Gamma-H2AX biodosimetry for use in large scale radiation incidents: comparison of a rapid '96 well lyse/fix' protocol with a routine method

Determination of dose-response calibration curves for gamma radiation using gamma-H2AX immunofluorescence based biodosimetry

Background

Gamma-H2AX immunofluorescence assay has gained popularity as a DNA double strand break marker. In this work, we have investigated the potential use of gamma H2AX immunofluorescence assay as a biological dosimeter for estimation of dose in our institution.

Materials and methods

Seven healthy individuals were selected for the study and the blood samples collected from the first five individuals were irradiated to low doses (0-10 cGy) and high doses (50-500 cGy) in a telecobalt unit. All the samples were processed for gamma-H2AX immunofluorescence assay and the dose-response calibration curves for low and high doses were determined. In order to validate the determined dose-response calibration curves, the blood samples obtained from the sixth and seventh subjects were delivered a test dose of 7.5 cGy and 250 cGy. In addition, time and cost required to complete the assay were also reported.

Results

The goodness of fit (R2) values was found to be 0.9829 and 0.9766 for low and high dose-response calibration curves. The time required to perform the gamma-H2AX immunofluorescence assay was found to be 7 hours and 30 minutes and the estimated cost per sample was 5000 rupees (~ 60 USD).

Conclusion

Based on this study we conclude that the individual dose-response calibration curves determined with gamma-H2AX immunofluorescence assay for both low and high dose ranges of gamma radiation can be used for biological dosimetry. Further, the gamma-H2AX immunofluorescence assay can be used as a rapid cost-effective biodosimetric tool for institutions with an existing confocal microscope facility.

In vitro study of radiosensitivity in colorectal cancer cell lines associated with Lynch syndrome

Introduction

Lynch syndrome patients have an inherited predisposition to cancer due to a deficiency in DNA mismatch repair (MMR) genes which could lead to a higher risk of developing cancer if exposed to ionizing radiation. This pilot study aims to reveal the association between MMR deficiency and radiosensitivity at both a CT relevant low dose (20 mGy) and a therapeutic higher dose (2 Gy).

Methods

Human colorectal cancer cell lines with (dMMR) or without MMR deficiency (pMMR) were analyzed before and after exposure to radiation using cellular and cytogenetic analyses i.e., clonogenic assay to determine cell reproductive death; sister chromatid exchange (SCE) assay to detect the exchange of DNA between sister chromatids; γH2AX assay to analyze DNA damage repair; and apoptosis analysis to compare cell death response. The advantages and limitations of these assays were assessed in vitro, and their applicability and feasibility investigated for their potential to be used for further studies using clinical samples.

Results

Results from the clonogenic assay indicated that the pMMR cell line (HT29) was significantly more radio-resistant than the dMMR cell lines (HCT116, SW48, and LoVo) after 2 Gy X-irradiation. Both cell type and radiation dose had a significant effect on the yield of SCEs/chromosome. When the yield of SCEs/chromosome for the irradiated samples (2 Gy) was normalized against the controls, no significant difference was observed between the cell lines. For the γH2AX assay, 0, 20 mGy and 2 Gy were examined at post-exposure time points of 30 min (min), 4 and 24 h (h). Statistical analysis revealed that HT29 was only significantly more radio-resistant than the MLH1-deficient cells lines, but not the MSH2-deficient cell line. Apoptosis analysis (4 Gy) revealed that HT29 was significantly more radio-resistant than HCT116 albeit with very few apoptotic cells observed.

Discussion

Overall, this study showed radio-resistance of the MMR proficient cell line in some assays, but not in the others. All methods used within this study have been validated; however, due to the limitations associated with cancer cell lines, the next step will be to use these assays in clinical samples in an effort to understand the biological and mechanistic effects of radiation in Lynch patients as well as the health implications.

Lessons on harmonization of scoring criteria for dicentric chromosome assay in South Korea

PURPOSE

Networking with other biodosimetry laboratories is necessary to assess the radiation exposure of many individuals in large-scale radiological accidents. The Korea biodosimetry network, K-BioDos, prepared harmonized scoring guidelines for dicentric chromosome assay to obtain homogeneous results within the network and investigated the efficiency of the guidelines.

MATERIALS AND METHODS

Three laboratories in K-BioDos harmonized the scoring guidelines for dicentric chromosome assay. The results of scoring dicentric chromosomes using the harmonized scoring guidelines were compared with the laboratories' results using their own methods. Feedback was collected from the scorers following the three intercomparison exercises in 3 consecutive years.

RESULTS

K-BioDos members showed comparable capacity to score dicentrics in the three exercises. However, the results of the K-BioDos guidelines showed no significant improvement over those of the scorers' own methods. According to the scorers, our harmonized guidelines led to more rejected metaphases and ultimately decreased the number of scorable metaphases compared with their own methods. Moreover, the scoring time was sometimes longer with the K-BioDos protocol because some scorers were not yet familiar with the guidelines, though most scorers reported that the time decreased or was unchanged. These challenges may cause low adherence to the guidelines. Most scorers expressed willingness to use the guidelines to select scorable metaphases or identify dicentrics for other biodosimetry works, whereas one did not want to use it due to the difference from their calibration curves.

CONCLUSIONS

We identified potential resistance to following the harmonized guidelines and received requests for more detailed methods. Our findings suggest that the harmonized criteria should be continually updated, and education and training should be provided for all scorers. These changes could allow members within the biodosimetry network to successfully collaborate and support each other in large-scale radiological accidents.

