TRANSGENIC MICE CARRYING THE HUMAN KRAS ONCOGENE UNDER THE CONTROL OFA THYROGLOBULIN PROMOTER - KRAS EXPRESSION IN THYROIDS ANALYZED BY IN-SITU HYBRIDIZATION

We have previously generated transgenic mice bearing a molecular const ruct obtained by fusing the rat thyroglobulin promoter with the human Kirsten ras oncogene (KRAS). These mice showed thyroid abnormalities, although at very low incidence and after a long latency period. A six- month thyrotropin (TSH) stimulation of thyroid glands, followed by a t wo-month suspension, induced a significant increase in the number of l esions in transgenic mice as compared with a nontransgenic control gro up. Our goal was to follow the progression and the reversion of the tu morigenesis process in relationship with the levels of expression of t he KRAS in this experimental model. In situ hybridization was used to detect expression of KRAS mRNA in sections of thyroids of the various groups of mice. A positive hybridization was observed in follicular ce lls of TSH-stimulated transgenic mice, whereas no expression could be appreciated in control nontransgenic mice. A positive signal was also observed in thyroid glands excised from transgenic mice after the 2-mo nth suspension of treatment; however, the number of expressing cells w as decreased compared with transgenic mice killed immediately after 6 months of a goitrogen regimen. Finally, every cell in the single thyro id carcinoma observed after the two-month suspension was positive for the transgene mRNA. This study further strengthens the role of the exp ression of mutated KRAS in the early stages of thyroid follicular cell transformation and indicates that when the expression of the mutated KRAS becomes independent of exogenous TSH stimulation, this event coin cides with a further progression towards tumorigenesis.