Residual γH2AX after irradiation of human lymphocytes and monocytes in vitro and its relation to late effects after prostate brachytherapy

Assessment of Individual Radiosensitivity in Breast Cancer Patients Using a Combination of Biomolecular Markers

About 5% of patients undergoing radiotherapy (RT) develop RT-related side effects. To assess individual radiosensitivity, we collected peripheral blood from breast cancer patients before, during and after the RT, and γH2AX/53BP1 foci, apoptosis, chromosomal aberrations (CAs) and micronuclei (MN) were analyzed and correlated with the healthy tissue side effects assessed by the RTOG/EORTC criteria. The results showed a significantly higher level of γH2AX/53BP1 foci before the RT in radiosensitive (RS) patients in comparison to normal responding patients (NOR). Analysis of apoptosis did not reveal any correlation with side effects. CA and MN assays displayed an increase in genomic instability during and after RT and a higher frequency of MN in the lymphocytes of RS patients. We also studied time kinetics of γH2AX/53BP1 foci and apoptosis after in vitro irradiation of lymphocytes. Higher levels of primary 53BP1 and co-localizing γH2AX/53BP1 foci were detected in cells from RS patients as compared to NOR patients, while no difference in the residual foci or apoptotic response was found. The data suggested impaired DNA damage response in cells from RS patients. We suggest γH2AX/53BP1 foci and MN as potential biomarkers of individual radiosensitivity, but they need to be evaluated with a larger cohort of patients for clinics.

Improving Patients' Life Quality after Radiotherapy Treatment by Predicting Late Toxicities

Personalized treatment and precision medicine have become the new standard of care in oncology and radiotherapy. Because treatment outcomes have considerably improved over the last few years, permanent side-effects are becoming an increasingly significant issue for cancer survivors. Five to ten percent of patients will develop severe late toxicity after radiotherapy. Identifying these patients before treatment start would allow for treatment adaptation to minimize definitive side effects that could impair their long-term quality of life. Over the last decades, several tests and biomarkers have been developed to identify these patients. However, out of these, only the Radiation-Induced Lymphocyte Apoptosis (RILA) assay has been prospectively validated in multi-center cohorts. This test, based on a simple blood draught, has been shown to be correlated with late radiation-induced toxicity in breast, prostate, cervical and head and neck cancer. It could therefore greatly improve decision making in precision radiation oncology. This literature review summarizes the development and bases of this assay, as well as its clinical results and compares its results to the other available assays.

Biomarkers for translational oncology - Peggy Olive's contribution

PURPOSE

Peggy Olive of the BC cancer research center (BCCRC), Vancouver, Canada, dedicated her career to improving the efficiency of radiation in the treatment of cancer. Keenly interested in the study of hypoxic cell radiosensitizers, she recognized the importance of DNA repair in improving the efficacy of radiotherapy. At the BCCRC she developed two methods for clinical practice that detect and quantitate DNA damage in mammalian cells. The alkaline comet assay and phosphorylated gamma histone H2AX (γH2AX) protein foci staining were two sensitive and attractive techniques that she attempted to apply in clinical practice.

CONCLUSION

Peggy Olive was able to establish the comet and the γH2AX assays as prospective predictive biomarkers in the application of personalized radiation treatment and improved cancer treatment outcomes. Nevertheless, several studies with a large number of samples are required before application of these biomarkers in routine radiotherapy could become a reality. The advent of 'omis' and microchip technologies envisage successful outcomes of future research in this direction.

Gamma-H2AX Foci Decay Ratio as a Stronger Predictive Factor of Late Radiation Toxicity Than Dose-Volume Parameters in a Prospective Cohort of Prostate Cancer Patients

PURPOSE

Late radiation toxicity is a major dose-limiting factor in curative cancer radiation therapy. Previous studies identified several risk factors for late radiation toxicity, including both dose-volume factors and genetic predisposition. Herein, we investigated the contribution of genetic predisposition, particularly compared with dose-volume factors, to the risk of late radiation toxicity in patients treated with highly conformal radiation therapy.

METHODS AND MATERIALS

We included 179 patients with prostate cancer who underwent treatment with curative external beam radiation therapy between 2009 and 2013. Toxicity was graded according to the Common Terminology Criteria for Adverse Events version 4.0. Transcriptional responsiveness of homologous recombination repair genes and γ-H2AX foci decay ratios (FDRs) were determined in ex vivo irradiated lymphocytes in a previous analysis. Dose-volume parameters were retrieved by delineating the organs at risk (OARs) on CT planning images. Associations between risk factors and grade ≥2 urinary and bowel late radiation toxicities were assessed using univariable and multivariable logistic regression analyses. The analyses were performed using the highest toxicity grade recorded during the follow-up per patient.

RESULTS

The median follow-up period was 31 months. One hundred and one patients (56%) developed grade ≥2 late radiation toxicity. Cumulative rates for urinary and bowel grade ≥2 late toxicities were 46% and 17%, respectively. In the multivariable analysis, factors significantly associated with grade ≥2 late toxicity were transurethral resection of the prostate (P = .013), γ-H2AX FDR <3.41 (P = .008), and rectum V70 >11.52% (P = .017).

CONCLUSIONS

Our results suggest that impaired DNA double-strand break repair in lymphocytes, as quantified by γ-H2AX FDR, is the most critical determining factor of late radiation toxicity. The limited influence of dose-volume parameters could be due to the use of increasingly conformal techniques, leading to improved dose-volume parameters of the organs at risk.

