Cancer Research CTRC-AACR San Antonio Breast Cancer Symposium Tumor Immunology: New Perspectives
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Cancer Research Clinical Cancer Research
Cancer Epidemiology Biomarkers & Prevention Molecular Cancer Therapeutics
Molecular Cancer Research Cancer Prevention Research
Cancer Prevention Journals Portal Cancer Reviews Online
Annual Meeting Education Book Meeting Abstracts Online
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Terzoudi, G. I.
Right arrow Articles by Iliakis, G.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Terzoudi, G. I.
Right arrow Articles by Iliakis, G.
[Cancer Research 65, 11292-11296, December 15, 2005]
© 2005 American Association for Cancer Research

Priority Reports

Checkpoint Abrogation in G2 Compromises Repair of Chromosomal Breaks in Ataxia Telangiectasia Cells

Georgia I. Terzoudi1, Kaliopi N. Manola1, Gabriel E. Pantelias1 and George Iliakis2

1 Institute of Nuclear Technology and Radiation Protection, National Centre for Scientific Research "Demokritos," Aghia Paraskevi Attikis, Athens, Greece and 2 Institute of Medical Radiation Biology, University of Duisburg-Essen Medical School, Essen, Germany

Requests for reprints: Gabriel E. Pantelias, Institute of Radioisotopes and Radiodiagnostic Products, National Centre for Scientific Research "Demokritos," Patriarchou Grigoriou and Neapoleos Strs., 15310 Aghia Paraskevi Attikis, Athens, Greece. Phone: 30-210-650-3848; Fax: 30-210-654-3526; E-mail: gabriel{at}ipta.demokritos.gr or George Iliakis, Medical Radiation Biology, University of Duisburg-Essen Medical School, Hufelandstr. 55, 45122 Essen, Germany. Phone: 49-201-723-4152 or 4153; Fax: 49-201-723-5966; E-mail: georg.iliakis{at}medizin.uni-essen.de.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results and Discussion
 References
 
Checkpoint abrogation in G2 compromises repair of DNA double-strand breaks (DSB) and confers enhanced G2 chromosomal radiosensitivity in ataxia telangiectasia (AT) cells. To directly test this hypothesis, we combined calyculin A–induced premature chromosome condensation with conventional cytogenetics to evaluate chromosome damage before and after the G2 checkpoint in irradiated primary AT and normal human lymphocytes and their lymphoblastoid derivatives. Direct analysis of radiation damage in G2 by premature chromosome condensation reveals practically indistinguishable levels of chromosomal breaks in AT and normal cells. Yet a 4-fold increase in metaphase chromosome damage is observed in AT cells as compared with normal cells which, in contrast to AT cells, exhibit a strong G2 arrest manifest as an abrupt reduction in the mitotic index. Thus, an active checkpoint facilitates repair of chromosomal breaks in normal cells. Treatment with caffeine that abrogates the G2 checkpoint without significantly affecting DSB rejoining increases metaphase chromosome damage of normal cells to the AT level but leaves unchanged interphase chromosome damage in G2. Caffeine has no effect on any of these end points in AT cells. These observations represent the first direct evidence that the G2 checkpoint facilitates repair of chromosome damage, presumably by supporting repair of DNA DSBs. Failure to arrest will lead to chromatin condensation and conversion of unrepaired DNA DSBs to chromosomal breaks during G2-to-M phase transition. (Cancer Res 2005; 65(24): 11292-6)


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results and Discussion
 References
 
Ataxia telangiectasia (AT) is a rare, autosomal recessive syndrome characterized by progressive cerebellar degeneration, immunodeficiency, genomic instability, defective cell cycle checkpoints, increased sensitivity to ionizing radiation, and cancer predisposition (1–3). The AT mutated (ATM) gene is located on chromosome 11q22.23 and the protein product belongs to the family of phosphatidylinositol 3 kinase–like family of protein kinases. The hypersensitivity of AT cells to ionizing radiation and radiomimetic agents, but not to UV or alkylating agents, suggests the rather specific involvement of ATM in the processing of DNA double-strand breaks (DSB), a particularly deleterious lesion generated by these agents.

