60Co-γ Radiation Alters Developmental Stages of Zeugodacus cucurbitae (Diptera: Tephritidae) Through Apoptosis Pathways Gene Expression.

Radiation is considered as a promising insect pest control strategy for minimizing postharvest yield losses. Among various techniques, irradiation is a method of choice as it induces lethal biochemical or molecular changes that cause a downstream cascade of abrupt physiological abnormalities at the cellular level. In this study, we evaluated the effect of 60Co-γ radiation on various developmental stages of Zeugodacus cucurbitae Coquillett and subsequent carry-over effects on the progeny. For this purpose, we treated eggs with 30- and 50-Gy radiation doses of 60Co-γ. We found that radiation significantly affected cellular antioxidants, insect morphology, and gene expression profiles. Our results indicate that in response to various doses of irradiation reactive oxygen species, catalase, peroxidase, and superoxide dismutase activities were increased along with a significant increase in the malondialdehyde (MDA) content. We observed higher mortality rates during the pupal stage of the insects that hatched from irradiated eggs (50 Gy). Furthermore, the life span of the adults was reduced in response to 50 Gy radiation. The negative effects carried over to the next generation were marked by significantly lower fecundity in the F1 generation of the irradiation groups as compared to control. The radiation induced morphological abnormalities at the pupal, as well as the adult, stages. Furthermore, variations in the gene expression following irradiation are discussed. Taken together, our results signify the utility of 60Co-γ radiation for fruit fly postharvest management.

Chromatin states responsible for the regulation of differentially expressed genes under 60Co~γ ray radiation in rice.

BACKGROUND: The role of histone modifications in the DNA damage response has been extensively studied in non-plant systems, including mammals and yeast. However, there is a lack of detailed evidence showing how chromatin dynamics, either an individual mark or combined chromatin states, participate in regulating differentially expressed genes in the plant DNA damage response. RESULTS: In this study, we used RNA-seq and ChIP-seq to show that differentially expressed genes (DEGs), in response to ionizing radiation (IR), might be involved in different pathways responsible for the DNA damage response. Moreover, chromatin structures associated with promoters, exons and intergenic regions are significantly affected by IR. Most importantly, either an individual mark or a certain chromatin state was found to be highly correlated with the expression of up-regulated genes. In contrast, only the chromatin states, as opposed to any individual marks tested, are related to the expression of the down-regulated genes. CONCLUSIONS: Our findings demonstrate that IR-related DEGs are modulated by distinct epigenetic mechanisms. Either chromatin states or distinct histone dynamics may act sequentially or in combination in regulating up-regulated genes, but the complex chromatin structure is mainly responsible for the expression of down-regulated genes. Thus, this study provides new insights into how up- and down-regulated genes are epigenetically regulated at the chromatin levels, thereby helping us to understand distinct epigenetic mechanisms that function in the plant DNA damage response.

New method for estimating clustering of DNA lesions induced by physical/chemical mutagens using fluorescence anisotropy.

We have developed a new method for estimating the localization of DNA damage such as apurinic/apyrimidinic sites (APs) on DNA using fluorescence anisotropy. This method is aimed at characterizing clustered DNA damage produced by DNA-damaging agents such as ionizing radiation and genotoxic chemicals. A fluorescent probe with an aminooxy group (AlexaFluor488) was used to label APs. We prepared a pUC19 plasmid with APs by heating under acidic conditions as a model for damaged DNA, and subsequently labeled the APs. We found that the observed fluorescence anisotropy (robs) decreases as averaged AP density (λAP: number of APs per base pair) increases due to homo-FRET, and that the APs were randomly distributed. We applied this method to three DNA-damaging agents, 60Co γ-rays, methyl methanesulfonate (MMS), and neocarzinostatin (NCS). We found that robs-λAP relationships differed significantly between MMS and NCS. At low AP density (λAP < 0.001), the APs induced by MMS seemed to not be closely distributed, whereas those induced by NCS were remarkably clustered. In contrast, the AP clustering induced by 60Co γ-rays was similar to, but potentially more likely to occur than, random distribution. This simple method can be used to estimate mutagenicity of ionizing radiation and genotoxic chemicals.