Evaluation of DNA double-strand break repair capacity in human cells: Critical overview of current functional methods

DNA double-strand breaks (DSBs) are highly deleterious lesions, responsible for mutagenesis, chromosomal translocation or cell death. DSB repair (DSBR) is therefore a critical part of the DNA damage response (DDR) to restore molecular and genomic integrity. In humans, this process is achieved through different pathways with various outcomes. The balance between DSB repair activities varies depending on cell types, tissues or individuals. Over the years, several methods have been developed to study variations in DSBR capacity. Here, we mainly focus on functional techniques, which provide dynamic information regarding global DSB repair proficiency or the activity of specific pathways. These methods rely on two kinds of approaches. Indirect techniques, such as pulse field gel electrophoresis (PFGE), the comet assay and immunofluorescence (IF), measure DSB repair capacity by quantifying the time-dependent decrease in DSB levels after exposure to a DNA-damaging agent. On the other hand, cell-free assays and reporter-based methods directly track the repair of an artificial DNA substrate. Each approach has intrinsic advantages and limitations and despite considerable efforts, there is currently no ideal method to quantify DSBR capacity. All techniques provide different information and can be regarded as complementary, but some studies report conflicting results. Parameters such as the type of biological material, the required equipment or the cost of analysis may also limit available options. Improving currently available methods measuring DSBR capacity would be a major step forward and we present direct applications in mechanistic studies, drug development, human biomonitoring and personalized medicine, where DSBR analysis may improve the identification of patients eligible for chemo- and radiotherapy.

NFATC2 Modulates Radiation Sensitivity in Dermal Fibroblasts From Patients With Severe Side Effects of Radiotherapy

Although it is well established that 5 to 15% of radiotherapy patients exhibit severe side-effects in non-cancerous tissues, the molecular mechanisms involved are still poorly known, and the links between cellular and tissue radiosensitivity are still debated. We here studied fibroblasts from non-irradiated skin of patients with severe sequelae of radiotherapy, to determine whether specific basal cell activities might be involved in susceptibility to side-effects in normal tissues. Compared to control cells, patient fibroblasts exhibited higher radiosensitivity together with defects in DNA repair. Transcriptome profiling of dermal fibroblasts from 16 radiotherapy patients with severe side-effects and 8 healthy individuals identified 540 genes specifically deregulated in the patients. Nuclear factor of activated T cells 2 (NFATC2) was the most differentially expressed gene, poorly expressed at both transcript and protein level, whereas the NFATC2 gene region was hypermethylated. Furthermore, NFATC2 expression correlated with cell survival after irradiation. Finally, silencing NFATC2 in normal cells by RNA interference led to increased cellular radiosensitivity and defects in DNA repair. This study demonstrates that patients with clinical hypersensitivity also exhibit intrinsic cellular radiosensitivity in their normal skin cells. It further reveals a new role for NFATC2 as a potential regulator of cellular sensitivity to ionizing radiation.

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

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

[Treatment of radiation-induced late effects: What's new?]

Mechanisms of late radio-induced lesions are the result of multiple and complex phenomena, with many entangled cellular and tissue factors. The biological continuum between acute and late radio-induced effects will be described, with firstly a break in homeostasis that leads to cellular redistributions. New insights into late toxicity will finally be addressed. Individual radiosensitivity is a primary factor for the development of late toxicity, and clinicians urgently need predictive tests to offer truly personalized radiation therapy. An update will be made on the various functional and genetic tests currently being validated. The management of radio-induced side effects remains a frequent issue for radiation oncologists, and an update will be made for certain specific clinical situations. Finally, an innovative management for patients with significant side effects after pelvic radiotherapy will be developed, involved mesenchymal stem cell transplantation, with the presentation of the "PRISME" protocol currently open to patients recruitment.

Radiobiology of brachytherapy: The historical view based on linear quadratic model and perspectives for optimization

Most preclinical studies examining the radiobiology of brachytherapy have focused on dose rate effects. Scarcer data are available on other major parameters of therapeutic index, such as cell cycle distribution, repopulation or reoxygenation. The linear quadratic model describes the effect of radiotherapy in terms of normal tissue or tumour response. It allows some comparisons between various irradiation schemes. This model should be applied cautiously for brachytherapy, because it relies on cell death analysis only, and therefore partially reflects the biological effects of an irradiation. Moreover, the linear quadratic model validity has not been demonstrated for very high doses per fraction. A more thorough analysis of mechanisms involved in radiation response is required to better understand the true effect of brachytherapy on normal tissue. The modulation of immune response is one promising strategy to be tested with brachytherapy. A translational approach applied to brachytherapy should lead to design trials testing pharmacological agents modulating radiation response, in order to improve not only local control, but also decrease the risk of distant failure. Here we review the radiobiology of brachytherapy, from the historical view based on linear quadratic model to recent perspectives for biological optimization.

Clinical and Functional Assays of Radiosensitivity and Radiation-Induced Second Cancer

Whilst the near instantaneous physical interaction of radiation energy with living cells leaves little opportunity for inter-individual variation in the initial yield of DNA damage, all the downstream processes in how damage is recognized, repaired or resolved and therefore the ultimate fate of cells can vary across the population. In the clinic, this variability is observed most readily as rare extreme sensitivity to radiotherapy with acute and late tissue toxic reactions. Though some radiosensitivity can be anticipated in individuals with known genetic predispositions manifest through recognizable phenotypes and clinical presentations, others exhibit unexpected radiosensitivity which nevertheless has an underlying genetic cause. Currently, functional assays for cellular radiosensitivity represent a strategy to identify patients with potential radiosensitivity before radiotherapy begins, without needing to discover or evaluate the impact of the precise genetic determinants. Yet, some of the genes responsible for extreme radiosensitivity would also be expected to confer susceptibility to radiation-induced cancer, which can be considered another late adverse event associated with radiotherapy. Here, the utility of functional assays of radiosensitivity for identifying individuals susceptible to radiotherapy-induced second cancer is discussed, considering both the common mechanisms and important differences between stochastic radiation carcinogenesis and the range of deterministic acute and late toxic effects of radiotherapy.

