Increased expression of a COOH-truncated nucleophosmin resulting from alternative splicing is associated with cellular resistance to ionizing radiation in HeLa cells

Nucleophosmin: A Nucleolar Phosphoprotein Orchestrating Cellular Stress Responses

Nucleophosmin (NPM1) is a key nucleolar protein released from the nucleolus in response to stress stimuli. NPM1 functions as a stress regulator with nucleic acid and protein chaperone activities, rapidly shuttling between the nucleus and cytoplasm. NPM1 is ubiquitously expressed in tissues and can be found in the nucleolus, nucleoplasm, cytoplasm, and extracellular environment. It plays a central role in various biological processes such as ribosome biogenesis, cell cycle regulation, cell proliferation, DNA damage repair, and apoptosis. In addition, it is highly expressed in cancer cells and solid tumors, and its mutation is a major cause of acute myeloid leukemia (AML). This review focuses on NPM1's structural features, functional diversity, subcellular distribution, and role in stress modulation.

Evaluation of in vitro intrinsic radiosensitivity and characterization of five canine high-grade glioma cell lines

Glioma is the most common primary brain tumor in dogs and predominantly affects brachycephalic breeds. Diagnosis relies on CT or MRI imaging, and the proposed treatments include surgical resection, chemotherapy, and radiotherapy depending on the tumor's location. Canine glioma from domestic dogs could be used as a more powerful model to study radiotherapy for human glioma than the murine model. Indeed, (i) contrary to mice, immunocompetent dogs develop spontaneous glioma, (ii) the canine brain structure is closer to human than mice, and (iii) domestic dogs are exposed to the same environmental factors than humans. Moreover, imaging techniques and radiation therapy used in human medicine can be applied to dogs, facilitating the direct transposition of results. The objective of this study is to fully characterize 5 canine glioma cell lines and to evaluate their intrinsic radiosensitivity. Canine cell lines present numerous analogies between the data obtained during this study on different glioma cell lines in dogs. Cell morphology is identical, such as doubling time, clonality test and karyotype. Immunohistochemical study of surface proteins, directly on cell lines and after stereotaxic injection in mice also reveals close similarity. Radiosensitivity profile of canine glial cells present high profile of radioresistance.

Genetic basis of acute myeloid leukemia (AML): The most common molecular changes in patients with normal karyotype

Acute myeloid leukemia (AML) is a clonal disorder that results from errors in proliferation and differentiation of bone marrow stem cells from myeloid lineage. According to the Gilliland “two-hit” model, genes of both groups related to proliferation (e.g., FLT3) and differentiation (e.g., CEBPA) must be mutated for full development of AML. The genetic background of AML is very complicated and varied, from single nucleotide mutations or changes in gene expression to cytogenetic aberrations. The DNA sequencing results enable identification of important gene alterations that occur first and may lead the whole leukemogenesis (driver mutations). Some of them have prognostic significance – that is, they are related to the overall survival (OS), complete remission rate, and event-free survival (EFS). The most common molecular changes in AML are mutations in NPM1, CEBPA, FLT3, and DNMT3A. Alterations in NPM1 gene are associated with a good prognosis but simultaneous mutation in FLT3 may change this prognosis. DNMT3A mutations are very often correlated with NPM1 mutations and are associated with short OS.

Splicing Genomics Events in Cervical Cancer: Insights for Phenotypic Stratification and Biomarker Potency

Gynaecological cancers are attributed to the second most diagnosed cancers in women after breast cancer. On a global scale, cervical cancer is the fourth most common cancer and the most common cancer in developing countries with rapidly increasing mortality rates. Human papillomavirus (HPV) infection is a major contributor to the disease. HPV infections cause prominent cellular changes including alternative splicing to drive malignant transformation. A fundamental characteristic attributed to cancer is the dysregulation of cellular transcription. Alternative splicing is regulated by several splicing factors and molecular changes in these factors lead to cancer mechanisms such as tumour development and progression and drug resistance. The serine/arginine-rich (SR) proteins and heterogeneous ribonucleoproteins (hnRNPs) have prominent roles in modulating alternative splicing. Evidence shows molecular alteration and expression levels in these splicing factors in cervical cancer. Furthermore, aberrant splicing events in cancer-related genes lead to chemo- and radioresistance. Identifying clinically relevant modifications in alternative splicing events and splicing variants, in cervical cancer, as potential biomarkers for their role in cancer progression and therapy resistance is scrutinised. This review will focus on the molecular mechanisms underlying the aberrant splicing events in cervical cancer that may serve as potential biomarkers for diagnosis, prognosis, and novel drug targets.

