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1.  Assessing the risk of second malignancies after modern radiotherapy 
Nature reviews. Cancer  2011;11(6):438-448.
Recent advances in radiotherapy have enabled the use of different types of particles, such as protons and heavy ions, as well as refinements to the treatment of tumours with standard sources (photons). However, the risk of second cancers arising in long-term survivors continues to be a problem. The long-term risks from treatments such as particle therapy have not yet been determined and are unlikely to become apparent for many years. Therefore, there is a need to develop risk assessments based on our current knowledge of radiation-induced carcinogenesis.
PMCID: PMC4101897  PMID: 21593785
2.  ATM Alters the Otherwise Robust Chromatin Mobility at Sites of DNA Double-Strand Breaks (DSBs) in Human Cells 
PLoS ONE  2014;9(3):e92640.
Ionizing radiation induces DNA double strand breaks (DSBs) which can lead to the formation of chromosome rearrangements through error prone repair. In mammalian cells the positional stability of chromatin contributes to the maintenance of genome integrity. DSBs exhibit only a small, submicron scale diffusive mobility, but a slight increase in the mobility of chromatin domains by the induction of DSBs might influence repair fidelity and the formation of translocations. The radiation-induced local DNA decondensation in the vicinity of DSBs is one factor potentially enhancing the mobility of DSB-containing chromatin domains. Therefore in this study we focus on the influence of different chromatin modifying proteins, known to be activated by the DNA damage response, on the mobility of DSBs. IRIF (ionizing radiation induced foci) in U2OS cells stably expressing 53BP1-GFP were used as a surrogate marker of DSBs. Low angle charged particle irradiation, known to trigger a pronounced DNA decondensation, was used for the defined induction of linear tracks of IRIF. Our results show that movement of IRIF is independent of the investigated chromatin modifying proteins like ACF1 or PARP1 and PARG. Also depletion of proteins that tether DNA strands like MRE11 and cohesin did not alter IRIF dynamics significantly. Inhibition of ATM, a key component of DNA damage response signaling, resulted in a pronounced confinement of DSB mobility, which might be attributed to a diminished radiation induced decondensation. This confinement following ATM inhibition was confirmed using X-rays, proving that this effect is not restricted to densely ionizing radiation. In conclusion, repair sites of DSBs exhibit a limited mobility on a small spatial scale that is mainly unaffected by depletion of single remodeling or DNA tethering proteins. However, it relies on functional ATM kinase which is considered to influence the chromatin structure after irradiation.
PMCID: PMC3961414  PMID: 24651490
3.  The effect of X-rays and C-ions on pluripotent embryonic stem cells 
Journal of Radiation Research  2014;55(Suppl 1):i55-i56.
Embryonic stem cells (ESC) are characterized by both the capacity of infinite self-renewal and the ability to give rise to all the three germ layers emphasizing the need to strictly control the genetic integrity. To date, ESC are a powerful tool in disease modeling, tissue engineering and drug testing. However, in the field of radiation research, their potential has not been exploited.
We used the mouse ESC line D3 as a model to examine the effects of X-rays or C-ions (spread out Bragg peak, energy 106–147 MeV/u, average LET = 75 keV/µm) [ 1]. Doses of 0.5–5 Gy were applied and endpoints such as cell cycle progression (measured by flow cytometry), apoptosis (microscopic analysis of cell nucleus morphology), induction of chromosome aberrations (mFISH analysis), presence of pluripotency markers Oct3/4 and SOX2 (western blotting) and differentiation capacity by means of an embryoid body formation assay were analyzed up to 17 days post-irradiation. The experiments show that cells undergo a transient G2 arrest following exposure. After G2 checkpoint release, an increase in the apoptotic index is observed for both radiation types (3.7-fold increase for 2 Gy X-ray and 2.4-fold increase for 2 Gy C-ions). C-ions induce more structural chromosomal aberrations in first cycle cells than X-rays. During subsequent cell divisions, the frequency of chromosome aberrations declines: After >7 population doublings (8 days after exposure), the aberration frequency in the progeny of X-ray exposed cells returns to the control level (7% aberrant cells), while the progeny of C-ion exposed cells still harbor significantly more aberrations than control cells, which is mainly due to transmissible translocations.