A Single Exposure to Low- or High-LET Radiation Induces Persistent Genomic Damage in Mouse Epithelial Cells In Vitro and in Lung Tissue

Exposures to low- and high-linear energy transfer (LET) radiation induce clustered damage in DNA that is difficult to repair. These lesions are manifested as DNA-associated foci positive for DNA repair proteins and have been shown to persist in vitro and in vivo for days in several cell types and tissues in response to low-LET radiation. Although in some experimental conditions these residual foci have been linked with genomic instability and chromosomal aberrations, it remains poorly understood what type of damage they represent. Because high-LET radiation induces complex DNA lesions more efficiently than low-LET radiation, we compared the efficacy of several heavy ions (oxygen, silicon and iron) in a range (17 , 70 and 175 keV/μm, respectively) of LET and X rays at a 1 Gy dose. Persistent genomic damage was measured by γ-H2AX-53BP1-positive residual foci and micronucleus levels during the first three days and up to a week after in vitro and in vivo irradiation in lung cells and tissue. We demonstrate that in an in vitro irradiated mouse bronchial epithelial cell line, the expression of residual foci is readily detectable at 24 h with levels declining in the following 72 h postirradiation, but still persisting elevated over background at day 7. At this time, foci numbers are low but significant and proportional to the dose and quality of the radiation. The expression of residual foci in vitro was mirrored by increased micronuclei generation measured in cytokinesis-blocked cells, indicating long-term, persistent effects of genomic damage in this cell type. We also tested the expression of residual foci in lung tissue of C57BL/6 mice that received whole-body X-ray or heavy-ion irradiation. We found that at day 7 postirradiation, Clara/Club cells, but not pro-SPC-positive pneumocytes, contained a subpopulation of cells expressing γ-H2AX-53BP1-positive foci in a radiation quality-dependent manner. These findings suggest that in vivo persistent DNA repair foci reflect the initial genotoxic damage induced by radiation and a differential vulnerability among cells in the lung.

Data-Based Radiation Oncology: Design of Clinical Trials in the Toxicity Biomarkers Era

The ability to stratify patients using a set of biomarkers, which predict that toxicity risk would allow for radiotherapy (RT) modulation and serve as a valuable tool for precision medicine and personalized RT. For patients presenting with tumors with a low risk of recurrence, modifying RT schedules to avoid toxicity would be clinically advantageous. Indeed, for the patient at low risk of developing radiation-associated toxicity, use of a hypofractionated protocol could be proposed leading to treatment time reduction and a cost-utility advantage. Conversely, for patients predicted to be at high risk for toxicity, either a more conformal form or a new technique of RT, or a multidisciplinary approach employing surgery could be included in the trial design to avoid or mitigate RT when the potential toxicity risk may be higher than the risk of disease recurrence. In addition, for patients at high risk of recurrence and low risk of toxicity, dose escalation, such as a greater boost dose, or irradiation field extensions could be considered to improve local control without severe toxicities, providing enhanced clinical benefit. In cases of high risk of toxicity, tumor control should be prioritized. In this review, toxicity biomarkers with sufficient evidence for clinical testing are presented. In addition, clinical trial designs and predictive models are described for different clinical situations.

Local tumor control and DNA-PK activity of peripheral blood lymphocytes in prostate cancer patients receiving radiotherapy

Repair of DNA damage is critical for genomic stability, and DNA-dependent protein kinase (DNA-PK) has an important role in repairing double-strand breaks. We examined whether the DNA-PK activity of peripheral blood lymphocytes (PBLs) was related to biochemical (prostate-specific antigen: PSA) relapse and radiation toxicity in prostate cancer patients who have received radiotherapy. A total of 69 patients with localized adenocarcinoma of the prostate participated in this study. Peripheral blood was collected 2 years or later after radiotherapy and centrifuged, then DNA-PK activity was measured by a filter binding assay. The high DNA-PK activity group had a significantly higher PSA relapse-free survival rate than the low DNA-PK activity group. The 10-year PSA relapse-free survival was 87.0% in the high DNA-PK activity group, whereas it was 52.7% in the low DNA-PK activity group. Multivariate analysis showed the Gleason score and the level of DNA-PK activity were significant predictors of PSA relapse after radiotherapy. In addition, the low DNA-PK activity group tended to have a higher incidence of Grade 1-2 urinary toxicity than the high DNA-PK activity group. Prostate cancer patients with low DNA-PK activity had a higher rate of PSA relapse and a higher incidence of urinary toxicity. DNA-PK activity in PBLs might be a useful marker for predicting PSA relapse and urinary toxicity, possibly contributing to personalized treatment of prostate cancer.

