Chromosome Aberrations In Human Lymphocytes Research Articles

Oral subchronic toxicity study and genetic toxicity evaluation of mitoquinone mesylate

AbstractMitochondrial dysfunction and excessive reactive oxygen species production contributes to the pathophysiology of aging. Coenzyme Q10 is thought to protect mitochondria from oxidative damage; thus, mitoquinone was developed as mitochondria‐targeted analogue with similar antioxidant activity. Mitoquinone is the oxidized form of mitoquinol. Mitoquinone/mitoquinol mesylate has been proposed as a food ingredient. As part of the safety analysis, we performed genotoxicity assays and a 39‐week toxicity study to determine overall toxicity potential. Mitoquinone mesylate showed no evidence of genotoxic potential in two in vitro assays, bacterial reverse mutation and human lymphocyte chromosome aberration, nor in the in vivo micronucleus test in rats. In the 39‐week study in dogs, there were no findings observed, which were considered to represent adverse systemic toxicity; therefore, the high dose level (40 mg/kg/day) was considered the NOAEL. The principal findings in this study were fecal disturbances and vomiting. These findings were considered to be due to a local, possibly irritant effect of the test substance on the gastrointestinal tract and were not considered adverse as there were no impacts on clinical or histopathology. This highest dose exceeds the expected daily human intake more than 100‐fold. Data from well‐designed clinical trials actively collecting safety endpoints corroborate that 20 mg/day can be safely consumed and is not likely to result in significant gastrointestinal complaints. These results support the conclusion that the use of mitoquinone/mitoquinol mesylate as a food ingredient is safe.

Assessing the Association of the Degree of DNA Methylation and the Frequency of Chromosomal Aberrations in Human Lymphocytes in a Single Irradiation of Blood In Vitro

Assessing the Association of the Degree of DNA Methylation and the Frequency of Chromosomal Aberrations in Human Lymphocytes in a Single Irradiation of Blood In Vitro

Assessing the Association of the Degree of DNA Methylation and the Frequency of Chromosomal Aberrations in Human Lymphocytes in a Single Irradiation of Blood <i>in vitro</i>

The most sensitive biomolecule under radiation exposure is DNA, whose damage manifests itself in the form of chromosomal aberrations (CA). The processes of DNA methylation, which are involved in the regulation of gene expression, replication, DNA repair, etc., are also affected by gamma radiation. The aim of the study was to evaluate the relationship between the degree of DNA methylation and the frequency of CA after acute in vitro irradiation of human blood lymphocytes with gamma radiation. The study involved 10 conditionally healthy workers of the Siberian Chemical Combine, in whose blood lymphocytes the degree of methylation of CpG-dinucleotides (wide-genome bisulfite sequencing, XmaI-Reduced representation bisulfite sequencing – XmaI-RRBS) and the frequency of CA (cytogenetic study) after acute in vitro blood irradiation with doses of 0 and 1.5 Gy were evaluated. After acute exposure to gamma radiation in lymphocytes, the frequency of aberrant cells, dicentric chromosomes, chromatid and chromosomal fragments increased. Correlation analysis of the status of CpG-dinucleotide methylation and the frequency of CA revealed changes in the degree of methylation of 97 genes, which strongly correlated positively (56 genes) or negatively (41 genes) with an increased frequency of CA. A primary genome-wide screening of genes whose methylation is correlates with a high frequency of CA was carried out. Many of the identified genes are promising as potential markers of radiation exposure and to study the mechanisms of formation of radiosensitivity of the body and radioresistance of tumors during radiation therapy.

Dose-Dependent Transmissibility of Chromosome Aberrations in Human Lymphocytes at First Mitosis. II. Biological Effectiveness of Heavy Charged Particles Versus Gamma Rays.

