Chromosomal radiosensitivity: a study of the chromosomal G(2) assay in human blood lymphocytes indicating significant inter-individual variability

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.

Inter-individual variability in the response of human peripheral blood lymphocytes to ionizing radiation: comparison of the dicentric and micronucleus assays

Ionizing radiation can induce a wide range of DNA damage that leads to chromosomal aberrations. Some of those aberrations (dicentrics and micronuclei) are applied in biodosimetry. Biological dosimetry assumes similar radiosensitivity of each donor, but it does not exclude inter-individual variations in radiation susceptibility. Therefore, for biological reasons, it is always challenging to investigate inter-individual variability in response to radiation. For mechanistic reasons, it is also interesting to investigate the correlation between dicentric and micronuclei formation in response to radiation. In this experiment, irradiated blood specimens from 14 healthy male and female donors have been used to evaluate inter-individual variability in response to the genotoxic effects of X-ray radiation, as well as the dose-response relationship and test sensitivity using two endpoints (dicentrics and micronuclei). The results showed similar patterns of cytogenetic biomarker distribution between donors, but differences in the response of some donors at some doses. Data also showed that responses of male donors were better detected using the dicentric test, while for females, micronucleus frequencies were higher in response to the same dose of radiation. No influence of smoking status or age on specific responses was observed. Group variability in response to radiation was evaluated using coefficient of variation for each group of individuals irradiated with the same doses; as the dose increases, group variability becomes substantially lower. Despite sporadic inter-individual variability, trend of radiation-induced changes was similar. Produced calibration curves for both types of damage revealed dicentrics as genetic damage more typical for radiation than micronuclei.

A 24-color metaphase-based radiation assay discriminates heterozygous BRCA2 mutation carriers from controls by chromosomal radiosensitivity

Numerous allelic variants identified in the familial breast cancer and DNA repair genes BRCA1 and BRCA2 are of unknown impact on protein function or clinical relevance, referred to as unclassified variants (UCV). Lymphocytes from pathogenic BRCA1/2 mutation carriers exhibit an increased level of chromosomal damage after irradiation. We established a radiation assay for the discrimination of pathogenic BRCA2 variants versus controls based on the level of chromosomal damage upon irradiation (p < 0.001). As a consequence, lymphocytes from UCV carriers could be separated into two distinct groups with normal or diminished DNA double strand break repair capacity. Our results suggested that all five UCV tested were benign and that one family carried a putative mutation in an as yet undetected DNA-repair gene. Thus, our test may serve as a valuable tool that aids the classification of BRCA2 UCV, but very likely also of BRCA1 UCV or aberrations in other genes involved in the DNA-repair system.

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.

Chromatin dynamics during cell cycle mediate conversion of DNA damage into chromatid breaks and affect formation of chromosomal aberrations: biological and clinical significance

The formation of diverse chromosomal aberrations following irradiation and the variability in radiosensitivity at different cell-cycle stages remain a long standing controversy, probably because most of the studies have focused on elucidating the enzymatic mechanisms involved using simple DNA substrates. Yet, recognition, processing and repair of DNA damage occur within the nucleoprotein complex of chromatin which is dynamic in nature, capable of rapid unfolding, disassembling, assembling and refolding. The present work reviews experimental work designed to investigate the impact of chromatin dynamics and chromosome conformation changes during cell-cycle in the formation of chromosomal aberrations. Using conventional cytogenetics and premature chromosome condensation to visualize interphase chromatin, the data presented support the hypothesis that chromatin dynamic changes during cell-cycle are important determinants in the conversion of sub-microscopic DNA lesions into chromatid breaks. Consequently, the type and yield of radiation-induced chromosomal aberrations at a given cell-cycle-stage depends on the combined effect of DNA repair processes and chromatin dynamics, which is cell-cycle-regulated and subject to up- or down-regulation following radiation exposure or genetic alterations. This new hypothesis is used to explain the variability in radiosensitivity observed at various cell-cycle-stages, among mutant cells and cells of different origin, or among different individuals, and to revisit unresolved issues and unanswered questions. In addition, it is used to better understand hypersensitivity of AT cells and to provide an improved predictive G2-assay for evaluating radiosensitivity at individual level. Finally, experimental data at single cell level obtained using hybrid cells suggest that the proposed hypothesis applies only to the irradiated component of the hybrid.

