have been observed in trophoblast cell nuclei of all embryos studied (perhaps both male and female). In the proliferative trophoblast cell population characterized by low ploidy levels (2-16c) and in the highly polyploid population of secondary giant trophoblast cells (32-256c) the total DNA content in GCB increased proportionally to the ploidy level. In individual GCBs the DNA content also rose proportionally to the ploidy level in nuclei both with one and with two GCBs in both trophoblast cell populations. Some increase in percentage of nuclei with 2-3 GCBs was shown in nuclei of the placenta junctional zone; this may be accounted for by genome multiplication via uncompleted mitoses. In nuclei of the secondary giant trophoblast cells (16-256c) the number of GCBs did not exceed 2, and the fraction of nuclei with two GCBs did not increase, which suggests the polytene nature of sex chromosomes in these cells. In all classes of ploidy the DNA content in trophoblast cell nuclei with the single GCB was lower than in nuclei with two and more GCBs. This can indicate that the single GCB in many cases does not derive from fusion of two GCBs. The measurements in individual GCBs suggest that different heterochromatized regions of the X- and Y-chromosome may contribute in GCB formation.

Epigenetic mechanisms for primary differentiation in mammalian embryos

This review examines main developments related to the interface between primary mammalian cell differentiation and various aspects of chromosomal structure changes, such as heterochromatin dynamics, DNA methylation, mitotic recombination, and inter- and intrachromosomal differentiation. In particular, X chromosome difference, imprinting, chromosomal banding, methylation pattern, single-strand DNA breaks, sister chromatid exchanges (SCEs), and sister chromatid asymmetry are considered. A hypothesis is put forward which implies the existence of an epigenetic asymmetry versus mirror symmetry of sister chromatids for any DNA sequences. Such epigenetic asymmetry appears as a result of asymmetry of sister chromatid organization and of SCE and is a necessary (not sufficient) condition for creating cell diversity. The sister chromatid asymmetry arises as a result of consecutive rounds of active and passive demethylation which leads after chromatin assembly events to chromatid difference. Single-strand DNA breaks that emerge during demethylation trigger reparation machinery, provend as sister chromatid exchanges, which are not epigenetically neutral in this case. Taken together, chromatid asymmetry and SCE lead to cell diversity regarding their future fate. Such cells are considered pluripotent stem cells which after interplay between a set of chromosomal domains and certain substances localized within the cytoplasmic compartments (and possibly cell interactions) can cause sister cells to express different gene chains. A model is suggested that may be useful for stem cell technology and studies of carcinogenesis.

Scanning near field optical/atomic force microscopy of bromodeoxyuridine-incorporated human chromosomes

The present study applied scanning near field optical/atomic force microscopy (SNOM/AFM) to the observation of human chromosomes immunostained with an anti-BrdU antibody after incorporation of BrdU into DNA. Human lymphocytes were cultured in BrdU for 72 h and their chromosomes were prepared with a standard method for light microscopy. After additional fixation with 15% formalin in phosphate buffered saline, the specimens were denatured with 2N HCI with 0.1% Triton-X 100, immunostained with the anti-BrdU antibody, and observed both by fluorescence microscopy and by SNOM/AFM. The preparation technique used in the present study enabled the differential staining of sister chromatids in each chromosome, and sister chromatid exchanges (SCEs) were recognized in some chromosomes of the metaphase spread. Observations of the specimens by SNOM/AFM further provided the simultaneous collection of topographical and fluorescent images of the same portions of BrdU-incorporated chromosomes. The resolution of the fluorescence images by SNOM/AFM was greater than that obtained by fluorescence microscopy. Superimposition of topographical and fluorescent images of the chromosomes is useful for the precise analysis of the fine structure of chromosomes in relation to the SCEs. The application of SNOM/AFM to the BrdU-incorporated chromosomes is thus useful for the analysis of the fine structure of chromosomes in relation to their function.

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.

