mFISH analysis of chromosomal damage in bone marrow cells collected from CBA/CaJ mice following whole body exposure to heavy ions (56Fe ions)

Simulating Space Conditions Evokes Different DNA Damage Responses in Immature and Mature Cells of the Human Hematopoietic System

The impact of space radiation and microgravity on DNA damage responses has been discussed controversially, largely due to the variety of model systems engaged. Here, we performed side-by-side analyses of human hematopoietic stem/progenitor cells (HSPC) and peripheral blood lymphocytes (PBL) cultivated in a 2D clinostat to simulate microgravity before, during and after photon and particle irradiation. We demonstrate that simulated microgravity (SMG) accelerates the early phase of non-homologous end joining (NHEJ)-mediated repair of simple, X-ray-induced DNA double-strand breaks (DSBs) in PBL, while repair kinetics in HSPC remained unaltered. Repair acceleration was lost with increasing LET of ion exposures, which increases the complexity of DSBs, precluding NHEJ and requiring end resection for successful repair. Such cell-type specific effect of SMG on DSB repair was dependent on the NF-кB pathway pre-activated in PBL but not HSPC. Already under unperturbed growth conditions HSPC and PBL suffered from SMG-induced replication stress associated with accumulation of single-stranded DNA and DSBs, respectively. We conclude that in PBL, SMG-induced DSBs promote repair of radiation-induced damage in an adaptive-like response. HSPC feature SMG-induced single-stranded DNA and FANCD2 foci, i.e., markers of persistent replication stress and senescence that may contribute to a premature decline of the immune system in space.

Fluorescence in situ hybridization analysis of chromosomal aberrations in mouse splenocytes at one- and two-months after total body exposure to iron-56 (Fe) ion particles or X-rays

High atomic number and energy (HZE) particles such as iron-56 (Fe) ions are a major contributor to health risks in long-term manned space exploration. The aim of this study is to understand radiation-induced differential genotoxic effects between HZE particles and low linear energy transfer (LET) photons. C57BL/6J Jms female mice of 8 weeks old were exposed to total body irradiation of accelerated Fe-particles with a dose ranging from 0.1 to 3.0 Gy or of X-rays with a dose ranging from 0.1 to 5.0 Gy. Chromosomal aberrations (CAs) in splenocytes were examined by fluorescence in situ hybridization at 1- and 2-months after exposure. Clonal expansions of cells with CAs were found to be induced only by X-rays but not by Fe-particles. Dose-dependent increase in the frequencies of stable-type CAs was observed at 1- as well as 2-months after exposure to both radiation types. The frequencies of stable-type CAs in average were much higher in mice exposed to X-rays than those to Fe-particles and did not change significantly between 1- and 2-months after exposure to both radiation types. On the other hand, the frequencies of unstable-type CAs induced by X-rays and Fe-particles were not much different, and they appeared to decrease with time from 1- to 2-months after exposure. These results suggested that larger fraction of stable-type CAs induced by Fe-particles might be non-transmissible than those by X-rays because of some associating lethal alterations on themselves or on other chromosomes in the same cells and that these cells might be removed by 1-month after Fe-TBI. We also demonstrated that exposure to Fe-particles induced insertions at relatively higher frequency to other stable-type CAs than X-rays. Our findings suggest that insertions can be used as indicators of past exposure to high-LET particle radiation.

Synergistic Effects of Chronic Restraint-Induced Stress and Low-Dose 56Fe-particle Irradiation on Induction of Chromosomal Aberrations in Trp53-Heterozygous Mice

