Mutations Induced by Heavy Charged Particles

Isolation and identification of high-yielding alkaline phosphatase strain: a novel mutagenesis technique and optimization of fermentation conditions

Isolating and screening enzyme-producing strains from microorganisms and the commercial production of ALPs from microorganisms are of increasing interest. In this work, isolation and identification of high-yielding alkaline phosphatase strain were carried out using atmospheric and room temperature plasma mutagenesis (ARTP) for optimization of fermentation conditions. A strain of alkaline phosphatase-producing bacteria was screened from soil and identified by 16S rRNA gene sequencing as Bacillus amyloliquefaciens and named S-1. This strain had an alkaline phosphatase activity of 2594.73 U/L. Later, mutagenesis breeding of the alkaline phosphatase-producing S-1 strain was conducted using (ARTP), from which a higher alkaline phosphatase-producing positive mutant strain S-52 was screened. A central combination of five factors, including corn starch, yeast extract, metal ions, fermentation temperature and inoculum ratio, was then used to influence the activity of alkaline phosphatase. Results from the response surface methodology showed that the maximum enzyme activity of alkaline phosphatase was 12,110.6 U/L at corn starch, yeast extract and magnesium ions concentrations of 17.48 g/L, 18.052 g/L and 0.744 g/L, respectively; fermentation temperature of 37.192 °C; and inoculation ratio of 5.59%. This study is important for further exploring ARTP mutagenesis in B. amyloliquefaciens and the commercialization of ALPs.

Lethal and mutagenic effects of different LET radiations on Bacillus subtilis spores

New, useful microorganism resources have been generated by ionizing radiation breeding technology. However, the mutagenic effects of ionizing radiation on microorganisms have not been systematically clarified. For a deeper understanding and characterization of ionizing radiation-induced mutations in microorganisms, we investigated the lethal effects of seven different linear energy transfer (LET) radiations based on the survival fraction (SF) and whole-genome sequencing analysis of the mutagenic effects of a dose resulting in an SF of around 1% in Bacillus subtilis spores. Consequently, the lower LET radiations (gamma [surface LET: 0.2 keV/µm] and 4He2+ [24 keV/µm]) showed low lethality and high mutation frequency (MF), resulting in the major induction of single-base substitutions. Whereas higher LET radiations (12C5+ [156 keV/µm] and 12C6+ [179 keV/µm]) showed high lethality and low MF, resulting in the preferential induction of deletion mutations. In addition, 12C6+ (111) ion beams likely possess characteristics of both low- and high-LET radiations simultaneously. A decrease in the relative biological effectiveness and an evaluation of the inactivation cross section indicated that 20Ne8+ (468 keV/µm) and 40Ar13+ (2214 keV/µm) ion beams had overkill effects. In conclusion, in the mutation breeding of microorganisms, it should be possible to regulate the proportions, types, and frequencies of induced mutations by selecting an ionizing radiation of an appropriate LET in accordance with the intended purpose.

Key biological mechanisms involved in high-LET radiation therapies with a focus on DNA damage and repair

DNA damage and repair studies are at the core of the radiation biology field and represent also the fundamental principles informing radiation therapy (RT). DNA damage levels are a function of radiation dose, whereas the type of damage and biological effects such as DNA damage complexity, depend on radiation quality that is linear energy transfer (LET). Both levels and types of DNA damage determine cell fate, which can include necrosis, apoptosis, senescence or autophagy. Herein, we present an overview of current RT modalities in the light of DNA damage and repair with emphasis on medium to high-LET radiation. Proton radiation is discussed along with its new adaptation of FLASH RT. RT based on α-particles includes brachytherapy and nuclear-RT, that is proton-boron capture therapy (PBCT) and boron-neutron capture therapy (BNCT). We also discuss carbon ion therapy along with combinatorial immune-based therapies and high-LET RT. For each RT modality, we summarise relevant DNA damage studies. Finally, we provide an update of the role of DNA repair in high-LET RT and we explore the biological responses triggered by differential LET and dose.

