A model of chromosome aberration induction and chronic myeloid leukaemia incidence at low doses

Some chromosome aberration types, generally translocations, are correlated with specific cancers. An example is provided by chronic myeloid leukemia (CML) cells, most of which carry a translocation involving the ABL gene on chromosome 9 and the BCR gene on chromosome 22. The hypothesis of a causal relationship between CML and the chimeric protein product of the BCR-ABL translocation has recently received strong support. In this framework, a mechanistic model and Monte-Carlo code simulating radiation-induced chromosome aberrations in human lymphocytes will be presented. The current version of the model can predict dose-response curves for the main aberration types following acute irradiation with gamma rays and light ions of different energies. The model is based on the assumption that only clustered DNA lesions can lead to aberrations and that only lesion free ends in neighbouring chromosome territories can join and form exchanges. Such lesions are distributed within the cell nucleus according to the radiation track structure, i.e. randomly for low-LET radiation and along straight lines for high-LET light ions. Interphase chromosome territories are explicitly simulated and background aberrations are taken into account. Very good agreement was found with experimental data taken from the literature that provided a further validation of the model. As an application, yields of BCR-ABL translocations were calculated. Preliminary results led to a CML induction dose-response that is approximately quadratic below 0.1 Gy and essentially linear at higher doses up to 1 Gy. The numerical values obtained for the probability of CML induction are consistent with values obtained by other groups with different approaches.Dedicated to Herwig Paretzke on the occasion of his 60th birthday.     

Cell type-specific quantitative predictions of radiation-induced chromosome aberrations: a computer model approach

A quantitative computer model was applied to simulate the three-dimensional (3D) spatial organization of chromatin in human cell nuclei under defined conditions of virtual irradiation to explore the implications of spatial organization on chromosome aberrations. To calibrate the virtual irradiation algorithm, a dose-dependent spectrum of radiation-induced chromosome aberrations such as dicentrics, translocations and centric rings was calculated for low-LET radiation doses ranging from 0.5 to 5 Gy. This was compared with the results from experimental studies. While the dose-response curves calculated from model simulations agree well with experimental dose-response curves for dicentrics and translocations, centric rings are significantly more frequent in the model simulation than in experiments despite taking into account exclusive arm territories in the applied Spherical 1 Mbp Chromatin Domain (SCD) computer model explicitly. Taking into account the non-random positioning of chromosome territories observed in lymphocyte cell nuclei (a so-called gene density-correlated arrangement of chromosome territories), aberration frequencies were calculated with the calibrated irradiation algorithm to investigate the impact of chromosome territory neighborhood effects (proximity effects). The absolute frequencies of pairwise exchanges agree well with those found in an experimental study. In conclusion, the results obtained using the computer model approach presented here based on only a few adjustable parameters correlated well with those of experimental studies of chromosome aberration frequencies. Thus the model may be a useful tool in radiation-induced cancer risk estimates in combination with epidemiological studies.     

Mechanistic modelling allows to assess pathways of DNA lesion interactions underlying chromosome aberration formation

The knowledge of radiation-induced chromosomal aberration (CA) mechanisms is required in many fields of radiation genetics, radiation biology, biodosimetry, etc. However, these mechanisms are yet to be quantitatively characterised. One of the reasons is that the relationships between primary lesions of DNA/chromatin/chromosomes and dose-response curves for CA are unknown because the pathways of lesion interactions in an interphase nucleus are currently inaccessible for direct experimental observation. This article aims for the comparative analysis of two principally different scenarios of formation of simple and complex interchromosomal exchange aberrations: by lesion interactions at chromosome territories?? surface vs. in the whole space of the nucleus. The analysis was based on quantitative mechanistic modelling of different levels of structures and processes involved in CA formation: chromosome structure in an interphase nucleus, induction, repair and interactions of DNA lesions. It was shown that the restricted diffusion of chromosomal loci, predicted by computational modelling of chromosome organization, results in lesion interactions in the whole space of the nucleus being impossible. At the same time, predicted features of subchromosomal dynamics agrees well with in vivo observations and does not contradict the mechanism of CA formation at the surface of chromosome territories. On the other hand, the ??surface mechanism?? of CA formation, despite having certain qualities, proved to be insufficient to explain high frequency of complex exchange aberrations observed by mFISH technique. The alternative mechanism, CA formation on nuclear centres is expected to be sufficient to explain frequent complex exchanges.     

