The G2 chromosomal radiosensitivity assay

A 4-Gene Signature of CDKN1, FDXR, SESN1 and PCNA Radiation Biomarkers for Prediction of Patient Radiosensitivity

The quest for the discovery and validation of radiosensitivity biomarkers is ongoing and while conventional bioassays are well established as biomarkers, molecular advances have unveiled new emerging biomarkers. Herein, we present the validation of a new 4-gene signature panel of CDKN1, FDXR, SESN1 and PCNA previously reported to be radiation-responsive genes, using the conventional G2 chromosomal radiosensitivity assay. Radiation-induced G2 chromosomal radiosensitivity at 0.05 Gy and 0.5 Gy IR is presented for a healthy control (n = 45) and a prostate cancer (n = 14) donor cohort. For the prostate cancer cohort, data from two sampling time points (baseline and Androgen Deprivation Therapy (ADT)) is provided, and a significant difference (p > 0.001) between 0.05 Gy and 0.5 Gy was evident for all donor cohorts. Selected donor samples from each cohort also exposed to 0.05 Gy and 0.5 Gy IR were analysed for relative gene expression of the 4-gene signature. In the healthy donor cohort, there was a significant difference in gene expression between IR dose for CDKN1, FXDR and SESN1 but not PCNA and no significant difference found between all prostate cancer donors, unless they were classified as radiation-induced G2 chromosomal radiosensitive. Interestingly, ADT had an effect on radiation response for some donors highlighting intra-individual heterogeneity of prostate cancer donors.

Women's contributions to radiobiology in Ireland; from small beginnings…

OBJECTIVES

To describe the contribution of women radiobiologists in Ireland to the development of the discipline internationally and at home and to discuss the history of radiobiology in Ireland to date. This parallels the history of the evolution of a small radiobiology group in Kevin Street, Dublin Institute of Technology (DIT) which was formerly part of the City of Dublin Vocational Education Committee. There followed years of development first as a radiobiological research center which evolved in the FOCAS Research Institute now embedded within Technological University Dublin (TU Dublin).

CONCLUSIONS

Over the last 45 years, the women of the Radiation and Environmental Science Centre (RESC) contributed to the major paradigm shift in low dose radiobiology contributing exciting new research concerning non-targeted effects, including discovery of lethal mutations, medium transfer bystander mechanisms, and signaling pathways. They also developed translational research using human explant culture systems with unique immunocytochemical methods and more recently evolved to molecular and spectroscopic analysis of clinical samples. The RESC also developed unique in vitro research methods into effects of radiation on non-human species of concern in ecosystems.

G2/M Checkpoint Abrogation With Selective Inhibitors Results in Increased Chromatid Breaks and Radiosensitization of 82-6 hTERT and RPE Human Cells

While technological advances in radiation oncology have led to a more precise delivery of radiation dose and a decreased risk of side effects, there is still a need to better understand the mechanisms underlying DNA damage response (DDR) at the DNA and cytogenetic levels, and to overcome tumor resistance. To maintain genomic stability, cells have developed sophisticated signaling pathways enabling cell cycle arrest to facilitate DNA repair via the DDR-related kinases and their downstream targets, so that DNA damage or DNA replication stress induced by genotoxic therapies can be resolved. ATM, ATR, and Chk1 kinases are key mediators in DDR activation and crucial factors in treatment resistance. It is of importance, therefore, as an alternative to the conventional clonogenic assay, to establish a cytogenetic assay enabling reliable and time-efficient results in evaluating the potency of DDR inhibitors for radiosensitization. Toward this goal, the present study aims at the development and optimization of a chromosomal radiosensitivity assay using the DDR and G2-checkpoint inhibitors as a novel modification compared to the classical G2-assay. Also, it aims at investigating the strengths of this assay for rapid radiosensitivity assessments in cultured cells, and potentially, in tumor cells obtained from biopsies. Specifically, exponentially growing RPE and 82-6 hTERT human cells are irradiated during the G2/M-phase transition in the presence or absence of Caffeine, VE-821, and UCN-1 inhibitors of ATM/ATR, ATR, and Chk1, respectively, and the induced chromatid breaks are used to evaluate cell radiosensitivity and their potency for radiosensitization. The increased yield of chromatid breaks in the presence of DDR inhibitors, which underpins radiosensitization, is similar to that observed in cells from highly radiosensitive AT-patients, and is considered here as 100% radiosensitive internal control. The results highlight the potential of our modified G2-assay using VE-821 to evaluate cell radiosensitivity, the efficacy of DDR inhibitors in radiosensitization, and reinforce the concept that ATM, ATR, and Chk1 represent attractive anticancer drug targets in radiation oncology.

Predicting chromosome damage in astronauts participating in international space station missions

Space radiation consists of energetic protons and other heavier ions. During the International Space Station program, chromosome aberrations in lymphocytes of astronauts have been analyzed to estimate received biological doses of space radiation. More specifically, pre-flight blood samples were exposed ex vivo to varying doses of gamma rays, while post-flight blood samples were collected shortly and several months after landing. Here, in a study of 43 crew-missions, we investigated whether individual radiosensitivity, as determined by the ex vivo dose-response of the pre-flight chromosome aberration rate (CAR), contributes to the prediction of the post-flight CAR incurred from the radiation exposure during missions. Random-effects Poisson regression was used to estimate subject-specific radiosensitivities from the preflight dose-response data, which were in turn used to predict post-flight CAR and subject-specific relative biological effectiveness (RBEs) between space radiation and gamma radiation. Covariates age, gender were also considered. Results indicate that there is predictive value in background CAR as well as radiosensitivity determined preflight for explaining individual differences in post-flight CAR over and above that which could be explained by BFO dose alone. The in vivo RBE for space radiation was estimated to be approximately 3 relative to the ex vivo dose response to gamma irradiation. In addition, pre-flight radiosensitivity tended to be higher for individuals having a higher background CAR, suggesting that individuals with greater radiosensitivity can be more sensitive to other environmental stressors encountered in daily life. We also noted that both background CAR and radiosensitivity tend to increase with age, although both are highly variable. Finally, we observed no significant difference between the observed CAR shortly after mission and at > 6 months post-mission.

