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Tamizh Selvan G, Venkatachalam P. Ataxia Telengectesia Protein Influences Bleomycin-Induced DNA Damage in Human Fibroblast Cells. Cell Biochem Biophys 2024; 82:1235-1242. [PMID: 38696104 DOI: 10.1007/s12013-024-01275-z] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 04/08/2024] [Indexed: 08/25/2024]
Abstract
Human cancer is caused mainly by exposure to genotoxic chemicals; therefore, cellular defence mechanisms against genotoxic stress are crucial. Genetic factors are essential to maintaining genome stability and play a vital role in overcoming this by repairing the genome damage caused by any agent in order to prevent chromosomal instability. To examine the influence of the genetic makeup in specific ataxia-telangiectasia (ATM), we have examined non-cancerous fibroblast cell lines (HLF, AG1522 and L6) and cells with ATM mutated deficiency (GM4405). Cell lines were exposed in vitro to bleomycin (0, 40 and 80 µg/mL). The induced DNA damages were measured using endpoints including the micronucleus assay (MN) to measure chromosome damage and gamma-H2AX (γ-H2AX) assay to measure DNA damage/repair foci formation. An increase in DNA damage were observed in bleomycin-treated cells compared to unexposed controls (p < 0.05). A concentration-dependent increase of MN and γ-H2AX foci was observed and the sensitivity differed among the cell lines as follows: GM4405 > HLF > AG1522 > L6 for MN frequency and HLF > AG1522 > GM4405 > L6 for γ-H2AX foci. These findings suggest that the genetic makeup of the cellular genome would play an essential role in repairing bleomycin-induced DNA damage. Signalling of DNA damage, and the genes responsible for the repair process, could contribute to the differential susceptibility of different tissues to carcinomas induced by environmental mutagens.
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Affiliation(s)
- G Tamizh Selvan
- Central Research Laboratory, K S Hegde Medical Academy, Nitte (Deemed to be University), Mangalore, Karnataka, India.
| | - P Venkatachalam
- Department of Human Genetics, Sri Ramachandra Institute of Higher Education and Research (Deemed to be University), Chennai, Tamilnadu, India
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Liu C, Kawata T, Zhou G, Furusawa Y, Kota R, Kumabe A, Sutani S, Fukada J, Mishima M, Shigematsu N, George K, Cucinotta F. Comparison of the repair of potentially lethal damage after low- and high-LET radiation exposure, assessed from the kinetics and fidelity of chromosome rejoining in normal human fibroblasts. JOURNAL OF RADIATION RESEARCH 2013; 54:989-997. [PMID: 23674607 PMCID: PMC3823769 DOI: 10.1093/jrr/rrt031] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 08/25/2012] [Revised: 03/09/2013] [Accepted: 03/12/2013] [Indexed: 06/02/2023]
Abstract
Potentially lethal damage (PLD) and its repair (PLDR) were studied in confluent human fibroblasts by analyzing the kinetics of chromosome break rejoining after X-ray or heavy-ion exposures. Cells were either held in the non-cycling G0 phase of the cell cycle for 12 h, or forced to proliferate immediately after irradiation. Fusion premature chromosome condensation (PCC) was combined with fluorescence in situ hybridization (FISH) to study chromosomal aberrations in interphase. The culture condition had no impact on the rejoining kinetics of PCC breaks during the 12 h after X-ray or heavy-ion irradiation. However, 12 h after X-ray and silicon irradiation, cycling cells had more chromosome exchanges than non-cycling cells. After 6 Gy X-rays, the yield of exchanges in cycling cells was 2.8 times higher than that in non-cycling cells, and after 2 Gy of 55 keV/μm silicon ions the yield of exchanges in cycling cells was twice that of non-cycling cells. In contrast, after exposure to 2 Gy 200-keV/μm or 440-keV/μm iron ions the yield of exchanges was similar in non-cycling and cycling cells. Since the majority of repair in G0/G1 occurs via the non-homologous end joining process (NHEJ), increased PLDR in X-ray and silicon-ion irradiated cells may result from improved cell cycle-specific rejoining fidelity through the NHEJ pathway, which is not the case in high-LET iron-ion irradiated cells.
