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C H Ockey

Publications and source records attributed to C H Ockey.

At least 19 recordsLinked to original sources
PubMed1989

Fine-structural aspects of bromodeoxyuridine incorporation in sister chromatid differentiation and replication banding.

The structure of harlequin-stained chromosomes following substitution with low levels of 5-bromodeoxyuridine (BrdUrd) over two cell cycles and high levels over the last part of one cycle (replication banding) was studied in Chinese hamster ovary (CHO) cells. By using correlative light (LM) and scanning electron microscopy (SEM), it was shown that the effects of both the ultraviolet light (u.v.) and hot SSC treatment steps of the harlequin staining procedure were necessary to obtain sister-chromatid differentiation (SCD) or replication banding. u.v. treatment alone resulted in dark Giemsa staining of both chromatids with SEM morphology of short compact protuberances and an overall flattened smooth appearance in both the unsubstituted and BrdUrd-substituted chromatids, a morphology essentially similar to that of untreated chromosomes. SSC alone on the other hand resulted in dark-staining chromatids with an SEM morphology of raised, loosely packed loops of fibres in both types of chromatids. u.v. and SSC treatment together resulted in differentiation, with dark-staining unifilarly (TB) chromatids in the LM corresponding to raised loosely packed loops in the SEM and pale bifilarly (BB) chromatids corresponding to the smooth compact flattened SEM appearance. Where the BrdUrd-substituted strand became the template (BT), or when the nascent strand TB contained high levels of BrdUrd substitution in replication banding, the chromatid stained pale and showed the compact smooth appearance in the SEM. The Giemsa staining ability and ultrastructural morphology of harlequin staining is discussed with respect to putative DNA loss and also in terms of preferential protein-protein, protein-DNA cross-linkage in BrdUrd-containing DNA. These changes are also compared with the ultrastructural morphology observed after other banding methods, where deterioration of protein and DNA-protein interaction resulting in aggregation of chromatin fibres appears to be the major mechanism.

Animals↗
PubMed1987

Sister chromatid induction by beta-irradiation from incorporated 3H-thymidine: a paradox explained.

Sister chromatid exchanges (SCE's) induced by [3H]thymidine (3HdT) of increasing specific activities incorporated over one cycle and 5-bromodeoxyuridine (BrdUrd) over the two following cycles were investigated in synchronised Chinese hamster ovary (CHO) cells. SCEs induced during the first cycle on a T.T template (SCE 1) show little increase with dose compared with those induced in the second cycle on a 3HT.T template (SCE 2) where the linear increase with dose reflects that seen after X irradiation. During the third cycle, SCEs 3.1 and 3.2 are induced on unlabelled T.B or labelled 3HT.B templates respectively. These templates are theoretically present in a 1:1 ratio after random segregation at second metaphase. Over practically the entire dose range however, the ratio 3.1/3.2, which decreased with dose, was greater than 1.0 and similar to the high values obtained by other workers. At increasing times after BrdUrd introduction, the ratio decreased from greater than 1.0 to less than 1.0. Measurements showed that the expected 50% level of labelled chromosomes at metaphase in the samples could vary between 42%-59%. Cells with greater than 50% labelled chromosomes were more delayed in the cell cycle due to the 3H-irradiation than those with less than 50%. Early fixations therefore favoured SCE 3.1 while late favoured SCE 3.2. SCEs due to BrdUrd in 3HT.B and T.B templates showed no synergistic interaction with irradiation-induced SCEs. When these BrdUrd-induced SCEs were removed from the totals then the 3H-induced SCE levels in 3HT.T, and 3HT.B templates (SCE 2 and 3.2) were similar and increased at a similar rate with dose. This was 2-3 times faster than in SCE 1 and 3.1 where the SCE levels due to irradiation were again similar but lower than for 2 and 3.2.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

The sizes of cellular deoxynucleoside 5'-triphosphate pools in relation to sensitivity to electron irradiation using sensitive and resistant cell lines.

