Pattern of condensation of mouse and Chinese hamster chromosomes in G2 and mitosis of 33258-Hoechst-treated cells.

The fluorochrome 33258-Hoechst which binds to DNA and preferentially to A-T-rich regions, inhibits drastically the condensation of the centromeric heterochromatic regions in mouse cell lines. Condensation of all other regions of the chromosomes is also inhibited to some extent. The kinetics of condensation-inhibition of the C-heterochromatin indicates that these regions are being condensed at specific time intervals in the G2 period with a specific order of condensation. The C-heterochromatic regions of mouse chromosomes nos. 9, 12, 14, 15 and 16 condense late in G2 and complete their condensation about 30 min before metaphase. Condensation in G2 of Chinese hamster chromosomes is also inhibited by 33258-H treatment.     

Condensation-inhibition by 33258-Hoechst of centromeric heterochromatin in prematurely condensed mouse chromosomes

The phenomenon of premature chromosome condensation has been applied to study the kinetics of condensation-inhibition exerted by the fluorochrome 33258-Hoechst (33258-H) on the centromeric heterochromatic regions of mouse chromosomes. Asynchronous mouse A-9 cells in culture were fused with mitotic HeLa cells in the presence of 33258-H. Pronounced condensation-inhibition of the c-heterochromatin was observed in prematurely condensed early G2, S and late G1 chromosomes in the 33258-H-treated cells. It is concluded that the c-heterochromatic regions begin to condense quite early in G2, decondense again late in G1 and remain decondensed in the S phase.     

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.     

The pleiotropic effects of 33258-Hoechst on the cell cycle in Chinese hamster cells in vitro

The fluorochrome 33258-Hoechst which binds to double-stranded DNA (dsDNA) has been previously shown to inhibit in several mammalian cell cultures the condensation of chromosomes in phase G2 and early mitosis. We have now found that this drug affects the cell cycle of Chinese hamster cells grown in vitro in several other ways. In cells treated with the drug, phase G2 is prolonged, the rate of DNA replication is drastically reduced and the cells are arrested most probably at very late S phase.     

A third common allele in the transferrin system,Tf C3 , detected by isoelectric focusing

Summary The fluorochrome Hoechst 33258 which binds preferentially to A-T base pairs, drastically inhibits the condensation of A-T-rich centromeric heterochromatin regions in mouse cell lines. The condensation of all other regions of these chromosomes is also inhibited to some extent. The human Y chromosome contains a large heterochromatic region, which is also rich in A-T base pairs. This chromosome is not affected by Hoechst 33258 in human leukocyte cell cultures. On the other hand, condensation of the multiple copies of human Y chromosome in the mouse-human cell hybrid RH-28Y-23 is inhibited and the chromosomes appear distorted in Hoechst 33258-treated cells.     

Chromatid repulsion associated with Roberts/SC phocomelia syndrome is reduced in malignant cells and not expressed in interspecies somatic-cell hybrids.

Different cell types from a female patient with Roberts/SC phocomelia syndrome were evaluated quantitatively for the presence of repulsion of heterochromatin and satellite regions of mitotic chromosomes. Whereas EBV-transformed lymphoblasts from an established cell line revealed these phenomena at frequencies equal to those in PHA-stimulated lymphocytes and cultured skin fibroblasts, aneuploid cells from a metastatic melanoma displayed them at 50% lower frequency. Cocultivation of the patient's fibroblasts with either an immortal Chinese hamster cell line or with a human male fibroblast strain carrying a t(4;6)(p14;q21) translocation showed that the phenomenon was not corrected or induced by a diffusible factor or by cell-to-cell contact. In each experiment, only the patient's metaphase spreads revealed chromatid repulsion. In fusion hybrids between the patient's fibroblasts and an established Chinese hamster cell line, the human chromosomes behaved perfectly normally, suggesting that the gene product which is missing or mutant in Roberts/SC phocomelia syndrome is supplied by the Chinese hamster genome.     

DNA image-fluorimetry of individual human chromosomes

A microfluorimetric method has been developed for determination of DNA content in individual human chromosomes. The method is based on a preliminary identification of chromosomes with Hoechst 33258 followed by staining of the chromosomes with Feulgen reaction by using Schiff’s reagent type ethidium bromide-SO₂ and then by measuring the fluorescence intensity of the chromosomes by using an image analyzer. The method allows determining the DNA content of individual chromosomes with an accuracy up to 4.5 fg. The DNA content of individual human chromosomes and their p-and q-arms, as well as homologous chromosomes, were measured by using the developed method. It has been shown that the DNA content in chromosomes of the normal human karyotype is unstable and can fluctuate in some chromosomes within 35–40 fg.     