Development of high-throughput systems for biodosimetry

Biomarkers for ionising radiation exposure have great utility in scenarios where there has been a potential exposure and physical dosimetry is missing or in dispute, such as for occupational and accidental exposures. Biomarkers that respond as a function of dose are particularly useful as biodosemeters to determine the dose of radiation to which an individual has been exposed. These dose measurements can also be used in medical scenarios to track doses from medical exposures and even have the potential to identify an individual's response to radiation exposure that could help tailor treatments. The measurement of biomarkers of exposure in medicine and for accidents, where a larger number of samples would be required, is limited by the throughput of analysis (i.e. the number of samples that could be processed and analysed), particularly for microscope-based methods, which tend to be labour-intensive. Rapid analysis in an emergency scenario, such as a large-scale accident, would provide dose estimates to medical practitioners, allowing timely administration of the appropriate medical countermeasures to help mitigate the effects of radiation exposure. In order to improve sample throughput for biomarker analysis, much effort has been devoted to automating the process from sample preparation through automated image analysis. This paper will focus mainly on biological endpoints traditionally analysed by microscopy, specifically dicentric chromosomes, micronuclei and gamma-H2AX. These endpoints provide examples where sample throughput has been improved through automated image acquisition, analysis of images acquired by microscopy, as well as methods that have been developed for analysis using imaging flow cytometry.

Evaluation of γ-H2AX foci distribution among different peripheral blood mononucleated cell subtypes

INTRODUCTION

The detection of γ-H2AX foci in peripheral blood mononucleated cells (PBMCs) has been incorporated as an early assay for biological dosimetry. However, overdispersion in the γ-H2AX foci distribution is generally reported. In a previous study from our group, it was suggested that overdispersion could be caused by the fact that when evaluating PBMCs, different cell subtypes are analyzed, and that these could differ in their radiosensitivity. This would cause a mixture of different frequencies that would result in the overdispersion observed.

OBJECTIVES

The objective of this study was to evaluate both the possible differences in the radiosensitivities of the different cell subtypes present in the PBMCs and to evaluate the distribution of γ-H2AX foci in each cell subtype.

MATERIALS AND METHODS

Peripheral blood samples from three healthy donors were obtained and total PBMCs, and CD3+, CD4+, CD8+, CD19+, and CD56+ cells were separated. Cells were irradiated with 1 and 2 Gy and incubated at 37 °C for 1, 2, 4, and 24 h. Sham-irradiated cells were also analyzed. γ-H2AX foci were detected after immunofluorescence staining and analyzed automatically using a Metafer Scanning System. For each condition, 250 nuclei were considered.

RESULTS

When the results from each donor were compared, no observable significant differences between donors were observed. When the different cell subtypes were compared, CD8+ cells showed the highest mean of γ-H2AX foci in all post-irradiation time points. The cell type that showed the lowest γ-H2AX foci frequency was CD56+. The frequencies observed in CD4+ and CD19+ cells fluctuated between CD8+ and CD56+ without any clear pattern. For all cell types evaluated, and at all post-irradiation times, overdispersion in γ-H2AX foci distribution was significant. Independent of the cell type evaluated the value of the variance was four times greater than that of the mean.

CONCLUSION

Although different PBMC subsets studied showed different radiation sensitivity, these differences did not explain the overdispersion observed in the γ-H2AX foci distribution after exposure to IR.

High-Throughput γ-H2AX Assay Using Imaging Flow Cytometry

The γ-H2AX assay is a sensitive and reliable method to evaluate radiation-induced DNA double-strand breaks. The conventional γ-H2AX assay detects individual nuclear foci manually, but is labor-intensive and time-consuming, and hence unsuitable for high-throughput screening in cases of large-scale radiation accidents. We have developed a high-throughput γ-H2AX assay using imaging flow cytometry. This method comprises (1) sample preparation from small volumes of blood in the Matrix™ 96-tube format, (2) automated image acquisition of cells stained with immunofluorescence-labeled γ-H2AX using ImageStream®X, and (3) quantification of γ-H2AX levels and batch processing using the Image Data Exploration and Analysis Software (IDEAS®). This enables the rapid analysis of γ-H2AX levels in several thousand of cells from a small volume of blood with accurate and reliable quantitative measurements for γ-H2AX foci and mean fluorescence levels. This high-throughput γ-H2AX assay could be a useful tool not only for radiation biodosimetry in mass casualty events, but also for large-scale molecular epidemiological studies and individualized radiotherapy.

Application of a semi-automated dicentric scoring system in triage and monitoring occupational radiation exposure

The dicentric chromosome assay (DCA) is considered the gold standard for radiation biodosimetry, but it is limited by its long dicentric scoring time and need for skilled scorers. The automation of scoring dicentrics has been considered a strategy to overcome the constraints of DCA. However, the studies on automated scoring methods are limited compared to those on conventional manual DCA. Our study aims to assess the performance of a semi-automated scoring method for DCA using ex vivo and in vivo irradiated samples. Dose estimations of 39 blind samples irradiated ex vivo and 35 industrial radiographers occupationally exposed in vivo were estimated using the manual and semi-automated scoring methods and subsequently compared. The semi-automated scoring method, which removed the false positives of automated scoring using the dicentric chromosome (DC) scoring algorithm, had an accuracy of 94.9% in the ex vivo irradiated samples. It also had more than 90% accuracy, sensitivity, and specificity to distinguish binary dose categories reflecting clinical, diagnostic, and epidemiological significance. These data were comparable to those of manual DCA. Moreover, Cohen's kappa statistic and McNemar's test showed a substantial agreement between the two methods for categorizing in vivo samples into never and ever radiation exposure. There was also a significant correlation between the two methods. Despite of comparable results with two methods, lower sensitivity of semi-automated scoring method could be limited to assess various radiation exposures. Taken together, our findings show the semi-automated scoring method can provide accurate dose estimation rapidly, and can be useful as an alternative to manual DCA for biodosimetry in large-scale accidents or cases to monitor radiation exposure of radiation workers.