The expression of pluripotency markers is maintained in cells surviving X-ray or C-ion exposure. This finding is supported by examining the differentiation capacity of ESC through the formation of embryoid bodies. Our experiments show that after X-ray or C-ion exposure, cells are able to develop spontaneous beating activity, indicating the differentiation ability into mesodermal cell lineages, i.e. beating cardiomyocytes. However, following C-ion exposure, the formation of beating clusters was delayed compared with control cells.
Moreover, our chromosome studies revealed that unexposed cells carry a high frequency of numerical aberrations. These comprise trisomies of chromosome 8 and 11 with a frequency of 29 ± 8% and 26 ± 6% respectively, as well as nullisomy of chromosome Y with a frequency of 35 ± 3%. Aneuploidy is a typical feature of mouse ESC and has been related to cell culture methods [ 2] and passage number. Because aneuploidy may affect gene expression and influence the properties of a cell population, the relevance of experiments based on mouse ESC is limited.
To overcome this problem, we recently extended our studies to human ESC. Human ESC are known to be cytogenetically more stable than mouse ESC, and represent a model that is closer to human embryonic development. Indeed, first investigations revealed a lower faction of cells with numerical and structural aberrations in the human ESC line H9 [ 3] compared with the mouse ESC line D3 (2% vs. 73% and 3% vs. 7%, respectively).
PMCID: PMC3941490
embryonic stem cells; pluripotency; genomic integrity
4.  A Model of Photon Cell Killing Based on the Spatio-Temporal Clustering of DNA Damage in Higher Order Chromatin Structures 
PLoS ONE  2014;9(1):e83923.
We present a new approach to model dose rate effects on cell killing after photon radiation based on the spatio-temporal clustering of DNA double strand breaks (DSBs) within higher order chromatin structures of approximately 1–2 Mbp size, so called giant loops. The main concept of this approach consists of a distinction of two classes of lesions, isolated and clustered DSBs, characterized by the number of double strand breaks induced in a giant loop. We assume a low lethality and fast component of repair for isolated DSBs and a high lethality and slow component of repair for clustered DSBs. With appropriate rates, the temporal transition between the different lesion classes is expressed in terms of five differential equations. These allow formulating the dynamics involved in the competition of damage induction and repair for arbitrary dose rates and fractionation schemes. Final cell survival probabilities are computable with a cell line specific set of three parameters: The lethality for isolated DSBs, the lethality for clustered DSBs and the half-life time of isolated DSBs.
By comparison with larger sets of published experimental data it is demonstrated that the model describes the cell line dependent response to treatments using either continuous irradiation at a constant dose rate or to split dose irradiation well. Furthermore, an analytic investigation of the formulation concerning single fraction treatments with constant dose rates in the limiting cases of extremely high or low dose rates is presented. The approach is consistent with the Linear-Quadratic model extended by the Lea-Catcheside factor up to the second moment in dose. Finally, it is shown that the model correctly predicts empirical findings about the dose rate dependence of incidence probabilities for deterministic radiation effects like pneumonitis and the bone marrow syndrome. These findings further support the general concepts on which the approach is based.
PMCID: PMC3879277  PMID: 24392100
5.  Chromosome Damage in Human Cells by γ Rays, α Particles and Heavy Ions: Track Interactions in Basic Dose-Response Relationships 
Radiation research  2012;179(1):9-20.
We irradiated normal human lymphocytes and fibroblasts with 137Cs γ rays, 3.5 MeV α particles and 1 GeV/amu 56Fe ions and measured the subsequent formation of chromosome-type aberrations by mFISH at the first mitosis following irradiation. This was done for the purposes of characterizing the shape of dose-response relationships and determining the frequency distribution of various aberration types with respect to the parameters of dose, radiation quality and cell type. Salient results and conclusions include the following. For low-LET γ rays, lymphocytes showed a more robust dose response for overall damage and a higher degree of upward curvature compared to fibroblasts. For both sources of high-LET radiation, and for both cell types, the response for simple and complex exchanges was linear with dose. Independent of all three parameters considered, the most likely damage outcome was the formation of a simple exchange event involving two breaks. However, in terms of the breakpoints making up exchange events, the majority of damage registered following HZE particle irradiation was due to complex aberrations involving multiple chromosomes. This adds a decidedly nonlinear component to the overall breakpoint response, giving it a significant degree of positive curvature, which we interpret as being due to interaction between ionizations of the primary HZE particle track and long-range δ rays produced by other nearby tracks. While such track interaction had been previously theorized, to the best of our knowledge, it has never been demonstrated experimentally.
