title: Cytoplasmic localization of NPM in myeloid leukemias is dictated by gain-of-function mutations that create a functional nuclear export signal.
abstract: Nucleophosmin (NPM) is a nucleus-cytoplasmic shuttling protein that is implicated in centrosome duplication, cell cycle progression and stress response. At the steady state, NPM localizes mainly in the nucleolus, where it forms a complex with different cellular proteins. One-third of acute myeloid leukemias (AML) are characterized by aberrant cytoplasmic localization of NPM, due to mutations within its last coding exon (exon 12) that cause a frameshift and the formation of novel C-termini. We report here our investigations on the molecular basis for the aberrant localization of mutated NPM. Alignment of the C-terminus of the various NPM mutants revealed the obligatory presence of four amino-acid residues that match a CRM1-dependent nuclear export signal (NES). Single alanine-substitutions at these sites provoked nuclear re-localization, while fusion of the mutated C-terminus to a heterologous nuclear protein induced CRM1-dependent cytoplasmic localization. Molecular characterization of one exceptional AML carrying cytoplasmic NPM and germ line exon 12 revealed a somatic mutation in the splicing donor site of exon 9 that caused the formation of a functional NES. It appears, therefore, that AMLs are frequently characterized by gain-of-function mutations of NPM that create functional NES, suggesting that alterations of nuclear export might represent a general mechanism of leukemogenesis and a novel target for therapeutic intervention.
authors.affiliation: Department of Experimental Oncology, European Institute of Oncology, Milan, Italy.
authors.forename: A R
authors.initials: AR
authors.lastname: Mariano
authors.forename: E
authors.initials: E
authors.lastname: Colombo
authors.forename: L
authors.initials: L
authors.lastname: Luzi
authors.forename: P
authors.initials: P
authors.lastname: Martinelli
authors.forename: S
authors.initials: S
authors.lastname: Volorio
authors.forename: L
authors.initials: L
authors.lastname: Bernard
authors.forename: N
authors.initials: N
authors.lastname: Meani
authors.forename: R
authors.initials: R
authors.lastname: Bergomas
authors.forename: M
authors.initials: M
authors.lastname: Alcalay
authors.forename: P G
authors.initials: PG
authors.lastname: Pelicci
mesh_terms: D000208:Acute Disease; D001483:Base Sequence; D003593:Cytoplasm / Q000378:metabolism*; D006801:Humans; D007951:Leukemia, Myeloid / Q000378:metabolism*; D008969:Molecular Sequence Data; D009154:Mutation; D049790:Nuclear Export Signals / Q000235:genetics*; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism*; D000090243:Nucleophosmin
chemical_list: C000716967:NPM1 protein, human; D049790:Nuclear Export Signals; D009687:Nuclear Proteins; D000090243:Nucleophosmin