Molecular contribution of BRCA1 and BRCA2 to genome instability in breast cancer patients: review of radiosensitivity assays

BACKGROUND

DNA repair pathways, cell cycle arrest checkpoints, and cell death induction are present in cells to process DNA damage and prevent genomic instability caused by various extrinsic and intrinsic ionizing factors. Mutations in the genes involved in these pathways enhances the ionizing radiation sensitivity, reduces the individual's capacity to repair DNA damages, and subsequently increases susceptibility to tumorigenesis.

BODY

BRCA1 and BRCA2 are two highly penetrant genes involved in the inherited breast cancer and contribute to different DNA damage pathways and cell cycle and apoptosis cascades. Mutations in these genes have been associated with hypersensitivity and genetic instability as well as manifesting severe radiotherapy complications in breast cancer patients. The genomic instability and DNA repair capacity of breast cancer patients with BRCA1/2 mutations have been analyzed in different studies using a variety of assays, including micronucleus assay, comet assay, chromosomal assay, colony-forming assay, γ -H2AX and 53BP1 biomarkers, and fluorescence in situ hybridization. The majority of studies confirmed the enhanced spontaneous & radiation-induced radiosensitivity of breast cancer patients compared to healthy controls. Using G2 micronucleus assay and G2 chromosomal assay, most studies have reported the lymphocyte of healthy carriers with BRCA1 mutation are hypersensitive to invitro ionizing radiation compared to non-carriers without a history of breast cancer. However, it seems this approach is not likely to be useful to distinguish the BRCA carriers from non-carrier with familial history of breast cancer.

CONCLUSION

In overall, breast cancer patients are more radiosensitive compared to healthy control; however, inconsistent results exist about the ability of current radiosensitive techniques in screening BRCA1/2 carriers or those susceptible to radiotherapy complications. Therefore, developing further radiosensitivity assay is still warranted to evaluate the DNA repair capacity of individuals with BRCA1/2 mutations and serve as a predictive factor for increased risk of cancer mainly in the relatives of breast cancer patients. Moreover, it can provide more evidence about who is susceptible to manifest severe complication after radiotherapy.

Individual Radiosensitivity in Oncological Patients: Linking Adverse Normal Tissue Reactions and Genetic Features

Introduction: Adverse effects of radiotherapy (RT) significantly affect patient's quality of life (QOL). The possibility to identify patient-related factors that are associated with individual radiosensitivity would optimize adjuvant RT treatment, limiting the severity of normal tissue reactions, and improving patient's QOL. In this study, we analyzed the relationships between genetic features and toxicity grading manifested by RT patients looking for possible biomarkers of individual radiosensitivity.

Methods: Early radiation toxicity was evaluated on 143 oncological patients according to the Common Terminology Criteria for Adverse Events (CTCAE). An individual radiosensitivity (IRS) index defining four classes of radiosensitivity (highly radiosensitive, radiosensitive, normal, and radioresistant) was determined by a G₂-chromosomal assay on ex vivo irradiated, patient-derived blood samples. The expression level of 15 radioresponsive genes has been measured by quantitative real-time PCR at 24 h after the first RT fraction, in blood samples of a subset of 57 patients, representing the four IRS classes.

Results: By applying univariate and multivariate statistical analyses, we found that fatigue was significantly associated with IRS index. Interestingly, associations were detected between clinical radiation toxicity and gene expression (ATM, CDKN1A, FDXR, SESN1, XPC, ZMAT3, and BCL2/BAX ratio) and between IRS index and gene expression (BBC3, FDXR, GADD45A, and BCL2/BAX).

Conclusions: In this prospective cohort study we found that associations exist between normal tissue reactions and genetic features in RT-treated patients. Overall, our findings can contribute to the identification of biological markers to predict RT toxicity in normal tissues.

A method for the cell-cycle-specific analysis of radiation-induced chromosome aberrations and breaks

The classical G₂-assay is widely used to assess cell-radiosensitivity and cancer phenotype: Cells are exposed to low doses of ionizing-radiation (IR) and collected for cytogenetic- analysis ˜1.5 h later. In this way, chromosome-damage is measured in cells irradiated in G₂-phase, without retrieving information regarding kinetics of chromosome-break-repair. Modification of the assay to include analysis at multiple time-points after IR, has enabled kinetic-analysis of chromatid-break-repair and assessment of damage in a larger proportion of G₂-phase cells. This modification, however, increases the probability that at later time points not only cells irradiated in G₂-phase, but also cells irradiated in S-phase will reach metaphase. However, the response of cells irradiated in G₂-phase can be mechanistically different from that of cells irradiated in S-phase. Therefore, indiscriminate analysis may confound the interpretation of experiments designed to elucidate mechanisms of chromosome-break-repair and the contributions of the different DSB-repair-pathways in this response. Here we report an EdU based modification of the assay that enables S- and G₂-phase specific analysis of chromatid break repair. Our results show that the majority of metaphases captured during the first 2 h after IR originate from cells irradiated in G₂-phase (EdU^- metaphases) in both rodent and human cells. Metaphases originating from cells irradiated in S-phase (EdU⁺ metaphases) start appearing at 2 h and 4 h after IR in rodent and human cells, respectively. The kinetics of chromatid-break-repair are similar in cells irradiated in G₂- and S-phase of the cell-cycle, both in rodent and human cells. The protocol is applicable to classical-cytogenetic experiments and allows the cell-cycle specific analysis of chromosomal-aberrations. Finally, the protocol can be applied to the kinetic analysis of chromosome-breaks in prematurely-condensed-chromosomes of G₂-phase cells. In summary, the developed protocol provides means to enhance the analysis of IR-induced-cytogenetic-damage by providing information on the cell-cycle phase where DNA damage is inflicted.

