Qualitative and quantitative difference in mutation induction between carbon- and neon-ion beams in normal human cells

LET dependence on killing effect and mutagenicity in the model filamentous fungus Neurospora crassa

PURPOSE

To assess the unique biological effects of different forms of ionizing radiation causing DNA double-strand breaks (DSBs), we compared the killing effect, mutagenesis frequency, and mutation type spectrum using the model filamentous fungus Neurospora.

MATERIALS AND METHODS

Asexual spores of wild-type Neurospora and two DSB repair-deficient strains [one homologous recombination- and the other non-homologous end-joining (NHEJ) pathway-deficient] were irradiated with argon (Ar)-ion beams, ferrous (Fe)-ion beams, or X-rays. Relative biological effectiveness (RBE), forward mutation frequencies at the ad-3 loci, and mutation spectra at the ad-3B gene were determined.

RESULTS

The canonical NHEJ (cNHEJ)-deficient strain showed resistance to higher X-ray doses, while other strains showed dose-dependent sensitivity. In contrast, the killing effects of Ar-ion and Fe-ion beam irradiation were dose-dependent in all strains tested. The rank order of RBE was Ar-ion > Fe-ion > C-ion. Deletion mutations were the most common, but deletion size incremented with the increasing value of linear energy transfer (LET).

CONCLUSIONS

We found marked differences in killing effect of a cNHEJ-deficient mutant between X-ray and high-LET ion beam irradiations (Ar and Fe). The mutation spectra also differed between irradiation types. These differences may be due to the physical properties of each radiation and the repair mechanism of induced damage in Neurospora crassa. These results may guide the choice of irradiation beam to kill or mutagenize fungi for agricultural applications or further research.

Particles with similar LET values generate DNA breaks of different complexity and reparability: a high-resolution microscopy analysis of γH2AX/53BP1 foci

Biological effects of high-LET (linear energy transfer) radiation have received increasing attention, particularly in the context of more efficient radiotherapy and space exploration. Efficient cell killing by high-LET radiation depends on the physical ability of accelerated particles to generate complex DNA damage, which is largely mediated by LET. However, the characteristics of DNA damage and repair upon exposure to different particles with similar LET parameters remain unexplored. We employed high-resolution confocal microscopy to examine phosphorylated histone H2AX (γH2AX)/p53-binding protein 1 (53BP1) focus streaks at the microscale level, focusing on the complexity, spatiotemporal behaviour and repair of DNA double-strand breaks generated by boron and neon ions accelerated at similar LET values (∼135 keV μm^-1) and low energies (8 and 47 MeV per n, respectively). Cells were irradiated using sharp-angle geometry and were spatially (3D) fixed to maximize the resolution of these analyses. Both high-LET radiation types generated highly complex γH2AX/53BP1 focus clusters with a larger size, increased irregularity and slower elimination than low-LET γ-rays. Surprisingly, neon ions produced even more complex γH2AX/53BP1 focus clusters than boron ions, consistent with DSB repair kinetics. Although the exposure of cells to γ-rays and boron ions eliminated a vast majority of foci (94% and 74%, respectively) within 24 h, 45% of the foci persisted in cells irradiated with neon. Our calculations suggest that the complexity of DSB damage critically depends on (increases with) the particle track core diameter. Thus, different particles with similar LET and energy may generate different types of DNA damage, which should be considered in future research.

Early and Delayed Induction of DSBs by Nontargeted Effects in ICR Mouse Lymphocytes after In Vivo X Irradiation

The goal of this study was to determine whether in vivo X irradiation induces nontargeted effects, such as delayed effects and bystander effects in ICR mouse lymphocytes. We first examined the generation of DNA double-strand breaks (DSBs) in lymphocytes, isolated from ICR mice exposed to 1 Gy X irradiation, by enumeration of p53 binding protein 1 (53BP1) foci, and observed that the number of 53BP1 foci reached their maximum 3 days postirradiation and decreased to background level 30 days postirradiation. However, the number of 53BP1 foci was significantly increased in lymphocytes isolated from ICR mice 90-365 days postirradiation. This result indicates that in vivo X irradiation induced delayed DSBs in ICR mouse lymphocytes. We next counted the number of 53BP1 foci in lymphocytes isolated from sham-irradiated ICR mice that had been co-cultured with lymphocytes isolated from 1 Gy X-irradiated ICR mice, and observed a significant increase in the number of 53BP1 foci 1-7 days postirradiation. This result indicates that in vivo X irradiation induced bystander effects in ICR mouse lymphocytes. These findings suggest that in vivo X irradiation induces early and delayed nontargeted effects in ICR mouse lymphocytes.