Differential gene expression in primary fibroblasts induced by proton and cobalt-60 beam irradiation.

INTRODUCTION: Proton beam therapy delivers a more conformal dose distribution than conventional radiotherapy, thus improving normal tissue sparring. Increasing linear energy transfer (LET) along the proton track increases the relative biological effectiveness (RBE) near the distal edge of the Spread-out Bragg peak (SOBP). The severity of normal tissue side effects following photon beam radiotherapy vary considerably between patients. AIM: The dual study aim was to identify gene expression patterns specific to radiation type and proton beam position, and to assess whether individual radiation sensitivity influences gene expression levels in fibroblast cultures irradiated in vitro. METHODS: The study includes 30 primary fibroblast cell cultures from patients previously classified as either radiosensitive or radioresistant. Cells were irradiated at three different positions in the proton beam profile: entrance, mid-SOBP and at the SOBP distal edge. Dose was delivered in three fractions × 3.5 Gy(RBE) (RBE 1.1). Cobalt-60 (Co-60) irradiation was used as reference. Real-time qPCR was performed to determine gene expression levels for 17 genes associated with inflammation response, fibrosis and angiogenesis. RESULTS: Differences in median gene expression levels were observed for multiple genes such as IL6, IL8 and CXCL12. Median IL6 expression was 30%, 24% and 47% lower in entrance, mid-SOBP and SOBP distal edge groups than in Co-60 irradiated cells. No genes were found to be oppositely regulated by different radiation qualities. Radiosensitive patient samples had the strongest regulation of gene expression; irrespective of radiation type. CONCLUSIONS: Our findings indicate that the increased LET at the SOBP distal edge position did not generally lead to increased transcriptive response in primary fibroblast cultures. Inflammatory factors were generally less extensively upregulated by proton irradiation compared with Co-60 photon irradiation. These effects may possibly influence the development of normal tissue damage in patients treated with proton beam therapy.

Melatonin may play a role in modulation of bax and bcl-2 expression levels to protect rat peripheral blood lymphocytes from gamma irradiation-induced apoptosis.

The close relationship between free radicals effects and apoptosis process has been proved. Melatonin has been reported as a direct free radical scavenger. We investigated the capability of melatonin in the modification of radiation-induced apoptosis and apoptosis-associated upstream regulators expression in rat peripheral blood lymphocytes. Rats were irradiated with a single whole body Cobalt 60-gamma radiation dose of 8Gy at a dose rate of 101cGy/min with or without melatonin pretreatments at different concentrations of 10 and 100mg/kg body weight. The rats were divided into eight groups of control, irradiation-only, vehicle-only, vehicle plus irradiation, 10mg/kg melatonin alone, 10mg/kg melatonin plus irradiation, 100mg/kg melatonin alone and 100mg/kg melatonin plus irradiation. Rats were given an intraperitoneal (IP) injection of melatonin or the same volume of vehicle alone 1h prior to irradiation. Blood samples were taken 4, 24, 48 and 72h after irradiation for evaluation of flow cytometric analysis of apoptotic lymphocytes using Annexin V/PI assay and measurement of bax and bcl-2 expression using quantitative real-time PCR (RT(2)qPCR). Irradiation-only and vehicle plus irradiation showed an increase in the percentage of apoptotic lymphocytes significantly different from control group (P<0.01), while melatonin pretreatments in a dose-dependent manner reduced it as compared with the irradiation-only and vehicle plus irradiation groups (P<0.01) in all time points. This reduced apoptosis by melatonin was related to the downregulation of bax, upregulation of bcl-2, and therefore reduction of bax/bcl-2 ratio. Our results suggest that melatonin in these doses may provide modulation of bax and bcl-2 expression as well as bax/bcl-2 ratio to protect rat peripheral blood lymphocytes from gamma irradiation-induced apoptosis.