Compromized DNA repair as a basis for identification of cancer radiotherapy patients with extreme radiosensitivity

A small percentage of cancer radiotherapy patients develop abnormally severe side effects as a consequence of intrinsic radiosensitivity. We analysed the γ-H2AX response to ex-vivo irradiation of peripheral blood lymphocytes (PBL) and plucked eyebrow hair follicles from 16 patients who developed severe late radiation toxicity following radiotherapy, and 12 matched control patients. Longer retention of the γ-H2AX signal and lower colocalization efficiency of repair factors in over-responding patients confirmed that DNA repair in these individuals was compromised. Five of the radiosensitive patients harboured LoF mutations in DNA repair genes. An extensive range of quantitative parameters of the γ-H2AX response were studied with the objective to establish a predictor for radiosensitivity status. The most powerful predictor was the combination of the fraction of the unrepairable component of γ-H2AX foci and repair rate in PBL, both derived from non-linear regression analysis of foci repair kinetics. We introduce a visual representation of radiosensitivity status that allocates a position for each patient on a two-dimensional "radiosensitivity map". This analytical approach provides the basis for larger prospective studies to further refine the algorithm, ultimately to triage capability.

Correlation between DNA damage responses of skin to a test dose of radiation and late adverse effects of earlier breast radiotherapy

AIM

To correlate residual double strand breaks (DSB) 24h after 4Gy test doses to skin in vivo and to lymphocytes in vitro with adverse effects of earlier breast radiotherapy (RT).

PATIENTS AND METHODS

Patients given whole breast RT ⩾5years earlier were identified on the basis of moderate/marked or minimal/no adverse effects despite the absence ('RT-Sensitive', RT-S) or presence ('RT-Resistant', RT-R) of variables predisposing to late adverse effects. Residual DSB were quantified in skin 24h after a 4Gy test dose in 20 RT-S and 15 RT-R patients. Residual DSB were quantified in lymphocytes irradiated with 4Gy in vitro in 30/35 patients.

RESULTS

Mean foci per dermal fibroblast were 3.29 (RT-S) vs 2.80 (RT-R) (p=0.137); 3.28 (RT-S) vs 2.60 (RT-R) in endothelium (p=0.158); 2.50 (RT-S) vs 2.41 (RT-R) in suprabasal keratinocytes (p=0.633); 2.70 (RT-S) vs 2.35 (RT-R) in basal epidermis (p=0.419); 12.1 (RT-S) vs 10.3 (RT-R) in lymphocytes (p=0.0052).

CONCLUSIONS

Residual DSB in skin following a 4Gy dose were not significantly associated with risk of late adverse effects of breast radiotherapy, although exploratory analyses suggested an association in severely affected individuals. By contrast, a significant association was detected based on the in vitro response of lymphocytes.

The γH2AX DNA damage assay from a drop of blood

DNA double-strand breaks (DSB) and blocked replication forks activate the DNA damage response (DDR), a signaling pathway marked by phosphorylation of histone 2AX (H2AX). The phosphorylated form, γH2AX, accumulates at the site of damage and can be detected as foci by immunocytochemistry. Therefore, γH2AX is a sensitive and robust biomarker of DNA damage, notably DSB. Cells from peripheral blood are often used for studies on genotoxic exposure of humans. They are limited, however, by the amount of blood required and the costly blood purification method. Here, we present a method that enables the detection of DNA damage by the analysis of γH2AX foci in a drop of blood. The blood drop method (BDM) is simple, fast, inexpensive and allows large series of blood sampling and storage over time. It can be combined with genotoxic treatment of cells in the collected blood sample for experimental purposes on DNA damage induction and repair. The BDM is suitable for rapid and large-scale screenings of genetic damage in human and animal populations.

The yield of DNA double strand breaks determined after exclusion of those forming from heat-labile lesions predicts tumor cell radiosensitivity to killing

BACKGROUND AND PURPOSE

The radiosensitivity to killing of tumor cells and in-field normal tissue are key determinants of radiotherapy response. In vitro radiosensitivity of tumor- and normal-tissue-derived cells often predicts radiation response, but high determination cost in time and resources compromise utility as routine response-predictor. Efforts to use induction or repair of DNA double-strand-breaks (DSBs) as surrogate-predictors of cell radiosensitivity to killing have met with limited success. Here, we re-visit this issue encouraged by our recent observations that ionizing radiation (IR) induces not only promptly-forming DSBs (prDSBs), but also DSBs developing after irradiation from the conversion to breaks of thermally-labile sugar-lesions (tlDSBs).

MATERIALS AND METHODS

We employ pulsed-field gel-electrophoresis and flow-cytometry protocols to measure total DSBs (tDSB=prDSB+tlDSBs) and prDSBs, as well as γH2AX and parameters of chromatin structure.

RESULTS

We report a fully unexpected and in many ways unprecedented correlation between yield of prDSBs and radiosensitivity to killing in a battery of ten tumor cell lines that is not matched by yields of tDSBs or γH2AX, and cannot be explained by simple parameters of chromatin structure.

CONCLUSIONS

We propose the introduction of prDSBs-yield as a novel and powerful surrogate-predictor of cell radiosensitivity to killing with potential for clinical application.

DNA damage foci: Meaning and significance

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

Relation between Ku80 and microRNA-99a expression and late rectal bleeding after radiotherapy for prostate cancer

BACKGROUND AND PURPOSE

Late rectal bleeding is one of the severe adverse events after radiotherapy for prostate cancer. New biomarkers are needed to allow a personalized treatment.

MATERIALS AND METHODS

Four patients each with grade 0-1 or grade 2-3 rectal bleeding were randomly selected for miRNA array to examine miRNA expression in peripheral blood lymphocytes (PBLs). Based on results of miRNA array, 1 of 348 miRNAs was selected for microRNA assays. Then, expression of DNA-dependent protein kinase mRNA and miR-99a was analyzed in the PBLs of 97 patients. PBLs were exposed to 4Gy of X-ray ex-vivo.