DeepFoci: Deep learning-based algorithm for fast automatic analysis of DNA double-strand break ionizing radiation-induced foci

DNA double-strand breaks (DSBs), marked by ionizing radiation-induced (repair) foci (IRIFs), are the most serious DNA lesions and are dangerous to human health. IRIF quantification based on confocal microscopy represents the most sensitive and gold-standard method in radiation biodosimetry and allows research on DSB induction and repair at the molecular and single-cell levels. In this study, we introduce DeepFoci - a deep learning-based fully automatic method for IRIF counting and morphometric analysis. DeepFoci is designed to work with 3D multichannel data (trained for 53BP1 and γH2AX) and uses U-Net for nucleus segmentation and IRIF detection, together with maximally stable extremal region-based IRIF segmentation. The proposed method was trained and tested on challenging datasets consisting of mixtures of nonirradiated and irradiated cells of different types and IRIF characteristics - permanent cell lines (NHDFs, U-87) and primary cell cultures prepared from tumors and adjacent normal tissues of head and neck cancer patients. The cells were dosed with 0.5-8 Gy γ-rays and fixed at multiple (0-24 h) postirradiation times. Under all circumstances, DeepFoci quantified the number of IRIFs with the highest accuracy among current advanced algorithms. Moreover, while the detection error of DeepFoci remained comparable to the variability between two experienced experts, the software maintained its sensitivity and fidelity across dramatically different IRIF counts per nucleus. In addition, information was extracted on IRIF 3D morphometric features and repair protein colocalization within IRIFs. This approach allowed multiparameter IRIF categorization of single- or multichannel data, thereby refining the analysis of DSB repair processes and classification of patient tumors, with the potential to identify specific cell subclones. The developed software improves IRIF quantification for various practical applications (radiotherapy monitoring, biodosimetry, etc.) and opens the door to advanced DSB focus analysis and, in turn, a better understanding of (radiation-induced) DNA damage and repair.

Cytogenetic biological dosimetry assays: recent developments and updates

Biological dosimetry is the measurement of radiation-induced changes in the human to measure short and long-term health risks. Biodosimetry offers an independent means of obtaining dose information and also provides diagnostic information on the potential for “partial-body” exposure information using biological indicators and otherwise based on computer modeling, dose reconstruction, and physical dosimetry. A variety of biodosimetry tools are available and some features make some more valuable than others. Among the available biodosimetry tool, cytogenetic biodosimetry methods occupy an exclusive and advantageous position. The cytogenetic analysis can complement physical dosimetry by confirming or ruling out an accidental radiological exposure or overexposures. We are discussing the recent developments and adaptability of currently available cytogenetic biological dosimetry assays.

Detection and quantification of γ-H2AX using a dissociation enhanced lanthanide fluorescence immunoassay

Phosphorylation of the histone protein H2AX to form γ-H2AX foci directly represents DNA double-strand break formation. Traditional γ-H2AX detection involves counting individual foci within individual nuclei. The novelty of this work is the application of a time-resolved fluorescence assay using dissociation-enhanced lanthanide fluorescence immunoassay for quantitative measurements of γ-H2AX. For comparison, standard fluorescence detection was employed and analyzed either by bulk fluorescent measurements or by direct foci counting using BioTek Spot Count algorithm and Gen 5 software. Etoposide induced DNA damage in A549 carcinoma cells was compared across all test platforms. Time resolved fluorescence detection of europium as a chelated complex enabled quantitative measurement of γ-H2AX foci with nanomolar resolution. Comparative bulk fluorescent signals achieved only micromolar sensitivity. Lanthanide based immunodetection of γ-H2AX offers superior detection and a user-friendly workflow. These approaches have the potential to improve screening of compounds that either enhance DNA damage or protect against its deleterious effects.

Automated Cytogenetic Biodosimetry at Population-Scale

The dicentric chromosome (DC) assay accurately quantifies exposure to radiation; however, manual and semi-automated assignment of DCs has limited its use for a potential large-scale radiation incident. The Automated Dicentric Chromosome Identifier and Dose Estimator (ADCI) software automates unattended DC detection and determines radiation exposures, fulfilling IAEA criteria for triage biodosimetry. This study evaluates the throughput of high-performance ADCI (ADCI-HT) to stratify exposures of populations in 15 simulated population scale radiation exposures. ADCI-HT streamlines dose estimation using a supercomputer by optimal hierarchical scheduling of DC detection for varying numbers of samples and metaphase cell images in parallel on multiple processors. We evaluated processing times and accuracy of estimated exposures across census-defined populations. Image processing of 1744 samples on 16,384 CPUs required 1 h 11 min 23 s and radiation dose estimation based on DC frequencies required 32 sec. Processing of 40,000 samples at 10 exposures from five laboratories required 25 h and met IAEA criteria (dose estimates were within 0.5 Gy; median = 0.07). Geostatistically interpolated radiation exposure contours of simulated nuclear incidents were defined by samples exposed to clinically relevant exposure levels (1 and 2 Gy). Analysis of all exposed individuals with ADCI-HT required 0.6–7.4 days, depending on the population density of the simulation.

Prediction of the Acute or Late Radiation Toxicity Effects in Radiotherapy Patients Using Ex Vivo Induced Biodosimetric Markers: A Review

A search for effective methods for the assessment of patients' individual response to radiation is one of the important tasks of clinical radiobiology. This review summarizes available data on the use of ex vivo cytogenetic markers, typically used for biodosimetry, for the prediction of individual clinical radiosensitivity (normal tissue toxicity, NTT) in cells of cancer patients undergoing therapeutic irradiation. In approximately 50% of the relevant reports, selected for the analysis in peer-reviewed international journals, the average ex vivo induced yield of these biodosimetric markers was higher in patients with severe reactions than in patients with a lower grade of NTT. Also, a significant correlation was sometimes found between the biodosimetric marker yield and the severity of acute or late NTT reactions at an individual level, but this observation was not unequivocally proven. A similar controversy of published results was found regarding the attempts to apply G2- and γH2AX foci assays for NTT prediction. A correlation between ex vivo cytogenetic biomarker yields and NTT occurred most frequently when chromosome aberrations (not micronuclei) were measured in lymphocytes (not fibroblasts) irradiated to relatively high doses (4-6 Gy, not 2 Gy) in patients with various grades of late (not early) radiotherapy (RT) morbidity. The limitations of existing approaches are discussed, and recommendations on the improvement of the ex vivo cytogenetic testing for NTT prediction are provided. However, the efficiency of these methods still needs to be validated in properly organized clinical trials involving large and verified patient cohorts.