Improved fish lymphocyte culture for chromosome preparation

Cytogenetic methodology is still underdeveloped in fishes compared with mammals. Culture condition for fish lymphocytes was optimized to improve chromosome preparation using the rainbow trout (Oncorhynchus mykiss) as a model after changing the combination of parameters such as mitogens, incubation periods, media, cell components, and freshness of blood. The optimized culture condition included isolation of lymphocytes from fresh blood by a stirring method, their culture in medium 199 supplemented with 10% FBS, 18 microg/ml of phytohemagglutinin (PHA-W) and 100 microg/ml of lipopolysaccharide (LPS) as mitogens, and harvested at 6 days after culture. This condition provided a notably increased mitotic index (MI) of 4.3-10.0% in rainbow trout lymphocytes. In addition, the condition was highly reproducible as shown by the similar level of MI in cultured lymphocytes from 181 individuals without failure. Applicability of this method in a wide range of fish groups was also proven with Ml of 1.1-13.3% in cultured lymphocytes from other 16 freshwater species of Acipenseridae, Anguillidae, Solmonidae, Cyprinidae, and Centrarchidae, and five marine species of Sparidae, Kyphosidae, Paralichthyidae, and Scorpaenidae. Chromosome preparations of improved quality by the present method were successfully applied for the replication R-banding with incorporation of 5-bromo-2'-deoxyuridine and direct R-banding fluorescence in situ hybridization.

In vivo repair during G1 of DNA lesions eliciting sister chromatid exchanges induced by methylnitrosourea or ethylnitrosourea in BrdU substituted or unsubstituted DNA in murine salivary gland cells

The difference in efficiency of methylnitrosourea (MNU) and ethylnitrosourea (ENU) to induce SCE in early or late G1 was determined in synchronized murine salivary gland cells in vivo, as a measure of the capacity of this tissue to repair the lesions involved in SCE formation during G1. The repair during G1 was determined by treating the cells in early or late G1. Treatment was in the first cycle (G1 before incorporation of 5-bromodeoxyuridine (BrdU)) or in G1 of the second cycle (after a single round of BrdU incorporation). It was observed that 50% of the lesions induced by MNU that elicit SCE are repaired during G1. BrdU incorporation into DNA increases the sensitivity of the cell to SCE induction by MNU nearly 40%; however under this circumstance a slightly lower SCE frequency was observed in the cells exposed to MNU at early G1, indicating that during G1 only few lesions are repaired. The ENU-induced DNA-lesions involved in SCE production are nearly 100% persistent along G1; besides, a slight but significantly higher SCE frequency was observed in cells exposed at early G1, suggesting the formation of SCE-inducing lesions during G1. BrdU incorporation to DNA sensitizes the cell to SCE induction by ENU, increasing the SCE frequency to nearly to a 40%, although these additional lesions involved in SCE induction seem to be susceptible to repair during G1.

Chromosome methylation patterns during mammalian preimplantation development

DNA methylation patterns were evaluated during preimplantation mouse development by analyzing the binding of monoclonal antibody to 5-methylcytosine (5-MeC) on metaphase chromosomes. Specific chromosome patterns were observed in each cell stage. A banding pattern predominated in chromosomes at the one-cell stage. Banding was replaced at the two-cell stage by an asymmetrical labeling of the sister chromatids. Then, the proportion of asymmetrical chromosomes decreased by one-half at each cell division until the blastocyst stage, and chromosomes became progressively symmetrical and weakly labeled. Our results indicate that chromosome demethylation is associated with each DNA replication and suggest that a passive mechanism predominates during early development.

Genotoxicity of selected pesticides in the mouse bone-marrow micronucleus test and in the sister-chromatid exchange test with human lymphocytes in vitro

Selected pesticides (aldicarb, 1,3-dichloropropene, methidathion, parathion, triadimefon, vinclozolin) were tested for their clastogenic and aneugenic activities in the mouse bone-marrow micronucleus (MN) test in vivo and for their sister-chromatid exchange-inducing activities in human lymphocytes in vitro in the presence and absence of an exogenous metabolizing system from rat-liver S9. 1,3-Dichloropropene significantly increased the frequencies of micronucleated polychromatic erythrocytes (PCE) in bone-marrow cells of female mice from 3.3 MN/1000 PCE to 15.3 MN/1000 PCE (187 mg per kg body weight). 1,3-Dichloropropene (100 microM) induced 16.0 SCE/metaphase after 24 h of incubation as compared with the basal rate of 11.2 SCE/metaphase (-S9) and of 15.4 SCE/metaphase as compared with 10.5 SCE/metaphase of the control (+S9). These values were statistically significantly different from each other. The other pesticides tested did neither increase the rate of micronuclei significantly in polychromatic erythrocytes in male nor in female animals. Aldicarb and methidathion induced a significant increase in SCEs in human lymphocytes in vitro only without the metabolic activating system: aldicarb, 5 microM, 24 h incubation: 15.5 SCE/metaphase; control: 12.6 SCE/metaphase; methidathion, 100 microM, 24 h incubation: 15.8 SCE/metaphase, control: 11.1 SCE/metaphase. Parathion, triadimefon and vinclozolin did not have any SCE-inducing effects.