Individual Radiosensitivity Assessment of the Families of Ataxia-Telangiectasia Patients by G2-Checkpoint Abrogation

Objectives

Ataxia-telangiectasia (A-T) is an autosomal recessive multisystem disorder characterised by cerebellar degeneration, telangiectasia, radiation sensitivity, immunodeficiency, oxidative stress and cancer susceptibility. Epidemiological research has shown that carriers of the heterozygous ataxia-telangiectasia mutated (ATM) gene mutation are radiosensitive to ionising irradiation and have a higher risk of cancers, type 2 diabetes and atherosclerosis. However, there is currently no fast and reliable laboratory-based method to detect heterozygous ATM carriers for family screening and planning purposes. This study therefore aimed to evaluate the ability of a modified G2-assay to identify heterozygous ATM carriers in the families of A-T patients.

Methods

This study took place at the Tehran University of Medical Sciences, Tehran, Iran, between February and December 2017 and included 16 A-T patients, their parents (obligate heterozygotes) and 30 healthy controls. All of the subjects underwent individual radiosensitivity (IRS) assessment using a modified caffeine-treated G2-assay with G2-checkpoint abrogation.

Results

The mean IRS of the obligate ATM heterozygotes was significantly higher than the healthy controls (55.13% ± 5.84% versus 39.03% ± 6.95%; P <0.001), but significantly lower than the A-T patients (55.13% ± 5.84% versus 87.39% ± 8.29%; P = 0.001). A receiver operating characteristic (ROC) curve analysis of the G2-assay values indicated high sensitivity and specificity, with an area under the ROC curve of 0.97 (95% confidence interval: 0.95-1.00).

Conclusion

The modified G2-assay demonstrated adequate precision and relatively high sensitivity and specificity in detecting heterozygous ATM carriers.

Ataxia telangiectasia syndrome: moonlighting ATM

INTRODUCTION

Ataxia-telangiectasia (A-T) a multisystem disorder primarily characterized by cerebellar degeneration, telangiectasia, immunodeficiency, cancer susceptibility and radiation sensitivity. Identification of the gene defective in this syndrome, ataxia-telangiectasia mutated gene (ATM), and further characterization of the disorder together with a greater insight into the function of the ATM protein have expanded our knowledge about the molecular pathogenesis of this disease. Area covered: In this review, we have attempted to summarize the different roles of ATM signaling that have provided new insights into the diverse clinical phenotypes exhibited by A-T patients. Expert commentary: ATM, in addition to DNA repair response, is involved in many cytoplasmic roles that explain diverse phenotypes of A-T patients. It seems accumulation of DNA damage, persistent DNA damage response signaling, and chronic oxidative stress are the main players in the pathogenesis of this disease.

Seeking genetic signature of radiosensitivity--a novel method for data analysis in case of small sample sizes

Background

The identification of polymorphisms and/or genes responsible for an organism's radiosensitivity increases the knowledge about the cell cycle and the mechanism of the phenomena themselves, possibly providing the researchers with a better understanding of the process of carcinogenesis.

Aim

The aim of the study was to develop a data analysis strategy capable of discovering the genetic background of radiosensitivity in the case of small sample size studies.

Results

Among many indirect measures of radiosensitivity known, the level of radiation-induced chromosomal aberrations was used in the study. Mathematical modelling allowed the transformation of the yield-time curve of radiation-induced chromosomal aberrations into the exponential curve with limited number of parameters, while Gaussian mixture models applied to the distributions of these parameters provided the criteria for mouse strain classification. A detailed comparative analysis of genotypes between the obtained subpopulations of mice followed by functional validation provided a set of candidate polymorphisms that might be related to radiosensitivity. Among 1857 candidate relevant SNPs, that cluster in 28 genes, eight SNPs were detected nonsynonymous (nsSNP) on protein function. Two of them, rs48840878 (gene Msh3) and rs5144199 (gene Cc2d2a), were predicted as having increased probability of a deleterious effect. Additionally, rs48840878 is capable of disordering phosphorylation with 14 PKs. In silico analysis of candidate relevant SNP similarity score distribution among 60 CGD mouse strains allowed for the identification of SEA/GnJ and ZALENDE/EiJ mouse strains (95.26% and 86.53% genetic consistency respectively) as the most similar to radiosensitive subpopulation

Conclusions

A complete step-by-step strategy for seeking the genetic signature of radiosensitivity in the case of small sample size studies conducted on mouse models was proposed. It is shown that the strategy, which is a combination of mathematical modelling, statistical analysis and data mining methodology, allows for the discovery of candidate polymorphisms which might be responsible for radiosensitivity phenomena.

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.

