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Background: For radiological protection and control, the International Commission on Ra- diological Protection (ICRP) provides the nominal risk coefficients related to radiation expo- sure, which can be extrapolated using the excess relative risk and excess absolute risk obtained from the Life Span Study of atomic bomb survivors in Hiroshima and Nagasaki with the dose and dose-rate effectiveness factor (DDREF). Materials and Methods: Since it is impossible to directly estimate the radiation risk at doses less than approximately 100 mSv only from epidemiological knowledge and data, support from radiation biology is absolutely imperative, and thus, several national and international bodies have advocated the importance of bridging knowledge between biology and epidemiology. Be- cause of the accident at the Tokyo Electric Power Company (TEPCO)’s Fukushima Daiichi Nu- clear Power Station in 2011, the exposure of the public to radiation has become a major concern and it was considered that the estimation of radiation risk should be more realistic to cope with the prevailing radiation exposure situation. Results and Discussion: To discuss the issues from wide aspects related to radiological protec- tion, and to realize bridging knowledge between biology and epidemiology, we have established a research group to develop low-dose and low-dose-rate radiation risk estimation methodology, with the permission of the Japan Health Physics Society. Conclusion: The aim of the research group was to clarify the current situation and issues relat- ed to the risk estimation of low-dose and low-dose-rate radiation exposure from the viewpoints of different research fields, such as epidemiology, biology, modeling, and dosimetry, to identify a future strategy and roadmap to elucidate a more realistic estimation of risk against low-dose and low-dose-rate radiation exposure. Keywords: Radiation Risk Estimation, Low Dose and Low Dose Rate, Epidemiology, Biology, Modeling, Dosimetry

Stem cell competition could shed light on the tissue-based quality control mechanism that prevents carcinogenesis. To quantitatively evaluate stem cell competition in vitro, we developed a two-color intestinal organoid forming system. First, we improved a protocol of culturing organoids from intestinal leucine-rich-repeat containing G-protein-coupled receptor 5 (Lgr5)- enhanced green fluorescent protein (EGFP)high stem cells directly sorted on Matrigel without embedding. The organoid-forming potential (OFP) was 25% of Lgr5-EGFPhigh cells sorted at one cell per well. Using this culture protocol with lineage tracing, we established a two-color organoid culture system by mixing stem cells expressing different fluorescent colors. To analyze stem cell competition, two-color organoids were formed by mixing X-ray-irradiated and non-irradiated intestinal stem cells. In the two-color organoids, irradiated stem cells exhibited a growth disadvantage, although the OFP of irradiated cells alone did not decrease significantly from that of non-irradiated cells. These results suggest that stem cell competition can be evaluated quantitively in vitro using our new system.

Radiation exposure such as A-bomb or radiation therapy is considered a major health-risk factor for cardiovascular disease. In order to understand the molecular mechanisms underlying the inflammatory reaction frequently encountered in the vascular system after exposure to ionizing radiation, we carried out a global scale microarray and computational gene expression analyses on human umbilical endothelial cells (HUVECs) exposed to X-ray (2.5Gy). The gene ontology analysis revealed that the down-regulated genes were associated with cell cycle regulation, whereas the up-regulated genes were associated with inflammatory responses, in particular, the type 1 interferon response. The computational analysis using ingenuity pathway analysis also identified a gene network containing the interferon response factor 7 (IRF7) and its transcriptional targets such as interferon-induced transcripts (IFITs) and Mx1, which have been known to be associated with inflammation in endothelial cells. The up-regulated genes and the gene network identified here may explain the inflammatory response induced by X-irradiation. These findings uncover part of the molecular basis of the mechanism(s) of the inflammatory disorder in response to X-irradiation in HUVECs. The dataset is publicly available at the Gene Expression Omnibus (GEO) repository (http://www.ncbi.nlm.nih.gov/geo/) with accession number GSE76484.

The microcolony assay developed by Withers and Elkind has been a gold standard to assess the surviving fraction of small intestinal stem cells after exposure to high (?8 Gy) doses of ionizing radiation (IR), but is not applicable in cases of exposure to lower doses. Here, we developed a novel in vitro assay that enables assessment of the surviving fraction of small intestinal stem cells after exposure to lower IR doses. The assay includes in vitro culture of small intestinal stem cells, which allows the stem cells to develop into epithelial organoids containing all four differentiated cell types of the small intestine. We used Lgr5-EGFP-IRES-CreERT2/ROSA26-tdTomato mice to identify Lgr5+ stem cells and their progeny. Enzymatically dissociated single crypt cells from the duodenum and jejunum of mice were irradiated with 7.25, 29, 101, 304, 1000, 2000 and 4000 mGy of X-rays immediately after plating, and the number of organoids was counted on Day 12. Organoid-forming efficiency of irradiated cells relative to that of unirradiated controls was defined as the surviving fraction of stem cells. We observed a significant decrease in the surviving fraction of stem cells at ?1000 mGy. Moreover, fluorescence-activated cell sorting analyses and passage of the organoids revealed that proliferation of stem cells surviving IR is significantly potentiated. Together, the present study demonstrates that the in vitro assay is useful for quantitatively assessing the surviving fraction of small intestinal stem cells after exposure to lower doses of IR as compared with previous examinations using the microcolony assay., Journal of Radiation Research, 55(2), pp.381-390; 2014

Accepted: 2010-04-21, 資料番号: SA1002706000

Essentially all eukaryotic cells contain circular extrachromosomal DNA as a result of excision from the chromosomes. To obtain insight into the nature of recombination associated with the occurrence of such DNA species and its biological significance, we analyzed a library enriched in recombination junctions which was constructed by a novel DNA subtraction technique; in-gel competitive reassociation (IGCR). Furthermore, we also introduced inverse PCR to characterize chromosomal DNA fragments containing the recombination junctions. At least 45% of the clones in the library constructed by the IGCR procedure comprised DNA with recombination junctions. Nucleotide sequence analysis of the recombination junctions indicated that three of four extrachromosomal DNAs thus analyzed were produced through recombination between sequences with a 3–5 bp homology in the chromosomes. One extrachromosomal DNA was apparently generated through non-homologous recombination, possibly by end-to-end joining. These results have demonstrated the usefulness of IGCR in concentrating recombination junctions, which provide the most direct evidence for the mechanism of the recombinational events involved, from highly complex genomes.