Characterizing the marker-dye correction for Gafchromic(®) EBT2 film: a comparison of three analysis methods.

PURPOSE: Gafchromic(®) EBT2 film has a yellow marker dye incorporated into the active layer of the film that can be used to correct the film response for small variations in thickness. This work characterizes the effect of the marker-dye correction on the uniformity and uncertainty of dose measurements with EBT2 film. The effect of variations in time postexposure on the uniformity of EBT2 is also investigated. METHODS: EBT2 films were used to measure the flatness of a (60)Co field to provide a high-spatial resolution evaluation of the film uniformity. As a reference, the flatness of the (60)Co field was also measured with Kodak EDR2 films. The EBT2 films were digitized with a flatbed document scanner 24, 48, and 72 h postexposure, and the images were analyzed using three methods: (1) the manufacturer-recommended marker-dye correction, (2) an in-house marker-dye correction, and (3) a net optical density (OD) measurement in the red color channel. The field flatness was calculated from orthogonal profiles through the center of the field using each analysis method, and the results were compared with the EDR2 measurements. Uncertainty was propagated through a dose calculation for each analysis method. The change in the measured field flatness for increasing times postexposure was also determined. RESULTS: Both marker-dye correction methods improved the field flatness measured with EBT2 film relative to the net OD method, with a maximum improvement of 1% using the manufacturer-recommended correction. However, the manufacturer-recommended correction also resulted in a dose uncertainty an order of magnitude greater than the other two methods. The in-house marker-dye correction lowered the dose uncertainty relative to the net OD method. The measured field flatness did not exhibit any unidirectional change with increasing time postexposure and showed a maximum change of 0.3%. CONCLUSIONS: The marker dye in EBT2 can be used to improve the response uniformity of the film. Depending on the film analysis method used, however, application of a marker-dye correction can improve or degrade the dose uncertainty relative to the net OD method. The uniformity of EBT2 was found to be independent of the time postexposure.

[Exploration of relationship between the expression level of DNA polymerase beta and 60Co gamma-ray radiosensitivity].

OBJECTIVE: To explore the relationship between the expression level of DNA polymerase beta (pol beta) and 60Co gamma-ray radiosensitivity and provide a basis on improving the efficiency of radiotherapy theoretically. METHODS: pol beta wild-type cells (pol beta +/+), pol beta null cells (pol beta -/-) and pol beta overexpressed cells (polp beta oe) were applied as a model system. The radiosensitivity of 60Co gamma-ray on the cell was detected by MTT assay and clone formation assay. The DCFH-DA fluorescent probe was used to examine the cellular ROS after 60Co gamma-rays radiation. RESULTS: MTT assay showed that after radiation by 60Co gamma-rays followed with 72 h incubation, the cell viabilities in the three kinds of cells decreased significantly with a dose-response relationship (r-/+ = -0.976, r-/- = -0.977, r(oe) = -0.982, P<0.05). In addition, the viability of pol beta -/- cell was lower than those of other two kinds of cells at the same dose (P<0.05). Likewise, the colony number and colony formation rate in all tested cells also decreased after exposure to 60Co gamma-rays. The ROS level in the three kinds of cells was enhanced after treatment with 60Co gamma-ray, and the ROS level in pol beta -/- cells was much higher than that in the other two kinds of cells (P<0.05). CONCLUSION: Cell death caused by 60Co gamma-ray may associated with the DNA oxidative damage mediated by ROS; Overexpression of pol beta could protect against oxidative DNA damage, thus the cell apoptosis/death, thereby leading to reducing the radiosensitivity of 60Co gamma-rays, while null of DNA pol beta could increase radiosensitivity of 60Co gamma-rays by compromising the DNA repair.

Homogeneity of Gafchromic EBT2 film.

