A single formula to describe radiation-induced protein relocalization: Towards a mathematical definition of individual radiosensitivity

The impact of dose rate on responses of human lens epithelial cells to ionizing irradiation

The knowledge on responses of human lens epithelial cells (HLECs) to ionizing radiation exposure is important to understand mechanisms of radiation cataracts that are of concern in the field of radiation protection and radiation therapy. However, biological effects in HLECs following protracted exposure have not yet fully been explored. Here, we investigated the temporal kinetics of γ-H2AX foci as a marker for DNA double-strand breaks (DSBs) and cell survival in HLECs after exposure to photon beams at various dose rates (i.e., 150 kVp X-rays at 1.82, 0.1, and 0.033 Gy/min, and 137Cs γ-rays at 0.00461 Gy/min (27.7 cGy/h) and 0.00081 Gy/min (4.9 cGy/h)), compared to those in human lung fibroblasts (WI-38). In parallel, we quantified the recovery for DSBs and cell survival using a biophysical model. The study revealed that HLECs have a lower DSB repair rate than WI-38 cells. There is no significant impact of dose rate on cell survival in both cell lines in the dose-rate range of 0.033-1.82 Gy/min. In contrast, the experimental residual γ-H2AX foci showed inverse dose rate effects (IDREs) compared to the model prediction, highlighting the importance of the IDREs in evaluating radiation effects on the ocular lens.

Effect of bisphosphonates and statins on the in vitro radiosensitivity of breast cancer cell lines

BACKGROUND

Early-stage breast cancer is usually treated with breast-conserving surgery followed by adjuvant radiation therapy. Acute skin toxicity is a common radiation-induced side effect experienced by many patients. Recently, a combination of bisphosphonates (zoledronic acid) and statins (pravastatin), or ZOPRA, was shown to radio-protect normal tissues by enhancing DNA double-strand breaks (DSB) repair mechanism. However, there are no studies assessing the effect of ZOPRA on cancerous cells. The purpose of this study is to characterize the in vitro effect of the zoledronic acid (ZO), pravastatin (PRA), and ZOPRA treatment on the molecular and cellular radiosensitivity of breast cancer cell lines.

MATERIALS

Two breast cancer cell lines, MDA MB 231 and MCF-7, were tested. Cells were treated with different concentrations of pravastatin (PRA), zoledronate (ZO), as well as their ZOPRA combination, before irradiation. Anti-γH2AX and anti-pATM immunofluorescence were performed to study DNA DSB repair kinetics. MTT assay was performed to assess cell proliferation and viability, and flow cytometry was performed to analyze the effect of the drugs on the cell cycle distribution. The clonogenic assay was used to assess cell survival.

RESULTS

ZO, PRA, and ZOPRA treatments were shown to increase the residual number of γH2AX foci for both cell lines. ZOPRA treatment was also shown to reduce the activity of the ATM kinase in MCF-7. ZOPRA induced a significant decrease in cell survival for both cell lines.

CONCLUSIONS

Our findings show that pretreatment with ZOPRA can decrease the radioresistance of breast cancer cells at the molecular and cellular levels. The fact that ZOPRA was previously shown to radioprotect normal tissues, makes it a good candidate to become a therapeutic window-widening drug.

Response of Fibroblasts from Menkes' and Wilson's Copper Metabolism-Related Disorders to Ionizing Radiation: Influence of the Nucleo-Shuttling of the ATM Protein Kinase

Menkes' disease (MD) and Wilson's disease (WD) are two major copper (Cu) metabolism-related disorders caused by mutations of the ATP7A and ATP7B ATPase gene, respectively. While Cu is involved in DNA strand breaks signaling and repair, the response of cells from both diseases to ionizing radiation, a common DNA strand breaks inducer, has not been investigated yet. To this aim, three MD and two WD skin fibroblasts lines were irradiated at two Gy X-rays and clonogenic cell survival, micronuclei, anti-γH2AX, -pATM, and -MRE11 immunofluorescence assays were applied to evaluate the DNA double-strand breaks (DSB) recognition and repair. MD and WD cells appeared moderately radiosensitive with a delay in the radiation-induced ATM nucleo-shuttling (RIANS) associated with impairments in the DSB recognition. Such delayed RIANS was notably caused in both MD and WD cells by a highly expressed ATP7B protein that forms complexes with ATM monomers in cytoplasm. Interestingly, a Cu pre-treatment of cells may influence the activity of the MRE11 nuclease and modulate the radiobiological phenotype. Lastly, some high-passage MD cells cultured in routine may transform spontaneously becoming immortalized. Altogether, our findings suggest that exposure to ionizing radiation may impact on clinical features of MD and WD, which requires cautiousness when affected patients are submitted to radiodiagnosis and, eventually, radiotherapy.

The Radiobiological Characterization of Human and Porcine Lens Cells Suggests the Importance of the ATM Kinase in Radiation-Induced Cataractogenesis

Studies about radiation-induced human cataractogenesis are generally limited by (1) the poor number of epithelial lens cell lines available (likely because of the difficulties of cell sampling and amplification) and (2) the lack of reliable biomarkers of the radiation-induced aging process. We have developed a mechanistic model of the individual response to radiation based on the nucleoshuttling of the ATM protein (RIANS). Recently, in the frame of the RIANS model, we have shown that, to respond to permanent endo- and exogenous stress, the ATM protein progressively agglutinates around the nucleus attracted by overexpressed perinuclear ATM-substrate protein. As a result, perinuclear ATM crowns appear to be an interesting biomarker of aging. The radiobiological characterization of the two human epithelial lens cell lines available and the four porcine epithelial lens cell lines that we have established showed delayed RIANS. The BFSP2 protein, found specifically overexpressed around the lens cell nucleus and interacting with ATM, may be a specific ATM-substrate protein facilitating the formation of perinuclear ATM crowns in lens cells. The perinuclear ATM crowns were observed inasmuch as the number of culture passages is high. Interestingly, 2 Gy X-rays lead to the transient disappearance of the perinuclear ATM crowns. Altogether, our findings suggest a strong influence of the ATM protein in radiation-induced cataractogenesis.