Nucleophosmin in Its Interaction with Ligands

Nucleophosmin (NPM1) is a mainly nucleolar protein that shuttles between nucleoli, nucleoplasm and cytoplasm to fulfill its many functions. It is a chaperone of both nucleic acids and proteins and plays a role in cell cycle control, centrosome duplication, ribosome maturation and export, as well as the cellular response to a variety of stress stimuli. NPM1 is a hub protein in nucleoli where it contributes to nucleolar organization through heterotypic and homotypic interactions. Furthermore, several alterations, including overexpression, chromosomal translocations and mutations are present in solid and hematological cancers. Recently, novel germline mutations that cause dyskeratosis congenita have also been described. This review focuses on NPM1 interactions and inhibition. Indeed, the list of NPM1 binding partners is ever-growing and, in recent years, many studies contributed to clarifying the structural basis for NPM1 recognition of both nucleic acids and several proteins. Intriguingly, a number of natural and synthetic ligands that interfere with NPM1 interactions have also been reported. The possible role of NPM1 inhibitors in the treatment of multiple cancers and other pathologies is emerging as a new therapeutic strategy.

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.

Function, clinical application, and strategies of Pre-mRNA splicing in cancer

Pre-mRNA splicing is a fundamental process that plays a considerable role in generating protein diversity. Pre-mRNA splicing is also the key to the pathology of numerous diseases, especially cancers. In this review, we discuss how aberrant splicing isoforms precisely regulate three basic functional aspects in cancer: proliferation, metastasis and apoptosis. Importantly, clinical function of aberrant splicing isoforms is also discussed, in particular concerning drug resistance and radiosensitivity. Furthermore, this review discusses emerging strategies how to modulate pathologic aberrant splicing isoforms, which are attractive, novel therapeutic agents in cancer. Last we outline current and future directions of isoforms diagnostic methodologies reported so far in cancer. Thus, it is highlighting significance of aberrant splicing isoforms as markers for cancer and as targets for cancer therapy.

Interaction of CacyBP/SIP with NPM1 and its influence on NPM1 localization and function in oxidative stress

Calcyclin (S100A6) binding protein/Siah-1 interacting protein (CacyBP/SIP) is mainly a cytoplasmic protein; however, some literature data suggested its presence in the nucleus. In this work we examined more precisely the nuclear localization and function of CacyBP/SIP. By applying mass spectrometry, we have identified several nuclear proteins, among them is nucleophosmin (NPM1), that may interact with CacyBP/SIP. Subsequent assays revealed that CacyBP/SIP forms complexes with NPM1 in the cell and that the interaction between these two proteins is direct. Interestingly, although CacyBP/SIP exhibits phosphatase activity, we have found that its overexpression favors phosphorylation of NPM1 on S125. In turn, the RNA immunoprecipitation assay indicated that the altered CacyBP/SIP level has an impact on the amount of 28S and 18S rRNA bound to NPM1. The overexpression of CacyBP/SIP resulted in a significant increase in the binding of 28S and 18S rRNA to NPM1, whereas silencing of CacyBP/SIP expression decreased 28S rRNA binding and had no effect on the binding of 18S rRNA. Further studies have shown that under oxidative stress, CacyBP/SIP overexpression alters NPM1 distribution in cell nuclei. In addition, staining for a nucleolar marker, fibrillarin, revealed that CacyBP/SIP is indispensable for maintaining the nucleolar structure. These results are in agreement with data obtained by western blot analysis, which show that upon oxidative stress the NPM1 level decreases but that CacyBP/SIP overexpression counteracts the effect of stress. Altogether, our results show for the first time that CacyBP/SIP binds to and affects the properties of a nuclear protein, NPM1, and that it is indispensable for preserving the structure of nucleoli under oxidative stress.