Potential application of γ-H2AX as a biodosimetry tool for radiation triage

Radiation triage and biological dosimetry are two initial steps in the medical management of exposed individuals following radiological accidents. Well established biodosimetry methods such as the dicentric (DC) assay, micronucleus (MN) assay, and fluorescence in-situ hybridization (FISH) translocation assay (for residual damage) have been used for this purpose for several decades. Recent advances in scoring methodology and networking among established laboratories have increased triage capacity; however, these methods still have limitations in analysing large sample numbers, particularly because of the ∼ 48 h minimum culture time required prior to analysis. Hence, there is a need for simple, and high throughput markers to identify exposed individuals in case of radiological/nuclear emergencies. In recent years, a few markers were identified, one being phosphorylated histone 2AX (γ-H2AX), which measured a nuclear foci or nuclear staining intensity that was found to be suitable for triage. Measurement of γ-H2AX foci formed at and around the sites of DNA double-strand breaks is a rapid and sensitive biodosimetry method which does not require culturing and is thus promising for the analysis of a large number of samples. In this review, we have summarized the recent developments of γ-H2AX assay in radiation triage and biodosimetry, focusing chiefly on: i) the importance of baseline frequency and reported values among different laboratories, ii) the influence of known and unknown variables on dose estimation, iii) quality assurance such as inter-laboratory comparison between scorers and scoring methods, and iv) current limitations and potential for future development.

Induction of oxidative stress biomarkers following whole-body irradiation in mice

Dose assessment is an important issue for radiation emergency medicine to determine appropriate clinical treatment. Hematopoietic tissues are extremely vulnerable to radiation exposure. A decrease in blood cell count following radiation exposure is the first quantitative bio-indicator using hematological techniques. We further examined induction of oxidative stress biomarkers in residual lymphocytes to identify new biomarkers for dosimetry. In vivo whole-body radiation to mice exposed to 5 Gy significantly induces DNA double-strand breaks, which were visualized by γ-H2AX in mouse blood cells. Mouse blood smears and peripheral blood mononuclear cells (PBMC) isolated from irradiated mice were used for immunostaining for oxidative biomarkers, parkin or Nrf2. Parkin is the E3 ubiquitin ligase, which is normally localized in the cytoplasm, is relocated to abnormal mitochondria with low membrane potential (ΔΨm), where it promotes clearance via mitophagy. Nrf2 transcription factor controls the major cellular antioxidant responses. Both markers of oxidative stress were more sensitive and persistent over time than nuclear DNA damage. In conclusion, parkin and Nrf2 are potential biomarkers for use in radiation dosimetry. Identification of several biological markers which show different kinetics for radiation response is essential for radiation dosimetry that allows the assessment of radiation injury and efficacy of clinical treatment in emergency radiation incidents. Radiation-induced oxidative damage is useful not only for radiation dose assessment but also for evaluation of radiation risks on humans.

A faster and easier biodosimetry method based on calyculin A-induced premature chromosome condensation (PCC) by scoring excess objects

Dicentric analysis and the ring PCC assay as established biodosimetry methods both have limitations in the estimation of absorbed doses in suspected overexposure cases between 5 and 10 Gy. The proposed method based on calyculin A-induced PCC overcomes these limitations by scoring excess objects as the endpoint. This new scoring method can potentially serve as a faster and up-scalable approach that complements the existing methods with higher accuracy at different dose ranges. It can also potentially be performed by less skilled workers when no automated system is available in mass casualty emergency cases to assist with the triage of patients. Additionally, it offers the possibility to further reduce the sample size and PCC induction time. In this pilot study, a calibration curve for excess objects was constructed using the new scoring method for the first time and a blind validation test composed of three unknown doses was carried out. Almost all the dose estimates were within the 95% confidence limits of the actual test doses by scoring only 50-100 PCC spreads. This method was found to be more accurate than ring PCC for doses below 10 Gy.

Identification of Potential Biomarkers of Radiation Exposure in Blood Cells by Capillary Electrophoresis Time-of-Flight Mass Spectrometry

Biodosimetry is a useful method for estimating personal exposure doses to ionizing radiation. Studies have identified metabolites in non-cellular biofluids that can be used as markers in biodosimetry. Levels of metabolites in blood cells may reflect health status or environmental stresses differentially. Here, we report changes in the levels of murine blood cell metabolites following exposure to X-rays in vivo. Levels of blood cell metabolites were measured by capillary electrophoresis time-of-flight mass spectrometry. The levels of 100 metabolites were altered substantially following exposure. We identified 2-aminobutyric acid, 2'-deoxycytidine, and choline as potentially useful markers of radiation exposure and established a potential prediction panel of the exposure dose using stepwise regression. Levels of blood cell metabolites may be useful biomarkers in estimating exposure doses during unexpected radiation incidents.

Effect of dose and dose rate on temporal γ-H2AX kinetics in mouse blood and spleen mononuclear cells in vivo following Cesium-137 administration

Background

Cesium-137 (¹³⁷Cs) is one of the major and most clinically relevant radionuclides of concern in a radiological dispersal device, “dirty bomb” scenario as well as in nuclear accidents and detonations. In this exposure scenario, a significant amount of soluble radionuclide(s) may be dispersed into the atmosphere as a component of fallout. The objectives of the present study were to investigate the effect of protracted ¹³⁷Cs radionuclide exposures on DNA damage in mouse blood and spleen mononuclear cells (MNCs) in vivo using the γ-H2AX biomarker, and to develop a mathematical formalism for these processes.