The deoxynucleoside 5'-triphosphate (dNTP) pool sizes have been determined before and after electron (e-) irradiation in sets of radiation sensitive and resistant cell lines. In the L5178Y mouse lymphoma radiosensitive line (LS), the dTTP pool fell 50% following irradiation, whilst the three other dNTP pools remained unaltered. On the other hand, for the radioresistant line (AII) all four dNTP pools increased by 2-to 3-fold. The dNTP pools of the Chinese hamster radiosensitive (V79) line and radioresistant (V79/79) lines were unaltered by the radiation, but a difference in pool size was present before irradiation, with the pools of the V79 cells being approximately twice those of the V79/79 cells. Two out of the three ataxia telangiectasia cell lines studied show reduced dNTP pools when compared with those of normal human fibroblasts and these pools were also unaltered by the radiation. In the L5178Y and Chinese hamster cells the levels of enzymes involved in the biosynthesis of dNTPs have been determined. In general the higher the level of ribonucleoside diphosphate reductase (RDR) the larger the cellular pools. The observed levels of RDR could, in part, explain the observed results. Increasing the dTTP pool by the addition of deoxythymidine and deoxycytidine to the cell culture with the V79/79 cells reduced their sensitivity to the radiation. These results indicate a relationship between a cell's sensitivity to e- irradiation and the sizes of the cellular dNTP pools. However, the exact nature of any such relationship is unknown.

Animals↗
PubMed1986

Molecular analysis of X-ray-induced mutants at the HPRT locus in V79 Chinese hamster cells.

Spontaneous and X-ray-induced mutants at the hypoxanthine phosphoribosyl transferase (HPRT) locus have been isolated from V79 Chinese hamster cells and characterized at the biochemical and cytogenetic levels. Fourteen spontaneous and 24 X-ray-induced clones were azaguanine and thioguanine resistant, did not grow in HAT medium (AZRTGRHATS) and failed to incorporate significant levels of [14C]hypoxyanthine. Cytogenetic analysis of two spontaneous and eight X-ray-induced mutants revealed no major X chromosome rearrangements. In two induced mutants, one of which was hypotetraploid (mode 35-39) with 2 X chromosomes, the short arm of the chromosome (Xp) was slightly shorter than normal. A third mutant was hyperdiploid (mode 22-23) compared with the parental clone (mode 21). When compared with wild-type clones, no other cytogenetic changes were evident in the remaining mutants. Analysis at the DNA level using a Chinese hamster HPRT cDNA probe showed major deletion of HPRT sequences in two and partial deletion in another two induced mutants. In two of the mutants with deletions of HPRT sequences there was a visible shortening of the Xp arm. In the other six mutants two spontaneous and four induced) no karyotypic changes or alterations in restriction fragment patterns were detected suggesting that they carry small deletions or point mutations at the HPRT locus.

Animals↗
PubMed1986

Delayed DNA maturation, a possible cause of the elevated sister-chromatid exchange in Bloom's syndrome.

Differences in behaviour between the 5-bromodeoxyuridine (BrdU)-substituted template strands in Bloom's syndrome (BS) and normal human fibroblasts have been investigated in order to elucidate the mechanism responsible for the elevated baseline sister-chromatid exchange (SCE) frequency in BS. Alkaline sucrose gradient analysis of the normal and BrdU-substituted DNA strands showed the former to be of higher mol. wt. and of mature size while the latter were of lower molecular size, resulting from breaks introduced during the repair of the BrdU with no differences discernible between BS and normal cells. The rates of removal of BrdU were similar in BS and normal cells, which indicates that the increased SCE level in BS is not due to different rates of repair of the BrdU. The maturation of newly synthesized DNA on a normal template is delayed in BS cells compared with normal cells although it is complete at 18 h, the time it is acting as a template for DNA synthesis. In the presence of a BrdU-substituted template the maturation although further delayed is complete in normal cells by 12 h but in BS cells is not complete even by 30 h, when the newly synthesized strand, due to cell cycle delay produced by the incorporation of BrdU, becomes a template in the next round of DNA synthesis. It is suggested that a similar delay in maturation probably occurs when a new strand containing BrdU is synthesized on a normal template in BS cells. When these strands act as a template they will contain two types of breaks--those due to BrdU repair and those due to delayed maturation. The latter will be responsible for the elevated SCEs in BS cells as the DNA replication forks move through them in a manner similar to that previously reported. The possible implications of differential delays in cell proliferation in BrdU, rates of BrdU removal and extent of DNA maturation in this syndrome are discussed.

