Difference between diploid and aneuploid Chinese hamster cells in replication at mid-S-phase

Molecular characterization of methyl methanesulphonate (MMS)-induced HPRT mutations in V79 cells

Spontaneous and methyl methanesulphonate-induced HPRT-deficient mutants were analysed for changes in the hprt gene structure using multiplex polymerase chain reaction. The PCR amplification pattern of 21 MMS-induced mutations revealed one total deletion of the hprt coding exons and one small deletion within exon 5, while 19 mutants showed the V79 wild-type pattern. Molecular analysis of 30 spontaneous mutations revealed no mutants with amplification patterns which differed from those of wild-type cells. We further analysed MMS-induced mutants in a different V79 cell line with a high (40%) spontaneous deletion frequency. MMS caused a dose-dependent increase in the mutant frequency but the incidence of deletions was reduced to 6% at 2 x 10(-4) M and to 13% at 5 x 10(-4) M indicating that mainly point mutations were induced. The repair inhibitor cytosine arabinoside (araC) enhanced mutation induction by MMS but did not change the proportion of deletions in the mutation spectrum. The results indicate that different V79 cell lines spontaneously produce different amounts of deletion mutations. The frequency of MMS-induced deletions does not depend on the frequency of spontaneous deletions in a given cell line. The MMS-induced mutation spectrum seems to be unchanged even at high concentrations with a strong cytotoxic effect. Deletions are not increased as a consequence of araC-inhibited repair of MMS-induced lesions.

Replication asynchrony between homologs 15q11.2: cytogenetic evidence for genomic imprinting

Replication kinetics of the Prader-Willi syndrome critical region (15q11.2) was investigated in seven normal healthy adult females using RBG replication bands. Replication asynchrony between homologs 15q11.2 was identified consistently in about 40% of cells in all individuals. It was limited to the stages in which Xp22, Xp11, Xq13 and Xq24/26 were visible in the late-replicating X chromosome. This asynchrony suggested that replication timing overlapped between 15q11.2 and the early replicating R-bands of the late X chromosome in some cells, and that the difference in replication timing between homologs was probably related to genomic imprinting; the latter has been suggested as a pathogenetic basis of Prader-Willi syndrome. As a result of an analysis of the proportions of asynchronous and synchronous cells in each replication stage, two types of cells were deduced providing 1:1 methylation mosaicism of genomic imprinting was assumed. The first type was composed of cells with normal replication in one homolog and delayed replication in the other. The second type was composed of cells with normal replication in both homologs. Our results provide cytogenetic evidence of methylation mosaicism for mammalian genomic imprinting.

BrdU pulse/reverse staining protocols for investigating chromosome replication

By using a reverse Giemsa staining procedure (TT chromatin pale, TB chromatin dark) it is possible to detect replication in metaphase chromosomes with short (approximately 10 min) 5-bromodeoxyuridine (BrdU) pulses. A pulse protocol allows us to consider the question "What is replicating at this point in time?" and we have investigated replication patterns during cycle transit in stimulated human female lymphocytes. A clear-cut demarcation between R-zone early and G-zone late was not found. Instead, whilst replication commences (with a very staggered start) in R-zones, activity soon appears to transgress band boundaries and gives rise to cells with unclassifiable patterns where chromosomes take on a mottled or reticulate appearance. Replication in R-zones dies out leaving a clear G-zone pattern persisting for the remainder of S which terminates with a very staggered finish. When pulse duration is increased (approximately 1 h) the frequency of unclassifiable cells falls and occasional "mixed-pattern" cells appear which have, within the same cell, typical R- and G-zone regions. The existence of such cells indicates that if a mid-S replication pause exists (and the absence of any mid-S wave of pale stained cells suggests that it does not) it does not make exclusive separation between dark R- and G-band zones.

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 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.