Topoisomerase IIα levels and G2 radiosensitivity in T-lymphocytes of women presenting with breast cancer

Previous studies from our laboratory have identified a link between intracellular topoisomerase IIα (topo IIα) levels and chromosomal radiosensitivity, as measured by the frequencies of chromatid breaks in the so-called G2-assay. Lower topo IIα levels were associated with reduced chromosomal radiosensitivity in cultured human cells. These findings supported a model, in which it is proposed that such chromatid breaks are the result of radiation-induced errors made by topoisomerase IIα during decatenation of chromatids. Studies from our and other laboratories, using the G2-assay, have shown that phytohaemagglutinin (PHA)-stimulated peripheral blood T-lymphocytes from 40% of female breast cancer cases show elevated chromatid break frequencies when exposed to a small standard dose of ionizing radiation, i.e. elevated above the 90th percentile of a group of female control samples. In the present study we have used a modified G2-assay to test whether elevated frequency of chromatid breaks in breast cancer cases is linked with elevated intracellular topo IIα level in PHA-stimulated T-lymphocytes, and also whether there is a general correlation between chromosomal radiosensitivity and topo IIα level. Our results confirm previous studies that 40% of breast cancer cases show elevated radiosensitivity as compared with controls. Also, the mean chromatid break frequency in breast cancer cases was significantly higher than in controls (P = 0.0001). We found that the mean topo IIα level in the cohort of breast cancer cases studied was significantly raised, as compared with controls (P = 0.0016), which could indicate a genetic propensity towards a raised intracellular production of topo IIα in these individuals. There was no direct correlation between chromosomal radiosensitivity and topo IIα level for individual samples either in the breast cancer cohort or in controls. However, a comparison between control and breast cancer samples shows a higher mean topo IIα level in breast cancer samples that correlates with the elevated mean chromatid break frequency seen in these patient samples. We found no meaningful correlations between either chromatid break frequency or topo IIα level and either tumour grade or hormone status. We conclude that elevated intracellular topo IIα level is likely to be a significant factor in determining the chromosomal response of stimulated T-lymphocytes from certain breast cancer cases.

Intra-individual variation in G2 chromosomal radiosensitivity

Intra-individual variation in G(2) chromosomal radiosensitivity was examined by repeatedly taking blood samples from two individuals. Two healthy female volunteers provided a total of 44 blood samples, Donor 1 gave 28 samples in four time periods between 2001 and 2006 and Donor 2 gave 16 samples in two of the same time periods. Lymphocytes were cultured for 72 h prior to irradiation with 0.5 Gy, 300 kV X-rays. Colcemid was added 30 min post-irradiation. Cultures were harvested 90 min post-irradiation and analysed for chromatid gaps and breaks. Donor 1 exhibited significant intra-individual variation in G(2) chromosomal radiosensitivity for two of the four time periods. Variation was not significant for Period 1 (13 samples, P = 0.111) and Period 2 (six samples, P = 0.311) but was significant for Period 3 (two samples, P = 0.030) and Period 4 (seven samples, P = 0.005). Significant intra-individual variation was observed for both time periods involving Donor 2, these being Period 2 (nine samples, P = 0.002) and Period 4 (seven samples, P < 0.001). The combined data from all time periods exhibited a significant intra-individual variation for Donor 1 (P < 0.001) and Donor 2 (P < 0.001). These findings led to the conclusion that too much reliance should not be placed on the result from a single sample when assessing individual radiosensitivity status.

Impact of heterozygous c.657-661del, p.I171V and p.R215W mutations in NBN on nibrin functions

Nibrin, product of the NBN gene, together with MRE11 and RAD50 is involved in DNA double-strand breaks (DSBs) sensing and repair, induction of apoptosis and cell cycle control. Biallelic NBN mutations cause the Nijmegen breakage syndrome, a chromosomal instability disorder characterised by, among other things, radiosensitivity, immunodeficiency and an increased cancer risk. Several studies have shown an association of heterozygous c.657-661del, p.I171V and p.R215W mutations in the NBN gene with a variety of malignancies but the data are controversial. Little is known, however, whether and to what extent do these mutations in heterozygous state affect nibrin functions. We examined frequency of chromatid breaks, DSB repair, defects in S-phase checkpoint and radiosensitivity in X-ray-irradiated cells from control individuals, NBS patients and heterozygous carriers of the c.657-661del, p.I171V and p.R215W mutations. While cells homozygous for c.657-661del displayed a significantly increased number of chromatid breaks and residual γ-H2AX foci, as well as abrogation of the intra-S-phase checkpoint following irradiation, which resulted in increased radiosensitivity, cells with heterozygous c.657-661del, p.I171V and p.R215W mutations behaved similarly to control cells. Significant differences in the frequency of spontaneous and ionising radiation-induced chromatid breaks and the level of persistent γ-H2AX foci were observed when comparing control and mutant cells heterozygous for c.657-661del. However, it is still possible that heterozygous NBN mutations may contribute to cancer development.

Error-prone nonhomologous end joining repair operates in human pluripotent stem cells during late G2

Genome stability of human embryonic stem cells (hESC) is an important issue because even minor genetic alterations can negatively impact cell functionality and safety. The incorrect repair of DNA double-stranded breaks (DSBs) is the ultimate cause of the formation of chromosomal aberrations. Using G2 radiosensitivity assay, we analyzed chromosomal aberrations in pluripotent stem cells and somatic cells. The chromatid exchange aberration rates in hESCs increased manifold 2 hours after irradiation as compared with their differentiated derivatives, but the frequency of radiation-induced chromatid breaks was similar. The rate of radiation-induced chromatid exchanges in hESCs and differentiated cells exhibited a quadratic dose response, revealing two-hit mechanism of exchange formation suggesting that a non-homologous end joining (NHEJ) repair may contribute to their formation. Inhibition of DNA-PK, a key NHEJ component, by NU7026 resulted in a significant decrease in radiation-induced chromatid exchanges in hESCs but not in somatic cells. In contrast, NU7026 treatment increased the frequency of radiation-induced breaks to a similar extent in pluripotent and somatic cells. Thus, DNA-PK dependent NHEJ efficiently participates in the elimination of radiation-induced chromatid breaks during the late G2 in both cell types and DNA-PK activity leads to a high level of misrejoining specifically in pluripotent cells.

