Acute myeloid leukemia with mutated NPM1: diagnosis, prognosis and therapeutic perspectives

The clinical impact of the molecular landscape of acute myeloid leukemia

Research into the underlying pathogenic mechanisms of acute myeloid leukemia (AML) has led to remarkable advances in our understanding of the disease. Mutations now allow us to explore the enormous diversity among cytogenetically defined subsets of AML, particularly the large subset of cytogenetically normal AML. Despite the progress in unraveling the tumor genome, only a small number of recurrent mutations have been incorporated into risk-stratification schemes and have been proven to be clinically relevant, targetable lesions. The current World Health Organization Classification of myeloid neoplasms and leukemia includes eight AML categories defined by recurrent genetic abnormalities as well as three categories defined by gene mutations. We here discuss the utility of molecular markers in AML in prognostication and treatment decision-making. New therapies based on targetable markers include IDH inhibitors (ivosidenib, enasidenib), venetoclax-based therapy, FLT3 inhibitors (midostaurin, gilteritinib, and quizartinib), gemtuzumab ozogamicin, magrolimab and menin inhibitors.

The genomics of acute myeloid leukemia in children

Acute myeloid leukemia (AML) is a clinically, morphologically, and genetically heterogeneous disorder. Like many malignancies, the genomic landscape of pediatric AML has been mapped recently through sequencing of large cohorts of patients. Much has been learned about the biology of AML through studies of specific recurrent genetic lesions. Further, genetic lesions have been linked to specific clinical features, response to therapy, and outcome, leading to improvements in risk stratification. Lastly, targeted therapeutic approaches have been developed for the treatment of specific genetic lesions, some of which are already having a positive impact on outcomes. While the advances made based on the discoveries of sequencing studies are significant, much work is left. The biologic, clinical, and prognostic impact of a number of genetic lesions, including several seemingly unique to pediatric patients, remains undefined. While targeted approaches are being explored, for most, the efficacy and tolerability when incorporated into standard therapy is yet to be determined. Furthermore, the challenge of how to study small subpopulations with rare genetic lesions in an already rare disease will have to be considered. In all, while questions and challenges remain, precisely defining the genomic landscape of AML, holds great promise for ultimately leading to improved outcomes for affected patients.

Human MYC G-quadruplex: From discovery to a cancer therapeutic target

Overexpression of the MYC oncogene is a molecular hallmark of both cancer initiation and progression. Targeting MYC is a logical and effective cancer therapeutic strategy. A special DNA secondary structure, the G-quadruplex (G4), is formed within the nuclease hypersensitivity element III₁ (NHE III₁) region, located upstream of the MYC gene's P1 promoter that drives the majority of its transcription. Targeting such G4 structures has been a focus of anticancer therapies in recent decades. Thus, a comprehensive review of the MYC G4 structure and its role as a potential therapeutic target is timely. In this review, we first outline the discovery of the MYC G4 structure and evidence of its formation in vitro and in cells. Then, we describe the functional role of G4 in regulating MYC gene expression. We also summarize three types of MYC G4-interacting proteins that can promote, stabilize and unwind G4 structures. Finally, we discuss G4-binding molecules and the anticancer activities of G4-stabilizing ligands, including small molecular compounds and peptides, and assess their potential as novel anticancer therapeutics.

Nucleophosmin1 (NPM1) abnormality in hematologic malignancies, and therapeutic targeting of mutant NPM1 in acute myeloid leukemia

Nucleophosmin (NPM1) is an abundant nucleolar protein that is implicated in a variety of biological processes and in the pathogenesis of several human malignancies. For hematologic malignancies, approximately one-third of anaplastic large-cell non-Hodgkin’s lymphomas were found to express a fusion between NPM1 and the catalytic domain of anaplastic lymphoma receptor tyrosine kinase. About 50–60% of acute myeloid leukemia patients with normal karyotype carry NPM1 mutations, which are characterized by cytoplasmic dislocation of the NPM1 protein. Nevertheless, NPM1 is overexpressed in various hematologic and solid tumor malignancies. NPM1 overexpression is considered a prognostic marker of recurrence and progression of cancer. Thus, NPM1 abnormalities play a critical role in several types of hematologic malignancies. This has led to intense interest in the development of an NPM1 targeting strategy for cancer therapy. The aim of this review is to summarize present knowledge on NPM1 origin, pathogenesis, and therapeutic interventions in hematologic malignancies.

