Cytoplasmic Nucleophosmin Research Articles

Inhibition of CRM1-Mediated Nuclear Export of Transcription FACTORS by Leukemogenic NUP98 Fusion Proteins.

Abstract 1959Poster Board I-982NUP98 is a nucleoporin that plays complex roles in the nucleocytoplasmic trafficking of macromolecules. Rearrangements of the NUP98 gene in human leukemia result in the expression of numerous fusion oncoproteins whose effect on nucleocytoplasmic trafficking is poorly understood. The present study was undertaken to determine the effects of leukemogenic NUP98 fusion proteins on CRM1-mediated nuclear export. NUP98-HOXA9, a prototypic NUP98 fusion, inhibited the nuclear export of two known CRM1 substrates: mutated cytoplasmic nucleophosmin (NPMc) and HIV-1 Rev. In-vitro binding assays revealed that NUP98-HOXA9 binds CRM1 through the FG-repeat motif in a Ran-GTP dependent manner similar to but stronger than the interaction between CRM1 and its export substrates. Two NUP98 fusions, NUP98-HOXA9 and NUP98-DDX10, whose fusion partners are structurally and functionally unrelated, interacted with endogenous CRM1 in cells as shown by co-immunoprecipitation. These leukemogenic NUP98 fusion proteins interacted with CRM1, Ran, and the nucleoporin NUP214 in a manner fundamentally different from that of wild-type NUP98. NUP98-HOXA9 and NUP98-DDX10 formed characteristic aggregates within the nuclei of myeloid cells and caused aberrant localization of CRM1 to these aggregates. These NUP98 fusions caused nuclear accumulation of two transcription factors, NFAT and NFΚB, that are regulated by CRM1-mediated export. The nuclear entrapment of NFAT and NFΚB correlated with enhanced transcription from promoters responsive to these transcription factors. Taken together, the results suggest a new mechanism by which NUP98 fusions dysregulate transcription and cause leukemia, namely, inhibition of CRM1-mediated nuclear export with aberrant nuclear retention of transcriptional regulators. Disclosures:No relevant conflicts of interest to declare.

Cytoplasmic localization of nucleophosmin in bone marrow blasts of acute myeloid leukemia patients is not completely concordant with NPM1 mutation and is not predictive of prognosis

Nucleophosmin (NPM1) gene mutations are reported to predict a favorable prognosis in acute myeloid leukemia (AML) patients. Aberrant cytoplasmic localization of nucleophosmin (NPM) protein is reported be a surrogate for NPM1 gene mutation. Using immunohistochemical (IHC) analysis, we assessed for NPM (clone 376) expression in formalin-fixed, formic acid-decalcified bone marrow biopsy specimens. DNA sequencing of exon 12 of NPM1 gene was performed in 104 patients. The study included 252 AML patients: 192 de novo AML, 33 AML preceded by either myelodysplastic syndrome or chronic myelomonocytic leukemia, and 27 therapy-related AML. The median age was 62 years and 115 patients were <or=60 years old. All patients received intensive chemotherapy. Cytoplasmic NPM was detected in 59 of 252 (23%) patients, including 48 of 192 (25%) de novo AML and 33 of 94 (35%) with a normal karyotype. DNA sequencing identified NPM1 mutations in 30 of 38 cases with cytoplasmic NPM and 10 of 66 cases with nuclear NPM. Cytoplasmic NPM was associated with young patient age (P=.024), FLT3/ITD (P=.005), CD34 negative blasts (P<.001), high peripheral blood blast count (P=.041), and high serum albumin level (P=.028). No statistical differences in overall or event-free survival were found on the basis of NPM localization. Similar results were obtained in patients<or=60 years old with normal karyotype and wild-type FLT3 (P=.768). IHC assessment for NPM localization did not predict prognosis in this patient cohort. The discordance between immunohistochemistry and DNA sequencing results indicates that DNA sequencing cannot be replaced by IHC assessment.