PURPOSE: The self-developing Gafchromic EBT film is a radiochromic film, widely used for relative photon dosimetry. Recently, the manufacturer has replaced the well-investigated EBT film by the new Gafchromic EBT2 film. It has the same sensitive component and, in addition, it contains a yellow marker dye in order to protect the film against ambient light exposure and to serve as a base for corrections of small differences in film response. Furthermore, the configuration of the film layers as well as the binder material have been changed in comparison to the EBT film. When investigating the properties of EBT2 film, all characteristics were found to be similar to those of EBT film, except for the film response homogeneity. Thus, in this article special focus was put on examining the homogeneity of EBT2 film. METHODS: A scan protocol established for EBT film and published previously was used. The uniformity of the film coloration was investigated for unirradiated and irradiated EBT2 film sheets. The dose response of EBT2 film was measured and the influence of film inhomogeneities on dose determination was evaluated. RESULTS: Inhomogeneities in pixel values of up to +/- 3.7% within one film were detected. The relative inhomogeneities were found to be approximately independent of the dose. Nonuniformities of the film response lead to uncertainties in dose determination of +/- 8.7% at 1 Gy. When using net optical densities for dose calibration, uncertainties in dose determination amount to more than +/- 6%. CONCLUSIONS: EBT2 films from the lot investigated in this study show response inhomogeneities, which lead to uncertainties in dose determination exceeding the commonly accepted tolerance levels. It is important to test further EBT2 lots regarding homogeneity before using the film in clinical routine.

A novel salt-tolerant mutant YWL-01 for the treatment of saline wastewater.

The treatment of high-saline wastewater from some salt-end markets including agro-food industry is a serious problem yet to be solved in some coastal cities. The conventional physical-chemical techniques are energy-consuming and their startup and running costs are still high. Biological methods using salt-tolerant bacterial strains for the treatment of hypersaline wastewater provide one possible solution. In this study, one salt-tolerant mutant named YWL-01 was screened out by sewage treatment and proved to be a genetically stable salt-tolerant strain for saline wastewater treatment. First, combined mutagenesis was done on an isolated sewage treatment strain Bacillus Y for the screening of salt tolerance, and 11 mutants were obtained after subculture for many times. Then, a secondary screening test was performed for COD (chemical oxygen demand) and TOC (total organic carbon) removal efficiency analyses. At last, the best mutant YWL-01 with increased capacity to treat saline wastewater was chosen for use. RAPD (Random Amplified Polymorphic DNA) analysis of genetic stability on the mutant YWL-01 showed that it is a hereditary mutant for the treatment of high-saline wastewater.

IN VIVO BIODOSIMETRY OF PORCINE T-LYMPHOCYTE SUBSETS AND NK CELLS.

The aim of the present study was to evaluate the biodosimetric potential of peripheral blood lymphocytes, particularly of T-cell subsets (null and T helper) and natural killer cells (NK), upon exposure to gamma irradiation (60Co) in vivo. For this purpose, the change in relative numbers of NK cells and T-lymphocyte subsets, as well as in the H2AX phosphorylation rate, were evaluated as potential early markers of the lymphocytic response to irradiation in vivo. These experiments were performed on a Large White Pig model. As a result, significant but not dose-dependent changes in the proportion of lymphocyte subpopulations (NK cells, null and T helper cells) were found after exposure to ionising radiation in vivo. On the other hand, circulating NK cells showed relatively higher radioresistance capacity when compared to the T-lymphocyte subsets; however, gamma-H2AX expression showed no significant difference between the evaluated lymphocyte subsets.

Comparison of Coding Transcriptomes in Fibroblasts Irradiated With Low and High LET Proton Beams and Cobalt-60 Photons.