Late Effects of 1H + 16O on Short-Term and Object Memory, Hippocampal Dendritic Morphology and Mutagenesis

The space extending beyond Earth’s magnetosphere is subject to a complex field of high-energy charged nuclei, which are capable of traversing spacecraft shielding and human tissues, inducing dense ionization events. The central nervous system is a major area of concern for astronauts who will be exposed to the deep-space radiation environment on a mission to Mars, as charged-particle radiation has been shown to elicit changes to the dendritic arbor within the hippocampus of rodents, and related cognitive-behavioral deficits. We exposed 6-month-old male mice to whole-body ¹H (0.5 Gy; 150 MeV/n; 18–19 cGy/minute) and an hour later to ¹⁶O (0.1Gy; 600 MeV/n; 18–33 Gy/min) at NASA’s Space Radiation Laboratory as a galactic cosmic ray-relevant model. Animals were housed with bedding which provides cognitive enrichment. Mice were tested for cognitive behavior 9 months after exposure to elucidate late radiation effects. Radiation induced significant deficits in novel object recognition and short-term spatial memory (Y-maze). Additionally, we observed opposing morphological differences between the mature granular and pyramidal neurons throughout the hippocampus, with increased dendritic length in the dorsal dentate gyrus and reduced length and complexity in the CA1 subregion of the hippocampus. Dendritic spine analyses revealed a severe reduction in mushroom spine density throughout the hippocampus of irradiated animals. Finally, we detected no general effect of radiation on single-nucleotide polymorphisms in immediate early genes, and genes involved in inflammation but found a higher variant allele frequency in the antioxidants thioredoxin reductase 2 and 3 loci.

Interstitial chromosomal deletion of the tuberous sclerosis complex 2 locus is a signature for radiation-associated renal tumors in Eker rats

Ionizing radiation can damage DNA and, therefore, is a risk factor for cancer. Eker rats, which carry a heterozygous germline mutation in the tumor-suppressor gene tuberous sclerosis complex 2 (Tsc2), are susceptible to radiation-induced renal carcinogenesis. However, the molecular mechanisms involved in Tsc2 inactivation are unclear. We subjected Fischer 344 × Eker (Long Evans Tsc2+/- ) F1 hybrid rats to gamma-irradiation (2 Gy) at gestational day 19 (GD19) or postnatal day 5 (PND5) and investigated the patterns of genomic alterations in the Tsc2 allele of renal tumors that developed at 1 year after irradiation (N = 24 tumors for GD19, N = 10 for PND5), in comparison with spontaneously developed tumors (N = 8 tumors). Gamma-irradiation significantly increased the multiplicity of renal tumors. The frequency of LOH at the chromosome 10q12 region, including the Tsc2 locus, was 38%, 29% and 60% in renal carcinomas developed from the nonirradiated, GD19 and PND5 groups, respectively. Array comparative genomic hybridization analysis revealed that the LOH patterns on chromosome 10 in renal carcinomas were classified into chromosomal missegregation, mitotic recombination and chromosomal deletion types. LOH of the interstitial chromosomal deletion type was observed only in radiation-associated carcinomas. Sequence analysis for the wild-type Tsc2 allele in the LOH-negative carcinomas identified deletions (nonirradiated: 26%; GD19: 21%) and base-substitution mutations (GD19: 4%). Reduced expression of Tsc2 was also observed in the majority of the LOH-negative carcinomas. Our results suggest that interstitial chromosomal deletion is a characteristic mutagenic event caused by ionizing radiation, and it may contribute to the assessment of radiation-induced cancer risk.

Comparison and Characterization of Mutations Induced by Gamma-Ray and Carbon-Ion Irradiation in Rice (Oryza sativa L.) Using Whole-Genome Resequencing

Gamma-rays are the most widely used mutagenic radiation in plant mutation breeding, but detailed characteristics of mutated DNA sequences have not been clarified sufficiently. In contrast, newly introduced physical mutagens, e.g., heavy-ion beams, have attracted geneticists’ and breeders’ interest and many studies on their mutation efficiency and mutated DNA characteristics have been conducted. In this study, we characterized mutations induced by gamma rays and carbon(C)-ion beams in rice (Oryza sativa L.) mutant lines at M₅ generation using whole-genome resequencing. On average, 57.0 single base substitutions (SBS), 17.7 deletions, and 5.9 insertions were detected in each gamma-ray-irradiated mutant, whereas 43.7 single SBS, 13.6 deletions, and 5.3 insertions were detected in each C-ion-irradiated mutant. The structural variation (SV) analysis detected 2.0 SVs (including large deletions or insertions, inversions, duplications, and reciprocal translocations) on average in each C-ion-irradiated mutant, while 0.6 SVs were detected on average in each gamma-ray-irradiated mutant. Furthermore, complex SVs presumably having at least two double-strand breaks (DSBs) were detected only in C-ion-irradiated mutants. In summary, gamma-ray irradiation tended to induce larger numbers of small mutations than C-ion irradiation, whereas complex SVs were considered to be the specific characteristics of the mutations induced by C-ion irradiation, which may be due to their different radiation properties. These results could contribute to the application of radiation mutagenesis to plant mutation breeding.