RESULTS

In the discovery cohort, grade 2-3 rectal bleeding was significantly higher in the Ku80 <1.09 expression group compared with ⩾1.09 group (P=0.011). In radiation-induced expression of miR-99a, grade 2-3 rectal bleeding was significantly higher in the miR-99a IR(+)/IR(-) >0.93 group compared with ⩽0.93 group (P=0.013). Most patients with grade 2-3 rectal bleeding were in the group with low Ku80 and high miR-99a expression. In the validation cohort, similar results were obtained.

CONCLUSION

A combination of low Ku80 expression and highly-induced miR-99a expression could be a promising marker for predicting rectal bleeding after radiotherapy.

Repair of ionizing radiation-induced DNA damage and risk of second cancer in childhood cancer survivors

The study's purpose was to assess whether individuals who developed a second malignant neoplasm (SMN) after treatment for a first malignant neoplasm (FMN) had a lower ability to repair DNA double-strand breaks (DSBs) using a bioassay with γH2AX intensity as a surrogate endpoint. In a case-control study nested in a cohort of childhood cancer survivors, lymphoblastoid cell lines (LCLs) were established from blood samples collected from 94 cases (SMN) and 94 matched controls (FMN). LCLs were irradiated with ionizing radiation (2 and 5 Gy) and γH2AX intensities measured 1, 3, 5 and 24h post-irradiation. Differences in mean γH2AX intensity between cases and controls were compared using Kruskal-Wallis tests. Generalized linear models for repeated measures and conditional logistic regressions for SMN risk estimates were performed. The mean baseline γH2AX intensity measured without irradiation was 9.1 [95% confidence interval (95% CI): 8.5-9.7] in the LCLs from cases and 6.4 (95% CI: 6.0-6.8) from controls (P < 0.001). Markedly higher γH2AX intensity, particularly at 1 h post-irradiation, was also found in the LCLs from the cases compared with the controls for all FMNs and for different types of FMN. Chemotherapy and radiation doses received by bone marrow and thymus for FMN treatment showed a non-significant effect on γH2AX intensity. This case-control study shows that higher baseline and post-irradiation levels of DNA DSBs, as measured by γH2AX intensity, are associated with the risk of SMN in childhood cancer survivors. Further investigations in a prospective setting are warranted to confirm this association.

γH2AX as a marker of DNA double strand breaks and genomic instability in human population studies

DNA double strand breaks (DSB) are the gravest form of DNA damage in eukaryotic cells. Failure to detect DSB and activate appropriate DNA damage responses can cause genomic instability, leading to tumorigenesis and possibly accelerated aging. Phosphorylated histone H2AX (γH2AX) is used as a biomarker of cellular response to DSB and its potential for monitoring DNA damage and repair in human populations has been explored in this review. A systematic search was conducted in PubMed for articles, in English, on human studies reporting γH2AX as a biomarker of either DNA repair or DNA damage. A total of 68 publications were identified. Thirty-four studies (50.0%) evaluated the effect of medical procedures or treatments on γH2AX levels; 20 (29.4%) monitored γH2AX in specific pathological conditions with a case/control or case/case design; 5 studies (7.4%) evaluated the effect of environmental genotoxic exposures, and 9 (13.2%) were descriptive studies on cancer and aging. Peripheral blood lymphocytes (44.6%) or biopsies/tissue specimens (24.3%) were the most commonly used samples. γH2AX was scored by optical microscopy as immunostained foci (78%), or by flow cytometry (16%). Critical features affecting the reliability of the assay, including protocols heterogeneity, specimen, cell cycle, kinetics, study design, and statistical analysis, are hereby discussed. Because of its sensitivity, efficiency and mechanistic relevance, the γH2AX assay has great potential as a DNA damage biomarker; however, the technical and epidemiological heterogeneity highlighted in this review infer a necessity for experimental standardization of the assay.

Correlation of in vitro lymphocyte radiosensitivity and gene expression with late normal tissue reactions following curative radiotherapy for breast cancer

BACKGROUND AND PURPOSE

Identification of mechanisms of late normal tissue responses to curative radiotherapy that discriminate individuals with marked or mild responses would aid response prediction. This study aimed to identify differences in gene expression, apoptosis, residual DNA double strand breaks and chromosomal damage after in vitro irradiation of lymphocytes in a series of patients with marked (31 cases) or mild (28 controls) late adverse reaction to adjuvant breast radiotherapy.

MATERIALS AND METHODS

Gene expression arrays, residual γH2AX, apoptosis, G2 chromosomal radiosensitivity and G0 micronucleus assay were used to compare case and control lymphocyte radiation responses.

RESULTS

Five hundred and thirty genes were up-regulated and 819 down-regulated by ionising radiation. Irradiated samples were identified with an overall cross-validated error rate of 3.4%. Prediction analyses to classify cases and controls using unirradiated (0Gy), irradiated (4Gy) or radiation response (4-0Gy) expression profiles correctly identified samples with, respectively, 25%, 22% or 18.5% error rates. Significant inter-sample variation was observed for all cellular endpoints but cases and controls could not be distinguished.

CONCLUSIONS

Variation in lymphocyte radiosensitivity does not necessarily correlate with normal tissue response to radiotherapy. Gene expression analysis can predict of radiation exposure and may in the future help prediction of normal tissue radiosensitivity.

