Long time persistence of residual 53BP1/γ-H2AX foci in human lymphocytes in relationship to apoptosis, chromatin condensation and biological dosimetry

The Molecular Mechanisms in Senescent Cells Induced by Natural Aging and Ionizing Radiation

This review discusses the relationship between cellular senescence and radiation exposure. Given the wide range of ionizing radiation sources encountered by people in professional and medical spheres, as well as the influence of natural background radiation, the question of the effect of radiation on biological processes, particularly on aging processes, remains highly relevant. The parallel relationship between natural and radiation-induced cellular senescence reveals the common aspects underlying these processes. Based on recent scientific data, the key points of the effects of ionizing radiation on cellular processes associated with aging, such as genome instability, mitochondrial dysfunction, altered expression of miRNAs, epigenetic profile, and manifestation of the senescence-associated secretory phenotype (SASP), are discussed. Unraveling the molecular mechanisms of cellular senescence can make a valuable contribution to the understanding of the molecular genetic basis of age-associated diseases in the context of environmental exposure.

Effects of low-dose ionizing radiation on genomic instability in interventional radiology workers

Interventional radiologists are chronically exposed to low-dose ionizing radiation (IR), which may represent a health risk. The aim of the present study was to evaluate genomic instability by analyzing chromosomal aberrations, micronuclei, and 53BP1 DNA repair foci in peripheral blood lymphocytes of radiologists. Based on the IAEA guidelines on biodosimetry using dicentrics, the average protracted whole-body dose in radiologists were estimated. Since preleukemic fusion genes (PFG) are the primary events leading to leukemia, we also studied their presence by RT-qPCR and FISH. No significant difference in 53BP1 foci and incidence of PFG (MLL-AF4, MLL-AF9, AML1-ETO, BCR-ABL p190) was found in cells of interventional radiologists in comparison to controls. However, our results showed an increased frequency of micronuclei and various types of chromosomal aberrations including dicentrics in interventional radiologists. The average protracted whole body estimated dose was defined at 452.63 mGy. We also found a significantly higher amplification of the MLL gene segment and increased RNA expression in cells of interventional radiologists in comparison to controls. In conclusion, our results showed that long-term low-dose IR induces genomic instability in interventional radiologists.

Chemotherapy and cryopreservation affects DNA repair foci in lymphocytes of breast cancer patients

PURPOSE

Although breast cancer (BC) patients benefit from radiotherapy (RT), some radiosensitive (RS) patients suffer from side effects caused by ionizing radiation in healthy tissues. It is thought that RS is underlaid by a deficiency in the repair of DNA double-strand breaks (DSB). DNA repair proteins such as p53-binding protein 1 (53BP1) and phosphorylated histone H2AX (γH2AX), form DNA repair foci at the DSB locations and thus serve as DSB biomarkers. Peripheral blood lymphocytes (PBL) are commonly believed to be an appropriate cell system for RS assessment using DNA repair foci. The amount of DSB may also be influenced by chemotherapy (CHT), which is often chosen as the first treatment modality before RT. As it is not always possible to analyze blood samples immediately after collection, there is a need for cryopreservation of PBL in liquid nitrogen. However, cryopreservation may potentially affect the number of DNA repair foci. In this work, we studied the effect of cryopreservation and CHT on the amount of DNA repair foci in PBL of BC patients undergoing radiotherapy.

MATERIALS AND METHODS

The effect of cryopreservation was studied by immunofluorescence analysis of 53BP1 and γH2AX proteins at different time intervals after in vitro irradiation. The effect of chemotherapy was analyzed by fluorescent labelling of 53BP1 and γH2AX proteins in PBL collected before, during, and after RT.

RESULTS

Higher number of primary 53BP1/γH2AX foci was observed in frozen cells indicating that cryopreservation affects the formation of DNA repair foci in PBL of BC patients. In CHT-treated patients, a higher number of foci were found before RT, but no differences were observed during and after the RT.

CONCLUSIONS

Cryopreservation is the method of choice for analyzing DNA repair residual foci, but only similarly treated and preserved cells should be used for comparison of primary foci. CHT induces DNA repair foci in PBL of BC patients, but this effect disappears during radiotherapy.

Transcriptomes of Wet Skin Biopsies Predict Outcomes after Ionizing Radiation Exposure with Potential Dosimetric Applications in a Mouse Model

Countermeasures for radiation diagnosis, prognosis, and treatment are trailing behind the proliferation of nuclear energy and weaponry. Radiation injury mechanisms at the systems biology level are not fully understood. Here, mice skin biopsies at h2, d4, d7, d21, and d28 after exposure to 1, 3, 6, or 20 Gy whole-body ionizing radiation were evaluated for the potential application of transcriptional alterations in radiation diagnosis and prognosis. Exposure to 20 Gy was lethal by d7, while mice who received 1, 3, or 6 Gy survived the 28-day time course. A Sammon plot separated samples based on survival and time points (TPs) within lethal (20 Gy) and sublethal doses. The differences in the numbers, regulation mode, and fold change of significantly differentially transcribed genes (SDTGs, p < 0.05 and FC > 2) were identified between lethal and sublethal doses, and down and upregulation dominated transcriptomes during the first post-exposure week, respectively. The numbers of SDTGs and the percentages of upregulated ones revealed stationary downregulation post-lethal dose in contrast to responses to sublethal doses which were dynamic and largely upregulated. Longitudinal up/downregulated SDTGs ratios suggested delayed and extended responses with increasing IR doses in the sublethal range and lethal-like responses in late TPs. This was supported by the distributions of common and unique genes across TPs within each dose. Several genes with potential dosimetric marker applications were identified. Immune, fibrosis, detoxification, hematological, neurological, gastric, cell survival, migration, and proliferation radiation response pathways were identified, with the majority predicted to be activated after sublethal and inactivated after lethal exposures, particularly during the first post-exposure week.

