Effect of signal transduction inhibition in adapted lymphocytes: micronuclei frequency and DNA repair

Evaluation of the in vivo genotoxic effects of gamma radiation on the peripheral blood leukocytes of head and neck cancer patients undergoing radiotherapy

The present study aimed to evaluate the genotoxic effects of ionizing radiation on non-target cells of Head and Neck Squamous Cell Carcinoma (HNSCC) patients exposed to various cumulative doses of gamma rays during radiotherapy. The ten patients (P1-P10) were treated with cobalt 60 gamma radiation (External Beam Radiotherapy) for a period of five to six weeks with a daily fraction of 2Gy for 5 days each week. The genotoxic effects of radiation (single strand breaks - SSBs) in these patients were analyzed using the alkaline single cell gel electrophoresis (SCGE) technique, with the Olive Tail Moment (OTM) as the critical parameter. A sample of each patient's peripheral blood before starting with radiotherapy (pre-therapy) served as the control, and blood collected at weekly time intervals during the course of the radiotherapy served as treated (10, 20, 30, 40, 50 and 60Gy) samples. In vivo radiosensitivity of these patients, as indicated by SSB's after the cumulative radiation doses at the various times, was assessed using Student's t-test. Significant DNA damage relative to the individual patient's pre-therapy baseline data was observed in all patients. Inter-individual variation of the genotoxic effects was analyzed using two-way ANOVA. The correlation between doses for the means of smoker and non-smoker patients was calculated using the Pearson test. The results of this study may indicate the need to reduce the daily radiotherapy dose further to prevent genotoxic effects on non-target cells, thus improving safety. Furthermore, these results may indicate that the estimation of DNA damage following exposure to a gamma radiation, as measured by the comet assay in whole blood leukocytes, can be used to screen human populations for radiation-induced genetic damage at the molecular level.

Radiation hormesis: Autophagy and other cellular mechanisms

PURPOSE

To review the cellular mechanisms of hormetic effects induced by low dose and low dose rate ionising radiation in model systems, and to call attention to the possible role of autophagy in some hormetic effects.

RESULTS AND CONCLUSIONS

Very low radiation doses stimulate cell proliferation by changing the equilibrium between the phosphorylated and dephosphorylated forms of growth factor receptors. Radioadaptation is induced by various weak stress stimuli and depends on signalling events that ultimately decrease the molecular damage expression at the cellular level upon subsequent exposure to a moderate radiation dose. Ageing and cancer result from oxidative damage under oxidative stress conditions; nevertheless, ROS are also prominent inducers of autophagy, a cellular process that has been shown to be related both to ageing retardation and cancer prevention. A balance between the signalling functions and damaging effects of ROS seems to be the most important factor that decides the fate of the mammalian cell when under oxidative stress conditions, after exposure to ionising radiation. Not enough is yet known on the pre-requirements for maintaining such a balance. Given the present stage of investigation into radiation hormesis, the application of the conclusions from experiments on model systems to the radiation protection regulations would not be justified.

An approach to estimate radioadaptation from DSB repair efficiency

In this review, we would like to introduce a unique approach for the estimation of radioadaptation. Recently, we proposed a new methodology for evaluating the repair efficiency of DNA double-strand breaks (DSB) using a model system. The model system can trace the fate of a single DSB, which is introduced within intron 4 of the TK gene on chromosome 17 in human lymphoblastoid TK6 cells by the expression of restriction enzyme I-SceI. This methodology was first applied to examine whether repair of the DSB (at the I-SceI site) can be influenced by low-dose, low-dose rate gamma-ray irradiation. We found that such low-dose IR exposure could enhance the activity of DSB repair through homologous recombination (HR). HR activity was also enhanced due to the pre-IR irradiation under the established conditions for radioadaptation (50 mGy X-ray-6 h-I-SceI treatment). Therefore, radioadaptation might account for the reduced frequency of homozygous loss of heterozygosity (LOH) events observed in our previous experiment (50 mGy X-ray-6 h-2 Gy X-ray). We suggest that the present evaluation of DSB repair using this I-SceI system, may contribute to our overall understanding of radioadaptation.

