An optimized method for detecting gamma-H2AX in blood cells reveals a significant interindividual variation in the gamma-H2AX response among humans

Genotoxicity of pesticide mixtures present in the diet of the French population

Consumers may be simultaneously exposed to several pesticide residues in their diet. A previous study identified the seven most common pesticide mixtures to which the French population was exposed through food consumption in 2006. The aim of this study was to investigate if the seven mixtures are potentially cytotoxic and genotoxic and if so, whether compounds in a same mixture have a combined effect. The cytotoxicity and genotoxicity of the seven mixtures were investigated with a new assay (γ-H2AX) using four human cell lines (ACHN, SH-SY5Y, LS-174T, and HepG2). Mixtures were tested at equimolar concentrations and also at concentrations reflecting their actual proportion in the diet. Irrespective of the cell line tested, parallel cytotoxicity of the seven mixtures was observed. Only one mixture was genotoxic for the HepG2 cells at concentrations = 3 μM in equimolar proportion and at 30 μM in actual proportion. Caspase 3/7 activity, the comet assay, and reactive oxygen species production were also investigated using the same mixture and HepG2 cells. Our results suggest that pesticide metabolites from the mixture generated by HepG2 cells were responsible for the observed damage to DNA. Among the five compounds in the genotoxic mixture, only fludioxonil and cyprodinil were genotoxic for HepG2 cells alone at concentrations = 4 and 20 μM, respectively. Our data suggest a combined genotoxic effect of the mixture at low concentrations with a significantly higher effect of the mixture of pesticides than would be expected from the response to the individual compounds. Environ. Mol. Mutagen. 2012. © 2012 Wiley Periodicals, Inc.

The activation of DNA damage detection and repair responses in cleavage-stage rat embryos by a damaged paternal genome

Male germ cell DNA damage, after exposure to radiation, exogenous chemicals, or chemotherapeutic agents, is a major cause of male infertility. DNA-damaged spermatozoa can fertilize oocytes; this is of concern because there is limited information on the capacity of early embryos to repair a damaged male genome or on the fate of these embryos if repair is inadequate. We hypothesized that the early activation of DNA damage response in the early embryo is a critical determinant of its fate. The objective of this study was to assess the DNA damage response and mitochondrial function as a measure of the energy supply for DNA repair and general health in cleavage-stage embryos sired by males chronically exposed to an anticancer alkylating agent, cyclophosphamide. Male rats were treated with saline or cyclophosphamide (6 mg/kg/day) for 4 weeks and mated to naturally cycling females. Pronuclear two- and eight-cell embryos were collected for immunofluorescence analysis of mitochondrial function and biomarkers of the DNA damage response: γH2AX foci, 53BP1 reactivity, and poly(ADP-ribose) polymer formation. Mitochondrial activities did not differ between embryos sired by control- and cyclophosphamide-exposed males. At the two-cell stage, there was no treatment-related increase in DNA double-strand breaks; by the eight-cell stage, a significant increase was noted, as indicated by increased medium and large γH2AX foci. This was accompanied by a dampened DNA repair response, detected as a decrease in the nuclear intensity of poly(ADP-ribose) polymers. The micronuclei formed in cyclophosphamide-sired embryos contained large γH2AX foci and enhanced poly(ADP-ribose) polymer and 53BP1 reactivity compared with their nuclear counterparts. Thus, paternal cyclophosphamide exposure activated a DNA damage response in cleavage-stage embryos. Furthermore, this damage response may be useful in assessing embryo quality and developmental competence.

Novel pathway of centrosome amplification that does not require DNA lesions

Centrosome amplification (also known as centrosome overduplication) is common in cancer cells and can be induced by DNA damaging agents. However, the mechanism and significance of centrosome amplification during carcinogenesis or after DNA damage are not clear. Previously, we showed that centrosome amplification could be induced by 3-aminobenzamide (3-AB), an inhibitor of poly(ADP-ribose) polymerases (PARPs) in mouse embryonic fibroblasts. In this paper, we determined if the effect of 3-AB on centrosome amplification was dependent on DNA damage in CHO-K1 cells. We used the well-known mutagen/carcinogen N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Ten micromolar MNNG and 10 mM 3-AB induced significant centrosome amplification in 18.1 ± 1.1% and 19.4 ± 1.8% of CHO-K1 cells, respectively, compared to 7.0 ± 0.5% of untreated CHO-K1 cells. AG14361, another potent inhibitor of PARPs, also induced centrosome amplification. We then used γ-H2AX analysis and alkaline comet assays to show that 10 μM MNNG induced a significant number of DNA lesions and cell cycle arrest at the G(2) /M phase. However, 10 mM 3-AB neither induced DNA lesions nor altered cell cycle progression. In the umu test, 10 μM MNNG was mutagenic, but 10 mM 3-AB was not. In addition, 10 μM MNNG induced significant accumulation of ataxia telangiectasia mutated protein in the nuclei, but 10 mM 3-AB did not. Thus, we found no association between apparent DNA lesions and centrosome amplification after 3-AB treatment. Therefore, we propose the presence of a novel pathway for centrosome amplification that does not necessarily require DNA lesions but involves regulation of epigenetic changes or post-translational modifications including polyADP-ribosylation.