Evaluation of different biomarkers to predict individual radiosensitivity in an inter-laboratory comparison--lessons for future studies

Radiotherapy is a powerful cure for several types of solid tumours, but its application is often limited because of severe side effects in individual patients. With the aim to find biomarkers capable of predicting normal tissue side reactions we analysed the radiation responses of cells from individual head and neck tumour and breast cancer patients of different clinical radiosensitivity in a multicentric study. Multiple parameters of cellular radiosensitivity were analysed in coded samples of peripheral blood lymphocytes (PBLs) and derived lymphoblastoid cell lines (LCLs) from 15 clinical radio-hypersensitive tumour patients and compared to age- and sex-matched non-radiosensitive patient controls and 15 lymphoblastoid cell lines from age- and sex- matched healthy controls of the KORA study. Experimental parameters included ionizing radiation (IR)-induced cell death (AnnexinV), induction and repair of DNA strand breaks (Comet assay), induction of yH2AX foci (as a result of DNA double strand breaks), and whole genome expression analyses. Considerable inter-individual differences in IR-induced DNA strand breaks and their repair and/or cell death could be detected in primary and immortalised cells with the applied assays. The group of clinically radiosensitive patients was not unequivocally distinguishable from normal responding patients nor were individual overreacting patients in the test system unambiguously identified by two different laboratories. Thus, the in vitro test systems investigated here seem not to be appropriate for a general prediction of clinical reactions during or after radiotherapy due to the experimental variability compared to the small effect of radiation sensitivity. Genome-wide expression analysis however revealed a set of 67 marker genes which were differentially induced 6 h after in vitro-irradiation in lymphocytes from radio-hypersensitive and non-radiosensitive patients. These results warrant future validation in larger cohorts in order to determine parameters potentially predictive for clinical radiosensitivity.

Assessing 'radiosensitivity' with kinetic profiles of γ-H2AX, 53BP1 and BRCA1 foci

BACKGROUND AND PURPOSE

DNA repair assays to identify radiosensitive patients have had limited clinical implementation due to long turn-around times or limited specificity. This study evaluates γ-H2AX-Irradiation Induced Foci (IRIF) kinetics as a more rapid surrogate for the 'gold standard' colony survival assay (CSA) using several known DNA repair disorders as reference models.

MATERIALS AND METHODS

Radiosensitive cells of known and unknown etiology were studied. γ-H2AX-IRIFs were quantified over 24 h, and the curves were fitted by combining logarithmic growth and exponential decay functions. Fitted values that differed from radionormal controls were considered aberrant and compared to CSA results.

RESULTS

We observed 87% agreement of IRIF data with the CSA for the 14 samples tested. Analysis of γ-H2AX-IRIF kinetics for known repair disorders indicated similarities between an RNF168(-/-) cell line and an RS cell of unknown etiology. These cell lines were further characterized by a reduction in BRCA1-IRIF formation and G2/M checkpoint activation.

CONCLUSIONS

γ-H2AX-IRIF kinetics showed high concordance with the CSA in RS populations demonstrating its potential as a more rapid surrogate assay. This method provides a means to globally identify defective DNA repair pathways in RS cells of unknown etiology through comparison with known DNA repair defects.

Rectal toxicity and rectal dosimetry in low-dose-rate (125)I permanent prostate implants: a long-term study in 1006 patients

OBJECTIVE

To describe the acute and late rectal toxicity in 1006 prostate brachytherapy patients implanted 1998-2003. To determine whether rectal dose-volume histogram as well as patient and treatment factors were associated with rectal toxicity.

METHODS AND MATERIALS

Median followup was 60.7 months. Rectal dosimetry was calculated as dose-volume histogram of the rectum using Day 28 CT-based dosimetry and expressed as volume of the rectum in cc receiving 50%, 100%, and 150% of the prescription dose (VR(50cc), VR(100cc), and VR(150cc), respectively). Univariate and multivariate analyses were performed to examine the influence of patient, implant, dosimetry, and learning curve factors on the development of acute and late toxicities using a modified Radiation Therapy Oncology Group (RTOG) scale. Acute toxicity was analyzed using logistic regression and late toxicity using Cox proportional hazards regression. Analysis of variance was used to examine the association between rectal toxicity and rectal dose.

RESULTS

Rectal dosimetry in 93.5% and rectal toxicity in 96.2% have been recorded. Median VR(100)=1.05cc. Late RTOG Grades 0, 1, 2, 3, and 4 were recorded in 68%, 23%, 7.3%, 0.9%, and 0.2% patients, respectively. On multivariate analysis, acute RTOG ≥2 rectal toxicity was associated with urinary retention (p=0.036) and learning curve (p=0.015); late RTOG ≥2 was associated with the presence of acute toxicity (p=0.0074), higher VR(100) (p=0.030) and learning curve (p=0.027).

CONCLUSIONS

Late rectal RTOG ≥2 rectal toxicity in this cohort was 8%. Increased VR(100), presence of acute rectal toxicity, and learning curve were associated with higher rate of late RTOG ≥2 toxicity. Severe late rectal toxicity after prostate brachytherapy was rare.

Retention of γH2AX foci as an indication of lethal DNA damage

The application of biological responses of tumours to predict clinical responses to treatment represents a challenging goal with the potential to inform treatment decisions and improve outcome. If tumour cell death is the result of the inability of a cell to repair complex DNA damage, and if γH2AX foci mark sites of unrepaired double-strand breaks, then it may be possible to use residual γH2AX foci to identify treatment-resistant tumour cells early in the course of therapy. This review will highlight some of the evidence that supports the idea that residual γH2AX foci, within certain limitations, may be useful as an early indicator of tumour response to radiotherapy in situ, either alone or in combination with chemotherapy.

