NPM mutations in acute myelogenous leukemia

Npm1 haploinsufficiency in collaboration with MEIS1 is sufficient to induce AML in mice

NPM1 is among the most frequently mutated genes in acute myeloid leukemia (AML). Mutations in the NPM1 gene result in the increased export of NPM1 to the cytoplasm (NPM1c) and are associated with multiple transforming events including the aberrant upregulation of MEIS1 that maintains stem cell and cell cycle-associated pathways in NPM1c AML. However, another consequence of the NPM1c mutation is the inadequate levels of NPM1 wild-type in the nucleus and nucleolus, caused by the loss of one wild-type allele in addition to enforced NPM1 nuclear export. The contribution of NPM1 haploinsufficiency independently of the NPM1 mutation to AML development and its relationship with MEIS1 function is poorly understood. Using mouse models, our study shows that NPM1 haploinsufficiency paired with MEIS1 overexpression is sufficient to induce a fully penetrant AML in mice that transcriptionally resembles human NPM1c AML. NPM1 haploinsufficiency alters MEIS1-binding occupancies such that it binds the promoter of the oncogene structural maintenance of chromosome protein 4 (SMC4) in NPM1 haploinsufficient AML cells but not in NPM1 wild-type-harboring Hoxa9/Meis1-transformed cells. SMC4 is higher expressed in haploinsufficient and NPM1c+ AML cells, which are more vulnerable to the disruption of the MEIS1-SMC4 axis compared with AML cells with nonmutated NPM1. Taken together, our study underlines that NPM1 haploinsufficiency on its own is a key factor of myeloid leukemogenesis and characterizes the MEIS1-SMC4 axis as a potential therapeutic target in this AML subtype.

Myeloid leukemia factor 1: A "double-edged sword" in health and disease

The occurrence and development of malignancies are closely related to abnormal cell cycle regulation. Myeloid leukemia factor 1 (MLF1) is a small nucleocytoplasmic shuttling protein associated with cell cycle exit, apoptosis, and certain immune functions. Therefore, it is pertinent to explore the role of MLF1 in health and diseases. Studies to date have suggested that MLF1 could act as a double-edged sword, regulating biochemical activities directly or indirectly. In hematopoietic cells, it serves as a protective factor for the development of lineages, and in malignancies, it serves as an oncogenesis factor. The diversity of its functions depends on the binding partners, including tumor inhibitors, scaffolding molecules, mitochondrial membrane proteins, and transcription factors. Emerging evidence indicates that MLF1 influences immune responses as well. This paper reviews the structure, biological function, and research progress on MLF1 in health and diseases to provide new insights for future research.

The Role of Nucleophosmin 1 (NPM1) Mutation in the Diagnosis and Management of Myeloid Neoplasms

Nucleophosmin (NPM1) is a multifunctional protein with both proliferative and growth-suppressive roles in the cell. In humans, NPM1 is involved in tumorigenesis via chromosomal translocations, deletions, or mutation. Acute myeloid leukemia (AML) with mutated NPM1, a distinct diagnostic entity by the current WHO Classification of myeloid neoplasm, represents the most common diagnostic subtype in AML and is associated with a favorable prognosis. The persistence of NPM1 mutation in AML at relapse makes this mutation an ideal target for minimal measurable disease (MRD) detection. The clinical implication of this is far-reaching because NPM1-mutated AML is currently classified as being of standard risk, with the best treatment strategy (transplantation versus chemotherapy) yet undefined. Myeloid neoplasms with NPM1 mutations and <20% blasts are characterized by an aggressive clinical course and a rapid progression to AML. The pathological classification of these cases remains controversial. Future studies will determine whether NPM1 gene mutation may be sufficient for diagnosing NPM1-mutated AML independent of the blast count. This review aims to summarize the role of NPM1 in normal cells and in human cancer and discusses its current role in clinical management of AML and related myeloid neoplasms.

Acceleration or Brakes: Which Is Rational for Cell Cycle-Targeting Neuroblastoma Therapy?

Unrestrained proliferation is a common feature of malignant neoplasms. Targeting the cell cycle is a therapeutic strategy to prevent unlimited cell division. Recently developed rationales for these selective inhibitors can be subdivided into two categories with antithetical functionality. One applies a “brake” to the cell cycle to halt cell proliferation, such as with inhibitors of cell cycle kinases. The other “accelerates” the cell cycle to initiate replication/mitotic catastrophe, such as with inhibitors of cell cycle checkpoint kinases. The fate of cell cycle progression or arrest is tightly regulated by the presence of tolerable or excessive DNA damage, respectively. This suggests that there is compatibility between inhibitors of DNA repair kinases, such as PARP inhibitors, and inhibitors of cell cycle checkpoint kinases. In the present review, we explore alterations to the cell cycle that are concomitant with altered DNA damage repair machinery in unfavorable neuroblastomas, with respect to their unique genomic and molecular features. We highlight the vulnerabilities of these alterations that are attributable to the features of each. Based on the assessment, we offer possible therapeutic approaches for personalized medicine, which are seemingly antithetical, but both are promising strategies for targeting the altered cell cycle in unfavorable neuroblastomas.