Search for clastogenic factors in the plasma of locally irradiated individuals

In studies reported in the 1960s and in several investigations since, plasma from irradiated individuals was shown to induce chromosomal aberrations when transferred into normal blood cultures. In the present study, the aim was to investigate the occurrence of these clastogenic factors (CF) using markers representing DNA damage produced in reporter lymphocytes that are treated with plasma from locally exposed individuals. Blood plasma was obtained from clinical patients with benign conditions before and after they had received radiation to small treatment volumes. Three patient groups were studied: (I) marginal resected basal cell carcinoma, (II) painful osteoarthritis of the knee, and (III) painful tendinitis of the elbow or the heel. Patients in each treatment group obtained the same fractionated treatment regimen, ranging from a total dose of 40 Gy (8 × 5 Gy, 2 factions/week) to a very small volume (1-3.5 cm³) in group I to a total dose of 6 Gy (6 × 1 Gy, 2 fractions/week) for groups II and III (treatment volumes 800-1150 cm³ and 80-160 cm³, respectively). The presence of CF in the plasma was investigated through cytogenetic (chromosomal aberrations, micronuclei) assays and kinetics of early DNA damage (γ-H2AX foci) in reporter cells. With the experimental settings applied, local radiation exposure had no apparent effect on the induction of CF in patient plasma; no deviations in chromosomal aberrations or micronucleus or focus induction were observed in reporter cells treated with postexposure plasma with respect to pre-exposure samples when the mean values of the groups were compared. However, there was a large interindividual variation in the plasma-induced DNA-damaging effects. Steroid treatment of patients was demonstrated to be the most influential factor affecting the occurrence of plasma factors; plasma from patients treated with steroids led to significant reductions of γ-H2AX foci and reduced numbers of chromatid aberrations in reporter cells. In addition to the locally exposed patients, newly obtained plasma samples from three radiological accident victims exposed in 1994 were examined. In contrast to the patient data, a significant increase in chromosomal aberrations was induced with plasma from two accident victims.

Gamma-H2AX-based dose estimation for whole and partial body radiation exposure

Most human exposures to ionising radiation are partial body exposures. However, to date only limited tools are available for rapid and accurate estimation of the dose distribution and the extent of the body spared from the exposure. These parameters are of great importance for emergency triage and clinical management of exposed individuals. Here, measurements of γ-H2AX immunofluorescence by microscopy and flow cytometry were compared as rapid biodosimetric tools for whole and partial body exposures. Ex vivo uniformly X-irradiated blood lymphocytes from one donor were used to generate a universal biexponential calibration function for γ-H2AX foci/intensity yields per unit dose for time points up to 96 hours post exposure. Foci – but not intensity – levels remained significantly above background for 96 hours for doses of 0.5 Gy or more. Foci-based dose estimates for ex vivo X-irradiated blood samples from 13 volunteers were in excellent agreement with the actual dose delivered to the targeted samples. Flow cytometric dose estimates for X-irradiated blood samples from 8 volunteers were in excellent agreement with the actual dose delivered at 1 hour post exposure but less so at 24 hours post exposure. In partial body exposures, simulated by mixing ex vivo irradiated and unirradiated lymphocytes, foci/intensity distributions were significantly over-dispersed compared to uniformly irradiated lymphocytes. For both methods and in all cases the estimated fraction of irradiated lymphocytes and dose to that fraction, calculated using the zero contaminated Poisson test and γ-H2AX calibration function, were in good agreement with the actual mixing ratios and doses delivered to the samples. In conclusion, γ-H2AX analysis of irradiated lymphocytes enables rapid and accurate assessment of whole body doses while dispersion analysis of foci or intensity distributions helps determine partial body doses and the irradiated fraction size in cases of partial body exposures.

In-solution staining and arraying method for the immunofluorescence detection of γH2AX foci optimized for clinical applications

Immunofluorescence quantification of γH2AX foci is a powerful approach to quantify DNA double-strand breaks induced by cancer therapy or accidental exposure to ionizing radiation. Here we report a modification to the γH2AX immunofluorescence labeling method, whereby cells are stained in-solution before being spotted and fixed onto microscope slides. Our modified method allows arraying of 16 patient samples/slide ready for foci counting in 2 h and demonstrated reliably detection of γH2AX foci in mononuclear cells prepared from patients who had undergone radiation therapy.

Priming and potentiation of DNA damage response by fibronectin in human colon cancer cells and tumor-derived myofibroblasts

We have previously shown that the genotoxin-induced apoptosis in mouse embryo fibroblasts was enhanced by the extracellular matrix protein fibronectin (FN). In the present study, we tested the hypothesis that FN regulates the DNA damage response (DDR) signaling pathways in HCT116 (p53-wt) and HT29 (p53-mut) human colon cancer cells and tumor-derived myofibroblasts. DNA damage recognition mechanisms were analyzed by immunofluorescence staining, cell cycle analysis and immunoblotting addressed at specific molecular sensors and executors involved in the DDR pathways. The results show that FN, but not collagen type IV or Matrigel, initiates and potentiates the DDR to the anticancer drug cisplatin in an α5 integrin and cell cycle-dependent manner (S and G2/M phases) in human colon cancer cells. Accordingly, we demonstrate that adhesion of HCT116 cells to FN upregulated the expression of α5 integrin FN receptors at the cell surface. These FN-induced DDR pathways include the concerted phosphorylation of histone H2AX on Ser139 detected as nuclear foci (γ-H2AX, 15 and 25 kDa forms), of ataxia telangiectasia mutated (ATM-Ser1981), checkpoint kinase 2 (CHK2-Thr68, 62 and 67 kDa) and p53-Ser15. These FN-induced γ-H2AX signals were interrupted or attenuated by selective inhibitors acting on the DDR pathway kinases, including wortmannin (targeting the phosphatidylinositol-3-kinase-related protein kinases, PIKK), KU55933 (ATM), NU7026 (DNA-dependent protein kinase catalytic subunit, DNA-PK-cs) and SP600125 (JNK2, stress activated protein kinase/c-Jun N-terminal kinase-2). Adhesion to FN also potentiated the γ-H2AX signals and the cytotoxic effects of cisplatin in human colon tumor-derived myofibroblasts. These data provide evidence that FN promotes DNA damage recognition and chemosensitization to cisplatin via the potentiation of the DNA damage signaling responses in human colon cancer cells and tumor-derived myofibroblasts.