Astronauts can develop psychological stress (PS) during space flights due to the enclosed environment, microgravity, altered light-dark cycles, and risks of equipment failure or fatal mishaps. At the same time, they are exposed to cosmic rays including high atomic number and energy (HZE) particles such as iron-56 (Fe) ions. Psychological stress or radiation exposure can cause detrimental effects in humans. An earlier published pioneering study showed that chronic restraint-induced psychological stress (CRIPS) could attenuate Trp53 functions and increase carcinogenesis induced by low-linear energy transfer (LET) γ rays in Trp53-heterozygous (Trp53+/-) mice. To elucidate possible modification effects from CRIPS on high-LET HZE particle-induced health consequences, Trp53+/- mice were received both CRIPS and accelerated Fe ion irradiation. Six-week-old Trp53+/- C57BL/6N male mice were restrained 6 h per day for 28 consecutive days. On day 8, they received total-body Fe-particle irradiation (Fe-TBI, 0.1 or 2 Gy). Metaphase chromosome spreads prepared from splenocytes at the end of the 28-day restraint regimen were painted with the fluorescence in situ hybridization (FISH) probes for chromosomes 1 (green), 2 (red) and 3 (yellow). Induction of psychological stress in our experimental model was confirmed by increase in urinary corticosterone level on day 7 of restraint regimen. Regardless of Fe-TBI, CRIPS reduced splenocyte number per spleen at the end of the 28-day restraint regimen. At 2 Gy, Fe-TBI alone induced many aberrant chromosomes and no modifying effect was detected from CRIPS on induction of aberrant chromosomes. Notably, neither Fe-TBI at 0.1 Gy nor CRIPS alone induced any increase in the frequency of aberrant chromosomes, while simultaneous exposure resulted in a significant increase in the frequency of chromosomal exchanges. These findings clearly showed that CRIPS could enhance the frequency of chromosomal exchanges induced by Fe-TBI at a low dose of 0.1 Gy.

Late Effects of Low-Dose Radiation on the Bone Marrow, Lung, and Testis Collected From the Same Exposed BALB/cJ Mice

We used 3 biological metrics highly relevant to health risks, that is, cell death, inflammation, and global DNA methylation, to determine the late effects of low doses (0.05 or 0.1 Gy) of 137Cs γ rays on the bone marrow, lung, and testis collected at 6 months post-irradiation from the same exposed BALB/cJ mouse. This integrative approach has not been used for such a purpose. Mice exposed to 0 or 1 Gy of radiation served as a sham or positive control group, respectively. The results could deliver information for better health risk assessment across tissues, including better scientific basis for radiation protection and clinical application. We found no changes in the levels of all studied biological metrics (except a significant increase in the levels of an anti-inflammatory cytokine, ie, interleukin 10) in tissues of 0.05-Gy exposed mice, when compared to those in sham controls. In contrast, significantly increased levels of cell death and inflammation, including a significant loss of global 5-hydroxymethylcytosine, were found in all tissues of the same mice exposed to 0.1 or 1.0 Gy. Our data demonstrated not only no harm but also hormesis in the 0.05-Gy exposed mice. However, the hormetic effect appears to be dependent on biological metrics and tissue.

MMR Deficiency Does Not Sensitize or Compromise the Function of Hematopoietic Stem Cells to Low and High LET Radiation

One of the major health concerns on long-duration space missions will be radiation exposure to the astronauts. Outside the earth's magnetosphere, astronauts will be exposed to galactic cosmic rays (GCR) and solar particle events that are principally composed of protons and He, Ca, O, Ne, Si, Ca, and Fe nuclei. Protons are by far the most common species, but the higher atomic number particles are thought to be more damaging to biological systems. Evaluation and amelioration of risks from GCR exposure will be important for deep space travel. The hematopoietic system is one of the most radiation-sensitive organ systems, and is highly dependent on functional DNA repair pathways for survival. Recent results from our group have demonstrated an acquired deficiency in mismatch repair (MMR) in human hematopoietic stem cells (HSCs) with age due to functional loss of the MLH1 protein, suggesting an additional risk to astronauts who may have significant numbers of MMR deficient HSCs at the time of space travel. In the present study, we investigated the effects gamma radiation, proton radiation, and ⁵⁶ Fe radiation on HSC function in Mlh1^+/+ and Mlh1^-/- marrow from mice in a variety of assays and have determined that while cosmic radiation is a major risk to the hematopoietic system, there is no dependence on MMR capacity. Stem Cells Translational Medicine 2018;7:513-520.

DNA Methylation in Radiation-Induced Carcinogenesis: Experimental Evidence and Clinical Perspectives

Ionizing radiation is a valuable tool in many spheres of human life. At the same time, it is a genotoxic agent with a well-established carcinogenic potential. Progress achieved in the last two decades has demonstrated convincingly that ionizing radiation can also target the cellular epigenome. Epigenetics is defined as heritable changes in the expression of genes that are not due to alterations of DNA sequence but consist of specific covalent modifications of chromatin components, such as methylation of DNA, histone modifications, and control performed by non-coding RNAs. Accumulating evidence suggests that DNA methylation, a key epigenetic mechanism involved in the control of expression of genetic information, may serve as one of the driving mechanisms of radiation-induced carcinogenesis. Here, we review the literature on the effects of ionizing radiation on DNA methylation in various biological systems, discuss the role of DNA methylation in radiation carcinogenesis, and provide our opinion on the potential utilization of this knowledge in radiation oncology.