Chromosomal radiosensitivity and acute radiation side effects after radiotherapy in tumour patients--a follow-up study

Background

Radiotherapists are highly interested in optimizing doses especially for patients who tend to suffer from side effects of radiotherapy (RT). It seems to be helpful to identify radiosensitive individuals before RT.

Thus we examined aberrations in FISH painted chromosomes in in vitro irradiated blood samples of a group of patients suffering from breast cancer. In parallel, a follow-up of side effects in these patients was registered and compared to detected chromosome aberrations.

Methods

Blood samples (taken before radiotherapy) were irradiated in vitro with 3 Gy X-rays and analysed by FISH-painting to obtain aberration frequencies of first cycle metaphases for each patient. Aberration frequencies were analysed statistically to identify individuals with an elevated or reduced radiation response. Clinical data of patients have been recorded in parallel to gain knowledge on acute side effects of radiotherapy.

Results

Eight patients with a significantly elevated or reduced aberration yield were identified by use of a t-test criterion. A comparison with clinical side effects revealed that among patients with elevated aberration yields one exhibited a higher degree of acute toxicity and two patients a premature onset of skin reaction already after a cumulative dose of only 10 Gy. A significant relationship existed between translocations in vitro and the time dependent occurrence of side effects of the skin during the therapy period.

Conclusions

The results suggest that translocations can be used as a test to identify individuals with a potentially elevated radiosensitivity.

The heritability of G2 chromosomal radiosensitivity and its association with cancer in Danish cancer survivors and their offspring

PURPOSE

To investigate the relationship between chromosomal radiosensitivity and early-onset cancer under the age of 35 years and to examine the heritability of chromosomal radiosensitivity.

MATERIALS AND METHODS

Peripheral blood lymphocytes were cultured for 72 hours prior to being irradiated with 0.5 Gy, 300 kV X-rays. Colcemid was added to cultures 30 min post-irradiation. Cultures were harvested 90 min post-irradiation and analysed for chromatid gaps and breaks. Heritability was estimated using Sequential Oligogenic Linkage Analysis Routines (SOLAR) software and by segregation analysis.

RESULTS

Elevated radiosensitivity was seen for seven out of 29 (24.1%) cancer survivors, three out of 29 (10.3%) partners and 10 out of 53 (20.8%) offspring. Although the proportion of individuals displaying enhanced radiosensitivity was twice as high in both the cancer survivor and offspring groups than the partner controls, neither reached statistical significance. Heritability analysis of the radiosensitive phenotype suggested 57.9-78.0% of the variance could be attributed to genetic factors.

CONCLUSION

An association between G(2) chromosomal radiosensitivity and childhood and young adult cancer is suggested but was not statistically significant. In contrast, there is strong evidence for heritability of the radiosensitive phenotype. The cancer survivors included a broad range of malignancies and future studies should focus on specific cancers with known or likely faults in deoxyribonucleic acid (DNA) damage recognition and repair mechanisms.

Increased chromosomal radiosensitivity in women carrying BRCA1/BRCA2 mutations assessed with the G2 assay

PURPOSE

Several in vitro studies suggest that BRCA1 and BRCA2 mutation carriers present increased sensitivity to ionizing radiation. Different assays for the assessment of deoxyribonucleic acid double-strand break repair capacity have been used, but results are rather inconsistent. Given the concerns about the possible risks of breast screening with mammography in mutation carrier women and the potentially damaging effects of radiotherapy, the purpose of this study was to further investigate the radiosensitivity of this population.

METHODS AND MATERIALS

The G2 chromosomal radiosensitivity assay was used to assess chromosomal breaks in lymphocyte cultures after exposure to 1 Gy. A group of familiar breast cancer patients carrying a mutation in the BRCA1 or BRCA2 gene (n = 15) and a group of healthy mutation carriers (n = 5) were investigated and compared with a reference group of healthy women carrying no mutation (n = 21).

RESULTS

BRCA1 and BRCA2 mutation carriers had a significantly higher number of mean chromatid breaks per cell (p = 0.006) and a higher maximum number of breaks (p = 0.0001) as compared with their matched controls. Both healthy carriers and carriers with a cancer history were more radiosensitive than controls (p = 0.002 and p = 0.025, respectively). Age was not associated with increased radiosensitivity (p = 0.868).

CONCLUSIONS

Our results indicate that BRCA1 and BRCA2 mutation carriers show enhanced radiosensitivity, presumably because of the involvement of the BRCA genes in deoxyribonucleic acid repair and cell cycle control mechanisms.

G2-phase chromosomal radiosensitivity of primary fibroblasts from hereditary retinoblastoma family members and some apparently normal controls

We previously described an enhanced sensitivity for cell killing and gamma-H2AX focus induction after both high-dose-rate and continuous low-dose-rate gamma irradiation in 14 primary fibroblast strains derived from hereditary-type retinoblastoma family members (both affected RB1(+/-) probands and unaffected RB1(+/+) parents). Here we present G(2)-phase chromosomal radiosensitivity assay data for primary fibroblasts derived from these RB family members and five Coriell cell bank controls (four apparently normal individuals and one bilateral RB patient). The RB family members and two normal Coriell strains had significantly higher ( approximately 1.5-fold, P < 0.05) chromatid-type aberration frequencies in the first postirradiation mitosis after doses of 50 cGy and 1 Gy of (137)Cs gamma radiation compared to the remaining Coriell strains. The induction of chromatid-type aberrations by high-dose-rate G(2)-phase gamma irradiation is significantly correlated to the proliferative ability of these cells exposed to continuous low-dose-rate gamma irradiation (reported in Wilson et al., Radiat. Res. 169, 483-494, 2008). Our results suggest that these moderately radiosensitive individuals may harbor hypomorphic genetic variants in genomic maintenance and/or DNA repair genes or may carry epigenetic changes involving genes that more broadly modulate such systems, including G(2)-phase-specific DNA damage responses.

