Manned interplanetary travel is imminent, but currently inhibited by uncertainty surrounding the long-term human health risks associated with space radiation exposure, for which there is currently no effective means of shielding. Terrestrial exposure to ionizing radiation, such as γ- or X-rays, increases the age-related risk of most common human cancers, however the extent to which these risks can be extrapolated to astronauts in deep space remains unclear. Compared to terrestrial radiation, space radiation contains high charge and energy (HZE) ions with a high linear energy transfer (high-LET). As they traverse the cell, HZE ions interact directly with macromolecules along the particle trajectory, and emit high-energy secondary electrons (δ rays) that extend laterally for several microns, resulting in a wide and densely ionizing core track. HZE radiation creates a complex mixture of DNA damage (double strand breaks, single strand breaks, oxidative base damage, etc.) in tight clusters along the particle track that can be difficult to repair. Consequently, research thus far has focused on the influence of both terrestrial and space radiation on DNA. Here we sought to determine the acute impact and long-term consequences of space radiation exposure on the epigenome. Immortalized human bronchial epithelial cells (3KT) were exposed to low LET (X-ray) or HLET radiation (28Si, 56Fe). Cells were harvested after 48hrs or maintained in continuous culture for an additional 3 months, and DNA methylation patterns using the Illumina Human 450K platform. A linear mixed-effects model was applied to identify changes in DNA methylation significantly associated with dose, source, or time after exposure. Significantly, we found that radiation-induced methylation changes occur early and persist over time, reflecting a stable and heritable change to the epigenome. CpG sites affected by high and low LET radiation exposure also tended to affect CpG sites in different genomic compartments. Fe-affected CpG sites were enriched in CpG island ‘shores’ and underrepresented in gene bodies, whereas those affected by X-ray exposure were enriched in gene bodies and intergenic regions. Interestingly, Fe affected sites tended to affect CpG sites in regions with features of ‘open’ chromatin, such as promoters and distal enhancer elements, whereas sites affected by Si were overrepresented in heterochromatic regions. To probe the significance of our findings with respect to human lung cancer, we examined the methylation status of the radiation-sensitive CpG sites in primary human lung cancers analyzed in The Cancer Genome Atlas (TCGA) project. We found that the methylation status of Fe radiation sensitive sites could discriminate tumor from normal tissue for both lung adenocarcinomas and squamous cell lung carcinomas. No such relationship existed for the sites affected by low LET X-rays or 28Si radiation exposure. Thus, the high LET radiation exposure ‘signature’ reflects a cancer-specific methylation pattern observed in human primary lung cancers. From these data, we suggest that the stable imprint of a prior high LET radiation exposure is reflected in the DNA methylation pattern, and may prove useful as a biomarker of long-term, individual cancer risk.
Citation Format: E. Kennedy, D.R. Powell, Z. Li, J.S.K. Bell, H. Feng, W. Dynan, B. Dwivedi, J. Kowalski, K.N. Conneely and P.M. Vertino. Space radiation exposure induces stable epigenome alterations relevant to human lung cancer. [abstract]. In: Proceedings of the AACR Special Conference on Chromatin and Epigenetics in Cancer; Sep 24-27, 2015; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2016;76(2 Suppl):Abstract nr B08.