As a component in cell signaling, ATM phosphorylates p53, Chk2, Brac1, Nbs1, Smc1, and other down stream effectors and seems to serve as a central hub in the development of the early responses to DNA DSBs (1–3). As a result, AT cells exhibit strongly diminished checkpoints when irradiated in G1 or S and a practically complete defect in checkpoint activation when irradiated in G2 (4). Cells irradiated in G1 or S display a normal or even enhanced checkpoint response on arrival in G2, suggesting the operation of distinct, ATM-independent mechanisms of activation (4). The complete abrogation of checkpoint response in G2-irradiated AT cells is particularly interesting as it allows a "clean" examination of the consequences of checkpoint activation (or abrogation) on the response to ionizing radiation.

In the present article, we describe experiments designed to investigate the interplay between DNA DSB repair defect and checkpoint abrogation and its contribution to the enhanced G2 chromosomal radiosensitivity observed in AT cells. The developed experimental design allows us to focus on the G2 phase of the cell cycle and the G2-to-M phase transition to take advantage of the clear and practically sole dependence of the G2 checkpoint on ATM (4). Specifically, we combine calyculin A–induced premature chromosome condensation (5) with conventional cytogenetics to evaluate radiation-induced chromosome damage in interphase, before the G2 checkpoint, and compare this damage to that scored at metaphase. This protocol allows us to examine directly the role of an active G2 checkpoint in the repair of chromosomal breaks. Furthermore, we use caffeine to abrogate the G2 checkpoint, without significantly affecting DSB rejoining, and to force cells with DNA damage into mitosis, which converts DNA DSBs into chromosomal breaks via the associated maturation-promoting factor activation and chromosome condensation (6–9).


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results and Discussion
 References
 
Three EBV-transformed lymphoblastoid cell lines, GM 15786, GM03188A, and GM09899, derived from an AT patient, an obligatory ATM heterozygote, and a normal individual, respectively, were used. Cells were maintained in RPMI (Life Technologies, Invitrogen, Paisley, Scotland) supplemented with HEPES and sodium bicarbonate, 15% fetal bovine serum (FBS), 1% L-glutamine (2 mmol/L), and antibiotics (penicillin, 100 units/mL; streptomycin, 100 µg/mL). All incubations were at 37°C in a humidified incubator in an atmosphere of 5% CO2 and 95% air. To study peripheral blood lymphocytes (PBL), 5-mL blood was drawn, after obtaining consent, from four clinically characterized AT homozygotes, three obligate AT heterozygotes, and four healthy controls. None of these individuals were under treatment at the time of blood sampling. For PBL culture, 0.5 mL of whole blood was added to 5 mL of McCoy's 5A medium supplemented with 10% FBS, 1% phytohemagglutinin (PHA), 1% glutamine, and antibiotics. Cultures were incubated at 37°C for 72 hours before use in experiments.

Irradiation was carried out in a GammaCell 220 irradiator (Atomic Energy of Canada Ltd., Ottawa, Canada) at room temperature and at a dose rate of 1 Gy/min. Calyculin A was prepared as a 1 mmol/L solution in ethanol. PHA was dissolved in water at a concentration of 0.24 mg/mL. Caffeine was prepared as a 100 mmol/L stock solution in PBS. All chemicals were from Sigma Chemical, Co., (St. Louis, MO) unless stated otherwise.