Elevated G2 chromosomal radiosensitivity in Irish breast cancer patients: a comparison with other studies

Previous studies have shown that a significant proportion of breast cancer patients exhibit elevated G2 chromosomal radiosensitivity in contrast to controls (approximately 40%). In this study, the G2 assay was applied to a small number of Irish breast cancer patients who were recorded as sporadic cases and they were compared with a control group to compare and contrast with the previous documented studies. Lymphocyte cultures were set up on whole blood samples and stimulated with phytohaemagglutinin. The cultures were irradiated 74 h later with 0.5 Gy gamma-radiation and cells were arrested in metaphase by treating the cultures with colcemid. The chromosomes were harvested and the aberrations scored per 100 metaphases to assign a G2 score. The assay was first carried out on four donor controls to estimate intra-individual variation and then ten controls for inter-individual variation to measure assay reproducibility. The G2 assay was then applied to 27 breast cancer patients. Good intrinsic assay reproducibility was observed in the coefficient of variation (CV) data in three out of four controls. Intra-individual variation was similar in three out of four of the donors (4.6 - 5.1%) with one donor showing a higher CV compared with the others (22.9%). Inter-individual variation was calculated at 30.5% for all controls. No significant difference was observed between intra- and inter-individual variation using the variance ratio F-test. A G2 radiosensitivity cut-off of 110 aberrations/100 metaphases was calculated from the controls, and from this 70.4% of breast cancer patients and 7.7% of controls were calculated as G2 radiosensitive. This proportion of G2-sensitive breast cancer patients is the highest recorded in studies to date. It is thought that the G2 radiosensitivity assay is a biomarker of breast cancer predisposition genes of low penetrance, suggesting the presence of these genes in the Irish breast cancer patients used in this study who were recorded as sporadic cases. A larger number of Irish patients would be required to consolidate these findings and be representative of the Irish breast cancer population.

Suppression of topoisomerase IIalpha expression and function in human cells decreases chromosomal radiosensitivity

The mechanism behind chromatid break formation is as yet unclear, although it is known that DNA double-strand breaks (DSBs) are the initiating lesions. Chromatid breaks formed in cells in the G2-phase of the cell-cycle disappear ('rejoin') as a function of time between radiation exposure and cell fixation. However, the kinetics of disappearance of chromatid breaks does not correspond to those of DSB rejoining, leading us to seek alternative models. We have proposed that chromatid breaks could be formed indirectly from DSB and that the mechanism involves topoisomerase IIalpha. In support of this hypothesis we have recently shown that frequencies of radiation-induced chromatid breaks are lower in two variant human promyelocytic leukaemic cell lines with reduced topoisomerase IIalpha expression. Here we report that suppression of topoisomerase IIalpha in human hTERT-RPE1 cells, either by its abrogation using specific siRNA or by inhibition of its catalytic activity with the inhibitor ICRF-193, causes a reduction in frequency of chromatid breaks in radiation-exposed cells. The findings support our hypothesis for the involvement of topoisomerase IIalpha in the formation of radiation-induced chromatid breaks, and could help explain inter-individual variation in human chromosomal radiosensitivity; elevation of which has been linked with cancer susceptibility.

Suppression of topoisomerase IIalpha expression and function in human cells decreases chromosomal radiosensitivity

The mechanism behind chromatid break formation is as yet unclear, although it is known that DNA double-strand breaks (DSBs) are the initiating lesions. Chromatid breaks formed in cells in the G2-phase of the cell-cycle disappear ('rejoin') as a function of time between radiation exposure and cell fixation. However, the kinetics of disappearance of chromatid breaks does not correspond to those of DSB rejoining, leading us to seek alternative models. We have proposed that chromatid breaks could be formed indirectly from DSB and that the mechanism involves topoisomerase IIalpha. In support of this hypothesis we have recently shown that frequencies of radiation-induced chromatid breaks are lower in two variant human promyelocytic leukaemic cell lines with reduced topoisomerase IIalpha expression. Here we report that suppression of topoisomerase IIalpha in human hTERT-RPE1 cells, either by its abrogation using specific siRNA or by inhibition of its catalytic activity with the inhibitor ICRF-193, causes a reduction in frequency of chromatid breaks in radiation-exposed cells. The findings support our hypothesis for the involvement of topoisomerase IIalpha in the formation of radiation-induced chromatid breaks, and could help explain inter-individual variation in human chromosomal radiosensitivity; elevation of which has been linked with cancer susceptibility.

A role for topoisomerase II alpha in the formation of radiation-induced chromatid breaks

Chromatid breaks in cells exposed to low dose irradiation are thought to be initiated by DNA double-strand breaks (DSB), and the frequency of chromatid breaks has been shown to increase in DSB rejoining deficient cells. However, the underlying causes of the wide variation in frequencies of G2 chromatid breaks (or chromatid ‘radiosensitivity’) in irradiated T-lymphocytes from different normal individuals and cancer cases are as yet unclear. Here we report evidence that topoisomerase IIα expression level is a factor determining chromatid radiosensitivity. We have exposed the promyelocytic leukaemic cell line (HL60) and two derived variant cell lines (MX1 and MX2) that have acquired resistance to mitoxantrone and low expression of topoisomerase II α, to low doses of γ-radiation and scored the induced chromatid breaks. Chromatid break frequencies were found to be significantly lower in the variant cell lines, compared with their parental HL60 cell line. Rejoining of DSB in the variant cell lines was similar to that in the parental HL60 strain. Our results indicate the indirect involvement of topoisomerase IIα in the formation of radiation-induced chromatid breaks from DSB, and suggest topoisomerase IIα as a possible factor in the inter-individual variation in chromatid radiosensitivity.