Lethal and mutagenic effects of ion beams and γ-rays in Aspergillus oryzae

Aspergillus oryzae is a fungus that is used widely in traditional Japanese fermentation industries. In this study, the lethal and mutagenic effects of different linear energy transfer (LET) radiation in freeze-dried conidia of A. oryzae were investigated. The lethal effect, which was evaluated by a 90% lethal dose, was dependent on the LET value of the ionizing radiation. The most lethal ionizing radiation among that tested was (12)C(5+) ion beams with an LET of 121keV/μm. The (12)C(5+) ion beams had a 3.6-times higher lethal effect than low-LET (0.2keV/μm) γ-rays. The mutagenic effect was evaluated by the frequency of selenate resistant mutants. (12)C(6+) ion beams with an LET of 86keV/μm were the most effective in inducing selenate resistance. The mutant frequency following exposure to (12)C(6+) ion beams increased with an increase in dose and reached 3.47×10(-3) at 700Gy. In the dose range from 0 to 700Gy, (12)C(5+) ion beams were the second most effective in inducing selenate resistance, the mutant frequency of which reached a maximum peak (1.67×10(-3)) at 400Gy. To elucidate the characteristics of mutation induced by ionizing radiation, mutations in the sulphate permease gene (sB) and ATP sulfurylase gene (sC) loci, the loss of function of which results in a selenate resistant phenotype, were compared between (12)C(5+) ion beams and γ-rays. We detected all types of transversions and transitions. For frameshifts, the frequency of a +1 frameshift was the highest in all cases. Although the incidence of deletions >2bp was generally low, deletions >20bp were characteristic for (12)C(5+) ion beams. γ-rays had a tendency to generate mutants carrying a multitude of mutations in the same locus. Both forms of radiation also induced genome-wide large-scale mutations including chromosome rearrangements and large deletions. These results provide new basic insights into the mutation breeding of A. oryzae using ionizing radiation.

Molecular nature of mutations induced by high-LET irradiation with argon and carbon ions in Arabidopsis thaliana

Linear energy transfer (LET) is an important parameter to be considered in heavy-ion mutagenesis. However, in plants, no quantitative data are available on the molecular nature of the mutations induced with high-LET radiation above 101-124keVμm(-1). In this study, we irradiated dry seeds of Arabidopsis thaliana with Ar and C ions with an LET of 290keVμm(-1). We analyzed the DNA alterations caused by the higher-LET radiation. Mutants were identified from the M(2) pools. In total, 14 and 13 mutated genes, including bin2, egy1, gl1, gl2, hy1, hy3-5, ttg1, and var2, were identified in the plants derived from Ar- and C-ions irradiation, respectively. In the mutants from both irradiations, deletion was the most frequent type of mutation; 13 of the 14 mutated genes from the Ar ion-irradiated plants and 11 of the 13 mutated genes from the C ion-irradiated plants harbored deletions. Analysis of junction regions generated by the 2 types of irradiation suggested that alternative non-homologous end-joining was the predominant pathway of repair of break points. Among the deletions, the proportion of large deletions (>100bp) was about 54% for Ar-ion irradiation and about 64% for C-ion irradiation. Both current results and previously reported data revealed that the proportions of the large deletions induced by 290-keVμm(-1) radiations were higher than those of the large deletions induced by lower-LET radiations (6% for 22.5-30.0keVμm(-1) and 27% for 101-124keVμm(-1)). Therefore, the 290keVμm(-1) heavy-ion beams can effectively induce large deletions and will prove useful as novel mutagens for plant breeding and analysis of gene functions, particularly tandemly arrayed genes.