Quantitative Assay of Mutated Nucleophosmin in Acute Myeloid Leukemia

Background

In our previous work, we provided strong evidence that nucleophosmin (NPM) gene mutation has an important role in leukemogenesis of primary acute myeloid leukemia (AML). Furthermore, we speculated a new targeted therapy in patients with primary AML and bearing mutated NPM (mNPM). Based on these results together with findings of other researchers, it was essential to develop a method for accurate detection of mNPM.

Methods

Our method based on utilizing the most recent flow cytometeric techniques and instruments in measuring mNPM. Attributed to their availability and technical feasibility, we used human leukemia cell lines to validate our method.

Results

The main findings were differential expression of wild-type NPM (wtNPM) within the same sample. Furthermore flow cytometry (FCM) was a simple straightforward tool for quantitative assay of mNPM.

Conclusions

In this work we developed an innovative technique that could enable quantitative assay of mNPM, and ease its use as a biomarker in cytogenetic and molecular prognostication of primary AML. In addition the study suggested that FCM could differentiate mNPM expression within cells of the same patient thus could be used for monitoring of minimal residual disease.

Reclassification of Acute Myeloid Leukemia According to the 2016 WHO Classification

We reviewed our leukemia database to reclassify 610 patients previously diagnosed as having acute myeloid leukemia (AML) according to the updated 2016 WHO classification. Nine patients were categorized as having myelodysplastic syndrome and myeloid neoplasms with germline predisposition. AML with recurrent genetic abnormalities accounted for 57.4% (345/601) of the patients under the 2016 WHO classification. AML with mutated NPM1 was the most common form (16.5%), with the majority associated with monocytic differentiation (63.6%). AML with double CEBPA mutations accounted for 8.3% of these cases, and the majority were previously diagnosed as AML with/without maturation (78.0%). These newly classified mutations were mutually exclusive without overlapping with other forms of AML with recurrent genetic abnormalities. AML with mutated NPM1 and AML with myelodysplasia-related changes comprised the oldest patients, whereas AML with RUNX1-RUNX1T1 included the youngest patients. The leukocyte count was highest in AML with mutated NPM1, and the percentage of peripheral blood blasts was the highest in AML with double CEBPA mutations. Our results indicate that implementation of the 2016 WHO classification of AML would not pose major difficulties in clinical practice. Hematopathologists should review and prepare genetic tests for the new classification, according to their clinical laboratory conditions.

[Study of the effects and mechanism of all-trans retinoic acid on leukemic cell line U937 cells with NPM1 mutation]

Objective: To investigate the effect and mechanism of all-trans retinoic acid (ATRA) on leukemic cell line U937 cells with NPM1 mutation. Methods: Human acute myeloid leukemia cell line U937 was explored, NPM1 mutated (A type) plasmids were transfected into U937 to form stable clones A1 and A2, which were identified by Western blot and Co-immunoprecipitation. The cell proliferation was measured by methylthiazolyl tetrazolium bromide (MTT) ; cell cycle and cell apoptosis were explored by flow cytometric; cell colony formation was measured by microscope count, the molecular pathways related to cell proliferation were measured by Western blot. Results: ①The cell proliferations of mutant A1 and A2 were inhibited significantly by 52.6% and 35.8% (P<0.05) , respectively under ATRA exposure. ②The percentages of G(0)/G(1) stage of mutant A1 and A2 increased by 20.1% and 35.8%, respectively under ATRA exposure. ③All the U937 leukemic cells were inhibited under ATRA exposure; the decreased percentages of vector, wild-type and mutant NPM1 cells were 32.7%, 57.9% and 90.9% respectively. ④p-ERK decreased obviously after ATRA exposure in NPM1 mutated leukemic cells. ⑤More mutant NPM1 cells inclined to apoptosis under the exposure of ATRA and cytotoxic drugs than cytotoxic drugs alone, meanwhile more cells apoptosis occurred when ATRA was administrated after cytotoxic drugs exposure. Conclusions: ATRA could inhibit cell proliferation and colony formation, blocked the cell cycle in the G(0)/G(1) stage accompanied by the significant reduction of p-ERK in U937 leukemic cells with NPM1 mutation. Besides, ATRA could synergize with drugs to suppress the leukemic cells survival more effectively when ATRA was administered after the cytotoxic drugs exposure in U937 leukemic cells with NPM1 mutation.