Chromosomal radiosensitivity in head and neck cancer patients: evidence for genetic predisposition?

The association between chromosomal radiosensitivity and genetic predisposition to head and neck cancer was investigated in this study. In all, 101 head and neck cancer patients and 75 healthy control individuals were included in the study. The G(2) assay was used to measure chromosomal radiosensitivity. The results demonstrated that head and neck cancer patients had a statistically higher number of radiation-induced chromatid breaks than controls, with mean values of 1.23 and 1.10 breaks per cell, respectively (P<0.001). Using the 90th percentile of the G(2) scores of the healthy individuals as a cutoff value for chromosomal radiosensitivity, 26% of the cancer patients were radiosensitive compared with 9% of the healthy controls (P=0.008). The mean number of radiation-induced chromatid breaks and the proportion of radiosensitive individuals were highest for oral cavity cancer patients (1.26 breaks per cell, 38%) and pharynx cancer patients (1.27 breaks per cell, 35%). The difference between patients and controls was most pronounced in the lower age group (<or=50 years, 1.32 breaks per cell, 38%) and in the non- and light smoking patient group (<or=10 pack-years, 1.28 breaks per cell, 46%). In conclusion, enhanced chromosomal radiosensitivity is a marker of genetic predisposition to head and neck cancer, and the genetic contribution is highest for oral cavity and pharynx cancer patients and for early onset and non- and light smoking patients.

Lymphocyte radiosensitivity in BRCA1 and BRCA2 mutation carriers and implications for breast cancer susceptibility

There is conflicting evidence as to whether individuals who are heterozygous for germ-line BRCA1 or BRCA2 mutations have an altered phenotypic cellular response to irradiation. To investigate this, chromosome breakage and apoptotic response were measured after irradiation in peripheral blood lymphocytes from 26 BRCA1 and 18 BRCA2 mutation carriers without diagnosed breast cancer, and 38 unaffected age, ethnically and sex-matched controls. To assess the role of BRCA1 and BRCA2 in homologous recombination, an S phase enrichment chromosome breakage assay was used. BrdUrd incorporation studies allowed verification of the correct experimental settings. We found that BRCA1 mutation carriers without cancer had increased chromosome breaks as well as breaks and gaps per cell post irradiation using the classical G2 assay (p = 0.01 and 0.004, respectively) and the S phase enrichment assay (p = 0.01 and 0.01, respectively) compared to age-matched unaffected controls. BRCA2 mutation carriers without cancer had increased breaks as well as breaks and gaps per cell post irradiation using the S phase enrichment assay (p = 0.045 and 0.012, respectively). No difference was detected using the G2 assay (p = 0.88 and 0.40 respectively). BRCA1 and BRCA2 mutation carriers had normal cell cycle kinetics and apoptotic response to irradiation compared to age-matched controls. Our results show a demonstrable impairment in irradiation induced DNA repair in women with heterozygous germline BRCA1 and BRCA2 mutations prior to being diagnosed with breast cancer.

TPD52 and NFKB1 gene expression levels correlate with G2 chromosomal radiosensitivity in lymphocytes of women with and at risk of hereditary breast cancer

PURPOSE

To evaluate a transcriptomic approach to identify healthy women at increased risk of breast cancer due to G2-radiosensitivity and look at transcripts that are differentially expressed between individuals.

MATERIALS AND METHODS

We perform the first study to assess the association of G2 radiosensitivity with basal gene expression in cultured T-lymphocytes from 11 women with breast cancer and 12 healthy female relatives using Affymetrix GeneChips.

RESULTS

Transcripts associated with radiosensitivity and breast cancer risk were predominantly involved in innate immunity and inflammation, such as interleukins and chemokines. Genes differentially expressed in radiosensitive individuals were more similarly expressed in close family members than in un-related individuals, suggesting heritability of the trait. The expression of tumour protein D52 (TPD52), a gene implicated in cell proliferation, apoptosis, and vesicle trafficking was the most strongly correlated with G2 score while nuclear factor (kappa)-B (NFKB1) was highly inversely correlated with G2 score. NFKB1 is known to be activated by irradiation and its inhibition has been previously shown to increase radiosensitivity.

CONCLUSIONS

Gene expression analysis of lymphocytes may provide a quantitative measure of radiation response potential and is a promising marker of breast cancer susceptibility.

G(2) chromosomal radiosensitivity in Danish survivors of childhood and adolescent cancer and their offspring

In order to investigate the relationship between chromosomal radiosensitivity and early-onset cancer, the G(2) chromosomal radiosensitivity assay was undertaken on a group of 23 Danish survivors of childhood and adolescent cancer, a control group comprising their partners and a group of 38 of their offspring. In addition, the previously reported in-house control group from Westlakes Research Institute (WRI) was extended to 27 individuals. When using the 90th percentile cutoff for the WRI control group, the proportion of individuals with elevated radiosensitivity was 11, 35, 52 and 53% for the WRI control, partner control, cancer survivor and the offspring groups, respectively, with significant differences between the WRI control group and the cancer survivor group (P=0.002) and the offspring group (P<0.001). However, while the comparisons with the WRI control group support an association of chromosomal radiosensitivity with cancer predisposition, when the partner control group was used to define the radiosensitivity cutoff point, no significant differences in radiosensitivity profiles were found between the partner control group and either the cancer survivor group or the offspring group. The failure to distinguish between the G(2) aberration profiles of the apparently normal group of partners and the cancer survivor group suggests that any association with cancer should be viewed with caution, but also raises questions as to the suitability of the partners of cancer survivors to act as an appropriate control group. Heritability of the radiosensitive phenotype was examined by segregation analysis of the Danish families and suggested that 67.3% of the phenotypic variance of G(2) chromosomal radiosensitivity is attributable to a putative major gene locus with dominant effect.

