MODULATION OF STEM CELL SIGNALLING PATHWAYS BY NUCLEOPHOSMIN AND ITS LEUKEMOGENIC MUTANT

About one third of acute myeloid leukemias (AML) is characterized by the aberrant cytoplasmic localization of nucleophosmin (NPM1), a ubiquitously expressed phosphoprotein that acts as a molecular chaperone and shuttles between nucleus and cytoplasm. Although several animal models have been generated to unravel the mechanism of action of the cytoplasmic mutant NPMc+, it remains poorly understood. In this thesis, we identified a novel function of both wild type NPM1 and NPMc+ in the modulation of Wnt signaling during zebrafish development and primitive hematopoiesis. The injection of NPMc+ and human NPM1 mRNAs in one cell stage zebrafish embryos reveals an opposite effect of the two proteins in the modulation of the Wnt signaling: NPM1 can inhibit the pathways whereas the mutant can activate it. Furthermore, NPM1 and NPMc+ have an opposite effect on the expression of dkk1b, a well known inhibitor of the Wnt pathway, and the co-injection of NPM1 and NPMc+ mRNA rescues the phenotype, suggesting a dominant negative effect of the mutant on the wild-type. Through whole mount in situ hybridization, markers of hematopoiesis have been studied revealing that the myeloproliferative effect of NPMc+ can be overcome by the co-injection of dkk1b, suggesting that the mutant can act by activating the pathway. Moreover, we generated an NPMc+ expressing mammalian in vitro system using a non-transformed hematopoietic/progenitor cell line (EML-C1). Although NPMc+ is strongly and stably expressed in EML-C1, we did not observe any phenotype or alteration in Wnt signaling. Taken together, data presented in this thesis showed that NPMc+, the leukemogenic mutant of NPM1, is able to act in a dominant negative fashion on NPM1 and displays a myeloproliferative effect during primitive zebrafish hematopoiesis. We showed that the proliferative effect of NPMc+ can be overcome by the simultaneous inhibition of the Wnt pathway through overexpression of dkk1b, suggesting that NPMc+ can activate Wnt signaling and that the pathway may be involved in the mechanism of NPMc+ AML establishment and/or progression.