DNA fragments induction in human fibroblasts by radiations of different qualities

Development of modified microdosimetric kinetic model for relative biological effectiveness in proton therapy

To predict the biological effects of ionising radiation, the quantity of biological dose is introduced instead of the physical absorbed dose. In proton therapy, a constant relative biological effectiveness (RBE) of 1.1 is usually applied clinically as recommended by the International Commission of Radiation Units and Measurements. This study presents a new model, based on the modified microdosimetric kinetic model (MMKM), for calculating variable RBE values based on experimental data on the induction of DNA double-strand breaks (DSBs) within cells. The MMKM was proposed based on experimental data for the yield of DSBs in mammalian cells, which allows modification of the yield of primary lesions in the MKM. In this approach, a unique function named f(LET), which describes the relation between RBE and linear energy transfer (LET), was considered for charged particles. In the presented model (DMMKM), the MMKM approach was developed further by considering different f(LET)s for different relevant ions involved in energy deposition events in proton therapy. Although experimental data represent the dependence of the yield of primary lesions on the ion species, the DSB yield (assumed as the main primary lesion) is assumed independent of the ion species in the MMKM. In the DMMKM, by considering the yield of primary lesions as a function of the ion species, the α and β values are in better agreement with the experimental data as compared to those of the MKM and MMKM approaches. The biological dose in the DMMKM is predicted to be lower than that in the MMKM. Further, in the proposed model, the variation of the β parameter is higher than the constant value assumed in the MKM, at the distal end of the spread-out Bragg peak (SOBP). Moreover, the level of cell death estimated by the MMKM at the SOBP region is higher than that obtained based on the DMMKM. It is concluded that considering modified f(LET)s in the model developed here is more consistent with experimental results than when MMKM and MKM approaches are considered. The DMMKM examines the biological effects with full detail and will, therefore, be effective in improving proton therapy.

Review of microbial resistance to chronic ionizing radiation exposure under environmental conditions

Ionizing radiation (IR) produces multiple types of damage to nucleic acids, proteins and other crucial cellular components. Nevertheless, various microorganisms from phylogenetically distant taxa (bacteria, archaea, fungi) can resist IR levels many orders of magnitude above natural background. This intriguing phenomenon of radioresistance probably arose independently many times throughout evolution as a byproduct of selective pressures from other stresses (e.g. desiccation, UV radiation, chemical oxidants). Most of the literature on microbial radioresistance is based on acute γ-irradiation experiments performed in the laboratory, typically involving pure cultures grown under near-optimal conditions. There is much less information about the upper limits of radioresistance in the field, such as in radioactively-contaminated areas, where several radiation types (e.g. α and β, as well as γ) and other stressors (e.g. non-optimal temperature and nutrient levels, toxic chemicals, interspecific competition) act over multiple generations. Here we discuss several examples of radioresistant microbes isolated from extremely radioactive locations (e.g. Chernobyl and Mayak nuclear plant sites) and estimate the radiation dose rates they were able to tolerate. Some of these organisms (e.g. the fungus Cladosporium cladosporioides, the cyanobacterium Geitlerinema amphibium) are widely-distributed and colonize a variety of habitats. These examples suggest that resistance to chronic IR and chemical contamination is not limited to rare specialized strains from extreme environments, but can occur among common microbial taxa, perhaps due to overlap between mechanisms of resistance to IR and other stressors.

Proton Beams Inhibit Proliferation of Breast Cancer Cells by Altering DNA Methylation Status

Proton beam therapy has been gaining popularity in the management of a wide spectrum of cancers. However, little is known about the effect of proton beams on epigenetic alterations. In this study, the effects of proton beams on DNA methylation were evaluated in the breast cell lines MCF-10A and MCF-7. Pyrosequencing analysis of the long interspersed element 1 (LINE1) gene indicated that a few specific CpG sites were induced to be hypermethylated by proton beam treatment from 64.5 to 76.5% and from 57.7 to 60.0% (p < 0.05) in MCF-10A and MCF-7, respectively. Genome-wide methylation analysis identified “Developmental Disorder, Hereditary Disorder, Metabolic Disease” as the top network in the MCF-7 cell line. The proliferation rate significantly decreased in proton beam-treated cells, as judged by colony formation and cell proliferation assay. Upon treatment with the proton beam, expression of selected genes (MDH2, STYXL1, CPE, FAM91A1, and GPR37) was significantly changed in accordance with the changes of methylation level. Taken together, the findings demonstrate that proton beam-induced physiological changes of cancer cells via methylation modification assists in establishing the epigenetic basis of proton beam therapy for cancer.