Analysis of NPM1 splice variants reveals differential expression patterns of prognostic value in acute myeloid leukemia

Mutations of the nucleophosmin-1 (NPM1) gene in cytogenetically normal (CN) acute myeloid leukemia (AML) identify a group of patients with more favorable prognosis. NPM1 encodes three main alternatively spliced isoforms R1(B23.1), R2(B23.2), and R3(B23.3). The expression of splice variants R1, R2 and R3 were higher in AML patients compared to normal cells of healthy volunteers (HVs), although RNA-seq analysis revealed enhanced R2 expression also in less differentiated cells of HVs as well as in AML cells. The variant R2, which lacks exons 11 and 12 coding for the nucleolar localization domain, might behave similar to the mutant form of NPM1 (NPM1mut). In accordance, in CN-AML high R2 expression was associated with favorable impact on outcome. Moreover, functional studies showed nucleolar localization of the eGFP-NPM1 wildtype and cytoplasmic localization of the eGFP-NPM1 mut protein. While the eGFP-NPM1 R2 splice variant localized predominantly in the nucleoplasm, we also could detect cytoplasmic expression for the R2 variant. These results support a unique biological consequence of R2 overexpression and in part explain our clinical observation, where that high R2 variant expression was associated with a better prognosis in CN-AML patients.

Nucleophosmin: from structure and function to disease development

Nucleophosmin (NPM1) is a critical cellular protein that has been implicated in a number of pathways including mRNA transport, chromatin remodeling, apoptosis and genome stability. NPM1 function is a critical requirement for normal cellular biology as is underlined in cancer where NPM1 is commonly overexpressed, mutated, rearranged and sporadically deleted. Consistent with a multifunctional role within the cell, NPM1 can function not only as a proto-oncogene but also as a tumor suppressor. The aim of this review is to look at the less well-described role of NPM1 in the DNA repair pathways as well as the role of NPM1 in the regulation of apoptosis and its mutation in cancers.

FGFR1 Induces Glioblastoma Radioresistance through the PLCγ/Hif1α Pathway

FGF2 signaling in glioblastoma induces resistance to radiotherapy, so targeting FGF2/FGFR pathways might offer a rational strategy for tumor radiosensitization. To investigate this possibility, we evaluated a specific role for FGFR1 in glioblastoma radioresistance as modeled by U87 and LN18 glioblastomas in mouse xenograft models. Silencing FGFR1 decreased radioresistance in a manner associated with radiation-induced centrosome overduplication and mitotic cell death. Inhibiting PLCγ (PLCG1), a downstream effector signaling molecule for FGFR1, was sufficient to produce similar effects, arguing that PLCγ is an essential mediator of FGFR1-induced radioresistance. FGFR1 silencing also reduced expression of HIF1α, which in addition to its roles in hypoxic responses exerts an independent effect on radioresistance. Finally, FGFR1 silencing delayed the growth of irradiated tumor xenografts, in a manner that was associated with reduced HIF1α levels but not blood vessel alterations. Taken together, our results offer a preclinical proof of concept that FGFR1 targeting can degrade radioresistance in glioblastoma, a widespread problem in this tumor, prompting clinical investigations of the use of FGFR1 inhibitors for radiosensitization. Cancer Res; 76(10); 3036-44. ©2016 AACR.

Examination of the FLT3 and NPM1 mutational status in patients with acute myeloid leukemia from southeastern Poland

INTRODUCTION

Acute myeloid leukemia (AML) is a genetically heterogeneous disease at both the cytogenetic and molecular levels. In AML cells many chromosomal aberrations are observed, some of them being characteristic of a particular subtype of patients, and others being less significant. Besides chromosomal abnormalities, the leukemic cells can have a variety of mutations involving individual genes. The aim of this work was to investigate the frequencies of molecular alterations with the focus on FLT3-ITD and NPM1 mutations in AML patients of different age groups living in a southeastern region of Poland.

MATERIAL AND METHODS

The study group comprised 50 consecutive AML patients. We analyzed bone marrow samples by conventional cytogenetics. Cytogenetic evaluation in selected cases was complemented by the FISH technique. The internal tandem mutation in the FLT3 gene was identified using polymerase chain reaction (PCR), and the NPM1 mutation was assessed by direct nucleotide sequencing.