Structural insights into amyloid structures of the C-terminal region of nucleophosmin 1 in type A mutation of acute myeloid leukemia

Acute myeloid leukemia (AML) is a clinically and a molecularly heterogeneous disease characterized by the accumulation of undifferentiated and uncontrolled proliferation of hematopoietic progenitor cells. The sub-group named "AML with gene mutations" includes mutations in nucleophosmin (NPM1) assumed as a distinct leukemic entity. NPM1 is an abundant multifunctional protein belonging to the nucleoplasmin family of nuclear chaperones. AML mutated protein is translocated into the cytoplasm (NPM1c+) retaining all functional domains except the loss of a unique NoLs (nucleolar localization signal) at the C-term domain (CTD) and the subsequent disruption of a three helix bundle as tertiary structure. The oligomeric state of NPM1 is of outmost importance for its biological roles and our previous studies linked an aggregation propensity of distinct regions of CTD to leukomogenic potentials of AML mutations. Here we investigated a polypeptide spanning the third and second helices of the bundle of type A mutated CTD. By a combination of several techniques, we ascertained the amyloid character of the aggregates and of fibrils resulting from a self-recognition mechanism. Further amyloid assemblies resulted cytoxic in MTT assay strengthening a new idea of a therapeutic strategy in AML consisting in the self-degradation of mutated NPM1.

Clinical implications of molecular markers in acute myeloid leukemia

The recently updated World Health Organization (WHO) Classification of myeloid neoplasms and leukemia reflects the fact that research in the underlying pathogenic mechanisms of acute myeloid leukemia (AML) has led to remarkable advances in our understanding of the disease. Gene mutations now allow us to explore the enormous diversity among cytogenetically defined subsets of AML, particularly the large subset of cytogenetically normal AML. Despite the progress in unraveling the tumor genome, only a small number of recurrent mutations have been incorporated into risk-stratification schemes and have been proven to be clinically relevant, targetable lesions. We here discuss the utility of molecular markers in AML in prognostication and treatment decision making, specifically highlighting the aberrations included in the current WHO classification.

The human nucleophosmin 1 mutation A inhibits myeloid differentiation of leukemia cells by modulating miR-10b

Mutations in the nucleophosmin 1 (NPM1) gene are the most frequent genetic alteration in acute myeloid leukemia (AML). Here, we showed that enforced expression of NPM1 mutation type A (NPM1-mA) inhibits myeloid differentiation of leukemia cells, whereas knockdown of NPM1-mA has the opposite effect. Our analyses of normal karyotype AML samples from The Cancer Genome Atlas (TCGA) dataset revealed that miR-10b is commonly overexpressed in NPM1-mutated AMLs. We also found high expression of miR-10b in primary NPM1-mutated AML blasts and NPM1-mA positive OCI-AML3 cells. In addition, NPM1-mA knockdown enhanced myeloid differentiation, while induced expression of miR-10b reversed this effect. Finally, we showed that KLF4 is downregulated in NPM1-mutated AMLs. These results demonstrated that miR-10b exerts its effects by repressing the translation of KLF4 and that NPM1-mA inhibits myeloid differentiation through the miR-10b/KLF4 axis. This sheds new light on the effect of NPM1 mutations' on leukemogenesis.

Potential for subsets of wt-NPM1 primary AML blasts to respond to retinoic acid treatment

Acute myeloid leukemia (AML) has high mortality rates, perhaps reflecting a lack of understanding of the molecular diversity in various subtypes and a lack of known actionable targets. There are currently 12 open clinical trials for AML using combination therapeutic modalities including all-trans retinoic acid (RA). Mutant nucleophosmin-1, proposed as a possible marker for RA response, is the criterion for recruiting patients in three active RA phase 3 clinical trials. We tested the ability of RA alone or in combination with either bosutinib (B) or 6-formylindolo(3,2-b) carbazole (F) to induce conversion of 12 de novo AML samples toward a more differentiated phenotype. We assessed levels of expression of cell surface markers associated with differentiation, aldehyde dehydrogenase activity, and glucose uptake activity. Colony formation capacity was reduced with the combined treatment of RA and B or F, and correlated with modulation of a c-Cbl/Lyn/c-Raf-centered signalsome. Combination treatment was in most cases more effective than RA alone. Based on their responses to the treatments, some primary leukemic samples cluster closer to HL-60 cells than to other primary samples, suggesting that they may represent a hitherto undefined AML subtype that is potentially responsive to RA in a combination differentiation therapy.

