Continued expression of a tissue specific activated oncogene in the early steps of radiation-induced human thyroid carcinogenesis

Ionizing radiation is a well-known risk factor of cancer development, but the mechanism of radiation induced carcinogenesis is not clear. Chromosomal rearrangements induced by radiation most likely are one of the principal genetic alterations resulting in malignant transformation. The chimeric BCR-ABL associated with chronic myelogenous leukemia (CML) and H4-RET oncogenes associated with thyroid papillary carcinoma are the result of a translocation and inversion, respectively. In vitro studies showed these genes were induced by high-doses of X-irradiation in cell lines. Studies also show that therapeutic external X-ray doses as high as 60 Gy for treatment of various childhood cancers including Hodgkin's disease significantly increase the risk of thyroid cancer. Therefore, we examined the induction and persistence of these chimeric genes in human thyroid tissues transplanted in scid mice after 50 Gy exposure as a function of time for 2 months to elucidate the early events of thyroid carcinogenesis. The H4-RET genes were detected on day 2 and throughout the 2 month period. On the other hand, BCR-ABL genes were detected on day 2 and were undetectable subsequently. These results suggest that ionizing radiation causes various oncogene activations, but cells with only specific gene alteration uniquely associated with thyroid carcinogenesis are selectively retained demonstrating one of the early events in the beginnings of radiation carcinogenesis in human thyroid tissues.