Phosphorylation of histone H2AX as a measure of radiosensitivity

Ribonucleotide reductase inhibition enhances chemoradiosensitivity of human cervical cancers

For repair of damaged DNA, cells increase de novo synthesis of deoxyribonucleotide triphosphates through the rate-limiting, p53-regulated ribonucleotide reductase (RNR) enzyme. In this study we investigated whether pharmacological inhibition of RNR by 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, NSC #663249) enhanced chemoradiation sensitivity through a mechanism involving sustained DNA damage. RNR inactivation by 3-AP and resulting chemoradiosensitization were evaluated in human cervical (CaSki, C33-a) cancer cells through study of DNA damage (γ-H2AX signal) by flow cytometry, RNR subunit p53R2 and p21 protein steady-state levels by Western blot analysis and laser scanning imaging cytometry, and cell survival by colony formation assays. 3-AP treatment led to sustained radiation- and cisplatin-induced DNA damage (i.e. increased γ-H2AX signal) in both cell lines through a mechanism of inhibited RNR activity. Radiation, cisplatin and 3-AP exposure resulted in significantly elevated numbers and persistence of γ-H2AX foci that were associated with reduced clonogenic survival. DNA damage was associated with a rise in p53R2 but not p21 protein levels 6 h after treatment with radiation and/or cisplatin plus 3-AP. We conclude that blockage of RNR activity by 3-AP impairs DNA damage responses that rely on deoxyribonucleotide production and thereby may substantially increase chemoradiosensitivity of human cervical cancers.

Quantification of gamma-H2AX foci in human lymphocytes: a method for biological dosimetry after ionizing radiation exposure

Recent studies have suggested that visualization of gamma-H2AX nuclear foci can be used to estimate exposure to very low doses of ionizing radiation. Although this approach is widely used for various purposes, its suitability for individual human biodosimetry has not yet been assessed. We therefore conducted such an assessment with the help of available software for observing and automatically scoring gamma-H2AX foci. The presence of gamma-H2AX foci was evaluated in human peripheral blood lymphocytes exposed ex vivo to gamma rays in a dose range of 0.02 to 2 Gy. We analyzed the response of gamma-H2AX to ionizing radiation in relation to dose, time after exposure, and individual variability. We constructed dose-effect calibration curves at 0.5, 8 and 16 h after exposure and evaluated the threshold of detection of the technique. The results show the promise of automatic gamma-H2AX scoring for a reliable assessment of radiation doses in a dose range of 0.6 Gy to 2 Gy up to 16 h after exposure. This gamma-H2AX-based assay may be useful for biodosimetry, especially for triage to distinguish promptly among individuals the ones who have received negligible doses from those with significantly exposures who are in need of immediate medical attention. However, additional in vivo experiments are needed for validation.

Induction of in situ DNA double-strand breaks and apoptosis by 200 MeV protons and 10 MV X-rays in human tumour cell lines

PURPOSE

To clarify the properties of clinical high-energy protons by comparing with clinical high-energy X-rays.

MATERIALS AND METHODS

Human tumor cell lines, ONS76 and MOLT4, were irradiated with 200 MeV protons or 10 MV X-rays. In situ DNA double-strand breaks (DDSB) induction was evaluated by immunocytochemical staining of phosphorylated histone H2AX (γ-H2AX). Apoptosis was measured by flow-cytometry after staining with Annexin V. The relative biological effectiveness (RBE) was obtained by clonogenic survival assay.

RESULTS

DDSB induction was significantly higher for protons than X-rays with average ratios of 1.28 (ONS76) and 1.59 (MOLT4) at 30 min after irradiation. However the differences became insignificant at 6 h. Also, apoptosis induction in MOLT4 cells was significantly higher for protons than X-rays with an average ratio of 2.13 at 12 h. However, the difference became insignificant at 20 h. RBE values of protons to X-rays at 10% survival were 1.06 ± 0.04 and 1.02 ± 0.15 for ONS76 and MOLT4, respectively.

CONCLUSIONS

Cell inactivation may differ according to different timings and/or endpoints. Proton beams demonstrated higher cell inactivation than X-rays in the early phases. These data may facilitate the understanding of the biological properties of clinical proton beams.

Dynamics of ionizing radiation-induced DNA damage response in reconstituted three-dimensional human skin tissue

The ATM-dependent DNA damage checkpoint plays a pivotal role in cellular response to ionizing radiation. Although amplification of the DNA damage signal through multifactorial protein complex formation of DNA damage checkpoint factors is crucial for proper DNA damage response in two-dimensionally cultured cells, the dynamics of the DNA damage response in three-dimensional tissues or organs remained to be determined. Here we used a model of reconstituted human skin and investigated the spatiotemporal dynamics of focus formation of DNA damage checkpoint factors after X irradiation. We found that DNA damage-induced foci were differentially formed in different layers. All cells in basal layers and approximately 40% of cells in spinous layers displayed foci. In basal cells, the foci showed linear dose relationships, and the number of foci decreased with increasing time after irradiation. We found that the initial foci grew within a few hours after irradiation, and persistent signals developed large foci. Colocalization of phosphorylated ATM, phosphorylated histone H2AX, MDC1 and 53BP1 foci was detected, and all of them showed simultaneous focus growth, indicating amplification of DNA damage signals. These results confirmed a dynamic DNA damage response in three-dimensional tissue, which provides a practical model for studying DNA damage response in vivo.

