111-2-06). “
“Nucleophosmin (NPM1) is a nucleolar multifunctional phosphoprotein involved in RNA metabolism [1], [2] and [3], regulation of the p19/ARF-p53 tumor-suppressor pathway [4] and [5] and c-Myc turnover through Fbw7γ [6]. Under physiological conditions,

the protein shuttles between nucleus and cytoplasm. In about one-third of adult patients with AML with normal karyotype, it has been demonstrated that AML cells bear mutations in the last coding exon of the NPM1 gene (exon 12) [7], [8] and [9]. More than 40 heterozygous different mutations have been described. SP600125 purchase The mutations result in frame shift and the loss of the two tryptophan residues located in the C-terminal portion of the protein that are necessary for nucleolar localization. The insertion of short nucleotide stretches of eleven amino acids generates the de novo formation of a Chromosomal Region Maintenance 1 (CRM1)/Exportin 1-dependent NES responsible Belnacasan for mutant NPM1 cytoplasmic delocalization (NPMc+) [10], [11] and [12]. Although a correlation between NPM1 cytoplasmic accumulation and leukemia initiation and progression has been recently demonstrated in vivo in murine models [13] and [14], so far there is no direct molecular

evidence of the mechanism by which NPMc+ can induce pathological Rho conditions. It has been suggested that NPMc+ could form

hetero-octamers with NPM1 inducing its delocalization and that of proteins normally associated to NPM1, such as p19/ARF and Fbw7γ [4], [5], [6] and [15]. A monoclonal antibody (T26) specific for the cytoplasmic mutation has been demonstrated helpful to confirm the connection between NPMc+ expression and AML in patients [16]. However, when we performed a double staining to identify both NPM1 and NPMc+ localization, it turned out that a significant portion of the wild type protein was still located in the nucleoli [17], questioning the hypothesis of a massive NPM1 migration to the cytoplasm. Nevertheless, both the shuttling and the residential activities of NPM1 are necessary for the normal metabolism since NPM1 seems to be the rate-limiting nuclear export shuttle for ribosome components in mammalian cells and an indispensable regulator of protein synthesis [18]. The diminished NPM1 shuttling capacity impairs the regular ribosome assembly, places genetic pressure upon p19/ARF/p53 pathway, and leads to mutations resulting in cellular transformation [18]. This means that NPM1 shuttling must be preserved as well as its predominant nucleolar accumulation.