DISSECTING THE ROLE OF THE CYTOPLASMIC MUTANT NUCLEOPHOSMIN IN ACUTE MYELOID LEUKAEMIA DEVELOPMENT

Acute myeloid leukaemia (AML) is a genetic heterogeneous group of diseases, with the largest subgroup showing a mutation in the Nucleophosmin gene (NPM1). Normally the NPM protein localizes mainly in the nucleolus, but in AML blasts it is aberrant localized to the cytoplasm (NPMc+AML). Notably, NPMc+AML patients show peculiar gene expression profiles, treatment response and prognosis. Hence, it has been proposed as an independent category for leukaemia classification according to WHO in 2008. In view of the relevance of NPMc+ mutation to AML pathogenesis and prognosis, understanding its role in leukaemia development represents a major issue in the field. The aim of this PhD project is to get further insight into the relevance of NPMc+ mutations to AML development. To this scope, here it is reported a characterization of a novel mouse model expressing the mutated protein. The hematopoietic restricted expression of the protein induces leukaemia in mice. This data definitively clarify that NPMc+ is an initiating mutation for leukaemia development. However, the long latency and low penetrance of disease onset strongly support the need of cooperating mutations. Since, the high frequency of FLT3-ITD mutations in NPMc+AML, we genetically tested the synergisms between the two abnormalities. To this scope, NPMc+ mice were crossed with FLT3-ITD mice (Lee, 2007). Double mutated mice developed leukaemia with sort latency and full penetrance indicating effective cooperation. Moreover, our data support the two hits model of tumourigenesis, where functional complementary mutations contribute to disease onset. Another major challenge of this project is to understand how NPMc+ affect the biology of normal HSPC and imposes the transition from normal to cancer stem cells. We found that NPMc+ expression perturbs the homeostasis of HSCP and expand the number of LT-HSC by increasing the proliferation rate. However, this enhanced proliferation is not associated to loss of quiescent and functional HSC, which may represent a reservoir of persistent pre-malignant cells available for the accumulation of additional genetic alteration. Further investigation into the biology of per-leukaemic stem cells may give insights into the molecular mechanisms imposed by the oncogene for malignancy transformation and finally may contribute for the development of new therapeutic strategies.