Role of peroxiredoxin I in rectal cancer and related to p53 status

BACKGROUND

Neoadjuvant chemoradiotherapy is widely accepted for the treatment of localized rectal cancer. Although peroxiredoxin I (PrxI) and p53 have been implicated in carcinogenesis and cancer treatment, the role of PrxI and its interaction with p53 in the prognosis and treatment response of rectal cancer remain relatively unstudied.

METHODS AND MATERIALS

In the present study, we examined the levels of PrxI and p53 in rectal cancer patients using membrane arrays and compared them with normal population samples. To demonstrate the biologic changes after manipulation of PrxI expression, we established stable transfectants of HCT-116 (wild-type p53) and HT-29 (mutant p53) cells with a PrxI silencing vector. The predictive capacities of PrxI and p53 were also assessed by relating the immunohistochemical staining of a retrospective series of rectal cancer cases to the clinical outcome.

RESULTS

The membrane array and immunochemical staining data showed that PrxI, but not p53, was significantly associated with the tumor burden. Our immunochemistry findings further indicated that PrxI positivity was linked to a poor response to neoadjuvant therapy and worse survival. In cellular and animal experiments, the inhibition of PrxI significantly decreased tumor growth and sensitized the tumor to irradiation, as indicated by a lower capacity to scavenge reactive oxygen species and more extensive DNA damage. The p53 status might have contributed to the difference between HCT-116 and HT-29 after knockdown of PrxI.

CONCLUSION

According to our data, the level of PrxI combined with the p53 status is relevant to the prognosis and the treatment response. We suggested that PrxI might be a new biomarker for rectal cancer.

Enhancement of radiation response in osteosarcoma and rhabdomyosarcoma cell lines by histone deacetylase inhibition

PURPOSE

Histone deacetylase inhibitors (HDACIs) can enhance the sensitivity of cells to photon radiation treatment (XRT) by altering numerous molecular pathways. We investigated the effect of pan-HDACIs such as suberoylanilide hydroxamic acid (SAHA) on radiation response in two osteosarcoma (OS) and two rhabdomyosarcoma (RMS) cell lines.

METHODS AND MATERIALS

Clonogenic survival, cell cycle analysis, and apoptosis were examined in OS (KHOS-24OS, SAOS2) and RMS (A-204, RD) cell lines treated with HDACI and HDACI plus XRT, respectively. Protein expression was investigated via immunoblot analysis, and cell cycle analysis and measurement of apoptosis were performed using flow cytometry.

RESULTS

SAHA induced an inhibition of cell proliferation and clonogenic survival in OS and RMS cell lines and led to a significant radiosensitization of all tumor cell lines. Other HDACI such as M344 and valproate showed similar effects as investigated in one OS cell line. Furthermore, SAHA significantly increased radiation-induced apoptosis in the OS cell lines, whereas in the RMS cell lines radiation-induced apoptosis was insignificant with and without SAHA. In all investigated sarcoma cell lines, SAHA attenuated radiation-induced DNA repair protein expression (Rad51, Ku80).

CONCLUSION

Our results show that HDACIs enhance radiation action in OS and RMS cell lines. Inhibition of DNA repair, as well as increased apoptosis induction after exposure to HDACIs, can be mechanisms of radiosensitization by HDACIs.

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.

Prostate-specific natural health products (dietary supplements) radiosensitize normal prostate cells

PURPOSE

Prostate-specific health products (dietary supplements) are taken by cancer patients to alleviate the symptoms linked with poor prostate health. However, the effect of these agents on evidence-based radiotherapy practice is poorly understood. The present study aimed to determine whether dietary supplements radiosensitized normal prostate or prostate cancer cell lines.

METHODS AND MATERIALS

Three well-known prostate-specific dietary supplements were purchased from commercial sources available to patients (Trinovin, Provelex, and Prostate Rx). The cells used in the study included normal prostate lines (RWPE-1 and PWR-1E), prostate tumor lines (PC3, DU145, and LNCaP), and a normal nonprostate line (HaCaT). Supplement toxicity was assessed using cell proliferation assays [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] and cellular radiosensitivity using conventional clonogenic assays (0.5-4Gy). Cell cycle kinetics were assessed using the bromodeoxyuridine/propidium iodide pulse-labeling technique, apoptosis by scoring caspase-3 activation, and DNA repair by assessing gammaH2AX.