Chromatin and the Cellular Response to Particle Radiation-Induced Oxidative and Clustered DNA Damage

Exposure to environmental ionizing radiation is prevalent, with greatest lifetime doses typically from high Linear Energy Transfer (high-LET) alpha particles via the radioactive decay of radon gas in indoor air. Particle radiation is highly genotoxic, inducing DNA damage including oxidative base lesions and DNA double strand breaks. Due to the ionization density of high-LET radiation, the consequent damage is highly clustered wherein ≥2 distinct DNA lesions occur within 1–2 helical turns of one another. These multiply-damaged sites are difficult for eukaryotic cells to resolve either quickly or accurately, resulting in the persistence of DNA damage and/or the accumulation of mutations at a greater rate per absorbed dose, relative to lower LET radiation types. The proximity of the same and different types of DNA lesions to one another is challenging for DNA repair processes, with diverse pathways often confounding or interplaying with one another in complex ways. In this context, understanding the state of the higher order chromatin compaction and arrangements is essential, as it influences the density of damage produced by high-LET radiation and regulates the recruitment and activity of DNA repair factors. This review will summarize the latest research exploring the processes by which clustered DNA damage sites are induced, detected, and repaired in the context of chromatin.

Persistent DNA Double-Strand Breaks After Repeated Diagnostic CT Scans in Breast Epithelial Cells and Lymphocytes

DNA double-strand break (DSB) induction and repair have been widely studied in radiation therapy (RT); however little is known about the impact of very low exposures from repeated computed tomography (CT) scans for the efficiency of repair. In our current study, DSB repair and kinetics were investigated in side-by-side comparison of RT treatment (2 Gy) with repeated diagnostic CT scans (≤20 mGy) in human breast epithelial cell lines and lymphoblastoid cells harboring different mutations in known DNA damage repair proteins. Immunocytochemical analysis of well known DSB markers γH2AX and 53BP1, within 48 h after each treatment, revealed highly correlated numbers of foci and similar appearance/disappearance profiles. The levels of γH2AX and 53BP1 foci after CT scans were up to 30% of those occurring 0.5 h after 2 Gy irradiation. The DNA damage repair after diagnostic CT scans was monitored and quantitatively assessed by both γH2AX and 53BP1 foci in different cell types. Subsequent diagnostic CT scans in 6 and/or 12 weeks intervals resulted in elevated background levels of repair foci, more pronounced in cells that were prone to genomic instability due to mutations in known regulators of DNA damage response (DDR). The levels of persistent foci remained enhanced for up to 6 months. This “memory effect” may reflect a radiation-induced long-term response of cells after low-dose x-ray exposure.

DNA damage response and apoptosis induced by hyperthermia in human umbilical cord blood lymphocytes

While hyperthermia (HT) is a promising modality for cancer treatment, the knowledge on mechanisms of its effect on cells is still limited. We have investigated DNA double-strand break (DSB) and apoptosis induced by HT. Umbilical cord blood lymphocytes (UCBL) were subjected to HT at 43 °C. We have treated cells for 1 h (1 h HT), 2 h (2 h HT) and by combined HT and ice treatment (both lasting 1 h). Enumeration of DSB by 53BP1/γH2AX DNA repair focus formation and early apoptosis by γH2AX pan-staining was conducted by automated fluorescent microscopy. Apoptotic stages and viability were assessed by the annexin/propidium iodide (PI) assay using flow cytometry 0, 18, and 42 h post-treatment. HT induced either immediate (2 h HT) or postponed (1 h HT) DNA damage. The levels of 53BP1 and γH2AX foci differed under the same treatment conditions, suggesting that the ratio of co-localized γH2AX/53BP1 foci to all γH2AX and also to all 53BP1 foci could be a valuable marker. The ratio of co-localized foci increased immediately after 2 h HT regardless the way of assessment. For the first time we show, by both annexin/PI and γH2AX pan-staining assay that apoptosis can be induced during or immediately after the 2 h HT treatment. Our results suggest that HT may induce DSB in dependence on treatment duration and post-treatment time due to inhibition of DNA repair pathways and that HT-induced apoptosis might be dependent or associated with DSB formation in human lymphocytes. Assessment of γH2AX pan-staining in lymphocytes affected by HT may represent a valuable marker of HT treatment side effects.