High-LET Radiation Increases Tumor Progression in a K-Ras-Driven Model of Lung Adenocarcinoma

High-linear energy transfer (LET) radiation encountered by astronauts in space generates clustered DNA damage that is potentially oncogenic. Analysis of the impact of exposure to space radiation on cancer formation is necessary to determine the best ways to prepare astronauts for space travel so they are protected for the duration of the space mission. A mouse model of lung adenocarcinoma driven by oncogenic K-Ras was used to ascertain the effect of low- and high-LET radiation on tumor formation. We observed increased tumor progression and tumor cell proliferation after single dose or fractionated high-LET doses, which was not observed in mice exposed to low-LET radiation. Location of the tumor nodules was not affected by radiation, indicating that the cell of origin of K-Ras-driven tumors was the same in irradiated or nonirradiated mice. Gene expression analysis revealed an upregulation of genes involved in cell proliferation and DNA damage repair. This study provides evidence that exposure to a single dose or fractionated doses of high-LET radiation induces molecular and cellular changes that accelerate lung tumor growth.

Detection of Inter-chromosomal Stable Aberrations by Multiple Fluorescence In Situ Hybridization (mFISH) and Spectral Karyotyping (SKY) in Irradiated Mice

Ionizing radiation (IR) induces numerous stable and unstable chromosomal aberrations. Unstable aberrations, where chromosome morphology is substantially compromised, can easily be identified by conventional chromosome staining techniques. However, detection of stable aberrations, which involve exchange or translocation of genetic materials without considerable modification in the chromosome morphology, requires sophisticated chromosome painting techniques that rely on in situ hybridization of fluorescently labeled DNA probes, a chromosome painting technique popularly known as fluorescence in situ hybridization (FISH). FISH probes can be specific for whole chromosome/s or precise sub-region on chromosome/s. The method not only allows visualization of stable aberrations, but it can also allow detection of the chromosome/s or specific DNA sequence/s involved in a particular aberration formation. A variety of chromosome painting techniques are available in cytogenetics; here two highly sensitive methods, multiple fluorescence in situ hybridization (mFISH) and spectral karyotyping (SKY), are discussed to identify inter-chromosomal stable aberrations that form in the bone marrow cells of mice after exposure to total body irradiation. Although both techniques rely on fluorescent labeled DNA probes, the method of detection and the process of image acquisition of the fluorescent signals are different. These two techniques have been used in various research areas, such as radiation biology, cancer cytogenetics, retrospective radiation biodosimetry, clinical cytogenetics, evolutionary cytogenetics, and comparative cytogenetics.

Late-occurring chromosome aberrations and global DNA methylation in hematopoietic stem/progenitor cells of CBA/CaJ mice exposed to silicon ((28)Si) ions

Although myeloid leukemia (ML) is one of the major health concerns from exposure to space radiation, the risk prediction for developing ML is unsatisfactory. To increase the reliability of predicting ML risk, a much improved understanding of space radiation-induced changes in the target cells, i.e. hematopoietic stem/progenitor cells (HSPCs), is important. We focused on the in vivo induction of late-occurring damage in HSPCs of mice exposed to (28)Si ions since such damage is associated with radiation-induced genomic instability (a key event of carcinogenesis). We gave adult male CBA/CaJ mice, known to be sensitive to radiation-induced ML, a whole-body exposure (2 fractionated exposures, 15 days apart, that totaled each selected dose, delivered at the dose-rate of 1 cGy/min) to various doses of 300 MeV/n (28)Si ions, i.e. 0 (sham controls), 0.1, 0.25, or 0.5 Gy. At 6 months post-irradiation, we collected bone marrow cells from each mouse (five mice per treatment-group) for obtaining the myeloid-lineage of HSPC-derived clones for analyses. We measured the frequencies of late-occurring chromosome aberrations (CAs), using the genome-wide multicolor fluorescence in situ hybridization method. The measurement of CAs was coupled with the characterization of the global DNA methylation patterns, i.e. 5-methylcytosine (5 mC) and 5-hydroxymethylcytosine (5 hmC). A dose-dependent increase in the frequencies of CAs was detected (Analysis of Variance or ANOVA, p<0.01), indicating the induction of genomic instability after exposure of mice to 300 MeV/n (28)Si ions. Slight increases in the levels of 5 mC were observed in all treatment groups, as compared to the sham-control level. In contrast, there was a significant reduction in levels of 5 hmC (ANOVA, p<0.01). Since these endpoints were evaluated in the same mouse, our data suggested for the first time a link between a reduction in 5 hmC and genomic instability in HSPC-derived myeloid colonies of CBA/CaJ mice exposed to 300 MeV/n (28)Si ions.