RESULTS

The cell growth and radiosensitivity of the malignant PC3, DU145, and LNcaP cells were not affected by any of the dietary prostate supplements (Provelex [2 microg/mL], Trinovin [10 microg/mL], and Prostate Rx [50 microg/mL]). However, both Trinovin (10 microg/mL) and Prostate Rx (6 microg/mL) inhibited the growth rate of the normal prostate cell lines. Prostate Rx increased cellular radiosensitivity of RWPE-1 cells through the inhibition of DNA repair.

CONCLUSION

The use of prostate-specific dietary supplements should be discouraged during radiotherapy owing to the preferential radiosensitization of normal prostate cells.

Microwaves from Mobile Phones Inhibit 53BP1 Focus Formation in Human Stem Cells More Strongly Than in Differentiated Cells: Possible Mechanistic Link to Cancer Risk

BACKGROUND

It is widely accepted that DNA double-strand breaks (DSBs) and their misrepair in stem cells are critical events in the multistage origination of various leukemias and tumors, including gliomas.

OBJECTIVES

We studied whether microwaves from mobile telephones of the Global System for Mobile Communication (GSM) and the Universal Global Telecommunications System (UMTS) induce DSBs or affect DSB repair in stem cells.

METHODS

We analyzed tumor suppressor TP53 binding protein 1 (53BP1) foci that are typically formed at the sites of DSB location (referred to as DNA repair foci) by laser confocal microscopy.

RESULTS

Microwaves from mobile phones inhibited formation of 53BP1 foci in human primary fibroblasts and mesenchymal stem cells. These data parallel our previous findings for human lymphocytes. Importantly, the same GSM carrier frequency (915 MHz) and UMTS frequency band (1947.4 MHz) were effective for all cell types. Exposure at 905 MHz did not inhibit 53BP1 foci in differentiated cells, either fibroblasts or lymphocytes, whereas some effects were seen in stem cells at 905 MHz. Contrary to fibroblasts, stem cells did not adapt to chronic exposure during 2 weeks.

CONCLUSIONS

The strongest microwave effects were always observed in stem cells. This result may suggest both significant misbalance in DSB repair and severe stress response. Our findings that stem cells are most sensitive to microwave exposure and react to more frequencies than do differentiated cells may be important for cancer risk assessment and indicate that stem cells are the most relevant cellular model for validating safe mobile communication signals.

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.

Early increase of radiation-induced γH2AX foci in a human Ku70/80 knockdown cell line characterized by an enhanced radiosensitivity

A better understanding of the underlying mechanisms of DNA repair after exposure to ionizing radiation represents a research priority aimed at improving the outcome of clinical radiotherapy. Because of the close association with DNA double strand break (DSB) repair, phosphorylation of the histone H2AX protein (γH2AX), quantified by immunodetection, has recently been used as a method to study DSB induction and repair at low and clinically relevant radiation doses. However, the lack of consistency in literature points to the need to further validate the role of H2AX phosphorylation in DSB repair and the use of this technique to determine intrinsic radiosensitivity. In the present study we used human mammary epithelial MCF10A cells, characterized by a radiosensitive phenotype due to reduced levels of the Ku70 and Ku80 repair proteins, and investigated whether this repair-deficient cell line displays differences in the phosphorylation pattern of H2AX protein compared to repair-proficient MCF10A cells. This was established by measuring formation and disappearance of γH2AX foci after irradiating synchronized cell populations with (60)Co γ-rays. Our results show statistically significant differences in the number of γH2AX foci between the repair-deficient and -proficient cell line, with a higher amount of γH2AX foci present at early times post-irradiation in the Ku-deficient cell line. However, the disappearance of those differences at later post-irradiation times questions the use of this assay to determine intrinsic radiosensitivity, especially in a clinical setting.

Unsuitability of lymphoblastoid cell lines as surrogate of cryopreserved isolated lymphocytes for the analysis of DNA double-strand break repair activity

As first task of a comprehensive investigation on DNA repair genotype-phenotype correlations, the suitability of Epstein-Barr virus (EBV)-transformed lymphoblastoid cell lines (LCLs) as surrogate of cryopreserved peripheral blood mononuclear cells (PBMCs) in DNA repair phenotypic assays was evaluated. To this aim the amount of DNA damage induced by gamma-rays and DNA repair capacity were evaluated in unstimulated (G(0)) and mitogen-simulated (G(2)) PBMC from 20 healthy subjects and in EBV-transformed LCL obtained from the same individuals. Phosphorylation of histone H2AX, micronuclei and chromosomal aberrations were the end-points investigated. The results obtained show higher basal frequencies of binucleated cells bearing micronuclei and nucleoplasmic bridge (NPB) in LCL with respect to PBMC, suggesting that EBV transformation may be associated with chromosomal instability. After irradiation, higher levels of micronuclei were induced in G(0)-treated PBMC compared to cycling LCL; conversely, NPB were more frequent in LCL than in PBMC. Moreover, higher levels of chromosomal aberrations were observed in G(2)-treated PBMC compared to LCL. Concerning gamma-H2AX measurements, phosphorylation levels 1h after treatment and dephosphorylation kinetics were basically similar in LCL and in PBMC. However, while Spearman's test showed a strong correlation between the results obtained in replicated experiments with PBMC, high inter-experimental variability and poor reproducibility was observed in the experiments performed with LCL, possibly due to the intrinsic instability of LCL. In summary, both the analysis of gamma-H2AX and the evaluation of chromosome damage highlighted a larger inter-experimental variability in the results obtained with LCL compared to PBMC. Noteworthy, the two set of results proved to lack any significant correlation at the individual level. These results indicate that LCL may be unsuitable for investigating genotype-phenotype correlations with phenotypic DNA repair assays, especially when low impact functional genetic variants are involved.