Irradiation induced foci (IRIF) as a biomarker for radiosensitivity

It has long been known that the level of radiosensitivity between individuals covers a considerable range. This range is reflected in analysis of patient cell lines with some cell lines showing significantly reduced sensitivity to in vitro radiation exposure. Our increased exposure to radiation from diagnostic medical procedures and other life style changes has raised concerns that there may be individuals who are at an elevated risk from the harmful impact of acute or chronic low dose radiation exposure. Additionally, a subset of patients show an enhanced normal tissue response following radiotherapy, which can cause significant discomfort and, at the extreme, be life threatening. It has long been realised that the ability to identify sensitive individuals and to understand the mechanistic basis underlying the range of sensitivity within the population is important. A reduced ability to efficiently repair DNA double strand breaks (DSB) and/or activate the DSB damage response underlies some, although not necessarily all, of this sensitivity. In this article, we consider the utility of the recently developed γH2AX foci analysis to provide insight into radiation sensitivity within the population. We consider the nature of sensitivity including the impact of radiation on cell survival, tissue responses and carcinogenesis and the range of responses within the population. We overview the current utility of the γH2AX assay for assessing the efficacy of the DNA damage response to low and high dose radiation and its potential future exploitation.

Targeting the DNA double strand break repair machinery in prostate cancer

Regardless of the achievable remissions with first line hormone therapy in patients with prostate cancer (CaP), the disease escapes the hormone dependent stage to a more aggressive status where chemotherapy is the only effective treatment and no treatment is curative. This makes it very important to identify new targets that can improve the outcome of treatment. ATM and DNA-PK are the two kinases responsible for signalling and repairing double strand breaks (DSB). Thus, both kinases are pertinent targets in CaP treatment to enhance the activity of the numerous DNA DSB inducing agents used in CaP treatment such as ionizing radiation (IR). Colony formation assay was used to assess the sensitivity of hormone dependent, p53 wt (LNCaP) and hormone independent p53 mutant (PC3) CaP cell lines to the cytotoxic effect of IR and Doxorubicin in the presence or absence of Ku55933 and NU7441 which are small molecule inhibitors of ATM and DNA-PK, respectively. Flow cytometry based methods were used to assess the effect of the two inhibitors on cell cycle, apoptosis and H2AX foci formation. Neutral comet assay was used to assess the induction of DNA DSBs. Ku55933 or NU7441 alone increased the sensitivity of CaP cell lines to the DNA damaging agents, however combining both inhibitors together resulted in further enhancement of sensitivity. The cell cycle profile of both cell lines was altered with increased cell death, DNA DSBs and H2AX foci formation. This study justifies further evaluation of the ATM and DNA-PK inhibitors for clinical application in CaP patients. Additionally, the augmented effect resulting from combining both inhibitors may have a significant implication for the treatment of CaP patients who have a defect in one of the two DSB repair pathways.

Use of the hollow fiber assay for the evaluation of DNA damaging agents

INTRODUCTION

The preclinical development and clinical progression of potential anticancer agents are highly time and resource-intensive. Traditionally, promising compounds in vitro undergo further screening in xenograft models, a long process that uses large numbers of animals. In order to hasten compound progression, the hollow fiber assay (HFA) was developed by the US National Cancer Institute as an additional filtering step in drug development, bridging the gap between in vitro and xenograft compound screening. The HFA demonstrates a good correlation in terms of clinical predictivity, and has significant reduction and refinement benefits for animal usage. In addition, the assay enables the study of basic pharmacological properties of compounds under investigation. The HFA has been mainly used as a rapid in vivo cytotoxicity screen, but has also been shown to be amenable to study drug/target interactions in vivo. One of the challenges of the HFA is the small sample sizes obtained, which can limit sensitivity.

METHODS

Here we specifically focus on the detection of DNA double-strand breaks, monitoring the effects of standard and novel anti-cancer agents on human lung, colon and breast cancer cell lines using immunoblotting and flow cytometry techniques for γ-H2A.X. This presented a further challenge due to the low abundance of the target event.

RESULTS

We found a good correlation between techniques in terms of rate of detection and sensitivity confirming the ability to use the HFA for detection of these specific drug-target interactions.

DISCUSSION

The results demonstrate good sensitivity and predictability for drug behavior in an assay where cell number is limited. In contrast to conventional xenograft studies, this short-term assay also enables analysis of pharmacodynamic endpoints in tumor cells in vivo. Importantly, there is a significant impact on reduction and refinement of the use of animals in incorporating this assay into the drug development process.

Prolonged expression of the γ-H2AX DNA repair biomarker correlates with excess acute and chronic toxicity from radiotherapy treatment

The normal tissue tolerance levels to fractionated radiotherapy have been appreciated by a century of careful clinical observations and radiobiological studies in animals. During clinical fractionated radiotherapy, these normal tissue tolerance levels are respected, and severe sequelae of radiotherapy are avoided in the majority of patients. Notwithstanding, a minority of patients experience unexpectedly severe normal tissue reactions. The ability to predict which patients might form this minority would be important. We have conducted a study to develop a rapid and reliable diagnostic test to predict excessive normal tissue toxicity (NTT) in radiotherapy patients. A flow cytometric immunocytochemical assay was used to measure DNA damage in peripheral blood lymphocytes (PBL) from cancer patients exposed to 2-Gy gamma radiation. DNA damage and repair was measured by induction of cellular γ-H2AX in unirradiated and exposed cells at specific time points following exposure. In 12 cancer patients that experienced severe atypical NTT following radiotherapy, there was a failure to repair DNA double-strand breaks (DSB) as measured by γ-H2AX induction and persistence. In ten cancer patients that experienced little or no NTT and in seven normal (noncancer controls), efficient repair of DNA DSB was observed in the γ-H2AX assay. We conclude that a flow cytometric assay based on γ-H2AX induction in PBL of radiotherapy patients may represent a robust, rapid and reliable biomarker to predict NTT during radiotherapy. Further research is required with a larger patient cohort to validate this important study.

