Chromosome aberration yields from multiple fixation regimens

Hypothermia differentially modulates the formation and decay of NBS1, γH2AX and 53BP1 foci in U2OS cells exposed to gamma radiation

In studies on the mechanism of DNA damage response where ionizing radiation is used as the DNA damaging agent, cells are often exposed to ionizing radiation on melting ice (corresponding to 0.8 °C). The purpose of this procedure is to inhibit cellular processes i.e. DNA repair. Low temperature at exposure has been shown to act in a radioprotective manner at the level of cytogenetic damage, but its mechanisms of action are poorly understood. The aim of the study was to analyze the effect of hypothermia at the level of formation and decay of NBS1, γH2AX, and 53BP1 foci, micronuclei, survival, cell cycle progression and oxidative stress in U2OS cells. The results show that hypothermia alone induced oxidative stress and foci. When applied in combination with radiation but only during the exposure time, it potentiated the formation of γH2AX and 53BP1 but not of NBS1 foci. When applied during irradiation and subsequent repair time, 53BP1 and NBS1 foci formed and decayed, but the levels were markedly lower than when repair was carried out at 37 °C. The frequency of micronuclei was elevated in cells irradiated at 0.8 °C, but only when analysed 20 h after irradiation which is likely due to a reduced G₂ cell cycle block. Hypothermia reduced cell survival, both with and without radiation exposure. The temperature effect should be considered when cooling cells on melting ice to inhibit DNA repair in the induction of DNA damage.

Biological effectiveness of very high gamma dose rate and its implication for radiological protection

Many experimental studies are carried out to compare biological effectiveness of high dose rate (HDR) with that of low dose rate (LDR). The rational for this is the uncertainty regarding the value of the dose rate effectiveness factor (DREF) used in radiological protection. While a LDR is defined as 0.1 mGy/min or lower, anything above that is seen as HDR. In cell and animal experiments, a dose rate around 1 Gy/min is usually used as representative for HDR. However, atomic bomb survivors, the reference cohort for radiological protection, were exposed to tens of Gy/min. The important question is whether gamma radiation delivered at very high dose rate (VHDR-several Gy/min) is more effective in inducing DNA damage than that delivered at HDR. The aim of this investigation was to compare the biological effectiveness of gamma radiation delivered at VHDR (8.25 Gy/min) with that of HDR (0.38 Gy/min or 0.79 Gy/min). Experiments were carried out with human peripheral mononuclear cells (PBMC) and the human osteosarcoma cell line U2OS. Endpoints related to DNA damage response were analysed. The results show that in PBMC, VHDR is more effective than HDR in inducing gene expression and micronuclei. In U2OS cells, the repair of 53BP1 foci was delayed after VHDR indicating a higher level of damage complexity, but no VHDR effect was observed at the level of micronuclei and clonogenic cell survival. We suggest that the DREF value may be underestimated when the biological effectiveness of HDR and LDR is compared.

Application of cell sorting for enhancing the performance of the cytokinesis-block micronucleus assay

Among the numerous methods available to assess genotoxicity, the cytokinesis-block micronucleus (CBMN) assay is very popular due its relative simplicity and power to detect both clastogenic and aneugenic compounds. A problem with the CBMN assay is that all DNA damaging agents also inhibit the ability of cells to progress through mitosis, leading to a low number of binucleated cells (BNCs). One method to resolve this issue is to ensure a sufficient proportion of BNCs in the samples. In the current study, the applicability of a cell sorting system capable of isolating cell fractions containing abundant BNCs was investigated. Furthermore, to investigate the relationship between the cell division delay due to radiation exposure and the generation of BNCs and micronuclei (MN), we assessed a series of lag times between radiation exposure and addition of cytochalasin-B (Cyt-B). Cells from the human chronic myelogenous leukemia cell line K562 were exposed to X-rays (2 Gy and 4 Gy), and Cyt-B was subsequently added at 0, 6 and 12 h following irradiation. After treatment with Cyt-B for 24 h, the percentage of BNCs, the MN frequency and the cell cycle distribution were analyzed. In addition, cells displaying the DNA contents corresponding to BNCs were isolated and analyzed. The results indicate that applying the cell sorter to the CBMN assay increased the percentage of BNCs compared with the standard method. Thus, this technique is a promising way of enhancing the capacity of the CBMN assay.

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.

Radioprotective effect of hypothermia on cells - a multiparametric approach to delineate the mechanisms

PURPOSE

Low temperature (hypothermia) during irradiation of cells has been reported to have a radioprotective effect. The mechanisms are not fully understood. This study further investigates the possible mechanisms behind hypothermia-mediated radioprotection.

