Mechanisms of radiation-induced chromosome aberrations

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.

Hypothermia modulates the DNA damage response to ionizing radiation in human peripheral blood lymphocytes

PURPOSE

Low temperature at exposure has been shown to act in a radioprotective manner at the level of cytogenetic damage. It was suggested to be due to an effective transformation of DNA damage to chromosomal damage at low temperature. The purpose of the study was to analyze the kinetics of aberration formation during the first hours after exposing human peripheral blood lymphocytes to ionizing radiation at 0.8 °C and 37 °C.

MATERIALS AND METHODS

To this end, we applied the technique of premature chromosome condensation. In addition, DNA damage response was analyzed by measuring the levels of phosphorylated DNA damage responsive proteins ATM, DNA-PK and p53 and mRNA levels of the radiation-responsive genes BBC3, FDXR, GADD45A, XPC, MDM2 and CDKN1A.

RESULTS

A consistently lower frequency of chromosomal breaks was observed in cells exposed at 0.8 °C as compared to 37 °C already after 30 minutes postexposure. This effect was accompanied by elevated levels of phosphorylated ATM and DNA-PK proteins and a reduced immediate level of phosphorylated p53 and of the responsive genes.

CONCLUSIONS

Low temperature at exposure appears to promote DNA repair leading to reduced transformation of DNA damage to chromosomal aberrations.

Cytogenetic dose-response in vitro for biological dosimetry after exposure to high doses of gamma-rays

The dose response for dicentrics plus centric rings and total unstable chromosome-type aberrations was studied in the first mitoses of cultured human peripheral blood lymphocytes irradiated in vitro to doses of ∼2, 4, 6, 8, 10, 16 and 20 Gy of acute (60)Со gamma-rays. A dose-dependent increase of aberration yield was accompanied by a tendency to the underdispersion of dicentrics and centric rings among cells distributions compared with Poisson statistics at doses ≥6 Gy. The formal fitting of the data to a linear-quadratic model resulted in an equation with the linear and quadratic coefficients ranged 0.098-0.129×cell(-1)×Gy(-1) and 0.039-0.034×cell(-1)×Gy(-2), respectively, depending on the fitting method. The actual radiation-induced aberration yield was markedly lower than expected from a calibration curve, generated earlier within a lower dose range. Interlaboratory variations in reported dicentric yields induced by medium-to-high radiation doses in vitro are discussed.

DNA repair gene polymorphisms in relation to chromosome aberration frequencies in retired radiation workers

Polymorphic variation in DNA repair genes was examined in a group of retired workers from the British Nuclear Fuels plc facility at Sellafield in relation to previously determined translocation frequencies in peripheral blood lymphocytes. Variation at seven polymorphisms in four genes involved in the base excision repair (XRCC1 R194W, R399Q and a [AC]n microsatellite in the 3' UTR) and double strand break repair (XRCC3 T241M and a [AC]n microsatellite in intron 3 of XRCC3, XRCC4 I134T, and a GACTAn microsatellite located 120 kb 5' of XRCC5) pathways was determined for 291 retired radiation workers who had received cumulative occupational external radiation doses of between 0 and 1873 mSv. When the interaction between radiation dose and each DNA repair gene polymorphism was examined in relation to translocation frequency there was no evidence for any of the polymorphisms studied influencing the response to occupational exposure. A positive interaction observed between genotype (individuals with at least one allele > or =20 repeat units) at a microsatellite locus in the XRCC3 gene and smoking status should be interpreted cautiously because interactions were investigated for seven polymorphisms and two exposures. Nonetheless, further research is warranted to examine whether this DNA repair gene variant might be associated with a sub-optimal repair response to smoking-induced DNA damage and hence an increased frequency of translocations.

Biological effectiveness of accelerated particles for the induction of chromosome damage measured in metaphase and interphase human lymphocytes

Chromosome aberrations were investigated in human lymphocytes after in vitro exposure to 1H-, 3He-, 12C-, 40Ar-, 28Si-, 56Fe-, or 197Au-ion beams, with LET ranging from approximately 0.4-1393 keV/microm in the dose range of 0.075-3 Gy. Dose-response curves for chromosome exchanges, measured at the first mitosis postirradiation using fluorescence in situ hybridization (FISH) with whole-chromosome probes, were fitted with linear or linear-quadratic functions. The relative biological effectiveness (RBE) was estimated from the initial slope of the dose-response curve for chromosomal damage with respect to low- or high-dose-rate gamma rays. Estimates of RBEmax values for mitotic spreads, which ranged from near 0.7 to 11.1 for total exchanges, increased with LET, reaching a maximum at about 150 keV/microm, and decreased with further increase in LET. RBEs for complex aberrations are undefined due to the lack of an initial slope for gamma rays. Additionally, the effect of mitotic delay on RBE values was investigated by measuring chromosome aberrations in interphase after chemically induced premature chromosome condensation (PCC), and values were up to threefold higher than for metaphase analysis.