Nitro musk compounds genotoxic activity : Genotoxicity testing of nitro musks with the SOS-chromotest and the sister-chromatid exchange test

Five nitro musk compounds are widely used as fragrance ingredients in perfumes, lotions and detergents; as food additives in cigarettes and fish baits, and in such technical products as herbicide formulations and explosives. Several studies identified nitro musk compounds in aquatic environment samples, human milk and fat samples as highly lipophilic and persistent bioaccumulating environmental pollutants. To examine the compounds for genotoxic activity, musk xylene (1-tert.-butyl-3, 5-dimethyl-2, 4, 6-trinitrobenzene), musk ketone (4-tert.-butyl-3, 5-dinitro-2, 6-dimethylacetophenone), musk ambrette (l-tert.-butyl-4-methyl-6-methoxy-3, 5-dinitrobenzene), musk moskene (l, 1, 3, 3, 5-pemamethyl-4, 6-di-nitroindane) and musk tibetene (1-tert.-butyl-3, 4, 5-trimethyl-2, 6-dinitrobenzene) were tested for SOS inducing potency in the SOS chromotest with E. coli PQ37 and for sister-chromatid exchange inducing activities in human lymphocytes in vitro both in the presence and absence of an exogenous metabolizing system from rat liver S9-Mix. Nitro musks revealed no genotoxicity either in the SOS chromotest with E. coli PQ37 or in the sister-chromatid exchange test with human lymphocytes.

Routine cytogenetic prenatal diagnosis using dynamic banding (RBG-GBG): a highly reproducible method for amniocytes, fetal cord blood, and chorionic villus investigations

Dynamic banding (RBG-GBG) using pulse 5-bromodeoxyuridine (5-BrdU) incorporation during part of the last S-phase before harvesting has been used in prenatal investigations. This method has already been routinely applied in 1344 cytogenetic investigations. GBG and RBG bandings produced almost identical patterns to classical G- and R-banding methods except for heterochromatic portions and some euchromatic segments. Nevertheless, these discordances may be somewhat helpful for cytogenetic diagnosis (i.e., X numerical abnormalities). The results showed particularly good contrast and staining; 5-BrdU incorporation did not prevent additional staining. Likewise, previous RBG or GBG disclosure allowed further chromosomal identification with C-banding or nucleolar organizer staining. Simplicity and reproducibility were very helpful in cases with a low mitotic index. 5-BrdU had no significant effect on in-vitro damage because only 0.31 percent of cells were affected; so, we believe that dynamic banding should be used more extensively in cytogenetic investigations. Moreover, the staining and contrast qualities were very suitable for automatic methods of analysis now in use: i.e., metaphase finding and computer-assisted karyogram creation.

The genetic toxicology of 5-bromodeoxyuridine in mammalian cells

The thymidine analog, BrdUrd, induces many biological responses which are of importance to the field of genetic toxicology and related disciplines. These include the induction of SCE, specific-locus mutations, and toxicity, inhibition of cell proliferation, and the expression of fragile sites in the human genome. In early models which addressed the mechanisms of the biological effects of BrdUrd exposure, two pathways were proposed to account for the induction of the biological responses. Incorporation of the enol form of BrdUrd into the nascent DNA strand after pairing with deoxyguanosine was proposed as one pathway, whereas the incorporation of BrdUrd opposite adenosine in place of thymidine was proposed as the second pathway. Many novel and sophisticated techniques have been applied to the study of the mechanism of the induction of biological effects by BrdUrd leading to a substantial increase in our understanding of these mechanisms. However, the experimental evidence clearly supports the contention that BrdUrd exerts its effects on eukaryotic cells through mechanisms similar to those originally proposed to explain the genotoxicity of BrdUrd.