γH2AX foci as a measure of DNA damage: a computational approach to automatic analysis

The γH2AX focus assay represents a fast and sensitive approach for the detection of one of the critical types of DNA damage - double-strand breaks (DSB) induced by various cytotoxic agents including ionising radiation. Apart from research applications, the assay has a potential in clinical medicine/pathology, such as assessment of individual radiosensitivity, response to cancer therapies, as well as in biodosimetry. Given that generally there is a direct relationship between numbers of microscopically visualised γH2AX foci and DNA DSB in a cell, the number of foci per nucleus represents the most efficient and informative parameter of the assay. Although computational approaches have been developed for automatic focus counting, the tedious and time consuming manual focus counting still remains the most reliable way due to limitations of computational approaches. We suggest a computational approach and associated software for automatic focus counting that minimises these limitations. Our approach, while using standard image processing algorithms, maximises the automation of identification of nuclei/cells in complex images, offers an efficient way to optimise parameters used in the image analysis and counting procedures, optionally invokes additional procedures to deal with variations in intensity of the signal and background in individual images, and provides automatic batch processing of a series of images. We report results of validation studies that demonstrated correlation of manual focus counting with results obtained using our computational algorithm for mouse jejunum touch prints, mouse tongue sections and human blood lymphocytes as well as radiation dose response of γH2AX focus induction for these biological specimens.

Lack of a correlation between γH2AX foci kinetics in lymphocytes and the severity of acute normal tissue reactions during IMRT treatment for head and neck cancer

PURPOSE

To investigate the phosphorylated histone H2A isoform X (γH2AX) foci kinetics as an indicator for the development of acute normal tissue complications during Intensity-Modulated Radiotherapy (IMRT) for head and neck cancer (HNC) patients.

MATERIALS AND METHODS

Microscopic scoring of the γH2AX foci was used to evaluate the DNA-double-strand-break repair capacity in from Ataxia-Telangiectasia (A-T) patients derived lymphoblastoid cell lines (LCL) and T-lymphocytes isolated from 31 IMRT treated HNC patients. Cells were irradiated in vitro with 0.5 Gy given at high-dose-rate (HDR) and examined at several times up to 24 h after irradiation. The patients were subdivided in three groups showing mild, moderate and severe acute normal tissue complications based on their Common Toxicity Criteria grades for dysphagia, mucositis and dermatitis during the radiotherapy course.

RESULTS

For the ATM (Ataxia-Telangiectasia-Mutated) defective LCL, a lower number of radiation-induced foci and a somewhat less efficient repair capacity was observed. No correlation was found between the γH2AX foci kinetics pattern and the risk for acute normal tissue complications among the three patient subgroups.

CONCLUSIONS

Scoring of γH2AX foci after in vitro irradiation of isolated T-lymphocytes of HNC patients cannot be applied to predict for the development of acute normal tissue complications.

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.

Double-strand breaks and the concept of short- and long-term epigenetic memory

Double-strand breaks represent an extremely cytolethal form of DNA damage and thus pose a serious threat to the preservation of genetic and epigenetic information. Though it is well-known that double-strand breaks such as those generated by ionising radiation are among the principal causative factors behind mutations, chromosomal aberrations, genetic instability and carcinogenesis, significantly less is known about the epigenetic consequences of double-strand break formation and repair for carcinogenesis. Double-strand break repair is a highly coordinated process that requires the unravelling of the compacted chromatin structure to facilitate repair machinery access and then restoration of the original undamaged chromatin state. Recent experimental findings have pointed to a potential mechanism for double-strand break-induced epigenetic silencing. This review will discuss some of the key epigenetic regulatory processes involved in double-strand break (DSB) repair and how incomplete or incorrect restoration of chromatin structure can leave a DSB-induced epigenetic memory of damage with potentially pathological repercussions.

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.

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.

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.

Sequence variant discovery in DNA repair genes from radiosensitive and radiotolerant prostate brachytherapy patients

PURPOSE

The presence of intrinsic radiosensitivity within prostate cancer patients may be an important factor contributing to development of radiation toxicity. We investigated whether variants in genes responsible for detecting and repairing DNA damage independently contribute to toxicity following prostate brachytherapy.

EXPERIMENTAL DESIGN

Genomic DNA was extracted from blood samples of 41 prostate brachytherapy patients, 21 with high and 20 with low late toxicity scores. For each patient, 242 PCR amplicons were generated containing 173 exons of eight candidate genes: ATM, BRCA1, ERCC2, H2AFX, LIG4, MDC1, MRE11A, and RAD50. These amplicons were sequenced and all sequence variants were subjected to statistical analysis to identify those associated with late radiation toxicity.

RESULTS

Across 41 patients, 239 sites differed from the human genome reference sequence; 170 of these corresponded to known polymorphisms. Sixty variants, 14 of them novel, affected protein coding regions and 43 of these were missense mutations. In our patient population, the high toxicity group was enriched for individuals with at least one LIG4 coding variant (P = 0.028). One synonymous variant in MDC1, rs28986317, was associated with increased radiosensitivity (P = 0.048). A missense variant in ATM, rs1800057, associated with increased prostate cancer risk, was found exclusively in two high toxicity patients but did not reach statistical significance for association with radiosensitivity (P = 0.488).

CONCLUSIONS

Our data revealed new germ-line sequence variants, indicating that existing sequence databases do not fully represent the full extent of sequence variation. Variants in three DNA repair genes were linked to increased radiosensitivity but require validation in larger populations.

Kinetics of H2AX phosphorylation after exposure to cisplatin

BACKGROUND

Cisplatin is a widely used cancer chemotherapeutic drug that causes DNA crosslinking and stimulates H2AX phosphorylation. Our goal was to assess the potential of gammaH2AX to help predict tumor response to cisplatin treatment.

METHODS

The kinetics of cisplatin-induced DNA interstrand crosslinks was measured using the alkaline comet assay and compared with gammaH2AX formation and clonogenic cell survival in several DNA repair proficient or deficient human and rodent cell lines.