MATERIALS AND METHODS

Human lymphoblastoid TK6 cells were incubated for 20 min at 0.8 or 37°C and subsequently exposed to 1 Gy of γ- or X-rays. The influence of ataxia telangiectasia mutated (ATM)-mediated double-strand break signalling and histone deacetylase-dependent chromatin condensation was investigated using the micronucleus assay. Furthermore, the effect of hypothermia was investigated at the level of phosphorylated histone 2AX (γH2AX) foci, clonogenic cell survival and micronuclei in sequentially-harvested cells.

RESULTS

The radioprotective effect of hypothermia (called the temperature effect [TE]) was evident only at the level of micronuclei at a single fixation time, was not influenced by the inhibition of ATM kinase activity and completely abolished by the histone deacetylase inhibition. No TE was seen at the level of γH2AX foci and cell survival.

CONCLUSIONS

We suggest that low temperature during irradiation can induce a temporary cell cycle shift, which could lead to a reduced micronucleus frequency. Future experiments focused on cell cycle progression are needed to confirm this hypothesis.

Modeling radiation-induced cell cycle delays

Ionizing radiation is known to delay the cell cycle progression. In particular after particle exposure significant delays have been observed and it has been shown that the extent of delay affects the expression of damage, such as chromosome aberrations. Thus, to predict how cells respond to ionizing radiation and to derive reliable estimates of radiation risks, information about radiation-induced cell cycle perturbations is required. In the present study we describe and apply a method for retrieval of information about the time-course of all cell cycle phases from experimental data on the mitotic index only. We study the progression of mammalian cells through the cell cycle after exposure. The analysis reveals a prolonged block of damaged cells in the G2 phase. Furthermore, by performing an error analysis on simulated data valuable information for the design of experimental studies has been obtained. The analysis showed that the number of cells analyzed in an experimental sample should be at least 100 to obtain a relative error <20%.

Correlation between mitotic delay and aberration burden, and their role for the analysis of chromosomal damage

The aim was to investigate further the relationship between radiation-induced mitotic delay and the expression of chromosome damage in V79 cells. Recently published data on the time-course of chromosome aberrations in V79 first-cycle metaphases after exposure to 10.4 MeV u(-1) Ar ions (LET = 1226 keV microm(-1)) were supplemented and reanalysed. A statistical analysis of the distribution of aberrations among cells was performed. Furthermore, cells were grouped into subpopulations carrying 0, 1 -2, 3-4, 5- 6 and 7 or more aberrations. Then, based on the mitotic index, the flux of each subgroup through the first mitosis was determined and the average entrance time to mitosis was estimated. For comparison, the flux of aberrant V79 cells generated by X-irradiation was analysed. Analysis of the Ar ion data revealed that the flux of each subpopulation through the first mitosis is strongly affected by its aberration burden, i.e. a positive correlation between the mitotic delay and the number of aberrations carried by a cell was observed. The distribution of aberrations among cells could be well described by Neyman-type A statistics; the corresponding fit parameters also reflect the damage-dependent mitotic delay. Interestingly, comparison of the flux of Ar ion and X-ray-irradiated V79 cells through mitosis revealed (1) that a direct correlation exists between the number of aberrations carried by a cell and its average entrance time to mitosis, and (2) that this effect is independent of the linear energy transfer. The role of these observations for radiation cytogenetics is discussed.

Cell cycle arrest and aberration yield in normal human fibroblasts. I. Effects of X‐rays and 195 MeV u−1C ions

PURPOSE

To examine the relationship between cell proliferation and the expression of chromosomal damage in normal human skin fibroblasts after X-ray and particle irradiation.

MATERIALS AND METHODS

Confluent G0/G1 AG1522B cells were exposed to X-rays or 195MeV u(-1) C ions with a linear energy transfer of 16.6 keV microm(-1) in the dose range 1-4 Gy. Directly after irradiation, cells were reseeded at a low density in medium containing 5-bromo-2'-deoxyuridine. At multiple time points post-irradiation, the cumulative BrdU-labelling index, mitotic index and aberration frequency were measured. Based on these data, the total amount of damage induced within the entire cell population was estimated by means of mathematical analysis.

RESULTS

Both types of radiation exposure exert a pronounced effect on the cell cycle progression of fibroblasts. They result in delayed entry of cells into S-phase and into the first mitosis, and cause a dramatic reduction in mitotic activity. Measurement of chromosomal damage in first-cycle cells at multiple time points post-irradiation shows that the frequencies of aberrant cells and aberrations increase with time up to twofold for the lower doses. However, for the higher doses, this effect is less pronounced or even disappears. When the data for the whole cell population are analysed, it becomes evident that only a few damaged fibroblasts can progress to the first mitosis, a response attributable at least in part to a long-term arrest of injured cells in the initial G0/G1-phase. As observed in other investigations, the effectiveness of 195 MeV u(-1) C ions was similar or slightly higher than X-rays for all endpoints studied leading to a relative biological effectiveness in the range 1.0-1.4.