Chromosome aberrations (CAs) are large scale structural rearrangements to the genome that have been used as a proxy endpoint of mutagenic and carcinogenic potential. And yet, many types of CAs are incapable of causing either of these effects simply because they are lethal. Using 24-color multi-fluor combinatorial painting (mFISH), we examined CAs in normal human lymphocytes exposed to graded doses of 1 GeV/nucleon accelerated 56Fe ions and 662 keV 137Cs gamma rays. As expected, the high-linear energy transfer (LET) heavy ions were considerably more potent per unit dose at producing total yields of CAs compared to low-LET gamma rays. As also anticipated, the frequency distribution of aberrations per cell exposed to 56Fe ions was significantly overdispersed compared to the Poisson distribution, containing excess numbers of cells devoid of aberrations. We used the zero-inflated negative binomial (ZINB) distribution to model these data. Based on objective cytogenetic criteria that are subject to caveats we discuss, each cell was individually evaluated in terms of likely survival (i.e., its ability to transmit to daughter cell progeny). For 56Fe ion irradiations, the frequency of surviving cells harboring complex aberrations represented a significant portion of aberration-bearing cells, while for gamma irradiation no survivable cells containing complex aberrations were observed. When the dose responses for the two radiation types were compared, and the analysis was limited to surviving cells that contained aberrations, we were surprised to find the high-LET 56Fe ions only marginally more potent than the low-LET gamma rays for doses less than 1 Gy. In fact, based on dose-response modeling, they were predicted to be less effective than gamma rays at somewhat higher doses. The major implication of these findings is that measures of relative biological effectiveness that fail to account for coincident lethality will tend to overstate the impact of transmissible chromosomal damage from high-LET particle exposure.

Evaluation of mutagenicity, genotoxicity and chronic toxicity of antiviral drug imidazolyl ethanamide pentandioic acid in <i>in vitro</i> and <i>in vivo</i> test systems

Introduction. The antiviral properties of imidazolyl ethanamide pentandioic acid (IPA), the active compound of the drug product, has been proven in various experimental models. However, the literature data on the toxicological properties of IPA are limited.Purpose. To evaluate mutagenic and genotoxic properties in in vitro and in vivo models, as well as to study the toxicity of IPA following chronic oral administration to rats and dogs.Materials and methods. Mutagenic and genotoxic properties of IPA were assessed using the Ames test, the test of chromosomal aberrations in human lymphocytes, and the micronucleus test in rats. The chronic toxicity of IPA was studied in Sprague Dawley rats and beagle dogs of both sexes, to which IPA was administered orally at doses of 30-300 mg/kg/day for 26 and 39 weeks, respectively.Results and discussion. In the Ames test, the addition of IPA up to the maximum dose (5000 mcg/plate) did not result in the increase in the number of revertant colonies. At a concentration of up to 5000 mcg/ml, IPA did not cause chromosomal aberrations in human leukocytes. At doses doses ≤ 2000 mg/kg, IPA did not increase the amount of micronuclei in the bone marrow of rats. In chronic experiments, animals tolerated the administration of IPA well: the dose without an observed effect (NOEL) for rats and dogs was 300 mg/kg/day.Conclusion. IPA did not show mutagenic and genotoxic properties in standard in vitro and in vivo tests. With chronic oral administration to rats and dogs, NOEL IPA equal to 300 mg/kg/day provided a systemic exposure that was 8-10 and 41-65 times higher than that in humans, respectively. The results obtained allow us to consider the safety profile of the prolonged use in humans as favorable.

Хромосомні аберації у лімфоцитах людини за зовнішнього та/або внутрішнього опромінення зразків крові 137Cs у модельних експериментах in vitro

The linear dependence of the yield of unstable chromosomal aberrations in human lymphocytes on the dose of prolonged external and/or internal irradiation of peripheral blood samples by 137Cs in the dose range of 0.09 - 0.54 Gy in vitro is presented. The higher cytogenetic efficiency of internal 137Cs irradiation is shown in comparison with external. It is shown that the cytogenetic efficiency of 137Cs in blood contamination is caused by influence on cells not only in the mitotic rest phase but during proliferation, within intracellular radioactivity through their cultivation. It is proposed to use the presented dose dependences of the yield of chromosomal aberrations in lymphocytes in vitro for human biological dosimetry during radiation accidents associated with prolonged external exposure or 137Cs incorporation to the body as a supplement to the results of physical dosimetry.