Tissue-specific heterogeneity in DNA replication patterns of human X chromosomes

Regional DNA replication kinetics in human X chromosomes have been analysed using BrdU-33258 Hoechst-Giemsa techniques in five cell types from human females: amniotic fluid cells, fetal and adult skin fibroblasts, and fetal and adult peripheral lymphocytes. In all cell types, the late-replicating X chromosome can be distinguished from its active, earlyreplicating homologue, and both the early and late X exhibit temporally and regionally characteristic internal sequences of DNA replication. The replication pattern of the early X in amniotic fluid cells and skin fibroblasts is similar to that of the early X in lymphocytes, although certain discrete regions are later-replicating in these monolayer tissue culture cells than are the corresponding regions in lymphocytes. However, DNA replication kinetics in late X chromosomes from amniotic fluid cells and skin fibroblasts are strikingly different from those observed in lymphocytes with respect both to the initiation and termination of DNA synthesis. The predominant late X pattern observed in 80–95% of lymphocytes, in which replication terminates in the long arm in bands Xq21 and Xq23, was never seen in amniotic fluid cells or skin fibroblasts. Instead, in these cell types, bands Xq25 and Xq27 are the last to complete DNA synthesis, while bands Xq21 and Xq23 are earlier-replicating; this pattern is similar to the alternative replication sequence observed in 5–20% of lymphocyte late X chromosomes. This replication sequence heterogeneity is consistent with the existence of tissue-specific influences on the control of DNA replication in human X chromosomes.     

Energy transfer and binding competition between dyes used to enhance staining differentiation in metaphase chromosomes

The ability of electronic energy transfer and direct binding competition between pairs of dyes to enhance contrast in human or bovine metaphase chromosome staining patterns is illustrated, and the relative effectiveness of these two mechanisms compared. The existence of energy transfer between quinacrine or 33258 Hoechst and 7-amino-actinomycin D in doubly stained chromosomes is demonstrated directly by microfluorometry. The ability of the dyes 7-amino-actinomycin D, methyl green, or netropsin, acting as counterstains, to displace quinacrine, 33258 Hoechst, or chromomycin A₃ from chromosomes, is estimated by quantitative analysis of energy transfer data, by photobleaching of the counterstains, or by selective removal of counter-stains by appropriate synthetic polynucleotides. Effects on the fluorescence of soluble 33258 Hoechst-DNA complexes due to energy transfer or binding displacement, by actinomycin D or netropsin, respectively, are further differentiated by nanosecond fluorescence decay measurements. Examples are presented of dye combinations for which (a) energy transfer is the primary mechanism operative, (b) binding competition exists, with consequences reinforcing those due to energy transfer, or (c) binding competition is the most important interaction. These analyses of mechanisms responsible for contrast enhancement in doubly stained chromosomes are used to derive information about the relationship between chromosome composition and banding patterns.     

Kinetochore formation in experimentally undercondensed chromosomes

Summary Treatment of human and mouse cell cultures with the cytidine analogue 5-azadeoxycytidine and the AT-specific DNA ligand Hoechst 33258 dramatically inhibited condensation of the pericentromeric heterochromatin in several chromosomes. When stained with antikinetochore autoimmune sera, these experimentally undercondensed chromosomes showed kinetochores with preserved antigenicity. The undercondensed and normally condensed chromosomes share the major antigenic determinants of the kinetochore.     

Relationship between the number and function of human ribosomal genes

Summary The relative number of ribosomal RNA genes of the acrocentric chromosomes in one individual was measured by counting grains after in situ hybridization of ³H-labeled human 18S rDNA to fixed metaphase chromosomes. The relative amount of ribosomal RNA gene activity of each of the same chromosomes was estimated by determining the frequency with which the chromosome's nucleolus organizer region (NOR) was silver stained, the size of the silver-stained region, and how often the chromosome was found in satellite association. Results were similar in phytohemagglutinin-stimulated T-lymphocytes, Epstein-Barr virus transformed lymphoblasts, and fibroblasts. One chromosome 21 had few gene copies and low activity. One chromosome 22 had many gene copies but low activity. Both chromosomes 14 had few gene copies but high activity. The level of expression that can be achieved by rRNA gene clusters can, therefore, be determined by factors other than the number of gene copies.     

