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

Weiss Lecture. Effects of radiations of different qualities on cells: molecular mechanisms of damage and repair

Studies of ionizing radiations of different quality are discussed with particular emphasis on damage to DNA of mammalian cells. Three related themes are followed. Firstly, inactivation and mutation experiments with ultrasoft X-rays and slow heavy ions, coupled with theoretical analyses of the structures of the radiation tracks, have emphasized the biological importance of localized track features over nanometre dimensions. This led to the suggestion that the critical physical features of the tracks are the stochastic clusterings of ionizations, directly in or very near to DNA, resulting in clustered initial molecular damage including various combinations of breaks, base damages, cross-links, etc. in the DNA. The quantitative hypotheses imply that final cellular effects from high-LET radiations are dominated by their more severe, and therefore less repairable, clustered damage, and that these are qualitatively different from the dominant low-LET damage. Second, relative effectiveness of different types of radiation led to questions on the mechanisms of induction of chromosome exchanges. The high efficiency of ultrasoft X-rays, despite their very short track lengths, suggested that single sites of DNA damage may lead to exchanges by a molecular process involving interaction with undamaged DNA. Also it is shown that a single site-specific DNA break, introduced by restriction enzymes, sometimes leads to a large deletion when misrepaired by cell extracts. These deletions occur between short DNA repeats, and are therefore a form of 'illegitimate' recombination, but clearly do not involve the interaction of two damage sites. Third, it was shown that cells from patients with the radiosensitive disorder ataxia-telangiectasia (AT) lack a post-irradiation recovery process. The sensitivity of AT cells to high LET radiations was found to be reduced relative to that for normal cells, reinforcing the concept that high LET damage is less easy to repair. AT patients are prone to lymphoreticular cancers, and their cells show characteristic chromosomal rearrangements, which may be associated with misrepair at specific genomic sequences. Similarly, studies of radiation-induced leukaemia in the mouse have implicated rearrangement at specific interstitial chromosome sites, which are rich in telomere-like repeat sequences.

The effect of X-irradiation on cell cycle progression and chromatid aberrations in stimulated human lymphocytes using cohort analysis studies

Stage sensitivity for the production of chromatid-type aberrations and mitotic delay has been investigated in a stimulated human lymphocyte population, following an absorbed dose of 1.5 Gy 250 kVp X-rays. BrdU replication banding was used to obtain a fine analysis of the cell cycle and to permit cohort analysis. Fluctuations in yield with sample time were found for all aberration categories, but these could not be related simply to either the developmental stage of the cells at time of exposure, or to the time-to-run to metaphase. In general G2 and late S cells had higher aberration yields than early S and pre S cell populations. Mitotic delay and perturbation at this dose extends to all sub-phases of S and is as great, if not greater, in the earliest S cells as it is in G2.

Demonstration of replication patterns in the last premeiotic S-phase of male Chinese hamsters after BrdU pulse labeling

Chromosome replication in the last premeiotic S-phase of male mammals has been previously studied by [3H]thymidine autoradiography and by a 5-bromodeoxyuridine (BrdU)/Giemsa technique. We used a recently developed BrdU-antibody technique (BAT) in this study. The following conclusions were drawn: (1) The replication patterns observed are similar to that of somatic cells. (2) The heterochromatin starts replication in early S-phase. (3) The euchromatic part of the X chromosome of the male Chinese hamster replicates together with the autosomes and therefore behaves isocyclicly and not allocyclicly as hitherto assumed. Hence, genetic inactivity of the X chromosome may be brought about by a mechanism different from that in somatic cells.