Studies on Application of Ion Beam Breeding to Industrial Microorganisms at TIARA

Mutation-breeding technologies are useful tools for the development of new biological resources in plants and microorganisms. In Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) at the National Institutes for Quantum and Radiological Science and Technology, Japan, ion beams were explored as novel mutagens. The mutagenic effects of various ion beams on eukaryotic and prokaryotic microorganisms were described and their application in breeding technology for industrial microorganisms were discussed. Generally, the relative biological effectiveness (RBE) depended on the liner energy transfer (LET) and the highest RBE values were obtained with 12C5+ ion beams. The highest mutation frequencies were obtained at radiation doses that gave 1%–10% of surviving fraction. By using 12C5+ ion beams in this dose range, many microorganisms have been improved successfully at TIARA. Therefore, ion-beam breeding technology for microorganisms will have applications in many industries, including stable food production, sustainable agriculture, environmental conservation, and development of energy resources in the near future. Moreover, genome analyses of the ion-beam-induced mutants are in progress to clear the differences of mutational functions induced by different LET radiations in microorganisms. Further characterizations of mutations induced by different LET radiations will facilitate more effective use of ion beams in microorganisms breeding.

Genome-Wide Comparisons of Mutations Induced by Carbon-Ion Beam and Gamma-Rays Irradiation in Rice via Resequencing Multiple Mutants

Physical mutagens, such as carbon-ion beams (CIBs) and gamma rays (GRs), induce mutations with high frequency at a relatively low dose and are more user-friendly and environment-friendly in mutation breeding. Previous studies showed that CIBs induced large sized deletions and insertions, and chromosomal rearrangements, whereas GRs induce shorter deletions and insertions, and more frequent base substitutions. However, the difference on the genomic level between CIB- and GR-induced mutations remains to be clarified. In the present study, we re-sequence six mutagenized lines derived from CIB irradiation and four mutagenized lines derived from GRs. A total of 283 and 381 variations are induced in these mutants by CIBs and GRs, respectively, including single base substitutions (SBSs), small insertion and deletions (InDels), multiple nucleotide variants (MNVs). SBSs are the most abundant type of mutation and single base transition is the main form for SBSs. CIB-induced InDels accounted for 25.44% of the total variations, while GR-induced InDels accounted for 17.85%. On the contrary, the frequency of MNVs induced by GRs was approximately 2.19 times that induced by CIBs, which indicates CIBs induced increased InDels, whereas GRs induced increased MNVs. Notably, multiple base deletions (≥5 bp) were induced at a much higher frequency by CIBs than by GRs. We also find mutations induced by CIBs and GRs are unevenly distributed on chromosomes. Unusual high-frequency (HF) and low-frequency (LF) mutation regions are discovered by analyzing mutations per 1Mb along the genome. The mutation frequency within the HF regions were significantly higher than the LF regions (P < 0.05). A large majority of SBSs, InDels, and MNVs induced by CIBs and GRs occurred in upstream and downstream regions. Our study compares difference of mutation profiles induced by the CIB irradiation and GR on rice genomes, and give some clues for understanding the mutagenesis mechanism of physical radiation and improving the mutagenesis efficiency.

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.

Simulated space radiation-induced mutants in the mouse kidney display widespread genomic change

Exposure to a small number of high-energy heavy charged particles (HZE ions), as found in the deep space environment, could significantly affect astronaut health following prolonged periods of space travel if these ions induce mutations and related cancers. In this study, we used an in vivo mutagenesis assay to define the mutagenic effects of accelerated ⁵⁶Fe ions (1 GeV/amu, 151 keV/μm) in the mouse kidney epithelium exposed to doses ranging from 0.25 to 2.0 Gy. These doses represent fluences ranging from 1 to 8 particle traversals per cell nucleus. The Aprt locus, located on chromosome 8, was used to select induced and spontaneous mutants. To fully define the mutagenic effects, we used multiple endpoints including mutant frequencies, mutation spectrum for chromosome 8, translocations involving chromosome 8, and mutations affecting non-selected chromosomes. The results demonstrate mutagenic effects that often affect multiple chromosomes for all Fe ion doses tested. For comparison with the most abundant sparsely ionizing particle found in space, we also examined the mutagenic effects of high-energy protons (1 GeV, 0.24 keV/μm) at 0.5 and 1.0 Gy. Similar doses of protons were not as mutagenic as Fe ions for many assays, though genomic effects were detected in Aprt mutants at these doses. Considered as a whole, the data demonstrate that Fe ions are highly mutagenic at the low doses and fluences of relevance to human spaceflight, and that cells with considerable genomic mutations are readily induced by these exposures and persist in the kidney epithelium. The level of genomic change produced by low fluence exposure to heavy ions is reminiscent of the extensive rearrangements seen in tumor genomes suggesting a potential initiation step in radiation carcinogenesis.