Chromosome aberration frequencies produced by a 70-MeV proton beam

The effectiveness of a 70-MeV proton beam in the induction of chromosome aberrations was studied. We employed peripheral lymphocytes and analyzed the frequencies of dicentrics and rings after irradiation at doses ranging from 0.1 to 8.0 Gy at various depths within a Lucite phantom. The frequency of chromosome aberrations after irradiation with an unmodulated proton beam at 5 mm showed a dose-response relationship similar to that of 60Co gamma rays. However, irradiation at greater depths with the spread-out Bragg peak induced higher aberration frequencies at doses lower than those with gamma rays. Furthermore, the distribution curve of chromosome aberration frequencies as a function of depth was found to be slightly different from the physically measured depth-dose curve. With the spread-out Bragg peak the biological effects were more marked at greater depths, resulting in a distribution of relative biological effectiveness values. The results obtained from chromosome aberration analysis may not be related directly to those for the relationship between dose and cell killing. Slight differences in values for relative biological effectiveness due to the change of dose and site of proton beam irradiation may not be important for practical proton beam therapy, but may be important in the prevention of late radiation injuries.     

Biological effectiveness of accelerated particles for the induction of chromosome damage measured in metaphase and interphase human lymphocytes

Chromosome aberrations were investigated in human lymphocytes after in vitro exposure to 1H-, 3He-, 12C-, 40Ar-, 28Si-, 56Fe-, or 197Au-ion beams, with LET ranging from approximately 0.4-1393 keV/microm in the dose range of 0.075-3 Gy. Dose-response curves for chromosome exchanges, measured at the first mitosis postirradiation using fluorescence in situ hybridization (FISH) with whole-chromosome probes, were fitted with linear or linear-quadratic functions. The relative biological effectiveness (RBE) was estimated from the initial slope of the dose-response curve for chromosomal damage with respect to low- or high-dose-rate gamma rays. Estimates of RBEmax values for mitotic spreads, which ranged from near 0.7 to 11.1 for total exchanges, increased with LET, reaching a maximum at about 150 keV/microm, and decreased with further increase in LET. RBEs for complex aberrations are undefined due to the lack of an initial slope for gamma rays. Additionally, the effect of mitotic delay on RBE values was investigated by measuring chromosome aberrations in interphase after chemically induced premature chromosome condensation (PCC), and values were up to threefold higher than for metaphase analysis.     

Studies on radiation-induced chromosome aberrations in mouse spermatocytes. I. Stage specificity and dose-response relationships of chromosome aberrations induced in mouse primary spermatocytes following X-irradiation

The cytological analysis of chromosome aberrations induced at diplotene, mid-pachytene, zygotene and leptotene stages following X-irradiation was performed at diakinesis-metaphase I in mouse spermatocytes. The dose-response relationships fitted well to linear equations for deletion-type aberrations at each stage, and to linear-quadratic equations for exchange-type aberrations at all stages except for leptotene. The radiosensitivity to chromosome aberration induction tended to increase gradually with progression through synaptic and post-synaptic stages, diplotene being the most sensitive. Chromatid exchanges were hardly observed at leptotene, the aberrations being mainly isochromatid fragments. On the contrary, chromatid exchanges and isochromatid deletions were mainly observed at later stages (zygotene-diplotene). The specificity of chromosome aberration induction in primary spermatocytes might be influenced by chromatin organization and chromosomal configuration peculiar to meiotic cells.     

~(60)Coγ线引起上海真厉螨染色体畸变的研究

本文首次报道用~(6O)Coγ线照射一种革螨——上海真厉螨引起的染色体畸变的研究。用~(6O)Coγ线(剂量1—50Krad)照射雌性革螨,引起的染色体畸变类型有:染色体裂隙、断片、微小体、环形染色体、粉碎化和多倍体,染色体断片是最常见的畸变类型,并观察到微核的形成。染色体畸变率随照射剂量增加而增高,辐射剂量与畸变率之间存在密切相关(相关系数为0.85,P<0.025),配得曲线回归方程为Y=3.27+14.49lg(X+1)。     

The effect of functional inactivation of TP53 by HPV-E6 transformation on the induction of chromosome aberrations by gamma rays in human tumor cells

