Heavy ion-induced DNA double-strand breaks with yeast as a model system

Aspergillus niger Spores Are Highly Resistant to Space Radiation

The filamentous fungus Aspergillus niger is one of the main contaminants of the International Space Station (ISS). It forms highly pigmented, airborne spores that have thick cell walls and low metabolic activity, enabling them to withstand harsh conditions and colonize spacecraft surfaces. Whether A. niger spores are resistant to space radiation, and to what extent, is not yet known. In this study, spore suspensions of a wild-type and three mutant strains (with defects in pigmentation, DNA repair, and polar growth control) were exposed to X-rays, cosmic radiation (helium- and iron-ions) and UV-C (254 nm). To assess the level of resistance and survival limits of fungal spores in a long-term interplanetary mission scenario, we tested radiation doses up to 1000 Gy and 4000 J/m². For comparison, a 360-day round-trip to Mars yields a dose of 0.66 ± 0.12 Gy. Overall, wild-type spores of A. niger were able to withstand high doses of X-ray (LD₉₀ = 360 Gy) and cosmic radiation (helium-ion LD₉₀ = 500 Gy; and iron-ion LD₉₀ = 100 Gy). Drying the spores before irradiation made them more susceptible toward X-ray radiation. Notably, A. niger spores are highly resistant to UV-C radiation (LD₉₀ = 1038 J/m²), which is significantly higher than that of other radiation-resistant microorganisms (e.g., Deinococcus radiodurans). In all strains, UV-C treated spores (1000 J/m²) were shown to have decreased biofilm formation (81% reduction in wild-type spores). This study suggests that A. niger spores might not be easily inactivated by exposure to space radiation alone and that current planetary protection guidelines should be revisited, considering the high resistance of fungal spores.

Short DNA Fragments Are a Hallmark of Heavy Charged-Particle Irradiation and May Underlie Their Greater Therapeutic Efficacy

Growing interest in proton and heavy ion therapy has reinvigorated research into the fundamental biological mechanisms underlying the therapeutic efficacy of charged-particle radiation. To improve our understanding of the greater biological effectiveness of high-LET radiations, we have investigated DNA double-strand breaks (DSBs) following exposure of plasmid DNA to low-LET Co-60 gamma photon and electron irradiation and to high-LET Beryllium and Argon ions with atomic force microscopy. The sizes of DNA fragments following radiation exposure were individually measured to construct fragment size distributions from which the DSB per DNA molecule and DSB spatial distributions were derived. We report that heavy charged particles induce a significantly larger proportion of short DNA fragments in irradiated DNA molecules, reflecting densely and clustered damage patterns of high-LET energy depositions. We attribute the enhanced short DNA fragmentation following high-LET radiations as an important determinant of the observed, enhanced biological effectiveness of high-LET irradiations.

Monte Carlo simulations of the relative biological effectiveness for DNA double strand breaks from 300 MeV u(-1) carbon-ion beams

Monte Carlo simulations are used to calculate the relative biological effectiveness (RBE) of 300 MeV u(-1) carbon-ion beams at different depths in a cylindrical water phantom of 10 cm radius and 30 cm long. RBE values for the induction of DNA double strand breaks (DSB), a biological endpoint closely related to cell inactivation, are estimated for monoenergetic and energy-modulated carbon ion beams. Individual contributions to the RBE from primary ions and secondary nuclear fragments are simulated separately. These simulations are based on a multi-scale modelling approach by first applying the FLUKA (version 2011.2.17) transport code to estimate the absorbed doses and fluence energy spectra, then using the MCDS (version 3.10A) damage code for DSB yields. The approach is efficient since it separates the non-stochastic dosimetry problem from the stochastic DNA damage problem. The MCDS code predicts the major trends of the DSB yields from detailed track structure simulations. It is found that, as depth is increasing, RBE values increase slowly from the entrance depth to the plateau region and change substantially in the Bragg peak region. RBE values reach their maxima at the distal edge of the Bragg peak. Beyond this edge, contributions to RBE are entirely from nuclear fragments. Maximum RBE values at the distal edges of the Bragg peak and the spread-out Bragg peak are, respectively, 3.0 and 2.8. The present approach has the flexibility to weight RBE contributions from different DSB classes, i.e. DSB0, DSB+ and DSB++.