Differential effect of ionizing radiation exposure on multipotent and differentiation-restricted bone marrow mesenchymal stem cells

Debilitating effects of bone marrow from ionizing radiation exposure has been well established for hematopoietic stem cells; however, radiation toxicity of mesenchymal stem cells (MSCs) has been controversial. The present study addressed if ionizing radiation exposure differently affected bone marrow MSCs with various differentiation commitments. Mouse bone-marrow-derived MSCs, D1 cells of early passages (≤ 5 passages; p5) maintained the complete characteristics of multipotent MSCs, whereas, after ≥ 45 passages (p45) the differentiation capability of D1 cells became partially restricted. Both p5 and p45 D1 cells were subjected to single dose irradiation by radioactive isotope (137)Cs. Radiation treatment impaired cell renewal and differentiation activities of p5 D1 cells; however, p45 D1 cells were less affected. Radiation treatment upregulated both pro- and anti-apoptotic genes of p5 D1 cells in a dose-dependent manner, potentially resulting in the various apoptosis thresholds. It was found that constitutive as well as radiation-induced phosphorylation levels of histone H2AX was significantly higher in p45 D1 cells than in p5 D1 cells. The increased repair activity of DNA double-strand breakage may play a role for p45 D1 cells to exhibit the relative radioresistance. In conclusion, the radiation toxicity predominantly affecting multipotent MSCs may occur at unexpectedly low doses, which may, in part, contribute to the catabolic pathology of bone tissue.

γ-H2AX detection in peripheral blood lymphocytes, splenocytes, bone marrow, xenografts, and skin

Measurement of DNA double-strand break (DSB) levels in cells is useful in many research areas, including those related to DNA damage and repair, tumorigenesis, anti-cancer drug development, apoptosis, radiobiology, environmental effects, and aging, as well as in the clinic. DSBs can be detected in the nuclei of cultured cells and tissues with an antibody to H2AX phosphorylated on serine residue 139 (γ-H2AX). DSB levels can be obtained either by measuring overall γ-H2AX protein levels in a cell population or by counting γ-H2AX foci in individual nuclei. Total levels can be obtained in extracts of cell populations by immunoblot analysis, and in cell populations by flow cytometry. Furthermore, with flow cytometry, the cell cycle distribution of a population can be obtained in addition to DSB levels, which is an advantage when studying anti-cancer drugs targeting replicating tumor cells. These described methods are used in genotoxicity assays of compounds of interest or in analyzing DSB repair after exposure to drugs or radiation. Immunocyto/immunohistochemical analysis can detect γ-H2AX foci in individual cells and is very sensitive (a single DSB can be visualized), permitting the use of extremely small samples. Measurements of γ-H2AX focal numbers can reveal subtle changes found in the radiation-induced tissue bystander response, low dose radiation exposure, and in cells with mutations in genomic stability maintenance pathways. In addition, marking DNA DSBs in a nucleus with γ-H2AX is a powerful tool to identify novel DNA repair proteins by their abilities to co-localize with γ-H2AX foci at the DSB site. This chapter presents techniques for γ-H2AX detection in a variety of human and mouse samples.

Adapting the γ-H2AX assay for automated processing in human lymphocytes. 1. Technological aspects

The immunofluorescence-based detection of γ-H2AX is a reliable and sensitive method for quantitatively measuring DNA double-strand breaks (DSBs) in irradiated samples. Since H2AX phosphorylation is highly linear with radiation dose, this well-established biomarker is in current use in radiation biodosimetry. At the Center for High-Throughput Minimally Invasive Radiation Biodosimetry, we have developed a fully automated high-throughput system, the RABIT (Rapid Automated Biodosimetry Tool), that can be used to measure γ-H2AX yields from fingerstick-derived samples of blood. The RABIT workstation has been designed to fully automate the γ-H2AX immunocytochemical protocol, from the isolation of human blood lymphocytes in heparin-coated PVC capillaries to the immunolabeling of γ-H2AX protein and image acquisition to determine fluorescence yield. High throughput is achieved through the use of purpose-built robotics, lymphocyte handling in 96-well filter-bottomed plates, and high-speed imaging. The goal of the present study was to optimize and validate the performance of the RABIT system for the reproducible and quantitative detection of γ-H2AX total fluorescence in lymphocytes in a multiwell format. Validation of our biodosimetry platform was achieved by the linear detection of a dose-dependent increase in γ-H2AX fluorescence in peripheral blood samples irradiated ex vivo with γ rays over the range 0 to 8 Gy. This study demonstrates for the first time the optimization and use of our robotically based biodosimetry workstation to successfully quantify γ-H2AX total fluorescence in irradiated peripheral lymphocytes.

In vitro and in vivo genotoxicity of radiofrequency fields

There has been growing concern about the possibility of adverse health effects resulting from exposure to radiofrequency radiations (RFR), such as those emitted by wireless communication devices. Since the introduction of mobile phones many studies have been conducted regarding alleged health effects but there is still some uncertainty and no definitive conclusions have been reached so far. Although thermal effects are well understood they are not of great concern as they are unlikely to result from the typical low-level RFR exposures. Concern rests essentially with the possibility that RFR-exposure may induce non-thermal and/or long-term health effects such as an increased cancer risk. Consequently, possible genetic effects have often been studied but with mixed results. In this paper we review the data on alleged RFR-induced genetic effects from in vitro and in vivo investigations as well as from human cytogenetic biomonitoring surveys. Attention is also paid to combined exposures of RFR with chemical or physical agents. Again, however, no entirely consistent picture emerges. Many of the positive studies may well be due to thermal exposures, but a few studies suggest that biological effects can be seen at low levels of exposure. Overall, however, the evidence for low-level genotoxic effects is very weak.

Use of the γH2AX assay for assessing the genotoxicity of polycyclic aromatic hydrocarbons in human cell lines

The development of in vitro genotoxic assays as an alternative method to animal experimentation is of growing interest in the context of the implementation of new regulations on chemicals. However, extrapolation of toxicity data from in vitro systems to in vivo models is hampered by the fact that in vitro systems vary in their capability to metabolize chemicals, and that biotransformation can greatly influence the experimental results. Therefore, much attention has to be paid to the cellular models used and experimental conditions. Polycyclic aromatic hydrocarbons (PAHs) are carcinogenic ubiquitous pollutants. Human exposure to PAHs is mainly from food origin. In this study, a detailed analysis of the biotransformation capabilities of three human cell lines commonly used for in vitro testing (HepG2, ACHN and Caco-2) was undertaken using 3 model PAHs (benzo(a)pyrene [B(a)P], fluoranthene [FLA] and 3-methylcholanthrene [3-MC]). Concomitantly the genotoxicity of these PAHs was investigated in different cell lines, using a new genotoxic assay (H2AX) in 96-well plates. The metabolic rates of B(a)P, FLA and 3-MC were similar in HepG2 and Caco-2 cell lines, respectively, though with the production of different metabolites. The ACHN cell line was shown to express very limited metabolic capabilities. We demonstrated that the PAHs having a high metabolic rate (B(a)P and 3-MC) were genotoxic from 10(-7) molar in both HepG2 and Caco-2 cells. The present study shows that H2AX measurement in human cell lines competent for the metabolism, is an efficient and sensitive genotoxic assay requiring less cells and time than other currently available tests.