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Affiliation(s)
- Cuihua Liu
- Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, Chiba 263-8555, Japan
| | - Tetsuya Kawata
- Department of Radiology, School of Medicine, Keio University, Tokyo, 160-8582, Japan
| | - Guangming Zhou
- Department of Space Radiobiology, Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730-000, China
| | - Yoshiya Furusawa
- Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, Chiba 263-8555, Japan
| | - Ryuichi Kota
- Department of Radiology, School of Medicine, Keio University, Tokyo, 160-8582, Japan
| | - Atsuhiro Kumabe
- Department of Radiology, School of Medicine, Keio University, Tokyo, 160-8582, Japan
| | - Shinya Sutani
- Department of Radiology, School of Medicine, Keio University, Tokyo, 160-8582, Japan
| | - Junichi Fukada
- Department of Radiology, School of Medicine, Keio University, Tokyo, 160-8582, Japan
| | - Masayo Mishima
- Department of Radiology, School of Medicine, Keio University, Tokyo, 160-8582, Japan
| | - Naoyuki Shigematsu
- Department of Radiology, School of Medicine, Keio University, Tokyo, 160-8582, Japan
| | - Kerry George
- Wyle Integrated Science and Engineering Group, Houston, Texas, USA
| | - Francis Cucinotta
- NASA Johnson Space Center, Radiation Biophysics, Houston, Texas, USA
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Autsavapromporn N, Suzuki M, Plante I, Liu C, Uchihori Y, Hei TK, Azzam EI, Murakami T. Participation of gap junction communication in potentially lethal damage repair and DNA damage in human fibroblasts exposed to low- or high-LET radiation. Mutat Res 2013; 756:78-85. [PMID: 23867854 PMCID: PMC4001089 DOI: 10.1016/j.mrgentox.2013.07.001] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/05/2013] [Accepted: 07/05/2013] [Indexed: 10/26/2022]
Abstract
Existing research has not fully explained how different types of ionizing radiation (IR) modulate the responses of cell populations or tissues. In our previous work, we showed that gap junction intercellular communication (GJIC) mediates the propagation of stressful effects among irradiated cells exposed to high linear energy transfer (LET) radiations, in which almost every cells is traversed by an IR track. In the present study, we conducted an in-depth study of the role of GJIC in modulating the repair of potentially lethal damage (PLDR) and micronuclei formation in cells exposed to low- or high-LET IR. Confluent human fibroblasts were exposed in the presence or absence of a gap junction inhibitor to 200kV X rays (LET∼1.7keV/μm), carbon ions (LET∼76keV/μm), silicon ions (LET∼113keV/μm) or iron ions (LET∼400keV/μm) that resulted in isosurvival levels. The fibroblasts were incubated for various times at 37°C. As expected, high-LET IR were more effective than were low-LET X rays at killing cells and damaging DNA shortly after irradiation. However, when cells were held in a confluent state for several hours, PLDR associated with a reduction in DNA damage, occurred only in cells exposed to X rays. Interestingly, inhibition of GJIC eliminated the enhancement of toxic effects, which resulted in an increase of cell survival and reduction in the level of micronucleus formation in cells exposed to high, but not in those exposed to low-LET IR. The experiment shows that gap-junction communication plays an important role in the propagation of stressful effects among irradiated cells exposed to high-LET IR while GJIC has only a minimal effect on PLDR and DNA damage following low-LET irradiation. Together, our results show that PLDR and induction of DNA damage clearly depend on gap-junction communication and radiation quality.
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Affiliation(s)
- Narongchai Autsavapromporn
- Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, Chiba 263-8555, Japan
| | - Masao Suzuki
- Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, Chiba 263-8555, Japan
| | - Ianik Plante
- University Space Research Association, NASA Johnson Space Center, Houston, TX 77058, USA
| | - Cuihua Liu
- Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, Chiba 263-8555, Japan
| | - Yukio Uchihori
- Radiation Measurement Research Section, Research, Development and Support Center, National Institute of Radiological Sciences, Chiba 263-8555, Japan
| | - Tom K. Hei
- Center of Radiological Research, Department of Radiation Oncology, College of Physicians and Surgeons, Columbia University Medical Center, New York, NY 10032, USA
| | - Edouard I. Azzam
- Department of Radiology, New Jersey Medical School Cancer Center-University of Medicine and Dentistry of New Jersey, Newark, NJ 07103, USA
| | - Takeshi Murakami
- Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, Chiba 263-8555, Japan