Deregulation of the HOXA9/MEIS1 axis in acute leukemia

PURPOSE OF REVIEW

HOXA9 is a homeodomain transcription factor that plays an essential role in normal hematopoiesis and acute leukemia, in which its overexpression is strongly correlated with poor prognosis. The present review highlights recent advances in the understanding of genetic alterations leading to deregulation of HOXA9 and the downstream mechanisms of HOXA9-mediated transformation.

RECENT FINDINGS

A variety of genetic alterations including MLL translocations, NUP98-fusions, NPM1 mutations, CDX deregulation, and MOZ-fusions lead to high-level HOXA9 expression in acute leukemias. The mechanisms resulting in HOXA9 overexpression are beginning to be defined and represent attractive therapeutic targets. Small molecules targeting MLL-fusion protein complex members, such as DOT1L and menin, have shown promising results in animal models, and a DOT1L inhibitor is currently being tested in clinical trials. Essential HOXA9 cofactors and collaborators are also being identified, including transcription factors PU.1 and C/EBPα, which are required for HOXA9-driven leukemia. HOXA9 targets including IGF1, CDX4, INK4A/INK4B/ARF, mir-21, and mir-196b and many others provide another avenue for potential drug development.

SUMMARY

HOXA9 deregulation underlies a large subset of aggressive acute leukemias. Understanding the mechanisms regulating the expression and activity of HOXA9, along with its critical downstream targets, shows promise for the development of more selective and effective leukemia therapies.

Nucleophosmin Mutations Induce Chemosensitivity in THP-1 Leukemia Cells by Suppressing NF-κB Activity and Regulating Bax/Bcl-2 Expression

Nucleophosmin (NPM1) - a gene that encodes for a nuclear protein with multiple functions. Mutations in NPM1 are seen in approximately one-third of acute myeloid leukemia (AML) and are generally associated with good response to induction chemotherapy. However, the mechanisms underlying this chemosensitivity are still unknown. Recent studies have established that nuclear factor-κB (NF-κB) activation is a key response of leukemia cell to chemotherapy. In this study, we transfected human monocytic leukemia THP-1 cells with the vector expressing NPM1 mutation variant (NPM1mA), and confirmed overexpression of NPM1mA at mRNA and protein levels by reverse transcription PCR (RT-PCR) and immunohistochemistry, respectively. The effects of NPM1 mutations on chemotherapeutical agents induced apoptosis, NF-κB activity and gene expression were examined using flow cytometry, luciferase reporter assays, quantitative real time PCR (qRT-PCR) and Western blot. We found that overexpression of NPM1mA in THP-1 cells sensitized these cells to apoptosis induced by chemotherapeutical agents such as daunorubicin (DNR) and cytarabine (Ara-C). Moreover, we demonstrated that expression of NPM1 mA reduced the NF-κB transcription activity of THP-1 cells upon drug treatment. In addition, restoration of NF-κB activity via TNF-α stimulation could attenuate the effect of NPM1mA overexpression on DNR-and Ara-C-induced apoptosis. Interestingly, expression of NPM1mA could upregulate Bax and downregulate Bcl-2 at mRNA and protein levels in THP-1 cells when treated with DNR or Ara-C. We also demonstrated that restoration of NF-κB activity via TNF-α pre-treatment reversed the effect of NPM1mA on the Bax/Bcl-2 expression. Furthermore, evaluation of gene expression data from The Cancer Genome Atlas (TCGA) dataset revealed that NPM1-mutated patients showed a higher expression of Bax and a lower expression of Bcl-2. These results suggest that the NPM1 gene mutations could confer increased sensitivity to chemotherapeutic agents, at least in part, by suppressing NF-κB activity and regulating Bax/Bcl-2 expression.

