Dicentric Aberrations Research Articles

Spectrum of chromosomal aberrations in peripheral lymphocytes of hospital workers occupationally exposed to low doses of ionizing radiation

Chromosome aberrations frequency was estimated in peripheral lymphocytes from hospital workers occupationally exposed to low levels of ionizing radiation and controls. Chromosome aberrations yield was analyzed by considering the effects of dose equivalent of ionizing radiation over time, and of confounding factors, such as age, gender and smoking status. Frequencies of aberrant cells and chromosome breaks were higher in exposed workers than in controls ( P=0.007, and P=0.001, respectively). Seven dicentric aberrations were detected in the exposed group and only three in controls, but the mean frequencies were not significantly different. The dose equivalent to whole body of ionizing radiation (Hwb) did appear to influence the spectrum of chromosomal aberrations when the exposed workers were subdivided by a cut off at 50 mSv. The frequencies of chromosome breaks in both subgroups of workers were significantly higher than in controls (≤50 mSv, P=0.041; >50 mSv, P=0.018). On the other hand, the frequency of chromatid breaks observed in workers with Hwb >50 mSv was significantly higher than in controls ( P=0.015) or workers with Hwb ≤50 mSv ( P=0.046). Regarding the influence of confounding factors on genetic damage, smoking status and female gender seem to influence the increase in chromosome aberration frequencies in the study population. Overall, these results suggested that chromosome breaks might provide a good marker for assessing genetic damage in populations exposed to low levels of ionizing radiation.

Chromosomal aberration analysis in chronically exposed radiation workers.

We performed a chromosomal aberration analysis on blood samples of 15 radiation workers by scoring dicentric aberrations. These workers were chronically exposed to cumulative doses of approximately 500 mSv over a period of two to three decades. The biological doses estimated using the linear coefficient of the in vitro dose/response curve based on dicentric frequency varied from 0 to 259 mGy, even though all the radiation workers had received approximately the same physical dose--i.e., 500 mSv. In all cases of chronic exposure, the estimated biological doses were found to be lower than the measured physical doses. The measured physical doses were corrected by applying the biphasic decay pattern of lymphocytes and also taking into consideration the time course of accumulation of doses in each individual. The corrected physical doses thus obtained were then compared with the estimated biological doses, and a reasonably good correlation was found between these two sets of values. However, on the whole, the corrected physical doses were found to be smaller than the estimated biological doses in most of the cases. This observation suggests that the kinetics of turnover of lymphocytes in conditions of chronic exposure may be slower than estimated so far in various studies involving exposures to high doses, wherein the estimation of life span of lymphocytes was carried out in patients undergoing radiotherapy.

Chromosome aberrations induced in human lymphocytes by heavy charged particles in track segment mode.

Human blood was irradiated with accelerated ions: 20 MeV 4He, 425 MeV 12C and 1480 MeV and 996 MeV 16O. For each ion, the blood was exposed to a range of doses as thin specimens in the track segment mode, so that irradiations took place at nearly constant LETs of 31.4, 61, 52 and 69 keV microm(-1), respectively. Lymphocytes were cultured to the first in vitro metaphase, analysed for chromosomal damage and the dicentric aberration frequencies fitted to the linear quadratic model of dose-response. For these high LET radiations, the linear (alpha) yield coefficient predominated and increased with LET, at least up to 60 keV microm(-1). Apart from the 996 MeV oxygen ions, the data indicated the presence of a quadratic (beta) coefficient, statistically consistent with values obtained with low LET radiations. However, the associated uncertainties on the measured beta values were large, illustrating the general problem that beta is more difficult to measure against a dominating and ever-increasing alpha term. The existence or otherwise of a beta component of the dose-response at these radiation qualities has important consequences for modelling mechanisms of aberration induction by radiation.

Biologically based risk estimation for radiation-induced CML. Inferences from BCR and ABL geometric distributions.