The influence of expression of TP53 (formerly known as p53) on the induction of chromosome aberrations by gamma rays was examined in an isogenic pair of human tumor cell lines where TP53 expression was normal or inactivated by human papillomavirus (HPV) type 16 E6 expression. Plateau-phase cultures were exposed to 0-8 Gy gamma rays and then either immediately released by subculture or held for 24 h prior to subculture and subsequent cytogenetic analysis. Aberration frequency was determined only in cells entering their first mitosis after irradiation, and cells were sampled over a 48-h period to include cells whose progression into mitosis was delayed. While aberration frequencies were similar at early harvest times, there was evidence for a subpopulation of more heavily damaged cells in the E6-transformed cells that cycled into late mitosis. Holding cells noncycling for 24 h to allow repair of potentially lethal damage eliminated this subpopulation of more heavily damaged cells. The E6-transformed cells also had higher levels of chromatid-type aberrations and sister chromatid exchanges, consistent with an additional defect in kinetics of repair of base damage that is associated with the E6 transformation. Holding cells noncycling for 24 h eliminated the elevated levels of chromatid-type aberrations and sister chromatid exchanges. These studies demonstrate that E6 transformation of human tumor cells will influence both the frequency and types of chromosome aberrations observed after radiation exposure, and that these effects are related to the expression of potentially lethal damage.     

The database for analysis of quantitative characteristics of chromosome aberration frequencies in the culture of human peripheral blood lymphocytes]

The data on spontaneous chromosome aberration rates in cultures of human peripheral blood lymphocytes obtained in the past 30 years have been collected to form a database. The database contains the results of analysis of more than 330,000 metaphases in lymphocytes from more than 1200 subjects. The frequency of aberrant metaphases in the control group has been estimated at 0.0213 +/- 0.00085. No differences between sexes have been found with respect to either the total chromosome aberration rate or the rates of individual aberration types. The total chromosome aberration rate did not depend on age; however, it has been found that the number of fragments increased and the number of exchanges decreased with age. Smoking has been found to increase the frequency of chromosome aberrations in individuals with occupational hazards, but not in those who are not occupationally exposed to radiation or chemicals. Alcohol consumption increased the frequency of paired fragments, whereas the frequencies of other aberrations did not differ from the control values.     

Distribution pattern and dose-response-relationship of chromosome aberrations in human lymphocytes induced in vitro by 60Co gamma-rays and 110 kV X-rays.

Peripheral blood lymphocyte culture system was used to construct reference dose-response curves for 60Co gamma-rays and 110 kV X-ray-induced chromosome aberrations at 6 dose points ranging from 0.25 to 4.0 Gy. Qualitative and quantitative differences between these two types of radiation for the yield of induced aberrations and their distribution pattern were analysed. Experimental data of aberration yields were compared after fitting them to five different dose-response models. The yields of chromosome aberrations in particular dicentrics, gave a good fit to linear-quadratic besides linear and power models. In this model, single-track events predominated over double-track events for both the qualities of radiation used. The pattern of distribution was mainly Poisson for dicentrics but gave a conflicting result for acentrics which was in excess.     

The Database for Analysis of Quantitative Characteristics of Chromosome Aberration Frequencies in the Culture of Human Peripheral Blood Lymphocytes

The data on spontaneous chromosome aberration rates in cultures of human peripheral blood lymphocytes obtained in the past 30 years have been collected to form a database. The database contains the results of analysis of more than 330 000 metaphases in lymphocytes from more than 1200 subjects. The frequency of aberrant metaphases in the control group has been estimated at 0.0213 ± 0.00085. No differences between sexes have been found with respect to either the total chromosome aberration rate or the rates of individual aberration types. The total chromosome aberration rate did not depend on age; however, it has been found that the number of fragments increased and the number of exchanges decreased with age. Smoking has been found to increase the frequency of chromosome aberrations in individuals with occupational hazards, but not in those who are not occupationally exposed to radiation or chemicals. Alcohol consumption increased the frequency of paired fragments, whereas the frequencies of other aberrations did not differ from the control values.     

Cell killing and chromatid damage in primary human bronchial epithelial cells irradiated with accelerated 56Fe ions

We examined cell killing and chromatid damage in primary human bronchial epithelial cells irradiated with high-energy 56Fe ions. Cells were irradiated with graded doses of 56Fe ions (1 GeV/nucleon) accelerated with the Alternating Gradient Synchrotron at Brookhaven National Laboratory. The survival curves for cells plated 1 h after irradiation (immediate plating) showed little or no shoulder. However, the survival curves for cells plated 24 h after irradiation (delayed plating) had a small initial shoulder. The RBE for 56Fe ions compared to 137Cs gamma rays was 1.99 for immediate plating and 2.73 for delayed plating at the D10. The repair ratio (delayed plating/immediate plating) was 1.67 for 137Cs gamma rays and 1.22 for 56Fe ions. The dose-response curves for initially measured and residual chromatid fragments detected by the Calyculin A-mediated premature chromosome condensation technique showed a linear response. The results indicated that the induction frequency for initially measured fragments was the same for 137Cs gamma rays and 56Fe ions. On the other hand, approximately 85% of the fragments induced by 137Cs gamma rays had rejoined after 24 h of postirradiation incubation; the corresponding amount for 56Fe ions was 37%. Furthermore, the frequency of chromatid exchanges induced by gamma rays measured 24 h after irradiation was higher than that induced by 56Fe ions. No difference in the amount of chromatid damage induced by the two types of radiations was detected when assayed 1 h after irradiation. The results suggest that high-energy 56Fe ions induce a higher frequency of complex, unrepairable damage at both the cellular and chromosomal levels than 137Cs gamma rays in the target cells for radiation-induced lung cancers.     