Matrix stiffness regulates α-TAT1-mediated acetylation of α-tubulin and promotes silica-induced epithelial-mesenchymal transition via DNA damage

Silicosis is characterized by silica exposure-induced lung interstitial fibrosis and formation of silicotic nodules, resulting in lung stiffening. The acetylation of microtubules mediated by α-tubulin N-acetyltransferase 1 (α-TAT1) is a posttranslational modification that promotes microtubule stability in response to mechanical stimulation. α-TAT1 and downstream acetylated α-tubulin (Ac-α-Tub) are decreased in silicosis, promoting the epithelial-mesenchymal transition (EMT); however, the underlying mechanisms are unknown. We found that silica, matrix stiffening or their combination triggered Ac-α-Tub downregulation in alveolar epithelial cells, followed by DNA damage and replication stress. α-TAT1 elevated Ac-α-Tub to limit replication stress and the EMT via trafficking of p53-binding protein 1 (53BP1, also known as TP53BP1). The results provide evidence that α-TAT1 and Ac-α-Tub inhibit the EMT and silicosis fibrosis by preventing 53BP1 mislocalization and relieving DNA damage. This study provides insight into how the cell cycle is regulated during the EMT and why the decrease in α-TAT1 and Ac-α-Tub promotes silicosis fibrosis.This article has an associated First Person interview with the first authors of the paper.

Triterpenoid CDDO-Me induces ROS generation and up-regulates cellular levels of antioxidative enzymes without induction of DSBs in human peripheral blood mononuclear cells

Ionizing radiation produces reactive oxygen species (ROS) leading to cellular DNA damage. Therefore, patients undergoing radiation therapy or first responders in radiological accident scenarios could both benefit from the identification of specifically acting pharmacological radiomitigators. The synthetic triterpenoid bardoxolone-methyl (CDDO-Me) has previously been shown to exert antioxidant, anti-inflammatory and anticancer activities in several cell lines, in part by enhancing the DNA damage response. In our study, we examined the effect of nanomolar concentrations of CDDO-Me in human peripheral blood mononuclear cells (PBMC). We observed increased cellular levels of the antioxidative enzymes heme oxygenase-1 (HO-1), NAD(P)H dehydrogenase (quinone1) and mitochondrial superoxide dismutase 2 by immunoblotting. Surprisingly, we found increased intracellular ROS-levels using imaging flow-cytometry. However, the radiation-induced DNA double-strand break (DSB) formation using the γ-H2AX + 53BP1 DSB focus assay and the cytokinesis-block micronucleus assay both revealed, that nanomolar CDDO-Me pre-treatment of PBMC for 2 h or 6 h ahead of X irradiation with 2 Gy did neither significantly affect γ-H2AX + 53BP1 DSB foci formation nor the frequency of micronuclei. CDDO-Me treatment also failed to alter the nuclear division index and the frequency of IR-induced PBMC apoptosis as investigated by Annexin V-labeled live-cell imaging. Our results indicate that pharmacologically increased cellular concentrations of antioxidative enzymes might not necessarily exert radiomitigating short-term effects in IR-exposed PBMC. However, the increase of antioxidative enzymes could also be a result of a defensive cellular mechanism towards elevated ROS levels.

Assessment of DNA damage by 53PB1 and pKu70 detection in peripheral blood lymphocytes by immunofluorescence and high-resolution transmission electron microscopy

Purpose

53BP1 foci detection in peripheral blood lymphocytes (PBLs) by immunofluorescence microscopy (IFM) is a sensitive and quantifiable DNA double-strand break (DSB) marker. In addition, high-resolution transmission electron microscopy (TEM) with immunogold labeling of 53BP1 and DSB-bound phosphorylated Ku70 (pKu70) can be used to determine the progression of the DNA repair process. To establish this TEM method in the PBLs of patients with cancer, we analyzed and characterized whether different modes of irradiation influence the formation of DSBs, and whether accompanying chemotherapy influences DSB formation.

Methods

We obtained 86 blood samples before and 0.1, 0.5, and 24 h after irradiation from patients (n = 9) with head and neck or rectal cancers receiving radiotherapy (RT; n = 4) or radiochemotherapy (RCT; n = 5). 53BP1 foci were quantified by IFM. In addition, TEM was used to quantify gold-labelled pKu70 dimers and 53BP1 clusters within euchromatin and heterochromatin of PBLs.

Results

IFM analyses showed that during radiation therapy, persistent 53BP1 foci in PBLs accumulated with increasing numbers of administered RT fractions. This 53BP1 foci accumulation was not influenced by the irradiation technique applied (3D conformal radiotherapy versus intensity-modulated radiotherapy), dose intensity per fraction, number of irradiation fields, or isodose volume. However, more 53BP1 foci were detected in PBLs of patients treated with accompanying chemotherapy. TEM analyses showed that DSBs, indicated by pKu70, were present for longer periods in PBLs of RCT patients than in PBLs of RT only patients. Moreover, not every residual 53BP1 focus was equivalent to a remaining DSB, since pKu70 was not present at every damage site. Persistent 53BP1 clusters, visualized by TEM, without colocalizing pKu70 likely indicate chromatin alterations after repair completion or, possibly, defective repair.

Conclusion

IFM 53BP1 foci analyses alone are not adequate to determine individual repair capacity after irradiation of PBLs, as a DSB may be indicated by a 53BP1 focus but not every 53BP1 focus represents a DSB.