Replication of X chromosomes in complete moles

Summary DNA replication patterns of X chromosomes in complete hydatidiform moles were studied using cultured fibroblasts from three 46,XX moles resulting from duplication of a haploid sperm, and from a 46,XY mole originating from dispermy. Control cultures included skin fibroblasts from an adult woman and a female fetus as well as PB lymphocytes from an adult woman. Cultures were treated with 5-bromodeoxyuridine for the last 2–4h of the S phase, and the chromosome slides prepared were stained by the Hoechst 33258-Giemsa procedure. Each of the three XX moles studied revealed one early-replicating and one late-replicating X chromosomes, while the XY mole revealed one early-replicating X chromosome. DNA replication patterns of molar X chromosomes were similar to those of adult and fetal fibroblasts, but different from those in adult lymphocytes. These findings indicate that DNA replication kinetics of molar fibroblasts are tissue-specific rather than origin- or developmental-stage specific.     

Human X chromosomes: synchrony of DNA replication in diploid and triploid fibroblasts with multiple active or inactive X chromosomes

We have analyzed patterns of DNA replication in X chromosomes from diploid cultured human fibroblasts and from three triploid 69,XXY fibroblast strains, using BrdU--33258 Hoechst--Giemsa techniques. Both X chromosomes in each of these Barr body-negative triploid strains were early-replicating. The results of gene dosage studies using (1) a histochemical stain to measure X-linked glucose-6-phosphate dehydrogenase (G6PD) activity in single cells and (2) cellulose acetate electrophoresis of G6PD activity in cell extracts also indicated that both Xs in these strains were genetically active. When we compared the synchrony of X chromosome DNA replication kinetics both between cells and within cells containing multiple inactive Xs, a marked variability and asynchrony was observed for late-replicating X chromosomes. In a culture of 47,XXX fibroblasts administered an 8-h terminal pulse of dT after growth in BrdU-containing medium, asynchrony was detected between the two late-replicating Xs in approximately 70% of cells examined. No such asynchrony was observed between the two early-replicating Xs in similarly cultured 69,XXY cells; in the triploid strains, the two Xs were distinguished by asynchronous replication in only approximately 15% of cells. The striking variability in late X chromosome replication kinetics appears, then, to be a property unique to inactive Xs and is not inherent to all X chromosomes.     

Identification of the microtubule-associated protein map2 and its binding sites on metaphase chromosomes from cultured cells

The microtubule-associated protein MAP₂, which binds preferentially to AT sequences of DNA, can bind to metaphase chromosomes. The binding pattern of MAP₂ to chromosomes is similar to that found for the binding of the bisbenzimidazole derivative 33258 Hoechst, which also binds preferentially to AT-rich regions.     

Immunofluorescent detection of histone 2B on metaphase chromosomes using flow cytometry

A monoclonal antibody against histone 2B (anti-H2B) was used as a reagent to stain isolated chromosomes for analysis using flow cytometry. Chromosome suspensions were treated with a mouse monoclonal antibody specific for the histone 2B (clone HBC-7) and then with a fluorescein-labeled goat anti-mouse-IgM antibody. The chromosomes were also stained for DNA content with either Hoechst 33258 or propidium iodide. The amount of antibody and the amount of DNA-specific stain bound to each chromosome were measured simultaneously using flow cytometry. The order of the steps in the staining protocol is important. Propidium iodide prevents anti-H2B from binding to chromosomes, and therefore must be added only after antibody labeling is completed. In contrast, the addition of Hoechst 33258 before antibody labeling reduces antibody binding by only 20%–30%. Binding of anti-H2B was proportional to the DNA content of both human and Chinese hamster chromosomes. Human chromosomes bind an average of three to four times more anti-H2B than do Chinese hamster or mouse chromosomes of the same DNA content. This was determined by analyzing mixtures of human and Chinese hamster chromosomes and human and mouse chromosomes. The results demonstrate that it is possible to label the proteins of chromosomes in suspension with fluorescent antibodies and to use these reagents for the analysis of chromosome structure by flow cytometry.     

Identification of human and mouse chromosomes in human-mouse hybrids by centromere fluorescence

Differences between the centromeric regions of mouse and human chromosomes have been revealed with a staining technique which is based on the quenching by 5-bromodeoxyuridine of the fluorescence of the dye 33258 Hoechst. The differences can be used to distinguish between human and mouse chromosomes in human-mouse hybrids.     

The different binding modes of Hoechst 33258 to DNA studied by electric linear dichroism.