Mechanism of the fluorescent light induced suppression of Curly phenotype in Drosophila melanogaster

A dominant mutation Curly (Cy), frequently used as a marker on the second chromosome in Drosophila melanogaster, was previously shown to be suppressed by several factors, including larval crowding, low temperature, and fluorescent light. While the first two factors affect this mutation only partially, fluorescent tube exposed flies exhibit an almost completely suppressed (wild type) phenotype. This suppressive effect is the result of a combination of the electric field and light, both factors being produced by common fluorescent tubes. In this study, experiments were carried out to clarify the basic mechanism of this unique phenomenon. Two fluorescent tube sensitive stages of Drosophila development were found in the second half of embryonic development and first half of the pupal stage. Riboflavin, which is administered to Drosophila larvae with yeast, and decomposed by light, seems to play a key role in this phenomenon. In a medium lacking riboflavin caused by light exposure, Cy expression is inhibited by the action of electric field. Positive results of experiments with lithium ions, which block the opening of Ca(2+) channels, support the hypothesis that electromagnetic fields may alter ion currents during ontogenic development of Drosophila, and thus influence, expression of the Cy gene. Also, fluorescent light induces an overexpression of a specific protein in the imaginal wing disc of Cy pupae.

Radioprotective effects of the thiols GSH and WR-2721 against X-ray-induction of micronuclei in erythroblasts

The frequency of micronucleated polychromatic erythrocytes (MNPCEs) was assessed in the bone marrow and peripheral blood of adult male Swiss mice treated with reduced glutathione (GSH) and S-2-/3-aminopropylamino/ethyl phosphorothioic acid (WR-2721), at a dose of 400 mg/kg body weight, and exposed to 6 Gy X-rays. GSH or WR-2721 was applied alone, or 60 and 30 min, respectively, prior to X-ray-exposure. The number of MNPCEs was determined at 24 h after the thiol treatment and X-irradiation. The radioprotection and toxicity caused in the mouse erythroblasts by GSH and WR-2721, as indicated by the number of MNPCEs were dependent on the thiol applied. The stronger radioprotective effect is obtained following WR-2721 administration than after GSH application. WR-2721 showed greater toxicity than GSH. The combination of GSH and WR-2721 given before X-ray-exposure resulted in the most radioprotective effect as compared to the respective single-drug treatment of mice. Application of the both thiols, without subsequent X-irradiation appeared to be the most toxic, compared with administration of WR-2721 or GSH alone. The effective radioprotection by the combined action of GSH and WR-2721 against genomic instability induced in the mouse erythroblasts by X-rays was shown.

No evidence for radiation-induced clastogenic factors after in vitro or in vivo exposure of human blood

Experiments were performed with human plasma irradiated in vitro or in vivo in order to evaluate the extent to which clastogenic factors might disturb the adaptive response to DNA-damaging factors currently studied in our laboratory. The studies were carried out with plasma isolated from whole blood given 4 Gy of X-rays in vitro and with plasma from people receiving local radiotherapy at a total dose of about 60 Gy gamma rays. Addition of irradiated plasma to culture medium did not result in a statistically significant increase in structural aberrations in chromosomes of non-irradiated normal blood.

A brief survey of aberration origin theories

The salient points of three currently debated theories for chromosomal aberration origins (the Classic Breakage-and-Reunion theory, the Exchange theory, and the Molecular theory) are outlined, and some comments are made on each in the light of recent research.

Insight into sites

Spatial factors conditioning the formation of radiation-induced chromosome exchange aberrations are reviewed, and concepts such as 'rejoining distance' and 'site' are re-examined in the light of the unexpectedly high frequencies of multi-break ('Complex') exchanges being revealed by FISH painting. Given the anticipated densities of dsb within a nucleus, and assuming random 3-D break distribution, nearest-neighbour analysis indicates that the most likely break interaction distance is a well defined shell, several hundred nm from each break. The sharpness with which this shell is defined increases with break density, and therefore with dose. It is argued that random movement and chance meeting over such distances will not account for the Complex frequencies observed, and that other factors, or modes of formation, must be invoked.