Circadian rhythm influences genome-wide transcriptional responses to 131I in a tissue-specific manner in mice

BACKGROUND Circadian variation of gene expression is often neglected when ionizing radiation-induced effects are studied, whether in animal models or in cell culture. This study characterized diurnal variation of genome-wide transcriptional regulation and responses of potential biomarkers and signature genes in normal mouse tissues at 24 h after i.v. administration of (131)I. METHODS Female BALB/c nude mice were i.v. injected with 90 kBq (131)I at 9:00 a.m., 12:00 p.m., or 3:00 p.m. and killed after 24 h (n = 4/group). Paired control groups were mock-treated (n = 3-4/group). The kidneys, liver, lungs, spleen, and thyroid were excised, snap-frozen, and stored at -80 °C until extraction of total RNA. RNA microarray technology was used for genome-wide expression analysis. Enriched biological processes were categorized after cellular function. Signature genes for ionizing radiation and thyroid hormone-induced responses were taken from the literature. Absorbed dose was estimated using the Medical Internal Radiation Dose (MIRD) formalism. RESULTS The thyroid received an absorbed dose of 5.9 Gy and non-thyroid tissues received 0.75-2.2 mGy over 24 h. A distinct peak in the total number of significantly regulated transcripts was observed at 9:00 a.m. in the thyroid, but 3 h later in the kidney cortex, kidney medulla, and liver. Transcriptional regulation in the lungs and spleen was marginal. Associated cellular functions generally varied in quality and response strength between morning, noon, and afternoon. In the thyroid, 25 genes were significantly regulated at all investigated times of day, and 24 thereof showed a distinct pattern of pronounced down-regulation at 9:00 a.m. and comparatively weak up-regulation at later times. Eleven of these genes belonged to the species-specific kallikrein subfamily Klk1b. Responses in signature genes for thyroid hormone-induced responses were more frequent than for ionizing radiation, and trends persisted irrespective of time of day. CONCLUSION Diurnal variation of genome-wide transcriptional responses to 90 kBq (131)I was demonstrated for the thyroid, kidney cortex and medulla, and liver, whereas variation was only marginal in the lungs and spleen. Overall, significant detection of potential biomarkers and signature genes was validated at each time of day, although direction of regulation and fold-change differed between morning, noon, and afternoon. These findings suggest that circadian rhythm should be considered in radiation research and that biological and analytical endpoints should be validated for circadian robustness.