A.A. Edwards

The Relationship between Chromosome Aberrations and Low LET Radiation Dose to Human Lymphocytes

In vitro dose-response curves of unstable chromosome aberrations in human lymphocytes have been obtained for 250 kV X-rays and cobalt-60gamma-radiation. The aberration yields have been fitted to the quadratic function Y = alphaD +betaD2, which is consistent with the single-track and two-track model for aberration formation. The values of the coefficients alpha and beta support the hypothesis that the dose-rate effect is limited to the D2 term. The main difference between the coefficients for X- and gamma-radiation is in the alpha values, indicating that X-rays are slightly more efficient, at lower doses, in producing two lesions with a single ionizing track. The lower limits of dose estimate, with 500 cells analysed, are 4 rad for X-rays and 10 rad gamma-radiation. Further evidence is presented confirming that, for cytogenetic dosimetry, in vitro dose-response curves should be prepared by irradiating whole blood maintained at 37 degrees C and prior to PHA stimulation. Curves were plotted showing the variation of the number of cells without aberrations with radiation dose and the shape of these curves were compared with those from human cell survival experiments.

Radiation induced chromosome aberrations and the Poisson distribution.

SummaryData on the distribution of dicentrics and acentrics observed when human lymphocytes are cultured for 48 h after irradiation by X-rays,γ-rays, and neutrons are presented. Analysis shows that for dicentrics, the observed distribution for X-rays,γ-rays, and fission neutrons may be described by Poisson statistics but for higher energy neutrons overdispersion is observed. The phenomenon of overdispersion is also observed for acentrics irrespective of the radiation used. The possibility that overdispersion results from the variations of dose in sensitive sites leads to the conclusion that for dicentrics the site size is considerably larger than the 1–2 µm diameter derived by applying the dual action theory to the dose effect relationships. This larger site may well be the cell nucleus.

Radiation induction of micronuclei in human lymphocytes

An examination of the cytokinesis-blocked micronucleus technique confirmed its potential usefulness as a method of biological dosimetry for radiation accidents. Several advantages and disadvantages of the system are discussed. It has been demonstrated that under the conditions of these experiments, the blocking agent, cytochalasin B does not induce micronuclei or unstable chromosome aberrations. The induction of sister-chromatid exchanges proved just significant. Analysis of the dose response for 250 kVp X-rays indicates that although the Y = alpha D + beta D2 model fits the data, the relationship does not correspond to that for total aberration induction as might have been expected. The background frequency of micronuclei and the value of the alpha coefficient are higher than for total aberrations and the beta term is lower. This indicates that simple incorporation of acentric chromosome fragments into micronuclei may not wholly account for the phenomenon.

The role of cytogenetics in early triage of radiation casualties.

Preliminary dose estimates by chromosomal analysis can be made rapidly in order to supplement early triage of radiation casualties based on clinical signs. An in vitro simulation of an accident with many casualties receiving whole or partial body exposure in the range 0-8 Gy is described. Faced with an urgent need for rapid results, confirmation of clinical triage can generally be obtained from scoring 20 metaphases per subject. Scoring should be increased to 50 cells where there is disagreement with the initial assessments or evidence of significantly inhomogeneous exposure.

Fluorescence in Situ Hybridization Detection of Chromosomal Aberrations in Human Lymphocytes: Applicability to Biological Dosimetry

Human lymphocytes in G0 have been irradiated with X-ray doses from 0 to 4.0 Gy. Metaphase chromosomes 2, 3 and 5 and all centromeres were painted using fluorescence in situ hybridization (FISH) probe libraries. Dicentrics, centric rings and acentrics in the whole genome as well as translocations involving the painted chromosomes were recorded. The translocations were subdivided as complete or incomplete. Interstitial insertions and inversions were also noted. The observations were also recorded according to the Protocol for Aberration Identification and Nomenclature Terminology (PAINT) system of scoring. Given that the painted chromosomes comprise 20.4% of the genome it was found that the yield of bicoloured dicentrics was consistent with the yield of dicentrics in the whole genome. The yield of radiation-induced translocations was not significantly higher than that of bicoloured dicentrics. Of the translocations, 60% were complete and it was concluded that the majority of dicentrics and translocations are complete exchanges. Chromosome 5 took part in exchanges marginally more commonly than its length suggests, but it is not known if this is a property of the chromosome or whether it is a donor-dependent observation. The PAINT system of recording rearrangements was examined and the suggested numerical interpretation of this nomenclature was considered to be unsuitable for use in the estimation of dose for cases of accidental overexposure.

