DNA damage from oxidants: influence of lesion complexity and chromatin organization

DNA damage by reactive oxygen species results in a spectrum of DNA lesions including single-strand breaks (ssb) and double-strand breaks (dsb). However, most damage is not lethal, and the location and nature of the DNA damage, in addition to total number of breaks, are likely to be critical in determining ultimate survival. Generally associated only with ionizing radiation, multiply damaged sites (i.e., complex lesions and clusters of complex lesions in DNA) are more likely to be lethal because they are less easily repaired. We examined five drugs known to cause DNA adducts, strand breaks, and reactive oxygen species for their ability to produce complex lesions: 4-nitroquinoline-1-oxide (4NQO), H2O2, doxorubicin, Tirapazamine, and etoposide. As indicators of lesion complexity we compared 1) the ratio of ssb to dsb, 2) the rate of rejoining of single-strand breaks, 3) the relative lethality of the breaks (number of breaks per mean lethal dose), and 4) the ability to produce complex lesions. Tirapazamine, etoposide, and doxorubicin gave dsb/ssb ratios similar to that for X-rays, whereas 4NQO and H2O2 showed dsb/ssb ratios of 200 and 3250, respectively. The number of dsb per LD50 varied from 2.5 to 500 for different drugs. There was no apparent relation between ssb rejoining half-time (3.5-85 min) and relative lethality or lesion complexity. A modified (nonionic detergent) filter elution method confirmed that tirapazamine, like ionizing radiation, produced multiple dsb within single chromatin domains. These data indicate that complex lesions can be produced by a number of different chemicals and suggest that the damage that results in killing by these drugs may be related to production of multiply damaged sites in DNA.     

Rejoining of DNA single- and double-strand breaks in human white blood cells exposed to ionizing radiation

PURPOSE: To characterize inter- and intra-individual differences in X-ray-induced DNA strand break rejoining kinetics in human peripheral white blood cells (WBC) obtained from 10 healthy volunteers. MATERIALS AND METHODS: The alkaline and neutral versions of the comet assay were used to measure the rate of rejoining of predominantly single-strand breaks (ssb) following exposure to 8 Gy and double-strand breaks (dsb) following 75 Gy. RESULTS: All cells within a population responded in a similar fashion to induction of ssb and dsb; however, a subset of the WBC appeared to rejoin ssb more rapidly. For the 10 individuals examined, the percentage of ssb rejoined by the rapid component(s) was 47 +/- 16% and the rejoining half-time for the slow component was 1.3 +/- 0.4 h. By 24 h after 8 Gy, 4.9 +/- 3.8% of the initial ssb remained. For dsb rejoining, 58 +/- 11% of the initial damage was still present 4h after 75 Gy and by 24 h 32% of the initial level of damage was still detected. Heavily damaged cells present 24 h after 75 Gy varied from 4% to 50% and were excluded from the analysis of repair rates. CONCLUSIONS: Inter-individual variability exceeded intra-individual variability for 2 of 4 endpoints examined for ssb repair, but not for dsb repair. It was concluded that DNA damage measured using the comet assay could identify a range in the X-ray repair responses of WBC from different normal individuals. Whether these differences correlate with differences in cell killing by radiation remains to be determined.     

The study of responses to 'model' DNA breaks induced by restriction endonucleases in cells and cell-free systems: achievements and difficulties

The use of restriction endonucleases (RE) as a means of implicating DNA double-strand breaks (dsb) in cellular responses is reviewed. The introduction of RE into cells leads to many of the responses known to be characteristic of radiation damage--cell killing, chromosomal aberration, oncogenic transformation, gene mutation and amplification. Additionally, radiosensitive cell lines are hypersensitive to RE, including those from the human disorder ataxia-telangiectasia. However, quantitation of response and comparisons of the effectiveness of different RE are difficult, partly because of unknown activity and lifetime of RE in the cell. RE-induced dsb have also been used to reveal molecular mechanisms of repair and misrepair at specific sites in DNA. Dsb have been implicated in recombination processes including those leading to illegitimate rejoining (formation of deletions and rearrangements) at short sequence features in DNA. Also model dsb act as a signal to activate other cellular processes, which may influence or indirectly cause some responses, including cell death. In these signalling responses the detailed chemistry at the break site may not be very important, perhaps explaining why there is considerable overlap in responses to RE and to ionizing radiations.     