Normal Tissue Toxicity Prediction: Clinical Translation on the Horizon

Improvements in radiotherapy delivery have enabled higher therapeutic doses and improved efficacy, contributing to the growing number of long-term cancer survivors. These survivors are at risk of developing late toxicity from radiotherapy, and the inability to predict who is most susceptible results in substantial impact on quality of life and limits further curative dose escalation. A predictive assay or algorithm for normal tissue radiosensitivity would allow more personalized treatment planning, reducing the burden of late toxicity, and improving the therapeutic index. Progress over the last 10 years has shown that the etiology of late clinical radiotoxicity is multifactorial and informs development of predictive models that combine information on treatment (eg, dose, adjuvant treatment), demographic and health behaviors (eg, smoking, age), co-morbidities (eg, diabetes, collagen vascular disease), and biology (eg, genetics, ex vivo functional assays). AI has emerged as a useful tool and is facilitating extraction of signal from large datasets and development of high-level multivariable models. Some models are progressing to evaluation in clinical trials, and we anticipate adoption of these into the clinical workflow in the coming years. Information on predicted risk of toxicity could prompt modification of radiotherapy delivery (eg, use of protons, altered dose and/or fractionation, reduced volume) or, in rare instances of very high predicted risk, avoidance of radiotherapy. Risk information can also be used to assist treatment decision-making for cancers where efficacy of radiotherapy is equivalent to other treatments (eg, low-risk prostate cancer) and can be used to guide follow-up screening in instances where radiotherapy is still the best choice to maximize tumor control probability. Here, we review promising predictive assays for clinical radiotoxicity and highlight studies that are progressing to develop an evidence base for clinical utility.

X-rays-Induced Bystander Effect Consists in the Formation of DNA Breaks in a Calcium-Dependent Manner: Influence of the Experimental Procedure and the Individual Factor

Radiation-induced bystander effects (RIBE) describe the biological events occurring in non-targeted cells in the vicinity of irradiated ones. Various experimental procedures have been used to investigate RIBE. Interestingly, most micro-irradiation experiments have been performed with alpha particles, whereas most medium transfers have been done with X-rays. With their high fluence, synchrotron X-rays represent a real opportunity to study RIBE by applying these two approaches with the same radiation type. The RIBE induced in human fibroblasts by the medium transfer approach resulted in a generation of DNA double-strand breaks (DSB) occurring from 10 min to 4 h post-irradiation. Such RIBE was found to be dependent on dose and on the number of donor cells. The RIBE induced with the micro-irradiation approach produced DSB with the same temporal occurrence. Culture media containing high concentrations of phosphates were found to inhibit RIBE, while media rich in calcium increased it. The contribution of the RIBE to the biological dose was evaluated after synchrotron X-rays, media transfer, micro-irradiation, and 6 MeV photon irradiation mimicking a standard radiotherapy session: the RIBE may represent less than 1%, about 5%, and about 20% of the initial dose, respectively. However, RIBE may result in beneficial or otherwise deleterious effects in surrounding tissues according to their radiosensitivity status and their capacity to release Ca2+ ions in response to radiation.

Radiation-Induced Fibrosis in Patients with Head and Neck Cancer: A Review of Pathogenesis and Clinical Outcomes

Radiotherapy-related fibrosis remains one of the most challenging treatment related side effects encountered by patients with head and neck cancer. Several established and ongoing novel therapies have been studied with paucity of data in how to best treat these patients. This review aims to provide researchers and health care providers with a comprehensive review on the presentation, etiology, and therapeutic options for this serious condition.

DNA Double-Strand Breaks Induced in Human Cells by 6 Current Pesticides: Intercomparisons and Influence of the ATM Protein

A mechanistic model from radiobiology has emerged by pointing out that the radiation-induced nucleo-shuttling of the ATM protein (RIANS) initiates the recognition, the repair of DNA double-strand breaks (DSB), and the final response to genotoxic stress. More recently, we provided evidence in this journal that the RIANS model is also relevant for exposure to metal ions. To document the role of the ATM-dependent DSB repair and signaling after pesticide exposure, we applied six current pesticides of domestic and environmental interest (lindane, atrazine, glyphosate, permethrin, pentachlorophenol and thiabendazole) to human skin fibroblast and brain cells. Our findings suggest that each pesticide tested may induce DSB at a rate that depends on the pesticide concentration and the RIANS status of cells. At specific concentration ranges, the nucleo-shuttling of ATM can be delayed, which impairs DSB recognition and repair, and contributes to toxicity. Interestingly, the combination of copper sulfate and thiabendazole or glyphosate was found to have additive or supra-additive effects on DSB recognition and/or repair. A general mechanistic model of the biological response to metal and/or pesticide is proposed to define quantitative endpoints for toxicity.

