Role of Nucleophosmin Mutations in AML.

Significant advances have been made towards understanding the molecular pathogenesis and prognostic determinants in acute myelogenous leukemia of normal karytype (AMLNK). One of these, somatic mutation within exon 12 of the nucleophosmin gene (NPM1), is present in 50–60% of AML-NK and has been associated with favorable response to induction chemotherapy, overall survival, and event-free survival, but only in the absence of FLT3-ITD mutation. In addition to exon 12 mutation, NPM1 is disrupted in hematologic malignancies through fusion to partner proteins such as the anaplastic lymphoma kinase (ALK), myeloid leukemia factor 1 in myelodysplasia (MLF1), and retinoic acid receptor-α (RARα). The NPM1 gene encodes a 37-kDa protein that is predominantly localized to the nucleolus but also shuttles to the nucleoplasm and cytoplasm. A strong association (perhaps a 100% correlation) exists between NPM1 mutation and aberrant localization of the nucleophosmin protein in the cytoplasm. This mislocalization of nucleophosmin has been attributed to the loss of tryptophan residues 288 and 290 (or 290 only) in the carboxy terminus of this protein, and these motifs are required for nucleolar localization of nucleophosmin. Importantly, the NPM1 mutation also creates a de novo nuclear export signal within nucleophosmin. The functional role of wild-type nucleophosmin has been implicated in the regulation of cell growth control through p14ARF and p53 interactions, ribosome biogenesis, centrosome duplication, as well as other functions. Pediatric AML samples with NPM1 mutation were reported to have a distinct gene expression signature, including altered expression of homeobox (HOX) genes, and adult AML specimens carrying mutant NPM1 were reported to have a distinct microRNA expression signature. In addition to alterations in the expression of mRNA and microRNA species, the critical function of nucleophosmin in ribosome biogenesis, as well as its reported association with poly(A)(+) mRNA’s in vivo, suggests that mutant NPM1 could disrupt gene expression through aberrant translational control. Regulators of translational initiation can be rate-limiting for neoplasia in animal models, and we evaluated the hypothesis that cytoplasmic nucleophosmin promotes leukemogenesis by similarly altering the translational control of gene expression. Here, we present data to show that enforced expression of mutant nucleophosmin significantly alters the partitioning of mRNA’s to polyribosomes. Polyribosomal extracts were purified from cells that express wild-type or mutant nucleophosmin, RNA was extracted from this material, and the global profile of mRNA in these fractions was evaluated by gene expression microarray analysis. Enforced expression of cytoplasmic nucleophosmin significantly altered mRNA recruitment to polysomes. Moreover, we found common features in the polysome signature of cells expressing mutant NPM or the NPM-ALK fusion, suggesting that cytoplasmic NPM and the NPM-ALK fusion might disrupt translational initiation through partially overlapping mechanisms. These findings suggest that mutant nucleophosmin can perturb mRNA translational initiation in concert with other molecular mechanisms in the pathogenesis of AML-NK.