Nucleophosmin in leukemia: Consequences of anchor loss

Nucleophosmin (NPM), one of the most abundant nucleolar proteins, has crucial functions in ribosome biogenesis, cell cycle control, and DNA-damage repair. In human cells, NPM occurs mainly in oligomers. It functions as a chaperone, undergoes numerous interactions and forms part of many protein complexes. Although NPM role in carcinogenesis is not fully elucidated, a variety of tumor suppressor as well as oncogenic activities were described. NPM is overexpressed, fused with other proteins, or mutated in various tumor types. In the acute myeloid leukemia (AML), characteristic mutations in NPM1 gene, leading to modification of NPM C-terminus, are the most frequent genetic aberration. Although multiple mutation types of NPM are found in AML, they are all characterized by aberrant cytoplasmic localization of the mutated protein. In this review, current knowledge of the structure and function of NPM is presented in relation to its interaction network, in particular to the interaction with other nucleolar proteins and with proteins active in apoptosis. Possible molecular mechanisms of NPM mutation-driven leukemogenesis and NPM therapeutic targeting are discussed. Finally, recent findings concerning the immunogenicity of the mutated NPM and specific immunological features of AML patients with NPM mutation are summarized.

Exposure of Rat Neural Stem Cells to Ethanol Affects Cell Numbers and Alters Expression of 28 Proteins

Developing brain cells express many proteins but little is known of how their protein composition responds to chronic exposure to alcohol and/or how such changes might relate to alcohol toxicity. We used cultures derived from embryonic rat brain (previously shown to contain mostly neural stem cells; rat NSC, rNSC), exposed them to ethanol (25-100 mM) for up to 96 h and studied how they reacted. Ethanol (50 and 100 mM) reduced cell numbers indicating either compromised cell proliferation, cytotoxicity or both. Increased lipid peroxidation was consistent with the presence of oxidative stress accompanying alcohol-induced cytotoxicity. Proteomics revealed 28 proteins as altered by ethanol (50 mM for 96 h). Some were constituents of cytoskeleton, others were involved in transcription/translation, signal transduction and oxidative stress. Nucleophosmin (NPM1) and dead-end protein homolog 1 (DND1) were further studied by immunological techniques in cultured neurons and astrocytes (derived from brain tissue at embryonic ages E15 and E20, respectively). In the case of DND1 (but not NPM1) ethanol induced similar pattern of changes in both types of cells. Given the critical role of the protein NPM1 in cell proliferation and differentiation, its reduced expression in the ethanol-exposed rNSC could, in part, explain the lower cells numbers. We conclude that chronic ethanol profoundly alters protein composition of rNSC to the extent that their functioning-including proliferation and survival-would be seriously compromised. Translated to humans, such changes could point the way towards mechanisms underlying the fetal alcohol spectrum disorder and/or alcoholism later in life.

Hypoxia-inducible factors in cancer stem cells and inflammation

Hypoxia-inducible factors (HIF) mediate metabolic switches in cells in hypoxic environments, including those in both normal and malignant tissues with limited supplies of oxygen. Paradoxically, recent studies have shown that cancer stem cells (CSCs) and activated immune effector cells exhibit high HIF activity in normoxic environments and that HIF activity is critical in the maintenance of CSCs as well as the differentiation and function of inflammatory cells. Given that inflammation and CSCs are two major barriers to effective cancer therapy, targeting HIF may provide a new approach to developing such treatments.

Deep profiling of multitube flow cytometry data

MOTIVATION

Deep profiling the phenotypic landscape of tissues using high-throughput flow cytometry (FCM) can provide important new insights into the interplay of cells in both healthy and diseased tissue. But often, especially in clinical settings, the cytometer cannot measure all the desired markers in a single aliquot. In these cases, tissue is separated into independently analysed samples, leaving a need to electronically recombine these to increase dimensionality. Nearest-neighbour (NN) based imputation fulfils this need but can produce artificial subpopulations. Clustering-based NNs can reduce these, but requires prior domain knowledge to be able to parameterize the clustering, so is unsuited to discovery settings.

RESULTS

We present flowBin, a parameterization-free method for combining multitube FCM data into a higher-dimensional form suitable for deep profiling and discovery. FlowBin allocates cells to bins defined by the common markers across tubes in a multitube experiment, then computes aggregate expression for each bin within each tube, to create a matrix of expression of all markers assayed in each tube. We show, using simulated multitube data, that flowType analysis of flowBin output reproduces the results of that same analysis on the original data for cell types of >10% abundance. We used flowBin in conjunction with classifiers to distinguish normal from cancerous cells. We used flowBin together with flowType and RchyOptimyx to profile the immunophenotypic landscape of NPM1-mutated acute myeloid leukemia, and present a series of novel cell types associated with that mutation.