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Liu C, Kawata T, Furusawa Y, Zhou G, Inoue K, Fukada J, Kota R, George K, Cucinotta F, Okayasu R. Chromosome aberrations in normal human fibroblasts analyzed in G0/G1 and G2/M phases after exposure in G0 to radiation with different linear energy transfer (LET). MUTATION RESEARCH-GENETIC TOXICOLOGY AND ENVIRONMENTAL MUTAGENESIS 2013; 756:101-7. [DOI: 10.1016/j.mrgentox.2013.05.005] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/07/2013] [Accepted: 05/09/2013] [Indexed: 11/29/2022]
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Fujii Y, Yurkon CR, Maeda J, Genet SC, Kubota N, Fujimori A, Mori T, Maruo K, Kato TA. Comparative study of radioresistance between feline cells and human cells. Radiat Res 2013; 180:70-7. [PMID: 23772923 DOI: 10.1667/rr3194.1] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
Abstract
Radioresistance of cats has been seen in animal radiotherapy. Feline radioresistance and its underlying mechanism(s) were investigated in fibroblast cells and lymphocytes. We hypothesized that radioresistance was attributable to an increase in the cells ability to repair DNA damage. To investigate this hypothesis, fibroblast cells were exposed to various doses of X rays and then colony formation assays were performed. Survival curves showed that potential lethal damage repair (PLDR) for feline cells were greater than that for human cells. γ-H2AX foci assays were performed to evaluate DNA double-strand breaks (DSBs) formation and repair kinetics. After PLDR, feline cells displayed a decreased residual amount of γ-H2AX foci. Formation of chromosome aberrations (dicentrics) after PLDR as an indicator of radiation-induced DNA damage and repair; human, feline and canine lymphocytes were evaluated. Human and canine lymphocytes showed two to three times the number of dicentrics compared to feline lymphocytes. Finally, micronuclei assays were performed to further confirm the radioresistant nature of feline lymphocytes. In concordance with the results of the chromosome aberration assay, the number of micronuclei in feline lymphocytes was less than observed in human and canine lymphocytes. Taken together, these results show that DNA and chromosome damage induced by X irradiation is more effectively repaired in feline cells, resulting in less residual damage. Our results suggest that both feline fibroblasts and lymphocytes are more radioresistant compared to human cells of similar tissues, and this resistance can be contributed, at least in part, to greater ability for PLDR.
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Affiliation(s)
- Yoshihiro Fujii
- Department of Radiological Sciences, Ibaraki Prefectural University of Health Sciences, 4669-2 Ami, Inashiki, Ibaraki, Japan
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Hada M, Huff JL, Patel ZS, Kawata T, Pluth JM, George KA, Cucinotta FA. AT cells are not radiosensitive for simple chromosomal exchanges at low dose. Mutat Res 2011; 716:76-83. [PMID: 21889946 DOI: 10.1016/j.mrfmmm.2011.08.006] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2011] [Revised: 08/15/2011] [Accepted: 08/19/2011] [Indexed: 05/31/2023]
Abstract
Cells deficient in ATM (product of the gene that is mutated in ataxia telangiectasia patients) or NBS (product of the gene mutated in the Nijmegen breakage syndrome) show increased yields of both simple and complex chromosomal aberrations after high doses (>0.5Gy) of ionizing radiation (X-rays or γ-rays), however less is known on how these cells respond at low dose. Previously we had shown that the increased chromosome aberrations in ATM and NBS defective lines was due to a significantly larger quadratic dose-response term compared to normal fibroblasts for both simple and complex exchanges. The linear dose-response term for simple exchanges was significantly higher in NBS cells compared to wild type cells, but not for AT cells. However, AT cells have a high background level of exchanges compared to wild type or NBS cells that confounds the understanding of low dose responses. To understand the sensitivity differences for high to low doses, chromosomal aberration analysis was first performed at low dose-rates (0.5Gy/d), and results provided further evidence for the lack of sensitivity for exchanges in AT cells below doses of 1Gy. Normal lung fibroblast cells treated with KU-55933, a specific ATM kinase inhibitor, showed increased numbers of exchanges at a dose of 1Gy and higher, but were similar to wild type cells at 0.5Gy or below. These results were confirmed using siRNA knockdown of ATM. The present study provides evidence that the increased radiation sensitivity of AT cells for chromosomal exchanges found at high dose does not occur at low dose.
