Radioadaptive Response and Radiation-Induced Teratogenesis in the Late Period of Organogenesis in Mice: Involvement of p53-Dependent Apoptosis.

GENERAL INTRODUCTION In the past 5 years, a series of study was done at National Institute of Radiological Sciences to investigate the effects of radiation on the embryogenesis in mice with an emphasis on mechanisms involved in the radiation-induced adaptive response and the role of radiation-induced apoptosis played in teratogenesis in the late period of organogenesis. \nMATERIALS AND METHODS Animals: The pregnant ICR mice bearing p53(+/+) fetuses and pregnant C57BL/6 mice bearing p53(+/+), p53(+/-) and p53(-/-) fetuses were used. X-Irradiation: The animals were exposed to whole-body irradiation at room temperature with an X-ray machine, operated at 200 kVp and 20 mA, using a 0.5 mm Al + 0.5 mm Cu filter. For investigation on induction of limb defects, 1-5 Gy at 1.8 Gy/min was given on E11. For induction of radioadaptation, the priming dose of 0.05-0.50 Gy and challenging dose of 3 or 5 Gy were given on E11 and E12, respectively. The priming and challenging irradiation dose rates were 34 cGy/min and 1.8 Gy/min, respectively. Biological Endpoints: In both limb teratogenesis and induction of radioadaptation study, the dams were sacrificed by cervical dislocation. Fetuses were removed by cesarean section. The limb buds were obtained 4-6 h after irradiation, and apoptotic cells in the predigital regions were identified by the routine hematoxylin eosin staining method and quantified microscopically. The number of living fetuses and the percentage of living malformed fetuses in total living fetuses were scored on E18. \nRESULTS Radiation, via induction of apoptosis in a dose-dependent way, selectively killed the cells in the predigital regions of limb buds where the cells were undergoing differentiation. Radiation-induced apoptosis hardly occurred in the cells in the preinterdigital regions where cells had stopped DNA synthesis and were scheduled to go spontaneous apoptosis. Radiation-inducted apoptosis was responsible for both digital defects and the severity of limb teratogenesis in ICR p53(+/+) mice [1]. In C57BL/6 mice, susceptibility to radiation-induced apoptosis in the predigital regions and digital defects depended on both p53 gene dose and radiation dosage, namely, p53 wild-type (p53(+/+)) mice appeared the most sensitive, p53 heterozygous type (p53(+/-)) mice were intermediate, and p53 knockout (p53(-/-)) mice showed the most resistant. Therefore, it was concluded that prenatal radiation-induced limb defects were mediated by p53-dependent apoptosis [2]. In ICR p53(+/+) mice, prior to a challenging dose of 5.0 Gy on E12, a priming dose of 0.30 Gy on E11 significantly increased the number of the living fetuses per dam and decreased that of the external gross malformations among living fetuses on E19. The adapted dams could give some living births unexpectively, but the pups showed a high postnatal mortality, and the survivors suffered from various detrimental effects such as growth retardation and behavioral alterations. These indicated the existence of a radiation-induced adaptive response reducing prenatal death and limb defects induced by a challenging dose radiation in the late period of embryogenesis in mice [3,4]. For C57BL/6 mice, prior to a challenging dose of 3.0 Gy, the adaptive response could be induced with a priming dose at 0.05 or 0.30 Gy on E11, but this was only among the animals with p53(+/+) gene status. From these results, it was demonstrated that the adaptive response in embryogenesis was related to radiation-induced apoptosis and that induction of the radioadaptation required the involvement of p53 gene [5,6].\nREFERENCES1. B. Wang, K. Fujita, K. Watanabe, C. Ohhira, T. Odaka, I. Hayata , H. Mitani, H. Ohyama, T. Yamada and A. 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