Chronic myeloid leukemia (CML) invites biologically based radiation risk modeling because CML is simultaneously well-understood, homogeneous and prevalent. CML is known to be caused by a translocation involving the ABL and BCR genes, almost all CML patients have the BCR-ABL translocation, and CML is prevalent enough that its induction is unequivocally detected among Hiroshima A-bomb survivors. In a previous paper, a linear-quadratic-exponential (LQE) dose-response model was used to estimate the lifetime excess risk of CML in the limit of low doses of gamma-rays, R gamma. This estimate assumed that BCR-ABL translocation dose-response curves in stem cells for both neutrons and gamma-rays, differ only by a common proportionality constant from dicentric aberration dose-response curves in lymphocytes. In the present paper we challenge this assumption by predicting the BCR-ABL dose response. The predictions are based on the biophysical theory of dual radiation action (TDRA) as it applies to recent BCR-to-ABL distance data in G0 human lymphocytes; this data shows BCR and ABL geometric distributions that are not uniform and not independent, with close association of the two genes in some cells. The analysis speaks against the previous proportionality assumption. We compute 11 plausible LQE estimates of R gamma, 2 based on the proportionality assumption and 9 based on TDRA predictions. For each estimate of R gamma we also compute an associated estimate of the number of CML target cells, N; the biological basis of the LQE model allows us to form such estimates. Consistency between N and hematological considerations provides a plausibility check of the risk estimates. Within the group of estimates investigated, the most plausible lifetime excess risk estimates tend to lie near R gamma = 0.01 Gy-1, substantially higher than risk estimates based on the proportionality assumption.

Dose Estimates made by Dicentric Analysis for Some Belarussian Children Irradiated by the Chernobyl Accident

The results are presented of chromosomal aberration analyses in blood lymphocytes sampled soon after the Chernobyl accident from children resident in and near Bragin, evacuees from the 30 km exclusion zone and a control group in Minsk. Children from the contaminated areas showed significantly elevated levels of dicentric aberrations. Average estimates of whole-body doses have been made by reference to an in vitro gamma dose effect curve. They were 230 mGy for Bragin children, and 400 mGy for evacuees and both are substantially higher than those derived from physical reconstructions. The difference may be due to their early incorporation of short-lived radionuclides.

How radiation-specific is the dicentric assay?

Quantitative cytogenetic analysis of structural chromosome aberrations in human peripheral blood lymphocytes is widely used as an assay to detect and quantify exposure to a variety of clastogens. Unstable chromosome rearrangements, dicentric chromosomes (dic) and centric ring chromosomes (centric r), are routinely used as biomarkers to assess human exposure to ionizing radiation ("biological dosimetry"). In the moderate- to high-dose range many authors consider the dic assay sufficiently radiation-specific for both practical and legal purposes. However, specificity has never been evaluated in quantitative terms. The high sensitivity of the assay would in principle allow for applications in the range of low-dose exposure which has been a major concern in epidemiologic studies. Validity of the assay then critically depends on specificity. A mathematical model is proposed which includes as parameters the decline of dicentric aberrations in vivo, the linear dose coefficient for the induction of dic and the average total radiation exposure of the population. To assess radiation-specificity of the dic assay the equilibrium dicentric rate due to radiation is compared to measurements of the background rate of dic in unexposed controls. Environmental and medical radiation combined account for at least about 80% of the average background level of dicentrics. It is concluded that the dicentric assay is highly specific for ionizing radiation and can therefore be used to assess prior exposure in the dose range of interest in environmental epidemiology.

Issues in Cytogenetic Biological Dosimetry: Emphasis on Radiation Environments in Space

Issues central to the reliability of cytogenetic biodosimetry are presented. Although it now appears that cytogenetic biodosimetry can be used reliably to reconstruct radiation dose after acute, uniform whole-body exposure (albeit within certain dose ranges), additional data are required to fully validate cytogenetic biodosimetry for the protracted and complex exposure conditions in space. Approaches are presented that could be used to obtain the data necessary for validation. It also appears that the use of dicentric aberrations for biodosimetry on missions lasting several months or more may not be reliable because of the large variability observed among individuals in the rate of loss of cells with dicentrics, making back-extrapolations uncertain. This may be testable, however, by comparing the results for dicentrics and translocations obtained by biodosimetry with the radiation dosimetry obtained on board the spacecraft. In addition to these and several other issues, estimates are provided of the current limitations of detection of dicentrics and reciprocal translocations.