PMCID: PMC3580060  PMID: 23198992
6.  Biophysical characterization of a relativistic proton beam for image-guided radiosurgery 
Journal of radiation research  2012;53(4):620-627.
We measured the physical and radiobiological characteristics of 1 GeV protons for possible applications in stereotactic radiosurgery (image-guided plateau-proton radiosurgery). A proton beam was accelerated at 1 GeV at the Brookhaven National Laboratory (Upton, NY) and a target in polymethyl methacrylate (PMMA) was used. Clonogenic survival was measured after exposures to 1–10 Gy in three mammalian cell lines. Measurements and simulations demonstrate that the lateral scattering of the beam is very small. The lateral dose profile was measured with or without the 20-cm plastic target, showing no significant differences up to 2 cm from the axis A large number of secondary swift protons are produced in the target and this leads to an increase of approximately 40% in the measured dose on the beam axis at 20 cm depth. The relative biological effectiveness at 10% survival level ranged between 1.0 and 1.2 on the beam axis, and was slightly higher off-axis. The very low lateral scattering of relativistic protons and the possibility of using online proton radiography during the treatment make them attractive for image-guided plateau (non-Bragg peak) stereotactic radiosurgery.
PMCID: PMC3393345  PMID: 22843629
proton therapy; radiosurgery; image-guided radiotherapy; proton radiography; RBE
7.  Influence of chronic hypoxia and radiation quality on cell survival 
Journal of Radiation Research  2013;54(Suppl 1):i13-i22.
To investigate the influence of chronic hypoxia and anoxia on cell survival after low- and high-LET radiation, CHO-K1 cells were kept for 24 h under chronic hypoxia (94.5% N2; 5% CO2; 0.5% O2) or chronic anoxia (95% N2; 5% CO2). Irradiation was performed using 250 kVp X-rays or carbon ions with a dose average LET of 100 keV/μm either directly under the chronic oxygenation states, or at different time points after reoxygenation. Moreover, the cell cycle distribution for cells irradiated under different chronic oxic states was measured over 24 h during reoxygenation. The measurements showed a fairly uniform cell cycle distribution under chronic hypoxia, similar to normoxic conditions. Chronic anoxia induced a block in G1 and a strong reduction of S-phase cells. A distribution similar to normoxic conditions was reached after 12 h of reoxygenation. CHO cells had a similar survival under both acute and chronic hypoxia. In contrast, survival after irradiation under chronic anoxia was slightly reduced compared to that under acute anoxia. We conclude that, in hamster cells, chronic anoxia is less effective than acute anoxia in inducing radioresistance for both X-rays and carbon ions, whereas in hypoxia, acute and chronic exposures have a similar impact on cell killing.
PMCID: PMC3700502  PMID: 23824117
radiosensitivity; hypoxia; anoxia; carbon ions; cell cycle distribution
9.  Influence of acute hypoxia and radiation quality on cell survival 
Journal of Radiation Research  2013;54(Suppl 1):i23-i30.
To measure the effect of acute oxygen depletion on cell survival for different types of radiation, experiments have been performed using Chinese hamster ovary (CHO) cells and RAT-1 rat prostate cancer cells. A special chamber has been developed to perform irradiations under different levels of oxygenation. The oxygen concentrations used were normoxia (air), hypoxia (94.5% N2, 5% CO2, 0.5% O2) and anoxia (95% N2, 5% CO2). Cells were exposed to X-rays and to C-, N- or O-ions with linear energy transfer (LET) values ranging from 100–160 keV/µm. The oxygen enhancement ratio (OER) and relative biological effectiveness (RBE) values have been calculated from the measured clonogenic survival curves. For both cell lines, the X-ray OER depended on the survival level. For particle irradiation, OER was not dependent on the survival level but decreased with increasing LET. The RBE of CHO cells under oxic conditions reached a plateau for LET values above 100 keV/µm, while it was still increasing under anoxia. In conclusion, the results demonstrated that our chamber could be used to measure radiosensitivity under intermediate hypoxia. Measurements suggest that ions heavier than carbon could be of additional advantage in the irradiation, especially of radioresistant hypoxic tumor regions.