Monte Carlo simulations of the relative biological effectiveness for DNA double strand breaks from 300 MeV u(-1) carbon-ion beams

Monte Carlo simulations are used to calculate the relative biological effectiveness (RBE) of 300 MeV u(-1) carbon-ion beams at different depths in a cylindrical water phantom of 10 cm radius and 30 cm long. RBE values for the induction of DNA double strand breaks (DSB), a biological endpoint closely related to cell inactivation, are estimated for monoenergetic and energy-modulated carbon ion beams. Individual contributions to the RBE from primary ions and secondary nuclear fragments are simulated separately. These simulations are based on a multi-scale modelling approach by first applying the FLUKA (version 2011.2.17) transport code to estimate the absorbed doses and fluence energy spectra, then using the MCDS (version 3.10A) damage code for DSB yields. The approach is efficient since it separates the non-stochastic dosimetry problem from the stochastic DNA damage problem. The MCDS code predicts the major trends of the DSB yields from detailed track structure simulations. It is found that, as depth is increasing, RBE values increase slowly from the entrance depth to the plateau region and change substantially in the Bragg peak region. RBE values reach their maxima at the distal edge of the Bragg peak. Beyond this edge, contributions to RBE are entirely from nuclear fragments. Maximum RBE values at the distal edges of the Bragg peak and the spread-out Bragg peak are, respectively, 3.0 and 2.8. The present approach has the flexibility to weight RBE contributions from different DSB classes, i.e. DSB0, DSB+ and DSB++.

Increased effectiveness of carbon ions in the production of reactive oxygen species in normal human fibroblasts

The production of reactive oxygen species (ROS), especially superoxide anions (O2 (·-)), is enhanced in many normal and tumor cell types in response to ionizing radiation. The influence of ionizing radiation on the regulation of ROS production is considered as an important factor in the long-term effects of irradiation (such as genomic instability) that might contribute to the development of secondary cancers. In view of the increasing application of carbon ions in radiation therapy, we aimed to study the potential impact of ionizing density on the intracellular production of ROS, comparing photons (X-rays) with carbon ions. For this purpose, we used normal human cells as a model for irradiated tissue surrounding a tumor. By quantifying the oxidization of Dihydroethidium (DHE), a fluorescent probe sensitive to superoxide anions, we assessed the intracellular ROS status after radiation exposure in normal human fibroblasts, which do not show radiation-induced chromosomal instability. After 3-5 days post exposure to X-rays and carbon ions, the level of ROS increased to a maximum that was dose dependent. The maximum ROS level reached after irradiation was specific for the fibroblast type. However, carbon ions induced this maximum level at a lower dose compared with X-rays. Within ∼1 week, ROS decreased to control levels. The time-course of decreasing ROS coincides with an increase in cell number and decreasing p21 protein levels, indicating a release from radiation-induced growth arrest. Interestingly, radiation did not act as a trigger for chronically enhanced levels of ROS months after radiation exposure.