Usher Syndrome Belongs to the Genetic Diseases Associated with Radiosensitivity: Influence of the ATM Protein Kinase

Usher syndrome (USH) is a rare autosomal recessive disease characterized by the combination of hearing loss, visual impairment due to retinitis pigmentosa, and in some cases vestibular dysfunctions. Studies published in the 1980s reported that USH is associated with cellular radiosensitivity. However, the molecular basis of this particular phenotype has not yet been documented. The aim of this study was therefore to document the radiosensitivity of USH1—a subset of USH—by examining the radiation-induced nucleo-shuttling of ATM (RIANS), as well as the functionality of the repair and signaling pathways of the DNA double-strand breaks (DSBs) in three skin fibroblasts derived from USH1 patients. The clonogenic cell survival, the micronuclei, the nuclear foci formed by the phosphorylated forms of the X variant of the H2A histone (ɣH2AX), the phosphorylated forms of the ATM protein (pATM), and the meiotic recombination 11 nuclease (MRE11) were used as cellular and molecular endpoints. The interaction between the ATM and USH1 proteins was also examined by proximity ligation assay. The results showed that USH1 fibroblasts were associated with moderate but significant radiosensitivity, high yield of micronuclei, and impaired DSB recognition but normal DSB repair, likely caused by a delayed RIANS, suggesting a possible sequestration of ATM by some USH1 proteins overexpressed in the cytoplasm. To our knowledge, this report is the first radiobiological characterization of cells from USH1 patients at both molecular and cellular scales.

Avoidance or adaptation of radiotherapy in patients with cancer with Li-Fraumeni and heritable TP53-related cancer syndromes

The management of patients with cancer and Li-Fraumeni or heritable TP53-related cancer syndromes is complex because of their increased risk of developing second malignant neoplasms after genotoxic stresses such as systemic treatments or radiotherapy (radiosusceptibility). Clinical decision making also integrates the risks of normal tissue toxicity and sequelae (radiosensitivity) and tumour response to radiotherapy (radioresistance and radiocurability). Radiotherapy should be avoided in patients with cancer and Li-Fraumeni or heritable TP53 cancer-related syndromes, but overall prognosis might be poor without radiotherapy: radioresistance in these patients seems similar to or worse than that of the general population. Radiosensitivity in germline TP53 variant carriers seems similar to that in the general population. The risk of second malignant neoplasms according to germline TP53 variant and the patient's overall oncological prognosis should be assessed during specialised multidisciplinary staff meetings. Radiotherapy should be avoided whenever other similarly curative treatment options are available. In other cases, it should be adapted to minimise the risk of second malignant neoplasms in patients who still require radiotherapy despite its genotoxicity, in view of its potential benefit. Adaptations might be achieved through the reduction of irradiated volumes using proton therapy, non-ionising diagnostic procedures, image guidance, and minimal stray radiation. Non-ionising imaging should become more systematic. Radiotherapy approaches that might result in a lower probability of misrepaired DNA damage (eg, particle therapy biology and tumour targeting) are an area of investigation.

Individual Response to Radiation of Individuals with Neurofibromatosis Type I: Role of the ATM Protein and Influence of Statins and Bisphosphonates

Neurofibromatosis type 1 (NF1) is a disease characterized by high occurrence of benign and malignant brain tumours and caused by mutations of the neurofibromin protein. While there is an increasing evidence that NF1 is associated with radiosensitivity and radiosusceptibility, few studies have dealt with the molecular and cellular radiation response of cells from individuals with NF1. Here, we examined the ATM-dependent signalling and repair pathways of the DNA double-strand breaks (DSB), the key-damage induced by ionizing radiation, in skin fibroblast cell lines from 43 individuals with NF1. Ten minutes after X-rays irradiation, quiescent NF1 fibroblasts showed abnormally low rate of recognized DSB reflected by a low yield of nuclear foci formed by phosphorylated H2AX histones. Irradiated NF1 fibroblasts also presented a delayed radiation-induced nucleoshuttling of the ATM kinase (RIANS), potentially due to a specific binding of ATM to the mutated neurofibromin in cytoplasm. Lastly, NF1 fibroblasts showed abnormally high MRE11 nuclease activity suggesting a high genomic instability after irradiation. A combination of bisphosphonates and statins complemented these impairments by accelerating the RIANS, increasing the yield of recognized DSB and reducing genomic instability. Data from NF1 fibroblasts exposed to radiation in radiotherapy and CT scan conditions confirmed that NF1 belongs to the group of syndromes associated with radiosensitivity and radiosusceptibility.

Radiosensitizing Effect of Curcumin on Human Bladder Cancer Cell Lines: Impact on DNA Repair Mechanisms

Chemo-radiotherapy is one of the promising approaches to treat bladder cancer, but its effectiveness is limited to sensitive patients. Polyphenol curcumin has shown anticancer and radiosensitizing potentials, but the mechanism is not fully understood. Here, the In Vitro response of UM-UC5 and UM-UC6 bladder cell lines to curcumin and radiation treatments was evaluated. The effect of curcumin on the DNA double-strand breaks repair system after treatment with ionizing radiation (2 Gy) was determined by immunofluorescence. Cell viability, proliferation, and survival were performed using trypan blue, MTT, clonogenic, and sphere-forming assays. The migratory ability of both cells was assessed by wound healing. We showed that curcumin treatment increased the radiosensitivity by modifying the DNA double-strand breaks repair kinetics of the most radioresistant cells UM-UC6 without affecting the radiosensitive UM-UC5. Moreover, UM-UC6 cell survival and proliferation was significantly decreased after the combination of curcumin with radiation. Bladder cell migration was also inhibited considerably. Curcumin was also shown to reduce the number and the volume of bladder cancer spheres of both cell lines. This study revealed that curcumin was able to radiosensitize resistant bladder cell line without affecting the sensitive one with minimal side effects through enhancing DNA damage signaling and repair pathway.