Integration site preferences of endogenous retroviruses

Retroviruses have the ability to integrate into the genome of their host, in many cases with little apparent sequence or site specificity. However, relatively few studies have addressed more general features of chromosomal integration. In this study we directly visualized the chromosomal organization of three representative endogenous retroviruses by in situ hybridization. Because there are 50-1000 copies of each of these retroviruses in the genome, it was possible to evaluate repeated integration events. Each retroviral sequence exhibited a unique and markedly different integration pattern. In order to characterize more precisely the chromosomal domains targeted by each retrovirus, later replicating domains were differentially labeled. Additionally, prototypic SINES and LINES (short and long interspersed reiterated sequences), which are inhomogeneously distributed on chromosome arms, were simultaneously detected. Retroviral copies of greater than or equal to 2 kb were found (i) exclusively in a discrete set of later replicating domains, most of which have the staining characteristics of constitutive heterochromatin, (ii) widely represented in disparate types of chromosome domains, or (iii) almost completely confined to CpG Alu-rich regions that are known to be early replicating. Retroviral elements in Alu-rich domains would be expected to be actively transcribed in all cells. Surprisingly, hybridization to blots of brain RNA showed an approximately 25 fold lower level of transcripts from these Alu associated elements than from retroviral sequences restricted to later replicating, heterochromatic domains. Retroviral insertions may subvert more typical transcriptional characteristics of a domain. The present results indicate that there are highly specific integration patterns for each endogenous retrovirus that do not readily relate to their sequence or particle classification. Each host genome may utilize these elements for contrary, and possibly beneficial functions.

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.

Analysis of chromosome replication by a BrdU antibody technique

Chromosome replication was studied without synchronization in human lymphocyte and amniotic cell cultures visualizing very short 5-bromodeoxyuridine (BrdU) pulses by an immunologic technique (BAT). The findings agree in general with those facts known from earlier BrdU staining techniques. The very high sensitivity of BAT was shown to allow the detection of replication in a band where 1 in 200 nucleotides is replaced by BrdU. The main observations are: though the replication patterns after BAT appear strange the bands correspond to those described by the Paris Conference (1971). At the beginning of the S-phase a stepwise onset of replication in only a subset of R-bands is confirmed. There is a considerable difference in the sensitivity between early and late S (SE and SL) for the detection of BrdU pulses. This difference probably reflects a different spatial arrangement of chromatin in R-bands as compared with G-bands below the level of cytogenetic analysis. The use of short pulses did not reveal any additional subdivision of SE or SL. The correspondence between chromosomal bands and replicon clusters is discussed briefly with respect to the different time they need for replication.

Generalized blocking in S phase by methotrexate

An attempt was made to enhance the frequency of prometaphase cells for high-resolution-banding studies in untransformed Syrian hamster fibroblasts using a typical methotrexate (MTX) block/bromodeoxyuridine release schedule. The recovery 'wave' was serially sampled and detailed sub-phase analysis made using the replication bands resulting from bromodeoxyuridine uptake. Of the 3 batches of MTX used, one (Sigma greater than 2 years old) was found to have decayed to a non-toxic compound which produced almost no measurable perturbation of the cell cycle at any concentration used. The other 2 (new Sigma and new Lederle), whilst producing mitotic index fluctuations which could be construed to indicate blocking and "synchrony", gave absolutely no evidence of any specific blocking site, but rather a general stoppage (or slowing down) during MTX treatment and continuation in exactly the same order as untreated controls upon release.

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

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

The induction of chromosome exchange aberrations by carbon ultrasoft X-rays in V79 hamster cells

V79 hamster cells in plateau (extended G1) phase were irradiated with either 250 kV ('hard') X-rays or carbon K characteristic ultrasoft X-rays under conditions minimizing cell overlap. These cells were killed most effectively by the carbon X-rays, by a factor of about 3 relative to hard X-rays, in agreement with our previous findings with cells in exponential growth. Chromosome-type aberrations were measured at 3 fixation times within the first division cycle after irradiation, and an approximately uniform sensitivity to aberration induction was found for both radiations. The combined aberration data show that carbon X-rays are 2 or more times as effective as hard X-rays, depending on dose and/or data fit. Exchange aberrations require recombination between two separate chromosomes, but they are induced efficiently by carbon X-rays with a substantial linear component to the dose-response despite the very short electron tracks (approximately less than 7 nm) that they produce in the cell. This implies either that the participating DNA helices must be lying extremely close together at the time of radiation damage, so that one track can effectively damage both helices, or that only one radiation-damaged chromosome is needed to promote an exchange event.