Differences in the kinetics of gamma-H2AX fluorescence decay after exposure to low and high LET radiation

PURPOSE

In order to obtain more insight into heavy ion tumour therapy, some features of the underlying molecular mechanisms controlling the cellular response to high linear energy transfer (LET) radiation are currently analysed.

MATERIALS AND METHODS

We analysed the decay of the integrated fluorescence intensity of gamma-H2AX (phosphorylated histone H2AX) which is thought to reflect the repair kinetics of radiation-induced DNA double-strand breaks (DSB) using Laser-Scanning-Cytometry. Asynchronous human HeLa cells were irradiated with a single dose of either 1.89 Gy of 55 MeV carbon ions or 5 Gy of 70 kV X-rays.

RESULTS

Measurements of the gamma-H2AX-intensities from 15-60 min resulted in a 16 % decrease for carbon ions and in a 43 % decrease for X-rays. After 21 h, the decrease was 77 % for carbon ions and 85 % for X-rays. The corresponding time-effect relationship was fitted by a bi-exponential function showing a fast and a slow component with identical half-life values for both radiation qualities being 24 +/- 4 min and 13.9 +/- 0.7 h, respectively. Apparent differences in the kinetics following high and low LET irradiation could completely be attributed to quantitative differences in their contributions, with the slow component being responsible for 47 % of the repair after exposure to X-rays as compared to 80 % after carbon ion irradiation.

CONCLUSION

gamma-H2AX loss kinetics follows a bi-exponential decline with two definite decay times independent of LET. The higher contribution of the slow component determined for carbon ion exposure is thought to reflect the increased amount of complex DSB induced by high LET radiation.

H2AX phosphorylation screen of cells from radiosensitive cancer patients reveals a novel DNA double-strand break repair cellular phenotype

Background:

About 1–5% of cancer patients suffer from significant normal tissue reactions as a result of radiotherapy (RT). It is not possible at this time to predict how most patients’ normal tissues will respond to RT. DNA repair dysfunction is implicated in sensitivity to RT particularly in genes that mediate the repair of DNA double-strand breaks (DSBs). Phosphorylation of histone H2AX (phosphorylated molecules are known as γH2AX) occurs rapidly in response to DNA DSBs, and, among its other roles, contributes to repair protein recruitment to these damaged sites. Mammalian cell lines have also been crucial in facilitating the successful cloning of many DNA DSB repair genes; yet, very few mutant cell lines exist for non-syndromic clinical radiosensitivity (RS).

Methods:

Here, we survey DNA DSB induction and repair in whole cells from RS patients, as revealed by γH2AX foci assays, as potential predictive markers of clinical radiation response.

Results:

With one exception, both DNA focus induction and repair in cell lines from RS patients were comparable with controls. Using γH2AX foci assays, we identified a RS cancer patient cell line with a novel ionising radiation-induced DNA DSB repair defect; these data were confirmed by an independent DNA DSB repair assay.

Conclusion:

γH2AX focus measurement has limited scope as a pre-RT predictive assay in lymphoblast cell lines from RT patients; however, the assay can successfully identify novel DNA DSB repair-defective patient cell lines, thus potentially facilitating the discovery of novel constitutional contributions to clinical RS.

Focus on histone variant H2AX: to be or not to be

Phosphorylation of histone variant H2AX at serine 139, named gammaH2AX, has been widely used as a sensitive marker for DNA double-strand breaks (DSBs). gammaH2AX is required for the accumulation of many DNA damage response (DDR) proteins at DSBs. Thus it is believed to be the principal signaling protein involved in DDR and to play an important role in DNA repair. However, only mild defects in DNA damage signaling and DNA repair were observed in H2AX-deficient cells and animals. Such findings prompted us and others to explore H2AX-independent mechanisms in DNA damage response. Here, we will review recent advances in our understanding of H2AX-dependent and independent DNA damage signaling and repair pathways in mammalian cells.

GammaH2AX, an accurate marker that analyzes UV genotoxic effects on human keratinocytes and on human skin

The phosphorylated form of histone H2AX, gammaH2AX, is a component of the DNA repair system. Most studies have focused on the role of gammaH2AX during cell transformation and human cancer, but little is known about its role in keratinocytes and the skin during UV irradiation. We analyzed the response to UV irradiation focusing on the phosphorylation of histone H2AX both in vitro, in keratinocyte cultures and in artificial epidermis, and then in vivo, in human skin. Acute UVB irradiation of human keratinocytes increased the phosphorylation of H2AX in a dose-dependent manner; two types of gammaH2AX response were observed either in vitro or in vivo. After a low nonapoptotic UVB irradiation, cells contained phosphorylated H2AX and arrested their cell cycle to repair the DNA damages. For a stronger and proapoptotic UVB irradiation, keratinocytes dramatically increased the phosphorylation of H2AX and committed apoptosis. Our results indicate that gammaH2AX constitutes a highly sensitive marker relevant for studying subapoptotic doses as well as proapoptotic doses of UVB in human skin.

Monitoring drug-induced gammaH2AX as a pharmacodynamic biomarker in individual circulating tumor cells

PURPOSE

Circulating tumor cells (CTC) in peripheral blood of patients potentially represent a fraction of solid tumor cells available for more frequent pharmacodynamic assessment of drug action than is possible using tumor biopsy. However, currently available CTC assays are limited to cell membrane antigens. Here, we describe an assay that directly examines changes in levels of the nuclear DNA damage marker gammaH2AX in individual CTCs of patients treated with chemotherapeutic agents.

