Transcriptional response of BALB/c mouse thyroids following in vivo astatine-211 exposure reveals distinct gene expression profiles

Female BALB/c nude mice were intravenously injected with 0.064 to 42 kBq of 211At, resulting in absorbed doses of 0.05 to 32 Gy in the thyroid gland. Thyroids were removed 24 h after injection; total RNA was extracted from pooled thyroids and processed in triplicate using Illumina MouseRef-8 Whole-Genome Expression Beadchips.Thyroids exposed to 211At revealed distinctive gene expression profiles compared to non-irradiated controls. A larger number of genes were affected at low absorbed doses (0.05 and 0.5 Gy) compared to intermediate (1.4 Gy) and higher absorbed doses (11 and 32 Gy). The proportion of dose-specific genes increased with decreased absorbed dose. Additionally, 1.4 Gy often exerted opposite regulation on gene expression compared to the other absorbed doses. Using Gene Ontology data, an immunological effect was detected at 0.05 and 11 Gy. Effects on cellular response to external stress and cell cycle regulation and proliferation were detected at 1.4 and 11 Gy.Conclusively, the cellular response to ionizing radiation is complex and differs with absorbed dose. The response acquired at high absorbed doses cannot be extrapolated down to low absorbed doses or vice versa. We also demonstrated that the thyroid - already at absorbed doses similar to those obtained in radionuclide therapy - responds with expression of a high number of genes. Due to the increased heterogeneous irradiation at low absorbed doses, we suggest that this response partly originates from non-irradiated cells in the tissue, i.e., bystander cells.Since the first measurement of clonogenic cell survival of HeLa cells exposed to X-rays in 1955 [1] and up to the early 1990s, radiobiological research has mainly focused on cell death and survival with an approach from the target-cell hypothesis. It was believed that the direct killing of parenchymal and vascular endothelial cells successively led to organ failure [2,3]. In addition, the time interval between exposure and manifestation of late nor