Ionizing Particle Radiation as a Modulator of Endogenous Bone Marrow Cell Reprogramming: Implications for Hematological Cancers

Exposure of individuals to ionizing radiation (IR), as in the case of astronauts exploring space or radiotherapy cancer patients, increases their risk of developing secondary cancers and other health-related problems. Bone marrow (BM), the site in the body where hematopoietic stem cell (HSC) self-renewal and differentiation to mature blood cells occurs, is extremely sensitive to low-dose IR, including irradiation by high-charge and high-energy particles. Low-dose IR induces DNA damage and persistent oxidative stress in the BM hematopoietic cells. Inefficient DNA repair processes in HSC and early hematopoietic progenitors can lead to an accumulation of mutations whereas long-lasting oxidative stress can impair hematopoiesis itself, thereby causing long-term damage to hematopoietic cells in the BM niche. We report here that low-dose ¹H- and ⁵⁶Fe-IR significantly decreased the hematopoietic early and late multipotent progenitor (E- and L-MPP, respectively) cell numbers in mouse BM over a period of up to 10 months after exposure. Both ¹H- and ⁵⁶Fe-IR increased the expression of pluripotent stem cell markers Sox2, Nanog, and Oct4 in L-MPPs and 10 months post-IR exposure. We postulate that low doses of ¹H- and ⁵⁶Fe-IR may induce endogenous cellular reprogramming of BM hematopoietic progenitor cells to assume a more primitive pluripotent phenotype and that IR-induced oxidative DNA damage may lead to mutations in these BM progenitors. This could then be propagated to successive cell lineages. Persistent impairment of BM progenitor cell populations can disrupt hematopoietic homeostasis and lead to hematologic disorders, and these findings warrant further mechanistic studies into the effects of low-dose IR on the functional capacity of BM-derived hematopoietic cells including their self-renewal and pluripotency.

NF-κB-dependent DNA damage-signaling differentially regulates DNA double-strand break repair mechanisms in immature and mature human hematopoietic cells

Hematopoietic stem and progenitor cells (HSPC), that is, the cell population giving rise not only to all mature hematopoietic lineages but also the presumed target for leukemic transformation, can transmit (adverse) genetic events, such as are acquired from chemotherapy or ionizing radiation. Data on the repair of DNA double-strand-breaks (DSB) and its accuracy in HSPC are scarce, in part contradictory, and mostly obtained in murine models. We explored the activity, quality and molecular components of DSB repair in human HSPC as compared with mature peripheral blood lymphocytes (PBL). To consider chemotherapy/radiation-induced compensatory proliferation, we established cycling HSPC cultures. Comparison of pathway-specific repair activities using reporter systems revealed that HSPC were severely compromised in non-homologous end joining and homologous recombination but not microhomology-mediated end joining. We observed a more pronounced radiation-induced accumulation of nuclear 53BP1 in HSPC relative to PBL, despite evidence for comparable DSB formation from cytogenetic analysis and γH2AX signal quantification, supporting differential pathway usage. Functional screening excluded a major influence of phosphatidylinositol-3-OH-kinase (ATM/ATR/DNA-PK)- and p53-signaling as well as chromatin remodeling. We identified diminished NF-κB signaling as the molecular component underlying the observed differences between HSPC and PBL, limiting the expression of DSB repair genes and bearing the risk of an inaccurate repair.

