Spontaneous and 3-aminobenzamide-induced sister-chromatid exchange frequencies estimated by ring chromosome analysis

Drosophila ring chromosomes interact with sisters and homologs to produce anaphase bridges in mitosis

Ring chromosomes are known in many eukaryotic organisms, including humans. They are typically associated with a variety of maladies, including abnormal development and lethality. Underlying these phenotypes are anaphase chromatin bridges that can lead to chromosome loss, nondisjunction and breakage. By cytological examination of ring chromosomes in Drosophila melanogaster we identified five causes for anaphase bridges produced by ring chromosomes. Catenation of sister chromatids is the most common cause and these bridges frequently resolve during anaphase, presumably by the action of topoisomerase II. Sister chromatid exchange and chromosome breakage followed by sister chromatid union also produce anaphase bridges. Mitotic recombination with the homolog was rare, but was another route to generation of anaphase bridges. Most surprising, was the discovery of homolog capture, where the ring chromosome was connected to its linear homolog in anaphase. We hypothesize that this is a remnant of mitotic pairing and that the linear chromosome is connected to the ring by multiple wraps produced through the action of topoisomerase II during establishment of homolog pairing. In support, we showed that in a ring/ring homozygote the two rings are frequently catenated in mitotic metaphase, a configuration that requires breaking and rejoining of at least one chromosome.

Sister chromatid exchanges in ring chromosomes following X‐irradiation of human lymphocytes

PURPOSE

To examine whether X-rays induce sister chromatid exchanges (SCE).

MATERIALS AND METHODS

Peripheral lymphocytes irradiated in vitro or in vivo were cultured and treated with okadaic acid to generate premature chromosome condensation (PCC). When identical spreads were analysed using conventional Giemsa staining and pan-centromeric fluorescence in situ hybridization painting, ring chromosomes were observed.

RESULTS

In PCC preparations, cells in the late G(2) phase and late M phase were observed. In late M phase cells, 17-20% of ring chromosomes lacked one chromatid (single-chromatid ring), irrespective of dose. Both the distribution patterns of centromeres in rings and intercentromere distances in dicentric rings indicate that a considerable number of single-chromatid rings might be formed by SCE occurring in a chromosome-type ring, thereby joining strands of two rings, followed by a transformation into one ring. These single-chromatid rings were less stable in vivo than chromosome-type rings.

CONCLUSION

Single-chromatid rings visualized clearly using PCC techniques indicate SCE in the respective rings. Contrary to the conventional SCE-detecting technique, this approach does not require the use of bromodeoxyuridine, which itself leads to SCE. Some of the observed SCE might be secondary products resulting from the repair of radiation-induced DNA damage, while others may be spontaneous.

Growth-phase-dependent response to DNA damage in poly(ADP-ribose) polymerase deficient cell lines: basis for a new hypothesis describing the role of poly(ADP-ribose) polymerase in DNA replication and repair

We have studied the role of poly(ADP-ribose) polymerase in the repair of DNA damage induced by x-ray and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) by using V79 chinese hamster cells, and two derivative mutant cell lines, ADPRT54 and ADPRT351, that are deficient in poly(ADP-ribose) polymerase activity. Under exponentially growing conditions these mutant cell lines are hypersensitive to x-irradiation and MNNG compared to their parental V79 cells which could be interpreted to suggest that poly(ADP-ribose) polymerase is involved in the repair of DNA damage. However, the level of DNA strand breaks induced by x-irradiation and MNNG and their rates of repair are similar in all the cell lines, thus suggesting that it may not be the difference in strand break formation or in its rate of repair that is contributing to the enhanced cell killing in exponentially growing poly(ADP-ribose) polymerase deficient cell lines. In contrast, under growth-arrested conditions, all three cell lines become similarly sensitive to both x-irradiation and MNNG, thus suggesting that poly(ADP-ribose) polymerase may not be involved in the repair of DNA damage in growth-arrested cells. These paradoxical results could be interpreted to suggest that poly(ADP-ribose) polymerase is involved in DNA repair in a cell-cycle-dependent fashion, however, it is functionally active throughout the cell cycle. To resolve this dilemma and explain these results and those obtained by many others, we propose that the normal function of poly(ADP-ribose) polymerase is to prevent DNA recombination processes and facilitate DNA ligation.

Influence of low doses of BrdU and estimation of spontaneous SCE in CHO chromosomes: three-way differential staining and an immunoperoxidase method

The influence of low doses of 5-bromodeoxyuridine (BrdU) on the occurrence of sister chromatid exchanges (SCEs) during the first cell cycle, when unsubstituted DNA templates replicate in the presence of the halogenated nucleoside (SCE1) has been assessed in third mitosis (M3) Chinese hamster ovary (CHO) cells showing three-way differential (TWD) staining. In addition, lower concentrations of BrdU, not detectable by Giemsa staining, have been tested by a high resolution immunoperoxidase method (anti-BrdU monoclonal antibody) and SCEs were scored in second mitosis (M2) cells. Our findings was a dose-response curve for SCE1 that allows an estimated mean spontaneous yield of 1.32/cell per cell cycle by extrapolation to zero concentration of BrdU. On the other hand, when the total SCE frequency corresponding to the first and second rounds of replication (SCE1 + SCE2) found in M3 chromosomes was compared with the yield of SCEs scored in M2 cells grown in BrdU at doses lower than 1 microM no further reduction was achieved. This seems to indicate that SCEs can occur spontaneously in this cell line, though the estimated frequency is higher than that reported in vivo.