An IKKα-nucleophosmin axis utilizes inflammatory signaling to promote genome integrity

The inflammatory microenvironment promotes skin tumorigenesis. However, the mechanisms by which cells protect themselves from inflammatory signals are unknown. Downregulation of IKKα promotes skin tumor progression from papillomas to squamous cell carcinomas, which is frequently accompanied by genomic instability, including aneuploid chromosomes and extra centrosomes. In this study, we found that IKKα promoted oligomerization of nucleophosmin (NPM), a negative centrosome duplication regulator, which further enhanced NPM and centrosome association, inhibited centrosome amplification, and maintained genome integrity. Levels of NPM hexamers and IKKα were conversely associated with skin tumor progression. Importantly, proinflammatory cytokine-induced IKKα activation promoted the formation of NPM oligomers and reduced centrosome numbers in mouse and human cells, whereas kinase-dead IKKα blocked this connection. Therefore, our findings suggest a mechanism in which an IKKα-NPM axis may use inflammatory signals to suppress centrosome amplification, promote genomic integrity, and prevent tumor progression.

Cancer translocations in human cells induced by zinc finger and TALE nucleases

Chromosomal translocations are signatures of numerous cancers and lead to expression of fusion genes that act as oncogenes. The wealth of genomic aberrations found in cancer, however, makes it challenging to assign a specific phenotypic change to a specific aberration. In this study, we set out to use genome editing with zinc finger (ZFN) and transcription activator-like effector (TALEN) nucleases to engineer, de novo, translocation-associated oncogenes at cognate endogenous loci in human cells. Using ZFNs and TALENs designed to cut precisely at relevant translocation breakpoints, we induced cancer-relevant t(11;22)(q24;q12) and t(2;5)(p23;q35) translocations found in Ewing sarcoma and anaplastic large cell lymphoma (ALCL), respectively. We recovered both translocations with high efficiency, resulting in the expression of the EWSR1–FLI1 and NPM1–ALK fusions. Breakpoint junctions recovered after ZFN cleavage in human embryonic stem (ES) cell–derived mesenchymal precursor cells fully recapitulated the genomic characteristics found in tumor cells from Ewing sarcoma patients. This approach with tailored nucleases demonstrates that expression of fusion genes found in cancer cells can be induced from the native promoter, allowing interrogation of both the underlying mechanisms and oncogenic consequences of tumor-related translocations in human cells. With an analogous strategy, the ALCL translocation was reverted in a patient cell line to restore the integrity of the two participating chromosomes, further expanding the repertoire of genomic rearrangements that can be engineered by tailored nucleases.

Detection of nucleophosmin and FMS-like tyrosine kinase-3 gene mutations in acute myeloid leukemia

BACKGROUND AND OBJECTIVES

Nucleophosmin gene mutations are frequently reported in acute myeloid leukemia (AML) patients with normal karyotype, which is also frequently associated with internal tandem duplication mutations in the FMS-like tyrosine kinase-3 gene. We sought to detect the nucleophosmin and FMS-like tyrosine kinase-3 (FLT3) internal tandem duplication (ITD) mutations among Iranian patients with AML and to assess the relationship between these mutations and the subtypes of the disease.

DESIGN AND SETTING

Cross-sectional study of patients referred during 2007 through 2009.

PATIENTS AND METHODS

Bone marrow and peripheral blood samples of 131 AML patients were randomly collected at the time of diagnosis and prior to treatment and the DNA extracted. After amplifying the nucleophosmin and FLT3 gene regions, positive cases were screened by conformation-sensitive gel electrophoresis and agarose gel electrophoresis techniques.

RESULTS

Of 131 patients, 23 (17.5%) (0.95% CI=0.107-0.244) had nucleophosmin gene mutations. The highest frequency of such mutations was found among the subtypes of M4 (30.4%), M3 (21.7%) and M5 (17.4%). There was a high frequency of these mutations in the M3 subtype as well as a high frequency of allele D in all subtypes. Also, 21 (16.0%) samples (0.95% CI=0.092-0.229) had FLT3/ITD mutation, of which 8 samples had mutant nucleophosmin (8 of 23, 35%), and another 13 samples had wild-type nucleophosmin gene (13 of 108, 12%). There was a high degree of association between the occurrence of nucleophosmin and FLT3/ITD mutations (P=.012).

CONCLUSION

Our data showed a high frequency of NPM1 mutations in the monocytic subtypes of AML, as well as a high degree of association between the occurrence of NPM1 and FLT3/ITD mutations.

FLT3 and NPM1 mutations in myelodysplastic syndromes: Frequency and potential value for predicting progression to acute myeloid leukemia

We reviewed FLT3 and NPM1 mutation data in a large cohort of patients with myelodysplastic syndrome (MDS). The frequencies of FLT3 and NPM1 mutation were 2.0% and 4.4%, respectively, and mutations were restricted to cases of intermediate- and high-risk MDS. Cytogenetic abnormalities were identified in 46.9% of cases. FLT3 mutations were associated with a complex karyotype (P = .009), whereas NPM1 mutations were associated with a diploid karyotype (P < .001). FLT3 mutation (P < .001) was associated with progression to acute myeloid leukemia (AML), as were a higher bone marrow (BM) blast count (P < .001) and complex cytogenetics (P = .039). No patient with an NPM1 mutation alone had disease that progressed to AML. Cox proportional regression multivariate analysis indicated that FLT3 mutation, NPM1 mutation, complex cytogenetics, BM blast count, pancytopenia, and age were independent factors that correlated with progression-free survival. We conclude that FLT3 and NPM1 mutations are rare in MDS, but assessment of mutation status is potentially useful for predicting progression to AML.