EXPERIMENTAL DESIGN

An Alexa Fluor 488-conjugated monoclonal gammaH2AX antibody and epithelial cancer cell lines treated with topotecan and spiked into whole blood were used to measure DNA damage-dependent nuclear gammaH2AX signals in individual CTCs. Time-course changes in both CTC number and gammaH2AX levels in CTCs were also evaluated in blood samples from patients undergoing treatment.

RESULTS

The percentage of gammaH2AX-positive CTCs increased in a concentration-dependent manner in cells treated with therapeutically relevant concentrations of topotecan ex vivo. In samples from five patients, percent gammaH2AX-positive cells increased post-treatment from a mean of 2% at baseline (range, 0-6%) to a mean of 38% (range, 22-64%) after a single day of drug administration; this increase was irrespective of increases or decreases in the total CTC count.

CONCLUSIONS

These data show promise for monitoring dynamic changes in nuclear biomarkers in CTCs (in addition to CTC count) for rapidly assessing drug activity in clinical trials of molecularly targeted anticancer therapeutics as well as for translational research.

Radiation-induced DNA repair foci: spatio-temporal aspects of formation, application for assessment of radiosensitivity and biological dosimetry

Several proteins involved in DNA repair and DNA damage signaling have been shown to produce discrete foci in response to ionizing radiation. These foci are believed to co-localize to DSB and referred to as ionizing radiation-induced foci (IRIF) or DNA repair foci. Recent studies have revealed that some residual IRIF remain in cells for a relatively long time after irradiation, and have indicated a possible correlation between radiosensitivity of cells and residual IRIF. Remarkably, residual foci are significantly larger in size than the initial foci. Increase in the size of IRIF with time upon irradiation has been found in various cell types and has partially been correlated with dynamics and fusion of initial foci. Although it is admitted that the number of IRIF reflect that of DSB, several studies report a lack of correlation between kinetics for IRIF and DSB and a lack of co-localization between DSB repair proteins. These studies suggest that some proportion of residual IRIF that depend on cell type, dose, and post-irradiation time may represent alternations in chromatin structure after DSB have been repaired or misrepaired. While precise functions of residual foci are presently unknown, their possible link to remaining chromatin alternations, nuclear matrix, apoptosis, delayed repair and misrejoining of DSB, activity of several kinases, phosphatases, and checkpoint signaling has been suggested. Another intriguing possibility is that some of DNA repair foci may mark break-points at chromosomal aberrations (CA). While this possibility has not been confirmed substantially, the residual foci seem to be useful for biological dosimetry and estimation of individual radiosensitivity in radiotherapy of cancer.

Inter-individual variation in DNA double-strand break repair in human fibroblasts before and after exposure to low doses of ionizing radiation

DNA double-strand breaks (DSB) are generally considered the most critical lesion induced by ionizing radiation (IR) and may initiate carcinogenesis and other disease. Using an immunofluorescence assay to simultaneously detect nuclear foci of the phosphorylated forms of histone H2AX and ATM kinase at sites of DSBs, we examined the response of 25 apparently normal and 10 DNA repair-deficient (ATM, ATR, NBN, LIG1, LIG4, and FANCG) primary fibroblast strains irradiated with low doses of (137)Cs gamma-rays. Quiescent G(0)/G(1)-phase cultures were exposed to 5, 10, and 25 cGy and allowed to repair for 24h. The maximum level of IR-induced foci (0.15 foci per cGy, at 10 or 30 min) in the normal strains showed much less inter-individual variation (CV approximately 0.2) than the level of spontaneous foci, which ranged from 0.2-2.6 foci/cell (CV approximately 0.6; mean+/-SD of 1.00+/-0.57). Significantly slower focus formation post-irradiation was observed in seven normal strains, similar to most mutant strains examined. There was variation in repair efficiency measured by the fraction of IR-induced foci remaining 24h post-irradiation, curiously with the strains having slower focus formation showing more efficient repair after 25 cGy. Interestingly, the ranges of spontaneous and residual induced foci levels at 24h in the normal strains were as least as large as those observed for the repair-defective mutant strains. The inter-individual variation in DSB foci parameters observed in cells exposed to low doses of ionizing radiation in this small survey of apparently normal people suggests that hypomorphic genetic variants in genomic maintenance and/or DNA damage signaling and repair genes may contribute to differential susceptibility to cancer induced by environmental mutagens.

Titanium dioxide nanoparticles induce DNA damage and genetic instability in vivo in mice

Titanium dioxide (TiO(2)) nanoparticles are manufactured worldwide in large quantities for use in a wide range of applications including pigment and cosmetic manufacturing. Although TiO(2) is chemically inert, TiO(2) nanoparticles can cause negative health effects, such as respiratory tract cancer in rats. However, the mechanisms involved in TiO(2)-induced genotoxicity and carcinogenicity have not been clearly defined and are poorly studied in vivo. The present study investigates TiO(2) nanoparticles-induced genotoxicity, oxidative DNA damage, and inflammation in a mice model. We treated wild-type mice with TiO(2) nanoparticles in drinking water and determined the extent of DNA damage using the comet assay, the micronuclei assay, and the gamma-H2AX immunostaining assay and by measuring 8-hydroxy-2'-deoxyguanosine levels and, as a genetic instability endpoint, DNA deletions. We also determined mRNA levels of inflammatory cytokines in the peripheral blood. Our results show that TiO(2) nanoparticles induced 8-hydroxy-2'-deoxyguanosine, gamma-H2AX foci, micronuclei, and DNA deletions. The formation of gamma-H2AX foci, indicative of DNA double-strand breaks, was the most sensitive parameter. Inflammation was also present as characterized by a moderate inflammatory response. Together, these results describe the first comprehensive study of TiO(2) nanoparticles-induced genotoxicity in vivo in mice possibly caused by a secondary genotoxic mechanism associated with inflammation and/or oxidative stress. Given the growing use of TiO(2) nanoparticles, these findings raise concern about potential health hazards associated with TiO(2) nanoparticles exposure.