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 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.

Subdivision of S-phase and its use for comparative purposes in cultured human cells

Intranuclear DNA synthesis and concomitant chromosome duplication occur during a discrete period of the cell cycle termed S-phase. Using replication-banding and serial time sampling in asynchronous cell populations, it is possible to subdivide the S-phase into four or five chronological compartments termed "subphases". This paper discusses methods for analysing the sampling data to obtain the average duration of these subphases and the positions within S of the borders between them. Such information not only allows a more detailed analysis of the cell cycle, but also provides parameters which can be used for rigorous comparisons of cell populations from different sources and experimental conditions. Examples are given of application of the method to normal and chromosomally abnormal primary human fibroblasts and lymphocytes growing in short-term in vitro culture.

Cytological subdivision of the S phase of human cells in asynchronous culture

A method is described for subdividing S phase cells in asynchronous cell cultures on the basis of replication band patterns produced in chromosomes by bromodeoxyuridine incorporation. The criteria used for cell classification are objective, requiring the presence or absence of specific bands on particular chromosomes, and therefore lead to subdivisions amenable to quantitative analysis and for comparative purposes. Two schemes are given: key 1, based on bands in chromosomes 2 and 5, leads to five sub-phases; and key 4, based on bands in chromosomes 3 and 4, leads to four sub-phases. The order of the sub-phases, though not their relative durations, is identical in the six primary cell cultures (four fibroblast and two lymphocyte) tested. The technique provides for a detailed study of the programme of chromosome replication in normal and abnormal cells which, in time, should produce new criteria for diagnostic purposes.

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.

The disparity between homologous chromosomes during DNA replication

When the patterns of replicating chromosome bands of homologous chromosomes within a diploid cell during DNA synthesis phase are compared, the frequency of disparity (i.e. a band present on only one homologue) is less than expected on the basis of chance. This could be taken as implying some "link" between homologues which constrains their programmes of replication to keep in step. This paper develops a model showing that the observed disparities can be accommodated within a framework of homologue independence. Differences between cells in the time of appearance of bands lead in any sample to the summation of an infinitude of binomial distributions and hence to over-dispersion. The model fits observed data for bands replicating in euchromatic and heterochromatic chromosome regions obtained from Syrian hamster fibroblast cells growing in vitro.

Early-replication DNA patterns in a derivative chromosome in a Syrian hamster with only 42 chromosomes

From crosses within a 2n=43 line of Syrian hamsters (Mesocricetus auratus) lacking one derivative (der 11) of an 11;20 reciprocal translocation we have obtained homozygotes with only 42 chromosomes. These animals are homozygous deficient (nullisomic) for the centromere and short arm of chromosome 11 and for the bulk of the long arm of chromosome 20. -During cytogenetic studies, we investigated the frequency patterns of early-replicating bands in the surviving derivative (der 20) at two cytologically defined sub-phases of S using short-term fibroblast cultures. These patterns were compared with those observed in the component, untranslocated arms in normal 2n=44 cells at the same two sub-phases. -Very close agreement was found, indicating that neither the nullisomy, nor the new arm combination has interfered detectably with the pattern or programme of early band replication.

Differential effects of sulphur mustard on S-phase cells of primary fibroblast cultures from Syrian hamsters