Mechanisms of radiation-induced chromosome aberrations

We investigated the radioprotective efficacy of quercetin (QN), a naturally occurring flavonoid against gamma radiation-induced damage in human peripheral blood lymphocytes and plasmid DNA. In plasmid study, QN at different concentrations (3, 6, 12, 24 and 48 μM) were pre-incubated with plasmid DNA for 1 h followed by exposure of 6 Gy radiation. Among all concentrations of QN used, 24 μM showed optimum radioprotective potential. To establish the most effective protective concentration of QN in lymphocytes, the cells were pre-incubated with 3, 6, 12, 24 and 48 μM of QN for 30 min and then exposed to 4 Gy γ-radiation. The concentration-dependent effects of QN were evaluated by scoring micronuclei (MN) frequencies. The results showed that QN decreased the MN frequencies dose dependently, but the effect was more pronounced at 24 μM. Thus, 24 μM of QN was selected as the optimum concentration and was further used to evaluate its radioprotective effect in lymphocytes. For that a separate experiment was carried out, in which lymphocytes were incubated with QN (24 μM) for 30 min and exposed to different doses of radiation (1, 2, 3 and 4 Gy). Genetic damage (MN, dicentric aberration and comet attributes) and biochemical changes were measured to evaluate the effect of QN on γ-radiations (1–4 Gy). Radiation exposed showed significant increases in the genetic damage and thiobarbituric acid reactive substances (TBARS) accompanied by a significant decrease in the antioxidant status. QN pretreatment significantly decreased the genetic damage and TBARS and improved antioxidant status through its antioxidant potential. Altogether, our findings encourage further mechanistic and in vivo studies to investigate radioprotective efficacy of QN.

Dose-Effect Relationship for In Vivo and In Vitro Induction of Dicentric Aberrations in Blood Lymphocytes of Children

Chromosome aberrations induced in vivo were studied in nine children 5-12 years old treated with total-body high-energy photon irradiation (pulsed exposure from a LINAC) for different types of malignant diseases. Dose-effect relationships were obtained for each child by taking blood at different times during exposure. In vitro dose-effect relationships for chromosome aberrations in children and adults were obtained by exposing blood under the same conditions as the children. Exposure in vivo and in vitro yielded similar linear-quadratic dose-effect relationships for dicentric aberrations. The response in vitro was slightly greater than in vivo, but the difference was not very large. It is concluded that the dose-effect relationship for dicentric chromosome aberrations obtained in vitro for adults can be used for biological dosimetry in irradiated children. Some of the children displayed a high number of "rogue cells" before exposure; this may be due to the malignant disease as it was not found in the healthy controls.

Effect of LET on chromosomal aberration yields. I. Do long-lived, exchange-prone double strand breaks play a role?

Dicentric chromosomal aberrations produced by ionizing radiation probably result from pairwise interaction of DNA double strand breaks (dsbs). It has been suggested that high LET radiation may preferentially produce a subclass of 'severe' dsbs that are long-lived and/or exchange-prone, and that it is the production of these severe dsbs which account for the increased biological effectiveness of high-LET radiation. We present a quantitative formalism to describe the induction of these severe dsbs, and the subsequent production of exchange-type chromosomal aberrations. Using a Markov model and microdosimetric methods, we conclude that dicentric production by such severe dsbs has properties similar to those observed at high LET. Specifically, at high doses, the yield is nearly linear with dose even if dsbs from different tracks interact. The model is applied to published data on dicentric aberrations produced by irradiation of human lymphocytes in vitro. Corrections for the effects of interphase death are estimated. From comparisons with the experiments we conclude that interaction of severe dsbs could make a significant contribution to the observed dicentric production at high LET and also perhaps for low doses (though not high doses) at low LET. Proximity explanations of high-LET effects continue to offer the main prospect for obtaining a unified picture of chromosomal aberration formation by all ionizing radiation types, but a hybrid model in which severe dsbs contribute to the high-LET aberration yield cannot be ruled out. If all or part of high-LET radiation damage is qualitatively different from low-LET radiation damage, as this severe dsb model may suggest, there could be far-reaching implications for the field of radiation protection.

Chromosome aberrations induced in human lymphocytes by an X-radiation accident: results of a 4-year postirradiation analysis.