The use of Rasch analysis to produce scale-free measurement of functional ability

PURPOSE: The aim of the work was to compare critically the radiosensitivity of the supercoiled and relaxed forms of a plasmid DNA system commonly used in DNA damage assays. MATERIALS AND METHODS: The yields of single- (ssb) and double-strand breaks (dsb) in pBR322 DNA over a range of scavenging capacities were measured in the presence of Tris after irradiation with a single pulse of 400 keV electrons. The response was compared with DNA that had been given a preliminary dose of gamma-rays such that an average of one ssb per molecule was present. RESULTS: The yields of dsb were found to be enhanced in the pre-irradiated DNA when Tris was present during irradiation at concentrations varying between 10 and 100 mmol dm(-3) with a maximal enhancement ratio of 1.6 at 80 mmol x dm(-3) Tris. This increased yield was not observed when both the pre-irradiation and the experimental doses were given by gamma-irradiation. The increased response was not found in DNA that had been enzymatically relaxed by the introduction of a nick in each molecule using the enzyme gpII. CONCLUSIONS: The observations suggest that the conformation per se does not influence the radiosensitivity. The enhanced yields observed in the gamma-ray pre-irradiated DNA with pulsed irradiation appear to be a consequence of the high dose rate used. A proposed explanation is that the combination of high dose levels (< or =1500 Gy) and short irradiation times (approximately 5 ns) would allow some 'spurs' to overlap and it is proposed that the enhanced yields of dsb are related to this.     

The intraoperative administration of ketorolac tromethamine in evaluating length of stay in a same day surgery unit

Relative biological effectiveness (RBE), as a function of linear energy transfer (LET), is evaluated for different types of damage contributing to mammalian cell reproductive death. Survival curves are analysed assuming a linear-quadratic dose dependence of lethal lesions. The linear term represents lethal damage due to single particle tracks, the quadratic term represents lethality due to interaction of lesions from independent tracks. RBE-LET relationships of single-track lethal damage, sublethal damage, potentially lethal damage and DNA double-strand breaks (dsb) are compared. Single-track lethal damage is shown to be composed of two components: damage that remains unrepaired in an interval between irradiation and assay, characterized by a very strong dependence on LET, with RBEs up to 20, and potentially lethal damage, which is weakly dependent on LET with RBEs < 3. Potentially lethal damage and sublethal damage depend similarly on LET as DNA dsb. The identification of these different components of damage leads to an interpretation of differences in radiosensitivity and in RBEs among various types of cells.     

Association of thymoma and severe intestinal strongyloidiasis

The G2 chromosomal radiosensitivity of murine SCID (severe combined immunodeficient) and normal fibroblasts has been investigated. We have also investigated the G2 response of these cell lines to the restriction endonuclease PvuII. We show that chromatid breaks are induced linearly with radiation dose in both cell lines and SCID cells are approximately 1.6 times as radiosensitive as normal murine fibroblasts when tested using a G2 assay with a 2 h sampling time. The disappearance of chromatid breaks with time after irradiation was first order with a half-time of approximately 1.5 h in both cell lines. Thus, although SCID cells are deficient in the rejoining of double-strand breaks (dsb), they show similar kinetics of disappearance of chromatid breaks with time as normal CB17 cells, indicating that the 'rejoining' of chromatid breaks does not reflect dsb repair. When CB17 and SCID cells were treated with PvuII, which generates dsb in cellular DNA in the presence of streptolysin O (as a porating agent), approximately 3 times more chromatid breaks were observed in SCID than CB17 cells. We conclude that SCID cells convert a higher number of dsb into chromatid breaks than do CB17 cells. The conversion process is interpreted in terms of the recently proposed 'signal' model, whereby a signal, resulting from a single dsb, triggers the cell to make a recombinational exchange which, if incomplete, gives rise to a visible chromatid break. In terms of the signal model, elevated conversion of dsb into chromatid breaks results from altered signalling and the disappearance of chromatid breaks with time following irradiation represents the completion of recombinational exchanges rather than repair of dsb.     