Mutant nucleophosmin deregulates cell death and myeloid differentiation through excessive caspase-6 and -8 inhibition

In up to one-third of patients with acute myeloid leukemia, a C-terminal frame-shift mutation results in abnormal and abundant cytoplasmic accumulation of the usually nucleoli-bound protein nucleophosmin (NPM), and this is thought to function in cancer pathogenesis. Here, we demonstrate a gain-of-function role for cytoplasmic NPM in the inhibition of caspase signaling. The NPM mutant specifically inhibits the activities of the cell-death proteases, caspase-6 and -8, through direct interaction with their cleaved, active forms, but not the immature procaspases. The cytoplasmic NPM mutant not only affords protection from death ligand-induced cell death but also suppresses caspase-6/-8-mediated myeloid differentiation. Our data hence provide a potential explanation for the myeloid-specific involvement of cytoplasmic NPM in the leukemogenesis of a large subset of acute myeloid leukemia.

Development of a malignancy-associated proteomic signature for diffuse large B-cell lymphoma

The extreme pathological diversity of non-Hodgkin's lymphomas has made their accurate histological assessment difficult. New diagnostics and treatment modalities are urgently needed for these lymphomas, particularly in drug development for cancer-specific targets. Previously, we showed that a subset of B cell lymphoma, diffuse large B cell lymphoma, may be characterized by two major, orthogonal axes of gene expression: one set of transcripts that is differentially expressed between resting and proliferating, nonmalignant cells (ie, a "proliferative signature") and another set that is expressed only in proliferating malignant cells (ie, a "cancer signature"). A differential proteomic analysis of B cell proliferative states, similar to previous transcriptional profiling analyses, holds great promise either to reveal novel factors that participate in lymphomagenesis or to define biomarkers of onset or progression. Here, we use a murine model of diffuse large B cell lymphoma to conduct unbiased two-dimensional gel electrophoresis and mass spectrometry-based comparative proteomic analyses of malignant proliferating B cells and tissue-matched, normal resting, or normal proliferating cells. We show that the expression patterns of particular proteins or isoforms across these states fall into eight specific trends that provide a framework to identify malignancy-associated biomarkers and potential drug targets, a signature proteome. Our results support the central hypothesis that clusters of proteins of known function represent a panel of expression markers uniquely associated with malignancy and not normal proliferation.

Pediatric acute myeloid leukemia with NPM1 mutations is characterized by a gene expression profile with dysregulated HOX gene expression distinct from MLL-rearranged leukemias

Somatic mutations in nucleophosmin (NPM1) occur in approximately 35% of adult acute myeloid leukemia (AML). To assess the frequency of NPM1 mutations in pediatric AML, we sequenced NPM1 in the diagnostic blasts from 93 pediatric AML patients. Six cases harbored NPM1 mutations, with each case lacking common cytogenetic abnormalities. To explore the phenotype of the AMLs with NPM1 mutations, gene expression profiles were obtained using Affymetrix U133A microarrays. NPM1 mutations were associated with increased expression of multiple homeobox genes including HOXA9, A10, B2, B6 and MEIS1. As dysregulated homeobox gene expression is also a feature of MLL-rearranged leukemia, the gene expression signatures of NPM1-mutated and MLL-rearranged leukemias were compared. Significant differences were identified between these leukemia subtypes including the expression of different HOX genes, with NPM1-mutated AML showing higher levels of expression of HOXB2, B3, B6 and D4. These results confirm recent reports of perturbed HOX expression in NPM1-mutated adult AML, and provide the first evidence that the NPM1-mutated signature is distinct from MLL-rearranged AML. These findings suggest that mutated NPM1 leads to dysregulated HOX expression via a different mechanism than MLL rearrangement.

MN1 overexpression is an important step in the development of inv(16) AML

The gene encoding the transcriptional co-activator MN1 is the target of the reciprocal chromosome translocation (12;22)(p13;q12) in some patients with acute myeloid leukemia (AML). In addition, expression array analysis showed that MN1 was overexpressed in AML specified by inv(16), in some AML overexpressing ecotropic viral integration 1 site (EVI1) and in some AML without karyotypic abnormalities. Here we describe that mice receiving transplants of bone marrow (BM) overexpressing MN1 rapidly developed myeloproliferative disease (MPD). This BM also generated myeloid cell lines in culture. By mimicking the situation in human inv(16) AML, forced coexpression of MN1 and Cbfbeta-SMMHC rapidly caused AML in mice. These findings identify MN1 as a highly effective hematopoietic oncogene and suggest that MN1 overexpression is an important cooperative event in human inv(16) AML.

