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Au cours de cette étude, le rôle des miR-204-5p et miR-204-3p dans la tumorigenèse thyroïdienne, en particulier dans les carcinomes papillaires de la thyroïde (PTC), a été exploré. Bien que les PTC soient généralement associés à un pronostic favorable, certains évoluent vers des formes plus agressives. La transformation maligne de la thyroïde en PTC, induite par des mutations oncogéniques dans la voie des MAPK, s'accompagne de changements significatifs dans l'expression des miARNs parmi lesquels les miR-204-5p et miR-204-3p ont été précédemment identifiés dans les PTC comme étant sous régulés et associés à l’agressivité. Afin d’évaluer leur rôle précis dans la progression tumorale des PTC, trois lignées thyroïdiennes humaines, qui ont perdu l’expression de ces deux miARNs, ont été transfectées de manière transitoire à l’aide d’un mimique spécifique du miR-204-5p et du miR-204-3p et des tests fonctionnels de migration/d’invasion, de prolifération, de croissance et d’apoptose ont été réalisés 72h après transfection. La caractérisation fonctionnelle de ces deux miARNs a révélé que ceux-ci agissaient en tant que suppresseur de tumeurs en inhibant l'invasion cellulaire. De nouvelles cibles du miR-204-5p ont alors pu être identifiées dont HMGA2, SNAI2, SOX4, TGFBR2, MMP14, ADAMTS9, et sont toutes étroitement associées à la transition épithélio-mésenchymateuse (TEM). La caractérisation approfondie de la fonction de ce miARN a révélé que celui-ci pourrait être inversement corrélé à l’agressivité tumorale en contrôlant le processus de différenciation cellulaire. Nous avons montré que l’inhibition de l’invasion et le maintien de la différenciation cellulaire par le miR-204-5p impliquait la liaison directe et la répression de HMGA2, régulé par la voie des MAPK. Nos résultats ont également mis en évidence que des modifications épigénétiques, en particulier la méthylation de l'ADN, étaient responsables de la sous expression du miR-204-5p dans les PTC. Ensuite, la pertinence clin, During this study, the role of miR-204-5p and miR-204-3p in thyroid tumorigenesis, particularly in papillary thyroid carcinomas (PTC), was investigated. While PTCs are generally associated with a favorable prognosis, some progress to more aggressive forms. The malignant transformation of the thyroid into PTC, induced by oncogenic mutations in the MAPK pathway, is accompanied by significant changes in the expression of miRNAs, among which miR-204-5p and miR-204-3p have been previously identified in PTCs as downregulated and associated with aggressiveness. To assess their precise role in the tumor progression of PTCs, three human thyroid cell lines, which had lost the expression of these two miRNAs, were transiently transfected with specific mimics of miR-204-5p and miR-204-3p. Functional tests for migration/invasion, proliferation, growth, and apoptosis were performed 72 hours after transfection. The functional characterization of these two miRNAs revealed that they acted as tumor suppressors by inhibiting cell invasion.Novel targets of miR-204-5p were identified, including HMGA2, SNAI2, SOX4, TGFBR2, MMP14, ADAMTS9, all closely associated with the epithelial-mesenchymal transition (EMT). In-depth characterization of miR-204-5p's function revealed its potential inverse correlation with tumor aggressiveness by controlling the process of cellular differentiation. We demonstrated that the inhibition of invasion and maintenance of cellular differentiation by miR-204-5p involved direct binding and repression of HMGA2, regulated by the MAPK pathway.Our results also highlighted that epigenetic modifications, particularly DNA methylation, were responsible for the underexpression of miR-204-5p in PTCs. Subsequently, the clinical relevance of our findings was evaluated in vivo. Correlation analyses in a murine PTC model and in human PTCs revealed a negative correlation between the expression of miR-204-5p and HMGA2, as well as between the expression of HMGA2 and various genes, Doctorat en Sciences biomédicales et pharmaceutiques (Médecine), info:eu-repo/semantics/nonPublished

This work aimed to better understand the early molecular events occasioned in human thyrocytes following exposure to genotoxic agents than can lead to thyroid mutagenesis and cancer. Similar in vitro data in human physiological models remain scarce.The reported incidence of papillary thyroid carcinoma (PTC), has been increasing worldwide but the pathogenesis of sporadic PTC remains unravelled. H2O2 has long been postulated by the host laboratory and by other groups as a potential thyroid mutagen since it is capable to cause DNA damage and it affects γ radiation-induced DNA damage in thyrocytes. Thyrotropin (TSH) has recently been advocated as a potential risk factor for PTC: it has been associated with advanced stage disease and with progression of microcarcinoma during active surveillance. TSH signalling was found to be essential for BRAF-induced thyroid carcinogenesis in mouse models. Radiation (γ and β) is the only clearly established environmental risk factor for PTC; the cancer relative risk is rising linearly beyond a threshold of 100mGy thyroid absorbed dose. We studied in human thyrocytes in primary cultures the early molecular events following 131I exposure (β radiation) in comparison to γ radiation and H2O2 exposure, and their modulation by TSH. We assessed cell survival, DNA damage, DNA repair, gene expression, cell proliferation and apoptosis. Although we globally observed that the thyrocyte responses following exposure to β, γ radiation or H2O2 showed similarities, TSH, unlike other proliferative agents tested, specifically increased DNA damage both in non-exposed and in 131I-exposed thyrocytes. TSH did not influence γ radiation- or H2O2-induced damage. The effect of TSH on DNA damage in non-exposed thyrocytes decreased after incubation with an antioxidant agent and in 131I-exposed thyrocytes was partially alleviated after inhibition of iodide uptake. Therefore, in our experimental conditions, TSH seemed to predispose thyroid cells to greater DNA damage, Doctorat en Sciences médicales (Médecine), info:eu-repo/semantics/nonPublished