RESULTS

The studies using classical cytogenetics showed chromosomal aberrations in 32 (64%) patients. In 18 cases no changes in the karyotype were found by conventional karyotyping. FLT3-ITD mutation was detected in 4 (8%) patients and mutation of NPM1 in 3 patients with AML (6%).

CONCLUSIONS

The incidence of both mutations in our study group was lower than described elsewhere. We have confirmed that FLT3-ITD occurred more commonly in older patients and it was associated with shorter overall survival. By contrast, mutation of exon 12 of the NPM1 gene seems to be a good prognostic factor in AML patients with normal karyotype.

A HIERARCHICAL BAYESIAN MODEL FOR INFERENCE OF COPY NUMBER VARIANTS AND THEIR ASSOCIATION TO GENE EXPRESSION

A number of statistical models have been successfully developed for the analysis of high-throughput data from a single source, but few methods are available for integrating data from different sources. Here we focus on integrating gene expression levels with comparative genomic hybridization (CGH) array measurements collected on the same subjects. We specify a measurement error model that relates the gene expression levels to latent copy number states which, in turn, are related to the observed surrogate CGH measurements via a hidden Markov model. We employ selection priors that exploit the dependencies across adjacent copy number states and investigate MCMC stochastic search techniques for posterior inference. Our approach results in a unified modeling framework for simultaneously inferring copy number variants (CNV) and identifying their significant associations with mRNA transcripts abundance. We show performance on simulated data and illustrate an application to data from a genomic study on human cancer cell lines.

Adenoviral protein V promotes a process of viral assembly through nucleophosmin 1

Adenoviral infection induces nucleoplasmic redistribution of a nucleolar nucleophosmin 1/NPM1/B23.1. NPM1 is preferentially localized in the nucleoli of normal cells, whereas it is also present at the nuclear matrix in cancer cells. However, the biological roles of NPM1 during infection are unknown. Here, by analyzing a pV-deletion mutant, Ad5-dV/TSB, we demonstrate that pV promotes the NPM1 translocation from the nucleoli to the nucleoplasm in normal cells, and the NPM1 translocation is correlated with adenoviral replication. Lack of pV causes a dramatic reduction of adenoviral replication in normal cells, but not cancer cells, and Ad5-dV/TSB was defective in viral assembly in normal cells. NPM1 knockdown inhibits adenoviral replication, suggesting an involvement of NPM1 in adenoviral biology. Further, we show that NPM1 interacts with empty adenovirus particles which are an intermediate during virion maturation by immunoelectron microscopy. Collectively, these data implicate that pV participates in a process of viral assembly through NPM1.

Alternative transcript initiation and splicing as a response to DNA damage

Humans are exposed to the DNA damaging agent, ionizing radiation (IR), from background radiation, medical treatments, occupational and accidental exposures. IR causes changes in transcription, but little is known about alternative transcription in response to IR on a genome-wide basis. These investigations examine the response to IR at the exon level in human cells, using exon arrays to comprehensively characterize radiation-induced transcriptional expression products. Previously uncharacterized alternative transcripts that preferentially occur following IR exposure have been discovered. A large number of genes showed alternative transcription initiation as a response to IR. Dose-response and time course kinetics have also been characterized. Interestingly, most genes showing alternative transcript induction maintained these isoforms over the dose range and times tested. Finally, clusters of co-ordinately up- and down-regulated radiation response genes were identified at specific chromosomal loci. These data provide the first genome-wide view of the transcriptional response to ionizing radiation at the exon level. This study provides novel insights into alternative transcripts as a mechanism for response to DNA damage and cell stress responses in general.