Microscopic imaging of DNA repair foci in irradiated normal tissues

PURPOSE

It is now feasible to detect DNA double strand breaks (DSB) in tissues by measuring the induction and resolution of DNA repair foci, such as gamma-H2AX, using immunofluorescent microscopy and digital image analysis. This review will highlight principal tools and approaches to tissue microscopy and analysis. It will also discuss the practical considerations of using microscopy in vitro and in vivo in measuring intranuclear foci following irradiation.

CONCLUSIONS

Computer-based image analysis algorithms allow an objective and quantitative analysis of foci and protein-protein interactions using 3D confocal images. Finally, we review the literature in which DNA repair foci have been investigated as a biodosimeter or a biomarker of DNA repair in normal tissues.

Irradiation alters selection for oncogenic mutations in hematopoietic progenitors

Exposure to ionizing radiation and other DNA-damaging carcinogens is strongly associated with induction of malignancies. Prevailing paradigms attribute this association to the induction of oncogenic mutations, as the incidence of oncogenic events is thought to limit initiation and progression of cancers. On the other hand, random mutagenic and genotoxic effects of irradiation are likely to alter progenitor cell populations and the microenvironment, thus altering the selective effects of oncogenic mutations. Using competitive bone marrow transplantation experiments in mice, we show that ionizing irradiation leads to a persistent decline in the numbers and fitness of hematopoietic stem cells, in part resulting from persistent induction of reactive oxygen species. Previous irradiation dramatically alters the selective effects of some oncogenic mutations, substantially inhibiting clonal expansion and leukemogenesis driven by Bcr-Abl or activated N-Ras oncogenes but enhancing the selection for and leukemogenesis driven by the activated Notch1 mutant ICN. Irradiation-dependent selection for ICN expression occurs in a hematopoietic stem cell-enriched pool, which should facilitate the accumulation of additional oncogenic events at a committed T-progenitor stage critical for formation of T-lymphocytic leukemia stem cells. Enhancement of ICN-driven selection and leukemogenesis by previous irradiation is in part non-cell autonomous, as partial restoration of normal hematopoiesis can reverse these effects of irradiation. These studies show that irradiation substantially alters the adaptive landscape in hematopoietic progenitors and suggest that the causal link between irradiation and carcinogenesis might involve increased selection for particular oncogenic mutations.

Numerical analysis of etoposide induced DNA breaks

Background

Etoposide is a cancer drug that induces strand breaks in cellular DNA by inhibiting topoisomerase II (topoII) religation of cleaved DNA molecules. Although DNA cleavage by topoisomerase II always produces topoisomerase II-linked DNA double-strand breaks (DSBs), the action of etoposide also results in single-strand breaks (SSBs), since religation of the two strands are independently inhibited by etoposide. In addition, recent studies indicate that topoisomerase II-linked DSBs remain undetected unless topoisomerase II is removed to produce free DSBs.

Methodology/Principal Findings

To examine etoposide-induced DNA damage in more detail we compared the relative amount of SSBs and DSBs, survival and H2AX phosphorylation in cells treated with etoposide or calicheamicin, a drug that produces free DSBs and SSBs. With this combination of methods we found that only 3% of the DNA strand breaks induced by etoposide were DSBs. By comparing the level of DSBs, H2AX phosphorylation and toxicity induced by etoposide and calicheamicin, we found that only 10% of etoposide-induced DSBs resulted in histone H2AX phosphorylation and toxicity. There was a close match between toxicity and histone H2AX phosphorylation for calicheamicin and etoposide suggesting that the few etoposide-induced DSBs that activated H2AX phosphorylation were responsible for toxicity.

Conclusions/Significance

These results show that only 0.3% of all strand breaks produced by etoposide activate H2AX phosphorylation and suggests that over 99% of the etoposide induced DNA damage does not contribute to its toxicity.

Validation of the cell cycle G(2) delay assay in assessing ionizing radiation sensitivity and breast cancer risk

Genetic variations in cell cycle checkpoints and DNA repair genes are associated with prolonged cell cycle G(2) delay following ionizing radiation (IR) treatment and breast cancer risk. However, different studies reported conflicting results examining the association between post-IR cell cycle delay and breast cancer risk utilizing four different parameters: cell cycle G(2) delay index, %G(2)-M, G(2)/G(0)-G(1), and (G(2)/G(0)-G(1))/S. Therefore, we evaluated whether different parameters may influence study results using a data set from 118 breast cancer cases and 225 controls as well as lymphoblastoid and breast cancer cell lines with different genetic defects. Our results suggest that cell cycle G(2) delay index may serve as the best parameter in assessing breast cancer risk, genetic regulation of IR-sensitivity, and mutations of ataxia telangiectasia mutated (ATM) and TP53. Cell cycle delay in 21 lymphoblastoid cell lines derived from BRCA1 mutation carriers was not different from that in controls. We also showed that IR-induced DNA-damage signaling, as measured by phosphorylation of H2AX on serine 139 (γ-H2AX) was inversely associated with cell cycle G(2) delay index. In summary, the cellular responses to IR are extremely complex; mutations or genetic variations in DNA damage signaling, cell cycle checkpoints, and DNA repair contribute to cell cycle G(2) delay and breast cancer risk. The cell cycle G(2) delay assay characterized in this study may help identify subpopulations with elevated risk of breast cancer or susceptibility to adverse effects in normal tissue following radiotherapy.

Clinically relevant radioresistant cells efficiently repair DNA double-strand breaks induced by X-rays

Radiotherapy is one of the major therapeutic modalities for eradicating malignant tumors. However, the existence of radioresistant cells remains one of the most critical obstacles in radiotherapy and radiochemotherapy. Standard radiotherapy for tumor treatment consists of approximately 2 Gy once a day, 5 days a week, over a period of 5-8 weeks. To understand the characteristics of radioresistant cells and to develop more effective radiotherapy, we established a novel radioresistant cell line, HepG2-8960-R with clinical relevance from parental HepG2 cells by long-term fractionated exposure to 2 Gy of X-rays. HepG2-8960-R cells continued to proliferate with daily exposure to 2 Gy X-rays for more than 30 days, while all parental HepG2 cells ceased. After exposure to fractionated 2 Gy X-rays, induction frequencies of micronuclei and remaining foci of gamma-H2AX in HepG2-8960-R were less than those in HepG2. Flow cytometric analysis revealed that the proportion of cells in S- and G2/M-phase of the cell cycle was higher in HepG2-8960-R than in HepG2. These suggest that the response of clinically relevant radioresistant (CRR) cells to fractionated radiation is not merely an accumulated response to each fractionated radiation. This is the first report on the establishment of a CRR cell line from an isogenic parental cell line.