Rare chromosomal aberrations induced by vincristine. Partial endoreduplication and pseudoendoreduplication, segmentally endoreduplicated chromosomes, and segmental premature chromosome condensation

Vincristine (VCR) is capable of inducing a cell containing both conventional chromosomes (monochromosomes) and diplochromosomes. A total of 124 such metaphases were examined by 5-bromodeoxyuridine (BrdU) incorporation and fluorescence plus Giemsa (FPG) technique to analyze cell cycle kinetics. The majority of cells (119 metaphases) showed differential BrdU incorporation between the two kinds of chromosomes, indicating that partial endoreduplication occurred in these cells. In addition, existence of partially endoreduplicated cells with premature chromosome condensation (PCC) in either mono- or diplochromosomes suggests that the timing of monochromosome-replication was very variable in individual cells. On the other hand, the remaining five metaphases showed that both mono- and diplochromosomes incorporated BrdU similarly, indicating that diplochromosomes are formed by pseudoendoreduplication. Two kinds of chromosomal aberrations probably caused by delay of DNA synthesis on chromosome segments, segmental endoreduplication, and segmental PCC were also reported. Segmental endoreduplication was defined as endoreduplication that occurred on some segments of chromosomes. Out of 119 partially endoreduplicated cells, 3 contained a chromosome consisting of both mono- and diplochromosomal segments, indicating that the former segments missed one round of DNA synthesis. Segmental PCC was defined as PCC restricted to only some segments of chromosomes. Two types of segmental PCC, segmental S-PCC and G2-PCC, were observed in VCR-induced ordinary polyploidy. Although both segmental endoreduplication and segmental PCC occurred with very low frequency, these phenomena suggest that DNA synthesis was disturbed in some part of the nucleus.

Analysis of DNA replication during S-phase by means of dynamic chromosome banding at high resolution

The characteristic patterns of dynamic banding (replication banding) were analysed. Extremely high resolution (850 to 1,250 bands per genome) G- and R-band patterns were obtained after 5-bromo-2'-deoxyuridine (BrdUrd) incorporation either during the early or the late S-phase. We synchronized human lymphocytes with high concentrations of thymidine or BrdUrd as blocking agents, followed by low concentrations of BrdUrd or thymidine respectively as releasing agents, and obtained R- or G-band patterns respectively. The dynamic R- and G-band patterns were complementary for all chromosomes, even for the late-replicating X chromosome. There was no overlapping and every part of each chromosome was positively stained by one of the two banding procedures. The complementarity of the two patterns shows that both high thymidine and high BrdUrd concentrations blocked S-phase progression near the R-band to G-band replication transition in the middle of S-phase. Some bands of the inactive X chromosome replicate before this transition concurrently with R-band replication. The 48 different telomeric regions could be classified into 5 distinct morphotypes based upon the distribution of early and late-replicating DNA in each telomeric region. The dynamic band patterns are particularly useful for the study of the structural and physiological organization of chromosomes at high resolution and should prove invaluable for assessing the replication behavior of rearranged chromosomes.

Coincidence in fragile site expression with fluorodeoxyuridine and bromodeoxyuridine

Fragile site expression induced by 10 micrograms/ml or 20 micrograms/ml fluorodeoxyuridine (FudR) and 25 micrograms/ml or 50 micrograms/ml bromodeoxyuridine (BrdU) was studied in lymphocyte cultures of six healthy individuals. A significant decrease in mitotic indexes in respect to control cultures was observed with both FudR concentrations used. The cells showing chromosome aberrations and the total number of cytogenetic alterations were significantly increased both in FudR (p less than 0.001) and BrdU (25 micrograms/ml) (p less than 0.05) treated cultures with respect to the control culture. A site showing a gap or a break was defined as fragile if it appeared in 1% of the cells analyzed and in at least three of the six individuals studied with the same culture treatment. Using these criteria, fragile sites 4q31, 5q15, 6p22, 7p13, 7q32, 13q21, and 14q24 were induced in different proportions by both chemical agents. Although these drugs act via different mechanisms, they both substitute for thymidine in DNA. Our findings suggest that FudR is a more potent common fragile site inducer than BrdU.