PURPOSE: To identify differential cellular responses after proton and photon irradiation by comparing transcriptomes of primary fibroblasts irradiated with either radiation type. METHODS AND MATERIALS: A panel of primary dermal fibroblast cultures was irradiated with low and higher linear energy transfer (LET) proton beams. Cobalt-60 photon irradiation was used as reference. Dose was delivered in 3 fractions of 3.5 Gy (relative biological effectiveness) using a relative biological effectiveness of 1.1 for proton doses. Cells were harvested 2 hours after the final fraction was delivered, and RNA was purified. RNA sequencing was performed using Illumina NextSeq 500 with high-output kit. The edgeR package in R was used for differential gene expression analysis. RESULTS: Pairwise comparisons of the transcriptomes in the 3 treatment groups showed that there were 84 and 56 differentially expressed genes in the low LET group compared with the Cobalt-60 group and the higher LET group, respectively. The higher LET proton group and the Cobalt-60 group had the most distinct transcriptome profiles, with 725 differentially regulated genes. Differentially regulated canonical pathways and various regulatory factors involved in regulation of biological mechanisms such as inflammation, carcinogenesis, and cell cycle control were identified. CONCLUSIONS: Inflammatory regulators associated with the development of normal tissue complications and malignant transformation factors seem to be differentially regulated by higher LET proton and Cobalt-60 photon irradiation. The reported transcriptome differences could therefore influence the progression of adverse effects and the risk of developing secondary cancers.

RADIOPROTECTIVE EFFECT OF HYDROXYL RADICAL SCAVENGERS ON PROKARYOTIC AND EUKARYOTIC CELLS UNDER VARIOUS GAMMA IRRADIATION CONDITIONS.

The influence of various hydroxyl radical scavengers such as methanol, ethanol and dimethyl sulfoxide on radiation sensitivity of prokaryotic cells (bacteria Escherichia coli) and eukaryotic cells (yeast Saccharomyces cerevisiae and V79 cells-Chinese hamster pulmonary fibroblasts) irradiated by 60Co gamma radiation was investigated. The dependence of radiation sensitivity on dose rate in range from 1.8 to 100 Gy h-1 was evaluated. Survival of cells irradiated by increasing dose rates was followed using clonogenic assay. Specific protective effect was found to be a nonmonotonous function of dose rate with typical maximum at the dose rate range from 50 to 55 Gy h-1 in all studied cell types.

Response of an implantable MOSFET dosimeter to 192Ir HDR radiation.

New in vivo dosimetry methods would be useful for clinical HDR brachytherapy. An implantable MOSFET Dose Verification System designed by Sicel Technologies, Inc. was examined for use with 192Ir HDR applications. This investigation demonstrated that varying the dose rate from 22 to 84 cGy/min did not change detector response. The detectors exhibited a higher sensitivity to 192Ir energies than 60Co energies. A nonlinear accumulated dose effect was characterized by three third-order polynomials fit to data from detectors placed at three different distances from the source. The detectors were found to have minimal rotational angular dependence. A strong longitudinal angular dependence was found when the detector's copper coil and electronics assembly were aligned between the MOSFETs and incident radiation. This orientation showed a 16% decrease in response relative to other orientations tested.

PENLINAC: extending the capabilities of the Monte Carlo code PENELOPE for the simulation of therapeutic beams.

In this work we present PENLINAC, a code package developed to facilitate the use of the Monte Carlo code PENELOPE for the simulation of therapeutic beams, including high-energy electrons, photons and 60Co beams. The code simplifies the creation of the treatment machine geometry, allowing the modeling of their components from elementary geometric bodies and their further conversion to the quadric functions-based structure handled by PENELOPE. The code is implemented in various subroutines that allow the user to handle several models of radiation sources and phase spaces. The phase spaces are not part of the geometry and can store many variables of the particle in a relatively small data space. The set of subroutines does not alter the PENELOPE algorithms; thus, the main program implemented by the user can maintain its kind-of-particle-independent structure. A support program can handle and analyze the phase spaces to generate, among others, last interaction maps and probability distributions that can be used as sources in simulation. Results from simulations of a Clinac linear accelerator head are presented in order to demonstrate the package capabilities. Dose distributions calculated in a water phantom for a variety of beams of this accelerator showed good agreement with measurements.

Regulatory T Cells Promote ß-Catenin--Mediated Epithelium-to-Mesenchyme Transition During Radiation-Induced Pulmonary Fibrosis.