Non-random distribution of DNA double-strand breaks induced by particle irradiation

Induction of DNA double-strand breaks (dsbs) in mammalian cells is dependent on the spatial distribution of energy deposition from the ionizing radiation. For high LET particle radiations the primary ionization sites occur in a correlated manner along the track of the particles, while for X-rays these sites are much more randomly distributed throughout the volume of the cell. It can therefore be expected that the distribution of dsbs linearly along the DNA molecule also varies with the type of radiation and the ionization density. Using pulsed-field gel and conventional gel techniques, we measured the size distribution of DNA molecules from irradiated human fibroblasts in the total range of 0.1 kbp-10 Mbp for X-rays and high LET particles (N ions, 97 keV/microns and Fe ions, 150 keV/microns). On a mega base pair scale we applied conventional pulsed-field gel electrophoresis techniques such as measurement of the fraction of DNA released from the well (FAR) and measurement of breakage within a specific NotI restriction fragment (hybridization assay). The induction rate for widely spaced breaks was found to decrease with LET. However, when the entire distribution of radiation-induced fragments was analysed, we detected an excess of fragments with sizes below about 200 kbp for the particles compared with X-irradiation. X-rays are thus more effective than high LET radiations in producing large DNA fragments but less effective in the production of smaller fragments. We determined the total induction rate of dsbs for the three radiations based on a quantitative analysis of all the measured radiation-induced fragments and found that the high LET particles were more efficient than X-rays at inducing dsbs, indicating an increasing total efficiency with LET. Conventional assays that are based only on the measurement of large fragments are therefore misleading when determining total dsb induction rates of high LET particles. The possible biological significance of this non-randomness for dsb induction is discussed.     

Guanine is the target for direct ionisation damage in DNA, as detected using excision enzymes

Exposure of an aqueous, aerated solution (pH 7) of a double-stranded DNA to 193 nm light, of sufficient energy to ionise DNA, leads to selective, non-random modification at guanine in the form of frank single-strand break (ssb) and base modifications, revealed by treatment with either Escherichia coli formamidopyrimidine-DNA glycosylase (Fpg), Escherichia coli endonuclease III (Nth) or hot piperidine treatment. There is a similar neighbouring base sequence dependence for Fpg- and Nth-sensitive damage as that previously reported for both hot alkali-labile damage and prompt ssb. Low yields of photoproducts, namely pyrimidine dimers, are also revealed using the enzyme T4 endonuclease V (T4 endo V). Although irradiation of DNA with 193 nm light causes photoionisation of all the nucleic acid bases, these results indicate that guanine is the predominant site for localisation of the oxidative damage. These findings are consistent with migration of the radical cation to 'target' damage at guanine sites.     

Efficacy of novel antimicrobials against clinical isolates of opportunistic amebas

Using atomic force microscopy (AFM), we have investigated neutron-induced DNA double-strand breaks in plasmids in aqueous solution. AFM permits direct measurement of individual DNA molecules with an accuracy of a few nanometers. Furthermore, the analysis of the DNA fragment size distribution is non-parametric, whereas other methods are dependent on the model. Neutron irradiation of DNA results in the generation of many short fragments, an observation not made for damage induced by low-LET radiation. These data provide clear experimental evidence for the existence of clustered DNA double-strand breaks and demonstrate that short DNA fragments may be produced by such radiations in the absence of a nucleosomal DNA structure.     

DNA double-strand breaks measured by pulsed-field gel electrophoresis in irradiated lymphocytes from normal humans and those with Alzheimer's disease

We have previously found that radiation-induced chromosome aberrations (dicentrics) are more numerous in lymphocytes from Alzheimer's disease (AD) patients than in those from age-matched normal individuals (Tobi et al. 1990). To investigate this further, we have examined double-strand breaks (dsb) produced by gamma-irradiation in the DNA of AD and normal lymphocytes by using pulsed-field gel electrophoresis. The percentage of DNA migrating into the gels is an indirect measure of the number of dsb; we have assayed the DNA content of sequential slices of the gel by direct fluorometry and have found that the percentage migrating is dose dependent. Our results show that the level of damage is similar in AD and normal lymphocytes and preliminary assays of the rate of repair suggest that the half-time is also similar, the value being > 1 h. The latter is consistent with the known rate of rejoining of chromosome fragments in interphase lymphocytes (Pantelias and Maillie 1985). The results suggest that at a gross level dsb repair is not impaired in AD cells; however, we cannot exclude the possibility that there is misrepair or non-repair of a small fraction of the dsb, which might account for the greater radiosensitivity of the AD cells.     