Dynamic G- and R-banding of human chromosomes for electron microscopy

Synchronized human lymphocytes were exposed to 5-bromo-2'-deoxyuridine (BrdUrd) for incorporation in either G- or R-bands. The substituted bands were revealed by monoclonal anti-BrdUrd antibodies disclosed with either gold-labeled antibodies or with the protein A-gold complex. Sharp G- or R-banding, specific for electron microscopy (EM), was obtained. These banding patterns, referred to as GB-AAu (G-bands by BrdUrd using Antibodies and gold [Au]) and RB-AAu (R-bands by BrdUrd using Antibodies and gold [Au]), resemble dynamic band patterns (GBG and RBG) much more than they do morphologic band patterns (GTG and RHG). The G- and R-band patterns allow accurate chromosome identification and karyotyping. An actual karyotype of human GB-AAu-banded chromosomes at the 750 band level, photographed in the EM, is presented. The method produces excellent band separation and band contrast. Variations in band staining intensities were noted and correlated with BrdUrd enrichment. The C-band regions were positively stained after GB-AAu banding while they were negatively stained after RB-AAu banding. Telomeres appeared heterogeneous after GB-AAu banding suggesting that part of the telomeric bands might be late replicating.

A man with isochromosome Xq Klinefelter syndrome with lack of height increase and normal androgenization

We report on a patient with Klinefelter syndrome (KS) and the homogeneous aneuploidy 47,Xi(Xq)Y, or male trisomy Xq. He had many characteristics of classical KS: small testes, azoospermia, elevated FSH and LH, average intelligence, and normal androgenization, but his stature was not increased, compared with his father's and brothers'. The i(Xq), found in all cells analyzed, was late-replicating, monocentric, and also asymmetric for the RBG-banding of the two arms, indicating a different chronology of DNA synthesis in each arm. When indicated, in the seven previously reported cases, the level of plasma testosterone was always subnormal; it was normal (650 ng/100 ml) in our patient, who had normal masculinization. Thus the level of testosterone among patients with KS is not necessarily lower with an extra Xq. Furthermore, the sharp contrast in the height of KS patients with or without an i(Xq) is striking. It appears definitely possible to associate the isochromosome Xq Klinefelter syndrome with a lack of height increase.

Cell cycle studies in chorionic villi

We have studied the cell cycle of cells obtained from chorionic villi in direct and culture preparations by incorporation of the thymidine analogue BrdU to produce late-labelling or sister chromatid differentiation patterns. We have, therefore, been able to estimate the duration of the cell cycle and, more specifically, the length of some of its phases. While results for chorionic villus sample cells in culture resembled those obtained for fibroblasts, data for the spontaneously dividing trophoblastic cells in direct preparations were different. Villi exposed to BrdU immediately after sampling showed a slight delay in the incorporation of the analogue and a lower percentage of labelled cells compared to villi treated after an overnight incubation, probably due to a temporary effect of the sampling technique. Results from semi-direct protocols suggest that cells have a G2 of no more than 4 h, and a mid-S phase of 10-16 h. The G1 period is very variable. After 48 h incubation with BrdU, only 4% of cells reach their second generation, whereas this percentage increases up to 70% after 72 h, indicating that under these experimental conditions most cells have a cell cycle of approximately 36 h. The average number of sister chromatid exchanges was similar in both direct preparations and cultures: 5.2 +/- 2.1 SCE per cell.

Factors that influence formation of sister chromatid exchanges in human blood lymphocytes

Sister chromatid exchange (SCE) reflects an interchange of DNA sequences between helices in a replicating chromosome. This was initially accomplished by Taylor and colleagues (1957) using tritiated thymidine incorporation followed by autoradiography. The development of an elegant technique for differential staining of sister chromatids by incorporating a thymidine analog, 5-bromodeoxyuridine (BrdU) has greatly simplified the detection of SCEs in metaphase chromosomes. In recent years, the analysis of SCE has been considered to be a highly sensitive and additional (i.e., with chromosome aberrations) end point for measuring mutagenic/carcinogenic potential of various environmental agents and is increasingly being used to detect and differentiate among chromosome fragility human diseases that predispose to neoplasia. Attention has been focused to see if the induction of SCEs in lymphocyte cultures can be used as a reliable "biological dosimeter" for genetic risk assessment and to monitor the exposed populations. Several physical or preparatory as well as biological factors that modify the response and formation of SCEs make the monitoring difficult. The purpose of this article is to review and analyze these factors to facilitate an effective development of a standard protocol for SCE testing and for appropriate evaluation of test results. This may also provide clues to understand the yet unknown molecular mechanism(s) and biological significance of SCE formation.