Replication time of interspersed repetitive DNA sequences in hamsters

The replication time of 34 hamster genomic DNA segments containing interspersed repeat sequences was determined by probing the cloned segments with nick-translated early- and late-replicating hamster DNA. One-third of these cloned families replicated early, one-third replicated late, and one-third replicated without temporal bias. 19 different inserts from these clones along with the SINE, Alu, and the LINE, A36Fc, were used to probe Southern blots of early- and late-replicating hamster or human DNA. We report long interspersed repeats, LINEs, are selectively partitioned into late-replicating DNA and are often concertedly hypomethylated, while short interspersed repeats, SINEs, are selectively partitioned into early-replicating DNA. For some interspersed repeat families, this partitioning is complete or almost complete. The CCGG frequency is very low in late-replicating DNA. The mammalian chromosome's pattern of early-replicating R-bands and late-replicating G-bands reflects a differential distribution of LINEs and SINEs.

Cytogenetic replication studies with short thymidine pulses in bromodeoxyuridine-substituted chromosomes of different mouse cell lines

In normal diploid fibroblasts of the mouse, 3T3-SV-3T3-, and Meth A-cells, the chromosome replication patterns were studied by a bromodeoxyuridine (BrdU)-labelling technique. SV-3T3 is a subline of 3T3 transformed by SV 40 and Meth A is a permanent cell line from Balb c transformed by methylcholanthrene. The use of 1 h thymidine pulses permits high resolution of the S-phase after partial synchronization of the cells at G1/S in an otherwise BrdU-substituted S-phase. It could be shown that the autosomal heterochromatin of the mouse (Mus musculus) starts replication during the early S-phase (R-band replication), continues while R-band chromatin finishes, and still replicates when G-band chromatin starts. The heterochromatin finishes before the majority of G-bands have been replicated. There is no fundamental difference in the course of chromosome replication between the different cell lines studied here. It is concluded that there are no obligate changes in the course of the S-phase linked to the process of transformation.

Characterization of mitoses with sister chromatid differentiation (SCD) and consequences for the analysis of proliferation kinetics and sister chromatid exchanges in asynchronously growing cells

Labeling cells with bromodeoxyuridine (BrdU) permits the differentiation of mitoses of the first, second, and third generation after the addition of BrdU. The term "second mitoses" is used for those cells which have incorporated BrdU for two S-phases and which exhibit sister chromatid differentiation (SCD). However, SCD can also be obtained if the cell was in S-phase at the time of BrdU-addition and had already replicated part of its DNA. Such cells with incomplete BrdU-substitution in the first S-phase can only be differentiated from completely substituted ones by the quality of the SCD and are usually also grouped as "second mitoses" in the evaluation of experiments. Due to the heterogeneity of the evaluated "second mitoses", the determination of proliferation delay and the incidence of sister chromatid exchange-induction can depend on the time of chromosome preparation.

Characterization of chromosome replication during S-phase with bromodeoxyuridine labelling in Chinese hamster ovary and HeLa cells

Chinese hamster ovary (CHO) and HeLa cells were successively pulse labelled at 1-h intervals after the cultures were synchronized at the end of G1 (monitored by flow cytometry). The metaphases analysed afterwards showed R-type replication patterns after 1-h pulses during the early S-phase (SE; from h 1-5 after release) and replication of G- and C-bands in late S-phase (SL: from h 6-8 after release). The transition from SE to SL is abrupt, constituting a sudden switch of replication between different types of chromatin as has been described for human lymphocytes. The differences between these two cell lines and earlier results reported on a V79 Chinese hamster cell line and on normal diploid human and Chinese hamster fibroblasts are discussed.