FLT3-ITD expression levels and their effect on STAT5 in AML with and withoutNPMmutations

FLT3-internal tandem duplication (ITD) mutations are heterogeneous with regards to length and proportion of DNA harbouring the mutation and the expression level of FLT3 also varies widely, however very little is known about the biological effects of these variables. We studied FLT3-associated biological parameters in 322 acute myeloid leukaemia samples to establish their importance. Expression of total FLT3 transcripts was shown to be significantly higher in the FLT3-ITD cohort (n = 121) compared to the wild-type cohort (P = 0.004). Whilst phosphorylated signal transducer and activator of transcription 5 (phospho-STAT5) was not confined to FLT3-ITD samples, within the FLT3-ITD group phosphorylation correlated with adjusted FLT3-ITD levels assessed by determining the total transcripts and proportion of FLT3-ITD within a sample. Expression of the STAT5 downstream target Bcl-xl (an isoform of BCL2L1) was strongly correlated with FLT3 total and adjusted FLT3-ITD levels in FLT3-ITD samples (P < 0.001), however there was no association between Bcl-xl and phospho-STAT5 levels suggesting that STAT5 is not the sole regulator of Bcl-xl in FLT3-ITD cells. We further stratified our cohort by the presence/absence of a cytoplasmic nucleophosmin NPMc+ mutation. Samples co-expressing NPMc+ had longer FLT3-ITD mutations (P = 0.01) and there was a high occurrence of NPMc+ in samples that had >1 FLT3-ITD mutation. Phospho-STAT5 levels were reduced in the FLT3-ITD/NPMc+ group (P = 0.04) suggesting that NPMc+ may oppose the FLT3-ITD-dependent activation of STAT5.

Altered nucleophosmin transport in acute myeloid leukaemia with mutated NPM1: molecular basis and clinical implications

Nucleophosmin (NPM1) is a highly conserved nucleo-cytoplasmic shuttling protein that shows a restricted nucleolar localization. Mutations of NPM1 gene leading to aberrant cytoplasmic dislocation of nucleophosmin (NPMc+) occurs in about one third of acute myeloid leukaemia (AML) patients that exhibit distinctive biological and clinical features. We discuss the latest advances in the molecular basis of nucleophosmin traffic under physiological conditions, describe the molecular abnormalities underlying altered transport of nucleophosmin in NPM1-mutated AML and present evidences supporting the view that cytoplasmic nucleophosmin is a critical event for leukaemogenesis. We then outline how a highly specific immunohistochemical assay can be exploited to diagnose NPM1-mutated AML and myeloid sarcoma in paraffin-embedded samples by looking at aberrant nucleophosmin accumulation in cytoplasm of leukaemic cells. This procedure is also suitable for detection of haemopoietic multilineage involvement in bone marrow trephines. Moreover, use of immunohistochemistry as surrogate for molecular analysis can serve as first-line screening in AML and should facilitate implementation of the 2008 World Health Organization classification of myeloid neoplasms that now incorporates AML with mutated NPM1 (synonym: NPMc+ AML) as a new provisional entity. Finally, we discuss the future therapeutic perspectives aimed at reversing the altered nucleophosmin transport in AML with mutated NPM1.

Cytoplasmic nucleophosmin is not detected in blastic plasmacytoid dendritic cell neoplasm

Acute myeloid leukemia carrying cytoplasmic mutated nucleophosmin (NPMc(+) AML) and blastic plasmacytoid dendritic cell neoplasm have been included as new entities in the 4(th) edition (2008) WHO classification of myeloid neoplasms. These conditions may show clinical and pathological overlapping features (leukemic and skin involvement, and expression of macrophage markers). In this study, we provide evidence that aberrant cytoplasmic dislocation of nucleophosmin - the immunohistochemical surrogate for NPM1 mutations - allows the two entities to be genetically separated. In fact, nucleophosmin is consistently cytoplasmic in NPMc(+) AML (because of the presence of NPM1 mutations), whilst it is nucleus-restricted (predictive of a germline NPM1 gene) in blastic plasmacytoid dendritic cell neoplasm. Our results clearly point cytoplasmic nucleophosmin (a full predictor of NPM1 mutations) as a new marker for distinguishing NPMc(+) AML and blastic plasmacytoid dendritic cell neoplasm, further clarify the cell of origin of NPMc(+) AML, and justify the inclusion of these pathological conditions as separate entities in the new WHO classification.

Role of Nucleophosmin Mutations in AML.