Effects of bromodeoxyuridine substitution on metaphase chromosome structures examined by scanning electron microscopy

Chinese hamster chromosomes were differentially substituted with 50 microM 5-bromodeoxyuridine (BrdU) to obtain chromosomes with bifilarly and unifilarly substituted (BB-TB) and unifilarly and non-substituted (TB-TT) chromatid constitutions. To avoid the effect of Giemsa staining on the ultrastructure of chromosomes, unstained preparations were exclusively used. When TB-TT chromosomes were prepared with the conventional air-drying method followed by the osmium tetroxide-thiocarbohydrazide (OsO4-TCH) technique and examined by scanning electron microscopy (SEM), the TB-chromatid appeared somewhat more slender and showed more conspicuous spiral structures, thereby appearing more loosened compared to the TT-chromatid. At higher magnifications, however, 30 nm chromatin fibres which were seen to constitute both chromatids showed no discernible differences in dimension between the TT- and TB-chromatids. On the other hand, TB-TT chromosomes specially prepared for SEM without the process of air-drying appeared in their entirety less extended and no spiral configuration was observed even in the TB-chromatid. The TB-chromatid instead appeared rather less loosened than the TT-chromatid whereas thick fibre-like structures which in turn seemed to consist of 30 nm fibres were more easily discernible in the TT-chromatid compared to the TB. Such seemingly contradictory results obtained from the two different preparatory procedures were tentatively explained on the basis of our multiple coiling model (Taniguchi and Takayama 1986).

Different reactivity of Z-DNA antibodies with human chromosomes modified by actinomycin D and 5-bromodeoxyuridine

Antibodies against Z-DNA react with fixed metaphase chromosomes of man and other mammals. Indirect immunofluorescence staining shows that chromosomal segments corresponding to R- and T-bands preferentially fix Z-DNA antibodies. In this work Z-DNA antibodies were used as a probe for DNA conformation in euchromatin of fixed human chromosomes whose condensation or staining were modified by actinomycin D (AMD) and by 5-bromodeoxyuridine (BrdU). Treatments with AMD and BrdU were performed to induce a G-banding by modification of chromosomal segments corresponding to R- and T-bands. Long BrdU treatments were used to induce asymmetrical and partially undercondensed chromosomes by substitution of thymidine in one or both DNA strand. Our results show a clear difference of Z-DNA antibodies reactivity after AMD or BrdU treatment. The G-banding obtained after AMD treatment is not reversed by Z-DNA antibodies staining since these antibodies bind very weakly to the undercondensed R-bands. On the other hand, the G-banding obtained by BrdU is completely reversed giving typical R-banding, as on untreated chromosomes. For asymmetrical chromosomes an R-, T-banding pattern is always observed but there is a decrease of the fluorescence intensity proportional to the degree of BrdU incorporation. We conclude that AMD treatment greatly disturbs Z-DNA antibodies binding suggesting a change in DNA conformation, whereas BrdU treatments do not suppress but only weaken the specific binding of Z-DNA antibodies on R- and T-bands. The direct involvement of thymidine substitution in DNA sequences recognized by Z-DNA antibodies is discussed.

Differential inhibition of sister chromatid condensation induced by 5-azadeoxycytidine in human chromosomes

The deoxycytidine analogue 5-azadeoxycytidine (5-aza-dC) induces differential inhibition of sister chromatid condensation when cells are treated with this substance for two replication cycles, as the subsequent staining of metaphase chromosomes with Giemsa shows. The bifilarly substituted chromatid is dramatically longer than the unifilar one. A percentage of the metaphases treated with 5-aza-dC even show a complete undercondensation of the bifilarly substituted chromatid. The optimum conditions for inducing sister chromatid differentiation were determined. No method has been developed as yet to permit enhancement of the differential staining in 5-aza-dC-treated preparations. The interactions between 5-aza-dC and chromosomal DNA as well as the factors involved in the differential staining of sister chromatids are discussed.