Regulation of Neuronal Survival by Nucleophosmin 1 (NPM1) Is Dependent on Its Expression Level, Subcellular Localization, and Oligomerization Status

NPM1 (nucleophosmin 1) is a nucleolar phosphoprotein that regulates many cellular processes, including ribosome biogenesis, proliferation, and genomic integrity. Although its role in proliferating cell types and tissues has been extensively investigated, little is known about its function in neurons and in the brain where it is highly expressed. We report that NPM1 protein expression is increased selectively in the striatum in both the R6/2 transgenic and 3-nitropropionic acid-injected mouse models of Huntington's disease. Examination of the effect of ectopic expression on cultured neurons revealed that increasing NPM1 is toxic to otherwise healthy cerebellar granule and cortical neurons. Toxicity is dependent on its cytoplasmic localization and oligomerization status. Forced retention of NPM1 in the nucleus, as well as inhibiting its ability to oligomerize, not only neutralizes NPM1 toxicity but also renders it protective against apoptosis. Although not blocked by pharmacological inhibition of the pro-apoptotic molecules, JNK, glycogen synthase kinase 3 beta, or caspases, toxicity is blocked by compounds targeting cyclin-dependent kinases (CDKs), as well as by dominant-negative forms of CDK1 and CDK2 and the pan-CDK inhibitor, p21(Cip1/Waf1) Although induced in in vivo Huntington's disease models, NPM1 protein levels are unchanged in cultured cerebellar granule and cortical neurons induced to die by low potassium or homocysteic acid treatment, respectively. Moreover, and counterintuitively, knockdown of its expression or inhibition of endogenous NPM1 oligomerization in these cultured neurons is toxic. Taken together, our study suggests that although neurons need NPM1 for survival, an increase in its expression beyond physiological levels and its translocation to the cytoplasm leads to death through abortive cell cycle induction.

Pretransplant NPM1 MRD levels predict outcome after allogeneic hematopoietic stem cell transplantation in patients with acute myeloid leukemia

The objective was to evaluate the prognostic impact of pre-transplant minimal residual disease (MRD) as determined by real-time quantitative polymerase chain reaction in 67 adult NPM1-mutated acute myeloid leukemia patients receiving allogeneic hematopoietic stem cell transplantation (HSCT). Twenty-eight of the 67 patients had a FLT3-ITD (42%). Median age at transplantation was 54.7 years, median follow-up for survival from time of allografting was 4.9 years. At transplantation, 31 patients were in first, 20 in second complete remission (CR) and 16 had refractory disease (RD). Pre-transplant NPM1 MRD levels were measured in 39 CR patients. Overall survival (OS) for patients transplanted in CR was significantly longer as compared to patients with RD (P=0.004), irrespective of whether the patients were transplanted in first or second CR (P=0.74). There was a highly significant difference in OS after allogeneic HSCT between pre-transplant MRD-positive and MRD-negative patients (estimated 5-year OS rates of 40 vs 89% P=0.007). Multivariable analyses on time to relapse and OS revealed pre-transplant NPM1 MRD levels >1% as an independent prognostic factor for poor survival after allogeneic HSCT, whereas FLT3-ITD had no impact. Notably, outcome of patients with pre-transplant NPM1 MRD positivity >1% was as poor as that of patients transplanted with RD.

Minimal residual disease in acute myeloid leukemia--current status and future perspectives

In acute myeloid leukemia (AML), the achievement of a morphological complete remission (CR) is an important milestone on the road to cure. Still, the majority of patients who achieve a morphological CR will eventually relapse. Thus, morphological means are not sensitive enough to detect clinically relevant tumor burdens left behind after therapy. Over the last years, several methodologies, particularly multiparameter flow cytometry and polymerase chain reaction, have emerged that can detect, quantify, and monitor submicroscopic amounts of leukemia cells ("minimal residual disease", MRD). Newer techniques, such as next-generation sequencing, have not only changed our understanding of the molecular pathogenesis and clonal heterogeneity of AML but may also be used for MRD detection. Increasing evidence indicates that MRD could play an important role in dynamically refining disease risk and, perhaps, serve to fine-tune post-remission therapy in a risk-adapted manner, although the latter concept awaits validation through well-controlled trials. In this review, we discuss the current use of MRD measurements during AML treatment and highlight future perspectives.

Principles of analytical validation of next-generation sequencing based mutational analysis for hematologic neoplasms in a CLIA-certified laboratory

Targeted therapy based on mutational profiles is the current standard of practice for the management of patients with hematologic malignancies. Next-generation sequencing (NGS)- based analysis has been adopted by clinical laboratories for high-throughput mutational profiling of myeloid and lymphoid neoplasms. The technology is fairly novel and complex, hence both validation and test implementation in a CLIA-certified laboratory differ substantially from traditional sequencing platforms. Recently, organizations such as the American College of Medical Genetics, Centers for Disease Control and Prevention and College of American Pathologists have published principles and guidelines for NGS test development to ensure standardization of testing across institutions. Summarized here are the recommendations from these organizations as they pertain to targeted NGS-based testing of hematologic malignancies ('liquid tumors'), with particular emphasis on myeloid neoplasms.