Modulating radiation resistance by inhibiting ribonucleotide reductase in cancers with virally or mutationally silenced p53 protein

Therapeutic ionizing radiation damages DNA, increasing p53-regulated ribonucleotide reductase (RNR) activity required for de novo synthesis of the deoxyribonucleotide triphosphates used during DNA repair. This study investigated the pharmacological inhibition of RNR in cells of virally or mutationally silenced p53 cancer cell lines using 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, Triapine(R), NSC #663249), a chemotherapeutic radiosensitizer that equally inhibits RNR M2 and p53R2 small subunits. The effects of 3-AP on RNR inhibition and resulting radiosensitization were evaluated in cervical (CaSki, HeLa and C33-a) and colon (RKO, RKO-E6) cancer cells. 3-AP treatment significantly enhanced radiation-related cytotoxicity in cervical and colon cancer cells. 3-AP treatment significantly decreased RNR activity, caused prolonged radiation-induced DNA damage, and resulted in an extended G(1)/S-phase cell cycle arrest in all cell lines. Similar effects were observed in both RKO and RKO-E6 cells, suggesting a p53-independent mechanism of radiosensitization. We conclude that inhibition of ribonucleotide reductase by 3-AP enhances radiation-mediated cytotoxicity independent of p53 regulation by impairing repair processes that rely on deoxyribonucleotide production, thereby substantially increasing the radiation sensitivity of human cancers.

γ-H2AX Foci as a Biomarker for Patient X-Ray Exposure in Pediatric Cardiac Catheterization

Background— A better knowledge of patient x-ray dose and the associated radiation risk in pediatric interventional cardiology is warranted in view of the extensive use of x-rays and the higher radiosensitivity of children. In the present study, γ-H2AX foci were used as a biomarker for radiation-induced effects. Patient-specific dose was assessed and radiation risks were estimated according to the linear-no-threshold model, commonly used in radiation protection, and the γ-H2AX foci data.

Methods and Results— In 49 pediatric patients (median age, 0.75 years) with congenital heart disease who underwent cardiac catheterization procedures, blood samples were taken before and shortly after the procedure. γ-H2AX foci were determined in peripheral blood T lymphocytes. In each patient, a net increase in γ-H2AX foci, representing DNA double-strand breaks induced by interventional x-rays, was observed. In addition, a patient-specific Monte Carlo simulation of the procedure was performed, resulting in individual blood, organ, and tissue doses. Plotting of γ-H2AX foci versus blood dose indicated a low-dose hypersensitivity. Median effective doses calculated according to the International Commission on Radiological Protection 60 and 103 publications are 5.6 and 6.4 mSv, respectively. The lifetime-attributable risk of cancer mortality was calculated from the linear-no-threshold model and the γ-H2AX foci data. This resulted in lifetime-attributable risk values of 1% and 4%, respectively, for the patient population under study.

Conclusions— γ-H2AX foci as a biomarker for DNA damage indicate that radiation risk estimates according to the linear-no-threshold hypothesis are possibly underestimates. Great care should be taken to minimize and optimize patient radiation 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.

H2AX: functional roles and potential applications

Upon DNA double-strand break (DSB) induction in mammals, the histone H2A variant, H2AX, becomes rapidly phosphorylated at serine 139. This modified form, termed gamma-H2AX, is easily identified with antibodies and serves as a sensitive indicator of DNA DSB formation. This review focuses on the potential clinical applications of gamma-H2AX detection in cancer and in response to other cellular stresses. In addition, the role of H2AX in homeostasis and disease will be discussed. Recent work indicates that gamma-H2AX detection may become a powerful tool for monitoring genotoxic events associated with cancer development and tumor progression.

CD133+ glioblastoma stem-like cells are radiosensitive with a defective DNA damage response compared with established cell lines

PURPOSE

CD133+ glioblastoma tumor stem-like cells (TSC) have been defined as radioresistant. However, although previously classified relative to CD133- cells, the radiosensitivity of CD133+ TSCs with respect to the standard glioblastoma model, established glioma cell lines, has not been determined. Therefore, to better understand the radioresponse of this cancer stem cell, we have used established cell lines as a framework for defining their in vitro radioresponse.

EXPERIMENTAL DESIGN

The intrinsic radiosensitivity of CD133+ TSC cultures and established glioma cell lines was determined by clonogenic assay. The TSCs and established cell lines were also compared in terms of DNA double-strand break (DSB) repair capacity and cell cycle checkpoint activation.