Intestinal microbiota reduces genotoxic endpoints induced by high-energy protons

Ionizing space radiation causes oxidative DNA damage and triggers oxidative stress responses, and compromised DNA repair mechanisms can lead to increased risk of carcinogenesis. Young adult mice with developed innate and adaptive immune systems that harbored either a conventional intestinal microbiota (CM) or an intestinal microbiota with a restricted microbial composition (RM) were irradiated with a total dose of 1 Gy delivered by high-energy protons (2.5 GeV/n, LET = 0.2-2 keV/μm) or silicon or iron ions (850 MeV/n, LET ≈ 50 keV/μm and 1 GeV/n, LET = 150 keV/μm, respectively). Six hours after whole-body irradiation, acute chromosomal DNA lesions were observed for RM mice but not CM mice. High-throughput rRNA gene sequencing of intestinal mucosal bacteria showed that Barnesiella intestinihominis and unclassified Bacterodiales were significantly more abundant in male RM mice than CM mice, and phylotype densities changed in irradiated mice. In addition, Helicobacter hepaticus and Bacteroides stercoris were higher in CM than RM mice. Elevated levels of persistently phosphorylated γ-H2AX were observed in RM mice exposed to high-energy protons compared to nonirradiated RM mice, and they also were associated with a decrease of the antioxidant glutathione in peripheral blood measured at four weeks after irradiation. After radiation exposure, CM mice showed lower levels of γ-H2AX phosphorylation than RM mice and an increase in specific RM-associated phylotypes, indicating a down-regulating force on DNA repair by differentially abundant phylotypes in RM versus a radiation-sensitive complex CM.

Lack of genomic instability in bone marrow cells of SCID mice exposed whole-body to low-dose radiation

It is clear that high-dose radiation is harmful. However, despite extensive research, assessment of potential health-risks associated with exposure to low-dose radiation (at doses below or equal to 0.1 Gy) is still challenging. Recently, we reported that 0.05 Gy of 137Cs gamma rays (the existing limit for radiation-exposure in the workplace) was incapable of inducing significant in vivo genomic instability (measured by the presence of late-occurring chromosomal damage at 6 months post-irradiation) in bone marrow (BM) cells of two mouse strains, one with constitutively high and one with intermediate levels of the repair enzyme DNA-dependent protein-kinase catalytic-subunit (DNA-PKcs). In this study, we present evidence for a lack of genomic instability in BM cells of the severely combined-immunodeficiency (SCID/J) mouse (which has an extremely low-level of DNA-PKcs activity) exposed whole-body to low-dose radiation (0.05 Gy). Together with our previous report, the data indicate that low-dose radiation (0.05 Gy) is incapable of inducing genomic instability in vivo (regardless of the levels of DNA-PKcs activity of the exposed mice), yet higher doses of radiation (0.1 and 1 Gy) do induce genomic instability in mice with intermediate and extremely low-levels of DNA-PKcs activity (indicating an important role of DNA-PKcs in DNA repair).

Identification of proteins secreted into the medium by human lymphocytes irradiated in vitro with or without adaptive environments

There is increasing evidence to support the hypothesis of adaptive response, a phenomenon in which protection arises from a low-dose radiation (<0.1 Gy) against damage induced by subsequent exposure to high-dose radiation. The molecular mechanisms underlying such protection are poorly understood. The goal of this study was to fill this knowledge gap. Mass spectrometry-based proteomics was used to characterize global protein expression profiles in the medium collected from human lymphocyte cultures given sham irradiation (0 Gy) or a priming low dose of 0.03 Gy 137Cs γ rays 4 h prior to a challenging dose of 1 Gy 137Cs γ rays. Adaptive response was determined by decreased micronucleus frequencies in lymphocytes receiving low dose irradiation prior to high dose irradiation compared to those receiving only high dose irradiation. Adaptive response was found in these experiments. Proteomic analysis of media revealed: (a) 55 proteins with similar abundance in both groups; (b) 23 proteins in both groups, but 7 of them were high abundance in medium with adaptive environment, while 16 high abundance proteins were in medium without adaptive environment; (c) 17 proteins in medium with adaptive environment only; and (d) 8 proteins in medium without adaptive environment only. The results provide a foundation for improving understanding of the molecular mechanisms associated with the beneficial effects of low dose radiation that, in turn, will have an important impact on radiation risk estimation. Hence, these studies are highly relevant to radiation protection due to an increased use of low dose radiation in daily life (e.g., medical diagnosis or airport safety) or an unavoidable exposure to low level background radiation.