Electronic supplementary material

The online version of this article (10.1007/s00066-020-01576-1) contains supplementary material, which is available to authorized users.

Biodosimetry of Low Dose Ionizing Radiation Using DNA Repair Foci in Human Lymphocytes

Purpose: Ionizing radiation induced foci (IRIF) known also as DNA repair foci represent most sensitive endpoint for assessing DNA double strand breaks (DSB). IRIF are usually visualized and enumerated with the aid of fluorescence microscopy using antibodies to γH2AX and 53BP1. This study analyzed effect of low dose ionizing radiation on residual IRIF in human lymphocytes to the aim of potential biodosimetry and possible extrapolation of high-dose γH2AX/53BP1 effects to low doses and compared kinetics of DSB and IRIF. We also analyzed whether DNaseI, which is used for reducing of clumps, affects the IRIF level. Materials and Methods: The cryopreserved human lymphocytes from umbilical cord blood (UCB) were thawed with/without DNaseI, γ-irradiated at doses of 0, 5, 10, and 50 cGy and γH2AX/53BP1 foci were analyzed 30 min, 2 h, and 22 h post-irradiation using appropriate antibodies. We also analyzed kinetics of DSB using PFGE. Results: No significant difference was observed between data obtained by γH2AX foci evaluation in cells that were irradiated by low doses and data obtained by extrapolation from higher doses. Residual 53BP1 foci induced by low doses significantly outreached the data extrapolated from irradiation by higher doses. 53BP1 foci induced by low dose-radiation remain longer at DSB loci than foci induced by higher doses. There was no significant effect of DNaseI on DNA repair foci. Conclusions: Primary γH2AX, 53BP1 foci and their co-localization represent valuable markers for biodosimetry of low doses, but their usefulness is limited by short time window. Residual γH2AX and 53BP1 foci are more useful markers for biodosimetry in vitro. Effects of low doses can be extrapolated from high dose using γH2AX residual foci while γH2AX/53BP1 foci are valuable markers for evaluation of initial DSB induced by ionizing radiation. Residual IRIF induced by low doses persist longer time than those induced by higher doses.

Nanoscopic analysis of 53BP1, BRCA1 and Rad51 reveals new insights in temporal progression of DNA-repair and pathway choice

The accumulation and spatial distribution of 53BP1, BRCA1 and Rad51, key proteins in DNA double-strand break (DSB) repair, was investigated with high temporal resolution over a time span of 24 h, using STED nanoscopy. DNA lesions were induced by irradiation with high-LET (linear energy transfer) α-particles. We show that 53BP1 IRIF formation occurs quickly in almost all cells and after about 6 h the fraction of 53BP1 IRIF positive cells slowly declines. Against the expectations BRCA1 and Rad51 IRIF formation is only shortly delayed but with the maximum of cells showing foci after 6 and 8 h after irradiation. At this stage, almost all IRIF in a given Rad51-positive cell show Rad51 accumulation, suggesting that repair via homologous recombination is attempted at almost all residual DSB sites. The frequency of BRCA1 IRIF positive cells increases much earlier and remains high after Rad51 positive cells start to decline, supporting models claiming that functional roles of BRCA1 change over time. Correlation analysis showed a high degree of correlation of Rad51 with BRCA1, while the exclusion of 53BP1 from the actual resection-zone is demonstrated by anti-correlation of Rad51 and 53BP1. Interestingly, these correlation and anti-correlation patterns exhibit complementary temporal variation.

Inorganic Nitrate Alleviates Total Body Irradiation-Induced Systemic Damage by Decreasing Reactive Oxygen Species Levels

PURPOSE

To evaluate the protective effect of inorganic nitrate against systemic damage in a mouse model of total body gamma irradiation (TBI).

METHODS AND MATERIALS

C57BL/6 mice in the irradiation (IR) + NaNO₃ group were pretreated with 2 mmol/L NaNO₃ in their drinking water for 1 week before receiving 5 Gy irradiation. Animals that received only 5 Gy irradiation were designated as the IR group. Survival and body weight were monitored. The peripheral blood lymphocytes, heart, liver, lung, and submandibular gland were harvested and assessed. Reactive oxygen species (ROS) were measured in the lung and submandibular gland. We examined phosphorylated histone H2AX (p-H2AX) and p53-binding protein 1 (53BP1) as markers of early-stage DNA damage and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) and Bax/caspase 3 mRNA expression as markers of apoptosis.

RESULTS

No improvement of survival was observed in the IR + NaNO₃ group after TBI, but body weight loss after 5 Gy TBI was significantly attenuated in the IR + NaNO₃ group. The levels of peripheral blood erythrocytes, leukocytes, and platelets at 7 days postirradiation recovered with nitrate treatment; moreover, the p-H2AX level in the peripheral blood lymphocytes was much lower in the IR + NaNO₃ group at 2 and 4 hours post irradiation. In the lung and submandibular gland, the levels of p-H2AX, 53BP1 and ROS as well as TUNEL staining were significantly decreased in the IR + NaNO₃ group compared with those in the IR group. Gene expression of Bax and caspase 3 was decreased in both the lung and submandibular gland with nitrate treatment, indicating attenuation of apoptosis.