Measurement of DNA strand breakage and DNA repair induced with hydrogen peroxide using single cell gel electrophoresis, alkaline DNA unwinding and alkaline elution of DNA

Three techniques: single cell gel electrophoresis (SCGE), alkaline elution of DNA (AE), and alkaline DNA unwinding (ADU) were chosen to compare the sensitivity among these methods in detection of DNA damage and repair in human diploid VH10 cell line after short-term exposure to hydrogen peroxide. Using SCGE technique a dose-dependent increase in DNA migration was found in cells exposed to hydrogen peroxide in concentration range from 10 micromol/l to 100 micromol/l. Alkaline DNA unwinding method detected increased level of single strand breaks (ssb) in concentration range from 25 micromol/l to 100 micromol/l of H2O2, and alkaline elution of DNA estimated increased DNA elution rate from concentration 50 micromol/l of H2O2. In a time course study to evaluate the kinetics of DNA repair, both SCGE and ADU techniques showed that the repair of DNA strand breaks is very rapid; the level of ssb in treated cells has returned to near the background level within two hours. After this time damage remaining in the DNA was in the form of oxidised bases as revealed the incubation of treated cells with specific DNA repair endonuclease, formamidopyrimidine-DNA glycosylase.     

Weiss Lecture. Effects of radiations of different qualities on cells: molecular mechanisms of damage and repair

Studies of ionizing radiations of different quality are discussed with particular emphasis on damage to DNA of mammalian cells. Three related themes are followed. Firstly, inactivation and mutation experiments with ultrasoft X-rays and slow heavy ions, coupled with theoretical analyses of the structures of the radiation tracks, have emphasized the biological importance of localized track features over nanometre dimensions. This led to the suggestion that the critical physical features of the tracks are the stochastic clusterings of ionizations, directly in or very near to DNA, resulting in clustered initial molecular damage including various combinations of breaks, base damages, cross-links, etc. in the DNA. The quantitative hypotheses imply that final cellular effects from high-LET radiations are dominated by their more severe, and therefore less repairable, clustered damage, and that these are qualitatively different from the dominant low-LET damage. Second, relative effectiveness of different types of radiation led to questions on the mechanisms of induction of chromosome exchanges. The high efficiency of ultrasoft X-rays, despite their very short track lengths, suggested that single sites of DNA damage may lead to exchanges by a molecular process involving interaction with undamaged DNA. Also it is shown that a single site-specific DNA break, introduced by restriction enzymes, sometimes leads to a large deletion when misrepaired by cell extracts. These deletions occur between short DNA repeats, and are therefore a form of 'illegitimate' recombination, but clearly do not involve the interaction of two damage sites. Third, it was shown that cells from patients with the radiosensitive disorder ataxia-telangiectasia (AT) lack a post-irradiation recovery process. The sensitivity of AT cells to high LET radiations was found to be reduced relative to that for normal cells, reinforcing the concept that high LET damage is less easy to repair. AT patients are prone to lymphoreticular cancers, and their cells show characteristic chromosomal rearrangements, which may be associated with misrepair at specific genomic sequences. Similarly, studies of radiation-induced leukaemia in the mouse have implicated rearrangement at specific interstitial chromosome sites, which are rich in telomere-like repeat sequences.     

Role of auxiliary diagnostic tests in the clarification of the etiology of syncope: experience at an arrhythmia center

PURPOSE: A brief review is presented of the basic concepts in track structure and the relative merit of various theoretical approaches adopted in Monte-Carlo track-structure codes are examined. In the second part of the paper, a formal cluster analysis is introduced to calculate cluster-distance distributions. METHOD: Total experimental ionization cross-sections were least-square fitted and compared with the calculation by various theoretical methods. Monte-Carlo track-structure code Kurbuc was used to examine and compare the spectrum of the secondary electrons generated by using functions given by Born-Bethe, Jain-Khare, Gryzinsky, Kim-Rudd, Mott and Vriens' theories. The cluster analysis in track structure was carried out using the k-means method and Hartigan algorithm. RESULTS: Data are presented on experimental and calculated total ionization cross-sections: inverse mean free path (IMFP) as a function of electron energy used in Monte-Carlo track-structure codes; the spectrum of secondary electrons generated by different functions for 500 eV primary electrons; cluster analysis for 4 MeV and 20 MeV alpha-particles in terms of the frequency of total cluster energy to the root-mean-square (rms) radius of the cluster and differential distance distributions for a pair of clusters; and finally relative frequency distribution for energy deposited in DNA, single-strand break and double-strand breaks for 10MeV/u protons, alpha-particles and carbon ions. CONCLUSIONS: There are a number of Monte-Carlo track-structure codes that have been developed independently and the bench-marking presented in this paper allows a better choice of the theoretical method adopted in a track-structure code to be made. A systematic bench-marking of cross-sections and spectra of the secondary electrons shows differences between the codes at atomic level, but such differences are not significant in biophysical modelling at the macromolecular level. Clustered-damage evaluation shows: that a substantial proportion of dose ( 30%) is deposited by low-energy electrons; the majority of DNA damage lesions are of simple type; the complexity of damage increases with increased LET, while the total yield of strand breaks remains constant; and at high LET values nearly 70% of all double-strand breaks are of complex type.     