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Affiliation(s)
- Megumi Hada
- USRA Division of Life Sciences, Houston, TX 77058, USA
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Okayasu R. Repair of DNA damage induced by accelerated heavy ions--a mini review. Int J Cancer 2011; 130:991-1000. [PMID: 21935920 DOI: 10.1002/ijc.26445] [Citation(s) in RCA: 92] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/12/2011] [Accepted: 09/15/2011] [Indexed: 12/14/2022]
Abstract
Increasing use of heavy ions for cancer therapy and concerns from exposure to heavy charged particles in space necessitate the study of the basic biological mechanisms associated with exposure to heavy ions. As the most critical damage induced by ionizing radiation is DNA double strand break (DSB), this review focuses on DSBs induced by heavy ions and their repair processes. Compared with X- or gamma-rays, high-linear energy transfer (LET) heavy ion radiation induces more complex DNA damage, categorized into DSBs and non-DSB oxidative clustered DNA lesions (OCDL). This complexity makes the DNA repair process more difficult, partially due to retarded enzymatic activities, leading to increased chromosome aberrations and cell death. In general, the repair process following heavy ion exposure is LET-dependent, but with nonhomologous end joining defective cells, this trend is less emphasized. The variation in cell survival levels throughout the cell cycle is less prominent in cells exposed to high-LET heavy ions when compared with low LET, but this mechanism has not been well understood until recently. Involvement of several DSB repair proteins is suggested to underlie this interesting phenomenon. Recent improvements in radiation-induced foci studies combined with high-LET heavy ion exposure could provide a useful opportunity for more in depth study of DSB repair processes. Accelerated heavy ions have become valuable tools to investigate the molecular mechanisms underlying repair of DNA DSBs, the most crucial form of DNA damage induced by radiation and various chemotherapeutic agents.
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Affiliation(s)
- Ryuichi Okayasu
- International Open Laboratory and Heavy-ion Radiobiology Research Group, Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, Inage-ku, Chiba, Japan.
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Liu C, Kawata T, Shigematsu N, Cucinotta F, George K, Saito M, Uno T, Isobe K, Ito H. A comparison of chromosome repair kinetics in G(0) and G(1) reveals that enhanced repair fidelity under noncycling conditions accounts for increased potentially lethal damage repair. Radiat Res 2010; 174:566-73. [PMID: 20954858 DOI: 10.1667/rr2159.1] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
Abstract
Potentially lethal damage (PLD) and its repair were studied in confluent human fibroblasts by analyzing the kinetics of chromosome break rejoining and misrejoining in irradiated cells that were either held in noncycling G(0) phase or allowed to enter G(1) phase of the cell cycle immediately after 6 Gy irradiation. Virally mediated premature chromosome condensation (PCC) methods were combined with fluorescence in situ hybridization (FISH) to study chromosomal aberrations in interphase. Flow cytometry revealed that the vast majority of cells had not yet entered S phase 15 h after release from G(0). By this time some 95% of initially produced prematurely condensed chromosome breaks had rejoined, indicating that most repair processes occurred during G(1). The rejoining kinetics of prematurely condensed chromosome breaks was similar for each culture condition. However, under noncycling conditions misrepair peaked at 0.55 exchanges per cell, while under cycling conditions (G(1)) it peaked at 1.1 exchanges per cell. At 12 h postirradiation, complex-type exchanges were sevenfold more abundant for cycling cells (G(1)) than for noncycling cells (G(0)). Since most repair in G(0)/G(1) occurs via the non-homologous end-joining (NHEJ) process, increased PLD repair may result from improved cell cycle-specific rejoining fidelity of the NHEJ pathway.
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Affiliation(s)
- Cuihua Liu
- a Department of Radiology, Graduate School of Medicine, Chiba University, Chiba, Japan
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George KA, Hada M, Jackson LJ, Elliott T, Kawata T, Pluth JM, Cucinotta FA. Dose response of gamma rays and iron nuclei for induction of chromosomal aberrations in normal and repair-deficient cell lines. Radiat Res 2009; 171:752-63. [PMID: 19580482 DOI: 10.1667/rr1680.1] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
Abstract
We studied the effects of DNA double-strand break (DSB) repair deficiencies on chromosomal aberration frequency using low doses (<1 Gy) of gamma rays and high-energy iron ions (LET = 151 keV/microm). Chromosomal aberrations were measured using the fluorescence whole-chromosome painting technique. The cell lines included fibroblasts deficient in ATM (product of the gene that is mutated in ataxia telangiectasia patients) or NBS (product of the gene mutated in the Nijmegen breakage syndrome) and gliomablastoma cells proficient in or lacking DNA-dependent protein kinase (DNA-PK) activity. The yields of both simple and complex chromosomal aberrations were increased in DSB repair-defective cells compared to normal cells; the increase was more than twofold higher for gamma rays compared to iron nuclei. For gamma-ray-induced aberrations, the ATM- and NBS-defective lines were found to have significantly larger quadratic components compared to normal fibroblasts for both simple and complex aberrations, while the linear dose-response term was significantly higher only for the NBS cells. For simple and complex aberrations induced by iron nuclei, regression models preferred purely linear and quadratic dose responses, respectively, for each cell line studied. RBEs were reduced relative to normal cells for all of the DSB repair-defective lines, with the DNA-PK-deficient cells found to have RBEs near unity. The large increase in the quadratic dose-response terms in the DSB repair-deficient cell lines points to the importance of the functions of ATM and NBS in chromatin modifications to facilitate correct DSB repair and to minimize aberration formation. The differences found between AT and NBS cells at lower doses suggest important questions about the applicability of observations of radiation sensitivity at high doses to low-dose exposures.