Nucleophosmin and its complex network: a possible therapeutic target in hematological diseases

Nucleophosmin (NPM, also known as B23, numatrin or NO38) is a ubiquitously expressed phosphoprotein belonging to the nucleoplasmin family of chaperones. NPM is mainly localized in the nucleolus where it exerts many of its functions, but a proportion of the protein continuously shuttles between the nucleus and the cytoplasm. A growing number of cellular proteins have been described as physical interactors of NPM, and consequently, NPM is thought to have a relevant role in diverse cellular functions, including ribosome biogenesis, centrosome duplication, DNA repair and response to stress. NPM has been implicated in the pathogenesis of several human malignancies and intriguingly, it has been described both as an activating oncogene and a tumor suppressor, depending on cell type and protein levels. In fact, increased NPM expression is associated with different types of solid tumors whereas an impairment of NPM function is characteristic of a subgroup of hematolologic malignancies. A large body of experimental evidence links the deregulation of specific NPM functions to cellular transformation, yet the molecular mechanisms through which NPM contributes to tumorigenesis remain elusive. In this review, we have summarized current knowledge concerning NPM functions, and attempted to interpret its multifaceted and sometimes apparently contradictory activities in the context of both normal cellular homeostasis and neoplastic transformation.

The Multifunctional Nucleolar Protein Nucleophosmin/NPM/B23 and the Nucleoplasmin Family of Proteins

The nucleophosmin (NPM)/nucleoplasmin family of nuclear chaperones has three members: NPM1, NPM2, and NPM3. Nuclear chaperones serve to ensure proper assembly of nucleosomes and proper formation of higher order structures of chromatin. In fact, this family of proteins has such diverse functions in cellular processes such as chromatin remodeling, ribosome biogenesis, genome stability, centrosome replication, cell cycle, transcriptional regulation, apoptosis, and tumor suppression. Of the members of this family, NPM1 is the most studied and is the main focus of this review. NPM2 and NPM3 are less well characterized, and are also discussed wherever appropriate. The structure–function relationship of NPM proteins has largely been worked out. Other than the many processes in which NPM1 takes part, the major interest comes from its involvement in human cancers, particularly acute myeloid leukemia (AML). Its significance stems from the fact that AML with mutated NPM1 accounts for ∼30% of all AML cases and usually has good prognosis. Its clinical importance also comes from its involvement in virus replication, particularly in the era of outbreaks of infectious diseases.

Acute myeloid leukemia with mutated nucleophosmin (NPM1): molecular, pathological, and clinical features

The NPM1 gene encodes for nucleophosmin, a nucleolus-located shuttling protein that is involved in multiple cell functions, including regulation of ribosome biogenesis, control of centrosome duplication and preservation of ARF tumor suppressor integrity. The NPM1 gene is specifically mutated in about 30% acute myeloid leukemia (AML) but not in other human neoplasms. Mutations cause crucial changes at the C-terminus of the NPM1 protein that are responsible for the aberrant nuclear export and accumulation of NPM1 mutants in the cytoplasm of leukemic cells. Diagnosis of AML with mutated NPM1 can be done using molecular techniques, immunohistochemistry (looking at cytoplasmic dislocation of nucleophosmin that is predictive of NPM1 mutations) and Western blotting with antibodies specifically directed against NPM1 mutants. Because of its distinctive molecular, pathological, immunophenotypic and prognostic features, AML with mutated NPM1 (synonym: NPMc+ AML) has been included, as a new provisional entity, in the 2008 World Health Organization (WHO) classification of myeloid neoplasms.

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.

The natural product avrainvillamide binds to the oncoprotein nucleophosmin

Here we present evidence that (+)-avrainvillamide, a naturally occurring alkaloid with antiproliferative effects, binds to the nuclear chaperone nucleophosmin, a proposed oncogenic protein that is overexpressed in many different human tumors. Among other effects, nucleophosmin is known to regulate the tumor suppressor protein p53. A synthetic biotin-avrainvillamide conjugate, nearly equipotent to the natural product in inhibiting the growth of cultured T-47D cells, was used for affinity-isolation of a protein identified as nucleophosmin by MS sequencing and Western-blotting. Affinity-isolation of nucleophosmin was inhibited in the presence of iodoacetamide (10 mM), free (+)-avrainvillamide (100 microM), and a series of closely related structural analogues of (+)-avrainvillamide, the latter with inhibitory effects that appear to correlate with measured growth-inhibitory potencies. Using fluorescence microscopy, a synthetic dansyl-avrainvillamide conjugate was observed to localize within the nucleoli and the cytosol of treated cancer cells. Site-directed mutagenesis of each of the three cysteine residues of a truncated nucleophosmin coexpressed with native nucleophosmin in COS-7 cells revealed that the mutation cys275 --> ala275 effectively and uniquely reduced affinity-isolation of the truncated protein, suggesting that avrainvillamide targets cys275 of nucleophosmin. Finally, we show that treatment of adhered LNCaP or T-47D cells with (+)-avrainvillamide leads to an increase in cellular p53 concentrations, and that siRNA-promoted depletion of nucleophosmin in a population of HeLa S3 cells leads to increased sensitivity of that population toward apoptotic death upon treatment with (+)-avrainvillamide. Although potentially desirable as lead compounds for the development of novel anticancer therapies, nonpeptidic, synthetic small molecules that bind to nucleophosmin have not been described, prior to this report.