The radiosensitizing potential of glutaraldehyde on MCF7 breast cancer cells as quantified by means of the G2-chromosomal radiosensitivity assay

Glutaraldehyde (GA) is a high production volume chemical that is very reactive with a wide spectrum of medical, scientific and industrial applications. Concerning the genotoxic and carcinogenic effect of GA, controversial results have been reported, while in humans no studies with positive carcinogenic results for GA have been published. However, our previous study concerning the combined effects of exposure to both GA and ionising radiation (IR) in peripheral blood lymphocytes of healthy donors has shown that non-genotoxic doses of the chemical induces a statistically significant increase in chromosomal radiosensitivity. The lack of information concerning the radiosensitizing potential of GA on cancerous cells triggered us to test the radiosensitizing effect of GA on breast cancer cells (MCF7). For this purpose the G2-chromosomal radiosensitivity assay (G2-assay) was used. The assay involves G2-phase irradiation and quantitation of the chromosomal fragility in the subsequent metaphase. The experimental data show that 48 h exposure to GA, at doses that are not clastogenic to MCF7 breast cancer cells enhances G2-chromosomal radiosensitivity of this cell line. In an effort to evaluate whether the observed increase in GAs-induced G2-chromosomal radiosensitization is linked to GA-induced alterations in the cell cycle and feedback control mechanism, Mitotic Index analysis was performed. The results have shown that such a mechanism cannot be directly related to the observed GA-induced increase in G2-chromosomal radiosensitivity. Since increased G2-chromosomal radiosensitivity has been linked with cancer proneness, the radiosensitizing effect of GA at non-clastogenic doses highlights its potential carcinogenic profile.

The in vitro MN assay in 2011: origin and fate, biological significance, protocols, high throughput methodologies and toxicological relevance

Micronuclei (MN) are small, extranuclear bodies that arise in dividing cells from acentric chromosome/chromatid fragments or whole chromosomes/chromatids lagging behind in anaphase and are not included in the daughter nuclei at telophase. The mechanisms of MN formation are well understood; their possible postmitotic fate is less evident. The MN assay allows detection of both aneugens and clastogens, shows simplicity of scoring, is widely applicable in different cell types, is internationally validated, has potential for automation and is predictive for cancer. The cytokinesis-block micronucleus assay (CBMN) allows assessment of nucleoplasmic bridges, nuclear buds, cell division inhibition, necrosis and apoptosis and in combination with FISH using centromeric probes, the mechanistic origin of the MN. Therefore, the CBMN test can be considered as a "cytome" assay covering chromosome instability, mitotic dysfunction, cell proliferation and cell death. The toxicological relevance of the MN test is strong: it covers several endpoints, its sensitivity is high, its predictivity for in vivo genotoxicity requires adequate selection of cell lines, its statistical power is increased by the recently available high throughput methodologies, it might become a possible candidate for replacing in vivo testing, it allows good extrapolation for potential limits of exposure or thresholds and it is traceable in experimental in vitro and in vivo systems. Implementation of in vitro MN assays in the test battery for hazard and risk assessment of potential mutagens/carcinogens is therefore fully justified.

G2 checkpoint control and G2 chromosomal radiosensitivity in cancer survivors and their families

Significant inter-individual variation in G(2) chromosomal radiosensitivity, measured as radiation-induced chromatid-type aberrations in the subsequent metaphase, has been reported in peripheral blood lymphocytes of both healthy individuals and a range of cancer patients. One possible explanation for this variation is that it is driven, at least in part, by the efficiency of G(2)-M checkpoint control. The hypothesis tested in the current analysis is that increased G(2) chromosomal radiosensitivity is facilitated by a less efficient G(2)-M checkpoint. The study groups comprised 23 childhood and adolescent cancer survivors, their 23 partners and 38 of their offspring (Group 1) and 29 childhood and young adult cancer survivors (Group 2). Following exposure to 0.5 Gy of 300 kV X-rays, lymphocyte cultures were assessed for both G(2) checkpoint delay and G(2) chromosomal radiosensitivity. In Group 1, the extent of G(2) checkpoint delay was measured by mitotic inhibition. No statistically significant differences in G(2) checkpoint delay were observed between the cancer survivors (P = 0.660) or offspring (P = 0.171) and the partner control group nor was there any significant relationship between G(2) checkpoint delay and G(2) chromosomal radiosensitivity in the cancer survivors (P = 0.751), the partners (P = 0.634), the offspring (P = 0.824) or Group 1 taken as a whole (P = 0.379). For Group 2, G(2) checkpoint delay was assessed with an assay utilising premature chromosome condensation to distinguish cell cycle stage. No significant relationship between G(2) checkpoint delay and G(2) chromosomal radiosensitivity was found (P = 0.284). Thus, this study does not support a relationship between G(2)-M checkpoint efficiency and variation in G(2) chromosomal 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.