GammaH2AX and cancer

Histone H2AX phosphorylation on a serine four residues from the carboxyl terminus (producing gammaH2AX) is a sensitive marker for DNA double-strand breaks (DSBs). DSBs may lead to cancer but, paradoxically, are also used to kill cancer cells. Using gammaH2AX detection to determine the extent of DSB induction may help to detect precancerous cells, to stage cancers, to monitor the effectiveness of cancer therapies and to develop novel anticancer drugs.

An optimized method for measurement of gamma-H2AX in blood mononuclear and cultured cells

Phosphorylation of histone protein H2AX on serine 139 (gamma-H2AX) occurs at sites flanking DNA double-stranded breaks (DSBs) and can provide a measure of the number of DSBs within a cell. We describe a flow cytometry-based method optimized to measure gamma-H2AX in nonfixed mononuclear blood cells as well as in cultured cells, which is more sensitive and involves less steps compared with protocols involving fixed cells. This method can be used to monitor induction of gamma-H2AX in mononuclear cells from cancer patients undergoing radiotherapy and for detection of gamma-H2AX throughout the cell cycle in cultured cells. The method is based on the fact that H2AX like other histone proteins are retained in the nucleus when cells are lysed at physiological salt concentrations. Cells are therefore added without fixation to a solution containing detergent to lyse the cells along with a fluorescein isothiocyanate-labeled monoclonal gamma-H2AX antibody, DNA staining dye and blocking agents. The stained nuclei can be analyzed by flow cytometry to monitor the level of gamma-H2AX to determine the level of DSBs and DNA content and to determine the cell cycle stage. The omission of fixation simplifies staining and enhances the sensitivity. This protocol can be completed within 4-6 h.

Protective effect of hyaluronate on oxidative DNA damage in WI-38 and A549 cells

Progressive DNA damage in live cells by oxidants is the key factor contributing to cell aging and preconditioning to neoplastic transformation. The strategies to slow aging or prevent cancer rely on protection of DNA from the damage. Since cells reside within intercellular matrix it is of interest to know whether matrix constituents possess properties of modulating oxidative DNA damage. We explored, therefore, the effect of hyaluronate (HA), the ubiquitous component of the matrix, on extent of DNA damage induced by exogenous and endogenously generated oxidants. WI-38 and A549 cells were exposed to 200 microM H2O2 in the absence or presence of HA and induction of histone H2AX phosphorylation and activation of ATM, the reporters of DNA damage, was assessed by multiparameter cytometry. Also explored was effect of HA on constitutive H2AX phosphorylation that reflects DNA damage caused by endogenous oxidants generated during aerobic metabolism. HA of average MW 5.4 million (high MW) and 2 million (medium MW) at 0.1% (w/v) in culture medium totally prevented the H2O2-induced H2AX phosphorylation in both cell types whereas effect of 60,000 average MW (low MW) HA was somewhat less pronounced. Constitutive H2AX phosphorylation in WI-38 cells growing in the presence of 0.1% HA of low MW and medium MW was reduced by about 35 and 30%, respectively; no reduction was observed in A549 cells. The data indicate that HA protected DNA from damage caused by the exogenous oxidant H2O2. In WI-38 fibroblasts, the cells that express the HA-receptor CD44, HA also protected DNA from damage caused by endogenous oxidants. We postulate that expression of CD44 in some cell types such as stem cells may provide the means to internalize HA by endocytosis and one of the functions of the internalized HA may be protection of DNA from oxidants. The mechanism of protective effect of HA may either: i) involve entrapment of iron ions thereby inhibiting the Fenton's reaction that produces secondary oxidative species, and/or: ii) directly scavenging of primary and secondary ROIs, as an antioxidant, resulting in HA degradation. Since no significant degradation of HA upon its exposure in tissue culture medium to H2O2 was detected the scavenging may occur intracellularly.

Cytometry of ATM activation and histone H2AX phosphorylation to estimate extent of DNA damage induced by exogenous agents

This review covers the topic of cytometric assessment of activation of Ataxia telangiectasia mutated (ATM) protein kinase and histone H2AX phosphorylation on Ser139 in response to DNA damage, particularly the damage that involves formation of DNA double-strand breaks. Briefly described are molecular mechanisms associated with activation of ATM and the downstream events that lead to recruitment of DNA repair machinery, engagement of cell cycle checkpoints, and activation of apoptotic pathway. Examples of multiparameter analysis of ATM activation and H2AX phosphorylation vis-a-vis cell cycle phase position and induction of apoptosis that employ flow- and laser scanning-cytometry are provided. They include cells treated with a variety of exogenous genotoxic agents, such as ionizing and UV radiation, DNA topoisomerase I (topotecan) and II (mitoxantrone, etoposide) inhibitors, nitric oxide-releasing aspirin, DNA replication inhibitors (aphidicolin, hydroxyurea, thymidine), and complex environmental carcinogens such as present in tobacco smoke. Also presented is an approach to identify DNA replicating (BrdU incorporating) cells based on selective photolysis of DNA that triggers H2AX phosphorylation. Listed are strategies to distinguish ATM activation and H2AX phosphorylation induced by primary DNA damage by genotoxic agents from those effects triggered by DNA fragmentation that takes place during apoptosis. While we review most published data, recent new findings also are included. Examples of multivariate analysis of ATM activation and H2AX phosphorylation presented in this review illustrate the advantages of cytometric flow- and image-analysis of these events in terms of offering a sensitive and valuable tool in studies of factors that induce DNA damage and/or affect DNA repair and allow one to explore the linkage between DNA damage, cell cycle checkpoints and initiation of apoptosis.