Transcription Factors in the Cellular Response to Charged Particle Exposure

Charged particles, such as carbon ions, bear the promise of a more effective cancer therapy. In human spaceflight, exposure to charged particles represents an important risk factor for chronic and late effects such as cancer. Biological effects elicited by charged particle exposure depend on their characteristics, e.g., on linear energy transfer (LET). For diverse outcomes (cell death, mutation, transformation, and cell-cycle arrest), an LET dependency of the effect size was observed. These outcomes result from activation of a complex network of signaling pathways in the DNA damage response, which result in cell-protective (DNA repair and cell-cycle arrest) or cell-destructive (cell death) reactions. Triggering of these pathways converges among others in the activation of transcription factors, such as p53, nuclear factor κB (NF-κB), activated protein 1 (AP-1), nuclear erythroid-derived 2-related factor 2 (Nrf2), and cAMP responsive element binding protein (CREB). Depending on dose, radiation quality, and tissue, p53 induces apoptosis or cell-cycle arrest. In low LET radiation therapy, p53 mutations are often associated with therapy resistance, while the outcome of carbon ion therapy seems to be independent of the tumor's p53 status. NF-κB is a central transcription factor in the immune system and exhibits pro-survival effects. Both p53 and NF-κB are activated after ionizing radiation exposure in an ataxia telangiectasia mutated (ATM)-dependent manner. The NF-κB activation was shown to strongly depend on charged particles' LET, with a maximal activation in the LET range of 90-300 keV/μm. AP-1 controls proliferation, senescence, differentiation, and apoptosis. Nrf2 can induce cellular antioxidant defense systems, CREB might also be involved in survival responses. The extent of activation of these transcription factors by charged particles and their interaction in the cellular radiation response greatly influences the destiny of the irradiated and also neighboring cells in the bystander effect.

Efficient Rejoining of DNA Double-Strand Breaks despite Increased Cell-Killing Effectiveness following Spread-Out Bragg Peak Carbon-Ion Irradiation

Radiotherapy of solid tumors with charged particles holds several advantages in comparison to photon therapy; among them conformal dose distribution in the tumor, improved sparing of tumor-surrounding healthy tissue, and an increased relative biological effectiveness (RBE) in the tumor target volume in the case of ions heavier than protons. A crucial factor of the biological effects is DNA damage, of which DNA double-strand breaks (DSBs) are the most deleterious. The reparability of these lesions determines the cell survival after irradiation and thus the RBE. Interestingly, using phosphorylated H2AX as a DSB marker, our data in human fibroblasts revealed that after therapy-relevant spread-out Bragg peak irradiation with carbon ions DSBs are very efficiently rejoined, despite an increased RBE for cell survival. This suggests that misrepair plays an important role in the increased RBE of heavy-ion radiation. Possible sources of erroneous repair will be discussed.

Accelerated (48)Ti Ions Induce Autosomal Mutations in Mouse Kidney Epithelium at Low Dose and Fluence

Exposure to high-energy charged particles (HZE ions) at low fluence could significantly affect astronaut health after prolonged missions in deep space by inducing mutations and related cancers. We tested the hypothesis that the mutagenic effects of HZE ions could be detected at low fluence in a mouse model that detects autosomal mutations in vivo. Aprt heterozygous mice were exposed to 0.2, 0.4 and 1.4 Gy of densely ionizing (48)Ti ions (1 GeV/amu, LET = 107 keV/μm). We observed a dose-dependent increase in the Aprt mutant fraction in kidney epithelium at the two lowest doses (an average of 1 or 2 particles/cell nucleus) that plateaued at the highest dose (7 particles/cell nucleus). Mutant cells were expanded to determine mutation spectra and translocations affecting chromosome 8, which encodes Aprt. A PCR-based analysis for loss of heterozygosity (LOH) events on chromosome 8 demonstrated a significant shift in the mutational spectrum from Ti ion exposure, even at low fluence, by revealing "radiation signature" mutations in mutant cells from exposed mice. Likewise, a cytogenetic assay for nonreciprocal chromosome 8 translocations showed an effect of exposure. A genome-wide LOH assay for events affecting nonselected chromosomes also showed an effect of exposure even for the lowest dose tested. Considered in their entirety, these results show that accelerated (48)Ti ions induce large mutations affecting one or more chromosomes at low dose and fluence.