Cytogenetic Damage of Human Lymphocytes in Humanized Mice Exposed to Neutrons and X Rays 24 h After Exposure

Detonation of an improvised nuclear device highlights the need to understand the risk of mixed radiation exposure as prompt radiation exposure could produce significant neutron and gamma exposures. Although the neutron component may be a relatively small percentage of the total absorbed dose, the large relative biological effectiveness (RBE) can induce larger biological DNA damage and cell killing. The objective of this study was to use a hematopoietically humanized mouse model to measure chromosomal DNA damage in human lymphocytes 24 h after in vivo exposure to neutrons (0.3 Gy) and X rays (1 Gy). The human dicentric and cytokinesis-block micronucleus assays were performed to measure chromosomal aberrations in human lymphocytes in vivo from the blood and spleen, respectively. The mBAND assay based on fluorescent in situ hybridization labeling was used to detect neutron-induced chromosome 1 inversions in the blood lymphocytes of the neutron-irradiated mice. Cytogenetics endpoints, dicentrics and micronuclei showed that there was no significant difference in yields between the 2 irradiation types at the doses tested, indicating that neutron-induced chromosomal DNA damage in vivo was more biologically effective (RBE ∼3.3) compared to X rays. The mBAND assay, which is considered a specific biomarker of high-LET neutron exposure, confirmed the presence of clustered DNA damage in the neutron-irradiated mice but not in the X-irradiated mice, 24 h after exposure.

Assessment of the Genotoxic Potential of Cizolirtine a Substance-P and Calcitonin Gene-Related Peptide Release Modulator.

The analysis of the genotoxic potential of cizolirtine, a compound being developed as a drug for analgesia and for urinary incontinence, was carried out using a battery of in vitro and in vivo assays as recommended in the guidelines for medicinal products. Negative results were obtained in an Ames test (up to 5000 µg/plate), in a Mouse Lymphoma assay (up to 2000 µg/ml) and in a single dose mouse bone marrow micronucleus assay (up to 300 mg/kg). In a human lymphocyte chromosome aberration assay, a slight statistical increase in the frequency of cells with chromosome aberrations including gaps was reported for the concentrations of 200 and 1600 μg/ml at the 24-h sampling time. This minor increase in chromosome aberrations was considered of questionable biological relevance since it was moderate, was within the laboratory historical control values, did no show a dose-dependent effect and was not observed at similar concentrations in a repeat assay. Taking into considerations the results obtained in the different in vitro and in vivo assays and a weight-of-evidence analysis, it suggests that cizolirtine would not pose a genotoxic risk when administered to humans.

Modeling of chromosome aberration response functions induced by particle beams with different LET.

This study is based on our already published experimental data (Kowalska et al. in Radiat Environ Biophys 58:99-108, 2019) and is devoted to modeling of chromosome aberrations in human lymphocytes induced by 22.1MeV/u 11B ions, 199MeV/u 12C ions, 150MeV and spread-out Bragg peak (SOBP) proton beams as well as by 60Co γ rays. The curvature of the dose-effect curves determined by the linear-quadratic model was considered in the frame of a simple analytical approach taking into account increase in the irradiation dose due to overlapping interaction regions of ion tracks. The model enabled to estimate effective interaction radius which could be compared with the physical expectations. The results were also compared to the Amorphous Track Structure Model of Katz which allows to get some additional information about the ion track structure. The analysis showed that the curvature of the experimental dose-effect curves mainly results from highly efficient repair processes of the DNA damage.

Genotoxic properties of Betonica officinalis, Gratiola officinalis, Vincetoxicum luteum and Vincetoxicum hirundinaria extracts

Genotoxicity of B. officinalis, G. officinalis, V. luteum and V. hirundinaria extracts, which demonstrated strong antioxidant capacity, was tested using chromosome aberration, sister chromatid exchange (SCE), cytokinesis-block micronucleus and alkaline single-cell gel electrophoresis (comet) assays in human lymphocytes in vitro and Ames Salmonella/microsome test. All tested extracts were not mutagenic in S. typhimurium strains TA98 and TA100 with and without metabolic activation and did not induce chromosome aberrations in human lymphocytes in vitro. Extract from G. officinalis was the only one, which induced significant increase in micronuclei, indicating possible aneugenic effect. All investigated plant extracts induced DNA damage evaluated by the comet assay, while B. officinalis and V. luteum extracts induced slight increase in SCE values. The determined variation in response might be due to the plant extract tested and donor susceptibility.