Proliferating cells (lymphoblasts or lymphocytes) were exposed to radiation (1 Gy) and incubated for 30 minutes at 37°C to allow division of cells irradiated at mitosis. Subsequently, the culture was divided and one half was treated with colcemid for 1 hour to arrest dividing cells at metaphase whereas the remaining half was incubated at 37°C for 30 minutes in the presence of colcemid, and then 50 nmol/L calyculin A was added and allowed to act for an additional 30 minutes to induce premature chromosome condensation. At 90 minutes postirradiation, cells from all cultures were collected by centrifugation, treated in 75 mmol/L KCl for 10 minutes, fixed in methanol/glacial acetic acid (3:1, v/v), and processed for cytogenetics analysis. Standard procedures were used for chromosome preparation and staining (9). For each experimental point, ~100 cells were scored for chromatid damage based on standard criteria. For scoring, we considered chromatid breaks and gaps, the latter only when longer than a chromatid width. Light microscopy was coupled to an image analysis system (MetaSystems, Altlussheim, Germany) to facilitate scoring.


   Results and Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
References
 
Stimulated PBLs and actively growing lymphoblastoid cell lines were exposed to 0 or 1 Gy and returned to 37°C for 30 minutes to allow division of cells irradiated past the G2 checkpoint but before the colcemid metaphase arrest point (see Fig. 1A). Subsequently, colcemid was added for 1 hour to arrest at metaphase cells irradiated in G2 to analyze radiation-induced chromosome damage and to measure the mitotic index. To quantitatively evaluate the role of the G2 checkpoint in the repair of chromosome damage, we employed calyculin A to directly visualize and score damage in G2 phase, and thus presumably before the G2 checkpoint, in ATM-proficient or ATM-deficient cells. Calyculin A, a specific inhibitor of protein phosphatases type 1 and 2A (10), initiates premature chromosome condensation in all phases of the cell cycle and causes complete chromosome condensation in G2 phase, allowing analysis of chromosomal damage in G2-phase cells. Prematurely condensed chromosomes in G2 are morphologically similar to metaphase chromosomes but lack visible constriction at the centromeric region (Fig. 1B, left). This enables an easy and clear distinction in the same sample between cells at metaphase (Fig. 1B, right) and cells in G2 and allows parallel scoring of chromosome damage before and after the G2 checkpoint.



View larger version (35K):
[in this window]
[in a new window]
  		Figure 1. Outline of the experimental design and visualization of chromosome breaks at metaphase or in G2 phase. A, characteristics of the experimental design. The adopted experimental design allows analysis of chromosome damage in a well-defined window of the cell cycle that covers G2 and the end of S phase (gray area). For illustration purposes, a 24-hour cell cycle is assumed with 14-, 7-, 2-, and 1-hour duration for G1, S, G2, and M phase, respectively. Only the part of the cell cycle relevant for the experiments presented here is shown. X-axis, white and black triangles, approximate locations of the G2 checkpoint and the colcemid arrest point at metaphase. Horizontal bars, distribution of cells through the part of the cell cycle under investigation at the moment of ionizing radiation exposure, as well their anticipated distribution at time points relevant for the experimental design. Due to differences in response, progression through the cycle after irradiation is depicted separately for ATM+/+ and ATM–/– cells. Widening of the bars, accumulation in G2 phase as a result of checkpoint activation or accumulation in M phase as a result of colcemid treatment. Irradiation is at 0 minutes. During the first 30 minutes after irradiation, before addition of colcemid, cells past the G2 checkpoint traverse metaphase and are thus excluded from analysis. AT cells experience no delay in G2 during this period of time whereas normal cells begin arresting in G2 and display a reduction in mitotic index (narrowing of the bars). After an additional 60 minutes, AT cells accumulate at metaphase due to the action of colcemid but show no accumulation at the G2 checkpoint; normal cells accumulate at both blocks although accumulation at metaphase is reduced compared with AT cells. See text for details and justification of assumptions made in the drawing of this schematic. B, left, example of an irradiated cell treated with 50 nmol/L calyculin A during the G2 phase of the cell cycle. Note the complete condensation of chromosomes that allows visualization and analysis of chromosome damage. Note also the lack of pericentric constriction that allows distinction between G2 phase and metaphase cells. Right, metaphase appearance of a cell irradiated in G2 phase. Note the chromatid-type damage present as well as the clearly outlined pericentric constriction.