The effect of radiation quality on the risks of second malignancies

Abstract Purpose: Numerous studies have implicated elevated second cancer risks as a result of radiation therapy. Our aim in this paper was to contribute to an understanding of the effects of radiation quality on second cancer risks. In particular, we developed a biologically motivated model to study the effects of linear energy transfer (LET) of charged particles (including protons, alpha particles and heavy ions Carbon and Neon) on the risk of second cancer.

MATERIALS AND METHODS

A widely used approach to estimate the risk uses the so-called initiation-inactivation-repopulation model. Based on the available experimental data for the LET dependence of radiobiological parameters and mutation rate, we generalized this formulation to include the effects of radiation quality. We evaluated the secondary cancer risks for protons in the clinical range of LET, i.e., around 4-10 (KeV/μm), which lies in the plateau region of the Bragg peak.

RESULTS

For protons, at a fixed radiation dose, we showed that the increase in second cancer risks correlated directly with increasing values of LET to a certain point, and then decreased. Interestingly, we obtained a higher risk for proton LET of 10 KeV/μm compared to the lower LET of 4 KeV/μm in the low dose region. In the case of heavy ions, the risk was higher for Carbon ions than Neon ions (even though they have almost the same LET). We also compared protons and alpha particles with the same LET, and it was interesting to note that the second cancer risks were higher for protons compared to alpha particles in the low-dose region.

CONCLUSION

Overall, this study demonstrated the importance of including LET dependence in the estimation of second cancer risk. Our theoretical risk predictions were noticeably high; however, the biological end points should be tested experimentally for multiple treatment fields and to improve theoretical predictions.

Effect of high-LET Fe-ion beam irradiation on mutation induction in Arabidopsis thaliana

Heavy-ion beams are powerful mutagens. They cause a broad spectrum of mutation phenotypes with high efficiency even at low irradiation doses and short irradiation times. These mutagenic effects are due to dense ionisation in a localised region along the ion particle path. Linear energy transfer (LET; keV·μm(-1)), which represents the degree of locally deposited energy, is an important parameter in heavy-ion mutagenesis. For high LET radiation above 290 keV∙μm(-1), however, neither the mutation frequency nor the molecular nature of the mutations has been fully characterised. In this study, we investigated the effect of Fe-ion beams with an LET of 640 keV∙μm(-1) on both the mutation frequency and the molecular nature of the mutations. Screening of well-characterised mutants (hy and gl) revealed that the mutation frequency was lower than any other ion species with low LET. We investigated the resulting mutations in the 4 identified mutants. Three mutants were examined by employing PCR-based methods, one of which had 2-bp deletion, another had 178 bp of tandemly duplication, and other one had complicated chromosomal rearrangements with variable deletions in size at breakpoints. We also detected large deletions in the other mutant by using array comparative genomic hybridisation. From the results of the analysis of the breakpoints and junctions of the detected deletions, it was revealed that the mutants harboured chromosomal rearrangements in their genomes. These results indicate that Fe-ion irradiation tends to cause complex mutations with low efficiency. We conclude that Fe-ion irradiation could be useful for inducing chromosomal rearrangements or large deletions.

Effects of shielding on the induction of 53BP1 foci and micronuclei after Fe ion exposures

High atomic number and high-energy (HZE) particles in deep space are of low abundance but substantially contribute to the biological effects of space radiation. Shielding is so far the most effective way to partially protect astronauts from these highly penetrating particles. However, simulated calculations and measurements have predicted that secondary particles resulting from the shielding of cosmic rays produce a significant fraction of the total dose and dose equivalent. In this study, we investigated the biological effects of secondary radiation with two cell types, and with cells exposed in different phases of the cell cycle, by comparing the biological effects of a 200 MeV/u iron beam with a shielded beam in which the energy of the iron ion beam was decreased from 500 MeV/u to 200 MeV/u with PMMA, polyethylene (PE), or aluminum. We found that beam shielding resulted in increased induction of 53BP1 foci and micronuclei in a cell-type-dependent manner compared with the unshielded 200 MeV/u Fe ion beam. These findings provide experimental proof that the biological effects of secondary particles resulting from the interaction between HZE particles and shielding materials should be considered in shielding design.