DNA Double-Strand Breaks Induced in Human Cells by Twelve Metallic Species: Quantitative Inter-Comparisons and Influence of the ATM Protein

Despite a considerable amount of data, the molecular and cellular bases of the toxicity due to metal exposure remain unknown. Recent mechanistic models from radiobiology have emerged, pointing out that the radiation-induced nucleo-shuttling of the ATM protein (RIANS) initiates the recognition and the repair of DNA double-strand breaks (DSB) and the final response to genotoxic stress. In order to document the role of ATM-dependent DSB repair and signalling after metal exposure, we applied twelve different metal species representing nine elements (Al, Cu, Zn Ni, Pd, Cd, Pb, Cr, and Fe) to human skin, mammary, and brain cells. Our findings suggest that metals may directly or indirectly induce DSB at a rate that depends on the metal properties and concentration, and tissue type. At specific metal concentration ranges, the nucleo-shuttling of ATM can be delayed which impairs DSB recognition and repair and contributes to toxicity and carcinogenicity. Interestingly, as observed after low doses of ionizing radiation, some phenomena equivalent to the biological response observed at high metal concentrations may occur at lower concentrations. A general mechanistic model of the biological response to metal exposure based on the nucleo-shuttling of ATM is proposed to describe the metal-induced stress response and to define quantitative endpoints for toxicity and carcinogenicity.

Exposure of the cytoplasm to low-dose X-rays modifies ataxia telangiectasia mutated-mediated DNA damage responses

We recently showed that when a low X-ray dose is used, cell death is enhanced in nucleus-irradiated compared with whole-cell-irradiated cells; however, the role of the cytoplasm remains unclear. Here, we show changes in the DNA damage responses with or without X-ray microbeam irradiation of the cytoplasm. Phosphorylated histone H2AX foci, a surrogate marker for DNA double-strand breaks, in V79 and WI-38 cells are not observed in nucleus irradiations at ≤ 2 Gy, whereas they are observed in whole-cell irradiations. Addition of an ataxia telangiectasia mutated (ATM) kinase inhibitor to whole-cell irradiations suppresses foci formation at ≤ 2 Gy. ABL1 and p73 expression is upregulated following nucleus irradiation, suggesting the induction of p73-dependent cell death. Furthermore, CDKN1A (p21) is upregulated following whole-cell irradiation, indicating the induction of cell cycle arrest. These data reveal that cytoplasmic radioresponses modify ATM-mediated DNA damage responses and determine the fate of cells irradiated at low doses.

First radiobiological characterization of the McCune-Albright syndrome: influence of the ATM protein and effect of statins + bisphosphonates treatment

PURPOSE

MacCune-Albright syndrome (MAS) is a rare autosomal dominant osteo-hormonal disorder. MAS is characterized by a severe form of polyostotic fibrous dysplasia, 'café-au-lait' pigmentation of the skin and multiple endocrinopathies. MAS was shown to be caused by mosaic missense somatic mutations in the GNAS gene coding for the alpha-subunit of the stimulatory G-protein. MAS is also associated with radiation-induced malignant tumors, like osteosarcoma, fibrosarcoma and chondrosarcoma but their origin remains misunderstood. In parallel, bisphosphonates treatment was shown to improve the MAS patients' outcome, notably by increasing bone density but, again, the molecular mechanisms supporting these observations remain misunderstood.

MATERIALS AND METHODS

Here, by using fibroblast and osteoblast cell lines derived from 2 MAS patients, the major radiobiological features of MAS were investigated. Notably, the clonogenic cell survival, the micronuclei and the γH2AX, pATM and MRE11 immunofluorescence assays were applied to MAS cells.

RESULTS

It appears that cells from the 2 MAS patients are associated with a moderate but significant radiosensitivity, a delayed radiation-induced nucleoshuttling of the ATM kinase likely caused by its sequestration in cytoplasm, suggesting impaired DNA double-strand breaks (DSB) repair and signaling in both fibroblasts and osteoblasts. Such delay may be partially corrected by using bisphosphonates combined with statins, which renders cells more radioresistant.

CONCLUSIONS

Our findings represent the first radiobiological characterization of fibroblasts and osteoblasts providing from MAS patients. Although the number of studied cases is reduced, our findings suggest that the MAS cells tested belong to the group of syndromes associated with moderate but significant radiosensitivity. Further investigations are however required to secure the clinical transfer of the combination of bisphosphonates and statins, to reduce the disease progression and to better evaluate the potential risks linked to radiation exposure.

Fibroblasts from Retinoblastoma Patients Show Radiosensitivity Linked to Abnormal Localization of the ATM Protein

PURPOSE/AIM OF THE STUDY

Retinoblastoma (Rb) is a rare form of pediatric cancer that develops from retina cells. Bilateral and some unilateral forms of Rb are associated with heterozygous germline mutations of the (retinoblastoma 1) RB1 gene. RB1 mutations are also associated with a significant risk of secondary malignancy like head and neck tumors. Hence, to date, even if Rb patients are less subjected to radiotherapy to treat their primary ocular tumors, their healthy tissues may be exposed to significant doses of ionizing radiation during the treatment against their secondary malignancies with a significant risk of adverse tissue reactions (radiosensitivity) and/or radiation-induced cancer (radiosusceptibility). However, the biological role of the Rb protein in response to radiation remains misunderstood. Since the ataxia telangiectasia mutated (ATM) protein is a key protein of radiation response and since untransformed skin fibroblasts are a current model to quantify cellular radiosensitivity, we investigated here for the first time the functionality of the ATM-dependent signaling and repair pathway of the radiation-induced DNA double-strand breaks (DSB) in irradiated skin fibroblasts derived from Rb patients.