After occupational and medical radiation exposures structural chromosome aberrations may be induced in the lymphocytes of the irradiated persons. Many authors have estimated the radiation dose from the yields of dicentric aberrations. We analysed the influence on the dicentric yield of increasing time intervals between irradiation and blood sampling from a person involved in an X-radiation accident (radiography). During a 4-year follow-up we observed an approximately 7-fold decline of the dicentric yield up to the 25th month and thereafter an almost constant value. Since the decline did not commence until around the 10th month after the exposure, an exponential decrease of dicentric yield with time should be considered with reservation in this study, although it cannot be entirely ruled out. We conclude that in 'biological dosimetry' blood should be sampled as early as possible after the exposure. Furthermore, computation of 'half-times' of lymphocytes to allow for a delay in blood sampling seems uncertain after partial body exposures to high doses. Therefore in such cases dose estimates obtained 1 or more years after irradiation should be considered as minimum values.

Cytogenetic damage induced in human lymphocytes by low doses of 60Co gamma rays delivered at high and low dose rates.

Chromosomal aberrations were investigated in human peripheral blood lymphocytes after exposure to low doses of 60Co gamma rays delivered acutely or at low dose rates (0.1 or 0.03 Gy/h). Chromosome analysis was performed in cells collected after a 44 to 46 hour culture time in order to avoid scoring of second division cells, and the dose-related induction of aberrations was analysed by the maximum likelihood method. In all cases, the induction of dicentrics was well described by a linear-quadratic dose response model (Y = aD + bD2), the data obtained at a low dose rate being equally well fitted to a linear equation. According to earlier findings on the mechanisms of aberration formation, the two lesions originating from single ionizing tracks have to be produced within a period of approximately 5 hours, in order to interact and to give rise to dicentric aberrations, which could explain the decrease in the quadratic term at the low dose rate since the highest doses were delivered over a period of more than 5 hours.

Interpretation of Dose-Reponse Curves for Cell Killing and Induction of Chromosome Aberrations Using Calkins' Enzyme-Kinetic Model

Experimental data on Chinese hamster (V79) cells are presented which show that the number of dicentric aberrations produced by gamma radiation is reduced if the cells are held in stationary phase for 6 h after irradiation, before plating. This result suggests that misrepair does not take place at the same time as correct repair, as is assumed in the “lethal-potentially lethal” model, but that it takes place at the end of the repair period, and involves lesions which the enzyme system has not been able to repair within the time available. We investigate the use of Calkins' enzyme-kinetic model of radiation response and show that it can account for the shapes of low LET survival and dicentric aberration response curves, and for the repair of potentially lethal and sub-lethal damage. It predicts that repairable damage contributes to the inital slope of a dose-response curve.

Distribution of the various radiation-induced chromosomal rearrangements in relation to the dose and sampling time

The quantitative analysis of the chromosome rearrangements detected in 2128 R-bandedn metaphases, obtained from γ-irradiatedd human lymphocytes after 48 to 95 h in culture is reported. Depending on the culture time, and possibly on the dose of radiation (from 1 to 3 Gy), the frequent type of rearrangement was either dicentries or reciprocal translocations. In first generationd mitoses, the frequency of cells without rearrangement rangedd from 0.66 to 0.18, andd the meand number of rearrangedd chromosomes per cell from 0.79 to 3.28. The dose-response curve follows a quadratic function for dicentric aberration yields, but not for other rearrangements.

On the Microdosimetric Definition of Quality Factors

A formulation of the concept of quality factor based on the microdosimetric quantity lineal energy, y, is described. Toward this end, functions--denoted Specific Quality Functions (SQF)--are defined such that (a) they can be determined directly from observation and (b) a number of them can be used toward setting, by consensus, the values of a microdosimetrically-based quality factor. The determination of SQFs is exemplified by correlating data on the yields of dicentric aberrations in human lymphocytes with measured and calculated microdosimetric distributions. The implications of this approach to problems of radiation protection are discussed.