Results

C57BL/6 mice were injected with a range of ¹³⁷CsCl activities (5.74, 6.66, 7.65 and 9.28 MBq) to achieve total-body committed doses of ~ 4 Gy at Days 3, 5, 7, and 14. Close to 50% of ¹³⁷Cs was excreted by day 5, leading to a slower rate of decay for the remaining time of the study; ¹³⁷Cs excretion kinetics were independent of activity level within the tested range, and the absorbed radiation dose was determined by injected activity and time after injection. Measurements of γ-H2AX fluorescence in blood and spleen MNCs at each time point were used to develop a new biodosimetric mathematical formalism to estimate injected activity based on γ-H2AX fluorescence and time after injection. The formalism performed reasonably well on blood data at 2–5 days after injection: Pearson and Spearman’s correlation coefficients between actual and predicted activity values were 0.857 (p = 0.00659) and 0.929 (p = 0.00223), respectively.

Conclusions

Despite the complicated nature of the studied biological system and the time-dependent changes in radiation dose and dose rate due to radionuclide excretion and other processes, we have used the γ-H2AX repair kinetics to develop a mathematical formalism, which can relatively accurately predict injected ¹³⁷Cs activity 2–5 days after initial exposure. To determine the assay’s usefulness to predict retrospective absorbed dose for medical triage, further studies are required to validate the sensitivity and accuracy of the γ-H2AX response after protracted exposures.

Electronic supplementary material

The online version of this article (10.1186/s12860-019-0195-2) contains supplementary material, which is available to authorized users.

Radiation dosimetry and repair kinetics of DNA damage foci in mouse pachytene spermatocyte and round spermatid stages

Accurate quantification of DNA double strand breaks (DSB) in testicular germ cells is difficult because of cellular heterogeneity and the presence of endogenous γH2AX. Here, we used confocal microscopy to quantify DNA damage and repair kinetics following γ-irradiation (0.5-4 Gy) in three major mouse male germ cell stages, early and late pachytene spermatocytes and round spermatids (RSs), following a defined post irradiation time course. Dose-response curves showing linear best fit validated γH2AX focus as a rapid biodosimetric tool in these substages in response to whole body in vivo exposure. Stage specific foci yield/dose and repair kinetics demonstrated differential radiosensitivity and repair efficiency: early pachytenes (EP) repaired most rapidly and completely followed by late pachytene (LP) and RSs. Repair kinetics for all three stages followed 'exponential decay' in response to each radiation dose. In pachytenes immediate colocalisation of γH2AX and 53BP1, which participates in non-homologous end-joining repair pathway, was followed by dissociation from the major focal area of γH2AX by 4 h demonstrating ongoing DSB repair. These results confirm the differential radiosensitivity and repair kinetics of DSBs in male germ cells at different stages. Taken together, our results provide a simple and accurate method for assessing DNA damage and repair kinetics during spermatogenesis.

A statistical framework for radiation dose estimation with uncertainty quantification from the γ-H2AX assay

Over the last decade, the γ–H2AX focus assay, which exploits the phosphorylation of the H2AX histone following DNA double–strand–breaks, has made considerable progress towards acceptance as a reliable biomarker for exposure to ionizing radiation. While the existing literature has convincingly demonstrated a dose–response effect, and also presented approaches to dose estimation based on appropriately defined calibration curves, a more widespread practical use is still hampered by a certain lack of discussion and agreement on the specific dose–response modelling and uncertainty quantification strategies, as well as by the unavailability of implementations. This manuscript intends to fill these gaps, by stating explicitly the statistical models and techniques required for calibration curve estimation and subsequent dose estimation. Accompanying this article, a web applet has been produced which implements the discussed methods.

Pharmacological approach to increasing the retention of radiation-induced γ-H2AX foci using phosphatase inhibitors: significance in radiation biodosimetry

In a scenario of accidental mass radiation exposure transportation and analysis of samples may take some time, resulting in loss of biomarker information over this period. The present study aims to use phosphatase inhibitors for longer retention of focal signals to adopt γ-H2AX as a biodosimetric biomarker for the management of early triage. Peripheral blood lymphocytes isolated from healthy individuals were irradiated in vitro with x-rays and γ-H2AX foci were analysed using fluorescent microscopy and flow cytometric methods. Further, the effect of protein phosphatase 2A inhibitors such as calyculin A, fostriecin and okadiac acid on the retention of foci was studied. Fluorescent microscopy was found to be a more sensitive method than flow cytometry. Calyculin A showed significant retention of focal signals at 6 h with 1.5-fold increased retention compared to radiation alone; this may prove beneficial in early triage management because of a better dose approximation.

Ionizing radiation biomarkers in epidemiological studies - An update

Recent epidemiology studies highlighted the detrimental health effects of exposure to low dose and low dose rate ionizing radiation (IR): nuclear industry workers studies have shown increased leukaemia and solid tumour risks following cumulative doses of <100mSv and dose rates of <10mGy per year; paediatric patients studies have reported increased leukaemia and brain tumours risks after doses of 30-60mGy from computed tomography scans. Questions arise, however, about the impact of even lower doses and dose rates where classical epidemiological studies have limited power but where subsets within the large cohorts are expected to have an increased risk. Further progress requires integration of biomarkers or bioassays of individual exposure, effects and susceptibility to IR. The European DoReMi (Low Dose Research towards Multidisciplinary Integration) consortium previously reviewed biomarkers for potential use in IR epidemiological studies. Given the increased mechanistic understanding of responses to low dose radiation the current review provides an update covering technical advances and recent studies. A key issue identified is deciding which biomarkers to progress. A roadmap is provided for biomarker development from discovery to implementation and used to summarise the current status of proposed biomarkers for epidemiological studies. Most potential biomarkers remain at the discovery stage and for some there is sufficient evidence that further development is not warranted. One biomarker identified in the final stages of development and as a priority for further research is radiation specific mRNA transcript profiles.