MATERIALS AND METHODS

The major biomarkers of the DSB repair and signaling, namely clonogenic cell survival, micronuclei, nuclear foci of the phosphorylated forms of the X variant of the H2A histone (γH2AX), the phosphorylated forms of the ATM protein (pATM) and the meiotic recombination 11 nuclease (MRE11) were assessed in untransformed skin fibroblasts derived from three Rb patients.

RESULTS

Skin fibroblasts from Rb patients showed significant cellular radiosensitivity, incomplete DSB recognition, delay in the ATM nucleo-shuttling and exacerbated MRE11 nuclease activity. Treatment with statin and bisphosphonates led to significant complementation of these impairments.

CONCLUSIONS

Our findings strongly suggest the involvement of the ATM kinase in the radiosensitivity/radiosusceptibility phenotype observed in Rb cases.

Modulation of DNA Damage Response by Sphingolipid Signaling: An Interplay that Shapes Cell Fate

Although once considered as structural components of eukaryotic biological membranes, research in the past few decades hints at a major role of bioactive sphingolipids in mediating an array of physiological processes including cell survival, proliferation, inflammation, senescence, and death. A large body of evidence points to a fundamental role for the sphingolipid metabolic pathway in modulating the DNA damage response (DDR). The interplay between these two elements of cell signaling determines cell fate when cells are exposed to metabolic stress or ionizing radiation among other genotoxic agents. In this review, we aim to dissect the mediators of the DDR and how these interact with the different sphingolipid metabolites to mount various cellular responses.

Some mutations in the xeroderma pigmentosum D gene may lead to moderate but significant radiosensitivity associated with a delayed radiation-induced ATM nuclear localization

Purpose: Xeroderma Pigmentosum (XP) is a rare, recessive genetic disease associated with photosensitivity, skin cancer proneness, neurological abnormalities and impaired nucleotide excision repair of the UV-induced DNA damage. Less frequently, XP can be associated with sensitivity to ionizing radiation (IR). Here, a complete radiobiological characterization was performed on a panel of fibroblasts derived from XP-group D patients (XPD).Materials and methods: Cellular radiosensitivity and the functionality of the recognition and repair of chromosome breaks and DNA double-strand breaks (DSB) was evaluated by different techniques including clonogenic cell survival, micronuclei, premature chromosome condensation, pulsed-field gel electrophoresis, chromatin decondensation and immunofluorescence assays. Quantitative correlations between each endpoint were analyzed systematically.Results: Among the seven fibroblast cell lines tested, those derived from three non-relative patients holding the p.[Arg683Trp];[Arg616Pro] XPD mutations showed significant cellular radiosensitivity, high yield of residual micronuclei, incomplete DSB recognition, DSB and chromosome repair defects, impaired ATM, MRE11 relocalization, significant chromatin decondensation. Interestingly, XPD transduction and treatment with statins and bisphosphonates known to accelerate the radiation-induced ATM nucleoshuttling led to significant complementation of these impairments.Conclusions: Our findings suggest that some subsets of XPD patients may be at risk of radiosensitivity reactions and treatment with statins and bisphosphonates may be an interesting approach of radioprotection countermeasure. Different mechanistic models were discussed to better understand the potential specificity of the p.[Arg683Trp];[Arg616Pro] XPD mutations.

What Does the History of Research on the Repair of DNA Double-Strand Breaks Tell Us?-A Comprehensive Review of Human Radiosensitivity

Our understanding of the molecular and cellular response to ionizing radiation (IR) has progressed considerably. This is notably the case for the repair and signaling of DNA double-strand breaks (DSB) that, if unrepaired, can result in cell lethality, or if misrepaired, can cause cancer. However, through the different protocols, techniques, and cellular models used during the last four decades, the DSB repair kinetics and the relationship between cellular radiosensitivity and unrepaired DSB has varied drastically, moving from all-or-none phenomena to very complex mechanistic models. To date, personalized medicine has required a reliable evaluation of the IR-induced risks that have become a medical, scientific, and societal issue. However, the molecular bases of the individual response to IR are still unclear: there is a gap between the moderate radiosensitivity frequently observed in clinic but poorly investigated in the publications and the hyper-radiosensitivity of rare but well-characterized genetic diseases frequently cited in the mechanistic models. This paper makes a comprehensive review of semantic issues, correlations between cellular radiosensitivity and unrepaired DSB, shapes of DSB repair curves, and DSB repair biomarkers in order to propose a new vision of the individual response to IR that would be more coherent with clinical reality.

The Nucleoshuttling of the ATM Protein: A Unified Model to Describe the Individual Response to High- and Low-Dose of Radiation?

The evaluation of radiation-induced (RI) risks is of medical, scientific, and societal interest. However, despite considerable efforts, there is neither consensual mechanistic models nor predictive assays for describing the three major RI effects, namely radiosensitivity, radiosusceptibility, and radiodegeneration. Interestingly, the ataxia telangiectasia mutated (ATM) protein is a major stress response factor involved in the DNA repair and signaling that appears upstream most of pathways involved in the three precited RI effects. The rate of the RI ATM nucleoshuttling (RIANS) was shown to be a good predictor of radiosensitivity. In the frame of the RIANS model, irradiation triggers the monomerization of cytoplasmic ATM dimers, which allows ATM monomers to diffuse in nucleus. The nuclear ATM monomers phosphorylate the H2AX histones, which triggers the recognition of DNA double-strand breaks and their repair. The RIANS model has made it possible to define three subgroups of radiosensitivity and provided a relevant explanation for the radiosensitivity observed in syndromes caused by mutated cytoplasmic proteins. Interestingly, hyper-radiosensitivity to a low dose and adaptive response phenomena may be also explained by the RIANS model. In this review, the relevance of the RIANS model to describe several features of the individual response to radiation was discussed.