Chromosomal aberrations in the bone marrow cells of mice induced by accelerated (12)C(6+) ions

The whole bodies of 6-week-old male Kun-Ming mice were exposed to different doses of (12)C(6+) ions or X-rays. Chromosomal aberrations of the bone marrow (gaps, terminal deletions and breaks, fragments, inter-chromosomal fusions and sister-chromatid union) were scored in metaphase 9h after exposure, corresponding to cells exposed in the G(2)-phase of the first mitosis cycle. Dose-response relationships for the frequency of chromosomal aberrations were plotted both by linear and linear-quadratic equations. The data showed that there was a dose-related increase in the frequency of chromosomal aberrations in all treated groups compared to controls. Linear-quadratic equations were a good fit for both radiation types. The compound theory of dual radiation action was applied to decipher the bigger curvature (D(2)) of the dose-response curves of X-rays compared to those of (12)C(6+) ions. Different distributions of the five types of aberrations and different degrees of homogeneity were found between (12)C(6+) ion and X-ray irradiation and the possible underlying mechanism for these phenomena were analyzed according to the differences in the spatial energy deposition of both types of radiation.

No Evidence for the In Vivo Induction of Genomic Instability by Low Doses of CS Gamma Rays in Bone Marrow Cells of BALB/CJ and C57BL/6J Mice

In spite of extensive research, assessment of potential health risks associated with exposure to low-dose (≤ 0.1 Gy) radiation is still challenging. We evaluated the in vivo induction of genomic instability, expressed as late-occurring chromosome aberrations, in bone-marrow cells of two strains of mouse with different genetic background, i.e. the radiosensitive BALB/cJ and the radioresistant C57BL/6J strains following a whole-body exposure to varying doses of (137)Cs gamma rays (0, 0.05, 0.1, and 1.0 Gy). A total of five mice per dose per strain were sacrificed at various times post-irradiation up to 6 months for sample collections. Three-color fluorescence in situ hybridization for mouse chromosomes 1, 2, and 3 was used for the analysis of stable-aberrations in metaphase-cells. All other visible gross structural-abnormalities involving non-painted-chromosomes were also evaluated on the same metaphase-cells used for scoring the stable-aberrations of painted-chromosomes. Our new data demonstrated in bone-marrow cells from both strains that low doses of low LET-radiation (as low as 0.05 Gy) are incapable of inducing genomic instability but are capable of reducing specific aberration-types below the spontaneous rate with time post-irradiation. However, the results showed the induction of genomic instability by 1.0 Gy of (137)Cs gamma rays in the radiosensitive strain only.

Complex exchanges are responsible for the increased effectiveness of C-ions compared to X-rays at the first post-irradiation mitosis

The purpose of the present study was to investigate as to what extent differences in the linear energy transfer (LET) are reflected at the chromosomal level. For this study human lymphocytes were exposed to 9.5 MeV/u C-ions (1 or 2 Gy, LET=175 keV/microm) or X-rays (1-6 Gy), harvested at 48, 72 or 96 h post-irradiation and aberrations were scored in first cycle metaphases using 24 color fluorescence in situ hybridization (mFISH). Additionally, in selected samples aberrations were measured in prematurely condensed G2-phase cells. Analysis of the time-course of aberrations in first cycle metaphases showed a stable yield of simple and complex exchanges after X-ray irradiation. In contrast, after C-ion exposure the yields profoundly increased with harvesting time complicating the estimation of the frequency of aberrations produced by high LET particles within the entire cell population. This is especially true for the yield of complex exchanges. Complex aberrations dominate the aberration spectrum produced by C-ions. Their fraction was about 50% for the two measured doses. In contrast, isodoses of X-rays induced smaller proportions of complex aberrations (i.e. 5% and 15%, respectively). For both radiation qualities the fraction of complexes did not change with harvesting time. As expected from the different dose deposition of high and low LET radiation, complex exchanges produced by high LET C-ions involved more breaks and more chromosomes than those induced by isodoses of X-rays. Noteworthy, C-ions but not X-rays induced a small number of complex chromatid-isochromatid exchanges that are not expected for cells exposed in the G0-phase. The results obtained so far for cells arrested in G2-phase confirm these patterns. Altogether our data show that the increased effectiveness of C-ions for the induction of aberrations in first cycle cells is determined by complex exchanges, whereas for simple exchanges the relative biological effectiveness is about one.

Heavy ion carcinogenesis and human space exploration

Before the human exploration of Mars or long-duration missions on the Earth's moon, the risk of cancer and other diseases from space radiation must be accurately estimated and mitigated. Space radiation, comprised of energetic protons and heavy nuclei, has been shown to produce distinct biological damage compared with radiation on Earth, leading to large uncertainties in the projection of cancer and other health risks, and obscuring evaluation of the effectiveness of possible countermeasures. Here, we describe how research in cancer radiobiology can support human missions to Mars and other planets.