Production and distribution of chromosome aberrations in human lymphocytes by particle beams with different LET

We investigated induction of chromosome aberrations (CA) in human lymphocytes when exposed to 150 MeV and spread out Bragg peak (SOBP) proton beams, and 199 MeV/u carbon beam which are currently widely used for cancer treatment and simultaneously are important components of cosmic radiation. For a comparison, the boron ions of much lower energy 22 MeV/u and a 60Co γ rays were used. Dose–effect curves as well as the distributions of CA were studied using Poisson and Neyman type A statistics. Systematics of experimentally determined parameters, their dependence on applied doses and irradiation quality are presented.

Safety evaluation of the food enzyme glucose oxidase from a genetically modified Aspergillusoryzae (strain NZYM-KP).

The food enzyme is a glucose oxidase (beta‐d‐glucose:oxygen 1‐oxidoreductase; EC 1.1.3.4) produced with a genetically modified strain of Aspergillus oryzae strain NZYM‐KP by Novozymes A/S. The genetic modifications do not give rise to safety concerns. The food enzyme does not contain the production organism or DNA; therefore, there is no safety concern for the environment. The glucose oxidase is intended to be used in baking processes. Based on the maximum use levels recommended and individual consumption data from the EFSA Comprehensive European Food Consumption Database, dietary exposure to the food enzyme–total organic solids (TOS) was estimated to be up to 0.156 mg TOS/kg body weight (bw) per day in European populations. The food enzyme did not induce gene mutations in bacteria or chromosome aberrations in human lymphocytes. The subchronic toxicity was assessed by means of a repeated dose 90‐day oral toxicity study in rodents. A no‐observed‐adverse‐effect level was derived (341 mg TOS/kg bw per day), which compared with the estimated dietary exposure results in a sufficiently high margin of exposure. The allergenicity was evaluated by comparing the amino acid sequence to those of known allergens and one match with a fungal contact allergen was found. The Panel considered that, under the intended condition of use, the risk of allergic sensitisation and elicitation reactions by dietary exposure cannot be excluded, but the likelihood is considered low. Based on the microbial source, the genetic modifications, the manufacturing process, the compositional and biochemical data, the estimated dietary exposure and the findings in the toxicological studies, the Panel concluded that this food enzyme does not give rise to safety concerns under the intended conditions of use.

Safety evaluation of the food enzyme maltogenic amylase from a genetically modified Bacillussubtilis (strain NZYM-SM).

The food enzyme considered is a maltogenic amylase (glucan 1,4‐α‐maltohydrolase; EC 3.2.1.133) produced with the genetically modified Bacillus subtilis strain NZYM‐SM by Novozymes A/S. The food enzyme contains neither the production organism nor recombinant DNA. The maltogenic amylase is intended for use in baking processes and starch processing for glucose syrups production. Based on the maximum use levels recommended for the food processes and individual consumption data from the EFSA Comprehensive European Food Consumption Database, dietary exposure to the food enzyme–Total Organic Solids (TOS) was estimated to be up to 0.168 mg TOS/kg body weight (bw) per day in European populations. The food enzyme did not induce gene mutations in bacteria or chromosomal aberrations in human lymphocytes. The subchronic toxicity was assessed by means of a repeated dose 90‐day oral toxicity study in rodents. A no observed adverse effect level (NOAEL) was derived (320 mg TOS/kg bw per day), which, compared with the dietary exposure, results in a sufficiently high margin of exposure. The allergenicity was evaluated by searching for similarity of the amino acid sequence to those of known allergens. Three matches to occupational respiratory allergens were found, however, the Panel considered that there are no indications for food allergic reactions to the food enzyme. Based on the genetic modifications performed, the manufacturing process, the compositional and biochemical data provided, the dietary exposure assessment, the findings in the toxicological studies and allergenicity assessment, the Panel concluded that the food enzyme maltogenic amylase from Bacillus subtilis strain NZYM‐SM does not give rise to safety concerns under the intended conditions of use.