RESULTS

Based on clonogenic analysis, each of the six TSC cultures evaluated was more sensitive to radiation than the established glioma cell lines. Consistent with increased radiosensitivity, the DSB repair capacity as defined by neutral comet assay and gammaH2AX and Rad51 foci was significantly reduced in TSCs compared with the cell lines. Although G2 checkpoint activation was intact, in contrast to the cell lines, DNA synthesis was not inhibited in TSCs after irradiation, indicating the absence of the intra-S-phase checkpoint.

CONCLUSIONS

These data indicate that the mechanisms through which CD133+ TSCs respond to radiation are significantly different from those of the traditional glioblastoma in vitro model, established glioma cell lines. If TSCs play a critical role in glioblastoma treatment response, then such differences are likely to be of consequence in the development and testing of radiosensitizing agents.

gamma-H2AX foci formation in peripheral blood lymphocytes of tumor patients after local radiotherapy to different sites of the body: Dependence on the dose-distribution, irradiated site and time from start of treatment

PURPOSE

To evaluate the relationship between an estimated integral total body radiation dose delivered and phosphorylated histone H2AX protein (gamma-H2AX) foci formation in peripheral blood lymphocytes of cancer patients.

MATERIAL AND METHODS

gamma-H2AX formation was quantified as the mean number of foci per lymphocyte (N(meanH2AX)) and the percentage of lymphocytes with > or =n foci. The integrated total body radiation dose was estimated from the dose volume histogram of patient's body corrected for the proportion of the body scanned by computed tomography for 3D treatment planning.

RESULTS

There was a strong linear correlation between the mean number of gamma-H2AX foci per lymphocyte in the peripheral blood sample and integrated total body radiation dose (r = 0.83, p < 0.0001). The slope of the relationship was dependent on the site of body irradiated. In comparison to chest irradiation with a slope of 8.7 +/- 0.8 foci Gy(-1), the slopes for brain, upper leg and pelvic sites were significantly shallower by -4.7, -4.3, and -3.8 Gy(-1), respectively (p < 0.0001), while the slope for upper abdomen irradiation was significantly larger by 9.1 +/- 2.6 Gy(-1) (p = 0.0007). There was a slight time effect since the start of radiotherapy on the slopes of the in vivo dose responses leading to shallower slopes (-1.5 +/- 0.7 Gy(-1), p = 0.03) later (> or =10 day) during radiotherapy. After in vitro irradiation, lymphocytes showed 10.41 +/- 0.12 foci per Gy with no evidence of inter-individual heterogeneity.

CONCLUSIONS

gamma-H2AX measurements in peripheral lymphocytes after local radiotherapy allow the estimation of the applied integral body dose. The site and time dependence have to be considered.

Kinetics and dose-response of residual 53BP1/γ-H2AX foci: Co-localization, relationship with DSB repair and clonogenic survival

PURPOSE

Recent studies revealed that some foci produced by phosphorylated histone 2A family member X (gamma-H2AX) and tumor suppressor p53 binding protein 1 (53BP1) that co-localize with radiation-induced DNA double-strand breaks (DSB) remain in cells at relatively long times after irradiation and indicated a possible correlation between cellular radiosensitivity and residual foci. In this study, we investigated dose-responses and kinetics for radiation-induced 53BP1/gamma-H2AX foci formation in relation to their co-localization, DSB repair and cell survival.

MATERIALS AND METHODS

Cell survival, DSB and foci were analyzed by clonogenic assay, pulsed field gel electrophoresis (PFGE), and confocal laser microscopy, respectively, in normal human fibroblasts (VH-10) and in a cancer cell line (HeLa). Computer analysis was used to determine both the number and the area of foci.

RESULTS

We show that even at doses down to 1 cGy a statistically significant induction of 53BP1 foci is observed. While the number of foci was found to constantly decrease with post-irradiation time, the per-cell normalized area of foci does not change within a time window of approximately 4 h post-irradiation. Co-localization of gamma-H2AX and 53BP1 foci is shown to depend on dose and post-irradiation time. No clear correlations were established between radiosensitivity and foci formation because the dose response for 53BP1/gamma-H2AX foci may depend on time after irradiation and duration of the cell cycle. We show that the kinetics of foci disappearance within 24 h post-irradiation do not coincide with those of DSB repair.

CONCLUSIONS

The data suggest that the post-irradiation time used for estimation of radiosensitivity at therapeutically relevant low doses (e.g., <3 Gy) in proliferating cells by scoring residual foci should be limited by the duration of the cell cycle, and that direct comparison of the kinetics of DSB repair and disappearance of DSB-co-localizing foci is not possible. Therefore, results obtained from the counting of foci should be interpreted with caution in terms of DSB repair.

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.

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.