 
Figure 2A shows a 5- to 10-fold reduction in mitotic index in irradiated wild-type cells, indicative of a strong arrest in G2. ATM-deficient cells show a marginal reduction in mitotic index, in agreement with an abrogation of the G2 checkpoint. Stimulated lymphocytes from ATM homozygotes or normal individuals give results similar to those obtained with lymphoblastoid cell lines. PBLs of ATM heterozygotes show a response similar to that of normal controls (Fig. 2A). An auxiliary to the above discrimination in the same sample between mitotic and G2-phase cells is the ability to evaluate G2 arrest after ionizing radiation. This is because the ratio of cells in G2 to cells in G2 + M is expected to increase in cells sustaining checkpoint activation, but to remain unchanged in cells unable to develop this response, such as AT cells. This is indeed observed in the results shown in Fig. 2B. An increase in this ratio is seen in normal and AT heterozygote cells but no significant change is detected in AT cells.



View larger version (22K):
[in this window]
[in a new window]
  		Figure 2. Effect of 1-Gy {gamma}-irradiation during G2 on cell cycle progression and transition into M phase. A, relative mitotic indices of lymphoblastoid cell lines and PBLs of different ATM statuses measured 90 minutes after exposure either to 0- or 1-Gy {gamma}-radiation. Mitotic index is defined as the percent of cells at mitosis. The plotted values were calculated as the ratio of mitotic indices of irradiated and nonirradiated cells. Due to the timing of collection, cells analyzed have been exposed to radiation in the G2 phase of the cell cycle (see Fig. 1A). B, G2 to G2 + M ratio calculated by estimating the percent of cells in G2 and M using differences in pericentric morphology after treatment with calyculin A.

 
To evaluate the consequences of G2 checkpoint defect in AT cells, we scored chromatid aberrations at metaphase (see example in Fig. 1B, right). Normal controls show low levels of chromatid aberrations at metaphase, either in the lymphoblastoid cell line or the PBL cultures, whereas AT cells show an increased frequency of chromatid aberrations, in agreement with the increased radiosensitivity to killing (Fig. 3A). This is in agreement with the hypothesis that an intact, ATM-dependent G2 checkpoint prevents G2 cells from entering mitosis before repair reduces damage load (11). Figure 1B (left) shows an example of ionizing radiation–induced chromatid damage in G2 phase and Fig. 3B shows the results obtained from such analysis. Notably, chromatid damage in G2 under these experimental conditions is similar in AT and normal cells. Comparison between Fig. 3A and B shows that the level of damage measured in all types of cells in G2 is similar to the level of damage measured in AT metaphase cells. The 2- to 3-fold reduction in the level of chromosome damage in normal metaphase cells suggests a modulation of scorable damage at the checkpoint transition. Lymphoblastoid cell lines and lymphocytes give very similar results, suggesting that the underlying G2 checkpoint mechanism is not altered by immortalization and culturing.



View larger version (22K):
[in this window]
[in a new window]
  		Figure 3. Analysis of chromosomal breaks at metaphase or directly in G2 phase using premature chromosome condensation in cells of different ATM statuses. A, chromatid breaks scored at metaphase in normal cells, AT heterozygotes, and AT cells after 1-Gy {gamma}-irradiation and 90 minutes postirradiation incubation as indicated in Fig. 1A. B, chromatid breaks per cell after 1-Gy irradiation of lymphoblastoid cell lines or PBLs obtained from AT patients, carriers, and control donors, scored directly in G2 phase in prematurely condensed chromosomes visualized after treatment with calyculin A.