Radiation dose\u2013response curves: cell repair mechanisms vs. ion track overlapping

Chromosome aberrations in human lymphocytes exposed to different doses of particle radiation: 150 MeV and spread out Bragg peak proton beams, 22 MeV/u boron beam and 199 V/u carbon beam were studied. For comparison, an experiment with 60Co γ-rays was also performed. We investigated distributions of aberration frequency and the shape of dose–response curves for the total aberration yield as well as for exchange and non-exchange aberrations, separately. Applying the linear-quadratic model, we could derive a relation between the fitted parameters and the ion track radius which could explain experimentally observed curvature of the dose–response curves. The results compared with physical expectations clearly show that the biological effects of cell repair are much more important than the ion track overlapping.Graphical abstract

Effects of Valproic Acid on Radiation-Induced Chromosomal Aberrations in Human Lymphocytes.

One of the most widely employed histone deacetylases inhibitors in the clinic is the valproic acid (VA), proving to have a good tolerance and low side effects on human health. VA induces changes in chromatin structure making DNA more susceptible to damage induction and influence DNA repair efficiency. VA is also proposed as a radiosensitizing agent. To know if VA is suitable to sensitize human lymphocytes γ-irradiation in vitro, different types of chromosomal aberrations in the lymphocytes, either in the absence or presence of VA, were analyzed. For this purpose, blood samples from four healthy donors were exposed to γ-rays at a dose of 1.5 Gy and then treated with two different doses of VA (0.35 or 0.70 mM). Unstable and stable chromosomal aberrations were analyzed by means of fluorescence in situ hybridization. Human lymphocytes treated with VA alone did not show any increase in the frequency of chromosomal aberrations. However, a moderate degree of sensitization was observed, through the increase of chromosomal aberrations, when 0.35 mM VA was employed after γ-irradiation, whereas 0.70 mM VA did not modify chromosomal aberration frequencies. The lower number of chromosomal aberrations obtained when VA was employed at higher dose after γ-irradiation, could be related to the induction of a cell cycle arrest, a fact that should be taken into consideration when VA is employed in combination with physical or chemical agents.

Genotoxic and antigenotoxic effects of luteolin against MMC-induced chromosomal aberrations in human lymphocytes in vitro

Genotoxic and antigenotoxic effects of luteolin against MMC-induced chromosomal aberrations in human lymphocytes in vitro

Cytogenetic Effects of Low-Dose Tritiated Water in Human Peripheral Blood Lymphocytes—Experimental Studies on the Relative Biological Effectiveness and Chromosome Aberration Rate and CBMN in Human Blood Lymphocyte Irradiated by Tritium Low Dose Tritium <i>β</i>-Rays and <sup>60</sup>Co <i>γ</i>-Rays

In this study, the yields of unstable chromosome aberrations in human lymphocytes induced by β particles from low-dose HTO have been measured. HTO was mixed with heparinized blood in varying amounts so that a dose of 6.24 × 10－4 Gy to 1.23 Gy were delivered in 24 and 48 h, respectively. After 72-h culture, the dicentric yield was measured as a function of dose to the blood and compared with data from 60Co γ-radiation. Using a linear-quadratic dose-effect relation to fit the experimental data, a significant linear contribution Y = 0.062D + 0.053D2 was found. The main difference between the coefficients for β and γ-radiation was in the a values, indicating that HTO β-rays were more efficient, particularly at low doses. As per the theory of dual radiation, the relative biological effectiveness (RBE) of HTO β-particles relative to γ-rays is 2.21 at 0.06 Gy and decreases with increasing dose. Micronucleus yield at low doses was fitted to a linear equation Y = C + αD, indicating that the RBE value of MN for HTO β-rays irradiation was between 1.46 and 2.17, which is similar in shape to the chromosome aberrations experiments. Thus, β-rays were found to be more efficient in producing two lesions with single ionizing tracks at low dose.