γ-H2AX/53BP1/pKAP-1 foci and their linear tracks induced by in vitro exposure to radon and its progeny in human peripheral blood lymphocytes

The biodosimetric information is critical for evaluating the human health hazards caused by radon and its progeny. Here, we demonstrated that the formation of phosphorylated histone variant H2AX (γ-H2AX), p53-binding protein 1 (53BP1) and phosphorylated KRAB-associated protein 1 (pKAP-1) foci and their linear tracks in human peripheral blood lymphocytes (HPBLs) in vitro exposed to radon and its progeny were dependent on the cumulative absorbed dose of radon exposure but was unrelated to the concentration of radon. Among them, γ-H2AX foci and its linear tracks were the most sensitive indicators with the lowest estimable cumulative absorbed dose of 1.74 mGy from their linear dose-response curves and sustained for 12 h after termination of radon exposure. In addition, three types of foci showed an overdispersed non-Poisson distribution in HPBLs. The ratios of pKAP-1/γ-H2AX foci co-localization, 53BP1/γ-H2AX foci co-localization and 53BP1/pKAP-1 foci co-localization were significantly increased in HPBLs exposed to radon while they were unrelated to the cumulative dose of radon exposure, suggesting that γ-H2AX, pKAP-1 and 53BP1 play an important role in the repair of heterochromatic double-strand breaks. Altogether, our findings provide an experimental basis for estimating the biological dose of internal α-particle irradiation from radon and its progeny exposure in humans.

Frequency of gamma H2AX foci in healthy volunteers and health workers occupationally exposed to X-irradiation and its relevance in biological dosimetry

Gamma-H2AX (γ-H2AX) assay is a marker to measure double-strand breaks in the deoxyribonucleic acid. Variables such as age, oxidative stress, temperature, genetic factors and inter-individual variation have been reported to influence the baseline γ-H2AX focus levels. Therefore, knowledge on baseline frequency of γ-H2AX foci in a targeted population would facilitate reliable radiation triage and dose estimation. The objective of the present study was to establish the baseline data using blood samples from healthy volunteers (n = 130) differing in age, occupation and lifestyle as well as from occupationally exposed health workers (n = 20). The γ-H2AX focus assay was performed using epifluorescence microscopy. In vitro dose-response curve for γ-H2AX foci was constructed in blood samples (n = 3) exposed to X-rays (30 min post-exposure). The mean γ-H2AX focus frequency obtained in healthy volunteers was 0.042 ± 0.001 and showed an age-related increase (p < 0.001). Significantly higher (p < 0.005) focus frequencies were observed in health workers (0.066 ± 0.005) than in healthy volunteers. A sub-group analysis did not show a significant (p > 0.1) difference in γ-H2AX focus frequency among sexes. Blood exposed in vitro to X-rays showed dose-dependent increase in γ-H2AX foci frequency (Y = 0.1902 ± 0.1363 + 2.9020 ± 0.3240 * D). Baseline frequency of γ-H2AX foci obtained from different age groups showed a significant (p < 0.01) influence on the dose-response coefficients. The overall results demonstrated that the γ-H2AX assay can be used as a reliable biomarker for radiation triage and estimating the radiation absorbed dose by considering variables such as age, occupation and lifestyle factors.

Evaluating the Special Needs of The Military for Radiation Biodosimetry for Tactical Warfare Against Deployed Troops: Comparing Military to Civilian Needs for Biodosimetry Methods

The aim of this paper is to delineate characteristics of biodosimetry most suitable for assessing individuals who have potentially been exposed to significant radiation from a nuclear device explosion when the primary population targeted by the explosion and needing rapid assessment for triage is civilians vs. deployed military personnel. The authors first carry out a systematic analysis of the requirements for biodosimetry to meet the military's needs to assess deployed troops in a warfare situation, which include accomplishing the military mission. Then the military's special capabilities to respond and carry out biodosimetry for deployed troops in warfare are compared and contrasted systematically, in contrast to those available to respond and conduct biodosimetry for civilians who have been targeted by terrorists, for example. Then the effectiveness of different biodosimetry methods to address military vs. civilian needs and capabilities in these scenarios was compared and, using five representative types of biodosimetry with sufficient published data to be useful for the simulations, the number of individuals are estimated who could be assessed by military vs. civilian responders within the timeframe needed for triage decisions. Analyses based on these scenarios indicate that, in comparison to responses for a civilian population, a wartime military response for deployed troops has both more complex requirements for and greater capabilities to use different types of biodosimetry to evaluate radiation exposure in a very short timeframe after the exposure occurs. Greater complexity for the deployed military is based on factors such as a greater likelihood of partial or whole body exposure, conditions that include exposure to neutrons, and a greater likelihood of combined injury. These simulations showed, for both the military and civilian response, that a very fast rate of initiating the processing (24,000 d) is needed to have at least some methods capable of completing the assessment of 50,000 people within a 2- or 6-d timeframe following exposure. This in turn suggests a very high capacity (i.e., laboratories, devices, supplies and expertise) would be necessary to achieve these rates. These simulations also demonstrated the practical importance of the military's superior capacity to minimize time to transport samples to offsite facilities and use the results to carry out triage quickly. Assuming sufficient resources and the fastest daily rate to initiate processing victims, the military scenario revealed that two biodosimetry methods could achieve the necessary throughput to triage 50,000 victims in 2 d (i.e., the timeframe needed for injured victims), and all five achieved the targeted throughput within 6 d. In contrast, simulations based on the civilian scenario revealed that no method could process 50,000 people in 2 d and only two could succeed within 6 d.