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van Bree C, Rodermond HM, ten Cate R, de Vos J, Stalpers LJA, Haveman J, Medema JP, Franken NAP. G0 cell cycle arrest alone is insufficient for enabling the repair of ionizing radiation-induced potentially lethal damage. Radiat Res 2008; 170:184-91. [PMID: 18666809 DOI: 10.1667/rr0845.1] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2006] [Accepted: 03/27/2008] [Indexed: 11/03/2022]
Abstract
The repair of ionizing radiation-induced potentially lethal damage (PLD) is suggested to be important for the clinical response to radiotherapy. PLD repair is usually studied in quiescent cultures prepared by growing cells to confluence with an accumulation of cells in G(0) phase of the cell cycle, but the biological pathways enabling PLD repair are still unknown. In this study, we examined whether the controlled expression of two different inducers of G(0) cell cycle arrest, the human tumor suppressor gene growth arrest specific 1 (GAS1) in murine fibroblasts and the forkhead transcription factor FOXO3a in human colon carcinoma cells, is sufficient to enable PLD repair. We found that GAS1 and FOXO3a induced a cell cycle arrest in G(0) phase with a concomitant reduction of proliferation of log-phase cells. In both cell systems, this cell cycle arrest in G(0) phase did not enable PLD repair in log-phase cells. Significant PLD repair was found in all confluent cultures that showed similar cell cycle distributions, while GAS1 and FOXO3a in confluent cells did not influence PLD repair. No differences were found in cell cycle re-entry after replating cells with different capacities for PLD repair. Our data suggest that the induction of G(0) cell cycle arrest and the reduction of proliferation are not sufficient to enable PLD repair.
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Affiliation(s)
- Chris van Bree
- Academic Medical Center, Laboratory for Experimental Oncology and Radiobiology, Department of Radiation Oncology, University of Amsterdam, Amsterdam, The Netherlands.
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Bergs JWJ, ten Cate R, Haveman J, Medema JP, Franken NAP, van Bree C. Chromosome fragments have the potential to predict hyperthermia-induced radio-sensitization in two different human tumor cell lines. JOURNAL OF RADIATION RESEARCH 2008; 49:465-472. [PMID: 18413979 DOI: 10.1269/jrr.07133] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/26/2023]
Abstract
Cellular radiosensitivity, assessed by loss of clonogenicity, has been shown to correlate with the number of radiation-induced chromosomal aberrations. Also an increased radiosensitivity by hyperthermia has been shown to correlate with an increase in chromosomal aberrations. Therefore, determination of the number of chromosomal aberrations might be used as an assay to predict the radiosensitivity of tumors pre-treated with hyperthermia at clinically relevant temperatures. The use of premature chromosome condensation combined with fluorescent in situ hybridisation (PCC-FISH) has been shown to be clinically applicable. Therefore, the use of chromosomal aberrations as determined with PCC-FISH for the prediction of hyperthermia-induced radio-sensitization in human tumor cells was investigated. Confluent cultures of SW-1573 (human lung carcinoma) and RKO (human colorectal carcinoma) cells were treated with 1 h 41 degrees C or 43 degrees C hyperthermia prior to gamma-irradiation. Clonogenic cell survival and induction of chromosomal aberrations (unrejoined chromosomal fragments and translocations), by PCC-FISH, were studied at 24 h after treatment. Pre-treatment with hyperthermia at 41 degrees C for 1 h enhanced the radiosensitivity of RKO cells but not of SW-1573 cells. Increasing the temperature to 43 degrees C for 1 h enhanced the radiosensitivity of SW-1573 cells. When radio-sensitization was observed, a significant increase in the number of unrejoined chromosomal fragments was found but the frequency of translocations was not increased. Hyperthermia-induced radio-sensitization is correlated with an increase in unrejoined chromosomal fragments. This suggests that determination of the number of chromosomal fragments after hyperthermia and radiation treatment might be used for the prediction of combined treatment response in cancer patients.
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Affiliation(s)
- Judith W J Bergs
- Academic Medical Center, University of Amsterdam, Laboratory for Experimental Oncology and Radiobiology (LEXOR), Center for Experimental Molecular Medicine, Department of Radiation Oncology, Amsterdam, The Netherlands
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