Effect of bromodeoxyuridine on the proliferation and growth of ethyl methanesulfonate-exposed P3 cells: relationship to the induction of sister-chromatid exchanges

Although sister-chromatid exchange (SCE) analysis is recognized as an indicator of exposure to DNA-damaging agents, the results of these analyses have been confounded by the use of bromodeoxyuridine (BrdUrd) to differentially label the sister chromatids. Not only does BrdUrd itself induce SCE, it also modulates the frequency of SCE induced by certain DNA-damaging agents. In order to examine this effect of BrdUrd on SCE frequency, an indirect method which lends itself to measurements both with and without BrdUrd was employed. Human teratocarcinoma-derived (P3) cells were exposed to ethyl methanesulfonate (EMS) and cultured with increasing concentrations of BrdUrd for lengths of time corresponding to one, two, and three generations of cell growth. At each time point, the distribution of nuclei among the phases of the cell-cycle and cell growth were evaluated for each concentration and chemical. A statistical model was employed which tested both for the main effects of chemicals and culture times and for interactions between these factors. Both EMS and BrdUrd significantly affected the percentages of nuclei within the cell-cycle. Exposure to EMS resulted in decreases in the percentages of nuclei in G0 + G1 and increases in the G2 + M compartment. Exposure to BrdUrd affected the size of the G0 + G1 compartment as well as the percentage of S-phase nuclei. Cell growth was reduced as a consequence of increasing EMS concentration and as a function of BrdUrd concentration; the effects of these chemicals were more readily apparent at the later time points. Most importantly, for both the cell-cycle kinetics data and the cell growth data, no evidence of an interaction between the effects of EMS and the effects of BrdUrd was detected statistically. These results may be interpreted to mean that while both EMS and BrdUrd affect the induction of SCE, under the conditions of this experiment, the effects are additive rather than interactive.

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.

Poly(ADP-ribose)polymerase: a perplexing participant in cellular responses to DNA breakage

Poly(ADP-ribose) polymerase is a major nuclear protein of 116 kd, coded by a gene on chromosome 1, that plays a role in cellular responses to DNA breakage. The polymerase binds to DNA at single- and double-strand breaks and synthesizes long branched chains of poly(ADP-ribose), which covalently, but transiently, modifies itself and numerous other cellular proteins and depletes cells of NAD+. This much is known, but the physiological role of the polymerization-degradation cycle is still unclear. Poly(ADP-ribosyl)ation of proteins generally inhibits their function and can dissociated chromatin proteins from DNA. Inhibition of poly(ADP-ribose) polymerase increases to toxicity of alkylating agents and some other DNA-damaging agents and increases sister-chromatid exchange frequencies. During repair of alkylation damage, inhibition of poly(ADP-ribose) polymerase makes no change in excision of damaged products. increases the total number of repair patches, accelerates the rejoining of DNA breaks, and makes variable increases or decreases in net break frequencies. The polymerization cycle consequently is a major player in the response of cells to DNA breakage, but the game it plays is yet to be explained.

X-ray induction of methotrexate resistance due to dhfr gene amplification

The effect of ionizing radiation on methotrexate (MTX) resistance and gene amplification in cultured mammalian cells was investigated. X-irradiation of mouse EMT-6 cells induced cell killing and MTX resistance due to amplification of dihydrofolate reductase (dhfr) gene in a dose-dependent manner. The highest yields of mutant cells were obtained at approximately D37 (the dose at which 37% of the cells survive), where the frequency of MTX-resistant cells was four- to eightfold over that of the unirradiated population. The proportion of MTX-resistant cells among the survivors increased logarithmically with dose, up to a 1000-fold increase over unirradiated cells at 1000 cGy, the highest dose tested. The induced frequency of MTX resistance after X-irradiation was greater than the induced frequency of 8-azaguanine resistance, which indicates deletion of the hypoxanthine phosphoribosyltransferase gene. Inhibition of poly(ADP-ribose) polymerase by the addition of 3-aminobenzamide before irradiation increased both cell killing and MTX resistance. Metaphase spreads of chromosomes from EMT-6 cells that had been irradiated and subjected to stepwise increases in MTX concentration showed numerous double minutes. Pulsed-field gel electrophoresis of the DNA from cells containing radiation-induced double minutes showed that many copies of the dhfr gene were present on circular DNA molecules of 10(6), 2 x 10(6), and 3 x 10(6) base pairs. These results suggest a relationship between the induction of chromosome aberrations and the induction of gene amplification.