The R-banding pattern of the Chinese hamster Don cell line

Chinese hamster cells (Don line) were treated in vivo with 5-BrdU and 33258-Hoechst fluorochrome for obtaining the partial inhibition of condensation that causes the R-banding pattern. Untreated chromosomes were stained by a standard G-banding method. Statistical measurements show significant differences in the band numbers between the two treatments. The Don cell line in the authors' laboratory presents some karyotypical differences from Don cell lines studied by other authors.

Prefixation treatment with ethidium bromide for high resolution banding analysis of chromosomes from cultured human bone marrow cells

Prefixation treatment of cultured human bone marrow cells with a DNA intercalating agent, ethidium bromide (EBr), induced a dose- and time-related elongation of chromosomes. When compared with EBr-free cultures, a 2.9-fold increase in the yield of early mitotic cells with more than 400 bands per haploid set of chromosomes was achieved by simply adding 10 micrograms/ml of EBr during the last 2 hours of culture. The proportion of early mitotic cells was equal to that obtained in methotrexate synchronized cultures. Fluorescence banding methods using base composition specific agents actinomycin D/DAPI for AT base pairs and chromomycin A3/distamycin A for GC suggested that EBr does not have base specificity, because EBr did not alter the banding patterns of chromosomes obtained with these staining procedures.

Simultaneous production of R-bands and either replication patterns or sister chromatid differentiation

The dye triplet chromomycin/methyl green/DAPI and some related dye combinations were applied to bromodeoxyuridine (BrdU)-substituted and non-substituted chromosomes. In both kinds of chromosome preparations, tri-staining followed by observation at pH 7 resulted in well defined R-bands (excitation wavelength 436 nm) and a mixed DA-DAPI/Q-banding pattern (360 nm). Two approaches have given satisfactory expression both of reverse bands and of the differential BrdU-substitution present in metaphase chromosomes: (1) direct tri-staining at pH 7 and mounting of the preparations at pH 11; or (2) pretreatment of the preparations with Hoechst 33258 plus UV and hot buffer followed by tri-staining and mounting at pH 7. These methods should prove useful for routine chromosome analysis and, in combination with BrdU-labelling, in studies of chromosome structure and replication.

DNA hypomethylation causes an increase in DNase-I sensitivity and an advance in the time of replication of the entire inactive X chromosome

We have examined the effect of 5-azacytidine (5-aza-C) induced hypomethylation of DNA on the time of replication and DNase I sensitivity of the X chromosomes of female Gerbillus gerbillus (rodent) lung fibroblast cells. Using in situ nick translation to visualise the potential state of activity of large regions of metaphase chromosomes we show that 5-aza-C causes a dramatic increase in the DNase-I sensitivity of the entire inactive X chromosome of female G. gerbillus cells and this increase in nuclease sensitivity correlates with a large shift in the time of replication of the inactive X chromosome from late S phase to early S phase. These effects of 5-aza-C on the inactive X chromosome are associated with a 15% decrease in DNA methylation. Our results indicate that DNA methylation concomitantly affects both the time of replication and the chromatin conformation of the inactive X chromosome.

Q-banding of human chromosomes after BUdR and BCdR treatment

Two different Q patterns were found in BUdR and BCdR treated chromosomes of human lymphocyte cultures: X-type pattern, in which Giemsa and quinacrine banding both are reversed; Y-type pattern, in which Q-banding remains conventional in spite of reverse G-banding. Possible mechanisms of these findings are discussed.

Fragile (X) expression: relationship to the cell cycle

The fragile (X) chromosome demonstrable in individuals with one type of X-linked mental retardation is seldom, if ever, seen in more than 50% of cells of affected individuals. We have devised a model to explain this apparent 50% maximum, one essential feature of which is that the fragile (X) will not be seen in cells in their first division in thymidine-depleted media. The validity of our model was tested on lymphoblastoid cell lines from affected males by treating the cells with fluorodeoxyuridine (FUdR) to induce the marker and/or bromodeoxyuridine (BrdU) to determine the cell cycle. We have evidence that the fragile (X) is present in cells in the first and subsequent cell divisions in thymidine-depleted media. In light of these observations our model is not valid and the 50% expression of the fragile site at Xq(28) and other unusual properties of this region of the X chromosome remain unexplained.