Radiation adaptive response and cancer: From the statistical physics point of view

Elements of statistical physics formalism were applied to mutagenic and carcinogenic processes associated with cellular DNA; these are lesion (damage) creation, mutation creation, and cellular neoplastic (cancer) transformation. The probabilities of all state changes were strictly related to potential barrier heights between energetic states of DNA molecules. Barriers can be modified when radiation adaptive response mechanisms are applied, which are associated with a radiobiological quantity called radiosensitivity. It was discussed that radiosensitivity is determined by the cell's response to radiation resulting in three potential dose-response scenarios: linear, threshold, or hormetic. The type of dose-response is of critical importance in the development of radiation protection standards and individual radiation risk assessment. It is shown that the different scenarios describe different limits of the same underlying phenomena and the cell can respond in a linear, threshold, or hormetic way regarding its radiosensitivity. Finally, the dissipative adaptation mechanism is discussed in the context of proliferating cancer cells.

Influence of Linear Energy Transfer on the Nucleo-shuttling of the ATM Protein: A Novel Biological Interpretation Relevant for Particles and Radiation

PURPOSE

Linear energy transfer (LET) plays an important role in radiation response. Recently, the radiation-induced nucleo-shuttling of ATM from cytoplasm to the nucleus was shown to be a major event of the radiation response that permits a normal DNA double-strand break (DSB) recognition and repair. Here, we aimed to verify the relevance of the ATM nucleo-shuttling model for high-LET particles and various radiation types.

METHODS AND MATERIALS

ATM- and H2AX-immunofluorescence was used to assess the number of recognized and unrepaired DSB in quiescent fibroblast cell lines exposed to x-rays, γ-rays, 9- and 12-MeV electrons, 3- and 65-MeV protons and 75-MeV/u carbon ions.

RESULTS

The rate of radiation-induced ATM nucleo-shuttling was found to be specific to each radiation type tested. By increasing the permeability of the nuclear membrane with statin and bisphosphonates, 2 fibroblast cell lines exposed to high-LET particles were shown to be protected by an accelerated ATM nucleo-shuttling.

CONCLUSIONS

Our findings are in agreement with the conclusion that LET and the radiation/particle type influence the formation of ATM monomers in cytoplasm that are required for DSB recognition. A striking analogy was established between the DSB repair kinetics of radioresistant cells exposed to high-LET particles and that of several radiosensitive cells exposed to low-LET radiation. Our data show that the nucleo-shuttling of ATM provides crucial elements to predict radiation response in human quiescent cells, whatever the LET value and their radiosensitivity.

Ionizing radiation-induced cellular senescence promotes tissue fibrosis after radiotherapy. A review

Ionizing radiation-exposure induces a variety of cellular reactions, such as senescence and apoptosis. Senescence is a permanent arrest state of the cell division, which can be beneficial or detrimental for normal tissue via an inflammatory response and senescence-associated secretion phenotype. Damage to healthy cells and their microenvironment is considered as an important source of early and late complications with an increased risk of morbidity in patients after radiotherapy (RT). In addition, the benefit/risk ratio may depend on the radiation technique/dose used for cancer eradication and the irradiated volume of healthy tissues. For radiation-induced fibrosis risk, the knowledge of mechanisms and potential prevention has become a crucial point to determining radiation parameters and patients' intrinsic radiosensitivity. This review summarizes our understanding of ionizing radiation-induced senescent cell in fibrogenesis. This mechanism may provide new insights for therapeutic modalities for better risk/benefit ratios after RT in the new era of personalized treatments.

The Phosphorylated ATM Immunofluorescence Assay: A High-performance Radiosensitivity Assay to Predict Postradiation Therapy Overreactions

PURPOSE

The ability to identify, before treatment, those patients who will overreact to radiation therapy would have sound positive clinical implications. By focusing on DNA double-strand breaks recognition and repair proteins after irradiation, we recently demonstrated that the maximal number of phosphorylated ATM (pATM) nuclear foci in the first hour (pATMmax) after ex vivo irradiation correlated with postradiation therapy toxicity severity. We performed additional analyses of our whole collection of fibroblast lines to refine the predictive performance of our assay.

METHODS AND MATERIALS

Immunofluorescence experiments were performed on 117 primary skin fibroblast lines irradiated at 2 Gy. The toxicity response was split into 2 binary classes: 0 if the toxicity grade was <2 and 1 otherwise. To assess the relationship between the quantity of pATMmax foci and toxicity grade, we applied a correlation and then a supervised classification analysis. Training data sets from 13 radiosensitive patients randomly drawn using a random undersampling technique were constituted. Receiver operating characteristic analyses were performed using a Monte-Carlo method to estimate the optimal threshold and discriminate the responses for each data set. The discrimination cutoff was estimated as the maximum value of the 10⁴ thresholds computed from each training subset.

RESULTS

As expected, we confirmed a quasi-linear dependence between toxicity and pATMmax (Pearson correlation coefficient -0.85; P < 2.2e^-16). When taken as a binary predictive assay with the optimal cutoff value of 34.5 pATM foci/cell, our assay showed outstanding predictive performance (sensitivity, specificity, negative predictive value, positive predictive value, and area under the curve: 100%, 92%, 100%, 99%, and 0.987, respectively).

CONCLUSIONS

The results of these experiments allowed us to identify pATMmax as a high-performance predictive parameter of patients with postradiation therapy overreactions. Additional studies are in progress to confirm that this radiosensitivity assay reaches the same performance level in any condition to adapt clinical practice.

