Radiation quality and intra-chromosomal aberrations: Size matters

The shift from plant to mammalian cell models in radiation cytogenetics hastened the development of methods suitable for the analysis of chromosome-type aberrations. These included methods to detect interchanges that take place between different chromosomes (dicentrics and translocations), and intrachanges occurring within a given chromosome (rings, interstitial deletions and inversions). In this review we consider the relationship between chromosome-type interchanges and intrachanges in response to changes in ionization density (linear energy transfer; LET). In that context, we discuss advantages and disadvantages of more modern methods used to measure intrachanges, and the implications that their increased resolution of measurement may have on the inter-to-intrachange fraction (i.e., the F-ratio). We conclude that the premise of the F-ratio is supported by its biophysical assumptions, but its intended use as an LET-dependent measure of prior radiation exposure is hampered mainly by our inability to accurately assess, on a cell-by-cell basis, inversions and interstitial deletions whose small sizes are below the detection limits of conventional cytogenetic techniques.

Directional genomic hybridization: inversions as a potential biodosimeter for retrospective radiation exposure

Chromosome aberrations in blood lymphocytes provide a useful measure of past exposure to ionizing radiation. Despite the widespread and successful use of the dicentric assay for retrospective biodosimetry, the approach suffers substantial drawbacks, including the fact that dicentrics in circulating blood have a rather short half-life (roughly 1-2 years by most estimates). So-called symmetrical aberrations such as translocations are far more stable in that regard, but their high background frequency, which increases with age, also makes them less than ideal for biodosimetry. We developed a cytogenetic assay for potential use in retrospective biodosimetry that is based on the detection of chromosomal inversions, another symmetrical aberration whose transmissibility (stability) is also ostensibly high. Many of the well-known difficulties associated with inversion detection were circumvented through the use of directional genomic hybridization, a method of molecular cytogenetics that is less labor intensive and better able to detect small chromosomal inversions than other currently available approaches. Here, we report the dose-dependent induction of inversions following exposure to radiations with vastly different ionization densities [i.e., linear energy transfer (LET)]. Our results show a dramatic dose-dependent difference in the yields of inversions induced by low-LET gamma rays, as compared to more damaging high-LET charged particles similar to those encountered in deep space.

Effects of sparsely and densely ionizing radiation on plants

One of the main purposes leading botanists to investigate the effects of ionizing radiations is to understand plant behaviour in space, where vegetal systems play an important role for nourishment, psychological support and functioning of life support systems. Ground-based experiments have been performed with particles of different charge and energy. Samples exposed to X- or γ-rays are often used as reference to derive the biological efficiency of different radiation qualities. Studies where biological samples are exposed directly to the space radiation environment have also been performed. The comparison of different studies has clarified how the effects observed after exposure are deeply influenced by several factors, some related to plant characteristics (e.g. species, cultivar, stage of development, tissue architecture and genome organization) and some related to radiation features (e.g. quality, dose, duration of exposure). In this review, we report main results from studies on the effect of ionizing radiations, including cosmic rays, on plants, focusing on genetic alterations, modifications of growth and reproduction and changes in biochemical pathways especially photosynthetic behaviour. Most of the data confirm what is known from animal studies: densely ionizing radiations are more efficient in inducing damages at several different levels, in comparison with sparsely ionizing radiation.

Cellular effects of energetic heavy ions: from DNA breaks to chromosomal rearrangements

Risk from exposure to energetic heavy ions is considered one of the main problems for human space exploration. Late stochastic risk estimates, particularly cancer, are affected by large uncertainties. Basic cell biology studies to elucidate the mechanisms involved in genetic damage are necessary to reduce the uncertainty and eventually design effective countermeasures. To study the influence of nuclear architecture on the formation of chromosomal rearrangements, normal diploid human fibroblasts have been exposed to heavy ions in horizontal and vertical positions. Analysis of chromosomal aberrations by arm-specific mFISH shows that, at the same radiation dose, the yield of chromosomal damage is modified by the irradiation geometry. A clear difference is seen in the fraction of aberrant cells, owing to the different nuclear cross sections.

Complex exchanges are responsible for the increased effectiveness of C-ions compared to X-rays at the first post-irradiation mitosis

The purpose of the present study was to investigate as to what extent differences in the linear energy transfer (LET) are reflected at the chromosomal level. For this study human lymphocytes were exposed to 9.5 MeV/u C-ions (1 or 2 Gy, LET=175 keV/microm) or X-rays (1-6 Gy), harvested at 48, 72 or 96 h post-irradiation and aberrations were scored in first cycle metaphases using 24 color fluorescence in situ hybridization (mFISH). Additionally, in selected samples aberrations were measured in prematurely condensed G2-phase cells. Analysis of the time-course of aberrations in first cycle metaphases showed a stable yield of simple and complex exchanges after X-ray irradiation. In contrast, after C-ion exposure the yields profoundly increased with harvesting time complicating the estimation of the frequency of aberrations produced by high LET particles within the entire cell population. This is especially true for the yield of complex exchanges. Complex aberrations dominate the aberration spectrum produced by C-ions. Their fraction was about 50% for the two measured doses. In contrast, isodoses of X-rays induced smaller proportions of complex aberrations (i.e. 5% and 15%, respectively). For both radiation qualities the fraction of complexes did not change with harvesting time. As expected from the different dose deposition of high and low LET radiation, complex exchanges produced by high LET C-ions involved more breaks and more chromosomes than those induced by isodoses of X-rays. Noteworthy, C-ions but not X-rays induced a small number of complex chromatid-isochromatid exchanges that are not expected for cells exposed in the G0-phase. The results obtained so far for cells arrested in G2-phase confirm these patterns. Altogether our data show that the increased effectiveness of C-ions for the induction of aberrations in first cycle cells is determined by complex exchanges, whereas for simple exchanges the relative biological effectiveness is about one.