Seung Jin Choi

Differential expression of thymic DNA repair genes in low-dose-rate irradiated AKR/J mice

We previously determined that AKR/J mice housed in a low-dose-rate (LDR) (137Cs, 0.7 mGy/h, 2.1 Gy) γ-irradiation facility developed less spontaneous thymic lymphoma and survived longer than those receiving sham or high-dose-rate (HDR) (137Cs, 0.8 Gy/min, 4.5 Gy) radiation. Interestingly, histopathological analysis showed a mild lymphomagenesis in the thymus of LDR-irradiated mice. Therefore, in this study, we investigated whether LDR irradiation could trigger the expression of thymic genes involved in the DNA repair process of AKR/J mice. The enrichment analysis of Gene Ontology terms and Kyoto Encyclopedia of Genes and Genomes pathways showed immune response, nucleosome organization, and the peroxisome proliferator-activated receptors signaling pathway in LDR-irradiated mice. Our microarray analysis and quantitative polymerase chain reaction data demonstrated that mRNA levels of Lig4 and RRM2 were specifically elevated in AKR/J mice at 130 days after the start of LDR irradiation. Furthermore, transcriptional levels of H2AX and ATM, proteins known to recruit DNA repair factors, were also shown to be upregulated. These data suggest that LDR irradiation could trigger specific induction of DNA repair-associated genes in an attempt to repair damaged DNA during tumor progression, which in turn contributed to the decreased incidence of lymphoma and increased survival. Overall, we identified specific DNA repair genes in LDR-irradiated AKR/J mice.

Identification of radiation‐sensitive expressed genes in the ICR and AKR/J mouse thymus

We have investigated radiation‐sensitive expressed genes (EGs), their signal pathways, and the effects of ionizing radiation in the thymus of ICR and AKR/J mice. Whole‐body and relative thymus weights were taken and microarray analyses were done on the thymuses of high‐dose‐rate (HDR, 137Cs, 0.8 Gy/min, a single dose of 4.5 Gy) and low‐dose‐rate (LDR, 137Cs, 0.7 mGy/h, a cumulative dose of 1.7 Gy) irradiated ICR and AKR/J mice. Gene expression patterns were validated by quantitative polymerase chain reaction (qPCR). The effect of ionizing radiation on thymus cell apoptosis was measured terminal deoxynucleotidyl‐transferase‐mediated dUTP‐end labeling (TUNEL). LDR‐irradiation increased the mean whole‐body weight, but decreased the relative thymus weight of AKR/J mice. Radiation‐sensitive EGs were found by comparing HDR‐ and LDR‐irradiated ICR and AKR/J mice. qPCR analysis showed that 12 EGs had dose and dose‐rate dependent expression patterns. Gene‐network analysis indicated that Ighg, Igh‐VJ558, Defb6, Reg3g, and Saa2 may be involved in the immune response, leukocyte migration, and apoptosis. Our data suggest that expression of the HDR (Glut1, Glut4, and PKLR) and LDR radiation‐response genes (Ighg and Igh‐VJ558) can be dose or dose‐rate dependent. There was an increased number of apoptotic cells in HDR‐irradiated ICR mice and LDR‐irradiated AKR/J mice. Thus, changes of the mean whole‐body weight and relative thymus weight, EGs, signal pathways, and the effects of ionizing radiation on the thymus of ICR and AKR/J mice are described.

Differential expression of cancer pathway-related genes in low-versus high-dose-rate-irradiated AKR/J mice

To understand the biological effects of ionizing radiation on lymphomagenesis, we reared AKR/J mice for 130 days with exposure to either high-dose-rate (HDR, 0.8 Gy/min, a single dose of 4.5 Gy) or low-dose-rate (LDR, 0.7 mGy/h, a cumulative dose of 2.1 Gy) irradiation. After 130 days, we compared the mean thymus weight, analyzed the histological changes, and measured apoptotic cell numbers using the terminal deoxynucleotidyl transferase-mediated dUTP-end labeling (TUNEL) assay. We also used microarrays and quantitative polymerase chain reaction analysis (qPCR) to analyze the expression profiles of cancer pathway-related genes in the thymuses of the mice. The mean thymus weight of the LDR-irradiated mice decreased relative to Sham- and HDR-irradiated mice. Histopathological examination revealed that the neoplastic cells in the thymuses of the Sham- and HDR-irradiated mice were pleomorphic, with marked anisocytosis and anisokaryosis, whereas the cells and their nuclei were relatively small and uniform in size in the LDR-irradiated mice. Furthermore, TUNEL assays showed that the number of apoptotic cells was higher in the LDR-irradiated mice than in the Sham- and HDR-irradiated mice. Microarray analysis showed differentially expressed genes according to carcinogenic stage (DNA repair/genomic instability, DNA damage signaling pathway, cell cycle, cancer pathway, p53 signaling pathway, apoptosis, and T- and B-cell activation). qPCR data for cancer pathway-related genes showed that Cds1 gene expression was upregulated in the LDR-irradiated mice, whereas expression of the Itga4, Myc, and Itgb1 genes was upregulated in the irradiated mice. However, the functions of cancer pathway-related genes require further study and validation.