CONCLUSION

Inorganic nitrate delivery could effectively prevent TBI-induced systemic damage. Nitrate-mediated decreases in ROS levels may contribute to this systemic protective effect.

DNA damage in leukocytes after internal ex-vivo irradiation of blood with the α-emitter Ra-223

Irradiation with high linear energy transfer α-emitters, like the clinically used Ra-223 dichloride, severely damages cells and induces complex DNA damage including closely spaced double-strand breaks (DSBs). As the hematopoietic system is an organ-at-risk for the treatment, knowledge about Ra-223-induced DNA damage in blood leukocytes is highly desirable. Therefore, 36 blood samples from six healthy volunteers were exposed ex-vivo (in solution) to different concentrations of Ra-223. Absorbed doses to the blood were calculated assuming local energy deposition of all α- and β-particles of the decay, ranging from 0 to 142 mGy. γ-H2AX + 53BP1 co-staining and analysis was performed in leukocytes isolated from the irradiated blood samples. For DNA damage quantification, leukocyte samples were screened for occurrence of α-induced DNA damage tracks and small γ-H2AX + 53BP1 DSB foci. This revealed a linear relationship between the frequency of α-induced γ-H2AX damage tracks and the absorbed dose to the blood, while the frequency of small γ-H2AX + 53BP1 DSB foci indicative of β-irradiation was similar to baseline values, being in agreement with a negligible β-contribution (3.7%) to the total absorbed dose to the blood. Our calibration curve will contribute to the biodosimetry of Ra-223-treated patients and early after incorporation of α-emitters.

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.

Validation of JCountPro software for efficient assessment of ionizing radiation-induced foci in human lymphocytes

PURPOSE

Ionizing radiation-induced foci (IRIF) known also as DNA repair foci represent the most sensitive and specific assay for assessing DNA double-strand break (DSB). IRIF are usually visualized and enumerated with the aid of fluorescence microscopy using antibodies to phosphorylated γH2AX and 53BP1. Although several approaches and software packages were developed for quantification of IRIF, not one of them was commonly accepted and inter-laboratory variability in the outputs was reported. In this study, JCountPro software was validated for IRIF enumeration in two independent laboratories.

MATERIALS AND METHODS

Human lymphocytes were γ-irradiated at doses of 0, 2, 5, 10 and 50 cGy. The cells were fixed, permeabilized and IRIF were immunostained using appropriate antibodies. Cell images were acquired with automatic Metafer system. Endogenous and radiation-induced γH2AX and 53BP1 foci were enumerated using JCountPro. This analysis was performed from the same cell galleries by the researchers from two laboratories. Yield of foci was analyzed by either arithmetic mean (AM) value (foci/cell) or principal average (PA) derived from the approximation of foci distribution with Poisson statistics. Statistical analysis was performed using factorial ANOVA.

RESULTS

Enumeration of 53BP1, γH2AX and co-localized 53BP1/γH2AX foci by JCountPro was essentially the same between laboratories. IRIF were detected at all doses and linear dose response was obtained in the studied dose range. PA values from Poisson distribution fitted the data better as compared to AM values and were more powerful and sensitive for IRIF analysis than the AM values. All JCountPro data were confirmed by visual focus enumeration.

CONCLUSIONS

We concluded that the JCountPro software was efficient in objectively enumerating IRIF regardless of an individual researcher's bias and has a potential for usage in clinics and molecular epidemiology.

The profiles of gamma-H2AX along with ATM/DNA-PKcs activation in the lymphocytes and granulocytes of rat and human blood exposed to gamma rays

Establishing a rat model suitable for γ-H2AX biodosimeter studies has important implications for dose assessment of internal radionuclide contamination in humans. In this study, γ-H2AX, p-ATM and p-DNA-PKcs foci were enumerated using immunocytofluorescence method, and their protein levels were measured by Western blot in rat blood lymphocytes and granulocytes exposed to γ-rays compared with human blood lymphocytes and granulocytes. It was found that DNA double-strand break repair kinetics and linear dose responses in rat lymphocytes were similar to those observed in the human counterparts. Moreover, radiation induced clear p-ATM and p-DNA-PKcs foci formation and an increase in ratio of co-localization of p-ATM or p-DNA-PKcs with γ-H2AX foci in rat lymphocytes similar to those of human lymphocytes. The level of γ-H2AX protein in irradiated rat and human lymphocytes was significantly reduced by inhibitors of ATM and DNA-PKcs. Surprisingly, unlike human granulocytes, rat granulocytes with DNA-PKcs deficiency displayed a rapid accumulation, but delayed disappearance of γ-H2AX foci with essentially no change from 10 h to 48 h post-irradiation. Furthermore, inhibition of ATM activity in rat granulocytes also decreased radiation-induced γ-H2AX foci formation. In comparison, human granulocytes showed no response to irradiation regarding γ-H2AX, p-ATM or p-DNA-PKcs foci. Importantly, incidence of γ-H2AX foci in lymphocytes after total-body radiation of rats was consistent with that of in vitro irradiation of rat lymphocytes. These findings show that rats are a useful in vivo model for validation of γ-H2AX biodosimetry for dose assessment in humans. ATM and DNA-PKcs participate together in DSB repair in rat lymphocytes similar to that of human lymphocytes. Further, rat granulocytes, which have the characteristic of delayed disappearance of γ-H2AX foci in response to radiation, may be a useful experimental system for biodosimetry studies.