Ionizing radiation-induced bystander mutagenesis and adaptation: quantitative and temporal aspects

This work explores several quantitative aspects of radiation-induced bystander mutagenesis in WTK1 human lymphoblast cells. Gamma-irradiation of cells was used to generate conditioned medium containing bystander signals, and that medium was transferred onto naïve recipient cells. Kinetic studies revealed that it required up to 1h to generate sufficient signal to induce the maximal level of mutations at the thymidine kinase locus in the bystander cells receiving the conditioned medium. Furthermore, it required at least 1h of exposure to the signal in the bystander cells to induce mutations. Bystander signal was fairly stable in the medium, requiring 12-24h to diminish. Medium that contained bystander signal was rendered ineffective by a 4-fold dilution; in contrast a greater than 20-fold decrease in the cell number irradiated to generate a bystander signal was needed to eliminate bystander-induced mutagenesis. This suggested some sort of feedback inhibition by bystander signal that prevented the signaling cells from releasing more signal. Finally, an ionizing radiation-induced adaptive response was shown to be effective in reducing bystander mutagenesis; in addition, low levels of exposure to bystander signal in the transferred medium induced adaptation that was effective in reducing mutations induced by subsequent gamma-ray exposures.

Relationships between acute reactions to radiotherapy in head and neck cancer patients and parameters of radiation-induced DNA damage and repair in their lymphocytes

PURPOSE

To study the relationship between lymphocyte radiosensitivity measured in vitro and acute reactions to radiotherapy in patients with head and neck cancer.

MATERIALS AND METHODS

Acute reactions were measured in 34 patients using the Dische scale. Lymphocyte radiosensitivity was measured using the alkaline comet assay, the micronucleus assay, the nuclear division index and morphological assessment of apoptosis.

RESULTS

There was a weak, statistically significant correlation between in vitro radiosensitivity measured as the rate of DNA damage repair and the cumulative radiation dose exerting the maximum acute reaction scored (r = -0.366, p = 0.039, n = 34). Subgroup analyses showed that for patients with a low level of radiation-induced DNA damage there was a statistically significant relationship between lymphocyte radiosensitivity measured as inhibition of proliferation and acute toxicity (r = -0.621, p = 0.007, n = 18). For patients with a high level of residual DNA damage, there was a relationship between lymphocyte radiosensitivity measured using the micronucleus assay and acute toxicity (r = -0.597, p = 0.023, n = 14).

CONCLUSIONS

Combining two measures of radiosensitivity improves the ability to correlate in vitro lymphocyte radiosensitivity and acute radiotherapy toxicity data.

A modified alkaline comet assay for measuring DNA repair capacity in human populations

Trzeciak, A. R., Barnes, J. and Evans, M. K. A Modified Alkaline Comet Assay for Measuring DNA Repair Capacity in Human Populations. Radiat. Res. 169, 110-121 (2008). Use of the alkaline comet assay to assess DNA repair capacity in human populations has been limited by several factors, including lack of methodology for use of unstimulated cryopreserved peripheral blood mononuclear cells (PBMCs), insufficient control of interexperimental variability, and limited analysis of DNA repair kinetics. We show that unstimulated cryopreserved PBMCs can be used in DNA repair studies performed using the comet assay. We have applied data standardization for the analysis of DNA repair capacity using negative and positive internal standards as controls for interexperimental variability. Our standardization procedure also uses negative controls, which provides a way to minimize the interference of interindividual variation in baseline DNA damage levels on DNA repair capacity measurements in populations. DNA repair capacity was assessed in a small human cohort using the parameters described in the literature including initial DNA damage, half-time of DNA repair, and residual DNA damage after 30 and 60 min. We have also introduced new DNA repair capacity parameter, initial rate of DNA repair. There was no difference in DNA repair capacity between fresh and cryopreserved PBMCs when measured by the Olive tail moment and tail DNA. The use of DNA repair capacity parameters in assessment of fast and slow single-strand break repair components is discussed.

Mutagenic radioadaptation in a human lymphoblastoid cell line

We investigated the mutagenic radioadaptive response of human lymphoblastoid TK6 cells by pretreating them with a low dose (5 cGy) of X-rays followed by a high (2 Gy) dose 6h later. Pretreatment reduced the 2-Gy-induced mutation frequency (MF) of the thymidine kinase (TK) gene (18.3 x 10(-6)) to 62% of the original level (11.4 x 10(-6)). A loss of heterozygosity (LOH) detection analysis applied to the isolated TK(-) mutants revealed the mutational events as non-LOH (resulting mostly from a point mutation in the TK gene), hemizygous LOH (resulting from a chromosomal deletion), or homozygous LOH (resulting from homologous recombination (HR) between chromosomes). For non-LOH events, pretreatment decreased the frequency to 27% of the original level (from 7.1 x 10(-6) to 1.9 x 10(-6)). cDNAs prepared from the non-LOH mutants revealed that the decrease was due mainly to the repression of base substitutions. The frequency of hemizygous LOH events, however, was not significantly altered by pretreatment. Mapping analysis of chromosome 17 demonstrated that the distribution and the extent of hemizygous LOH events were also not significantly influenced by pretreatment. For homozygous LOH events, pretreatment reduced the frequency to 61% of the original level (from 5.1 x 10(-6) to 3.1 x 10(-6)), reflecting an enhancement in HR repair of DNA double-strand breaks. Our findings suggest that the radioadaptive response in TK6 cells follows mainly from mutations at the base-sequence level, not the chromosome level.