Radiobiological Characterization of Tuberous Sclerosis: a Delay in the Nucleo-Shuttling of ATM May Be Responsible for Radiosensitivity

The tuberous sclerosis complex (TSC) syndrome is associated with numerous cutaneous pathologies (notably on the face), epilepsy, intellectual disability and developmental retardation and, overall, high occurrence of benign tumors in several organs, like angiofibromas, giant cell astrocytomas, renal angiomyolipomas, and pulmonary lymphangioleiomyomatosis. TSC is caused by mutations of either of the hamartin or tuberin proteins that are mainly cytoplasmic. Some studies published in the 1980s reported that TSC is associated with radiosensitivity. However, its molecular basis in TSC cells is not documented enough. Here, we examined the functionality of the repair and signaling of radiation-induced DNA double-strand breaks (DSB) in fibroblasts derived from TSC patients. Quiescent TSC fibroblast cells elicited abnormally low rate of recognized DSB reflected by a low yield of nuclear foci formed by phosphorylated H2AX histones. Irradiated TSC cells also presented a delay in the nucleo-shuttling of the ATM kinase, potentially due to a specific binding of ATM to mutated TSC protein in cytoplasm. Lastly, TSC fibroblasts showed abnormally high MRE11 nuclease activity suggesting genomic instability. A combination of biphosphonates and statins complemented these impairments by facilitating the nucleoshuttling of ATM and increasing the yield of recognized DSB. Our results showed that TSC belongs to the group of syndromes associated with low but significant defect of DSB signaling and delay in the ATM nucleo-shuttling associated with radiosensitivity.

Individual response to ionizing radiation

The human response to ionizing radiation (IR) varies among individuals. The first evidence of the individual response to IR was reported in the beginning of the 20th century. Considering nearly one century of observations, we here propose three aspects of individual IR response: radiosensitivity for early or late adverse tissue events after radiotherapy on normal tissues (non-cancer effects attributable to cell death); radiosusceptibility for IR-induced cancers; and radiodegeneration for non-cancer effects that are often attributable to mechanisms other than cell death (e.g., cataracts and circulatory disease). All the molecular and cellular mechanisms behind IR-induced individual effects are not fully elucidated. However, some specific assays may help their quantification according to the dose and to the genetic status. Accumulated data on individual factors have suggested that the individual IR response cannot be ignored and raises some clinical and societal issues. The individual IR response therefore needs to be taken into account to better evaluate the risks related to IR exposure.

The nucleo-shuttling of the ATM protein as a basis for a novel theory of radiation response: resolution of the linear-quadratic model

PURPOSE

For 50 years, cellular radiosensitivity has been defined in vitro as the lack of clonogenic capacity of irradiated cells and its mathematical link with dose has been described by the target theory. Among the numerous formulas provided from the target theory, the linear-quadratic (LQ) model empirically describes cell survival as a negative exponential of a second degree polynomial dose-function in which αD is the linear component and βD(2) is the quadratic one. The LQ model is extensively used in radiobiology (to describe survival curves) and in radiotherapy (the α/β ratio indicates whether tissue reactions can occur early or late after the treatment). However, no biological interpretation of the LQ parameters was proposed to explain together the radiation response in a wide dose range, the radiosensitivity of some genetic syndromes caused by the mutation of cytoplasmic proteins and the hyper-radiosensitivity phenomenon specific to low-dose.

THE MODEL

From a solid amount of experimental data, we hypothesized that the major forms of ataxia telangiectasia mutated (ATM) are cytoplasmic dimers and that ionizing radiation induce ATM monomerization. The resulting ATM monomers diffuse into nucleus to facilitate double-strand-breaks (DSB) recognition and repair. Such hypotheses lead to a coherent molecular interpretation of the LQ model by considering the yield of recognized but unrepaired (α-type) DSB and the non-recognized (β-type) DSB. The notion of cell tolerance to unrepaired DSB was introduced by considering that not all DSB are lethal. Cell survival and DSB repair and signaling immunofluorescence data from 42 normal skin fibroblast and 18 tumor human cell lines were used to verify the validity of this biomathematical model proposed.

RESULTS

Our model is validated at different levels by one of the widest spectrum of radiosensitivity. That mathematical developments of the present model imply that β is a Lorentzian function of α was confirmed experimentally. Our model is also relevant to describe the hypersensitivity to low-dose phenomenon.

CONCLUSIONS

Our model provides a very general picture of human radiosensitivity, independently of the dose, the cell type and the genetic status.

Huntington Disease: A Disease of DNA Methylation or DNA Breaks?

This commentary highlights the article by Mollica et al that describes an interesting model for the clinical evolution of Huntington disease.

[Repeated radiation dose effect and DNA repair: Importance of the individual factor and the time interval between the doses]

The dose fractionation effect is a recurrent question of radiation biology research that remains unsolved since no model predicts the clinical effect only with the cumulated dose and the radiobiology of irradiated tissues. Such an important question is differentially answered in radioprotection, radiotherapy, radiology or epidemiology. A better understanding of the molecular response to radiation makes possible today a novel approach to identify the parameters that condition the fractionation effect. Particularly, the time between doses appears to be a key factor since it will permit, or not, the repair of certain radiation-induced DNA damages whose repair rates are of the order of seconds, minutes or hours: the fractionation effect will therefore vary according to the functionality of the different repair pathways, whatever for tumor or normal tissues.