RESULTS

The comet assay was effective in ranking cell lines according to their relative sensitivity to cisplatin based on reduced crosslink formation measured 6 h after drug exposure or by the failure of irs3 and UV41 cell lines to subsequently remove crosslinks. In comparison, the initial rate of phosphorylation of H2AX measured over the first 6 h after cisplatin treatment was unrelated to drug sensitivity or crosslinking proficiency. However, for proliferating cell cultures, the fraction of cells that retained gammaH2AX foci 24 h after cisplatin treatment was correlated with the fraction of cells that lost clonogenic potential (slope = 1.1, r(2) = 0.85).

CONCLUSIONS

H2AX phosphorylation occurs in response to replication fork damage caused by cisplatin induced DNA lesions, probably interstrand crosslinks. Although early kinetics of gammaH2AX formation was uninformative, retention of gammaH2AX foci 24 h after treatment was shown to be a useful indicator of cell response to killing by cisplatin. However, for gammaH2AX to serve as an indicator of cell viability after cisplatin treatment, cells must have the opportunity to transit S phase during the recovery period.

Sex-specific aspects of tumor therapy

There is increasing evidence that sex-specific differences in toxicity profiles and outcome after radiotherapy are accumulating in medical oncology, and that treatment strategies may require some modification. Furthermore, sex-specific differences in the sensitivity to genotoxic and therapeutical agents are also of general concern for risk estimation. This review is focussed on the specific influence of sex on these endpoints covering both a clinical and a biological point of view. In this paper, the literature was systematically reviewed with respect to sex-specific differences in tumor and normal tissue sensitivity after exposure to ionizing radiation, as well as to the relevant underlying molecular and cellular mechanisms. Although a number of data on sex-specific differences are available and remarkable differences on clinical, molecular, and cellular levels have been reported, a firm conclusion on any existing sex-specific differences is not yet possible. Future studies are required and should be focussed on this aspect of individual radiosensitivity.

Urinary symptom flare in 712 125I prostate brachytherapy patients: long-term follow-up

PURPOSE

To describe the late transient worsening of urinary symptoms ("urinary symptom flare") in 712 consecutive prostate brachytherapy patients, associated predictive factors, association with rectal and urinary toxicity, and the development of erectile dysfunction.

METHODS AND MATERIALS

Patients underwent implantation between 1998 and 2003 (median follow-up, 57 months). International Prostate Symptom Score (IPSS), Radiation Therapy Oncology Group (RTOG) toxicity, and erectile function data were prospectively collected. Flare was defined as an increase in IPSS of > or =5 and of > or =8 points greater than the post-treatment nadir. The relationships between the occurrence of flare and the patient, tumor, and treatment characteristics were examined. The Cox proportional hazards method was used to test individual variables and the multivariate models.

RESULTS

The incidence of flare was 52% and 30% using the flare definition of an IPSS of > or =5 and > or =8 points greater than the postimplant nadir, respectively. Of the patients with symptoms, 65% had resolution of their symptoms within 6 months and 91% within 1 year. Flares most commonly occurred 16-24 months after implantation. On multivariate analysis, a greater baseline IPSS and greater maximal postimplant IPSS were the predictors of flare, regardless of the flare definition used. Androgen suppression was a predictor for fewer flares (IPSS > or =5). Diabetes and prostate edema predicted for more frequent flares (IPSS >/=8). Patients with flare had a greater incidence of RTOG Grade 3 urinary toxicity and RTOG Grade 2 or greater rectal toxicity. No association was found between erectile dysfunction and the occurrence of flare.

CONCLUSION

Urinary symptom flare is a common, transient phenomenon after prostate brachytherapy. A greater baseline IPSS and maximal postimplant IPSS were the strongest predictive factors. Flare was associated with a greater incidence of late RTOG Grade 3 urinary toxicity and greater rate of late RTOG Grade 2 or greater rectal toxicity.

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.

Effect of prolonging radiation delivery time on retention of gammaH2AX

Background and purpose

Compared to conventional external beam radiotherapy, IMRT requires significantly more time to deliver the dose. Prolonging dose delivery potentially increases DNA repair which would reduce the biological effect. We questioned whether retention of γH2AX, a measure of lack of repair of DNA damage, would decrease when dose delivery was protracted.

Materials and methods

Exponentially growing SiHa cervical carinoma cells were irradiated with 6 MV photons in a water tank using a VarianEX linear accelerator. Cells held at 37°C received 2 Gy in 0.5 min and 4 Gy in 1 min. To evaluate effect of dose delivery prolongation, 2 and 4 Gy were delivered in 30 and 60 min. After 24 h recovery, cells were analyzed for clonogenic survival and for residual γH2AX as measured using flow cytometry.

Results

Increasing the dose delivery time from 0.5 or 1 min to 30 or 60 min produced a signficant increase in cell survival from 0.45 to 0.48 after 2 Gy, and from 0.17 to 0.20 after 4 Gy. Expression of residual γH2AX decreased from 1.27 to 1.22 relative to background after 2 Gy and 1.46 to 1.39 relative to background after 4 Gy, but differences were not statistically significant. The relative differences in the slopes of residual γH2AX versus dose for acute versus prolonged irradiation bordered on significant (p = 0.055), and the magnitude of the change was consistent with the observed increase in surviving fraction.

Conclusion

These results support the concept that DNA repair underlies the increase in survival observed when dose delivery is prolonged. They also help to establish the limits of sensitivity of residual γH2AX, as measured using flow cytometry, for detecting differences in response to irradiation.