DNA-PK-independent rejoining of DNA double-strand breaks in human cell extracts in vitro

PURPOSE: To investigate the role of DNA-dependent protein kinase (DNA-PK) in the rejoining of ionizing radiation-induced DNA double-strand breaks (dsb). MATERIALS AND METHODS: This study employed previously described in vitro assays that utilize nuclei or 'naked' DNA prepared from agarose-embedded cells as a substrate and S-HeLa cell extracts as a source of enzymes. Rejoining of dsb in these assays is absolutely dependent on cell extract and it proceeds, under optimal reaction conditions, to an extent similar to that observed in intact cells. Results were confirmed in a plasmid-based assay for in vitro rejoining of dsb. RESULTS: It is shown that concentrations of wortmannin completely inhibiting DNA-PK activity profoundly affect the rejoining of dsb in vivo, but have no effect on dsb rejoining in vitro. Furthermore, fractionation of cell extracts using ammonium sulphate precipitation, generates protein fractions that are able to support dsb rejoining, despite the fact that they do not contain detectable amounts of either DNA-PKcs or Ku80. Efficient rejoining of dsb in vitro is also observed with extracts of MO59J cells that lack DNA-PK activity. Finally, rejoining of dsb remains unaffected by wortmannin in a plasmid-based assay, and is also detectable with extracts of MO59J cells. CONCLUSIONS: These findings are in contrast with genetic studies demonstrating a requirement for DNA-PK activity for efficient rejoining of dsb in vivo. The difference between in vitro and in vivo results may not be attributed to chromatin structure since wortmannin was without an effect when using nuclei as a substrate. It is speculated that the differences between in vivo and in vitro results can be explained either by assuming the operation of multiple pathways in dsb rejoining, some of which do not require DNA-PK, or by postulating a purely regulatory/damage-sensing role for DNA-PK in intact cells but no direct involvement in dsb rejoining.     

An exercise on the feasibility of carrying out secondary economic analyses

The prevailing hypothesis on the mechanism of radiation-induced cell killing identifies the genetic material deoxyribonucleic acid (DNA) as the most important subcellular target at biologically relevant doses. In this review we present new data and summarize the role of the DNA double-strand breaks (dsb) induced by ionizing radiation and DNA dsb rejoining as determinants of cellular radiosensitivity. When cells were irradiated at high dose-rate, two molecular end-points were identified which often correlated with radiosensitivity: (1) the apparent number of DNA dsb induced per Gy per DNA unit and (2) the half-time of the fast component of the DNA dsb rejoining kinetics. These two molecular determinants, not mutually exclusive, may be linked through a common factor such as the conformation of DNA.     

Comparison of bond strengths of three denture base resins to treated nickel-chromium-beryllium alloy

The success of radiotherapy in eradicating tumours depends on the total radiation dose, but what limits this dose is the tolerance of the normal tissues within the treatment volume. Studies involving fibroblast survival have demonstrated the theoretical feasibility of a predictive assay of radiation sensitivity, but such an assay is still far from clinical application. Using pulsed-field gel electrophoresis (PFGE), we have quantified the initial "apparent" number of DNA double-strand breaks (dsb) induced by the radiation as an alternative measure of sensitivity in 2 different normal cell types from the same patients, epidermal skin cells and lymphocytes. We found significant inter-individual variation in the measured dsb (1-5 dsb/Gy/DNA unit). We also found a linear correlation between molecular damage in lymphocytes and skin samples from the same patient (slope = 0.83; r = 0.694; p = 0.0001). These results suggest that the initial number of dsb could be used as an indicator of the in vivo response to radiation.     

