Quebec platelet disorder (QPD) is a bleeding disorder characterized by a gain of function defect in fibrinolysis. The hallmark feature of QPD is the marked overexpression of urokinase plasminogen activator (uPA) in megakaryocytes (MK) and platelets. The genetic cause of QPD is a tandem duplication of a ~78 kb region that encompasses the uPA gene, PLAU. As the mechanism of PLAU overexpression is unknown, gene regulatory mechanisms specifically epigenetics were evaluated at the PLAU locus in QPD MK and granulocytes, a QPD unaffected lineage. The aims of the thesis were to assess if QPD is associated with 1) genome wide methylation changes of promoter CpG islands, particularly at PLAU and 2) genome wide changes of active histone modifications H3K27Ac, H3K36me3 and H3K4me2, particularly at the region of PLAU duplication. Methylation and active histone enrichment analysis revealed that in QPD and control subjects, PLAU promoter CpG island was characterized by unaltered hypo-methylation and changes in active histone peak enrichments that were within the realm of having one extra copy of PLAU in both MK and granulocytes. The findings imply that the PLAU CNV mutation does not induce altered promoter methylation status and/or significantly alter active histone markers as the reason for the marked PLAU overexpression in QPD MK. Instead, the rearrangement of an active enhancer element, particularly an H3K27Ac enhancer expressed in MK but not granulocytes, that is upstream of the second copy of PLAU might underlie the marked PLAU expression by differentiated QPD MK. The thesis provides novel insights into the epigenetic regulation of PLAU that will be crucial to identifying the mechanism underlying the aberrant PLAU expression in QPD. / Thesis / Master of Science (MSc)

Le cancer du sein est la principale cause de mortalité par cancer chez la femme. Deux des facteurs de transcription de la famille Fos, Fra-1 et Fra-2, sont surexprimés dans les cancers du sein agressifs et contribuent au phénotype tumoral en favorisant entre autres, la prolifération, la motilité et l'invasivité. De façon surprenante, les mécanismes moléculaires via lesquels Fra-1 et Fra-2 (et plus généralement le complexe transcriptionnel AP-1 dont ils sont des constituants) gouvernent la transcription de leurs gènes cibles sont quasi-inconnus. Dans ce contexte, en combinant diverses approches (immunoprécipitation de chromatine, interférence à l'ARN…), j'ai étudié les mécanismes moléculaires par lesquels Fra-1 et Fra-2 contrôlent la transcription dérégulée du gène de l'urokinase ou uPA (sérine protéase cruciale dans la progression tumorale et l'établissement de métastases) qui est l'un des nouveaux marqueurs utilisés en clinique pour la mise en place des choix thérapeutiques. Mes travaux montrent de façon originale que (i) Fra-1 et Fra-2 agissent de façon non redondante et coopèrent pour réguler l'expression d'uPA via leur fixation sur un enhancer AP-1 localisé à -1,9 kb du site d'initiation de la transcription (TSS), (ii) Fra-2 est nécessaire au recrutement de RNA Pol II au niveau de l'enhancer, tandis que Fra-1 stimule le passage de RNA Pol II de sa forme initiatrice à sa forme élongatrice et (iii) que la polymérase recrutée à l'enhancer rejoint le TSS par un mécanisme de « tracking », très rarement décrit dans la littérature, en produisant de petits ARNs non codants, bidirectionnels et instables. / Breast cancer is the most frequent malignant disease among women. Two transcription factors, Fra-1 and Fra-2, belonging to the Fos family members, are overexpressed in aggressive breast cancers and contribute to the tumorigenic phenotype by favoring proliferation, motility and invasion. Surprisingly, the molecular mechanisms governed by Fra-1 and Fra-2 (and more generally by the AP-1 transcriptional complex, which they are components of) for the transcription of their target genes are still largely unknown. In this context, by combining different approaches (chromatin immunoprecipitation, RNA interference…), I studied the molecular mechanisms orchestrated by Fra-1 and Fra-2 for the expression of the urokinase (or uPA) gene (encoding a serine protease crucial for tumor progression and metastasis), which is one of the new diagnostic markers now taken into consideration for deciding therapeutic strategies. Interestingly, my results show that (i) Fra-1 and Fra-2 have non redundant functions and cooperate for the transcriptional regulation of uPA through their binding to AP-1 enhancer located 1.9 kb upstream of the transcriptional start site (TSS), (ii) Fra-2 is required for the recruitment of RNA Pol II on this enhancer while Fra-1 allows the conversion of RNA Pol II initiating form into its elongating form and (iii) enhancer-recruited RNA Pol II reaches the TSS by a tracking mechanism, mechanism very rarely described in the literature, during which it synthetizes small, unstable bidirectional, non coding RNAs.