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Deux tiers des cancers du sein expriment le récepteur à l’estrogène alpha (REα) et leur croissance dépend des estrogènes alors que l’expression de REα induit la différenciation et la sénescence des cellules humaines mammaires épithéliales normales. Le développement embryonnaire et la différenciation de la glande mammaire adulte sont contrôlés par des facteurs de transcription, dont beaucoup sont aussi impliqués dans la tumorigenèse. Plusieurs études d'association pan-génomiques ont identifié le putatif facteur de transcription TOX high mobility group box family member 3 (TOX3) comme un nouveau gène de prédisposition au cancer du sein.L’objectif de cette étude était la caractérisation fonctionelle de TOX3 dans l’épithélium mammaire normal et le cancer du sein.J’ai examiné l’expression de TOX3 dans des tumeurs du sein humaines et dans le sein normal par analyse des données d’expression issues de puces à ADN. L’effet de TOX3 sur la différenciation des cellules épithéliales mammaires était analysé par des assays CFC et FACS. De plus, j’ai effectué une analyse microarray des cellules luminales cancéreuses MCF-7 exprimant TOX3 afin d’identifier les gènes cibles de TOX3 ainsi que la méthode de purification d’affinité en tandem (TAP) combinée à la spectrométrie de masse afin d’identifier des protéines interactants de TOX3. Finalement, j’ai étudié le potentiel oncogénique de TOX3 dans un modèle de xénogreffe. / Two thirds of breast cancers express the estrogen receptor alpha (ERα) and are estrogen-dependent for growth. In contrast, expression of ERα induces differentiation and senescence in normal human mammary epithelial cells. Both embryonic development and lineage commitment in the adult mammary gland are governed by transcriptional regulators, many of which have also been implicated in tumourigenesis. Genome-wide association studies have identified the previously uncharacterised putative transcription factor TOX high mobility group box family member 3 (TOX3) as a new candidate breast cancer susceptibility gene.In the present study, I aimed to characterise TOX3 function in the normal human mammary epithelium and in breast cancer.I have examined TOX3 expression in primary breast tumours and in the normal mammary gland using micorarray data. The influence of TOX3 expression on lineage commitment was investigated using the colony forming cell (CFC) assay and FACS analysis. I further carried out microarray analysis of luminal breast cancer cells ectopically expressing TOX3 to identify TOX3 target genes as well as tandem affinity purification of TOX3 to identify TOX3 interacting proteins. Finally, the oncogenic potential of TOX3 was investigated in a human-in-mouse xenograft model.

<p>Epigenetic mechanisms, including DNA methylation, play an important role in regulation of stem cell fate and tumorigenesis. The Ten-Eleven-Translocation 2 (TET2) is a core enzyme for DNA demethylation by catalyzing the conversion of 5-methylcytosine (5mC) to 5-hydromethylcytosine (5hmC). It has been shown that TET2 is the main regulator of hematopoietic stem cell homeostasis and loss of TET2 is highly associated with hematopoietic malignancies. Our previous work has also shown that loss of TET2 expression is linked to promotion of an epithelial-mesenchymal-transition phenotype and expansion of a breast cancer stem cell-like population with skewed asymmetric cell division in vitro; however, the in vivo role that TET2 plays in regulation of mammary stem cell (MaSC) fate and development of mammary pathology has yet to be determined. Here, using our newly established mammary-specific Tet2-knockout mouse model, the data reveals for the first time that TET2 plays a pivotal role in mammary gland development via directing MaSC to luminal lineage commitment in vivo. Furthermore, we find that TET2 coordinates with FOXP1 to target and demethylate FOXA1, GATA3, and ESR1, key transcription factors that orchestrate mammary luminal lineage specification and endocrine response and are often silenced by DNA methylation in aggressive human breast cancers. Finally, loss of TET2 expression leads to promotion of mammary tumor development with defective luminal cell differentiation and tamoxifen resistance in a PyMT;Tet2 deletion breast cancer mouse model. As a result, this study provides a previously unidentified role for TET2 in governing luminal lineage specification and endocrine response that underlies resistance to anti-estrogen treatments.</p>