Establishment of a γ-H2AX foci-based assay to determine biological dose of radon to red bone marrow in rats

The biodosimetric information is critical for assessment of cancer risk in populations exposed to high radon. However, no tools are available for biological dose estimation following radon exposure. Here, we established a γ-H2AX foci-based assay to determine biological dose to red bone marrow (RBM) in radon-inhaled rats. After 1–3 h of in vitro radon exposure, a specific pattern of γ-H2AX foci, linear tracks with individual p-ATM and p-DNA-PKcs foci, was observed, and the yield of γ-H2AX foci and its linear tracks displayed a linear dose-response manner in both rat peripheral blood lymphocytes (PBLs) and bone-marrow lymphocytes (BMLs). When the cumulative doses of radon inhaled by rats reached 14, 30 and 60 working level months (WLM), the yields of three types of foci markedly increased in both PBLs and BMLs, and γ-H2AX foci-based dose estimates to RBM were 0.97, 2.06 and 3.94 mGy, respectively. Notably, BMLs displayed a more profound increase of three types of foci than PBLs, and the absorbed dose ratio between BMLs and PBLs was similar between rats exposed to 30 and 60 WLM of radon. Taken together, γ-H2AX foci quantitation in PBLs is able to estimate RBM-absorbed doses with the dose-response curve of γ-H2AX foci after in vitro radon exposure and the ratio of RBM- to PBL-absorbed doses in rats following radon exposure.

γH2AX formation kinetics in PBMCs of rabbits exposed to acute and fractionated radiation and attenuation of focus frequency through preadministration of a combination of podophyllotoxin and rutin hydrate

DNA damage can be assessed by the quantitation of γH2AX foci that form at DSB sites. This study examines the generation and persistence of γH2AX foci, variability in foci size after acute and fractionated radiation exposure, and the effect of pretreatment with a safe radioprotective formulation termed G-003M on foci generation and persistence. G-003M contains a combination of podophyllotoxin and rutin hydrate, and was administered intramuscularly to rabbits 1 hr prior to Co(60) gamma irradiation. Rabbits were assigned to one of the following treatment groups: untreated, G-003M alone, irradiated (single dose 8 Gy, fractionated 2 Gy/day for 4 days or single dose 2 Gy) or G-003M preadministration followed by radiation exposure. Foci continuously persisted for a week in peripheral blood mononuclear cells of rabbits exposed to a single 8 Gy dose. However, the number of foci gradually decreased after reaching a maximum at 1 h. In rabbits exposed to fractionated radiation, foci detected 1 hr after the final exposure were significantly larger (P < 0.001) than in rabbits exposed to a single 8 Gy dose, but disappeared completely after 24 h. In both groups, foci reappeared on days 11-15 in terminally ill animals. G-003M pretreatment significantly (P < 0.05) attenuated the formation of γH2AX foci in all irradiated rabbits. This study reveals that γH2AX focus assessment could be used to confirm radiation exposure, that focus size reflects the type of radiation exposure (acute or fractionated), that the re-appearance of foci is a strong indicator of imminent death in animals, and that G-003M provides protection against radiation. Environ. Mol. Mutagen. 57:455-468, 2016. © 2016 Wiley Periodicals, Inc.

Early and Delayed Induction of DSBs by Nontargeted Effects in ICR Mouse Lymphocytes after In Vivo X Irradiation

The goal of this study was to determine whether in vivo X irradiation induces nontargeted effects, such as delayed effects and bystander effects in ICR mouse lymphocytes. We first examined the generation of DNA double-strand breaks (DSBs) in lymphocytes, isolated from ICR mice exposed to 1 Gy X irradiation, by enumeration of p53 binding protein 1 (53BP1) foci, and observed that the number of 53BP1 foci reached their maximum 3 days postirradiation and decreased to background level 30 days postirradiation. However, the number of 53BP1 foci was significantly increased in lymphocytes isolated from ICR mice 90-365 days postirradiation. This result indicates that in vivo X irradiation induced delayed DSBs in ICR mouse lymphocytes. We next counted the number of 53BP1 foci in lymphocytes isolated from sham-irradiated ICR mice that had been co-cultured with lymphocytes isolated from 1 Gy X-irradiated ICR mice, and observed a significant increase in the number of 53BP1 foci 1-7 days postirradiation. This result indicates that in vivo X irradiation induced bystander effects in ICR mouse lymphocytes. These findings suggest that in vivo X irradiation induces early and delayed nontargeted effects in ICR mouse lymphocytes.