PBX3 and MEIS1 Cooperate in Hematopoietic Cells to Drive Acute Myeloid Leukemias Characterized by a Core Transcriptome of the MLL-Rearranged Disease

Overexpression of HOXA/MEIS1/PBX3 homeobox genes is the hallmark of mixed lineage leukemia (MLL)-rearranged acute myeloid leukemia (AML). HOXA9 and MEIS1 are considered to be the most critical targets of MLL fusions and their coexpression rapidly induces AML. MEIS1 and PBX3 are not individually able to transform cells and were therefore hypothesized to function as cofactors of HOXA9. However, in this study, we demonstrate that coexpression of PBX3 and MEIS1 (PBX3/MEIS1), without ectopic expression of a HOX gene, is sufficient for transformation of normal mouse hematopoietic stem/progenitor cells in vitro. Moreover, PBX3/MEIS1 overexpression also caused AML in vivo, with a leukemic latency similar to that caused by forced expression of MLL-AF9, the most common form of MLL fusions. Furthermore, gene expression profiling of hematopoietic cells demonstrated that PBX3/MEIS1 overexpression, but not HOXA9/MEIS1, HOXA9/PBX3, or HOXA9 overexpression, recapitulated the MLL-fusion-mediated core transcriptome, particularly upregulation of the endogenous Hoxa genes. Disruption of the binding between MEIS1 and PBX3 diminished PBX3/MEIS1-mediated cell transformation and HOX gene upregulation. Collectively, our studies strongly implicate the PBX3/MEIS1 interaction as a driver of cell transformation and leukemogenesis, and suggest that this axis may play a critical role in the regulation of the core transcriptional programs activated in MLL-rearranged and HOX-overexpressing AML. Therefore, targeting the MEIS1/PBX3 interaction may represent a promising therapeutic strategy to treat these AML subtypes.

Concurrent detection of targeted copy number variants and mutations using a myeloid malignancy next generation sequencing panel allows comprehensive genetic analysis using a single testing strategy

Currently, comprehensive genetic testing of myeloid malignancies requires multiple testing strategies with high costs. Somatic mutations can be detected by next generation sequencing (NGS) but copy number variants (CNVs) require cytogenetic methods including karyotyping, fluorescence in situ hybidization and microarray. Here, we evaluated a new method for CNV detection using read depth data derived from a targeted NGS mutation panel. In a cohort of 270 samples, we detected pathogenic mutations in 208 samples and targeted CNVs in 68 cases. The most frequent CNVs were 7q deletion including LUC7L2 and EZH2, TP53 deletion, ETV6 deletion, gain of RAD21 on 8q, and 5q deletion, including NSD1 and NPM1. We were also able to detect exon-level duplications, including so-called KMT2A (MLL) partial tandem duplication, in 9 cases. In the 63 cases that were negative for mutations, targeted CNVs were observed in 4 cases. Targeted CNV detection by NGS had very high concordance with single nucleotide polymorphism microarray, the current gold standard. We found that ETV6 deletion was strongly associated with TP53 alterations and 7q deletion was associated with mutations in TP53, KRAS and IDH1. This proof-of-concept study demonstrates the feasibility of using the same NGS data to simultaneously detect both somatic mutations and targeted CNVs.

Nucleophosmin Mutants Promote Adhesion, Migration and Invasion of Human Leukemia THP-1 Cells through MMPs Up-regulation via Ras/ERK MAPK Signaling