Lack of increased DNA double-strand breaks in peripheral blood mononuclear cells of individuals from high level natural radiation areas of Kerala coast in India

The high level natural radiation area (HLNRA) of Kerala is a 55km long and 0.5km wide strip in south west coast of India. The level of background radiation in this area varies from <1.0mGy/year to 45.0mGy/year. It offers unique opportunity to study the effect of chronic low dose/low dose-rate radiation directly on human population. Spontaneous level of DNA double strand breaks (DSBs) was quantified in peripheral blood mononuclear cells of 91 random individuals from HLNRA (N=61, mean age: 36.1±7.43years) and normal level natural radiation area (NLNRA) (N=30, mean age: 35.5±6.35years) using gamma-H2AX as a marker. The mean annual dose received by NLNRA and HLNRA individuals was 1.28±0.086mGy/year and 8.28±4.96mGy/year, respectively. The spontaneous frequency of DSBs in terms of gamma-H2AX foci among NLNRA and HLNRA individuals were 0.095±0.009 and 0.084±0.004 per cell (P=0.22). The individuals from HLNRA were further classified as low dose group (LDG, 1.51-5.0mGy/year, mean dose: 2.63±0.76mGy/year) and high dose group (HDG, >5.0mGy/year, mean dose: 11.04±3.57mGy/year). The spontaneous frequency of gamma-H2AX foci per cell in NLNRA, LDG and HDG was observed to be 0.095±0.009, 0.096±0.008 and 0.078±0.004 respectively. Individuals belonging to HDG of HLNRA showed marginally lower frequency of DSBs as compared to NLNRA and LDG of HLNRA. This could be suggestive of either lower induction or better repair of DSBs in individuals from HDG of HLNRA. The present study indicated that 5.0mGy/year could be a possible threshold dose for DSB induction at chronic low-dose radiation exposure in vivo. However, further studies on DNA damage induction and repair kinetics are required to draw firm conclusions.

Radiation signature on exposed cells: Relevance in dose estimation

The radiation is considered as a double edged sword, as its beneficial and detrimental effects have been demonstrated. The potential benefits are being exploited to its maximum by adopting safe handling of radionuclide stipulated by the regulatory agencies. While the occupational workers are monitored by personnel monitoring devices, for general publics, it is not a regular practice. However, it can be achieved by using biomarkers with a potential for the radiation triage and medical management. An ideal biomarker to adopt in those situations should be rapid, specific, sensitive, reproducible, and able to categorize the nature of exposure and could provide a reliable dose estimation irrespective of the time of the exposures. Since cytogenetic markers shown to have many advantages relatively than other markers, the origins of various chromosomal abnormalities induced by ionizing radiations along with dose-response curves generated in the laboratory are presented. Current status of the gold standard dicentric chromosome assay, micronucleus assay, translocation measurement by fluorescence in-situ hybridization and an emerging protein marker the γ-H2AX assay are discussed with our laboratory data. With the wide choice of methods, an appropriate assay can be employed based on the net.

Direct measurement of the 3-dimensional DNA lesion distribution induced by energetic charged particles in a mouse model tissue

Charged particles are increasingly used in cancer radiotherapy and contribute significantly to the natural radiation risk. The difference in the biological effects of high-energy charged particles compared with X-rays or γ-rays is determined largely by the spatial distribution of their energy deposition events. Part of the energy is deposited in a densely ionizing manner in the inner part of the track, with the remainder spread out more sparsely over the outer track region. Our knowledge about the dose distribution is derived solely from modeling approaches and physical measurements in inorganic material. Here we exploited the exceptional sensitivity of γH2AX foci technology and quantified the spatial distribution of DNA lesions induced by charged particles in a mouse model tissue. We observed that charged particles damage tissue nonhomogenously, with single cells receiving high doses and many other cells exposed to isolated damage resulting from high-energy secondary electrons. Using calibration experiments, we transformed the 3D lesion distribution into a dose distribution and compared it with predictions from modeling approaches. We obtained a radial dose distribution with sub-micrometer resolution that decreased with increasing distance to the particle path following a 1/r2 dependency. The analysis further revealed the existence of a background dose at larger distances from the particle path arising from overlapping dose deposition events from independent particles. Our study provides, to our knowledge, the first quantification of the spatial dose distribution of charged particles in biologically relevant material, and will serve as a benchmark for biophysical models that predict the biological effects of these particles.

Mean frequency and relative fluorescence intensity measurement of γ-H2AX foci dose response in PBL exposed to γ-irradiation: An inter- and intra-laboratory comparison and its relevance for radiation triage

Measurement of γ-H2AX protein changes in the peripheral blood lymphocytes (PBL) of individuals exposed to ionizing radiation is a simple, sensitive, and rapid assay for radiation triage and early marker of dose estimation. The qualitative and quantitative measurements of the protein changes were examined using flow cytometry and microscopy. Whole blood and isolated lymphocytes were exposed in vitro between 0.1 and 5 Gy doses of (60) Co γ-radiation at a dose rate of 1 Gy/min. Radiation induced γ-H2AX foci frequency (n = 3) and relative fluorescence intensity (n = 7) in PBL was measured at 0.5 and 2 hrs postexposure. The observed dose response for γ-H2AX foci frequency at both time points, for whole blood and isolated lymphocytes did not show any significant (P > 0.05) differences. However, when compared with γ-H2AX foci frequency scored manually (microscopy), the semiautomated analysis (captured images) showed a better correlation (r(2) = 0.918) than that obtained with automated (Metafer) scoring (r(2) = 0.690). It is noteworthy to mention that, the γ-H2AX foci frequency quantified using microscopy showed a dose dependent increase up to 2 Gy and the relative fluorescence intensity (RFI) measured with flow cytometry revealed an increase up to 5 Gy in the PBL exposed in vitro. Moreover, a better correlation was observed between the γ-H2AX foci frequency obtained by manual scoring and RFI (r(2) = 0.910). Kinetic studies showed that the γ-H2AX foci remain more or less unchanged up to 4 hrs and reduces gradually over 48 hrs of postexposure at 37°C. Further, inter and intra-laboratory comparisons showed consistency in the scoring of γ-H2AX foci frequency by manual and semiautomated scoring. The overall results suggest that measurement of γ-H2AX (microscopy and flow cytometry) should be employed within 4 to 6 hrs for a reliable dosimetry either by sharing the work load between the laboratories or investing more manpower; however, triage can be possible even up to 48 hrs of postirradiation.