 
To test the hypothesis that in normal cells checkpoint function reduces chromosome damage at metaphase, we employed caffeine to abolish G2 checkpoint by inhibiting ATM (12, 13). Caffeine was administered 30 minutes before ionizing radiation to three lymphoblastoid cell lines or to lymphocytes obtained from two healthy donors, and chromosome damage was scored at metaphase. Figure 4A shows the results obtained. Treatment with caffeine of normal or AT heterozygotes increases ionizing radiation–induced chromosome damage at metaphase to the levels scored in AT cells. Notably, caffeine did not further increase chromosome damage measured in AT cells. This result is compatible with an abrogation by caffeine of the G2 checkpoint through inhibition of ATM. Because increase in chromosome damage at metaphase after caffeine treatment can, in principle, also be caused by inhibition of DNA repair (14), we used calyculin A to score chromosome damage in G2. We anticipated that if caffeine inhibited DNA repair during the 90 minutes allowed by the experimental protocol, it would increase damage scored in irradiated, treated G2-phase cells as compared with untreated G2-phase cells. The results in Fig. 4B indicate that chromosome damage scored in G2 phase in normal cells, heterozygotes, or AT cells is not affected by caffeine, confirming that the effect of caffeine in this phase of the cell cycle derives predominantly from abrogation of the G2 checkpoint.



View larger version (23K):
[in this window]
[in a new window]
  		Figure 4. Effect of caffeine on the yield of chromatid breaks in G2 after calyculin A–induced premature chromosome condensation, as well as by conventional metaphase analysis, in lymphoblastoid cell lines of different ATM statuses and PBLs of two normal donors. A, results obtained by conventional analysis at metaphase of cells exposed to 1-Gy and treated for 90 minutes in the presence or absence of 4 mmol/L caffeine. B, results obtained from calyculin A–treated samples in which G2 prematurely condensed chromosomes were analyzed together with metaphase chromosomes. The latter was achieved by taking advantage of the lack of centromeric constriction in the G2 chromosomes of calyculin A–treated samples.

 
Figure 4B also includes the analysis of chromosome damage at metaphase in the calyculin A–treated samples, carried out by exploiting the above indicated difference in centromeric morphology to differentiate mitotic from G2-phase cells. The results are similar to those obtained by conventional metaphase analysis carried out without calyculin A, confirming that centromeric morphology is a powerful discriminator between mitotic and G2-phase cells. This observation validates the use of this cytogenetic approach for visualizing and scoring chromosome damage before and after the G2 checkpoint.

The above presented results show the unique features of our cytogenetic approach that allow simultaneous analysis on the same preparation of chromosome repair, checkpoint function, and chromosome aberration (lethal lesion) production, and contribute to our understanding of the mechanism of ATM function in the following key end points. First, they provide strong evidence that the ATM-dependent checkpoint activated in cells irradiated in G2 contributes significantly to the reduction of chromosome damage. Second, because both AT and normal cells have the same time to carry out DNA repair before premature chromosome condensation, the similarity in the number of chromatid breaks scored in G2 suggests that the underlying lesion (i.e., the DNA DSB) is repaired with similar kinetics in both types of cells. This is in agreement with reports that repair of DNA DSBs in AT cells is compromised only in a small (~10%) subset of DNA DSBs that is hypothesized to represent complex lesions that are difficult to repair (15). Lastly, checkpoint abrogation and failure of cells to arrest in G2 phase will allow chromatin condensation, which can convert unrepaired DNA DSBs to chromosomal breaks during G2-to-M phase transition (6–9).

Because chromosome damage correlates with cell radiosensitivity to killing, we infer that the checkpoint will also contribute to cell radiosensitivity to killing. This observation is in line with observations by others indicating that G2 checkpoint activation in G2 relies exclusively on ATM (4). At this point, it is important to emphasize that the above conclusion is only valid for cells in G2 and seems to be in line with the low-dose hypersensitivity to killing (16) but cannot explain observations with AT cells exposed to radiation in other phases of the cell cycle (15, 17). The same holds true for the lack of caffeine effect in G2-irradiated AT cells as it is known that caffeine radiosensitizes exponentially growing AT cells to killing (14). This may be mediated either through an effect on the ATM-independent component of the G2 checkpoint evident in cells irradiated before G2 (4) or by an effect on DNA DSB repair by homologous recombination (18).