The binding mode of the bisbenzimidazole derivative Hoechst 33258 to a series of DNAs and polynucleotides has been investigated by electric linear dichroism. Positive reduced dichroisms were measured for the poly(dA-dT).poly(dA-dT)- and poly(dA).poly(dT)-Hoechst complexes in agreement with a deep penetration of the drug into the minor groove. Similarly, the drug displays positive reduced dichroism in the presence of the DNAs from calf thymus, Clostridium perfringens and Coliphage T4. Conversely, negative reduced dichroisms were obtained when Hoechst 33258 was bound to poly(dG-dC).poly(dG-dC), poly(dA-dC).poly(dG-dT) and poly(dG).poly(dC) as well as with the GC-rich DNA from Micrococcus lysodeikticus indicating that in this case minor groove binding cannot occur. Substitution of guanosines for inosines induces a reversal of the reduced dichroism from negative to positive. Therefore, as anticipated it is the 2-amino group of guanines protruding in this groove which prevents Hoechst 33258 from getting access to the minor groove of GC sequences. The ELD data obtained with the GC-rich biopolymers are consistent with an intercalative binding. Competition experiments performed with the intercalating drug proflavine lend credence to the involvement of an intercalative binding rather than to an external or major groove binding of Hoechst 33258 at GC sequences.     

A comparison of the effect of 5-bromodeoxyuridine substitution on 33258 Hoechst- and DAPI-fluorescence of isolated chromosomes by bivariate flow karyotyping

Application of the fluorescent DNA-intercalator propidium iodide for stabilization of the mitotic chromosome structure during isolation of chromosomes from V79 Chinese hamster cells and subsequent staining with the fluorochromes 33258 Hoechst or DAPI allowed bivariate flow karyotyping of isolated chromosomes. Fluorescence of 33258 Hoechst bound to isolated chromosomes containing 5-bromodeoxyuridine (BrdUrd) was quenched in comparison with the fluorescence of control chromosomes. Despite structural relationship and similarity of both absorption and fluorescence spectra of DAPI and 33258 Hoechst, reduction of fluorescence of DAPI-stained isolated chromosomes was not observed, by contrast with findings in conventional cytological metaphase preparations. It could be obtained, however, by preirradiation of the chromosomes with near-UV in the presence of DAPI. This led to a progressive destruction of the chromosomes. Destruction also occurred without BrdUrd, though at a slower rate. Preirradiation of chromosomes in the presence of 33258 Hoechst hardly affected the integrity of the chromosomes. Preirradiation of a 33258 Hoechst solution and its subsequent use as a stain resulted in a considerably decreased fluorescence of chromosomes. For DAPI this effect was small. Thus, whereas 33258 Hoechst itself is much more sensitive to near-UV irradiation than DAPI, DAPI bound to DNA in chromosomes renders the DNA much more sensitive to irradiation than 33258 Hoechst bound to DNA. Presumably, these differences can at least partly be reduced to the different molecular sizes of the dyes.     

Correction of combined β-galactosidase/neuraminidase deficiency in human fibroblasts

The combined deficiency of β-galactosidase and neuraminidase in human fibroblasts can be corrected to nearly normal values. This can be accomplished by addition of concentrated culture medium obtained after NH₄Cl stimulation of different types of human fibroblasts, including those with an isolated β-galactosidase or neuraminidase deficiency. The corrective factor is a macromolecular glycoprotein, which is labile at 60°C. Its uptake by human fibroblasts is competitively inhibited by mannose-6-phosphate and its corrective action within β-gal^?/neur^? fibroblasts continues during a “chase” of 72 hours.     

Cytochemical studies of metaphase chromosomes by flow cytometry

The cytochemical properties of metaphase chromosomes from Chinese hamster and human cells were studied by flow cytometry. This technique allows precise quantitation of the fluorescence properties of individual stained chromosome types. Chromosomes were stained with the following fluorescent DNA stains: Hoechst 33258, DAPI, chromomycin A₃, ethidium bromide, and propidium iodide. The relative fluorescence of individual chromosome types varied depending on the stain used, demonstrating that individual chromosome types differ in chemical properties. Flow measurements were performed as a function of stain and chromosome concentration to characterize the number and distribution of stain binding sites. Flow analysis of double stained chromosomes show that bound stains interact by energy transfer with little or no binding competition. For most hamster chromosomes, there is a strong correlation between relative fluorescence and stain base preference suggesting that staining differences may be determined primarily by differences in average base composition. A few hamster chromosome types exhibit anomalous staining which suggests that some other property, such as repetitive DNA sequences, also may be an important determinant of chromosomal staining.