CONCLUSIONS

Cell cycle arrests affect the aberration yield observable in normal human fibroblasts at mitosis. The data obtained for the cell population as a whole reveal that injured cells are rapidly removed from the mitotically active population through a chronic cell cycle arrest, which is consistent with other studies that indicate that this response is a specific strategy of fibroblasts to minimize the fixation and propagation of genetic alterations.

Enhanced chromosomal radiosensitivity in peripheral blood lymphocytes of larynx cancer patients

PURPOSE

The chromosomal radiosensitivity in peripheral blood lymphocytes of cancer patients was reported to be higher than that of healthy donors. This effect is especially prominent when aberrations induced in the G2 phase of the cell cycle are analyzed. The aim of our study was to investigate if the G2 aberration frequencies in lymphocytes of patients with larynx cancer are higher than in the case of control individuals. Also, we tested if the frequencies of G2 aberrations correlate with side effects of radiotherapy.

METHODS AND MATERIALS

Peripheral blood of 38 patients was collected before the onset of radiotherapy, cultured for 72 h, and irradiated with 2 Gy after 67 h. Lymphocytes of 40 healthy donors were treated in the same way.

RESULTS

The spontaneous and radiation-induced aberration frequencies in lymphocytes of patients were on average higher than in those of healthy donors. No statistically significant correlation was observed between aberration frequencies in lymphocytes and the degree of both early and late normal tissue reactions.

CONCLUSIONS

The chromosomal radiosensitivity of lymphocytes of patients with larynx cancer may be a marker of cancer predisposition; however, it does not appear to have a predictive value for the risk of developing side effects to radiotherapy.

Application of a multiple fixation regimen to study the adaptive response to ionizing radiation in lymphocytes of two human donors

The majority of experiments studying the adaptive response using chromosomal aberrations have been performed with proliferating lymphocytes. It is known that lymphocytes have variable cell cycle transit times and it has been pointed out that in such cases aberration scores obtained from a single harvest are not very meaningful because cells harvested together in metaphase at any one time after irradiation were in different parts of the cell cycle at the time of irradiation. The scored sample will thus always contain a mixture of cells having different radiosensitivities and any variations of cell proliferation will influence the aberration score. In order to get a more representative aberration score a multiple fixation regimen was applied to lymphocytes of two human donors. Cells receiving the adapting + challenging and the challenging dose were fixed at three intervals after the challenge. In lymphocytes of donor 1 no adaptive response was seen at any fixation time in two experiments. In lymphocytes of donor 2, however, a reduction of aberration frequencies was seen, but at different fixation times in the two experiments. In a third experiment, no adaptive response was detected. It is concluded that the response observed at some fixation times in lymphocytes of donor 2 is rather a result of some phenomenon associated with variations of cell cycle kinetics than of induced radiation resistance.

Genetic effects of the flavonols quercetin, kaempferol, and galangin on Chinese hamster ovary cells in vitro

The genotoxicity of selected flavonols was evaluated by multiple endpoints in Chinese hamster ovary (CHO) cells. Chromosomal aberrations, sister-chromatid exchange (SCE), and forward mutation at 4 gene loci were measured in a single population of cells exposed to quercetin, kaempferol, or galangin for 15 h with and without metabolic activation. The incidence of chromosomal aberrations was significantly increased by quercetin in the absence of activation and by kaempferol and galangin with and without activation. Flavonol treatment affected SCE and mutation at the hgprt, aprt, or Na+/K+-ATPase loci only marginally, but significantly increased mutation frequencies at the tk locus. The response at the tk locus suggests that the CHO cells may behave similarly to L5178Y cells, in which the tk locus is thought to reflect chromosomal lesions in addition to point mutation. These results indicate that, at least under the conditions examined, flavonols induce chromosomal aberrations in CHO cells, but have little effect on point mutation or SCE.

DNA crosslinking, sister-chromatid exchange and specific-locus mutations

Chinese hamster ovary cells were treated with the DNA-crosslinking chemicals, mitomycin C (MMC) and porfiromycin (POR), and their monofunctional derivative decarbamoyl mitomycin C (DCMMC). After exposure, the cells were studied for the induction of sister-chromatid exchanges (SCEs) and mutations at the hypoxanthine phosphoribosyltransferase and adenine phosphoribosyltransferase loci. The frequency of SCEs varied significantly in successive sampling intervals, requiring the weighting of each interval by the percentage of second-division mitosis in that interval to obtain the mean SCE frequency for each dose. All 3 compounds were potent inducers of SCEs but weakly mutagenic. All 3 chemicals by concentration were approximately equally effective in inducing SCEs or mutations. When the induced SCEs and mutations were compared at equal levels of survival, DCMMC was slightly more effective than MMC or POR in inducing SCEs and somewhat less mutagenic. These results indicate that the DNA interstrand crosslink is not the major lesion responsible for the induction of SCE or mutation by these compounds.