Induction and inhibition of the pan-nuclear gamma-H2AX response in resting human peripheral blood lymphocytes after X-ray irradiation

Human peripheral blood lymphocytes (HPBLs) are one of the most sensitive cells to ionizing radiation (IR) in the human body, and IR-induced DNA damage and functional impairment of HPBLs are the adverse consequences of IR accidents and major side effects of radiotherapy. Phosphorylated H2AX (γH2AX) is a sensitive marker for DNA double-strand breaks, but the role and regulation of the pan-nuclear γH2AX response in HPBLs after IR remain unclear. We herein demonstrated that the pan-nuclear γH2AX signals were increased in a time- and dose-dependent manner, colocalized with >94% of TUNEL apoptotic staining, and displayed a typical apoptotic pattern in resting HPBLs after low LET X-ray IR. In addition, the X-irradiation-induced pan-nuclear p-ATM and p-DNA-PKcs responses also occurred in resting HPBLs, and were colocalized with 92–95% of TUNEL staining and 97–98% of the pan-nuclear γH2AX signals, respectively, with a maximum at 6 h post irradiation, but disappeared at 24 h post irradiation. Moreover, ATM/DNA-PKcs inhibitor KU55933, p53 inhibitor PFT-μ and pan-caspase inhibitor ZVAD-fmk significantly decreased X-irradiation-induced pan-nuclear γH2AX signals and TUNEL staining, protected HPBLs from apoptosis, but decreased the proliferative response to mitogen in X-irradiated HPBLs. Notably, whereas both KU55933 and PFT-μ increased the IR-induced chromosome breaks and mis-repair events through inhibiting the formation of p-ATM, p-DNA-PKcs and γH2AX foci in X-irradiated HPBLs, the ZVAD-fmk did not increase the IR-induced chromosomal instability. Taken together, our data indicate that pan-nuclear γH2AX response represents an apoptotic signal that is triggered by the transient pan-nuclear ATM and DNA-PKcs activation, and mediated by p53 and pan-caspases in X-irradiated HPBLs, and that caspase inhibitors are better than ATM/DNA-PKcs inhibitors and p53 inhibitors to block pan-nuclear γH2AX response/apoptosis and protect HPBLs from IR.

Analysis of Residual DSBs in Ataxia-Telangiectasia Lymphoblast Cells Initiating Apoptosis

In order to examine the relationship between accumulation of residual DNA double-strand breaks (DSBs) and cell death, we have used a control and an ATM (Ataxia-Telangiectasia Mutated) defective cell line, as Ataxia-Telangiectasia (AT) cells tend to accumulate residual DSBs at long times after damage infliction. After irradiation, AT cells showed checkpoint impairment and a fraction of cells displayed an abnormal centrosome number and tetraploid DNA content, and this fraction increased along with apoptosis rates. At all times analyzed, AT cells displayed a significantly higher rate of radiation-induced apoptosis than normal cells. Besides apoptosis, 70-85% of the AT viable cells (TUNEL-negative) carried ≥ 10 γH2AX foci/cell, while only 12-27% of normal cells did. The fraction of AT and normal cells undergoing early and late apoptosis were isolated by flow cytometry and residual DSBs were concretely scored in these populations. Half of the γH2AX-positive AT cells undergoing early apoptosis carried ≥ 10 γH2AX foci/cell and this fraction increased to 75% in late apoptosis. The results suggest that retention of DNA damage-induced γH2AX foci is an indicative of lethal DNA damage, as cells undergoing apoptosis are those accumulating more DSBs. Scoring of residual γH2AX foci might function as a predictive tool to assess radiation-induced apoptosis.

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.

Dose-rate effect was observed in T98G glioma cells following BNCT

BACKGROUND

It is generally said that low LET radiation produce high dose-rate effect, on the other hand, no significant dose rate effect is observed in high LET radiation. Although high LET radiations are produced in BNCT, little is known about dose-rate effect of BNCT.

MATERIALS AND METHODS

T98G cells, which were tumor cells, were irradiated by neutron mixed beam with BPA. As normal tissue derived cells, Chinese hamster ovary (CHO-K1) cells and DNA double strand breaks (DNA-DSBs) repair deficient cells, xrs5 cells were irradiated by the neutrons (not including BPA). To DNA-DSBs analysis, T98G cells were stained immunochemically with 53BP1 antibody. The number of DNA-DSBs was determined by counting 53BP1 foci.

RESULTS

There was no dose-rate effect in xrs5 cells. D0 difference between 4cGy/min and 20cGy/min irradiation were 0.5 and 5.9 at the neutron and gamma-ray irradiation for CHO-K1, and 0.3 at the neutron for T98G cells. D0 difference between 20cGy/min and 80cGy/min irradiation for T98G cells were 1.2 and 0.6 at neutron irradiation plus BPA and gamma-ray. The differences between neutron irradiations at the dose rate in T98G cells were supported by not only the cell viability but also 53BP1 foci assay at 24h following irradiation to monitor DNA-DSBs.

CONCLUSION

Dose-rate effect of BNCT when T98G cells include 20ppm BPA was greater than that of gamma-ray irradiation. Moreover, Dose-rate effect of the neutron beam when CHO-K1 cells did not include BPA was less than that of gamma-ray irradiation These present results may suggest the importance of dose-rate effect for more efficient BNCT and the side effect reduction.

