Heavy ion space radiation triggers ongoing DNA base damage by downregulating DNA repair pathways
Introduction
The outer space environment is fraught with radiation exposure-associated health risk to traveling astronauts and thus remains a major challenge to space travelers’ well-being during and after long-duration space missions including missions to Mars. In outer space, it is a mixed radiation field consisting primarily of high-energy proton and heavy ion radiation (Hamilton et al., 2006; Townsend, 2005). While protons contribute significantly towards dose equivalence in sporadic solar particle events (SPE), the heavy ions such as iron and silicon have been reported to be a major contributor to the dose equivalence of the ambient galactic cosmic radiation (GCR) (Datta et al., 2012; Hamilton et al., 2006; Hayatsu et al., 2009; Suman et al., 2012a; Townsend, 2005). Characteristically, energetic heavy ions are high linear energy transfer (high-LET) and densely ionizing radiation possessing greater damaging potential compared to low-LET γ-rays (Durante and Cucinotta, 2008; Setlow, 2003; Sutherland et al., 2000).
Long-term health consequences of ionizing radiation have in part been attributed to persistent sub-lethal oxidative stress (Datta et al., 2000; Durante and Cucinotta, 2008; Kim et al., 2006; Lenarczyk et al., 2009; Robbins et al., 2002; Robbins and Zhao, 2004; Setlow, 2003; Sutherland et al., 2000). Notably, ionizing radiation and associated oxidative stress leads to a myriad of DNA damages (Fig. 1) including oxidatively-induced modifications to DNA bases and the 2’-deoxyribose moiety (reviewed in (Dizdaroglu and Jaruga, 2012)). Additionally, DNA base damage has also been associated with aging as well as neurodegeneration (Maynard et al., 2009; Kregel and Zhang, 2007; Garinis et al., 2008). Therefore, elaborate mechanisms are in place to repair oxidatively-induced modification of DNA bases and the 2’-deoxyribose moiety, and maintain genomic integrity. DNA base damage is predominantly repaired by base excision repair (BER), while nucleotide excision repair (NER) acts on 8,5’-cyclopurine-2’-deoxynucleosides (reviewed in Wallace et al., 2012). We have previously demonstrated that exposure to iron ion radiation induced oxidative stress that was detected in intestinal epithelial cells (IECs) up to twelve months after exposure (Datta et al., 2012; Datta et al., 2014). We also immunohistochemically detected increased levels of 8-hydroxyguanine (8-OH-Gua) in IECs at two months as well as at twelve months after radiation exposure (Datta et al., 2012; Datta et al., 2014). The purpose of the current study was to detect and quantify oxidatively-induced DNA base lesions using gas chromatography/tandem mass spectrometry (GC-MS/MS) two months after radiation exposure. We also assessed the status of the DNA damage processing pathways involved in repairing the identified DNA base lesions. Our data not only demonstrated quantitatively greater accumulation of some DNA base lesions, but also showed, compared to controls, greater downregulation of BER and NER pathways known to be involved in the repair of oxidatively-modified DNA bases.
Section snippets
Materials and methods
Mice irradiation and harvesting intestinal tissues. C57BL/6J mice (female, 6-8 weeks old, twenty mice per study group) were maintained and monitored as per criteria described earlier (Datta et al., 2013). All animal procedures were performed as per protocol approved by the Institutional Animal Care and Use Committee (IACUC) at Georgetown University (GU) and at Brookhaven National Laboratory (BNL), and we followed the Guide for the Care and Use of Laboratory Animals prepared by the Institute of
Results
DNA damage in C57BL/6J intestine after γ- and iron-radiation. Five DNA lesions, which showed increasing trend after 0.5 Gy of iron radiation exposure were identified and quantified. After 0.5 Gy of iron irradiation, the levels of 8-OH-Gua, R-cdG, and S-cdA were significantly greater than those in sham-irradiated control as well as γ-irradiated samples (Fig. 2A to C). However, these DNA lesions were not significantly higher in γ-irradiated samples relative to controls. Although increasing trend
Discussion
Indirect effects of radiation on cells and tissues have been attributed to generation of reactive oxygen species (ROS) and consequent oxidative stress. Our previous observations demonstrated that exposure to non-lethal doses of radiation caused persistent oxidative stress and DNA damage and that such stress and damage were more pronounced after exposure to high-LET heavy ion radiation (Datta et al., 2012; Datta et al., 2014). DNA is a critical target of radiation-induced oxidative stress
Declaration of Competing Interest
Authors have no conflict of interest to declare.
Acknowledgments
This study is supported in part by NASA grants NNX13AD58G and NNX09AU95G. We are very much thankful to the members of the NASA Space Radiation Laboratory (NSRL) especially to Drs. Peter Guida and Adam Rusek form Brookhaven National Laboratory for their excellent support in conducting heavy ion radiation exposures. We are thankful to Pelagie Ake for animal facility supports.
Certain commercial equipment or materials are identified in this paper in order to specify adequately the experimental
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