Mechanisms of the formation of radiation-induced chromosomal aberrations

Although much is now known about the mechanisms of radiation-induction of DNA double-strand breaks (DSB), there is less known about the conversion of DSB into chromosomal aberrations. In particular the induction and 'rejoining' of chromatid breaks has been a controversial topic for many years. However, its importance becomes clear in the light of the wide variation in the chromatid break response of human peripheral blood lymphocytes from different individuals when exposed to ionizing radiation, and the elevation of the frequency of radiation-induced chromatid breaks in stimulated peripheral blood lymphocytes of around 40% of breast cancer cases. A common assumption has been that chromatid breaks are merely expansions of initiating DSB, although the classic 'breakage-first' hypothesis (Sax, Ref. 44) was already challenged in the 50's by Revell [30] who maintained that chromatid breaks were formed as a result of an incomplete exchange process initiated by two interacting lesions of an unspecified nature. Here we argue that both these models of chromatid break formation are flawed and we suggest an alternative hypothesis, namely that a radiation-induced DSB initiates an indirect mechanism leading to a chromatid break. This mechanism we suggest involves the nuclear enzyme topoisomerase IIalpha and we present evidence from topoisomerase IIalpha expression variant human cell lines and from siRNA treatment of human cells that supports this hypothesis.

G0 and G2 chromosomal assays in the evaluation of radiosensitivity in a cohort of Italian breast cancer patients

Breast cancer (BC) is the most common type of malignancy in female patients and radio-treatment is the conventional therapy even if a great number of studies reported that enhanced sensitivity to ionizing radiation as measured as chromosome effects is present in a significant proportion of cancer patients, including breast cancer ones. In this study we analysed whether peripheral blood lymphocytes from sporadic BC patients and healthy subjects showed a different sensitivity to ionizing radiation and whether cytogenetic radiosensitivity may serve as a breast cancer risk biomarker. To test this hypothesis, the in vitro radiation sensitivity was measured by using both G(0) and G(2) chromosome radiosensitivity assays, on 46 subjects (23 BC patients and 23 healthy subjects). Results show that cancer patients are more radiosensitive than healthy controls and that G(2) assay could be more appropriate to define the individual radiosensitivity if compared to G(0) assay.

Response of human hematopoietic stem and progenitor cells to energetic carbon ions

PURPOSE

To characterise the radiation response of human hematopoietic stem and progenitor cells (HSPC) with respect to X and carbon ion irradiation.

MATERIALS AND METHODS

HSPC from peripheral blood of healthy donors treated with granulocyte-colony stimulating factor (G-CSF) were enriched for the transmembrane glycoprotein CD34 (cluster of differentiation) and irradiated with X rays or carbon ions (29 keV/microm monoenergetic beam and 60-85 keV/microm spread-out Bragg peak), mimicking radiotherapy conditions. Apoptotic cell death, cell cycle progression and the frequency of chromosomal aberrations were determined.

RESULTS

After radiation exposure no inhibition in the progression of the cell cycle was detected. However, an enhanced frequency of apoptotic cells and an increase in aberrant cells were observed, both effects being more pronounced for carbon ions than X rays, resulting in a relative biological effectiveness (RBE) of 1.4-1.7. The fraction of complex-type aberrations was higher following carbon ion exposure.

