Abstract A25: Intestinal tumorigenesis in APCMin/+ mice was higher after acute relative to fractionated proton radiation: Implications for space radiation-induced colorectal carcinogenesis
Authors: Suman Shubhankar , Moon Bo-Hyun , Fornace Albert J. , Datta Kamal 
Published: May 2018

Abstract
Ionizing radiation (IR) is a risk factor for colorectal cancer (CRC) based on epidemiologic studies. With increasing interest in exploring outer space, astronauts on prolonged space missions such as a mission to Mars will be exposed to IR doses that are expected to heighten the risk of CRC. Sources of space radiation beyond Earth's magnetosphere are galactic cosmic radiation (GCR) and solar particle event (SPE) and about 90% of the IR in outer space is energetic protons, which are more damaging than photon radiations such as γ-rays. While astronauts on deep-space missions are estimated to receive ~250 μGy/day of proton, radiation dose during SPE could reach up to ~2 Gy behind shielding. However, the risk of CRC after energetic proton radiation is not yet fully defined due to lack of human or animal model data. Limitations in obtaining adequate human data mandate that in vivo mechanistic data for CRC risk estimates be obtained using animal models and surrogate biologic and molecular end points relevant to human disease process. The purpose of the current study was to quantitatively and qualitatively compare and characterize differences in intestinal tumorigenesis between acute and fractionated proton irradiation in APCMin/+ mice, a well-established mouse model for human CRC. Mice were exposed to 1.88 Gy of proton radiation delivered in a single fraction or in 4 equal daily fractions (0.47 Gy x 4). Since proton dose to astronauts is variable based on space environment, in this initial study we chose a high single dose and a low daily dose pattern to assess intestinal tumorigenesis. Intestinal tumor frequency and grade were noted, and tumor and normal tissue samples were collected from irradiated and control mice euthanized 100 to 110 days after radiation exposure. Molecular analysis was focused on DNA damage, genomic instability, specific epigenetic changes and mutation analysis, and cellular proliferative signaling pathways relevant to CRC using immunoblots, immunohistochemistry, and PCR. Relative to control and fractionated protons, there was significantly higher intestinal tumor frequency and grade after acute proton irradiation. At the molecular level our data showed increased DNA damage and DNA double-strand breaks, decreased genomic stability, and upregulation of proliferative β-catenin and Akt signaling after acute proton exposure. Since increased DNA damage was not associated with concomitant increased apoptosis and there was increased microsatellite instability, our data are suggestive of continued proliferation of cells bearing sublethal damages. Mutational analysis showed significantly more tumors had higher overall mutation frequency and increased p53 hot spot mutations after acute relative to fractionated proton exposure. In fractionated proton groups, tumor frequency and grade as well as molecular changes were statistically comparable to control groups. When considered along with higher mutational frequency and altered promoter methylation of key CRC-related genes in tumor adjacent normal tissues, our data are suggestive of marked differences in carcinogenesis of acute vs. fractionated proton radiation with higher tumorigenic effects associated with acute exposures.
Citation Format: Shubhankar Suman, Bo-Hyun Moon, Albert J. Fornace, Jr., Kamal Datta. Intestinal tumorigenesis in APCMin/+ mice was higher after acute relative to fractionated proton radiation: Implications for space radiation-induced colorectal carcinogenesis [abstract]. In: Proceedings of the AACR Special Conference: Advances in Modeling Cancer in Mice: Technology, Biology, and Beyond; 2017 Sep 24-27; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(10 Suppl):Abstract nr A25.