Mutational effects of γ-rays and carbon ion beams on Arabidopsis seedlings

To assess the mutational effects of radiation on vigorously proliferating plant tissue, the mutation spectrum was analyzed with Arabidopsis seedlings using the plasmid-rescue method. Transgenic plants containing the Escherichia coli rpsL gene were irradiated with γ-rays and carbon ion beams (320-MeV (12)C(6+)), and mutations in the rpsL gene were analyzed. Mutant frequency increased significantly following irradiation by γ-rays, but not by 320-MeV (12)C(6+). Mutation spectra showed that both radiations increased the frequency of frameshifts and other mutations, including deletions and insertions, but only γ-rays increased the frequency of total base substitutions. These results suggest that the type of DNA lesions which cause base substitutions were less often induced by 320-MeV (12)C(6+) than by γ-rays in Arabidopsis seedlings. Furthermore, γ-rays never increased the frequencies of G:C to T:A or A:T to C:G transversions, which are caused by oxidized guanine; 320-MeV (12)C(6+), however, produced a slight increase in both transversions. Instead, γ-rays produced a significant increase in the frequency of G:C to A:T transitions. These results suggest that 8-oxoguanine has little effect on mutagenesis in Arabidopsis cells.

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 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.

The yield, processing, and biological consequences of clustered DNA damage induced by ionizing radiation

After living cells are exposed to ionizing radiation, a variety of chemical modifications of DNA are induced either directly by ionization of DNA or indirectly through interactions with water-derived radicals. The DNA lesions include single strand breaks (SSB), base lesions, sugar damage, and apurinic/apyrimidinic sites (AP sites). Clustered DNA damage, which is defined as two or more of such lesions within one to two helical turns of DNA induced by a single radiation track, is considered to be a unique feature of ionizing radiation. A double strand break (DSB) is a type of clustered DNA damage, in which single strand breaks are formed on opposite strands in close proximity. Formation and repair of DSBs have been studied in great detail over the years as they have been linked to important biological endpoints, such as cell death, loss of genetic material, chromosome aberration. Although non-DSB clustered DNA damage has received less attention, there is growing evidence of its biological significance. This review focuses on the current understanding of (1) the yield of non-DSB clustered damage induced by ionizing radiation (2) the processing, and (3) biological consequences of non-DSB clustered DNA damage.

Repair of HZE-particle-induced DNA double-strand breaks in normal human fibroblasts

DNA damage generated by high-energy and high-Z (HZE) particles is more skewed toward multiply damaged sites or clustered DNA damage than damage induced by low-linear energy transfer (LET) X and gamma rays. Clustered DNA damage includes abasic sites, base damages and single- (SSBs) and double-strand breaks (DSBs). This complex DNA damage is difficult to repair and may require coordinated recruitment of multiple DNA repair factors. As a consequence of the production of irreparable clustered lesions, a greater biological effectiveness is observed for HZE-particle radiation than for low-LET radiation. To understand how the inability of cells to rejoin DSBs contributes to the greater biological effectiveness of HZE particles, the kinetics of DSB rejoining and cell survival after exposure of normal human skin fibroblasts to a spectrum of HZE particles was examined. Using gamma-H2AX as a surrogate marker for DSB formation and rejoining, the ability of cells to rejoin DSBs was found to decrease with increasing Z; specifically, iron-ion-induced DSBs were repaired at a rate similar to those induced by silicon ions, oxygen ions and gamma radiation, but a larger fraction of iron-ion-induced damage was irreparable. Furthermore, both DNA-PKcs (DSB repair factor) and 53BP1 (DSB sensing protein) co-localized with gamma-H2AX along the track of dense ionization produced by iron and silicon ions and their focus dissolution kinetics was similar to that of gamma-H2AX. Spatial co-localization analysis showed that unlike gamma-H2AX and 53BP1, phosphorylated DNA-PKcs was localized only at very specific regions, presumably representing the sites of DSBs within the tracks. Examination of cell survival by clonogenic assay indicated that cell killing was greater for iron ions than for silicon and oxygen ions and gamma rays. Collectively, these data demonstrate that the inability of cells to rejoin DSBs within clustered DNA lesions likely contributes to the greater biological effectiveness of HZE particles.