Breast Cancer Induced by X-Ray Mammography Screening? A Review Based on Recent Understanding of Low-Dose Radiobiology

Screening mammography offers the possibility of discovering malignant diseases at an early stage, which is consequently treated early, thereby reducing the mortality rate. However, ionizing radiation as used in low-dose X-ray mammography may be associated with a risk of radiation-induced carcinogenesis. In the context of the harmful effects of ionizing radiation, this article reviewed novel radiobiological data and provided a simulation of the relative incidence of radiation-induced breast cancer due to screening against a background baseline incidence in a population of 100,000 individuals. The use of modern digital mammographic technology was assumed, giving rise to a glandular dose of 2.5 mGy from a 2-view per breast image. Assuming no latency time, this led to a ratio of induced incidence rate over baseline incidence rate of about 1.6‰ for biennial screening in women aged 50-74 years, although it cannot be excluded that the dose and dose rate effectiveness factor values relying on new radiobiological insights may lower this number to about 0.7‰. This carcinogenic risk is considered small in relation to the potential beneficial effects of screening, especially as latency time was not taken into consideration. However, individuals who are known to be carriers of risk-increasing genetic variations and/or have an inherited disposition of breast cancer should avoid ionizing radiation as much as possible and should be referred to ultrasound or magnetic resonance imaging. In addition, a significant, but difficult to quantify, risk of cancer is present for individuals who suffer from hypersusceptibility to ionizing radiation.

DNA double-strand break repair: a theoretical framework and its application

DNA double-strand breaks (DSBs) are formed as a result of genotoxic insults, such as exogenous ionizing radiation, and are among the most serious types of DNA damage. One of the earliest molecular responses following DSB formation is the phosphorylation of the histone H2AX, giving rise to γH2AX. Many copies of γH2AX are generated at DSBs and can be detected in vitro as foci using well-established immuno-histochemical methods. It has previously been shown that anti-γH2AX antibodies, modified by the addition of the cell-penetrating peptide TAT and a fluorescent or radionuclide label, can be used to visualize and quantify DSBs in vivo. Moreover, when labelled with a high amount of the short-range, Auger electron-emitting radioisotope, (111)In, the amount of DNA damage within a cell can be increased, leading to cell death. In this report, we develop a mathematical model that describes how molecular processes at individual sites of DNA damage give rise to quantifiable foci. Equations that describe stochastic mean behaviours at individual DSB sites are derived and parametrized using population-scale, time-series measurements from two different cancer cell lines. The model is used to examine two case studies in which the introduction of an antibody (anti-γH2AX-TAT) that targets a key component in the DSB repair pathway influences system behaviour. We investigate: (i) how the interaction between anti-γH2AX-TAT and γH2AX effects the kinetics of H2AX phosphorylation and DSB repair and (ii) model behaviour when the anti-γH2AX antibody is labelled with Auger electron-emitting (111)In and can thus instigate additional DNA damage. This work supports the conclusion that DSB kinetics are largely unaffected by the introduction of the anti-γH2AX antibody, a result that has been validated experimentally, and hence the hypothesis that the use of anti-γH2AX antibody to quantify DSBs does not violate the image tracer principle. Moreover, it provides a novel model of DNA damage accumulation in the presence of Auger electron-emitting (111)In that is supported qualitatively by the available experimental data.

Chromatin structure and radiation-induced DNA damage: from structural biology to radiobiology

Genomic DNA in eukaryotic cells is basically divided into chromosomes, each consisting of a single huge nucleosomal fiber. It is now clear that chromatin structure and dynamics play a critical role in all processes involved in DNA metabolism, e.g. replication, transcription, repair and recombination. Radiation is a useful tool to study the biological effects of chromatin alterations. Conversely, radiotherapy and radiodiagnosis raise questions about the influence of chromatin integrity on clinical features and secondary effects. This review focuses on the link between DNA damage and chromatin structure at different scales, showing how a comprehensive multiscale vision is required to understand better the effect of radiations on DNA. Clinical aspects related to high- and low-dose of radiation and chromosomal instability will be discussed. At the same time, we will show that the analysis of the radiation-induced DNA damage distribution provides good insight on chromatin structure. Hence, we argue that chromatin "structuralists" and radiobiological "clinicians" would each benefit from more collaboration with the other. We hope that this focused review will help in this regard.

Mutations of the Huntington's disease protein impact on the ATM-dependent signaling and repair pathways of the radiation-induced DNA double-strand breaks: corrective effect of statins and bisphosphonates

Huntington's disease (HD) is a neurodegenerative syndrome caused by mutations of the IT15 gene encoding for the huntingtin protein. Some research groups have previously shown that HD is associated with cellular radiosensitivity in quiescent cells. However, there is still no mechanistic model explaining such specific clinical feature. Here, we examined the ATM-dependent signaling and repair pathways of the DNA double-strand breaks (DSB), the key damage induced by ionizing radiation, in human HD skin fibroblasts. Early after irradiation, quiescent HD fibroblasts showed an abnormally low rate of recognized DSB managed by non-homologous end-joining reflected by a low yield of nuclear foci formed by phosphorylated H2AX histones and by 53BP1 protein. Furthermore, HD cells elicited a significant but moderate yield of unrepaired DSB 24 h after irradiation. Irradiated HD cells also presented a delayed nucleo-shuttling of phosphorylated forms of the ATM kinase, potentially due to a specific binding of ATM to mutated huntingtin in the cytoplasm. Our results suggest that HD belongs to the group of syndromes associated with a low but significant defect of DSB signaling and repair defect associated with radiosensitivity. A combination of biphosphonates and statins complements these impairments by facilitating the nucleo-shuttling of ATM, increasing the yield of recognized and repaired DSB.