Telomere staining of human chromosomes and the mechanism of radiation-induced dicentric formation

The majority of models of radiation action developed over the past half century hold that the curvilinear dose responses exhibited by eukaryotic cells to sparsely ionizing radiations result from the interaction of pairs of lesions produced in sensitive targets of the cell. Within this conceptual framework, chromosomal exchange aberrations (e.g., interchanges) are believed to occur through the interaction of damaged sites on both chromosomes participating in the exchange. In contrast, the model proposed by Chadwick and Leenhouts (as well as some other models) suggests that such exchanges arise from initial radiation damage to only one chromosome, which then becomes associated with an undamaged chromosome. A particular aspect of this theory is that asymmetrical exchanges, such as dicentrics, may be formed from the rejoining of a broken end of one chromosome to the telomere of another. By using a DNA probe that specifically hybridizes to the telomeric region of human chromosomes, we were able to test this assertion directly. After scanning more than 200 dicentrics produced in normal human fibroblasts by 6 Gy of 60Co gamma rays, virtually none were found that contained telomeres located between the centromeres of this aberration type. Therefore, since the proposed telomere-break rejoining process, per se, is not necessarily a central element of the Chadwick-Leenhouts model, we suggest the theory be modified to exclude this mechanism.     

Studies on radiation-induced chromosome aberrations in mouse spermatocytes. II. Dose-response relationships of chromosome aberrations induced at zygotene stage in mouse primary spermatocytes following fast neutron- and 60Co gamma-irradiations

The chromosome aberrations induced at zygotene stage in mouse spermatocytes following exposures to fast neutrons and 60Co gamma-rays were examined at diakinesis-metaphase I. The dose-response relationships were well fitted to linear equation for deletion-type aberrations and to linear-quadratic equation for exchange-type aberrations in 60Co gamma-irradiation group. In fast neutron-irradiation group, the dose-response relationships were well fitted to linear equations for deletion- and exchange-type aberrations. The rate of deletion-type aberrations was remarkably high for fast neutrons, about 6 times higher than that after 60Co gamma-irradiation. The main types of chromosome aberrations observed were iso-chromatid breaks or fragments and chromatid exchanges in both irradiation groups as well as X-irradiation. These results indicate that there is a possibility that two double-strand breaks are induced simultaneously at iso-locus position in sister chromatids by a single track of radiations. Production of such single-track-induced two double-strand breaks in iso-chromatids may be very frequently expressed as iso-chromatid-type deletions in the high LET fast neutron-irradiation group. On the contrary, in the low LET 60Co gamma- or X-irradiation group, the above-mentioned mechanism may not be so effective for contribution to chromosome aberration induction in mouse spermatocytes. This mechanism was discussed in detail.     

Karyotypes of human lymphocytes exposed to high-energy iron ions

Chromosomal aberrations were analyzed using multicolor fluorescence in situ hybridization (mFISH) in human peripheral blood lymphocytes after in vitro exposure to gamma rays or accelerated (56)Fe ions (1 GeV/nucleon, 145 keV/microm) at Brookhaven National Laboratory (Upton, NY). Doses of 0.3 and 3 Gy were used for both radiation types. Chromosomes were prematurely condensed by a phosphatase inhibitor (calyculin A) to avoid the population selection bias observed at metaphase as a result of the severe cell cycle delays induced by heavy ions. A total of 1053 karyotypes (G(2) and M phases) were analyzed in irradiated lymphocytes. Results revealed different distribution patterns for chromosomal aberrations after low- and high-LET radiation exposures: Heavy ions induced a much higher fraction of cells with multiple aberrations, while the majority of the aberrant cells induced by low doses of gamma rays contained a single aberration. The high fraction of complex-type exchanges after heavy ions leads to an overestimation of simple-type asymmetrical interchanges (dicentrics) from analysis of Giemsa-stained samples. However, even after a dose of 3 Gy iron ions, about 30% of the cells presented no complex-type exchanges. The involvement of individual chromosomes in exchanges was similar for densely and sparsely ionizing radiation, and no statistically significant evidence of a nonrandom involvement of specific chromosomes was detected.     