Recent reports implicate phosphorylation of Smc1 by ATM at Ser957 and Ser966 in the repair of ionizing radiation–induced chromosome breaks in the G2 phase of the cell cycle. Thus, knock-in mouse cells in which these phosphorylation sites are mutated to alanine show defects in the S-phase checkpoint with a magnitude equivalent to that of AT cells, reduced chromosome repair, and a marginal increase in radiosensitivity to killing (19). Smc1 is a component of cohesin and the recombination complex RC-1 and is phosphorylated after ionizing radiation at the sites of DNA DSBs by ATM in an Nbs1/Brca1–dependent manner (19). It will be informative to examine the effect of these knock-in mutations on chromosome repair and checkpoint response using the approaches described here.

Finally, there is evidence that the effects of ATM on checkpoint response and radiosensitivity to killing can be genetically separated (20). The results presented above indicate that the G2 phase may be unique in the sense that the checkpoint and survival functions of ATM coincide and emphasize the importance of a careful evaluation of radiation effects in the different phases of the cell cycle.


   Acknowledgments
 
Grant support: Hellenic Institute for Occupational Health and Safety, the German Research Foundation, and contract no FIGH-CT-2002-00218 awarded by the European Commission.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

We thank the AT patients and their families for their willingness to participate in this study and Prof. Dr. C. Dardoufas for referring the AT patients to our center.

Received 6/22/05. Revised 8/15/05. Accepted 9/15/05.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 References
 