Histone H2AX phosphorylation in response to changes in chromatin structure induced by altered osmolarity

DNA strand breaks trigger marked phosphorylation of histone H2AX (i.e. gamma-H2AX). While DNA double-strand breaks (DSBs) provide a strong stimulus for this event, the accompanying structural alterations in chromatin may represent the actual signal that elicits gamma-H2AX. Our data show that changes in chromatin structure are sufficient to elicit extensive gamma-H2AX formation in the relative absence of DNA strand breaks. Cells subjected to hypotonic (0.05 M) treatment exhibit gamma-H2AX levels that are equivalent to those found after the induction of 80-200 DNA DSBs (i.e. 2-5 Gy). Despite this significant increase in phosphorylation, cell survival remains relatively unaffected (<10% cytotoxicity), and there is no significant increase in apoptosis. Nuclear staining profiles indicate that gamma-H2AX-positive cells induced under altered tonicity exhibit variable levels of staining, ranging from uniform pan staining to discrete punctate foci more characteristic of DNA strand breakage. The capability to induce significant gamma-H2AX formation under altered tonicity in the relative absence of DNA strand breaks suggests that this histone modification evolved in response to changes in chromatin structure.

Rapid flow cytometry-based structural maintenance of chromosomes 1 (SMC1) phosphorylation assay for identification of ataxia-telangiectasia homozygotes and heterozygotes

BACKGROUND

No rapid reliable method exists for identifying ataxia-telangiectasia (A-T) homozygotes or heterozygotes. Heterozygotes are at an increased risk of cancer and are more sensitive to the effects of ionizing radiation (IR) than the general population. We report a rapid flow cytometry (FC)-based ataxia-telangiectasia mutated (ATM) kinase assay that measures ATM- dependent phosphorylation of structural maintenance of chromosomes 1 (SMC1) following DNA damage (FC-pSMC1 assay).

METHODS

After optimizing conditions with lymphoblastoid cell lines (LCLs), we studied peripheral blood mononuclear cells (PBMCs) isolated from 16 healthy donors (unknowns), 10 obligate A-T heterozygotes, and 6 unrelated A-T patients. One hour after DNA damage (by either IR or bleomycin), the cells were fixed and incubated with a primary antibody to SMC1pSer966. We analyzed the stained cells by FC to determine the difference in geometric mean fluorescence intensity (DeltaGMFI) of untreated and treated cells; this difference was expressed as a percentage of daily experimental controls.

RESULTS

The FC-pSMC1 assay reliably distinguished ATM heterozygotes and homozygotes from controls. Average DeltaGMFI percentages (SD) of daily controls were, for unknowns, 106.1 (37.6); for A-T heterozygotes, 37.0 (18.7); and for A-T homozygotes; -8.73 (16.2). Values for heterozygotes and homozygotes were significantly different from those of controls (P < 0.0001).

CONCLUSIONS

The FC-pSMC1 assay shortens the turnaround time for diagnosing A-T homozygotes from approximately 3 months to approximately 3 h. It also identifies A-T heterozygotes and can be used for prenatal counseling or for screening individuals in large study cohorts for potential ATM heterozygosity, which can then be confirmed by sequencing.

Active oral regimen for elderly adults with newly diagnosed acute myelogenous leukemia: a preclinical and phase 1 trial of the farnesyltransferase inhibitor tipifarnib (R115777, Zarnestra) combined with etoposide

The farnesyltransferase inhibitor tipifarnib exhibits modest activity against acute myelogenous leukemia. To build on these results, we examined the effect of combining tipifarnib with other agents. Tipifarnib inhibited signaling downstream of the farnesylated small G protein Rheb and synergistically enhanced etoposide-induced antiproliferative effects in lymphohematopoietic cell lines and acute myelogenous leukemia isolates. We subsequently conducted a phase 1 trial of tipifarnib plus etoposide in adults over 70 years of age who were not candidates for conventional therapy. A total of 84 patients (median age, 77 years) received 224 cycles of oral tipifarnib (300-600 mg twice daily for 14 or 21 days) plus oral etoposide (100-200 mg daily on days 1-3 and 8-10). Dose-limiting toxicities occurred with 21-day tipifarnib. Complete remissions were achieved in 16 of 54 (30%) receiving 14-day tipifarnib versus 5 of 30 (17%) receiving 21-day tipifarnib. Complete remissions occurred in 50% of two 14-day tipifarnib cohorts: 3A (tipifarnib 600, etoposide 100) and 8A (tipifarnib 400, etoposide 200). In vivo, tipifarnib plus etoposide decreased ribosomal S6 protein phosphorylation and increased histone H2AX phosphorylation and apoptosis. Tipifarnib plus etoposide is a promising orally bioavailable regimen that warrants further evaluation in elderly adults who are not candidates for conventional induction chemotherapy. These clinical studies are registered at www.clinicaltrials.gov as #NCT00112853.