Mapping of DNAase I sensitive regions on mitotic chromosomes

We have shown that in fixed mitotic chromosomes from female G. gerbillus cells the inactive X chromosome is distinctly less sensitive to DNAase I than the active X chromosome, as demonstrated by in situ nick translation. These results indicated that the specific chromatin conformation that renders potentially active genes sensitive to DNAase I is maintained in fixed mitotic chromosomes. We increased the sensitivity and accuracy of in situ nick translation using biotinylated dUTP and a specific detection and staining procedure instead of radioactive label and autoradiography and now show that in both human and CHO chromosomes, the DNAase I sensitive and insensitive chromosomal regions form a specific dark and light banding pattern. The DNAase I sensitive dark D-bands usually correspond to the light G-bands, but not all light G-bands are DNAase I sensitive. Identifiable regions of inactive constitutive heterochromatin are in a DNAase I insensitive conformation. Our methodology provides a new and important tool for studying the structural and functional organization of chromosomes.

The contribution of DNA single-strand breaks to the formation of chromosome aberrations and SCEs

The induction of sister chromatid exchanges (SCEs) and chromosomal aberrations (CAs) with bleomycin (BLM), hydrogen peroxide (H2O2), short-wave ultraviolet (UV)-irradiation, and long-wave UV-irradiation was investigated in V79 cells with BrdUrd-substituted DNA. The application of a Neurospora endonuclease (NE) which specifically cleaves single-stranded DNA after these treatments showed that DNA single-strand breaks (SSBs) are induced by these agents. The SSBs are converted to double-strand breaks (DSBs) by NE and become visible as CAs on metaphase chromosomes. H2O2 and both types of UV-irradiation also led to an induction of CAs and SCEs, whereas BLM only induced aberrations. Cysteine (Cys) reduced the frequency of the induced SSB-dependent CAs in all treatments, but had no influence on the SCE frequencies after BLM and H2O2 treatment and had only a slight effect on the UV-induced SCEs. The results confirm the opinion that directly induced SSBs can contribute to the induction of CAs in cells with BrdUrd-substituted DNA, but that these SSBs are not efficiently converted to SCEs. The more recent conceptions regarding the mechanism of SCE are in accordance with these findings and the conclusions derived therefrom.

Characterization of Giemsa dark- and light-band DNA

Using synchronized cultures of V79-8 Chinese hamster lung fibroblasts, we either alternately labeled early- and late-replicating DNA, or substituted one of these with bromodeoxyuridine to separate them in CsCl density gradients or to identify the bromodeoxyuridine-containing chromosome bands by fluorescence microscopy. The Giemsa light R bands were shown to replicate in the first half of S phase, and the dark G bands were shown to replicate in the last half of S phase. S phase was bimodal, with a distinct pause in the rate of DNA synthesis that separated the period of R-band DNA synthesis from that of G-band DNA synthesis. G-band DNA was found to be 3.2% richer in AT than R-band DNA. Surprisingly, G- and R-band DNA appeared equally transcriptionally active in that alternate labels in chromatin were digested with the same kinetics by DNAase I, and in reassociation experiments, total poly(A)+ RNA drove nick-translated G- and R-band DNA probes similarly. G- and R-band DNA also reassociated with identical kinetics, demonstrating that they contain equal proportions of all kinetic-complexity classes of sequences.

A comparative investigation of the antimitotic action of 2-mercaptoethanol and colcemid on V-79 Chinese hamster cells

The current study was performed to characterize the antimitotic action of 2-mercaptoethanol (MET) on mammalian cells. At concentrations of 2.5 x 10(-2) m, MET arrests V-79 Chinese hamster cells in metaphase. Smaller concentrations (from 5 x 10(-3) m) only produce a mitotic block after several hours, only arresting those mitoses which have gone through one cell cycle in the presence of MET. The accumulation of mitoses by MET is smaller in comparison with colcemid, explained by an effect reducing the number of cells which enter mitosis. In contrast to colcemid, MET-concentrations which do not lead to a mitotic block cause a delay in proliferation. It was shown, by means of the BUdR-labelling method that cells in the presence of colcemid concentrations which arrest mitosis again enter interphase and become polyploid, whereas MET leads to an irreversible arrest of mitosis and does not produce polyploidy in V-79 cells.