PMCID: PMC3700520  PMID: 23824123
hypoxia; LET; OER; radiosensitivity
10.  Clustered DNA damage induces pan-nuclear H2AX phosphorylation mediated by ATM and DNA–PK 
Nucleic Acids Research  2013;41(12):6109-6118.
DNA double-strand breaks (DSB) are considered as the most deleterious DNA lesions, and their repair is further complicated by increasing damage complexity. However, the molecular effects of clustered lesions are yet not fully understood. As the locally restricted phosphorylation of H2AX to form γH2AX is a key step in facilitating efficient DSB repair, we investigated this process after localized induction of clustered damage by ionizing radiation. We show that in addition to foci at damaged sites, H2AX is also phosphorylated in undamaged chromatin over the whole-cell nucleus in human and rodent cells, but this is not related to apoptosis. This pan-nuclear γH2AX is mediated by the kinases ataxia telangiectasia mutated and DNA-dependent protein kinase (DNA–PK) that also phosphorylate H2AX at DSBs. The pan-nuclear response is dependent on the amount of DNA damage and is transient even under conditions of impaired DSB repair. Using fluorescence recovery after photobleaching (FRAP), we found that MDC1, but not 53BP1, binds to the nuclear-wide γH2AX. Consequently, the accumulation of MDC1 at DSBs is reduced. Altogether, we show that a transient dose-dependent activation of the kinases occurring on complex DNA lesions leads to their nuclear-wide distribution and H2AX phosphorylation, yet without eliciting a full pan-nuclear DNA damage response.
PMCID: PMC3695524  PMID: 23620287
11.  Organotypic slice cultures of human glioblastoma reveal different susceptibilities to treatments 
Neuro-Oncology  2013;15(6):670-681.
Glioblastoma multiforme is the most common lethal brain tumor in human adults, with no major therapeutic breakthroughs in recent decades. Research is based mostly on human tumor cell lines deprived of their organotypic environment or inserted into immune-deficient animals required for graft survival. Here, we describe how glioblastoma specimens obtained from surgical biopsy material can be sectioned and transferred into cultures within minutes.
Slices were kept in 6-well plates, allowing direct observation, application of temozolomide, and irradiation. At the end of experiments, slice cultures were processed for histological analysis including hematoxylin-eosin staining, detection of proliferation (Ki67), apoptosis/cell death (cleaved caspase 3, propidium iodide), DNA double-strand breaks (γH2AX), and neural subpopulations. First clinical trials employed irradiation with the heavy ion carbon for the treatment of glioblastoma patients, but the biological effects and most effective dose regimens remain to be established. Therefore, we developed an approach to expose glioblastoma slice cultures to 12C and X-rays.
We found preservation of the individual histopathology over at least 16 days. Treatments resulted in activation of caspase 3, inhibition of proliferation, and cell loss. Irradiation induced γH2AX. In line with clinical observations, individual tumors differed significantly in their susceptibility to temozolomide (0.4%–2.5% apoptosis and 1%–15% cell loss).
Glioblastoma multiforme slice cultures provide a unique tool to explore susceptibility of individual tumors for specific therapies including heavy ions, thus potentially allowing more personalized treatments plus exploration of mechanisms of (and strategies to overcome) tumor resistance.
PMCID: PMC3661091  PMID: 23576601
glioblastoma multiforme; organotypic slice culture; human test system; heavy ions
13.  Spatiotemporal Dynamics of Early DNA Damage Response Proteins on Complex DNA Lesions 
PLoS ONE  2013;8(2):e57953.