Radiation-induced damage to normal tissues after radiotherapy in patients treated for gynecologic tumors: association with single nucleotide polymorphisms in XRCC1, XRCC3, and OGG1 genes and in vitro chromosomal radiosensitivity in lymphocytes

PURPOSE

To examine the association of polymorphisms in XRCC1 (194Arg/Trp, 280Arg/His, 399Arg/Gln, 632Gln/Gln), XRCC3 (5' UTR 4.541A>G, IVS5-14 17.893A>G, 241Thr/Met), and OGG1 (326Ser/Cys) with the development of late radiotherapy (RT) reactions and to assess the correlation between in vitro chromosomal radiosensitivity and clinical radiosensitivity.

METHODS AND MATERIALS

Sixty-two women with cervical or endometrial cancer treated with RT were included in the study. According to the Common Terminology Criteria for Adverse Events, version 3.0, scale, 22 patients showed late adverse RT reactions. Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assays were performed to examine polymorphic sites, the G2 assay was used to measure chromosomal radiosensitivity, and patient groups were compared using actuarial methods.

RESULTS

The XRCC3 IVS5-14 polymorphic allele was significantly associated with the risk of developing late RT reactions (odds ratio 3.98, p = 0.025), and the XRCC1 codon 194 variant showed a significant protective effect (p = 0.028). Patients with three or more risk alleles in XRCC1 and XRCC3 had a significantly increased risk of developing normal tissue reactions (odds ratio 10.10, p = 0.001). The mean number of chromatid breaks per cell was significantly greater in patients with normal tissue reactions than in patients with no reactions (1.16 and 1.34, respectively; p = 0.002). Patients with high chromosomal radiosensitivity showed a 9.2-fold greater annual risk of complications than patients with intermediate chromosomal radiosensitivity. Combining the G2 analysis with the risk allele model allowed us to identify 23% of the patients with late normal tissue reactions, without false-positive results.

CONCLUSION

The results of the present study showed that clinical radiosensitivity is associated with an enhanced G2 chromosomal radiosensitivity and is significantly associated with a combination of different polymorphisms in DNA repair genes.

Repair and chromosomal damage

Chromosomal aberrations in somatic cells link DNA damage with radiation-induced cell killing and individual susceptibility to oncogenesis, and are also potential markers of cancer susceptibility. While there is general acceptance that the DNA double-strand break (DSB) is the principal initiating lesion the complexity of the relationship between the induced frequency and the rates of repair and misjoining of DSB, and the production of chromosome and chromatid aberrations has led to much controversy. The principal models of chromosome aberrations are: the classical 'breakage-and-reunion' or 'breakage-first' model of Sax [Genetics 25 (1940) 41-68], the 'mis-recombination' model of Chadwick and Leenhouts [Mutat Res 404 (1998) 113-117] and the 'transcription-based' model of Radford [Int J Radiat Biol 78 (2002) 1081-1093]. Chromatid aberrations have also been variously interpreted on the 'breakage-first model', Revell's 'exchange' model [Proc R Soc B 150 (1959) 563-589] and the 'signal' model [Int J Radiat Biol 73 (1998) 243-251]. Recent evidence argues strongly for different mechanisms for chromosome (formed in G1 or Go) and chromatid (formed in G2) aberrations, i.e. there is little or no correspondence in the relative frequencies between chromosome and chromatid aberrations. The balance of evidence indicates that chromosome aberrations may be formed by a breakage-first type mechanism. Elevated frequencies of chromosomal aberrations occur to various extents in cell lines mutated in genes involved in both non-homologous DSB end-joining and homologous recombinational rejoining of DSB. Chromatid breaks, seem to be formed by a more complex mechanism since there is a lack of correspondence between the rates of DSB rejoining and chromatid break 'disappearance' (assumed by some to represent DSB repair). Thus, a model based on the dissociation of DSB rejoining from chromatid break rejoining is required to explain these data. A substantial proportion (approximately 20%) of both spontaneous and induced chromatid breaks visibly involve inter-chromatid rearrangements (determined using harlequin staining of chromatids). It is postulated that the remaining proportion may also involve rearrangements, but within a single chromatid (i.e. intra-chromatid rearrangements). Disappearance of chromatid breaks with time is postulated to result from the completion of rearrangements, i.e. rather than simply from repair of DSB.

Radiotherapy and the potential exploitation of bystander effects

Radiation-induced bystander effects are the subject of intense investigation in radiation protection. The effects predominate at low doses and have been discussed mainly in terms of the impact on low-dose risk assessment. Possible therapeutic implications have been alluded to, but not discussed in any detail. The purpose of this review was to consider bystander biology in areas of major importance or interest in radiotherapy. These include consideration of radiation-induced bystander effects during the cell cycle, under hypoxic conditions, when fractionated therapy modalities are used, or when combined radiochemotherapy is given. Also discussed are individual variations in toxicity of bystander factors and normal tissue "collateral" damage. The importance of considering the tumor in the context of the organ, and even the organism that supports it, is also discussed. Direct clinical radiotherapy studies that consider bystander effects are not in the public domain at the time of writing, but many in vitro studies are available that are relevant; some preliminary animal data have also been published. Because radiation-induced bystander effects appear to challenge many of the central assumptions that underlie radiotherapy practice, it is important to consider what unexplored treatment avenues might result from a consideration of these effects. The final part of this paper is devoted to this point.