Abstract 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.

Evidence for CD34+ Hematopoietic Progenitor Cell Involvement in Acute Myeloid Leukemia with NPM1 Gene Mutation: Implications for the Cell of Origin

Acute myeloid leukemia expressing mutated NPM1 gene and cytoplasmic nucleophosmin (NPMc+ AML) [Falini B et al, NEJM 2005; 352:254–266] is a new entity of WHO classification that shows distinctive biological and clinical features, including a unique molecular signature characterized by downregulation of CD34 and upregulation of most HOX genes [Falini B et al, Blood 2007; 109:874–885]. Involvement of HOX genes in the maintenance of the stem-cell phenotype strongly suggest that AML with mutated NPM1 originates from a multipotent hematopoietic progenitor (HSC). This view is also supported by immunohistological findings showing that AML with mutated NPM1 frequently displays multilineage involvement [Pasqualucci L et al, Blood 2006; 108:4146–4155]. On the other hand, the frequent negativity of NPMc+ AML for the HSC-associated antigen CD34 raises the question of whether the mutation event occurs in a CD34-negative HSC (these cells have been identified in mice) or whether a minimal pool of CD34-positive NPM1-mutated leukemic cells does exist. Currently, the hierarchical level of stem cell involvement in NPMc+ AML is unknown. To address this issue, we purified CD34+ cells from NPMc+ AML patients and detected NPM1 mutant protein in the sorted population by Western blot with anti-NPM mutant specific antibodies [Martelli MP et al, Leukemia 2008] (Figure 1A). We investigated 6 NPMc+ AML patients presenting at diagnosis with 0.12%, 0.14%, 0.38%, 5%, 22%, and 28% of CD34+ cells in the peripheral blood. In all cases, CD34+ fractions (purity >90%) harboured NPM1 mutant protein, indicating they belong to the leukemic clone (Figure 1B). The percentage of most undifferentiated CD34+/CD38− cells in the CD34+ fractions ranged from 5 to 97%. Notably, in at least one case, all CD34+ NPM1-mutated leukemic cells were CD38−negative. Moreover in all cases, CD34+ NPM1-mutated leukemic cells appeared to express CD123 (IL-3 receptor), considered a marker of the leukemic stem cell and target of potential therapy. Double staining of bone marrow biopsies with anti-CD34 and anti-NPM antibodies revealed that the rare CD34+ cells expressed NPM1 aberrantly in the cytoplasm. Inoculation of CD34+ NPM1-mutated AML cells into sublethally irradiated NOD/SCID mice resulted into leukemia engrafment in various body sites, especially bone marrow, spleen, lung and liver. Preliminary results showed that CD34+ leukemic cells reacquired the same leukemic phenotype as the original patient's, including CD34-negativity of the leukemic bulk in spite of any lack of differentiation. This finding suggests that NPM1 mutant protein may be involved in downregulation of CD34 antigen, while keeping a gene expression profile typical of the hematopoietic stem cell. These findings suggest the CD34+ fraction contains the SCID-leukemia initiating cells (SL-IC) and point to CD34+/CD38− HSC as the cell of origin of AML with mutated NPM1. [Display omitted]

Nucleophosmin (NPM) Has Increased Expression and Shuttling Activity in Chronic Lymphocytic Leukaemia without Somatic Hypermutation of the Immunoglubulin Gene (UM-CLL) and Is Linked to Ribosomal Proteins: A Possible Role for Increased Protein biosynthesis in the Adverse Outcome of UM-CLL.