The human Shwachman-Diamond syndrome protein, SBDS, associates with ribosomal RNA

Shwachman-Diamond syndrome (SDS) is an autosomal recessive disorder characterized by bone marrow failure, exocrine pancreatic dysfunction, and leukemia predisposition. Mutations in the SBDS gene are identified in most patients with SDS. SBDS encodes a highly conserved protein of unknown function. Data from SBDS orthologs suggest that SBDS may play a role in ribosome biogenesis or RNA processing. Human SBDS is enriched in the nucleolus, the major cellular site of ribosome biogenesis. Here we report that SBDS nucleolar localization is dependent on active rRNA transcription. Cells from patients with SDS or Diamond-Blackfan anemia are hypersensitive to low doses of actinomycin D, an inhibitor of rRNA transcription. The addition of wild-type SBDS complements the actinomycin D hypersensitivity of SDS patient cells. SBDS migrates together with the 60S large ribosomal subunit in sucrose gradients and coprecipitates with 28S ribosomal RNA (rRNA). Loss of SBDS is not associated with a discrete block in rRNA maturation or with decreased levels of the 60S ribosomal subunit. SBDS forms a protein complex with nucleophosmin, a multifunctional protein implicated in ribosome biogenesis and leukemogenesis. Our studies support the addition of SDS to the growing list of human bone marrow failure syndromes involving the ribosome.

Proteomics-based strategy to identify biomarkers and pharmacological targets in leukemias with t(4;11) translocations

Translocations and other aberrations involving the MLL (mixed lineage leukemia) gene result in aggressive forms of leukemias. Heterogeneity in partner genes, in chromosomal breakpoints, in MLL itself, and in the different partner genes results in heterogeneous fusion transcripts that can be alternatively spliced, which complicates deciphering a unifying mechanism of leukemogenesis. However, recent microarray studies completed with clinical leukemia specimens have uncovered several distinct mRNA signatures within MLL leukemia that differ from other types of leukemia. A global proteomics strategy using MV4-11 and RS4:11 cells in culture was employed to investigate possible protein signatures common to different MLL leukemias and to identify disease biomarkers and protein targets for pharmacological intervention. Initial proteomics screening experiments with two-dimensional differential in-gel electrophoresis revealed heat shock protein 90 alpha (HSP90alpha) as a potential target for pharmacological inhibition and nucleoside diphosphate kinase (nm23) as a biomarker for measuring treatment efficacy. Using a modified stable isotope labeling of amino acids in cell culture (SILAC) approach, coupled with two-dimensional liquid chromatography tandem mass spectrometry (2D-LC-MS/MS), changes in abundance for over 500 proteins were measured. In addition, decreased expression of the novel biomarker nm23 was observed during HSP90 inhibition with 17-allylamino-17-demethoxygeldanamycin (17-AAG) in the MV4-11 cell line. The present study validates the use of a global proteomics strategy to uncover novel biomarkers and pharmacological targets for leukemias with MLL translocations. Additionally, several proteins were found to be expressed in concordance with microarray studies of mRNA expression in specimens from patients showing the value in comparing mRNA transcript and proteomic profiles. This work represents one of the most comprehensive proteomics screens of MLL leukemias that have been conducted to date.

[The negative regulation of ribosome biogenesis: a new Arf-dependent pathway controlling cell proliferation?]

The nucleolar Arf protein has initially been shown to regulate cell cycle through the so-called Arf-mdm2-p53 pathway. In addition to this well characterized pathway, convergent data published since 2000 indicate that Arf can inhibit cell proliferation in absence of p53, suggesting the existence of a p53-independent pathway. Several partners have recently been described that could participate in an alternative regulatory process. Recent results show that : (1) Arf binds the rDNA promoter to inhibit the transcription of the 47S rRNA precursor and (2) Arf interacts with the nucleophosmin/B23 protein to negatively regulate rRNA maturation, it is assumed that the tumour suppressor may downregulate the cell cycle progression through the control of ribosome biogenesis, thus resulting in completion of cell cycle arrest.

Nucleophosmin: a versatile molecule associated with hematological malignancies

Nucleophosmin (NPM) is a nucleolar phosphoprotein that plays multiple roles in ribosome assembly and transport, cytoplasmic-nuclear trafficking, centrosome duplication and regulation of p53. In hematological malignancies, the NPM1 gene is frequently involved in chromosomal translocation, mutation and deletion. The NPM1 gene on 5q35 is translocated with the anaplastic lymphoma kinase (ALK) gene in anaplastic large cell lymphoma with t(2;5). The MLF1 and RARA genes are fused with NPM1 in myelodysplastic syndrome and acute myeloid leukemia (AML) with t(3;5) and acute promyelocytic leukemia with t(5;17), respectively. In each fused protein, the N-terminal NPM portion is associated with oligomerization of a partner protein leading to altered signal transduction or transcription. Recently, mutations of exon 12 have been found in a significant proportion of de novo AML, especially in those with a normal karyotype. Mutant NPM is localized aberrantly in the cytoplasm, but the molecular mechanisms for leukemia remain to be studied. Studies of knock-out mice have revealed new aspects regarding NPM1 as a tumor-suppressor gene. This review focuses on the clinical significance of the NPM1 gene in hematological malignancies and newly discovered roles of NPM associated with oncogenesis.