Mutagenic effects of carbon ions near the range end in plants

To gain insight into the mutagenic effects of accelerated heavy ions in plants, the mutagenic effects of carbon ions near the range end (mean linear energy transfer (LET): 425keV/μm) were compared with the effects of carbon ions penetrating the seeds (mean LET: 113keV/μm). Mutational analysis by plasmid rescue of Escherichia coli rpsL from irradiated Arabidopsis plants showed a 2.7-fold increase in mutant frequency for 113keV/μm carbon ions, whereas no enhancement of mutant frequency was observed for carbon ions near the range end. This suggested that carbon ions near the range end induced mutations that were not recovered by plasmid rescue. An Arabidopsis DNA ligase IV mutant, deficient in non-homologous end-joining repair, showed hyper-sensitivity to both types of carbon-ion irradiation. The difference in radiation sensitivity between the wild type and the repair-deficient mutant was greatly diminished for carbon ions near the range end, suggesting that these ions induce irreparable DNA damage. Mutational analysis of the Arabidopsis GL1 locus showed that while the frequency of generation of glabrous mutant sectors was not different between the two types of carbon-ion irradiation, large deletions (>∼30kb) were six times more frequently induced by carbon ions near the range end. When 352keV/μm neon ions were used, these showed a 6.4 times increase in the frequency of induced large deletions compared with the 113keV/μm carbon ions. We suggest that the proportion of large deletions increases with LET in plants, as has been reported for mammalian cells. The nature of mutations induced in plants by carbon ions near the range end is discussed in relation to mutation detection by plasmid rescue and transmissibility to progeny.

Mutagenic adaptive response to high-LET radiation in human lymphoblastoid cells exposed to low doses of heavy-ion radiation

Adaptive response (AR) and bystander effect are two important phenomena involved in biological responses to low doses of ionizing radiation (IR). Furthermore, there is a strong interest in better understanding the biological effects of high-LET radiation. We previously demonstrated the ability of low doses of X-rays to induce an AR to challenging heavy-ion radiation [8]. In this study, we assessed in vitro the ability of priming low doses (0.01Gy) of heavy-ion radiation to induce a similar AR to a subsequent challenging dose (1-4Gy) of high-LET IR (carbon-ion: 20 and 40keV/μm, neon-ion: 150keV/μm) in TK6, AHH-1 and NH32 cells. Our results showed that low doses of high-LET radiation can induce an AR characterized by lower mutation frequencies at hypoxanthine-guanine phosphoribosyl transferase locus and faster DNA repair kinetics, in cells expressing p53.

Mutagenic adaptive response to high-LET radiation in human lymphoblastoid cells exposed to X-rays

The ability of cells to adapt low-dose or low-dose rate radiation is well known. High-LET radiation has unique characteristics, and the data concerning low doses effects and high-LET radiation remain fragmented. In this study, we assessed in vitro the ability of low doses of X-rays to induce an adaptive response (AR) to a subsequent challenging dose of heavy-ion radiation. Lymphoblastoid cells (TK6, AHH-1, NH32) were exposed to priming 0.02-0.1Gy X-rays, followed 6h later by challenging 1Gy heavy-ion radiation (carbon-ion: 20 and 40keV/μm, neon-ion: 150keV/μm). Pre-exposure of p53-competent cells resulted in decreased mutation frequencies at hypoxanthine-guanine phosphoribosyl transferase locus and different H2AX phosphorylation kinetics, as compared to cells exposed to challenging radiation alone. This phenomenon did not seem to be linked with cell cycle effects or radiation-induced apoptosis. Taken together, our results suggested the existence of an AR to mutagenic effects of heavy-ion radiation in lymphoblastoid cells and the involvement of double-strand break repair mechanisms.

Ion beam radiobiology and cancer: time to update ourselves

High-energy protons and carbon ions exhibit an inverse dose profile allowing for increased energy deposition with penetration depth. Additionally, heavier ions like carbon beams have the advantage of a markedly increased biological effectiveness characterized by enhanced ionization density in the individual tracks of the heavy particles, where DNA damage becomes clustered and therefore more difficult to repair, but is restricted to the end of their range. These superior biophysical and biological profiles of particle beams over conventional radiotherapy permit more precise dose localization and make them highly attractive for treating anatomically complex and radioresistant malignant tumors but without increasing the severe side effects in the normal tissue. More than half a century since Wilson proposed their use in cancer therapy, the effects of particle beams have been extensively investigated and the biological complexity of particle beam irradiation begins to unfold itself. The goal of this review is to provide an as comprehensive and up-to-date summary as possible of the different radiobiological aspects of particle beams for effective application in cancer treatment.