Bloom Syndrome↗
PubMed1986

Chinese hamster cells harbouring the Escherichia coli O6-alkylguanine alkyltransferase gene are less susceptible to sister chromatid exchange induction and chromosome damage by methylating agents.

Clones of Chinese hamster V79 cells harbouring the Escherichia coli O6-alkylguanine (O6-AG) alkylphosphotriester (AP) alkyltransferase (ATase) gene (clone 8) or a subclone of it that codes only for O6-AG ATase activity (clone SB) have been exposed to increasing doses of N-methyl-N-nitrosourea (MNU) or methylmethanesulphonate (MMS) and the frequencies of induced sister chromatid exchanges (SCEs) measured. In control (clone 2) cells, SCE induction was almost linearly proportional to dose of MNU or MMS and at the highest doses used (15 or 80 micrograms/ml) SCE frequencies were 6 or 8 times background levels, respectively. Slightly lower levels of MMS-induced SCEs were seen in clone 8 and clone SB cells whilst, in contrast, MNU-induced SCE levels in these two clones were drastically reduced being less than twice background levels at 15 micrograms/ml. After treatment with N-butyl-N-nitrosourea, SCE frequency was similar in all three clones. At higher doses, MNU treatment produced less chromatid aberrations and micronuclei in clone SB than in clone 2 cells. These results suggest that ATase-repairable damage is involved in the induction of SCE, chromosome aberrations and micronuclei in V79 cells.

Alkylating Agents↗
PubMed1985

SCE induction and harlequin staining in mycoplasma-contaminated Chinese hamster cells.

Chinese hamster V79 and CHO cells infected with Mycoplasma hyorhinis show elevated sister-chromatid exchange (SCE) levels but normal cell proliferation and levels of chromosomal aberrations when compared with uninfected cells. Harlequin staining patterns differ from those seen with uninfected cells at similar levels of bromodeoxyuridine (BrdUrd), indicating that BrdUrd is rapidly depleted from the medium by the mycoplasmal uridine phosphorylase and therefore becomes unavailable over the two cell cycles necessary for harlequin staining. Continuous treatment with the antibiotic minocycline restores the SCE level and harlequin staining to that seen in uncontaminated cells. The results suggest that mycoplasma infection should be suspected if harlequin staining patterns indicate a sudden decrease in incorporation of BrdUrd in cells grown in normal levels of BrdUrd.

Animals↗
PubMed1985

Strand breaks arising from the repair of the 5-bromodeoxyuridine-substituted template and methyl methanesulphonate-induced lesions can explain the formation of sister chromatid exchanges.

5-Bromodeoxyuridine (BrdU)-induced sister chromatid exchanges (SCEs) are mainly determined during replication on a BrdU-substituted template. The BrdU, once incorporated, is rapidly excised as uracil (U), and the gap is repaired with the incorporation of BrdU from the medium, which leads to further repair. During the second S period in BrdU medium, this process continues as the strand acts as template. Experiments suggest that 3-aminobenzamide (3AB) delays the ligation of the gaps formed after U excision, resulting in enhanced SCE levels during the second cycle of BrdU incorporation. When normal templates of G1 cells are treated before BrdU introduction with methyl methanesulphonate (MMS), 3AB in the first cycle doubles the MMS-induced SCEs but has no effect on them during the second cycle. When the BrdU-substituted template is treated with MMS in G1 of the second cycle, 3AB again doubles the SCEs due to MMS and also enhances the SCEs resulting from delays in ligation of the gaps following U excision in the BrdU-substituted template. The repair processes of MMS lesions that are sensitive to 3AB and lead to SCEs take place rapidly, while the repair process of late repairing lesions that lead to SCEs appear to be insensitive to 3AB. A model for SCE induction is proposed involving a single-strand break or gap as the initial requirement for SCE initiation at the replicating fork. Subsequent events represent natural stages in the repair process of a lesion, ensuring replication without loss of genetic information.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1984

Modulation of enzyme activity in azaguanine-resistant V79 cells selected by chronic drug exposure.