The response of cells to ionizing radiation-induced DNA double-strand breaks (DSB) is determined by the activation of multiple pathways aimed at repairing the injury and maintaining genomic integrity. Densely ionizing radiation induces complex damage consisting of different types of DNA lesions in close proximity that are difficult to repair and may promote carcinogenesis. Little is known about the dynamic behavior of repair proteins on complex lesions. In this study we use live-cell imaging for the spatio-temporal characterization of early protein interactions at damage sites of increasing complexity. Beamline microscopy was used to image living cells expressing fluorescently-tagged proteins during and immediately after charged particle irradiation to reveal protein accumulation at damaged sites in real time. Information on the mobility and binding rates of the recruited proteins was obtained from fluorescence recovery after photobleaching (FRAP). Recruitment of the DNA damage sensor protein NBS1 accelerates with increasing lesion density and saturates at very high damage levels. FRAP measurements revealed two different binding modalities of NBS1 to damage sites and a direct impact of lesion complexity on the binding. Faster recruitment with increasing lesion complexity was also observed for the mediator MDC1, but mobility was limited at very high damage densities due to nuclear-wide binding. We constructed a minimal computer model of the initial response to DSB based on known protein interactions only. By fitting all measured data using the same set of parameters, we can reproduce the experimentally characterized steps of the DNA damage response over a wide range of damage densities. The model suggests that the influence of increasing lesion density accelerating NBS1 recruitment is only dependent on the different binding modes of NBS1, directly to DSB and to the surrounding chromatin via MDC1. This elucidates an impact of damage clustering on repair without the need of invoking extra processing steps.
PMCID: PMC3582506  PMID: 23469115
14.  Systematic analysis of RBE and related quantities using a database of cell survival experiments with ion beam irradiation 
Journal of Radiation Research  2012;54(3):494-514.
For tumor therapy with light ions and for experimental aspects in particle radiobiology the relative biological effectiveness (RBE) is an important quantity to describe the increased effectiveness of particle radiation. By establishing and analysing a database of ion and photon cell survival data, some remarkable properties of RBE-related quantities were observed. The database consists of 855 in vitro cell survival experiments after ion and photon irradiation. The experiments comprise curves obtained in different labs, using different ion species, different irradiation modalities, the whole range of accessible energies and linear energy transfers (LETs) and various cell types. Each survival curve has been parameterized using the linear-quadratic (LQ) model. The photon parameters, α and β, appear to be slightly anti-correlated, which might point toward an underlying biological mechanism. The RBE values derived from the survival curves support the known dependence of RBE on LET, on particle species and dose. A positive correlation of RBE with the ratio α/β of the photon LQ parameters is found at low doses, which unexpectedly changes to a negative correlation at high doses. Furthermore, we investigated the course of the β coefficient of the LQ model with increasing LET, finding typically a slight initial increase and a final falloff to zero. The observed fluctuations in RBE values of comparable experiments resemble overall RBE uncertainties, which is of relevance for treatment planning. The database can also be used for extensive testing of RBE models. We thus compare simulations with the local effect model to achieve this goal.
PMCID: PMC3650740  PMID: 23266948
relative biological effectiveness; cell survival; ions; local effect model; linear quadratic model
15.  Nanolesions induced by heavy ions in human tissues: Experimental and theoretical studies 
The biological effects of energetic heavy ions are attracting increasing interest for their applications in cancer therapy and protection against space radiation. The cascade of events leading to cell death or late effects starts from stochastic energy deposition on the nanometer scale and the corresponding lesions in biological molecules, primarily DNA. We have developed experimental techniques to visualize DNA nanolesions induced by heavy ions. Nanolesions appear in cells as “streaks” which can be visualized by using different DNA repair markers. We have studied the kinetics of repair of these “streaks” also with respect to the chromatin conformation. Initial steps in the modeling of the energy deposition patterns at the micrometer and nanometer scale were made with MCHIT and TRAX models, respectively.
PMCID: PMC3458601  PMID: 23019551
DNA repair; heavy ions; microdosimetry; Monte Carlo simulations; nanolesions; radiation-induced nanostructures
16.  Scanned carbon beam irradiation of moving films: comparison of measured and calculated response 
Treatment of moving target volumes with scanned particle beams benefits from treatment planning that includes the time domain (4D). Part of 4D treatment planning is calculation of the expected result. These calculation codes should be verified against suitable measurements. We performed simulations and measurements to validate calculation of the film response in the presence of target motion.
All calculations were performed with GSI's treatment planning system TRiP. Interplay patterns between scanned particle beams and moving film detectors are very sensitive to slight deviations of the assumed motion parameters and therefore ideally suited to validate 4D calculations. In total, 14 film motion parameter combinations with lateral motion amplitudes of 8, 15, and 20 mm and 4 combinations for lateral motion including range changes were used. Experimental and calculated film responses were compared by relative difference, mean deviation in two regions-of-interest, as well as line profiles.