Chronic lymphocytic leukaemia (CLL) is a neoplastic proliferation of mature B-lymphocytes that has a marked variability of clinical outcome. Two distinct groups of CLL may be distinguished by the presence (M-CLL) or absence (UM-CLL) of somatic hypermutation of the immunoglobulin heavy-chain gene. UM-CLL has an adverse outcome, but the precise biological differences that underlie this more aggressive clinical-course are unclear. Using a proteomic approach, we have found that M-CLL and UM-CLL are consistently distinguished according to protein expression pattern. Of those proteins, the most important predictor of CLL class was the protein nucleophosmin (NPM). A significant increase in expression of NPM was found in UM-CLL compared with M-CLL (3.2 fold p<0.05 Mann Whitney U test). This increased expression did not reflect differences in apoptosis or proliferation, or gene mutation. However, co-immunoprecipitation experiments identified the ribosomal proteins 60s (L19 and L24) and 40s (S9) to be associated with NPM in CLL. Ribosomal proteins are known to shuttle between the nucleoli, nucleus, and cytoplasm, of cells. Standard- and confocal-immunofluorescence microscopy identified NPM to be present, as expected, in the nucleoli of all CLL cells. Additionally however, NPM was consistently detected in the nucleoplasm of CLL, and in a proportion of cells from all CLL cases, NPM was also shown to be present in the cytoplasm (range =7–52% of cells expressed cytoplasmic NPM). Morphometric analysis suggested no net transfer from the nucleolus to the cytoplasm; but did suggest trafficking from the nucleoplasmic compartment to the cytoplasm consistent with shuttling between these two cellular regions. In UM-CLL the fluorescent intensity of the cytoplasmic NPM was found to be higher than that of the M-CLL (t-test p=0.009), and there was an approximate two fold increased proportion of cytoplasmic NPM-expression in the UM-CLL subgroup (p value< 0.001). We propose therefore that NPM, in association with ribosomal proteins, displays high nucleocytoplasmic shuttling in CLL, and may reflect increased protein biosynthesis. In particular, the higher levels of NPM expression and shuttling in UM-CLL may play a role in the adverse outcome of this form of the disorder.

Clonal Heterogeneity in the 5q- Syndrome: NPMc+ and p53 Expressing Progenitors Are Insensitive to Lenalidomide Treatment and Expand at Disease Progression

Clonal heterogeneity has not been described in patients with myelodysplastic syndrome (MDS) of the 5q- syndrome subtype, for which lenalidomide has emerged as a highly potent treatment. Interestingly, transformation to acute myeloid leukemia (AML) occurs more frequently in patients without a cytogenetic response to lenalidomide. We describe two patients with classical 5q- syndrome with complete erythroid and partial cytogenetic response to lenalidomide, who evolved to high-risk MDS and AML, respectively, with complex karyotypes including del(5q). Immunohistochemistry of pre-treatment marrow biopsies revealed small fractions of progenitors with aberrant cytoplasmic nucleophosmin (NPMc+) expression and in one patient also a TP53 mutation. Both lesions may induce a state of genomic instability, and represent novel findings in 5q- syndrome. These subclones remained stable during response, but expanded at transformation, suggesting that pre-existing cells with more malignant genotype may be insensitive to lenalidomide and responsible for disease progression. We are currently investigating a larger patient material for the presence of subclones with molecular lesions in order to assess the potential impact on the risk of leukemic evolution.

SMT3IP1, a nucleolar SUMO-specific protease, deconjugates SUMO-2 from nucleolar and cytoplasmic nucleophosmin

Sumoylation is reversibly regulated by SUMO-specific proteases. We characterized a nucleolar SUMO-specific protease, SMT3IP1, which has a preference for SUMO-2/3. To elucidate SMT3IP1 function, we screened for its interacting proteins that may be its substrates or regulate its activity. By using yeast two-hybrid screening, we identified nucleophosmin (NPM) as an SMT3IP1-binding partner. SMT3IP1 could preferentially remove SUMO-2 from sumoylated NPM. A catalytically inactive SMT3IP1 mutant increased intracellular accumulation of SUMO-2-modified NPM in a dominant-negative manner. Sumoylation of cytoplasmic mutated NPM was markedly elevated in an ARF-dependent manner. Despite the divergence in their localization, ectopic expression of SMT3IP1 could desumoylate a SUMO-2-modified NPM mutant. Additionally, genotoxic drugs caused the dissociation of NPM from nucleolar co-localization with SMT3IP1, but did not affect desumoylation of NPM by SMT3IP1. Our findings suggest that SMT3IP1-mediated desumoylation might control NPM physiological functions at both the nucleolus and other subcellular compartments.