Interaction between ROCK II and nucleophosmin/B23 in the regulation of centrosome duplication

Nucleophosmin (NPM)/B23 has been implicated in the regulation of centrosome duplication. NPM/B23 localizes between two centrioles in the unduplicated centrosome. Upon phosphorylation on Thr(199) by cyclin-dependent kinase 2 (CDK2)/cyclin E, the majority of centrosomal NPM/B23 dissociates from centrosomes, but some NPM/B23 phosphorylated on Thr(199) remains at centrosomes. It has been shown that Thr(199) phosphorylation of NPM/B23 is critical for the physical separation of the paired centrioles, an initial event of the centrosome duplication process. Here, we identified ROCK II kinase, an effector of Rho small GTPase, as a protein that localizes to centrosomes and physically interacts with NPM/B23. Expression of the constitutively active form of ROCK II promotes centrosome duplication, while down-regulation of ROCK II expression results in the suppression of centrosome duplication, especially delaying the initiation of centrosome duplication during the cell cycle. Moreover, ROCK II regulates centrosome duplication in its kinase and centrosome localization activity-dependent manner. We further found that ROCK II kinase activity is significantly enhanced by binding to NPM/B23 and that NPM/B23 acquires a higher binding affinity to ROCK II upon phosphorylation on Thr(199). Moreover, physical interaction between ROCK II and NPM/B23 in vivo occurs in association with CDK2/cyclin E activation and the emergence of Thr(199)-phosphorylated NPM/B23. All these findings point to ROCK II as the effector of the CDK2/cyclin E-NPM/B23 pathway in the regulation of centrosome duplication.

Global functional analysis of nucleophosmin in Taxol response, cancer, chromatin regulation, and ribosomal DNA transcription

Analysis of lung cancer response to chemotherapeutic agents showed the accumulation of a Taxol-induced protein that reacted with an anti-phospho-MEK1/2 antibody. Mass spectroscopy identified the protein as nucleophosmin/B23 (NPM), a multifunctional protein with diverse roles: ribosome biosynthesis, p53 regulation, nuclear-cytoplasmic shuttling, and centrosome duplication. Our work demonstrates that following cellular exposure to mitosis-arresting agents, NPM is phosphorylated and its chromatographic property is altered, suggesting changes in function during mitosis. To determine the functional relevance of NPM, its expression in tumor cells was reduced by siRNA. Cells with reduced NPM were treated with Taxol followed by microarray profiling accompanied by gene/protein pathway analyses. These studies demonstrate several expected and unexpected consequences of NPM depletion. The predominant downstream effectors of NPM are genes involved in cell proliferation, cancer, and the cell cycle. In congruence with its role in cancer, NPM is over-expressed in primary malignant lung cancer tissues. We also demonstrate a role for NPM in the expression of genes encoding SET (TAF1beta) and the histone methylase SET8. Additionally, we show that NPM is required for a previously unobserved G2/M upregulation of TAF1A, which encodes the rDNA transcription factor TAF(I)48. These results demonstrate multi-faceted functions of NPM that can affect cancer cells.

Two decades of cancer genetics: from specificity to pleiotropic networks

Modeling cancer in mice has reached an even greater relevance in the field of hematological malignancies, due to the already advanced characterization of the molecular basis of many hematological disorders. These mouse models have often allowed us to achieve insight into the pathogenesis of the human disease as well as to test novel therapeutic modalities in preclinical studies. However, one of the most rewarding cultural shifts triggered by these modeling efforts stems from what was originally perceived as background noise or modeling inaccuracy. Manipulation of the involved genes often triggered cancer susceptibility in cell types other than the hematopoietic lineages. This prompted us to challenge a fundamental misconception in cancer genetics that the approximately 200 genes directly involved in chromosomal translocations associated with hematopoietic malignancies are specifically and functionally restricted to leukemia/lymphoma pathogenesis only. The genetics underlying the pathogenesis of leukemia and lymphoma have historically been regarded as distinct from those underlying the pathogenesis of solid tumors because hematopoietic malignancies are often associated with characteristic chromosomal translocations that are leukemia- or lymphoma-specific. In this paper, we discuss how leukemia/lymphoma genes indeed participate in fundamental proto-oncogenic and growth-suppressive networks and may play a wider role in cancer pathogenesis. We focus on paradigmatic examples such as c-myc and PML, as well as on more recent findings from our laboratory concerning the role of NPM in tumorigenesis.

The NPM-RAR fusion protein associated with the t(5;17) variant of APL does not interact with PML

The PML protein localizes to regions of the nucleus known as nuclear bodies or PODs. However, in t(15;17) Acute Promyelocytic Leukemia (APL) blasts, PML is found in a micro-punctate pattern. In order to test the hypothesis that delocalization of PML from PODs is necessary for APL, we investigated the interaction of the t(5;17) APL fusion protein NPM-RAR with PML. NPM-RAR localizes diffusely throughout the nucleoplasm. NPM-RAR does not alter the localization of PML in transfected HeLa cells, and does not associate with PML in vitro. These studies suggest that NPM-RAR does not interact with PML.

DNA damage, p14ARF, nucleophosmin (NPM/B23), and cancer

The p53/p14ARF/mdm2 stress response pathway plays a central role in mediating cellular responses to oncogene activation, genome instability, and therapy-induced DNA damage. Abrogation of the pathway occurs in most if not all cancers, and may be essential for tumor development. The high frequency with which the pathway is disabled in cancer and the fact that the pathway appears to be incompatible with tumor cell growth, has made it an important point of focus in cancer research and therapeutics development. Recently, Nucleophosmin (NPM, B23, NO38 and numatrin), a multifunctional nucleolar protein, has emerged as a p14ARF binding protein and regulator of p53. While complex formation between ARF and NPM retains ARF in the nucleolus and prevents ARF from activating p53, DNA damaging treatments promote a transient subnuclear redistribution of ARF to the nucleoplasm, where it interacts with mdm2 and promotes p53 activation. The results add support to a recently proposed model in which the nucleolus serves as a p53-uspstream sensor of stress, and where ARF links nucleolar stress signals to nucleoplasmic effectors of the stress response. A better understanding of ARF's nucleolar interactions could further elucidate the regulation of the p53 pathway and suggest new therapeutic approaches to restore p53 function.