DNA repair capacity as a possible biomarker of breast cancer risk in female BRCA1 mutation carriers

The BRCA1 gene product helps to maintain genomic integrity through its participation in the cellular response to DNA damage: specifically, the repair of double-stranded DNA breaks. An impaired cellular response to DNA damage is a plausible mechanism whereby BRCA1 mutation carriers are at increased risk of breast cancer. Hence, an individual's capacity to repair DNA may serve as a useful biomarker of breast cancer risk. The overall aim of the current study was to identify a biomarker of DNA repair capacity that could distinguish between BRCA1 mutation carriers and non-carriers. DNA repair capacity was assessed using three validated assays: the single-cell alkaline gel electrophoresis (comet) assay, the micronucleus test, and the enumeration of gamma-H2AX nuclear foci. DNA repair capacity of peripheral blood lymphocytes from 25 cancer-free female heterozygous BRCA1 mutation carriers and 25 non-carrier controls was assessed at baseline and following cell exposure to gamma-irradiation (2 Gy). We found no significant differences in the mean tail moment, in the number of micronuclei or in the number of gamma-H2AX nuclear foci between the carriers and non-carriers at baseline, and following gamma-irradiation. These data suggest that these assays are not likely to be useful in the identification of women at a high risk for breast cancer.

Radiation-induced DNA damage and repair in peripheral blood mononuclear cells from Nijmegen breakage syndrome patients and carriers assessed by the Comet assay

Nijmegen breakage syndrome (NBS) patients and carriers are predisposed to malignancy and are often treated with X-irradiation. In the present study, the single-cell gel electrophoresis (Comet) assay was used to examine radiation-induced DNA damage and repair in peripheral blood mononuclear cells from NBS patients (n=13) and carriers (n=36) of six unrelated families. Cells from apparently healthy donors (n=10) and from breast cancer patients with normal clinical radiosensitivity (n=10) served as controls. Cells were irradiated with 5 Gy of X-rays and assayed for initial DNA damage and for residual DNA damage after 40 min of repair; the kinetics of DNA repair also was estimated. In addition, the nuclear area of unirradiated cells was extracted from the Comet data. The initial radiation-induced DNA fragmentation indicated that cells from members of two out of six NBS families were significantly more sensitive to X-irradiation than cells from the controls. Cells from four NBS families had longer DNA repair half-time values, while cells from five NBS families had significantly increased residual DNA damage following repair. The mean nuclear area of unirradiated cells processed in the Comet assay was 1.3-fold higher in cells from all NBS families than in the controls (P<0.05). Notably, the Comet assay parameters (initial and residual DNA damage and the repair kinetics) of irradiated NBS cells predicted the carrier status of the majority (86%) of blindly tested individuals. The prediction of NBS status was higher if the nuclear area of unirradiated cells was used as the endpoint. The results of this study suggest that the impaired radiation response of NBS cells should be taken into account if radiotherapy of NBS patients and carriers is required.

Adaptive response: stimulated DNA repair or decreased damage fixation?

The aim was to review the present state of knowledge on the adaptive response and attempt to redefine the acknowledged model in the framework of the transcription-based model of damage fixation of Radford (2002). Data are reviewed that suggest that the priming stimulus is the source of signalling that eventually leads to expression of the adaptive response. For a certain time, the 'primed' cell can then respond to the challenge dose by an increased recovery, as compared with the control one. An essential part of the adaptive response is generation or receipt and transmission of a signal that is the direct cause of initiation of a cellular response that diminishes the effects of DNA damage. The often accepted view that DNA repair is stimulated in the 'primed' and challenged cell is not supported by all the available data. Taking into account the abrogation of radio-adaptation by poly(ADP-ribosylation) inhibitors applied simultaneously with the challenge dose and the fact that adaptation is revealed as a decrease in chromosomal aberration frequency, one can apply to the adaptive response the same arguments as those that support the fixation model of Radford. Adaptive response (at least in part) is due to diminished fixation of double-strand breaks in the transcription factories by the mechanism proposed by Radford.