Induction of Chromosome Aberrations in G 0 Human Lymphocytes by Low Doses of Ionizing Radiations of Different Quality

Human peripheral blood lymphocytes from two donors were exposed to low doses (0.05 to 2.0 Gy) of gamma rays, X rays, or fast neutrons of different energies. Chromosome aberrations were analyzed in metaphase of first-division cells after a culture time of 45-46 hr. At this time, less than 5% of the cells were found in second division. Different dose-response relationships were fitted to the data by using a maximum likelihood method; best fits for radiation-induced dicentric aberrations were obtained with the linear-quadratic law for all radiations. The linear component of this equation predominated, however, for neutrons in the range of doses studied, and the frequency of dicentrics induced by d(16)+Be neutrons up to 1.0 Gy could also be described by a linear relationship. The relative biological efficiency (RBE) of X rays and d(16)+Be, d(33)+Be, and d(50)+Be neutrons compared to 60Co gamma rays in the low dose range was calculated from the dose-effect relationships for the dicentrics produced. The RBE increased with decreasing neutron dose and with decreasing neutron energy from d(50)+Be to d(16)-+Be neutrons. The limiting RBE at low doses (RBEo) was calculated to be about 1.5 for X rays and 14.0, 6.2, and 4.7 for the d(16)+Be, d(33)+Be, and d(50)+Be neutrons, respectively.

Kinetics of the effect of ara C on chromosome aberration yield in irradiated human lymphocytes.

In unstimulated lymphocytes the enhancing effect of ara C on the yield of X-ray-induced dicentric aberrations was maximal if ara C was added immediately or up to 2 h after irradiation. When ara C was added later the enhancing effect decreased and practically vanished by 5 h. In stimulated lymphocytes the ara C effect declined faster and practically vanished by 3 h. If ara C was added for 1 h immediately after irradiation, then the aberration yield observed in G0, early G1 and late G1 cells was similar whereas it increased significantly from G0 to late G1 cells without ara C post-treatment. The results are discussed in relation to the classical model of aberration formation.

Chromosome Aberrations Induced in Human Lymphocytes by D-T Neutrons

Unstable chromosome aberrations induced by in vitro irradiation with D-T neutrons have been analyzed in human blood lymphocytes. With respect to 250 kVp X rays a maximum limiting RBE at low doses of 4.1 was obtained for dicentric aberrations. Using aberrations as markers in mixed cultures of irradiated and unirradiated cells permits an assessment of interphase death plus mitotic delay. The low-dose RBE for this effect is 2.5. Assuming all unstable aberrations observed at metaphase would lead to cell death by nondisjunction allows an assessment of mitotic death. The low-dose RBE for this effect is 4.5. The data are compared with similar work obtained earlier with 242Cm alpha particles. The application of the present work to cytogenetic assessment of dose after accidental exposure to D-T neutrons is discussed.

Bleomycin-induced chromosomal aberrations in Down's syndrome lymphocytes

Blood samples from 4 Down's syndrome (DS) patients with a 47,XY,21 + karyotype and from 4 normal male probands were cultured for 72 h in the presence of BrdU and lymphocytes analysed at their first mitosis for chromosomal aberrations. The frequencies of spontaneous aberrations and the proportions of cells in the first or later mitoses in culture were not different between the groups. Treatment with various doses of bleomycin in vitro resulted in similar delays in cell development for both DS and normal lymphocytes and dose-dependent increases in the incidence of chromosome-type aberrations. However, the induction of both dicentric aberrations and acentric fragments was significantly enhanced in DS cells relative to cells of normal karyotype.

The incidence of unstable chromosome aberrations in peripheral blood lymphocytes from unirradiated and occupationally exposed people

Peripheral blood lymphocytes from unirradiated control subjects and workers exposed within permitted limits to γ-radiation, have been examined for the incidence of dicentric and acentric chromosome aberrations. The results are compared with a review of data published elsewhere. Background levels show inter-laboratory variation and possible reasons for this are discussed. By combining the present data with those from the literature the spontaneous incidence of dicentric aberrations is approx. 0.55 × 10 −3 and for acentrics is 3.7 × 10 −3. In occupationally exposed subjects a significantly higher incidence of aberrations was found. When allowance was made for the turnover of lymphocytes for the period over which each man had worked with radiation a linear dose-effect relationship was apparent. The incidence of dicentrics was 2.22 ± 0.94 × 10 −4 rad −1 and for all unstable aberrations 8.24 ± 2.8 × 10 −4 rad −1. These are in reasonable agreement with dose-response data obtained in vitro.