Abstract
Risk of radiation and chemicals in humans is estimated by epidemiological studies, animal experiments and in vitro cell culture studies. We have succeeded to examine direct effects of radiation and chemicals on human organs and tissues maintained in improved SCID (severe combined immunodeficient) mice for long period (up to 3 years).
In this study, thyroid tissues from Graves' disease patients were transplanted s.c. to C57BL/6J-scid mice, and effects of nuclear radiations (UTR-KINKI, 0.2 Gy of neutron and 0.2 Gy of γ-rays/hour, weekly one hour exposure) on human thyroid tissues were examined in comparison with those of 137Cs γ-rays (1.19 Gy/min, 0.23 mGy/min; biweekly one Gy exposure). This study is also aimed for the ground experiments of cosmic radiations.
Morphology and thyroid hormone secretion: Significant tissue damage and decrease of hormone secretion were observed by high dose rate γ-ray exposure (> 9 Gy) but not by low dose rate exposure. Similar changes were observed by neutron exposure (0.2 Gy x 4 times and more).
Gene mutation: High dose rate exposure to γ-rays (11-59 Gy, up to 2 years) induced significant increases of mutations in p53 and c-kit genes but not in K-ras, β-catenin, RET, bak and BRAF, although these mutations were not induced by low dose rate exposure and in unirradiated controls. Gene murations were not detected by neutron exposure (0.2 Gy x 6 times) during 5-13 months observation period).
Changes in gene expression: Changes in gene expression measured by GeneChip (Affymetrix) increased dose-dependently by 1-3 Gy of γ-ray exposure and by 2-4 times exposures to 0.2 Gy of neutron.
Thus, in human thyroid tissues, apparent dose rate effects were observed by γ-ray exposure, and neutron showed high RBE. (Supported by MEXT Japan and Japan Space Forum)
