Human Nucleophosmin (NPM1) Perturbs Myeloid Development in Zebrafish in Vivo

The most frequent genetic alterations in adult cases of Acute Myeloid Leukemia (AML) are mutations in the human nucleophosmin (hNPM1) gene. In about 30% of AMLs this nucleolar phosphoprotein is aberrantly localized to the cytoplasm (hNPMc) due to these mutations affecting the protein’s nuclear shuttling. hNPMc AMLs exhibit distinctive clinical and biological features that have led to its WHO classification as a distinct myeloid neoplasm. Despite its prominent association with AML, in vitro and in vivo models of hNPMc transformation in myeloid cells are lacking and its role in this process remains poorly understood. To further our understanding of hNPM function we are using the zebrafish vertebrate model system that is ideally suited for the in vivo analysis of cellular function and development during embryonic hematopoiesis. Importantly, the wide variety of blood cell types and key cellular factors regulating hematopoiesis are highly conserved between mammals and zebrafish. To investigate the in vivo role of hNPMc in hematopoiesis, we injected mRNAs encoding hNPM1wt and the leukemia-associated mutant hNPMc into one-cell stage zebrafish embryos. By fusing GFP to these hNPM proteins, we were able to follow the developmental expression of hNPM1 and its subcellular localization during embryogenesis. Analysis using confocal microscopy showed that NPMc is localized to the cell cytoplasm, while NPM1wt is found in nucleoli, as in human and mouse cells. These studies demonstrate the conservation of the nuclear-cytoplasmic transport functions of the human proteins in the zebrafish and further support its validity as a model to analyze and determine hNPM function. We also observed that hNPMc is expressed at far lower levels than its wild-type counterpart and is almost undetectable at 36hpf while hNPMwt continues to be expressed. Unlike mammals, two endogenous zebrafish NPM1 proteins were identified and named, zNpm1a and zNpm1b. Both zNpm1a and zNpm1b proteins are ubiquitously expressed in the embryo and demonstrated nucleolar localization. Expression of hNPMwt resulted in its colocalization with endogenous zNpm while hNPMc was able to bring about the export of both zebrafish proteins to the cytoplasm through heterotypic interactions. Co-immunoprecipitation experiments confirmed the interaction between human and zebrafish NPM1 proteins and zNpm1a and zNpm1b were both able to bind and co-immunoprecipitate with hNPM1 and hNPMc. These experiments suggest that transient hNPMc expression during zebrafish hematopoiesis may mimic its function in human leukemic blasts and provide clues to its functional role in AML. Comparable protein levels of either hNPMwt or hNPMc were expressed in embryos, confirmed by western blot at 22–24 hpf, and analyzed by whole mount in situ hybridization (WISH) using antisense RNA markers of specific hematopoietic lineages. Expression of hNPMc caused an increase in cMYB expression at 36 hpf, indicating an increase in the hematopoietic stem cell compartment. Furthermore, myeloid precursors (PU.1 at 22 hpf) also showed an increase upon hNPMc expression; however, mature myeloid cell (MPO and L-plastin at 26 hpf) levels were not increased relative to those in control hNPMwt injected embryos. Interestingly, the expression of hNPMc in p53 mutant embryos resulted in elevated levels of both PU.1 and MPO expressing cells, suggesting that hNPMc in zebrafish can activate p53 dependent cell cycle arrest, senescence or cell death in PU.1 cells prior to differentiation. These in vivo studies of hNPMc function during zebrafish hematopoietic differentiation suggest that hNPMc expression may increase the stem cell/ myeloid precursor compartment and can activate a p53 dependent cell death response in myeloid cells. Taking advantage of the zebrafish system in these continuing studies will further address how hNPMc expression may contribute to leukemogenesis.