PURPOSE: Radiation-induced pulmonary fibrosis results from thoracic radiation therapy and severely limits radiation therapy approaches. CD4(+)CD25(+)FoxP3(+) regulatory T cells (Tregs) as well as epithelium-to-mesenchyme transition (EMT) cells are involved in pulmonary fibrosis induced by multiple factors. However, the mechanisms of Tregs and EMT cells in irradiation-induced pulmonary fibrosis remain unclear. In the present study, we investigated the influence of Tregs on EMT in radiation-induced pulmonary fibrosis. METHODS AND MATERIALS: Mice thoraxes were irradiated (20 Gy), and Tregs were depleted by intraperitoneal injection of a monoclonal anti-CD25 antibody 2 hours after irradiation and every 7 days thereafter. Mice were treated on days 3, 7, and 14 and 1, 3, and 6 months post irradiation. The effectiveness of Treg depletion was assayed via flow cytometry. EMT and ß-catenin in lung tissues were detected by immunohistochemistry. Tregs isolated from murine spleens were cultured with mouse lung epithelial (MLE) 12 cells, and short interfering RNA (siRNA) knockdown of ß-catenin in MLE 12 cells was used to explore the effects of Tregs on EMT and ß-catenin via flow cytometry and Western blotting. RESULTS: Anti-CD25 antibody treatment depleted Tregs efficiently, attenuated the process of radiation-induced pulmonary fibrosis, hindered EMT, and reduced ß-catenin accumulation in lung epithelial cells in vivo. The coculture of Tregs with irradiated MLE 12 cells showed that Tregs could promote EMT in MLE 12 cells and that the effect of Tregs on EMT was partially abrogated by ß-catenin knockdown in vitro. CONCLUSIONS: Tregs can promote EMT in accelerating radiation-induced pulmonary fibrosis. This process is partially mediated through ß-catenin. Our study suggests a new mechanism for EMT, promoted by Tregs, that accelerates radiation-induced pulmonary fibrosis.

Effect of 60Co gamma-irradiation on the nonspecific cytotoxicity of alveolar macrophages in vitro.

This paper reports on the effect of radiation on the nonspecific cytotoxicity of rat alveolar macrophages (AM). AM (effector cells) of bacille Calmette-Guerin-activated Wistar rats were irradiated with 60Co gamma rays in vitro to give doses of 0, 100, 300, and 500 Gy. Three hours after irradiation, the AM were cultured with human lung adenocarcinoma AGZY83-a and HeLa target cells in 125I-deoxyuridine-containing media for 6 hr and the cytotoxicity indexes determined. The results indicated that the cytotoxicity indexes of AM against human lung adenocarcinoma cells and HeLa cells were 94.3 +/- 0.3% and 81.3 +/- 1.9%, respectively. The cytotoxicity indexes using an effector/target cell ratio (E/T) of 10 seemed to be greater than with ratios of 20 and 30. The cytotoxicity indexes of AM (7 rats), irradiated to give doses of 0, 100, 300, and 500 Gy, against adenocarcinoma cells at an E/T ratio of 10 were 87.9 +/- 8.4%, 65.4 +/- 14.1%, 47.5 +/- 17.5%, and 36.7 +/- 9.7%, respectively. The significance of the nonspecific cytotoxicity of AM in the immunological elimination of tumors and the inhibitory effect of radiation on AM cytotoxicity are discussed.

Effect of low doses of ionizing radiation on cells cultured from the hematopoietic tissue of the Dublin Bay prawn, Nephrops norvegicus.

Explant cultures from the hematopoietic tissue of the Dublin Bay prawn, Nephrops norvegicus, were exposed to low doses of (60)Co gamma radiation. Cells growing from the explants were examined 7 days after irradiation using light and transmission electron microscopy and were also tested for their ability to produce signals indicative of a bystander effect. The exposed cultures displayed pronounced damage and were orders of magnitude more sensitive than the data in the literature would suggest for arthropod cells. The cultures were also more sensitive than mammalian cells that were exposed to similar doses. Cellular abnormalities included damage to cytoplasmic organelles, particularly the cytoskeleton. Abnormal mitochondria were also prominent. At low doses (0.5 Gy), nuclear damage was not apparent in the cultures, but there was evidence of a dose-dependent increase in apoptosis. The irradiated cultures released a factor into the medium that was capable of inducing apoptosis and cell death in unirradiated fish and human cells. This bystander effect was of a similar magnitude to that reported for mammalian cell systems. It is suggested that these crustaceans may be highly sensitive to radiation, unlike terrestrial arthropods and certain other invertebrates, which are generally considered to be radioresistant.