  1. Lavin MF, Birrell G, Chen P, et al. ATM signaling and genomic stability in response to DNA damage. Mutat Res 2005;569:123–32.[Medline]
  2. Shiloh Y. ATM and related protein kinases: safeguarding genome integrity. Nat Rev Cancer 2003;3:155–68.[CrossRef][Medline]
  3. Abraham RT. Cell cycle checkpoint signaling through the ATM and ATR kinases. Genes Dev 2001;15:2177–96.[Free Full Text]
  4. Xu B, Kim S-T, Lim D-S, Kastan MB. Two molecularly distinct G2/M checkpoints are induced by ionizing irradiation. Mol Cell Biol 2002;22:1049–59.[Abstract/Free Full Text]
  5. Gotoh E, Kawata T, Durante M. Chromatid break rejoining and exchange aberration formation following {gamma}-ray exposure: analysis in G2 human fibroblasts by chemically induced premature chromosome condensation. Int J Radiat Biol 1999;75:1129–35.[CrossRef][Medline]
  6. Cheng X, Pantelias GE, Okayasu R, Cheong N, Iliakis G. MPF activity of inducer mitotic cells may affect the radiation yield of interphase chromosome breaks in the premature chromosome condensation assay. Cancer Res 1993;53:5592–6.[Abstract/Free Full Text]
  7. Terzoudi GI, Jung T, Hain J, et al. Increased G2 chromosomal radiosensitivity in cancer patients: the role of cdk1/cyclin-b activity level in the mechanisms involved. Int J Radiat Biol 2000;76:607–15.[CrossRef][Medline]
  8. Terzoudi GI, Malik SI, Pantelias GE, et al. A new cytogenetic approach for the evaluation of mutagenic potential of chemicals that induce cell cycle arrest in the G2 phase. Mutagenesis 2003;18:539–43.[Abstract/Free Full Text]
  9. Pantelias GE. Radiation-induced cytogenetic damage in relation to changes in interphase chromosome conformation. Radiat Res 1986;105:341–50.[CrossRef][Medline]
  10. Nazarov IB, Smirnova AN, Krutilina RI, et al. Dephosphorylation of histone {gamma}-H2AX during repair of DNA double-strand breaks in mammalian cells and its inhibition by calyculin A. Radiat Res 2003;160:309–17.[Medline]
  11. Iliakis G, Wang Y, Guan J, Wang H. DNA damage checkpoint control in cells exposed to ionizing radiation. Oncogene 2003;22:5834–47.[CrossRef][Medline]
  12. Hansson K, Natarajan AT, Kihlman BA. Effect of caffeine in G2 on X-ray-induced chromosomal aberrations and mitotic inhibition in ataxia telangiectasia fibroblast and lymphoblastoid cells. Hum Genet 1984;67:329–35.[CrossRef][Medline]
  13. Sarkaria JN, Busby EC, Tibbetts RS, et al. Inhibition of ATM and ATR KINASE activities by the radiosensitizing agent, caffeine. Cancer Res 1999;59:4375–82.[Abstract/Free Full Text]
  14. Asaad NA, Zeng Z-C, Guan J, Thacker J, Iliakis G. Homologous recombination as a potential target for caffeine radiosensitization in mammalian cells: reduced caffeine radiosensitization in xrcc2 and xrcc3 mutants. Oncogene 2000;19:5788–800.[CrossRef][Medline]
  15. Riballo E, Kühne M, Rief N, et al. A pathway of double-strand break rejoining dependent upon ATM, Artemis, and proteins locating to {gamma}-H2AX foci. Mol Cell 2004;16:715–24.[CrossRef][Medline]
  16. Marples B. Is low-dose hyper-radiosensitivity a measure of G2-phase cell radiosensitivity? Cancer Metastasis Rev 2004;23:197–207.[Medline]
  17. Loucas BD, Cornforth MN. Evidence that unrejoined DNA double-strand breaks are not predominantly responsible for chromosomal radiosensitivity of at fibroblasts. Radiat Res 2004;162:554–65.[CrossRef][Medline]
  18. Wang H, Böcker W, Wang H, et al. Caffeine inhibits homology-directed repair of I-SceI-induced DNA double-strand breaks. Oncogene 2004;23:824–34.[CrossRef][Medline]
  19. Kitagawa R, Bakkenist CJ, McKinnon PJ, Kastan MB. Phosphorylation of SMC1 is a critical downstream event in the ATM-NBS1-BRCA1 pathway. Genes Dev 2004;18:1423–38.[Abstract/Free Full Text]
  20. Jeggo PA, Carr AM, Lehmann AR. Splitting the ATM: distinct repair and checkpoint defects in ataxia-telangiectasia. Trends Genet 1998;14:312–6.[CrossRef][Medline]



This article has been cited by other articles:


Home page
 MutagenesisHome page
O. Shovman, A. C. Riches, D. Adamson, and P. E. Bryant
An improved assay for radiation-induced chromatid breaks using a colcemid block and calyculin-induced PCC combination
Mutagenesis, July 1, 2008; 23(4): 267 - 270.
[Abstract] [Full Text] [PDF]

Home page
 MutagenesisHome page
V. I. Hatzi, G. I. Terzoudi, V. Makropoulos, C. Maravelias, and G. E. Pantelias
Pre-irradiation exposure of peripheral blood lymphocytes to glutaraldehyde induces radiosensitization by increasing the initial yield of radiation-induced chromosomal aberrations
Mutagenesis, March 1, 2008; 23(2): 101 - 109.
[Abstract] [Full Text] [PDF]

Home page
 MutagenesisHome page
P. E. Bryant and H. Mozdarani
A comparison of G2 phase radiation-induced chromatid break kinetics using calyculin-PCC with those obtained using colcemid block
Mutagenesis, September 1, 2007; 22(5): 359 - 362.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Terzoudi, G. I.
Right arrow Articles by Iliakis, G.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Terzoudi, G. I.
Right arrow Articles by Iliakis, G.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Cancer Research Clinical Cancer Research
Cancer Epidemiology Biomarkers & Prevention Molecular Cancer Therapeutics
Molecular Cancer Research Cancer Prevention Research
Cancer Prevention Journals Portal Cancer Reviews Online
Annual Meeting Education Book Meeting Abstracts Online