Detailed cell cycle analysis in human lymphocytes; application to gamma-irradiated cells

A method based on BrdU incorporation for analyzing in detail the kinetics of the cell cycle is described. The S phase has been subdivided into five subphases, each recognizable by their BrdU incorporation pattern at metaphase. The method can be useful for the study of abnormal cell cycles, and may have particular application in mutagenesis studies concerning the various subphases of the S phase, without using synchronization techniques. An application of the method is described, showing that gamma-irradiation, during the course of the S phase, leads to a lack of cells which were in early S phase at the time of irradiation. This finding can be related either to a higher lethality at this stage of the cell cycle or to a delay in completion of DNA replication after irradiation.

Sister chromatid exchange studies in the chick embryo and neonate: actions of mutagens in a developing system

The embryonic and neonatal periods represent times when the disease process may be initiated as a result of exposure to environmental mutagens and teratogens. We are using the chick embryo/neonate as an experimental system to detect and study the genotoxicity of environmental chemicals in developing tissues and the resultant biological alterations in survivors of perinatal chemical exposure. In vivo bromodeoxyuridine (BrdUrd) labeling of replicating DNA has been employed to measure basal and induced sister chromatid exchanges (SCEs), a candidate cytogenetic endpoint in genetic toxicology testing. Additionally, SCE induction studies with model promutagens have permitted the detection and study of components of the developing mixed-function oxidase (MFO) system of the liver and other organs. The relationship between specific MFO enzyme induction and SCE generation by promutagens has been studied in ovo and using in vitro assays. The in vivo SCE induction potential of 53 compounds, including known mutagens and nonmutagens, was evaluated in the early chick embryo. About 90% of the mutagens induced SCEs; all nonmutagens failed to induce SCE above baseline. Clastogens such as bleomycin did not induce SCE but did cause massive chromosome damage that was easily detected. Gentian violet (GV) is a direct-acting clastogen that did not show any SCE induction. This agrees with the findings from in vitro mutation assays that incorporate rat liver S-9 preparations. Potency for inducing SCE in the chick embryo correlates well with true mutagenic potency, DNA inhibition, and to some extent with carcinogenic activity. Indirect-acting mutagen-carcinogens induced SCEs and also unscheduled DNA synthesis (UDS) in embryonic cells. Biochemical studies revealed that aryl hydrocarbon hydroxylase (AHH) activity develops in the early embryonic liver as it is first formed at 4-5 da of incubation. The level of AHH activity is sufficient to account for the dramatic SCE response. Enhanced SCE induction occurred in older stage embryos, correlating with the increased basal AHH level and enhanced induction capacity of the liver. Modulation of the MFO enzyme system with specific inducers resulted in altered SCE and UDS responses in vivo and in vitro using a chick microsome/chinese hamster ovary (CHO) mammalian cell assay. Viable embryos and neonates have been obtained following exposure to SCE-inducing levels of the mutagen-carcinogen aflatoxin B1 applied at either 6 da or 12 da of development.(ABSTRACT TRUNCATED AT 400 WORDS)

SCE and DNA methylation

The interrelationship between sister chromatid exchange (SCE) formation and DNA methylation was studied in Chinese hamster V79 and Indian muntjac cells. A DNA methylation inhibitor, 5-azacytidine (5AzaC), induced SCEs only when it had been present in cells for at least 2 rounds of DNA replication. This result suggests that SCEs are formed during replication of hemimethylated or demethylated DNA possessing 5AzaC, and that hypomethylated sites may become fully methylated after they pass 1 cell division. It also appears that hypomethylated DNA is not more sensitive to ultraviolet light (UV) or 3-aminobenzamide (3AMB) than normal chromosomes, but sensitized to mitomycin C (MMC) for the induction of SCEs. An analysis of sites of SCEs induced by 5AzaC within Indian muntjac chromosomes showed that the SCE frequency was enhanced at the 5 methylcytosine-rich regions where spontaneous SCEs were intensively suppressed. The SCE mechanism at the junction between contiguous replicons with different replication timing was discussed.