Chromosome aberrations induced in human lymphocytes by radiation from 252Cf.

In vitro dose--response curves of unstable chromosome aberrations in human lymphocytes have been obtained for 252Cf neutron radiation. The aberration yields fitted best to the linear function Y=aD, which is consistent with the single-track model of aberration formation for high LET radiation. The curves have been compared with others previously produced in this laboratory for several energies of neutrons and for 60Co gamma radiation. The r.b.e. for 252Cf with respect to 60Co is 27 at very low doses, decreasing to 6 at an aberration yield equivalent to 400 rad of 18 rad/hour gamma radiation. A profile of chromosome-aberration induction with depth in a perspex phantom was obtained by placing blood samples at several distances over the range 0.65-2.0 cm from the californium source. This profile was compared with depth-damage calculations for a radium needle. The r.b.e. of 252Cf radiation relative to 226Ra gamma radiation increased with the distance from the source, implying that californium is more effective at greater distances in destroying the ability of cells to divide, which may be an advantage in the treatment of large tumours.     

Stable intrachromosomal biomarkers of past exposure to densely ionizing radiation in several chromosomes of exposed individuals

A multicolor banding (mBAND) fluorescence in situ hybridization technique was used to investigate the presence inhuman populations of a stable biomarker-intrachromosomal chromosome aberrations-of past exposure to high-LET radiation. Peripheral blood lymphocytes were taken from healthy Russian nuclear workers occupationally exposed from 1949 onward to either plutonium, gamma rays or both. Metaphase spreads were produced and chromosomes 1 and 2 were hybridized with mBAND FISH probes and scored for intra-chromosomal aberrations. A large yield of intrachromosomal aberrations was observed in both chromosomes of the individuals exposed to high doses of plutonium, whereas there was no significant increase over the (low) background control rate in the population who were exposed to high doses of gamma rays.Interchromosome aberration yields were similar in both the high plutonium and the high gamma-ray groups. These results for chromosome 1 and 2 confirm and extend data published previously for chromosome 5. Intrachromosomal aberrations thus represent a potential biomarker for past exposure to high-LET radiations such as alpha particles and neutrons and could possibly be used as a biodosimeter to estimate both the dose and type of radiation exposure in previously exposed populations.     

mBAND analysis of chromosomal aberrations in human epithelial cells exposed to low- and high-LET radiation

Energetic heavy ions pose a potential health risk to astronauts who have participated in extended space missions. High-LET radiation is much more effective than low-LET radiation in the induction of biological effects, including cell inactivation, genetic mutations, cataracts and cancer. Most of these biological end points are closely correlated with chromosomal damage, which can be used as a biomarker for radiation damage. Multicolor banding in situ hybridization (mBAND) has proven to be highly useful for the study of intrachromosomal aberrations, which have been suggested as a biomarker of exposure to high-LET radiation. To investigate biological signatures of radiation quality and the complexity of intrachromosomal aberrations, we exposed human epithelial cells in vitro to (137)Cs gamma rays or iron ions (600 MeV/nucleon) and collected chromosomes using a premature chromosome condensation technique. Aberrations in chromosome 3 were analyzed using mBAND probes. The results of our study confirmed the observation of a higher incidence of inversions for high-LET radiation. However, detailed analysis of the inversion type revealed that both iron ions and gamma rays induced a low incidence of simple inversions. Half of the inversions observed in the low-LET-irradiated samples were accompanied by other types of intrachromosome aberrations, but few inversions were accompanied by interchromosome aberrations. In contrast, iron ions induced a significant fraction of inversions that involved complex rearrangements of both inter- and intrachromosome exchanges.     

Radiation-produced chromosome aberrations: colourful clues

Ionizing radiation produces many chromosome aberrations. A rich variety of aberration types can now be seen with the technique of chromosome painting. Apart from being important in medicine and public health, radiation-produced aberrations act as colorful molecular clues to damage-processing mechanisms and, because juxtaposition of different parts of the genome is involved, to interphase nuclear organization. Recent studies using chromosome painting have helped to identify DNA double-strand-break repair and misrepair pathways, to determine the extent of chromosome territories and motions, and to characterize different aberration patterns left behind by different kinds of radiation.