Investigations into the genotoxic potential of olive extracts

The phenolic anti-oxidant 3-hydroxytyrosol (HT) is a major constituent of olives and olive oil. Published data showed it was negative in the Ames test at concentrations up to 5μL per plate, but did induce chromosomal aberrations in human lymphocytes. HIDROX, an olive extract containing approximately 2.4% HT, was reported as both positive and equivocal in an Ames test in different papers from the same laboratory. Negative results for micronucleus induction in vivo in both an acute study and as part of a 90-day rat toxicity study were also reported for HIDROX.Given the widespread use and consumption of olives, olive oil and olive extracts, it was important to obtain more data. Here we confirm that pure HT, and an olive extract containing 15% HT, both induced micronuclei in cultured cells in vitro, but show that these responses were either due to high levels of cytotoxicity or to reaction of HT with culture medium components to produce hydrogen peroxide. Another extract (H40) containing 40% HT also induced micronuclei in vitro, probably via the same mechanism. However, both extracts were negative in robust Ames tests.The 15% HT formulated extract did not induce micronuclei in rat bone marrow after 4 weeks of dosing up to 561mg HT/kg/day. H40 produced increased rat bone marrow micronucleus frequencies at 250 and 500mg HT/kg/day in a 90-day toxicity study, but the results were questionable for various reasons. However, when two different batches of this extract were tested in acute micronucleus studies at doses up to 2000mg HT/kg, giving plasma exposures that exceeded those in the 90-day study, negative results were obtained. Based on weight of evidence it is concluded that the olive extracts tested are not genotoxic at high doses in vivo, and any genotoxic risks for human consumers are negligible.

MBAND analysis of early and late damages in the chromosome of human lymphocytes after exposures to gamma rays and Fe ions

Stable-type chromosome aberrations that survive multiple generations of cell division include translocation and inversions. An efficient method to detect an inversion is multi-color banding fluorescent in situ hybridization (mBAND) which allows identification of both inter- and intra-chromosome aberrations simultaneously. Post irradiation, chromosome aberrations may also arise after multiple cell divisions as a result of genomic instability. This study was aimed at investigating stable or late-arising chromosome aberrations in human lymphocytes induced after low- and high-LET radiation exposure.Human lymphocytes were exposed in vitro to gamma ray doses of 2 or 4 Gy using a 137Cs source at a dose rate of 0.5 Gy/min. For high LET radiation, cells were exposed to Fe ions (600 MeV/nucleon) of 0.05, 0.5 or 1 Gy at NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory (BNL). Chromosomes were collected at 48 h which represented the first mitosis, and 7 and 14 days after irradiation using a premature chromosome condensation (PCC) technique as described previously [ 1]. Chromosome 3 was painted with the XCyte3 mBAND kit (MetaSystems), and intra- and inter-chromosomal aberrations were analyzed with the mBAND analysis system (MetaSystems).With gamma irradiation, about half of the damages observed at first mitosis remained after 7- and 14-day culture, suggesting transmissibility of damages to the surviving progeny. With Fe ions irradiation, at the doses that produced similar frequencies of gamma-induced chromosome aberrations as observed at first mitosis, a significantly lower yield of aberrations remained at the same population doublings after Fe ion exposure. At these equitoxic doses, more complex-type aberrations were observed for Fe ions, indicating that Fe ion-induced initial chromosome damages are more severe and may lead to cell death.Comparison of the number of breaks indicates that Fe ions produced three or more breaks in ∼20% of the damaged chromosome 3, whereas ∼10% or a less fraction of cells with three or more breaks in chromosome 3 were found after gamma ray exposures. Unlike gamma rays, increasing doses of Fe ions did not produce more breaks in a damaged chromosome 3, indicating that damages from Fe ions exposures were mostly due to the single track effect, in agreement with the results of Hada et al. [ 1, 2].Increasing doses of exposure resulted in less fraction of chromosome break ends involved in intra-chromosomal exchange induced by either gamma rays or Fe ions in the first mitosis.Interestingly, simple inversions in chromosome 3 were found in only the 7 and 14-day samples after 4 Gy gamma irradiation. It is not clear whether cells containing simple inversions had already progressed through the first cell division at 48 h, or the simple inversions were induced after the first cell division.Detailed analysis of breaks participating in total chromosome exchanges within the first cell cycle post-irradiation revealed a common hot spot located in the 3p21 region, which is a known fragile site corresponding to Band 6 in the mBand analysis [ 2]. The breakpoint distribution in chromosomes collected at 7 days, but not at 14 days, post-irradiation appeared similar to the distribution in cells collected within the first cell cycle post-irradiation. The breakpoint distribution for human lymphocytes after radiation exposure was different from the previously published distribution for human mammary epithelial cells [ 2], indicating that interphase chromatin folding structures play a role in the distribution of radiation-induced breaks.

