Date Published: July 6, 2017
Publisher: Public Library of Science
Author(s): Mitchell S. Turker, Dmytro Grygoryev, Michael Lasarev, Anna Ohlrich, Furaha A. Rwatambuga, Sorrel Johnson, Cristian Dan, Bradley Eckelmann, Gwen Hryciw, Jian-Hua Mao, Antoine M. Snijders, Stacey Gauny, Amy Kronenberg, Albert J. Fornace.
Exposure to a small number of high-energy heavy charged particles (HZE ions), as found in the deep space environment, could significantly affect astronaut health following prolonged periods of space travel if these ions induce mutations and related cancers. In this study, we used an in vivo mutagenesis assay to define the mutagenic effects of accelerated 56Fe ions (1 GeV/amu, 151 keV/μm) in the mouse kidney epithelium exposed to doses ranging from 0.25 to 2.0 Gy. These doses represent fluences ranging from 1 to 8 particle traversals per cell nucleus. The Aprt locus, located on chromosome 8, was used to select induced and spontaneous mutants. To fully define the mutagenic effects, we used multiple endpoints including mutant frequencies, mutation spectrum for chromosome 8, translocations involving chromosome 8, and mutations affecting non-selected chromosomes. The results demonstrate mutagenic effects that often affect multiple chromosomes for all Fe ion doses tested. For comparison with the most abundant sparsely ionizing particle found in space, we also examined the mutagenic effects of high-energy protons (1 GeV, 0.24 keV/μm) at 0.5 and 1.0 Gy. Similar doses of protons were not as mutagenic as Fe ions for many assays, though genomic effects were detected in Aprt mutants at these doses. Considered as a whole, the data demonstrate that Fe ions are highly mutagenic at the low doses and fluences of relevance to human spaceflight, and that cells with considerable genomic mutations are readily induced by these exposures and persist in the kidney epithelium. The level of genomic change produced by low fluence exposure to heavy ions is reminiscent of the extensive rearrangements seen in tumor genomes suggesting a potential initiation step in radiation carcinogenesis.
The risk of cancer from exposure to charged particles that induce mutations is a long-term concern for astronauts on prolonged missions in deep space [1–4]. The most common charged particles in space are sparsely ionizing protons present in the trapped radiation belts, in solar particle events (SPE), and among the galactic cosmic rays (GCR) . However, charged particles with high linear energy transfer (LET) that are present in the GCR are of substantial concern for the induction of mutations due to their ability to deposit more energy locally generating the potential for even a single particle traversing a cell nucleus to cause complex DNA damage and resultant mutations.
Our in vivo mutagenesis system allows radiation-induced mutations to develop in the normal tissue environment, as would occur in the bodies of astronauts exposed to the space radiation environment to increase their risk for cancer. Aprt mutant frequencies from the Fe ion-exposed kidneys showed a linear increase with dose. This result is consistent with, and extends to lower doses, a prior study of Fe ion mutagenesis with doses ranging from 1.0 to 2.0 Gy . In contrast, no significant increase in Aprt mutant frequencies was observed with the proton doses tested here (0.5 and 1.0 Gy), consistent with a previously reported lack of an increase at 3.0 Gy and an exponential increase at 4.0 Gy and 5.0 Gy . The combined results for proton mutagenesis demonstrate a linear-quadratic mutagenic effect from these sparsely ionizing protons, which is consistent with the results of a study with B6D2F1 Aprt heterozygous kidney cells exposed in culture to the same proton beam . In a prior study with Ti ions (0.2 to 1.4 Gy) we observed a linear increase at the lowest doses and then a plateau in mutant frequency (i.e., a bending of the curve ). Thus, our results with three different charged particles demonstrate radiation quality effects for the induction of mutations in an epithelial cell type when a range of doses is considered. Moreover, the linear increases in mutant frequencies at lower doses for the high LET Fe and Ti ions suggest straightforward mutagenic mechanisms involving nuclear traversals leading to DNA damage and mutations.
We have shown that charged particle-induced mutant cells can carry large mutational loads affecting multiple chromosomes, that not all chromosomes are affected equally, and that cells with greatly perturbed genomes remain in the intact tissue for many months after irradiation. The results indicating extensive genomic damage following low fluence exposure to densely ionizing Fe ions are of concern for realistic exposures of astronauts to the galactic cosmic radiation during extended space flight. Moreover, the results for the lower dose proton exposures are also relevant, primarily for a rare subset of solar particle events where high-energy protons may be produced at high dose-rates . The linkage between these widespread genomic mutations and charged particle-induced tumors remains to be determined.