Biology, clinical relevance, and molecularly targeted therapy in acute leukemia with FLT3 mutation

Overexpression and activating mutations of receptor tyrosine kinases (RTKs) are known to be involved in the pathophysiology of several kinds of cancer cells. FMS-like receptor tyrosine kinase 3 (FLT3), together with KIT, FMS, and platelet-derived growth factor receptor, is a class III RTK. FLT3 mutations were first reported as internal tandem duplication (FLT3/ITD) of the juxtamembrane domain-coding sequence; subsequently, a missense point mutation at the D835 residue and point mutations, deletions, and insertions in the codons surrounding D835 within a FLT3 tyrosine kinase domain (FLT3/KDMs) have been found. FLT3 mutations are the most frequent genetic alterations so far reported in acute myeloid leukemia and are involved in the signaling pathway of autonomous proliferation and differentiation block in leukemia cells. Several large-scale studies have confirmed that FLT3/ITD is strongly associated with leukocytosis and a poor prognosis. Therefore, routine screening for FLT3 mutations is recommended to stratify patients into distinct risk groups. However, because high-dose chemotherapy and stem cell transplantation cannot overcome the adverse effects of FLT3 mutations, the development of FLT3 kinase inhibitors is expected to produce a more efficacious therapeutic strategy for leukemia therapy.

Inducible and reversible suppression of Npm1 gene expression using stably integrated small interfering RNA vector in mouse embryonic stem cells

The tetracycline (Tc)-inducible small interference RNA (siRNA) is a powerful tool for studying gene function in mammalian cells. However, the system is infrequently utilized in embryonic stem (ES) cells. Here, we present the first application of the Tc-inducible, stably integrated plasmid-based siRNA system in mouse ES cells to down-regulate expression of Npm1, an essential gene for embryonic development. The physiological role of Npm1 in ES cells has not been defined. Our data show that the knock-down of Npm1 expression by this siRNA system was not only highly efficient, but also Tc- dose- and induction time-dependent. Particularly, the down-regulation of Npm1 expression was reversible. Importantly, suppression of Npm1 expression in ES cells resulted in reduced cell proliferation. Taken together, this system allows for studying gene function in a highly controlled manner, otherwise difficult to achieve in ES cells. Moreover, our results demonstrate that Npm1 is essential for ES cell proliferation.

Leukemia with distinct phenotypes in transgenic mice expressing PML/RAR alpha, PLZF/RAR alpha or NPM/RAR alpha

Recurrent chromosomal translocations involving the RAR alpha locus on chromosome 17 are the hallmark of acute promyelocytic leukemia (APL). The RAR alpha gene fuses to variable partners (PML, PLZF, NPM, NuMA and STAT5B: X genes) leading to the expression of APL-specific fusion proteins with identical RAR alpha moieties. To analyse whether the variable X moiety could affect the activity of the fusion protein in vivo, we generated and characterized, on a comparative basis, NPM/RAR alpha transgenic mice (TM) in which the fusion gene is expressed under the control of a human Cathepsin G (hCG) minigene. We compared the features of the leukemia observed in these TM with those in hCG-PML/RAR alpha and hCG-PLZF/RAR alpha TM. In all three transgenic models, leukemia developed after a variably long latency, with variable penetrance. However, the three leukemias displayed distinct cytomorphological features. hCG-NPM/RAR alpha leukemic cells resembled monoblasts. This phenotype contrasts with what was observed in the hCG-PML/RAR alpha TM model in which the leukemic phase was characterized by the proliferation of promyelocytic blasts. Similarly, hCG-PLZF/RAR alpha TM displayed a different phenotype where terminally differentiated myeloid cells predominated. Importantly, the NPM/RAR alpha oncoprotein was found to localize in the nucleolus, unlike PML/RAR alpha and PLZF/RAR alpha, thus possibly interfering with the normal function of NPM. Similarly to what was observed in human APL patients, we found that NPM/RAR alpha and PML/RAR alpha, but not PLZF/RAR alpha leukemia, was responsive to all-trans retinoic acid (ATRA) or As2O3 treatments. Taken together, our results underscore the critical relevance of the X moiety in dictating the biology of the disease and the activity of the APL fusion oncoprotein.