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.

Acute myeloid leukemia carrying cytoplasmic/mutated nucleophosmin (NPMc+ AML): biologic and clinical features

The nucleophosmin (NPM1) gene encodes for a multifunctional nucleocytoplasmic shuttling protein that is localized mainly in the nucleolus. NPM1 mutations occur in 50% to 60% of adult acute myeloid leukemia with normal karyotype (AML-NK) and generate NPM mutants that localize aberrantly in the leukemic-cell cytoplasm, hence the term NPM-cytoplasmic positive (NPMc+ AML). Cytoplasmic NPM accumulation is caused by the concerted action of 2 alterations at mutant C-terminus, that is, changes of tryptophan(s) 288 and 290 (or only 290) and creation of an additional nuclear export signal (NES) motif. NPMc+ AML shows increased frequency in adults and females, wide morphologic spectrum, multilineage involvement, high frequency of FLT3-ITD, CD34 negativity, and a distinct gene-expression profile. Analysis of mutated NPM has important clinical and pathologic applications. Immunohistochemical detection of cytoplasmic NPM predicts NPM1 mutations and helps rationalize cytogenetic/molecular studies in AML. NPM1 mutations in absence of FLT3-ITD identify a prognostically favorable subgroup in the heterogeneous AML-NK category. Due to their frequency and stability, NPM1 mutations may become a new tool for monitoring minimal residual disease in AML-NK. Future studies should focus on clarifying how NPM mutants promote leukemia, integrating NPMc+ AML in the upcoming World Health Organization leukemia classification, and eventually developing specific antileukemic drugs.

Chromatin-related proteins in pluripotent mouse embryonic stem cells are downregulated after removal of leukemia inhibitory factor

Embryonic stem (ES) cells have generated enormous interest due to their capacity to self-renew and the potential for growing many different cell types in vitro. Leukemia inhibitory factor (LIF), bone morphogenetic proteins, octamer-binding protein 3 or 4, and Nanog are important factors in the maintenance of pluripotency in mouse ES cells. However, the mechanisms by which these factors regulate the pluripotency remain poorly understood. To identify other proteins involved in this process, we did a proteomic analysis of mouse ES cells that were cultured in the presence or absence of LIF. More than 100 proteins were found to be involved specifically in either the differentiation process or the maintenance of undifferentiated state. Among these, chromatin-related proteins were identified as the major proteins in nuclear extracts of undifferentiated cells. Analysis with real-time RT-PCR revealed that enrichment of these proteins in pluripotent ES cells was regulated at the transcriptional levels. These results suggest that specific chromatin-related proteins may be involved in maintaining the unique properties of pluripotent ES cells.

Nucleoplasmin regulates chromatin condensation during apoptosis

Although chromatin condensation is one of the hallmarks of apoptosis, its relationship with DNA fragmentation has been controversial. We show here that apoptotic chromatin condensation is regulated by nucleoplasmin, a protein that decondenses sperm chromatin during male pronuclear assembly. In Xenopus egg extracts, nucleoplasmin is tyrosine-dephosphorylated during apoptosis. This dephosphorylation inactivates the chromatin decondensation activity of nucleoplasmin and leads to its exclusion from the chromatin. Inhibition of tyrosine dephosphorylation prevents apoptotic chromatin condensation but not DNA fragmentation. Studies with mutant proteins indicate that dephosphorylation of nucleoplasmin at Tyr-124 regulates chromatin condensation through changes in the interaction of nucleoplasmin with chromatin and the loss of its chromatin decondensation activity. These results show that chromatin condensation and DNA fragmentation are independent processes.