CONCLUSIONS

RBE values of carbon ions are low, as expected for radiosensitive cells. The observed frequencies of apoptotic cells and chromosome aberrations in HSPC are similar to those reported for human peripheral blood lymphocytes suggesting that at least with respect to apoptosis and chromosomal aberrations mature lymphocytes reflect the respective radiation responses of their proliferating progenitors.

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.

Response of lymphocytes to radiation in untreated breast cancer patients as detected with three different genetic assays

OBJECTIVE

To detect the response of lymphocytes to radiation in untreated breast cancer patients with three different genetic assays.

METHODS

Blood samples were collected from 25 untreated patients and 25 controls. Each blood sample was divided into two parts: one was irradiated by 3-Gy X-ray (irradiated sample), the other was not irradiated (non-irradiated sample). The radiosensitivity of lymphocytes was assessed by comet assay, cytokinesis-block micronucleus (CBMN) assay and 6-TG-resistant cells scored (TG) assay.

RESULTS

The baseline values of micronucleated cell frequency (MCF) and micronucleus frequency (MNF) in the patients were significantly higher than those in the controls (P < 0.01), and 3-Gy X-ray induced genetic damage to lymphocytes in the patients increased significantly as compared with that in the controls as detected with the three genetic assays (P < 0.01). The proportion of radiosensitive cases in the patient group was 48% for the mean tail length (MTL), 40% for the mean tail moment (MTM), 40% for MCF, 44% for MNF, and 48% for mutation frequencies of the hprt gene (Mfs-hprt), respectively, whereas the proportion of radiosensitive cases in the control group was only 8% for all the parameters.

CONCLUSION

The difference in the lymphocyte radiosensitivity between the breast cancer patients and the controls is significant. Moreover, there are wide individual variations in lymphocyte radiosensitivity of patients with breast cancer. In some cases, the radiosensitivity of the same patient may be different as detected with the different assays. It is suggested that multiple assays should be used to assess the radiosensitivity of patients with breast cancer before therapy.

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.

Comparison of in vivo translocation frequencies with in vitro G2 radiosensitivity in radiation workers occupationally exposed to external radiation

A group of retired workers from the British Nuclear Fuels plc facility at Sellafield who had been studied for in vivo translocation frequencies in blood lymphocytes were resampled and analysed for in vitro chromosomal radiosensitivity. Significant variation in response to a dose of 0.5 Gy given at the G(2) stage of the cell cycle was observed between individuals (P < 0.001). In a regression analysis that included age, cumulative occupational radiation dose and in vitro G(2) radiation-induced aberration frequencies as independent variables, only cumulative occupational radiation dose had a significant influence on chromosomal translocation frequency (P = 0.0036). G(2) in vitro radiosensitivity is assumed to be a marker for genetic polymorphic variation in DNA damage recognition and repair genes. Therefore, since in vivo translocation frequencies can be considered a surrogate for cancer risk, this lack of association with G(2) in vitro radiosensitivity suggests that such genetic variation has no impact on the response to low dose chronic exposure.

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.

Influence of polymorphisms at loci encoding DNA repair proteins on cancer susceptibility and G2 chromosomal radiosensitivity

Sixteen candidate polymorphisms (13 SNPs and 3 microsatellites) in nine genes from four DNA repair pathways were examined in 83 subjects, comprising 23 survivors of childhood cancer, their 23 partners, and 37 offspring, all of whom had previously been studied for G(2) chromosomal radiosensitivity. Genotype at the Asp148Glu SNP site in the APEX gene of the base excision repair (BER) pathway was associated with childhood cancer in survivors (P = 0.001, significant even after multiple test adjustment), due to the enhanced frequency of the APEX Asp148 allele among survivors in comparison to that of their partners. Analysis of variance (ANOVA) of G(2) radiosensitivity in the pooled sample, as well as family-based association test (FBAT) of the family-wise data, showed sporadic suggestions of associations between G(2) radiosensitivity and polymorphisms at two sites (the Thr241Met SNP site in the XRCC3 gene of the homologous recombinational pathway by ANOVA, and the Ser326Cys site in the hOGG1 gene of the BER pathway by FBAT analysis), but neither of these remained significant after multiple-test adjustment. This pilot study provides an intriguing indication that DNA repair gene polymorphisms may underlie cancer susceptibility and variation in radiosensitivity.