Novel apparatus to measure hyperthermal heavy ion damage to DNA: strand breaks, base loss, and fragmentation

We have developed a novel apparatus that allows us to irradiate nonvolatile organic films of high mass (1-100 microg range) spread out over a large surface area (42 cm(2)) with low energy (kT-100 eV) heavy ions and to quantitatively analyze the film substance via standard biochemical techniques afterwards. Here we discuss the details of the apparatus and method and show that it allows us to measure substantial damage to double stranded DNA molecules (plasmids) and its fundamental subunits induced by heavy ions with unprecedented low energies, i.e., 2.5 eV/amu; these energies correspond to track end energies of stopping ions or secondary ions created along primary ion tracks. We find that hyperthermal Ar(+) ions interacting with plasmid DNA will lead to the formation of single and double strand breaks, as well as fragmentation of nucleosides, which also involve chemical modifications and site specific rupture along the N1-C1 glycosidic bond, resulting in base release. In cells, such localized clustered damage will enhance the severity of DNA strand lesions, thus making them harder to repair.

Heavy ion-induced DNA double-strand breaks in yeast

Induction of DSBs in the diploid yeast, Saccharomyces cerevisiae, was measured by pulsed-field gel electrophoresis (PFGE) after the cells had been exposed on membrane filters to a variety of energetic heavy ions with values of linear energy transfer (LET) ranging from about 2 to 11,500 keV/microm, (241)Am alpha particles, and 80 keV X rays. After irradiation, the cells were lysed, and the chromosomes were separated by PFGE. The gels were stained with ethidium bromide, placed on a UV transilluminator, and analyzed using a computer-coupled camera. The fluorescence intensities of the larger bands were found to decrease exponentially with dose or particle fluence. The slope of this line corresponds to the cross section for at least one double-strand break (DSB), but closely spaced multiple breaks cannot be discriminated. Based on the known size of the native DNA molecules, breakage cross sections per base pair were calculated. They increased with LET until they reached a transient plateau value of about 6 x 10(-7) microm(2) at about 300-2000 keV/microm; they then rose for the higher LETs, probably reflecting the influence of delta electrons. The relative biological effectiveness for DNA breakage displays a maximum of about 2.5 around 100-200 keV/microm and falls below unity for LET values above 10(3) keV/microm. For these yeast cells, comparison of the derived breakage cross sections with the corresponding cross section for inactivation derived from the terminal slope of the survival curves shows a strong linear relationship between these cross sections, extending over several orders of magnitude.

Induction of DNA double-strand breaks in mammalian cells and yeast

Induction of DNA double-strand breaks (dsb) and their distribution are dependent on the energy deposition pattern within the cell nucleus (physical structure) and the ultrastructure of the chromosomes and its variation by the cell cycle and gene activities (biological structure). For electron radiation very similar RBE-values are observed for mammalian and yeast cells (AlK, 1.5 keV, 15 keV/micrometer: 2.6 in mammalian cells and 2.2 in yeast; CK 0.278 keV, 23 keV/micrometer: approx. 2.5 in mammalian cells and 3.8 in yeast). In contrast, the RBE-values for the induction of dsb of 4He2+ and light ions in the LET range from about 100 keV/micrometer up to 1000 keV/micrometer are significantly higher for yeast cells compared to mammalian cells. For example, the RBE-value of alpha-particles (120 keV/micrometer) is about 1.2 for mammalian cells whereas for yeast the RBE-value is about 2.5. The yeast chromatin has less condensed fibres compared with mammalian cells. Since a single CK photoelectron can induce only one dsb, the different condensation of the mammalian and yeast chromatin has no influence. However, particles may induce more than one dsb when traversing a chromatin fibre. The probability for the induction of closely neighboured dsb is higher the more condensed the chromatin fibres are. Since small DNA fragments (50 bp up to several kbp) are lost by standard methods of lysis, the underestimation of dsb yields increases with fibre condensation, which is in accordance with the observes dsb yields in mammalian cells and yeast. In order to obtain relevant yields of dsb (and corresponding RBE-values) the measurement of all DNA fragments down to about 50 bp are needed.

A review of dsb induction data for varying quality radiations

PURPOSE

This short review summarizes the data obtained with various techniques for measuring the yields of double strand breaks (dsb) produced by particle radiations of differing linear energy transfer (LET) in order to obtain relative biological effectiveness (RBE) values.

RESULTS AND CONCLUSIONS

Studies aimed at understanding the interactions of different types of radiation with cellular DNA have monitored the yields of DNA dsb versus radiation quality. Several techniques have been used to measure dsb yields in mammalian cells, and these include: neutral sedimentation gradients, filter elution and more recently pulsed field gel electrophoresis techniques (PFGE). Recent developments in PFGE have allowed the measurement of both the yields and the distribution of breaks within the genome, which go part of the way to explaining the RBE values close to 1.0 previously measured using other approaches with various radiation qualities. It is clear that future studies to determine the effectiveness of radiations of differing LET must use techniques that determine both yields and distributions of dsb, and assays need to be developed to allow these measurements at biologically relevant doses.