DNA double-strand break repair of blood lymphocytes and normal tissues analysed in a preclinical mouse model: implications for radiosensitivity testing

PURPOSE

Radiotherapy is an effective cancer treatment, but a few patients suffer severe radiation toxicities in neighboring normal tissues. There is increasing evidence that the variable susceptibility to radiation toxicities is caused by the individual genetic predisposition, by subtle mutations, or polymorphisms in genes involved in cellular responses to ionizing radiation. Double-strand breaks (DSB) are the most deleterious form of radiation-induced DNA damage, and DSB repair deficiencies lead to pronounced radiosensitivity. Using a preclinical mouse model, the highly sensitive gammaH2AX-foci approach was tested to verify even subtle, genetically determined DSB repair deficiencies known to be associated with increased normal tissue radiosensitivity.

EXPERIMENTAL DESIGN

By enumerating gammaH2AX-foci in blood lymphocytes and normal tissues (brain, lung, heart, and intestine), the induction and repair of DSBs after irradiation with therapeutic doses (0.1-2 Gy) was investigated in repair-proficient and repair-deficient mouse strains in vivo and blood samples irradiated ex vivo.

RESULTS

gammaH2AX-foci analysis allowed to verify the different DSB repair deficiencies; even slight impairments caused by single polymorphisms were detected similarly in both blood lymphocytes and solid tissues, indicating that DSB repair measured in lymphocytes is valid for different and complex organs. Moreover, gammaH2AX-foci analysis of blood samples irradiated ex vivo was found to reflect repair kinetics measured in vivo and, thus, give reliable information about the individual DSB repair capacity.

CONCLUSIONS

gammaH2AX analysis of blood and tissue samples allows to detect even minor genetically defined DSB repair deficiencies, affecting normal tissue radiosensitivity. Future studies will have to evaluate the clinical potential to identify patients more susceptible to radiation toxicities before radiotherapy.

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.

DNA double-strand break rejoining in complex normal tissues

PURPOSE

The clinical radiation responses of different organs vary widely and likely depend on the intrinsic radiosensitivities of their different cell populations. Double-strand breaks (DSBs) are the most deleterious form of DNA damage induced by ionizing radiation, and the cells' capacity to rejoin radiation-induced DSBs is known to affect their intrinsic radiosensitivity. To date, only little is known about the induction and processing of radiation-induced DSBs in complex normal tissues. Using an in vivo model with repair-proficient mice, the highly sensitive gammaH2AX immunofluorescence was established to investigate whether differences in DSB rejoining could account for the substantial differences in clinical radiosensitivity observed among normal tissues.

METHODS AND MATERIALS

After whole body irradiation of C57BL/6 mice (0.1, 0.5, 1.0, and 2.0 Gy), the formation and rejoining of DSBs was analyzed by enumerating gammaH2AX foci in various organs representative of both early-responding (small intestine) and late-responding (lung, brain, heart, kidney) tissues.

RESULTS

The linear dose correlation observed in all analyzed tissues indicated that gammaH2AX immunofluorescence allows for the accurate quantification of DSBs in complex organs. Strikingly, the various normal tissues exhibited identical kinetics for gammaH2AX foci loss, despite their clearly different clinical radiation responses.

CONCLUSION

The identical kinetics of DSB rejoining measured in different organs suggest that tissue-specific differences in radiation responses are independent of DSB rejoining. This finding emphasizes the fundamental role of DSB repair in maintaining genomic integrity, thereby contributing to cellular viability and functionality and, thus, tissue homeostasis.

Gamma-H2AX in recognition and signaling of DNA double-strand breaks in the context of chromatin

DNA double-strand breaks (DSBs) are extremely dangerous lesions with severe consequences for cell survival and the maintenance of genomic stability. In higher eukaryotic cells, DSBs in chromatin promptly initiate the phosphorylation of the histone H2A variant, H2AX, at Serine 139 to generate γ-H2AX. This phosphorylation event requires the activation of the phosphatidylinositol-3-OH-kinase-like family of protein kinases, DNA-PKcs, ATM, and ATR, and serves as a landing pad for the accumulation and retention of the central components of the signaling cascade initiated by DNA damage. Regions in chromatin with γ-H2AX are conveniently detected by immunofluorescence microscopy and serve as beacons of DSBs. This has allowed the development of an assay that has proved particularly useful in the molecular analysis of the processing of DSBs. Here, we first review the role of γ-H2AX in DNA damage response in the context of chromatin and discuss subsequently the use of this modification as a surrogate marker for mechanistic studies of DSB induction and processing. We conclude with a critical analysis of the strengths and weaknesses of the approach and present some interesting applications of the resulting methodology.