The Induction of Chromosome Aberrations in Human Lymphocytes by Alpha-radiation

Human blood has been irradiated with alpha-particles from an external source of curium-242. The collimated alpha-particles entered the blood with an energy of 4-9 MeV and were almost completely absorbed by the blood. After culturing for 48 hours, the dicentric yield in the lymphocytes at the first metaphase was measured as a function of dose to the blood. The yield was linear with dose up to 400 rad with a slope of 28x6 X 10(4) dicentrics/cell per rad. This is equivalent to an initial slope r.b.e. of 17x9 with respect to cobalt-60 gamma-rays. This value disagrees with the only two other published values in the literature. Reasons for this disagreement are discussed. Compared with neutron r.b.e values obtained in this laboratory the alpha-particle values we observe are surprisingly low. A model is proposed which predicts low values of r.b.e. for chromosome aberration production using radiations of high LET. The low values occur because there is a distribution of specific energy between cells which causes a selective removal of cells likely to contain higher numbers of aberrations.

Chromosome Aberrations Induced in Human Lymphocytes by Neutron Irradiation

In vitro dose--response curves of unstable chromosome aberrations in human lymphocytes have been obtained for neutron spectra of mean energies 0-7, 0-9, 7-6 and 14-7 MeV. The aberration yields have been fitted to the quadratic function Y = alphaD + betaD2, which is consistent with the single-track and two-track model of aberration formation. However with high-LET radiation, the linear component of yield, corresponding to damage caused by single tracks, predominants, and this term becomes more dominant with increasing LET, so that for fission spectrum neutrons the relationship is linear, Y = alphaD. At low doses, such as those recieved by radiation workers, limiting r.b.e. values between 13 and 47 are obtained relative to 60Co gamma-radiation. At higher doses, as used in radiotherapy, the values are much lower; ranging from 2-7 to 8 at 200 rad of equivalent gamma-radiation. Both sets of r.b.e. values correlate well with track-averaged LET but not with dose-averaged LET. When the numbers of cells without aberrations are plotted against radiation dose, curves are obtained which are similar in shape to those for conventional cell-survival experiments with comparable neutron spectra. The Do values obtained in the present study are close to those from other cell system.

International study of factors affecting human chromosome translocations

Chromosome translocations in peripheral blood lymphocytes of normal, healthy humans increase with age, but the effects of gender, race, and cigarette smoking on background translocation yields have not been examined systematically. Further, the shape of the relationship between age and translocation frequency (TF) has not been definitively determined. We collected existing data from 16 laboratories in North America, Europe, and Asia on TFs measured in peripheral blood lymphocytes by fluorescence in situ hybridization whole chromosome painting among 1933 individuals. In Poisson regression models, age, ranging from newborns (cord blood) to 85 years, was strongly associated with TF and this relationship showed significant upward curvature at older ages versus a linear relationship (p<0.001). Ever smokers had significantly higher TFs than non-smokers (rate ratio (RR)=1.19, 95% confidence interval (CI), 1.09-1.30) and smoking modified the effect of age on TFs with a steeper age-related increase among ever smokers compared to non-smokers (p<0.001). TFs did not differ by gender. Interpreting an independent effect of race was difficult owing to laboratory variation. Our study is three times larger than any pooled effort to date, confirming a suspected curvilinear relationship of TF with age. The significant effect of cigarette smoking has not been observed with previous pooled studies of TF in humans. Our data provide stable estimates of background TF by age, gender, race, and smoking status and suggest an acceleration of chromosome damage above age 60 and among those with a history of smoking cigarettes.

A collaborative exercise on cytogenic dosimetry for simulated whole and partial body accidental irradiation

An experiment sponsored by the International Atomic Energy Agency was undertaken to compare dose estimation by cytogenetic analysis on aliquots of samples of irradiated blood sent by air to participating laboratories. Accidental acute whole-body irradiations to 0.7 and 2.34 Gy and half-body irradiations to 3.5 Gy were simulated with X- and gamma-rays. For the partial irradiations the size of the irradiated fraction and its dose were estimated by the Qdr and contaminated Poisson techniques. Each laboratorys in vitro dose-response data were fitted to the quadratic model by the iteratively reweighted least squares method. Interlaboratory variations in dose-response curves, and in the aberration yields and dose estimates for the simulated accidents were noted. However, in general, most participants consistently obtained results acceptably close to the true values.

The Induction of Chromosome Aberrations in Human Lymphocytes by in Vitro Irradiation with α-particles from Plutonium-239

The yields of unstable chromosome aberrations induced in human lymphocytes by alpha-particles from plutonium-239 have been measured. Plutonium citrate solution was mixed with heparinized blood so that doses of 13--160 rad were delivered in 24 hours. Dicentric aberration yields (Y) fitted best to the linear expression Y = 3 . 72 +/- 0 . 23 x 10(-3) rad-1. Inclusion of a 6 . 5 rad point resulting from a 1 . 7 hour irradiation raised the yield coefficient insignificantly to 3 . 75 +/- 0 . 24 x 10(-3). The aberration yields are in good agreement with data from curium-242 alpha-particles obtained in this laboratory but they are much lower than those obtained in two other laboratories. Reasons for this disagreement are examined.