Comparison between mutagenesis in normal and transformed Syrian hamster fibroblasts: difference in the temporal order of HPRT gene replication

A highly tumorigenic subdiploid cell line, BP6T, derived in our laboratory from Syrian hamster embryo (SHE) cells, is amenable to studies of somatic mutation in vitro. Cellular and biochemical characterization of clonally derived BP6T cells resistant to 6-thioguanine (TGr) or ouabain (Quar) demonstrated these mutants to be similar qualitatively to mutants of SHE cells characterized previously (Barrett et al., 1978). BP6T TGr mutants resistant to 6-thioguanine are cross-resistant to 8-azaguanine, lack HPRT activity, exhibit a low frequency of reversion and arise spontaneously at a rate of approximately 5 X 10(-7) mutants per cell per generation. BP6T Ouar mutants were shown to be highly resistant to ouabain-mediated inhibition of 86Rb influx, indicating an alteration in the Na+/K+ ATPase. These studies on the BP6T cell line provide the experimental basis for a comparative study of the mutagenic responses of normal, diploid SHE cells versus those of related, but transformed aneuploid cells. Highly synchronized cultures of these 2 cells were mutagenized by pulse treatment with BrdU during different periods of S phase, followed immediately by near-UV irradiation. The induced mutation frequencies so obtained provided information about the temporal order of replication of genes encoding HPRT and Na+/K+ ATPase in both SHE and BP6T cells. The temporal pattern of replication of Na+/K+ ATPase gene loci is similar in both cell types, but the temporal order of replication of the HPRT gene is significantly different between SHE and BP6T cells (mid-late S phase, versus early S phase, resp.). This observed difference emphasizes the caution required in the study of mutagenesis and DNA replication using transformed, aneuploid cells under the assumption that the underlying mechanisms are the same for normal, diploid cells.

Synergistic action of cysteamine and BrdU-substituted DNA in the induction of sister chromatid exchanges

The effect of different BrdU-concentrations on the cysteamine-induced SCE-rate was investigated in V-79 Chinese hamster cell monolayer cultures. Both cysteamine and its auto-oxidation product cystamine act synergistically with BrdU in the induction of SCEs. A given concentration of these substances produces a low SCE-frequency at low BrdU-concentrations -- but the incidence of SCEs is significantly increased at increased BrdU-concentrations. -- Using one-cycle substitution experiments and the determination of the relative level of substitution by means of 3HBrdU-incorporation, this synergism was shown to depend on the BrdU incorporated in the DNA and on the extent to which this incorporation takes place.

Asynchrony in late replication between homologous autosomes in primary cultures of Chinese hamster fibroblasts

Asynchronies in late replication of the autosomal chromosome pair No. 5, and to some extent of pair No. 4, were found after thymidine pulse labeling cultures of partially synchronized Chinese hamster lung fibroblasts from nine to nine and a half hours and from nine and a half to ten hours after block removal. In contrast to this, no asynchrony could be detected in the replication of homologous autosomes after continuous labeling for the last two hours of the S-phase. - G-banding and C-banding revealed no differences between the homologous autosomes. - These findings indicate that besides the known form of asynchronous replication in mammalian cells during S-phase on the chromosomal level, there also exists an asynchronous replication between homologous autosomes of the same complement.

Cytological sub-division of S-phase in the Syrian hamster (Mesocricetus auratus)

Using BrdU/Hoechst 33258/Giemsa methods for detecting replicating chromosome bands, a method is described by which the DNA synthesis phase may be sub-divided on the basis of distinctive patterns displayed by certain chromosomes.--Applied to asynchronous populations successively sampled through one cell cycle, cells in S can be "unscrambled" and replaced in their correct time sequence.--This helps to overcome the sampling-time variable inherent in such populations, and allows a clearer picture of the progression of events both qualitatively and quantitatively.