Repair of HZE-particle-induced DNA double-strand breaks in normal human fibroblasts

DNA damage generated by high-energy and high-Z (HZE) particles is more skewed toward multiply damaged sites or clustered DNA damage than damage induced by low-linear energy transfer (LET) X and gamma rays. Clustered DNA damage includes abasic sites, base damages and single- (SSBs) and double-strand breaks (DSBs). This complex DNA damage is difficult to repair and may require coordinated recruitment of multiple DNA repair factors. As a consequence of the production of irreparable clustered lesions, a greater biological effectiveness is observed for HZE-particle radiation than for low-LET radiation. To understand how the inability of cells to rejoin DSBs contributes to the greater biological effectiveness of HZE particles, the kinetics of DSB rejoining and cell survival after exposure of normal human skin fibroblasts to a spectrum of HZE particles was examined. Using gamma-H2AX as a surrogate marker for DSB formation and rejoining, the ability of cells to rejoin DSBs was found to decrease with increasing Z; specifically, iron-ion-induced DSBs were repaired at a rate similar to those induced by silicon ions, oxygen ions and gamma radiation, but a larger fraction of iron-ion-induced damage was irreparable. Furthermore, both DNA-PKcs (DSB repair factor) and 53BP1 (DSB sensing protein) co-localized with gamma-H2AX along the track of dense ionization produced by iron and silicon ions and their focus dissolution kinetics was similar to that of gamma-H2AX. Spatial co-localization analysis showed that unlike gamma-H2AX and 53BP1, phosphorylated DNA-PKcs was localized only at very specific regions, presumably representing the sites of DSBs within the tracks. Examination of cell survival by clonogenic assay indicated that cell killing was greater for iron ions than for silicon and oxygen ions and gamma rays. Collectively, these data demonstrate that the inability of cells to rejoin DSBs within clustered DNA lesions likely contributes to the greater biological effectiveness of HZE particles.

Histone H2AX phosphorylation as a molecular pharmacological marker for DNA interstrand crosslink cancer chemotherapy

The aims of this study were to investigate mechanisms of action involved in H2AX phosphorylation by DNA interstrand crosslinking (ICL) agents and determine whether gammaH2AX could be a suitable pharmacological marker for identifying potential ICL cellular chemosensitivity. In normal human fibroblasts, after treatment with nitrogen mustard (HN2) or cisplatin, the peak gammaH2AX response was detected 2-3 h after the peak of DNA ICLs measured using the comet assay, a validated method for detecting ICLs in vitro or in clinical samples. Detection of gammaH2AX foci by immunofluorescence microscopy could be routinely detected with 6-10 times lower concentrations of both drugs compared to detection of ICLs using the comet assay. A major pathway for repairing DNA ICLs is the initial unhooking of the ICL by the ERCC1-XPF endonuclease followed by homologous recombination. HN2 or cisplatin-induced gammaH2AX foci persisted significantly longer in both, ERCC1 or XRCC3 (homologous recombination) defective Chinese hamster cells that are highly sensitive to cell killing by ICL agents compared to wild type or ionising radiation sensitive XRCC5 cells. An advantage of using gammaH2AX immunofluorescence over the comet assay is that it appears to detect ICL chemosensitivity in both ERCC1 and HR defective cells. With HN2 and cisplatin, gammaH2AX foci also persisted in chemosensitive human ovarian cancer cells (A2780) compared to chemoresistant (A2780cisR) cells. These results show that gammaH2AX can act as a highly sensitive and general marker of DNA damage induced by HN2 or cisplatin and shows promise for predicting potential cellular chemosensitivity to ICL agents.

Hypoxia and metabolism. Hypoxia, DNA repair and genetic instability

Areas of hypoxic tumour tissue are known to be resistant to treatment and are associated with a poor clinical prognosis. There are several reasons why this might be, including the capacity of hypoxia to drive genomic instability and alter DNA damage repair pathways. Significantly, current models fail to distinguish between the complexities of the hypoxic microenvironment and the biological effects of acute hypoxia exposures versus longer-term, chronic hypoxia exposures on the transcription and translation of proteins involved in genetic stability and cell survival. Acute and chronic hypoxia might lead to different biology within the tumour and this might have a direct effect on the design of new therapies for the treatment of hypoxic tumours.

Biochemical kinetics model of DSB repair and induction of gamma-H2AX foci by non-homologous end joining

We developed a biochemical kinetics approach to describe the repair of double-strand breaks (DSBs) produced by low-LET radiation by modeling molecular events associated with non-homologous end joining (NHEJ). A system of coupled nonlinear ordinary differential equations describes the induction of DSBs and activation pathways for major NHEJ components including Ku70/80, DNA-PKcs, and the ligase IV-XRCC4 heterodimer. The autophosphorylation of DNA-PKcs and subsequent induction of gamma-H2AX foci observed after ionizing radiation exposure were modeled. A two-step model of regulation of repair by DNA-PKcs was developed with an initial step allowing access of other NHEJ components to breaks and a second step limiting access to ligase IV-XRCC4. Our model assumes that the transition from the first to the second step depends on DSB complexity, with a much slower rate for complex DSBs. The model faithfully reproduced several experimental data sets, including DSB rejoining as measured by pulsed-field gel electrophoresis (PFGE) at 10 min postirradiation or longer and quantification of the induction of gamma-H2AX foci. A process that is independent of DNA-PKcs is required for the model to reproduce experimental data for rejoining before 10 min postirradiation. Predictions are made for the behaviors of NHEJ components at low doses and dose rates, and a steady state is found at dose rates of 0.1 Gy/h or lower.