Evaluation of the gamma-H2AX assay for radiation biodosimetry in a swine model

There is a paucity of large animal models to study both the extent and the health risk of ionizing radiation exposure in humans. One promising candidate for such a model is the minipig. Here, we evaluate the minipig for its potential in γ-H2AX-based biodosimetry after exposure to ionizing radiation using both Cs137 and Co60 sources. γ-H2AX foci were enumerated in blood lymphocytes and normal fibroblasts of human and porcine origin after ex vivo γ-ray irradiation. DNA double-strand break repair kinetics in minipig blood lymphocytes and fibroblasts, based on the γ-H2AX assay, were similar to those observed in their human counterparts. To substantiate the similarity observed between the human and minipig we show that minipig fibroblast radiosensitivity was similar to that observed with human fibroblasts. Finally, a strong γ-H2AX induction was observed in blood lymphocytes following minipig total body irradiation. Significant responses were detected 3 days after 1.8 Gy and 1 week after 3.8 and 5 Gy with residual γ-H2AX foci proportional to the initial radiation doses. These findings show that the Gottingen minipig provides a useful in vivo model for validation of γ-H2AX biodosimetry for dose assessment in humans.

DNA damage response in CD133 + stem/progenitor cells from umbilical cord blood: low level of endogenous foci and high recruitment of 53BP1

Abstract Purpose: Human hematopoietic stem cells (HSC) are thought to be a major target of radiation-induced leukemogenesis and also provide a relevant cellular model for assessing cancer risk. Cluster of designation 133+ (CD133+) is a marker found in human progenitor and hematopoietic stem cells. Our study examined the repair of radiation-induced DNA double-strand breaks (DSB) in CD133 + umbilical cord blood cells (UCBC).

MATERIALS AND METHODS

After γ-irradiation, endogenous and induced DSB were evaluated in CD133 + UCBC, CD133 - UCBC and peripheral blood lymphocytes (PBL) in terms of phosphorylated histone 2A family member X (γH2AX) and tumor suppressor p53 binding protein 1 (53BP1) foci.

RESULTS

We found that repair signaling in CD133 + UCBC is different from CD133 - UCBC and PBL. These differences include lower endogenous DSB levels and higher 53BP1 recruitment.

CONCLUSIONS

Our data, together with a recent report on radiation-induced γH2AX and 53BP1 foci in CD34 + cells, indicate enhanced DNA repair capacity in HSC as compared to mature lymphocytes.

Inhibition of ATM blocks the etoposide-induced DNA damage response and apoptosis of resting human T cells

It is believed that normal cells with an unaffected DNA damage response (DDR) and DNA damage repair machinery, could be less prone to DNA damaging treatment than cancer cells. However, the anticancer drug, etoposide, which is a topoisomerase II inhibitor, can generate DNA double strand breaks affecting not only replication but also transcription and therefore can induce DNA damage in non-replicating cells. Indeed, we showed that etoposide could influence transcription and was able to activate DDR in resting human T cells by inducing phosphorylation of ATM and its substrates, H2AX and p53. This led to activation of PUMA, caspases and to apoptotic cell death. Lymphoblastoid leukemic Jurkat cells, as cycling cells, were more sensitive to etoposide considering the level of DNA damage, DDR and apoptosis. Next, we used ATM inhibitor, KU 55933, which has been shown previously to be a radio/chemo-sensitizing agent. Pretreatment of resting T cells with KU 55933 blocked phosphorylation of ATM, H2AX and p53, which, in turn, prevented PUMA expression, caspase activation and apoptosis. On the other hand, KU 55933 incremented apoptosis of Jurkat cells. However, etoposide-induced DNA damage in resting T cells was not influenced by KU 55933 as revealed by the FADU assay. Altogether our results show that KU 55933 blocks DDR and apoptosis induced by etoposide in normal resting T cells, but increased cytotoxic effect on proliferating leukemic Jurkat cells. We discuss the possible beneficial and adverse effects of drugs affecting the DDR in cancer cells that are currently in preclinical anticancer trials.

In vitro radiosensitization by gold nanoparticles during continuous low-dose-rate gamma irradiation with I-125 brachytherapy seeds

This communication reports the first experimental evidence of gold nanoparticle (AuNP) radiosensitization during continuous low-dose-rate (LDR) gamma irradiation with low-energy brachytherapy sources. HeLa cell cultures incubated with and without AuNP were irradiated with an I-125 seed plaque designed to produce a relatively homogeneous dose distribution in the plane of the cell culture slide. Four sets of irradiation experiments were conducted at low-dose rates ranging from 2.1 to 4.5cGy/h. Residual γH2AX was measured 24h after irradiation and used to compare radiation damage to the cells with and without AuNP. The data demonstrate that the biological effect when irradiating in the presence of 0.2mg/ml concentration of AuNP is about 70%-130% greater than without AuNP. Meanwhile, without radiation, the AuNP showed minimal effect on the cancer cells. These findings provide in vitro evidence that AuNP may be employed as radiosensitizers during continuous LDR brachytherapy.

FROM THE CLINICAL EDITOR

In this basic science paper, the application of gold nanoparticles as radiosensitizing agents for low dose rate gamma radiation therapy is discussed, demonstrating efficacy in cell culture models.