The Focinator - a new open-source tool for high-throughput foci evaluation of DNA damage

BACKGROUND

The quantitative analysis of foci plays an important role in many cell biological methods such as counting of colonies or cells, organelles or vesicles, or the number of protein complexes. In radiation biology and molecular radiation oncology, DNA damage and DNA repair kinetics upon ionizing radiation (IR) are evaluated by counting protein clusters or accumulations of phosphorylated proteins recruited to DNA damage sites. Consistency in counting and interpretation of foci remains challenging. Many current software solutions describe instructions for time-consuming and error-prone manual analysis, provide incomplete algorithms for analysis or are expensive. Therefore, we aimed to develop a tool for costless, automated, quantitative and qualitative analysis of foci.

METHODS

For this purpose we integrated a user-friendly interface into ImageJ and selected parameters to allow automated selection of regions of interest (ROIs) depending on their size and circularity. We added different export options and a batch analysis. The use of the Focinator was tested by analyzing γ-H2.AX foci in murine prostate adenocarcinoma cells (TRAMP-C1) at different time points after IR with 0.5 to 3 Gray (Gy). Additionally, measurements were performed by users with different backgrounds and experience.

RESULTS

The Focinator turned out to be an easily adjustable tool for automation of foci counting. It significantly reduced the analysis time of radiation-induced DNA-damage foci. Furthermore, different user groups were able to achieve a similar counting velocity. Importantly, there was no difference in nuclei detection between the Focinator and ImageJ alone.

CONCLUSIONS

The Focinator is a costless, user-friendly tool for fast high-throughput evaluation of DNA repair foci. The macro allows improved foci evaluation regarding accuracy, reproducibility and analysis speed compared to manual analysis. As innovative option, the macro offers a combination of multichannel evaluation including colocalization analysis and the possibility to run all analyses in a batch mode.

Persistent γH2AX: A promising molecular marker of DNA damage and aging

One of the earliest cellular responses to DNA double strand breaks (DSBs) is the phosphorylation of the core histone protein H2AX (termed γH2AX). Persistent γH2AX is the level of γH2AX above baseline, measured at a given time-point beyond which DNA DSBs are normally expected to be repaired (usually persist for days to months). This review summarizes the concept of persistent γH2AX in the context of exogenous source induced DNA DSBs (e.g. ionizing radiation (IR), chemotherapeutic drugs, genotoxic agents), and endogenous γH2AX levels in normal aging and accelerated aging disorders. Summary of the current literature demonstrates the following (i) γH2AX persistence is a common phenomenon that occurs in humans and animals; (ii) nuclei retain persistent γH2AX foci for up to several months after IR exposure, allowing for retrospective biodosimetry; (iii) the combination of various radiosensitizing drugs with ionizing radiation exposure leads to persistent γH2AX response, thus enabling the potential for monitoring cancer patients' response to chemotherapy and radiotherapy as well as tailoring cancer treatments; (iv) persistent γH2AX accumulates in telomeric DNA and in cells undergoing cellular senescence; and (v) increased endogenous γH2AX levels may be associated with diseases of accelerated aging. In summary, measurement of persistent γH2AX could potentially be used as a marker of radiation biodosimetry, evaluating sensitivity to therapeutic genotoxins and radiotherapy, and exploring the association of unrepaired DNA DSBs on telomeres with diseases of accelerated aging.

DNA damage foci: Meaning and significance

The discovery of DNA damage response proteins such as γH2AX, ATM, 53BP1, RAD51, and the MRE11/RAD50/NBS1 complex, that accumulate and/or are modified in the vicinity of a chromosomal DNA double-strand break to form microscopically visible, subnuclear foci, has revolutionized the detection of these lesions and has enabled studies of the cellular machinery that contributes to their repair. Double-strand breaks are induced directly by a number of physical and chemical agents, including ionizing radiation and radiomimetic drugs, but can also arise as secondary lesions during replication and DNA repair following exposure to a wide range of genotoxins. Here we aim to review the biological meaning and significance of DNA damage foci, looking specifically at a range of different settings in which such markers of DNA damage and repair are being studied and interpreted.

Defining Blood Processing Parameters for Optimal Detection of γ-H2AX Foci: A Small Blood Volume Method

Biodosimetric methods used to measure the effects of radiation are critical for estimating the health risks to irradiated individuals or populations. The direct measurement of radiation-induced γ-H2AX foci in peripheral blood lymphocytes is one approach that provides a useful end point for triage. Despite the documented advantages of the γ-H2AX assay, there is considerable variation among laboratories regarding foci formation in the same exposure conditions and cell lines. Taking this into account, the goal of our study was to evaluate the influence of different blood processing parameters on the frequency of γ-H2AX foci and optimize a small blood volume protocol for the γ-H2AX assay, which simulates the finger prick blood collection method. We found that the type of fixative, temperature and blood processing time markedly affect the results of the γ-H2AX assay. In addition, we propose a protocol for the γ-H2AX assay that may serve as a potential guideline in the event of large-scale radiation incidents.

Operational guidance for radiation emergency response organisations in Europe for using biodosimetric tools developed in EU MULTIBIODOSE project

In the event of a large-scale radiological emergency, the triage of individuals according to their degree of exposure forms an important initial step of the accident management. Although clinical signs and symptoms of a serious exposure may be used for radiological triage, they are not necessarily radiation specific and can lead to a false diagnosis. Biodosimetry is a method based on the analysis of radiation-induced changes in cells of the human body or in portable electronic devices and enables the unequivocal identification of exposed people who should receive medical treatment. The MULTIBIODOSE (MBD) consortium developed and validated several biodosimetric assays and adapted and tested them as tools for biological dose assessment in a mass-casualty event. Different biodosimetric assays were validated against the 'gold standard' of biological dosimetry-the dicentric assay. The assays were harmonised in such a way that, in an emergency situation, they can be run in parallel in a network of European laboratories. The aim of this guidance is to give a concise overview of the developed biodosimetric tools as well as how and when they can be used in an emergency situation.