Exposure of V79 cells to azaguanine (7-21 microM for 2-7 weeks) had little effect on growth or plating efficiency but resulted in gradual acquisition of resistance to 8-azaguanine (AZ) and 6-thioguanine (TG) and loss of ability to grow in HAT. The rate of evolution of the resistant phenotype was dependent on the concentration and duration of exposure to AZ. The increase in proportion of resistant cells was paralleled by a rise in phosphatase activity (pH optimum 7.0-7.5) expressed by intact cells and this preceded the fall in HGPRT activity. Elevated phosphatase activity and a resistant phenotype were stably expressed in clones isolated and cultured in the absence of AZ. Hypoxanthine guanine phosphoribosyl transferase (HGPRT) activity in cell extracts of three resistant clones ranged from 18 to 43% of wild-type levels but was unaltered with respect to substrate affinity and electrophoretic mobility. Mg2+-dependent activity dephosphorylated inosine 5'monophosphate (IMP), guanine 5'monophosphate (GMP), adenosine-5-monophosphate (AMP) and p-nitrophenylphosphate (PNPP) and was also elevated with respect to wild-type levels in resistant cell extracts. Purine nucleoside phosphorylase levels were similar in sensitive and resistant cell extracts. Cross-sensitivity studies with other purine analogues suggest that the elevated phosphatase activity does not contribute to the resistant phenotype. No karyotypic changes were observed in the resistant cell lines.

Animals↗
PubMed1982

Fibre autoradiography of repair and replication in DNA from single cells: the effect of DNA synthesis inhibitors.

DNA fibre autoradiography, after incorporation of high specific activity 3H-thymidine and 3H-deoxycytidine, has been used to investigate repair in DNA fibres from single cells following UV, or methyl-methane sulphonate (MMS) treatment. Asynchronously growing human fibroblasts, leucocytes, and HeLa cells at different phases of the cell cycle have been investigated. Isotope incorporation in repair could be differentiated from that involved in replication by the distribution and density of silver grains along the DNA fibres. Grain distribution due to repair was continuous over long stretches of the fibres and was at a low density, occasionally interspersed with short slightly denser segments. Replication labelling on the other hand, was dense and usually in short tandem segments. Repair labelling was of a similar overall density in fibres from a single cell, but differed in intensity from cell to cell. In mutagen treated Go (leucocytes) or G1 (HeLa cells), repair labelling was not increased by the presence of the DNA inhibitors, hydroxyurea (HU) or 5-fluorodeoxyuridine (FUdR). Repair was not detectable in S cells however, without the use of these inhibitors to reduce endogenous nucleoside production. FUdR enhanced the repair labelling in S cells only slightly, while HU increased it beyond that observed in UV irradiated, HU treated, G1 cells. The intensity of repair labelling in fibres from mutagen treated S cells appears to be proportional to the degree of reduction of DNA chain elongation in replicons.

Autoradiography↗
PubMed1979

Quantitative replicon analysis of DNA synthesis in cancer-prone conditions and the defects in Bloom's syndrome.