Cytogenetic methods for biodosimetry and risk individualisation after exposure to ionising radiation

Measurement of dicentric chromosomes in human lymphocytes has been applied to assess dose received by potentially overexposed people and estimate risk for health effects. Since the dicentrics in exposed people decrease with time, the introduction of fluorescent in situ hybridisation enables to measure stable translocations for biodosimetry and address old or long-term exposures. In addition, premature chromosome condensation, which enables analysis in interphase, offers several advantages for biodosimetry. However, dose and risk estimates derived using cytogenetics and adequate calibration curves are based on the assumption that all individuals respond equally to radiation. Since increased radiosensitivity has been associated with cancer proneness, there is particular interest for risk assessment at the individual level. Towards this end, the efficiency of dynamics that govern DNA repair and apoptosis, as well as the conserved cellular processes that have evolved to facilitate DNA damage recognition using signal transduction pathways to activate cell cycle arrest and preserve genomic integrity, are being investigated. Recent work in cancer cytogenetics and on the modulation of radiation effects at the chromosome level using changes in gene expression associated with proteins or factors such as caffeine or amifostine treatment during G(2) to M-phase transition, reconfirmed the importance of G(2) chekpoint in determining radiosensitivity and of the cdk1/cyclin-B activity in the conversion of DNA damage into chromatid breaks. G(2)-chromosomal radiosensitivity may offer, therefore, a basis for the identification or testing of key genetic targets for modulation of radiation effects and the establishment of a screening method to detect intrinsic radiosensitivity.

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.

Persistence of chromosome aberrations following acute radiation: I, PAINT translocations, dicentrics, rings, fragments, and insertions

Chromosome translocations are used to estimate the doses of radiation received following occupational or accidental exposure. Biodosimetry relies on the assumption that translocations are not cell-lethal and persist with little or no loss over time. While translocations do exhibit substantially greater persistence than other aberration types (e.g., dicentrics), there is evidence that translocation frequencies also decline over time, at least following acute doses above 1 Gy. To the extent that translocation frequencies decline, the predicted absorbed doses will be underestimated. Yet unknown is whether translocations induced by ionizing radiation at doses below 1 Gy also show significant declines. Here we report on the persistence of translocations induced by 137Cs gamma-rays at acute doses ranging from 0.2 to 4 Gy using peripheral blood lymphocytes from two unrelated healthy male donors. Chromosome aberrations were evaluated by simultaneously painting chromosomes 1, 2, and 4 in red and 3, 5, and 6 in green in cells harvested 2-7 days following exposure and were scored using the PAINT system. Translocations were also enumerated using several other methods and these results are reported separately by us in this issue. For comparison, the persistence of dicentrics, rings, acentric fragments, and color junctions was also evaluated and showed rapid losses with time. The results from both donors provide evidence that translocation frequencies decline with time in a statistically significant manner at doses as low as 0.2-0.3 Gy. The frequency of translocations for all dose groups declined from day 2 to 7 by averages of 39% and 26% for donors 1 and 2, respectively. These data emphasize the importance of considering translocation loss in biological dosimetry long times after exposure.

Chromosomal radiosensitivity and low penetrance predisposition to cancer

This mini-review summarises studies in this Institute on the sensitivity of cells of patients with common cancers to the chromosome-damaging effects of ionising radiation, in the context of related studies. Using the 90th percentile of healthy controls (n >200) as the cut-off point between a normal and a sensitive response, 40% of patients with breast cancer (n = 166) were sensitive when cells were irradiated in the G2 phase of the cell cycle. Smaller studies showed that patients with colorectal, head and neck (at < 45 years) and childhood cancers also exhibited degrees of enhanced sensitivity, whereas cervical and lung cancer cases did not. Cells from breast and head and neck cases irradiated in G(0) also showed increased sensitivity. We propose that such elevated sensitivity is a marker of low penetrance predisposition to cancer. The strongest support for this hypothesis was our demonstration of the Mendelian heritability of chromosomal radiosensitivity in 95 family members of breast cancer cases. Challenges for the future include more heritability studies, identification of the underlying genetic determinants, assessment of the associated cancer risk (spontaneous and radiogenic) and population screening for cancer prevention strategies.

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.