Nucleophosmin ( NPM1) Mutations in Acute Myeloid Leukemia: An Ongoing (Cytoplasmic) Tale of Dueling Mutations and Duality of Molecular Genetic Testing Methodologies

This Commentary highlights two articles that focus on molecular techniques to identify mutations in nucleophosmin (NPM1), which is the most frequently mutated gene in cytogenetically normal acute myeloid leukemia (CN-AML)

Distinctive microRNA signature of acute myeloid leukemia bearing cytoplasmic mutated nucleophosmin.

Acute myeloid leukemia (AML) carrying NPM1 mutations and cytoplasmic nucleophosmin (NPMc+ AML) accounts for about one-third of adult AML and shows distinct features, including a unique gene expression profile. MicroRNAs (miRNAs) are small noncoding RNAs of 19-25 nucleotides in length that have been linked to the development of cancer. Here, we investigated the role of miRNAs in the biology of NPMc+ AML. The miRNA expression was evaluated in 85 adult de novo AML patients characterized for subcellular localization/mutation status of NPM1 and FLT3 mutations using a custom microarray platform. Data were analyzed by using univariate t test within BRB tools. We identified a strong miRNA signature that distinguishes NPMc+ mutated (n = 55) from the cytoplasmic-negative (NPM1 unmutated) cases (n = 30) and includes the up-regulation of miR-10a, miR-10b, several let-7 and miR-29 family members. Many of the down-regulated miRNAs including miR-204 and miR-128a are predicted to target several HOX genes. Indeed, we confirmed that miR-204 targets HOXA10 and MEIS1, suggesting that the HOX up-regulation observed in NPMc+ AML may be due in part by loss of HOX regulators-miRNAs. FLT3-ITD+ samples were characterized by up-regulation of miR-155. Further experiments demonstrated that the up-regulation of miR-155 was independent from FLT3 signaling. Our results identify a unique miRNA signature associated with NPMc+ AML and provide evidence that support a role for miRNAs in the regulation of HOX genes in this leukemia subtype. Moreover, we found that miR-155 was strongly but independently associated with FLT3-ITD mutations.

NPM1 mutations and cytoplasmic nucleophosmin are mutually exclusive of recurrent genetic abnormalities: a comparative analysis of 2562 patients with acute myeloid leukemia

Acute myeloid leukemia carrying NPM1 mutations and cytoplasmic nucleophosmin (NPMc(+) acute myeloid leukemia) represents one-third of adult AML (50-60% of all acute myeloid leukemia with normal karyotype) and shows distinct biological, pathological and clinical features. We confirm in 2562 patients with acute myeloid leukemia our previous observation that NPM1 mutations and cytoplasmic nucleophosmin are mutually exclusive of recurrent genetic abnormalities. Taken together, these findings make NPMc+ acute myeloid leukemia a good candidate for inclusion in the upcoming World Health Organization classification.

Deguelin Represses Both the Expression of Nucleophosmin and some Nucleoporins: Nup88 and Nup214 in Jurkat Cells

Since the first report about cytoplasmic nucleophosmin (NPM) in acute myelogenous leukemia with a normal karyotype was announced, the shuttling activity of NPM and its proper subcellular localization have drawn many attentions. Mechanisms that regulate nucleocytoplasmic transport of proteins may provide novel opportunities for drug development. Here we show that, in Jurkat cells, strong fluorescence density of NPM prevails in the nucleus, while, some key nucleoporins: Nup88 and Nup214 localize mainly in the cytoplasm. Deguelin, a natural occurring rotenoid, presents powerful anti-leukemia effects through proliferation inhibition and apoptosis induction in Jurkat cells. Deguelin downregulates the expression of NPM, Nup88 and Nup214 in a dose-dependent manner and reverts the localization of Nup88 and Nup214 to nuclear rim. These results suggest that deguelin exhibit its strong anti-leukemia effects might through the regulation of some nucleoporins, thus influence the subsequent abnormal expressions or localizations of some key proteins involved in proliferation and/or apoptosis, such as: NPM.

Translational Regulation of Gene Expression by Cytoplasmic Nucleophosmin in AML: A Global Evaluation of RNA's Differentially Recruited to Polyribosomes.