Anesthetic-resistant spontaneous mutant of drosophila melanogaster: intensified response to 60Cobalt radiation damage.

Accumulating evidence suggests that the extent of acute damage by ionizing irradiation is closely related to the state of membrane orderliness. Decreased orderliness apparently protects organisms from ionizing irradiation. Because anesthetics decrease membrane orderliness, anesthesia is expected to affect damages caused by ionizing irradiation. The present study compared the effects of 60Co irradiation on Drosophila melanogaster between an anesthetic-resistant spontaneous mutant and an anesthetic-sensitive strain. We have previously established an anesthetic-resistant mutant strain, Eth-29, of Drosophila melanogaster. Eth-29 is resistant to diethylether, chloroform and halothane. The anesthetic-resistant strain was found to be radiosensitive when evaluated by survival at the eighth day after irradiation or by dyskinesia (knock-down) at the second day. The results indicate that anesthetic resistance may be related to an increase in orderliness. The findings in reciprocal crosses between Eth-29 and the control strain indicate that the mechanism of survival is different from that of knock-down. Presumably, knock-down is the direct sequela of irradiation, and the present result suggests that membrane damage may be involved in inducing knock-down.

Comparison of repair of DNA double-strand breaks caused by neutron or gamma radiation in cultured human cells.

The dose-response for the induction of initial double-strand breaks (dsb) in DNA of human epithelioid cells by JANUS 0.85 MeV fission-spectrum neutrons was parabolic as assayed by a calibrated neutral filter elution technique. The relative biological effectiveness (RBE) of these neutrons relative to 60Co gamma-rays was unity. The kinetics of repair after a 60 Gy gamma-ray exposure were biphasic. About 65% of these dsb were rapidly repaired (T 1/2 of approximately 2 min), and the remainder were almost completely removed after 150 min at a slower rate (T 1/2 = 30 min). After the same dose of JANUS neutrons, the rapid repair component was markedly reduced (possibly not a significant repair component), and the bulk of the dsb were sealed more slowly (T 1/2 = 90 min). After 150 min, 25% remained unsealed. Even after a lower neutron dose (20 Gy), a proportion of the dsb were refractory to repair. Thus, unrepaired (or irreparable) dsb induced by high energy neutrons might explain the high RBE of neutrons for cell killing.

Study of external low irradiation dose effects on induction of chromosome aberrations in Pisum sativum root tip meristem.

The effects of low doses of ionizing radiation have been a matter of important debate over the last few years. The point of discussion concerns the validity of the linear dose-response extrapolation for low doses, used by international organizations, to establish radio-protection norms. Here, we contributed to this discussion by investigating the induction of chromosome aberrations by low to moderate doses ranging from 0 to 10 Gy in root meristem cells of 6-day-old Pisum plantlets. After acute irradiation of plantlets by a (60)Co source, the percentage of root tip meristem cells displaying chromosome aberrations was estimated immediately after irradiation and after 20 h recovery time. The dose-effect curves show non-linear responses, especially in the low dose range (0- 1 Gy), which is of particular interest. After 20 h of recovery, a steep increase of aberrations was observed for cells exposed to 0.4 Gy, followed by a plateau for doses until 1 Gy. There was an irradiation effect on plant growth during the first and second generations, showing the persistence of cell division anomalies as a long term effect of acute irradiation. This result suggests the induction of a genomic instability. Our results, in agreement with some obtained in animals, show rather non-linear dose-effect responses, with notably higher biological effects of low doses than expected.