Cell line OCI/AML3 bears exon-12 NPM gene mutation-A and cytoplasmic expression of nucleophosmin

We recently identified a new acute myeloid leukemia (AML) subtype characterized by mutations at exon-12 of the nucleophosmin (NPM) gene and aberrant cytoplasmic expression of NPM protein (NPMc+). NPMc+ AML accounts for about 35% of adult AML and it is associated with normal karyotype, wide morphological spectrum, CD34-negativity, high frequency of FLT3-ITD mutations and good response to induction therapy. In an attempt to identify a human cell line to serve as a model for the in vitro study of NPMc+ AML, we screened 79 myeloid cell lines for mutations at exon-12 of NPM. One of these cell lines, OCI/AML3, showed a TCTG duplication at exon-12 of NPM. This mutation corresponds to the type A, the NPM mutation most frequently observed in primary NPMc+ AML. OCI/AML3 cells also displayed typical phenotypic features of NPMc+ AML, that is, expression of macrophage markers and lack of CD34, and the immunocytochemical hallmark of this leukemia subtype, that is, the aberrant cytoplasmic expression of NPM. The OCI/AML3 cell line easily engrafts in NOD/SCID mice and maintains in the animals the typical features of NPMc+ AML, such as the NPM cytoplasmic expression. For all these reasons, the OCI/AML3 cell line represents a remarkable tool for biomolecular studies of NPMc+ AML.

Nucleophosmin mutations in childhood acute myelogenous leukemia with normal karyotype

Nucleophosmin (NPM) is a nucleocytoplasmic shuttling protein involved in leukemia-associated chromosomal translocations, and it regulates the alternate reading frame (ARF)-p53 tumorsuppressor pathway. Recently, it has been demonstrated that mutations of the NPM1 gene alter the protein at its C-terminal, causing its cytoplasmic localization. Cytoplasmic NPM was detected in 35% of adult patients with primary non-French-American-British (FAB) classification M3 acute myeloid leukemia (AML), associated mainly with normal karyotype. We evaluated the prevalence of the NPM1 gene mutation in non-M3 childhood AML patients enrolled in the ongoing Associazione Italiana di Ematologia e Oncologia Pediatrica (AIEOP-AML02) protocol in Italy. NPM1 mutations were found in 7 (6.5%) of 107 successfully analyzed patients. NPM1- mutated patients carried a normal karyotype (7/26, 27.1%) and were older in age. Thus, the NPM1 mutation is a frequent abnormality in AML patients without known genetic marker; the mutation may represent a new target to monitor minimal residual disease in AML and a potential candidate for alternative and targeted treatments. (Blood. 2005;106:1419-1422)

Mutant nucleophosmin (NPM1) predicts favorable prognosis in younger adults with acute myeloid leukemia and normal cytogenetics: interaction with other gene mutations

To assess the prognostic relevance of mutations in the NPM1 gene encoding a nucleocytoplasmic shuttle protein in younger adults with acute myeloid leukemia (AML) and normal cytogenetics, sequencing of NPM1 exon 12 was performed in diagnostic samples from 300 patients entered into 2 consecutive multicenter trials of the AML Study Group (AMLSG). Treatment included intensive double-induction therapy and consolidation therapy with high cumulative doses of high-dose cytarabine. NPM1 mutations were identified in 48% of the patients including 12 novel sequence variants, all leading to a frameshift in the C-terminus of the nucleophosmin 1 (NPM1) protein. Mutant NPM1 was associated with specific clinical, phenotypical, and genetic features. Statistical analysis revealed a significant interaction of NPM1 and FLT3 internal tandem duplications (ITDs). NPM1 mutations predicted for better response to induction therapy and for favorable overall survival (OS) only in the absence of FLT3 ITD. Multivariable analysis for OS revealed combined NPM1-mutated/FLT3 ITD-negative status, CEBPA mutation status, availability of a human leukocyte antigen (HLA)-compatible donor, secondary AML, and lactate dehydrogenase (LDH) as prognostic factors. In conclusion, NPM1 mutations in the absence of FLT3 ITD define a distinct molecular and prognostic subclass of young-adult AML patients with normal cytogenetics.

Role of nucleophosmin in embryonic development and tumorigenesis

Nucleophosmin (also known as NPM, B23, NO38) is a nucleolar protein directly implicated in cancer pathogenesis, as the NPM1 gene is found mutated and rearranged in a number of haematological disorders. Furthermore, the region of chromosome 5 to which NPM1 maps is deleted in a proportion of de novo human myelodysplastic syndromes (MDS), and loss of chromosome 5 is extremely frequent in therapy-related MDS. NPM is a multifunctional protein, and its role in oncogenesis is controversial as NPM has been attributed with both oncogenic and tumour suppressive functions. To study the function of Npm in vivo, we generated a hypomorphic Npm1 mutant series (Npm1+/- < Npm1(hy/hy) < Npm1-/-) in mouse. Here we report that Npm is essential for embryonic development and the maintenance of genomic stability. Npm1-/- and Npm1(hy/hy) mutants have aberrant organogenesis and die between embryonic day E11.5 and E16.5 owing to severe anaemia resulting from defects in primitive haematopoiesis. We show that Npm1 inactivation leads to unrestricted centrosome duplication and genomic instability. We demonstrate that Npm is haploinsufficient in the control of genetic stability and that Npm1 heterozygosity accelerates oncogenesis both in vitro and in vivo. Notably, Npm1+/- mice develop a haematological syndrome with features of human MDS. Our findings uncover an essential developmental role for Npm and implicate its functional loss in tumorigenesis and MDS pathogenesis.