A quantitative method of replicon analysis of DNA fibre autoradiographs has been used to study the relationship between mean rate of DNA chain growth (R) and distance between adjacent replicons (ID) in fibroblasts from cancer-prone conditions. Results are expressed in terms of the mean linear regression R = delta +(K.ID)10-2. When replicon behaviour was examined in cells from patients with ataxia telangiectasia, basal cell naevus and Bloom's syndromes grown at high density after 48 h in culture, no significant differences could be found in replicon behaviour between these syndromes and normal cultures. However when Bloom's cells were grown at low density and examined 24 h earlier, the mean rate of chain growth R was reduced compared to normal cells at the same density. Both cell types at high densities at 24 h showed equal but lower R values than at 48 h after plating the cultures. The lower rate of chain growth in Bloom's was accompanied by a longer S-period and cell cycle. Studies of cell proliferation kinetics using consecutive mitoses after bromodeoxyuridine (BUdR) incorporation and harlequin banding showed that Bloom's cells at low cell density require a longer period to recover a normal cell cycle length after plating than do normal cells at the same density. Plating densities and using conditioned media shorten the recovery period in Bloom's cells, and when foetal calf serum/MEM is replaced by human AB serum/McCoy 5a medium as the growth media, cell cycle behaviour of low density Bloom's and normal cells are equal at a much earlier time. It is concluded that the slow rate of DNA chain growth in Bloom's cells is an artefact introduced by culture conditions and also may be present in normal cells at an earlier period. The behaviour of replicons during this recovery period appears to be similar in Bloom's and normal cells except for the time lag. As recovery proceeds, the DNA chain growth in the associated replicon pairs recover progressively. This alters both the mean R value from 0.4 to 0.8 micron/min, the slope of the regression K from less than 1.0 to approximately 1.0 while the distance between initiation sites (ID) remains constant throughout. Pretreatment of all cultures with fluorodeoxyuridine (FUdR) produced the same differential effect on release from DNA synthesis inhibition, that is a similar increase in the activation of normally inactive replicons and a slightly slower rate of chain growth over all replicons. No evidence of a substance released by Bloom's cells in culture capable of increasing the sister-chromatid frequency in normal cells could be found. Since SCE frequencies were found to increase with fixation time after BUdR introduction it is concluded that some of the reported changes could be due to differences in cell cycle kinetics brought about by the different media conditions.

Autoradiography↗
PubMed1978

Fluorescein excitation and emission polarization spectra in living cells: changes during the cell cycle.

Changes in the fluorescein fluorescence emission and excitation polarization spectra in synchronized cultured S3 fibroblasts at G1, mid-S, and mitosis, as well as in human lymphocytes before and after stimulation with mitogens, were studied. In contrast to those measured in aqueous solutions the emission and excitation polarization spectra in living cells exhibit a wavelength dependence characteristic for the state of the cell cycle. Changes in the temperature and in the amount of intracellular water result in quantitative wavelength-independent changes in the polarization spectra. Possible mechanisms for the qualitative wavelength-dependent changes in the fluorescein emission and excitation polarization spectra during the cell cycle are discussed.

Cell Cycle↗
PubMed1977

Analysis of karyotype variation following carcinogen treatment of Chinese hamster primary cell lines.

Chinese hamster primary fibroblasts derived from several embryos were treated with the carcinogens benzo(a)pyrene, 7,12-dimethylbenz(a)anthracene or N-methyl-N'-nitro-N-nitrosoguanidine. Karyotype analysis, sister chromatid exchange frequency, evidence of transformation by growth in agar, cell morphology and reaction to cytocholasin B were tested at regular intervals over many culture passages. Carcinogen treatment was found to shorten the time period before onset of permanent karyotypically changed stem and side lines and in vitro transformation. Chromosomes X, 6 and 10 were more frequently involved in all cultures in these karyotype changes which were usually preceded by a period of chromosome variation. Spontaneous chromatid aberrations and aneuploidy increased in frequency with time in culture and generally appeared prior to the expression of transformation. No specific chromosomes were involved with the different carcinogens. There was no correlation between in vitro transformation and karyotype evolution and the criteria for transformation were present independently of one another. It is suggested that the lack of correlation between the parameters tested indicates that the expression of in vitro transformation is a result of selection for growth advantage from a cell population expressing an increasing degree of genetic instability and variation with time in culture.

Aneuploidy↗