Molecular characterization of microbial mutations induced by ion beam irradiation

A positive selection system for gene disruption using a sucrose-sensitive transgenic rhizobium was established and used for the molecular characterization of mutations induced by ion beam irradiations. Single nucleotide substitutions, insertions, and deletions were found to occur in the sucrose sensitivity gene, sacB, when the reporter line was irradiated with highly accelerated carbon and iron ion beams. In all of the insertion lines, fragments of essentially the same sequence and of approximately 1188bp in size were identified in the sacB regions. In the deletion lines, iron ions showed a tendency to induce larger deletions than carbon ions, suggesting that higher LET beams cause larger deletions. We found also that ion beams, particularly "heavier" ion beams, can produce single gene disruptions and may present an effective alternative to transgenic approaches.

Epigenetic changes and nontargeted radiation effects--is there a link?

It is now well accepted that the effects of ionizing radiation (IR) exposure can be noticed far beyond the borders of the directly irradiated tissue. IR can affect neighboring cells in the proximity, giving rise to a bystander effect. IR effects can also span several generations and influence the progeny of exposed parents, leading to transgeneration effects. Bystander and transgeneration IR effects are linked to the phenomenon of the IR-induced genome instability that manifests itself as chromosome aberrations, gene mutations, late cell death, and aneuploidy. While the occurrence of the above-mentioned phenomena is well documented, the exact mechanisms that lead to their development have still to be delineated. Evidence suggests that the IR-induced genome instability, bystander, and transgeneration effects may be epigenetically mediated. The epigenetic changes encompass DNA methylation, histone modification, and RNA-associated silencing. Recent studies demonstrated that IR exposure alters epigenetic parameters in the directly exposed tissues and in the distant bystander tissues. Transgeneration radiation effects were also proposed to be of an epigenetic nature. We will discuss the role of the epigenetic mechanisms in radiation responses, bystander effects, and transgeneration effects.

Mutations Induced by Heavy Charged Particles

The relative biological-effectiveness of radiation is increased when cells or tissue are exposed to densely ionizing (high-LET) radiation. A large number of studies focus on the following aspects of the biological effects of high-LET radiation: (i) basic understanding of radiation damage and repair; (ii) developing radiotherapy protocols for accelerated charged particles; and (iii) estimation of human risks from exposure to high-LET heavy charged particles. The increased lethal effectiveness (cell inactivation) of high-LET radiation contributes to new methods for using radiation therapy, but it is also necessary to study the enhanced mutagenic effect of high LET radiation, because higher frequencies of mutation can be expected to provide higher rates of carcinogenicity with human exposure. It is important to note that both measures of biological effectiveness (lethality and mutagenicity) depend on the quality of radiation, the dose, dose-rate effects, and the biological endpoints studied. This paper is intended to provide a review of current research on the mutagenic effects of high-LET radiation, and is organized into three sections. First, are descriptions of the induced mutations studied with various detection systems (section 1) because the detectable mutations induced by ionizing radiation, including heavy-ions, depend largely on the detection system used. Second is a discussion of the biological significance of the dependence of induced mutations on LET (section 2). This is related to the molecular nature of radiation lesions and to the repair mechanisms used to help cells recover from such damage. Finally, applications of mutation detection systems for studies in space (section 3) are described, in which the carcinogenic effects of space environmental radiation are considered.

The Biological Effects of Space Radiation during Long Stays in Space

Many space experiments are scheduled for the International Space Station (ISS). Completion of the ISS will soon become a reality. Astronauts will be exposed to low-level background components from space radiation including heavy ions and other high-linear energy transfer (LET) radiation. For long-term stay in space, we have to protect human health from space radiation. At the same time, we should recognize the maximum permissible doses of space radiation. In recent years, physical monitoring of space radiation has detected about 1 mSv per day. This value is almost 150 times higher than that on the surface of the Earth. However, the direct effects of space radiation on human health are currently unknown. Therefore, it is important to measure biological dosimetry to calculate relative biological effectiveness (RBE) for human health during long-term flight. The RBE is possibly modified by microgravity. In order to understand the exact RBE and any interaction with microgravity, the ISS centrifugation system will be a critical tool, and it is hoped that this system will be in operation as soon as possible.