Irradiations of stationary films resulted in a mean relative difference of -1.52% ± 2.06% of measured and calculated responses. In comparison to this reference result, measurements with translational film motion resulted in a mean difference of -0.92% ± 1.30%. In case of irradiations incorporating range changes with a stack of 5 films as detector the deviations increased to -6.4 ± 2.6% (-10.3 ± 9.0% if film in distal fall-off is included) in comparison to -3.6% ± 2.5% (-13.5% ± 19.9% including the distal film) for the stationary irradiation. Furthermore, the comparison of line profiles of 4D calculations and experimental data showed only slight deviations at the borders of the irradiated area. The comparisons of pure lateral motion were used to determine the number of motion states that are required for 4D calculations depending on the motion amplitude. 6 motion states per 10 mm motion amplitude are sufficient to calculate the film response in the presence of motion.
By comparison to experimental data, the 4D extension of GSI's treatment planning system TRiP has been successfully validated for film response calculations in the presence of target motion within the accuracy limitation given by film-based dosimetry.
PMCID: PMC3342219  PMID: 22462523
17.  Human embryo stem cells and DNA repair 
Aging (Albany NY)  2011;3(6):564.
PMCID: PMC3164364  PMID: 21685511
18.  DNA double-strand breaks in heterochromatin elicit fast repair protein recruitment, histone H2AX phosphorylation and relocation to euchromatin 
Nucleic Acids Research  2011;39(15):6489-6499.
DNA double-strand breaks (DSBs) can induce chromosomal aberrations and carcinogenesis and their correct repair is crucial for genetic stability. The cellular response to DSBs depends on damage signaling including the phosphorylation of the histone H2AX (γH2AX). However, a lack of γH2AX formation in heterochromatin (HC) is generally observed after DNA damage induction. Here, we examine γH2AX and repair protein foci along linear ion tracks traversing heterochromatic regions in human or murine cells and find the DSBs and damage signal streaks bending around highly compacted DNA. Given the linear particle path, such bending indicates a relocation of damage from the initial induction site to the periphery of HC. Real-time imaging of the repair protein GFP-XRCC1 confirms fast recruitment to heterochromatic lesions inside murine chromocenters. Using single-ion microirradiation to induce localized DSBs directly within chromocenters, we demonstrate that H2AX is early phosphorylated within HC, but the damage site is subsequently expelled from the center to the periphery of chromocenters within ∼20 min. While this process can occur in the absence of ATM kinase, the repair of DSBs bordering HC requires the protein. Finally, we describe a local decondensation of HC at the sites of ion hits, potentially allowing for DSB movement via physical forces.
PMCID: PMC3159438  PMID: 21511815
19.  Dosimetric precision of an ion beam tracking system 
Scanned ion beam therapy of intra-fractionally moving tumors requires motion mitigation. GSI proposed beam tracking and performed several experimental studies to analyse the dosimetric precision of the system for scanned carbon beams.
A beam tracking system has been developed and integrated in the scanned carbon ion beam therapy unit at GSI. The system adapts pencil beam positions and beam energy according to target motion.
Motion compensation performance of the beam tracking system was assessed by measurements with radiographic films, a range telescope, a 3D array of 24 ionization chambers, and cell samples for biological dosimetry. Measurements were performed for stationary detectors and moving detectors using the beam tracking system.
All detector systems showed comparable data for a moving setup when using beam tracking and the corresponding stationary setup. Within the target volume the mean relative differences of ionization chamber measurements were 0.3% (1.5% standard deviation, 3.7% maximum). Film responses demonstrated preserved lateral dose gradients. Measurements with the range telescope showed agreement of Bragg peak depth under motion induced range variations. Cell survival experiments showed a mean relative difference of -5% (-3%) between measurements and calculations within the target volume for beam tracking (stationary) measurements.
The beam tracking system has been successfully integrated. Full functionality has been validated dosimetrically in experiments with several detector types including biological cell systems.
PMCID: PMC2907389  PMID: 20591160

Results 1-19 (19)