Endocrine Abstracts

Purpose: Exposure to ionizing radiation in childhood has been recognized as a risk factor for the development of thyroid cancer and possibly for other thyroid disorders. However, the effects of neonatal radiation exposure on thyroid functions have never been explored experimentally despite its potential importance. We have demonstrated that neonatal rat thyroid was sensitive to ionizing radiation, developing specific morphological changes characterized by smaller thyroid follicles. In the present study, we examined the effects on the gene expression potentially related to thyroid carcinogenesis in this model.

Materials and methods: One-week-old male Wistar rats were exposed to cervical X-irradiation at 6 and 12 Gy. For comparison, 8-week-old (adult) rats were cervically X-irradiated at the same doses. Apoptosis in the thyroid follicles was evaluated by the TUNEL staining. Expressions of the gene, including Mct8, Lat4, Braf1, Oct3, CD44, and Abcg2, were measured in the thyroid by the quantitative RT-PCR method.

Results: In rats that received cervical X-irradiation at 1 week old, the colloid size of thyroid follicles decreased at 8 weeks old and afterward. The number of apoptotic cells in the thyroid follicles was increased corresponding to the reduction in follicular size. Significant increases in Braf1, CD44, and Abcg2 mRNAs were noted in neonatally irradiated thyroid at 8 weeks old, while CD44 and Abcg2 expressions were still high at 16 weeks old.

Conclusion: Our results demonstrated that neonatal rat thyroid was sensitive to ionizing radiation, developing specific morphological changes that may be caused by the increase in apoptosis. Persistent elevation of CD44 and Abcg2 expressions in the thyroid after neonatal irradiation may play a role in thyroid cancer development in childhood.