Relative biological effectiveness (RBE) values for proton beam therapy. Variations as a function of biological endpoint, dose, and linear energy transfer

Proton therapy treatments are based on a proton RBE (relative biological effectiveness) relative to high-energy photons of 1.1. The use of this generic, spatially invariant RBE within tumors and normal tissues disregards the evidence that proton RBE varies with linear energy transfer (LET), physiological and biological factors, and clinical endpoint. Based on the available experimental data from published literature, this review analyzes relationships of RBE with dose, biological endpoint and physical properties of proton beams. The review distinguishes between endpoints relevant for tumor control probability and those potentially relevant for normal tissue complication. Numerous endpoints and experiments on sub-cellular damage and repair effects are discussed. Despite the large amount of data, considerable uncertainties in proton RBE values remain. As an average RBE for cell survival in the center of a typical spread-out Bragg peak (SOBP), the data support a value of ~1.15 at 2 Gy/fraction. The proton RBE increases with increasing LETd and thus with depth in an SOBP from ~1.1 in the entrance region, to ~1.15 in the center, ~1.35 at the distal edge and ~1.7 in the distal fall-off (when averaged over all cell lines, which may not be clinically representative). For small modulation widths the values could be increased. Furthermore, there is a trend of an increase in RBE as (α/β)x decreases. In most cases the RBE also increases with decreasing dose, specifically for systems with low (α/β)x. Data on RBE for endpoints other than clonogenic cell survival are too diverse to allow general statements other than that the RBE is, on average, in line with a value of ~1.1. This review can serve as a source for defining input parameters for applying or refining biophysical models and to identify endpoints where additional radiobiological data are needed in order to reduce the uncertainties to clinically acceptable levels.

The role of telomere length modulation in delayed chromosome instability induced by ionizing radiation in human primary fibroblasts

Telomere integrity is important for chromosome stability. The main objective of our study was to investigate the relationship between telomere length modulation and mitotic chromosome segregation induced by ionizing radiation in human primary fibroblasts. We used X-rays and low-energy protons because of their ability to induce different telomeric responses. Samples irradiated with 4 Gy were fixed at different times up to 6 days from exposure and telomere length, anaphase abnormalities, and chromosome aberrations were analyzed. We observed that X-rays induced telomere shortening in cells harvested at 96 hrs, whereas protons induced a significant increase in telomere length at short as well as at long harvesting times (24 and 96 hrs). Consistent with this, the analysis of anaphase bridges at 96 hrs showed a fourfold increase in X-ray- compared with proton-irradiated samples, suggesting a correlation between telomere length/dysfunction and chromosome missegregation. In line with these findings, the frequency of dicentrics and rings decreased with time for protons whereas it remained stable after X-rays irradiation. Telomeric FISH staining on anaphases revealed a higher percentage of bridges with telomere signals in X-ray-treated samples than that observed after proton irradiation, thus suggesting that the aberrations observed after X-ray irradiation originated from telomere attrition and consequent chromosome end-to-end fusion. This study shows that, beside an expected "early" chromosome instability induced shortly after irradiation, a delayed one occurs as a result of alterations in telomere metabolism and that this mechanism may play an important role in genomic stability.

Cell cycle perturbations and genotoxic effects in human primary fibroblasts induced by low-energy protons and X/gamma-rays

The effect of graded doses of high-linear energy transfer (LET) low-energy protons to induce cycle perturbations and genotoxic damage was investigated in normal human fibroblasts. Furthermore, such effects were compared with those produced by low-LET radiations. HFFF2, human primary fibroblasts were exposed to either protons (LET = 28.5 keV/microm) or X/gamma-rays, and endpoints related to cell cycle kinetics and DNA damage analysed. Following both type of irradiations, unsynchronized cells suffered an inhibition to entry into S-phase for doses of 1-4 Gy and remained arrested in the G(1)-phase for several days. The levels of induction of regulator proteins, such as TP53 and CDKN1A showed a clear LET-dependence. DSB induction and repair as measured by scoring for gamma-H2AX foci indicated that protons, with respect to X-rays, yielded a lower number of DSBs per Gy, which showed a slower kinetics of disappearance. Such result was in agreement with the extent of MN induction in binucleated cells after X-irradiation. No significant differences between the two types of radiations were observed with the clonogenic assay, resulting anyway the slope of gamma-ray curve higher than that the proton one. In conclusion, in normal human primary fibroblasts cell cycle arrest at the G(1)/S transition can be triggered shortly after irradiation and maintained for several hours post-irradiation of both protons and X-rays. DNA damage produced by protons appears less amenable to be repaired and could be transformed in cytogenetic damage in the form of MN.