Study of tumorigenesis by means of transgenic mouse models expressing RET/PTC3 rearrangement and E7 under control of bovine thyroglobulin promoter and CD1 mouse strain treated with acrylamide

Thyroid carcinomas are the most common endocrine tumors in humans. There are three major types of carcinomas of thyrocyte origin, including papillary, follicular, and anaplastic carcinomas. Papillary thyroid carcinoma (PTC) is the most common type of thyroid malignancy accounting 80% of thyroid cancer cases, and present several histologic variants, namely classical (45%), follicular (18%), solid, diffuse-sclerosing, cribriform, … . Specific genetic events represent early initiating and late triggering events. Several genetic lesions have been identified in various thyroid carcinomas and some of them are specifically associated to one type thyroid cancer. For instance, RET/PTC is the most common molecular event in the radiation-associated PTC in childhood. In the first part of the work, we studied two transgenic mouse models: the Tg-RET/PTC3 (Tg-RP3) mouse and the Tg-E7 mouse. Both strains express the human origin transgene (RET/PTC3 rearrangement or E7) exclusively in the thyroid under the control of the bovine thyroglobulin promoter. Our study of these two models showed:In both E7 and RET/PTC3 mouse models, the thyroids exhibited hyperplasia with own 'oncogene-dependent' follicular cell characteristics. Small follicular cells with hyperchromatic nuclei with an increased nucleus/cytoplasm ratio were numerous in the E7 mice, and large cells with convex apical border, a decreased nucleus/cytoplasm ratio, a pale nucleus and dispersed chromatin were found in the RET/PTC3 mice. At 6, 10 months and later on, E7 mice developed huge heterogeneous, normal functional thyroid goiter, with no tumor formation. As in previous studies on transgenic RET/PTC3 mouse models, the generally encountered features such as solid tumours were present. We also observed conventional variant of human PTC at late age (since 11 month-old) with quite low incidence (4%). In addition to solid and conventional variant PTCs, 28% of mice developed a peculiar big size thyroid tumor pattern with “proliferative papillary cystic changes with spindle cells and remodelling” and macrophage infiltration in the cysts at as early as 2 month of age; this kind of tumor histologically resembles the rare human young age 'diffused sclerosing' variant PTC (DSVP), but disappeared after 6 month. The other peculiar tumor exhibits morphological similarity with another rare human FAP-associated (Familial Adenomatous colonic polyposis) cribriform PTC, which showed a mixed architecture of several histological patterns (solid, follicular, cribriform). At 6 months, 26% of mice presented the cribriform tumor pattern.From the analyse of the proliferation index in the two models, we conclude that RET/PTC3 fusion protein over stimulates MAPK and Akt/PKB-signalling pathways, through Ras-Raf-Mek-Erk, Ras-PI3-K/Akt/PKB, particularly in the large cells which were strongly positive for three proliferation markers. E7 bypasses these two pathways, by directly binding to Rb1 protein and releasing the E2F transcription factor which induces cell proliferation. So RET/PTC3 and E7 mice present several morphologic features which mimic human PTC tumors; RET/PTC3 could therefore be used as a partial model for human PTCs. Further investigation of gene expression will allow the characterization of the molecular phenotype of the observed variants.In the second part of the work, we attempted to generate by xenobiotic administration an in vivo model of thyroid carcinoma. Chronic exposure of CD1 mice to acrylamide in the drinking water during 6 and 8 months at doses of 3mg/kg per day similar to those causing thyroid tumorigenesis after 2 years in rats, did not induce any thyroid tumors whatever the level of thyroid stimulation.