Genotoxicity in the eyes of bystander cells

The controversial use of a linear, no threshold extrapolation model for low dose risk assessment has become even more so in light of the recent reports on the bystander phenomenon. The answer to the question as to which of the two phenomena, bystander versus adaptive response, is more important has practical implication in terms of low dose radiation risk assessment. In this review, genotoxicity is used as an endpoint to introduce the two phenomena, provide some insight into the mechanisms of bystander effect and to bridge the two low dose phenomena which operate in opposite directions: the bystander effect tends to exaggerate the effect at low doses, by communicating damage from hit to non-hit cells whereas the adaptive response confers resistance to a subsequent challenging dose by an initial low priming dose.

Henry S. Kaplan Distinguished Scientist Award 2003. The crooked shall be made straight; dose-response relationships for carcinogenesis

Estimates of radiation-induced malignancies come principally from the atomic (A)-bomb survivors and show an excess incidence of carcinomas that is linearly related to dose from about 5 cGy to 2.5 Gy. Above and below this dose range there is considerable uncertainty about the shape of the dose-response relationship. Both the International Commission of Radiation Protected (ICRP) and the National Council of Radiation Protection (NCRP) suggest that cancer risks at doses lower than those at which direct epidemiological observations are possible should be obtained by a linear extrapolation from higher doses. The demonstrated bystander effect for irradiation exaggerates the consequences of small doses of radiation and implies that a linear extrapolation from high doses would underestimate low dose risks. It is possible to make estimates of the cancer risk of diagnostic radiological procedures. Helical computed tomography in children is of particular interest since it is rapidly increasing in use and the doses involved are close to the lower limit of significance in the A-bomb survivors. For example, an abdominal computed tomographic scan in a 1-year-old child can be estimated to result in a lifetime cancer risk of about 1:1000. In the context of radiotherapy, some normal tissues receive 70 Gy, while a larger volume receives a lower dose, but still far higher than the range for which data are available from the A-bomb survivors. Data are available for the risk of radiation-induced malignancies for patients who received radiotherapy, e.g. for prostate or cervical cancer. New technologies such as intensity modulated radiation therapy could result in a doubling of radiation-induced second cancers since the technique involves a larger total-body dose due to leakage radiation and the dose distribution obtained involves a larger volume of normal tissue exposed to lower radiation doses.

Adaptive response and the bystander effect induced by radiation in C3H 10T(1/2) cells in culture

This paper discusses two phenomena of importance at low doses that have an impact on the shape of the dose-response relationship. First, there is the bystander effect, the term used to describe the biological effects observed in cells that are not themselves traversed by a charged particle, but are neighbors of cells that are; this exaggerates the effect of small doses of radiation. Second, there is the adaptive response, whereby exposure to a low level of DNA stress renders cells resistant to a subsequent exposure; this reduces the effect of low doses of radiation. The present work was undertaken to assess the relative importance of the adaptive response and the bystander effect induced by radiation in C3H 10T(1/2) cells in culture. When the single-cell microbeam delivered from 1 to 12 alpha particles through the nuclei of 10% of C3H 10T(1/2) cells, more cells were inactivated than were actually traversed by alpha particles. The magnitude of this bystander effect increased with the number of particles per cell. An adaptive dose of 2 cGy of gamma rays, delivered 6 h beforehand, canceled out about half of the bystander effect produced by the alpha particles.

Neutron-induced adaptive response studied in go human lymphocytes using the comet assay

This study demonstrates that cells adapted to ionizing radiation developed reduced initial DNA damage when compared to non-adapted cells. The results were obtained by subjecting in vitro irradiated whole blood from 10 healthy volunteers (including 2 A-bomb survivors carrying 1.5-2 Gy in vivo exposure) in an unstimulated condition (G0) using the comet assay. The intensity of DNA damage was assessed by computing the 'tail moment'. Adaptive response (AR) was noticed in only donor 3, as indicated by reduced tail moment when the blood samples received priming + challenging doses over a 4 h interval. The priming dose was either 0.01 Gy 137Cs gamma-rays or 0.0025 Gy 252Cf neutrons. The delivered challenging dose was either 1 Gy 60Co g-rays or 0.25 Gy 252Cf neutrons. The irradiation was conducted using the HIRRAC facility. A prior exposure to 0.0025 Gy 252Cf neutrons nullified the excess tail moment caused by 0.25 Gy neutrons given during a 4 h gap. In a similar way, 0.01 Gy 137Cs gamma-rays offered a cross-adaptive response to the neutron challenging dose. The tail moment of A-bomb survivors after in vitro irradiation was less than that of the age-matched control and, at the same time, was not influenced by the priming dose. An altered subset and the immunological status of blood after A-bomb exposure were cited as possible factors. Because AR can affect the outcome of RBE, its individual variability only emphasizes the need to have individual biodosimetry for better risk assessment, especially in planning for a long space voyage.