Sulphur mustard (SM) (5 x 10(-8) M) given to primary Syrian hamster fibroblasts growing short-term in vitro produces a very sharp peak of chromatid aberrations 12-16 h after treatment. The composition of this peak has been investigated in relation to the cell cycle using the facility to divide S-phase into 5 cytologically defined sub-phases on the basis of replication band patterns following bromodeoxyuridine incorporation. It is shown that: (1) Considerable delay and perturbation of the cycle is present. (2) A major contribution to the aberration peak comes from pre-S cells, and the highest aberration frequencies are observed in such cells. (3) The bulk of the contribution from S-cells comes from the last subphase, SkV. (4) The majority of early S cells (sub-phases SkI-IV) fail to reach division within 36 h. Of the total S-cells scored, 54% are non-SkV in controls but only 14% after SM. (5) Aberrations are localized to late-replicating regions in SkV but are random in pre-S. (6) No measurable perturbation of replication programme was found in chromosome arms synthesizing in late SkV after SM. (7) The rapid fall in aberration frequency at later sampling times is consistent with a quick and efficient repair of DNA lesions which is known to occur after SM alkylation. The preferential loss of early S cells seems most likely to result from selective lethality, though differential delay and perturbation may make a contribution. It is interesting to note that the subphases missing are just those where the euchromatin replicates (early replicating R and G bands). The late-replicating chromatin, some of which is known to be dispensable in Syrian hamster, is confined to SkV, the sub-phase which appears to survive this dose of SM.

Cell-specific variation of X chromosome replication in the Syrian hamster (Mesocricetus auratus)

The sub-division of S-phase in Syrian hamsters, on the basis of Brd/U/Hoechst 33258/Giemsa banding, has allowed a quantitative comparison of the replication of individual chromosome bands within defined subphases of S. This analysis has shown that in hamsters, as has been reported in humans, there are distinct patterns of early replication in vitro in the early X, the late X in fibroblasts, and the late X in lymphocytes. In addition, it has been possible to show that, although the pattern of replication of the late X in fibroblasts differs from that in lymphocytes, the time in S at which bands first appear on this chromosome is the same in the two cell types. No significant heterogeneity can be ascribed to differences between individuals, adult or embryonic sources, culture media, or time of exposure to BrdU. The absence of any detectable heterogeneity in the replication band frequencies in autosomal heterochromatic arms suggests that the cell-specific variability of the late-replicating X is a feature of facultative rather than constitutive heterochromatin.

Chromosome replication in fibroblasts of the Syrian hamster (Mesocricetus auratus)

The BrdU/Hoechst 33258/Giemsa method for sub-dividing S-phase in asynchronous cell populations has been re-evaluated and modified to give better definition and more even distribution of sub-phases. A reference pattern of early-replicating euchromatic bands is given for all chromosomes at Sk2 in primary cultures of skin fibroblasts. The overall band patterns at each sub-phase have allowed more objective definitions of "early" and "late" replication for these cells, and show that in both classes of chromatin light G-bands preceed dark G-bands. Asynchrony between homologous bands is observed at all stages of S, albeit with a variable frequency. The observed in vitro replication patterns and programme for the chromosomes of skin fibroblasts does not appear to be affected by the age or sex of the source.

The induction of chromosomal aberrations by X irradiation during S-phase in cultured diploid Syrian hamster fibroblasts

The induction of chromosomal aberrations by 4.0 Gy of 25 kV X-rays in cell throughout S-phase has been investigated in untransformed diploid Syrian hamster fibroblasts. Using a method of subdividing S into cytologically defined stages (on the basis of replication band patterns displayed after bromo-deoxyuridine incorporation) it is shown that: (1) This dose does not perturb, measurably, the intracellular programme of synthesis at the chromosome band level, so that the cell classification criteria remain valid after radiation. (2) Mitotic delay and perturbation appears to be less for cells in very early S, but there is no evidence of a massive cell mixing of S cells. (3) S-phase is, in general, much less sensitive to aberration induction at all sub-phases than G2. (4) Both chromosome and chromatid-type aberrations are found in pre- S and S cells, but chromatid-types predominate in the latter at all sub-phases. (5) The frequency of chromatid-types, especially interchanges falls in early S. Syrian hamsters have large blocks of non-centromeric late-replicating autosomal chromatin, and unambiguous chromatid changes can be observed in these prior to detectable synthesis. Although the resolution afforded by the BrdU method is much better than that with radioactive labelling, it is still very poor compared with events at the molecular level, and it is felt that this finding cannot be taken as conclusive evidence that chromosomes appear double to radiation prior to synthesis.