Nucleophosmin (NPM) exon 12 mutations are found in 50–60 percent of acute myelogenous leukemia (AML) with normal karyotype, and the altered protein (termed NPMc) is relocalized from the nucleolus to the cytoplasm. Multiple functions have been attributed to NPM, and research by other investigators demonstrated that NPM serves a critical role in nucleolar-to-cytoplasmic shuttling of rRNA's and in ribosome biogenesis. An early indication of the role of NPM in hematopoietic neoplasia was its fusion to the ALK protein in patients with anaplastic large cell lymphoma (ALCL). We and others have demonstrated that the expression of NPM-ALK regulates mediators of translational initiation, including mTOR, S6K1, and 4E-BP1, and we showed that NPM-ALK expression significantly alters the partitioning of mRNA's to polyribosomes. Regulators of translational initiation can be rate-limiting for neoplasia in animal models, and we hypothesized that NPMc promotes leukemogenesis in a similar manner by altering the recruitment of specific mRNA's to polysomes and thereby altering their translation. To evaluate this hypothesis, we assessed the global profile of polysomal mRNA in cells engineered to express NPMc. We generated pTRE2-NPMc(A)-Ba/F3 murine hematopoietic cells which express NPMc under the control of a tetracycline-inducible promoter. These cells carry “variant A” NPMc, which contains an insertion of a TCTG tetranucleotide at positions 956 through 959 and is the most common NPMc variant in AML. Control cells contained the empty vector. We used these cell lines to compare in a global manner the partitioning of mRNA species to polysomes, which were isolated using sucrose gradient fractionation of post-nuclear supernatants. From this enriched polysomal material we purified RNA, and the global profile of mRNA in these fractions was assessed by using Affymetrix mouse genome microarrays. The data generated in this effort was analyzed using the GeneChip robust multichip analysis (GCRMA) method. Significance Analysis of Microarrays (SAM) and Linear Models for Microarray Data (Limma) were employed to identify significant differentially expressed genes. We identified 345 genes whose mRNA's exhibited altered polysomal partitioning, including 253 increased in cells with NPMc and 192 with decreased polysome association (FDR≤ 0.05). These translationally regulated mRNA's encode proteins that control transcription, post-translational modification of proteins, and proteins associated with hematopoietic neoplasia. One of these differentially recruited mRNA's encodes the Dub-1 protein, a ubiquitin peptidase that has been implicated previously in hematopoietic cell growth control, and the mRNA showed a 6-fold increase in polysome recruitment. Interestingly, in our previous evaluation comparing cells with NPM-ALK versus the kinase-deficient variant, there was a 50-fold increase in the partitioning of Dub-1 mRNA to polysomes. These findings suggest that alteration of Dub-1 expression through mRNA partitioning might be commonly utilized by both NPMc as well as NPM-ALK. Collectively, these findings suggest that concerted disruption of gene transcription and translation might broadly mediate the pathogenesis of hemotopoietic neoplasia.

The leukemia-associated cytoplasmic nucleophosmin mutant is an oncogene with paradoxical functions: Arf inactivation and induction of cellular senescence

Mutations leading to aberrant cytoplasmic localization of Nucleophosmin 1 (NPM1) have been recently identified as the most frequent genetic alteration in acute myelogenous leukemia. However, the oncogenic potential of this nucleophosmin mutant (NPMc+) has never been established, which casts doubt on its role in leukemogenesis. By performing classical transformation assays, we find that NPMc+, but not wild-type NPM, cooperates specifically with adenovirus E1A to transform primary mouse embryonic fibroblasts in soft agar. We demonstrate that NPMc+ blocks the p19(Arf) (Arf) induction elicited by E1A. Surprisingly, however, we find that NPMc+ induces cellular senescence and that E1A is able to overcome this response. We propose a model whereby the NPMc+ pro-senescence activity needs to be evaded for oncogenic transformation, even though NPMc+ can concomitantly blunt the Arf/p53 pathway. These findings identify for the first time NPMc+ as an oncogene and shed new unexpected light on its mechanism of action.