Chromosomal aberrations in human lymphocytes and fibroblasts after exposure to very low doses of high-LET radiation

Purpose: The relationship between biological effects and low doses of radiation is still uncertain, especially for high-LET radiation exposures. Estimates of risk from exposure to low doses and low dose rates are often extrapolated from the Japanese atomic bomb survivor data using either linear or linear-quadratic models fitted to dose–response data. In this study, we determined the dose–response for chromosome damage after exposure to very low doses of high-LET radiation and assessed the radiation qualities of Fe, Si and Oxygen ions.Materials and methods: Chromosomal aberrations (CA) were measured in human peripheral blood lymphocytes and normal skin fibroblasts after exposure to very low doses (0.01–0.20 Gy) of 77-MeV/u oxygen (LET = 55 keV/µm), 170-MeV/u 28Si (LET = 99 keV/µm), or 56Fe ions with energies of 600- or 450-MeV/u (LET = 180 or 195 keV/µm). These exposures included doses that, on average, produce fewer than one in five direct ion traversals per cell nucleus. Chromosomes were analyzed using the whole-chromosome fluorescence in situ hybridization (FISH) technique during the first cell division after irradiation, and CA were identified as either simple exchanges (translocations and dicentrics) or complex exchanges (involving more than two breaks in two or more chromosomes). The frequencies of CA in the painted chromosome(s) were evaluated as the ratio between aberrations scored and total cells analyzed. The dose–response for simple exchanges was assessed using a generalized linear model assuming binomial errors per number of chromosomes scored. The model coefficients were extrapolated to whole-genome equivalents. The linear dose–response denoted as the targete effects (TE) model considered the mean number of radiation tracks per cell. Two different non-targeted effect (NTE) models, P = P0 + αT + κ × I (NTE1), and P = P0 + αT (1 − e−T) + κe−T × I (NTE2), were compared with the simple linear model, P = P0 + αT. Akaike information criteria (AIC) and Bayes information criteria (BIC) were used to compare TE and NTE models for fitting chromosome aberrations in low dose range.Results: Doses that on average produce more than one ion traversal per cell nucleus showed a linear dose–response for CA in both lymphocytes and fibroblasts. However, for doses that produce fewer than one tracks per cell in fibroblasts, O, Si and Fe particles showed a dose-independent response for CA that was significantly elevated relative to background frequencies. For fibroblasts the NTE model 2, P = P0 + αT (1 − e−T) + κe−T × I, showed improved fit to CA in low dose range compared with TE model or NTE1 model. For lymphocytes, tests of the various models were less clear with TE model optimal for Si and Fe while the NTE2 model optimal for O particles. When low-dose exposures were fractionated with 2-h intervals, increased frequencies of both simple and complex exchanges were observed. Nitric oxide scavenger reduced CA induced by low doses of high-LET irradiation. Inhibition of transforming growth factor-β receptor-1 reduced the frequency of simple exchanges.Conclusions: The results show a non-linear dose–response for CA in fibroblasts after very low doses of high-LET exposure. Possible explanations for this could involve non-targeted effects due to aberrant cell signaling [ 1], perhaps involving nitric oxide and TGF-β, or could be due to delta-ray dose fluctuations [ 2] where CA are induced in cells that receive a significant dose from delta-rays emanating from the multiple ion tracks that do not directly traverse cell nuclei.