D.G. Papworth

Frequency and distribution studies of asymmetrical versus symmetrical chromosome aberrations

Two aspects of the relationship between Asymmetrical (A) and Symmetrical (S) radiation-induced chromosomal aberrations are considered in this paper. (1) Are A and S truly alternative modes of lesion interactions? Relative frequencies for chromatid-type and chromosome-type are examined, and new lymphocyte data using banding is used to look at this, and also for parallelism in chromosome participation of the two forms for various aberration categories. All the tests applied suggest that A and S are alternative interaction modes. (2) The long-term survival characteristics of A and S are discussed, and the differences in expected frequencies of derived S per surviving cell from chromosome-type and chromatid-types are stressed. Since many in vivo tissues have varying mixtures of potential chromatid and chromosome aberration-bearing target cells, ultimate cell survival and derived S frequencies may differ between tissues for the same absorbed dose. An Appendix gives Relative Corrected Lengths (RCL) for chromosomes of the human karyotype which should be used when testing the various exchange aberration categories for random chromosome participation.

The effect of variable G2 duration upon the interpretation of yield-time curves of radiation-induced chromatid aberrations

Abstract A variable G2 + prophase (G2P) transit time markedly influences the precision with which cells observed in a metaphase sample can be assigned to various positions in the compartment G2P, the profile of the yield time curve for radiation-induced chromatid aberrations, and the form of the curve of frequency of tritiated thymidine labelled metaphases (FLM). In hypothetical populations uncomplicated by extended metaphase durations and radiation induced cell perturbation, precision is poor when the parameters of a log-normal transit-time distribution are comparable to those found in experimental populations. To facilitate calculation, a very simple “all or nothing” radiosensitivity situation is assumed for G2P, namely that observable chromatid aberrations can only be produced in a cell during 10% of its transit of G2P, and not otherwise. When the position of this sensitive region is fixed, any alteration in transit-time distribution parameters modifies the profiles of both the yield-time aberration and FLM curves. Changes in the yield-time curve profile are even more dramatic if the position of the sensitive region in G2P is altered, whilst other parameters are held constant. We conclude that (1) neither the peak aberration yield, nor the integrated area under the yield-time curve are valid measures of radiation effect; (2) the profile of the yield-time curve does not reflect, in any simple manner, the pattern of intrinsic radiosensitivity throughout G2P; (3) the position of any observed aberration peak in relation to the known modal G2P duration is seldom reliable as an indication of the sensitive region location; (4) the true form of the transit-time distribution cannot be derived from the ascending limb of the FLM curve. Consideration is given to real populations where colchicine-extended metaphase, and treatment-induced cell perturbation produce substantial changes in the transit-time distribution, thus aggravating the already complex problem of interpretation.

Distortion hypothesis: An alternative to a limited number of sites for radiation-induced chromosome exchange

Abstract When two or more chromosome exchange events occur within a nucleus, there is a probability that a given chromosome arm will be involved more than once. Since an arm can never be observed at metaphase actually participating in more than one asymmetrical exchange, it follows that an increasing proportion of exchanges fails to be scored as the number of events (a function of radiation dose) rises. By sampling from a theoretical chromosome exchange population, the extent of the interactions between exchanges involving common arms, and their consequences, have been investigated for Campelia zanonia . It is shown that: 1. (1) The distortion introduced into the frequency distribution of exchanges between nuclei by this process is sufficient to account for the form and magnitude of the observed “binomial” distributions of exchange; there is no need to invoke a limited number of “sites” for exchange. 2. (2) The process of distortion leads to modification and eventual saturation of the dose response curve. Methods are given for deriving “distortion coefficients”, which enable the “true” yield of exchanges for a given radiation dose to be calculated. 3. (3) Interaction between exchanges leads to many configurational changes, so that what is observed seldom reflects what has actually taken place. 4. (4) Interaction also produces a marked, non-random loss of centric-rings leading to a spurious dicentric : centric-ring ratio. The implications of these findings upon some radiobiological problems are discussed.

Chromosome aberration yields from multiple fixation regimens.

Methods for obtaining quantitative information on chromosome aberration yields are examined. Single fixation regimens give valid results only for cell populations with a uniform sensitivity. Multiple fixation regimens are needed to sample adequately non-uniform populations. The resulting yield-time curves from such multiple fixation regimens do not directly reflect the profile of sensitivity throughout the cell cycle, and neither the peak aberration yield nor the integrated area under the yield-time curve is a valid measure of effect. Nevertheless, multiple fixation regimens can be used to determine the overall mean aberration frequency for an entire and identifiable population of cells. First the entire population is sampled in a series of non-overlapping fractions. For an “ideal” population this is done by fixing one sample and immediately treating the next with colchicine. The overall aberration yield is then a weighted average of the aberration frequencies in the samples. The w eighted a verage aberration y ield, W.A.Y. = ∑ i = 1 n f i A i , where n is the number of consecutive samples, Ai is the aberration frequency in the ith sample, and fi is the fraction of cells in the population which enter metaphase in the ith sampling interval. The metaphase fraction, fi, is normally determined indirectly as the product of two factors, the metaphase index in the ith sample, and a correction factor for changes in population size due to cell division. The determination of the W.A.Y. is described in detail for synchronous cell populations, and for S phase cells and G2 phase cells from asynchronous populations.

The site distribution parameter for radiation induced chromosome exchanges

Abstract The distribution between cells of radiation induced chromosome exchange aberrations in a number of species conforms to the terms of a binomial [ p + (1 − p )] n . The current explanation for this observation is that there are only a few places (termed “sites”) within the nucleus, where the participating chromosomal threads come close enough together for exchange to take place. p is taken as representing the probability of obtaining an exchange in a site, and n the number of “sites” in the cell available for exchange. Mathematical consideration is given to the case where not all cells in the population have the same number of “sites”, and it is concluded that n is a distribution parameter indicating the way in which cells with different numbers of sites are distributed in the population, and that it does not indicate directly the mean number of “sites” available. This conclusion is illustrated by calculated exchange distributions for a number of hypothetical populations having cells in the different site categories distributed in a known manner. It follows from this conclusion that: 1. (1) different types of aberration may have different mean numbers of “sites”, but the same distribution parameter; 2. (2) a Poissonian distribution of aberrations does not necessarily indicate that the mean number of “sites” per cell is unlimited or even large.

Some problems of sampling for chromosomal aberrations from synchronous populations.

The problems of obtaining compartmental chromosomal aberration yields from populations synchronized by mitotic shake-off and allowed to run on for sampling during the first post-synchrony mitotic wave are investigated. Using a simple model, where each cell has an independent rate invariant through the cycle, a lognormal developmental age distribution, and the assumption of an “all or nothing” radiosensitivity during 10% of the cells transit of a compartment, yield-time curves are derived from radiation given in G 2 or G 1 . Two basic methods of sampling are available: I, a single irradiation followed by serial sampling with time through the mitotic wave; II, irradiation given at intervals of time prior to sampling populations at a fixed time from release. Either method gives an infinitude of quite different yield time curves for the same fixed set of population parameters. The key factor is the “age” of the population, i.e. the time from release from synchrony to irradiation or observation. It is concluded that within rather narrow limits, sampling method II can give a reliable indication of the radiosensitivity situation obtaining in the G 1 compartment. There is no method of obtaining interpretable and usable quantitative chromosomal aberration data for the G 2 compartment.