Ku protein stimulates DNA end joining by mammalian DNA ligases: a direct role for Ku in repair of DNA double-strand breaks

Ku protein binds to DNA ends and is a cofactor for the DNA-dependent protein kinase. Both of these components are involved in DNA double-strand break repair, but it has not been clear if they function indirectly, by sensing DNA damage and activating other factors, or if they are more directly involved in the processing and rejoining of DNA breaks. We demonstrate that intermolecular ligation of DNA fragments is highly dependent on Ku under conditions designed to mimic those existing in the cell. This effect of Ku is specific to eukaryotic DNA ligases. Ku protein, therefore, has an activity consistent with a direct role in rejoining DNA breaks and independent of DNA-dependent protein kinase.     

Response of V79 cells to low doses of X-rays and negative pi-mesons: clonogenic survival and DNA strand breaks

Mammalian cells are hypersensitive to very low doses of X-rays (< 0.2 Gy), a response which is followed by increased radioresistance up to 1 Gy. Increased radioresistance is postulated to be a response to DNA damage, possibly single-strand breaks, and it appears to be a characteristic of low linear energy transfer (LET) radiation. Here we demonstrate a correspondence between the extent of the increased radioresistance and linear energy transfer of 250 kVp X-rays and plateau and Bragg peak negative pi-mesons. The results support our hypothesis since the size of the increased radioresistant response appears to correspond to the number of radiation induced single-strand breaks. Furthermore, since survival prior to the increased radioresistant response (< 0.2 Gy) was LET-independent, these data support the notion that the increased radioresistant response may dictate the overall survival response to higher doses. However, while these data provide further circumstantial evidence for the involvement of DNA strand breaks in the triggering of increased radioresistance, more direct conclusions cannot be made. The data are not accurate enough to detect structure in the single-strand break profiles, the production of single-strand breaks being apparently linear with dose.     

Effect of 60 Hz magnetic field exposure on c-fos expression in stimulated PC12 cells

Chromatid breaks are thought to result from DNA double-strand breaks (dsb) but the mechanisms are not yet understood. The early (but still prevailing) 'breakage-first' hypothesis fails to explain the large size of chromatid breaks; many of which are estimated to represent the apparent loss of between 15 and 45 Mbp (up to 30% of an average chromatid). The alternative 'exchange' hypothesis of Revell has potential for explaining the large sizes of deletions, but assumes the interaction of two lesions which therefore predicts a quadratic dependence of chromatid breaks on radiation dose. The exchange hypothesis is not tenable for mammalian cells since chromatid breaks are observed to be induced linearly with dose in both human and rodent cells. An alternative 'signal' model of chromatid breaks is outlined whereby a single dsb, occurring within a large looped chromatin domain, is signalled (possibly by molecules such as DNAPK or ATM protein) and triggers the cell to undergo a recombinational exchange, either within a chromatid or between sister chromatids. If incomplete, such recombinational exchanges would appear as chromatid breaks at metaphase. It is suggested that the large looped chromatin domains could be equivalent to one or more likely several replication 'factories' in which the DNA processing enzymes required for exchange formation would be located.     

Modern clinical testing strategies in cobalamin and folate deficiency

PURPOSE: To investigate the mechanisms underlying the induction of chromosome aberrations by ionizing radiation, focusing attention on DNA damage severity, interphase chromosome geometry and the distribution of DNA strand breaks. METHODS: An ab initio biophysical model of aberration induction in human lymphocytes specific for light ions was developed, based on the assumption that 'complex lesions' (clustered DNA breaks) produce aberrations, whereas less severe breaks are repaired. It was assumed that interphase chromosomes are spatially localized and that chromosome break free-ends rejoin pairwise randomly; the unrejoining of a certain fraction of free-ends was assumed to be possible, and small fragments were neglected in order to reproduce experimental conditions. The yield of different aberrations was calculated and compared with some data obtained using Giemsa or FISH techniques. RESULTS: Dose-response curves for dicentrics and centric rings (Giemsa) and for reciprocal, complex and incomplete exchanges (FISH) were simulated; the ratio between complex and reciprocal exchanges was also calculated as a function of particle type and LET. The results showed agreement with data from lymphocyte irradiation with light ions. CONCLUSIONS: The results suggest that clustered DNA breaks are a critical damage type for aberration induction and that interphase chromosome localization plays an important role. Moreover, the effect of a given particle type is related both to the number of induced complex lesions and to their spatial distribution.