Cytoplasmic nucleophosmin in myeloid sarcoma occurring 20 years after diagnosis of acute myeloid leukaemia

Falini and colleagues1 reported that 35% of adults with acute myeloid leukaemia (AML) carry mutations at exon 12 of the nucleophosmin gene (NPM1), which leads to the formation of a C-terminal nuclear-export signal and mutations at tryptophans 288 and 2901 that cause aberrant accumulation of nucleophosmin (NPM) in the cytoplasm.2 Mutation of NPM1 at exon 12 is the genetic lesion that arises most specifically and frequently in adults1 and children3 with AML who have a normal karyotype. These findings have been confirmed in about 1000 patients with AML.

Cytoplasmic localization of NPM in myeloid leukemias is dictated by gain-of-function mutations that create a functional nuclear export signal

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.

Mechanism of Altered Nucleo-Cytoplasmic Traffic of Nucleophosmin in Acute Myelogenous Leukemia Carrying Exon-12 NPM Mutations (NPMc+ AML).

We recently found that about one-third of adult AML (60% of all AML with normal karyotype) display aberrant cytoplasmic expression of nucleophosmin (NPM) which is due to mutations occurring at the exon-12 of NPM gene (Falini B, et al., Cytoplasmic nucleophosmin in acute myelogenous leukemia with a normal karyotype.. N Engl J Med 2005; 352: 254–266). Hereby, we clarify the molecular mechanism underlying cytoplasmic NPM accumulation that yet remained to be elucidated. AML-associated mutated NPM alleles encode abnormal NPM proteins (25 mutants so far identified) which have acquired at the C-terminus a nuclear export signal-(NES) motif and lost at least one of tryptophan residues 288 and 290 which determine nucleolar localization. Both alterations are crucial for mutant NPM export from nucleus to cytoplasm. In fact, the cytoplasmic NPM accumulation is blocked by leptomycin-B and ratjadones which are specific inhibitors of exportin-1/CRM1, and by re-insertion of tryptophan residues 288 and 290, which respectively relocate NPM mutants in the nucleoplasm and nucleoli. Thus, for cytoplasmic accumulation of NPM to occur, the NES motif and tryptophan mutations must act in concert. Possibly, when NPM mutans enter the nucleus by virtue of their nuclear localization signals (NLS), their capability to bind nucleoli must be hindered at least partially to become a CRM-1 target. Specific antibodies anti-NPM mutant proteins showed that the mutants localized exclusively in the cytoplasm and recruited in that site the wild-type NPM protein which is physiologically located in the nucleoli. These findings suggest that the NPM mutants may interfere with the functions of wild-type NPM and possibly contribute to leukemogenesis. Immunostaining of 393 AML cases using anti-NPM monoclonal antibodies predicted the presence of NPM exon-12 mutations in all 191 NPM-cytoplasmic positive cases. This finding is consistent with the fact that, despite genomic heterogeneity, all NPM mutants contain a NES motif which, in the presence of altered tryptophans, promotes their rapid export from the nucleus to the cytoplasm. The immunohistochemical test is diagnostically relevant since it can be used as simple first-step procedure in molecular-genetic characterization of AML and as a surrogate for mutational analysis in selected cases. These findings are also clinically relevant since cytoplasmic NPM/NPM mutations are predictors of good response to induction therapy and favourable prognosis in AML with a normal karyotype.

Acute myeloid leukemia bearing cytoplasmic nucleophosmin (NPMc+ AML) shows a distinct gene expression profile characterized by up-regulation of genes involved in stem-cell maintenance

AbstractApproximately one third of acute myeloid leukemias (AML55553) are characterized by aberrant cytoplasmic localization of nucleophosmin (NPMc+ AML), consequent to mutations in the NPM putative nucleolar localization signal. These events are mutually exclusive with the major AML-associated chromosomal rearrangements, and are frequently associated with normal karyotype, FLT3 mutations, and multilineage involvement. We report the gene expression profiles of 78 de novo AMLs (72 with normal karyotype; 6 without major chromosomal abnormalities) that were characterized for the subcellular localization and mutation status of NPM. Unsupervised clustering clearly separated NPMc+ from NPMc– AMLs, regardless of the presence of FLT3 mutations or non–major chromosomal rearrangements, supporting the concept that NPMc+ AML represents a distinct entity. The molecular signature of NPMc+ AML includes up-regulation of several genes putatively involved in the maintenance of a stem-cell phenotype, suggesting that NPMc+ AML may derive from a multipotent hematopoietic progenitor.