Acute myeloid leukemia (AML) with mutated nucleophosmin (NPM1) has been defined as a unique subgroup in the new classification of myeloid neoplasm, and the AML patients with mutated NPM1 frequently present extramedullary infiltration, but how NPM1 mutants regulate this process remains elusive. In this study, we found that overexpression of type A NPM1 gene mutation (NPM1-mA) enhanced the adhesive, migratory and invasive potential in THP-1 AML cells lacking mutated NPM1. NPM1-mA had up-regulated expression and gelatinolytic matrix metalloprotease-2 (MMP-2)/MMP-9 activity, as assessed by real-time PCR, western blotting and gelatin zymography. Following immunoprecipitation analysis to identify the interaction of NPM1-mA with K-Ras, we focused on the effect of NPM1-mA overexpression on the Ras/Mitogen-activated protein kinase (MAPK) signaling axis and showed that NPM1-mA increased the MEK and ERK phosphorylation levels, as evaluated by western blotting. Notably, a specific inhibitor of the ERK/MAPK pathway (PD98059), but not p38/MAPK, JNK/MAPK or PI3-K/AKT inhibitors, markedly decreased the cell invasion numbers in a transwell assay. Further experiments demonstrated that blocking the ERK/MAPK pathway by PD98059 resulted in reduced MMP-2/9 protein levels and MMP-9 activity. Additionally, NPM1-mA overexpression had down-regulated gene expression and protein production of tissue inhibitor of MMP-2 (TIMP-2) in THP-1 cells. Furthermore, evaluation of gene expression data from The Cancer Genome Atlas (TCGA) dataset revealed that MMP-2 was overexpressed in AML patient samples with NPM1 mutated and high MMP-2 expression associated with leukemic skin infiltration. Taken together, our results reveal that NPM1 mutations contribute to the invasive potential of AML cells through MMPs up-regulation via Ras/ERK MAPK signaling pathway activation and offer novel insights into the potential role of NPM1 mutations in leukemogenesis.

Fishing the targets of myeloid malignancies in the era of next generation sequencing

Recent advent in next generation sequencing (NGS) and bioinformatics has generated an unprecedented amount of genetic information in myeloidmalignancies. This information may shed lights to the pathogenesis, diagnosis and prognostication of these diseases and provide potential targets for therapeutic intervention. However, the rapid emergence of genetic information will quickly outpace their functional validation by conventional laboratory platforms. Foundational knowledge about zebrafish hematopoiesis accumulated over the past two decades and novel genomeediting technologies and research strategies in thismodel organismhavemade it a unique and timely research tool for the study of human blood diseases. Recent studies modeling human myeloid malignancies in zebrafish have also highlighted the technical feasibility and clinical relevance of thesemodels. Careful validation of experimental protocols and standardization among laboratorieswill further enhance the application of zebrafish in the scientific communities and provide important insights to the personalized treatment ofmyeloid malignancies.

Role of HOXA9 in leukemia: dysregulation, cofactors and essential targets

HOXA9 is a homeodomain-containing transcription factor that has an important role in hematopoietic stem cell expansion and is commonly deregulated in acute leukemias. A variety of upstream genetic alterations in acute myeloid leukemia lead to overexpression of HOXA9, which is a strong predictor of poor prognosis. In many cases, HOXA9 has been shown to be necessary for maintaining leukemic transformation; however, the molecular mechanisms through which it promotes leukemogenesis remain elusive. Recent work has established that HOXA9 regulates downstream gene expression through binding at promoter distal enhancers along with a subset of cell-specific cofactor and collaborator proteins. Increasing efforts are being made to identify both the critical cofactors and target genes required for maintaining transformation in HOXA9-overexpressing leukemias. With continued advances in understanding HOXA9-mediated transformation, there is a wealth of opportunity for developing novel therapeutics that would be applicable for greater than 50% of AML with overexpression of HOXA9.

Nucleophosmin contains amyloidogenic regions that are able to form toxic aggregates under physiological conditions

Nucleophosmin (NPM)-1 is a multifunctional protein involved in a variety of biologic processes and has been implicated in the pathogenesis of several human malignancies. To gain insight into the role of isolated fragments in NPM1 activities, we dissected the C-terminal domain (CTD) into its helical fragments. In this study, we observed the unexpected structural behavior of the peptide fragment corresponding to helix (H)2 (residues 264-277). This peptide has a strong tendency to form amyloidlike assemblies endowed with fibrillar morphology and β-sheet structure, under physiologic conditions, as shown by circular dichroism, thioflavin T, and Congo red binding assays; dynamic light scattering; and atomic force microscopy. The aggregates are also toxic to neuroblastoma cells, as determined using 3-(4;5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction and Ca(2+) influx assays. We also found that the extension of the H2 sequence beyond its N terminus, comprising the connecting loop with H1, delayed aggregation and its associated cytotoxicity, suggesting that contiguous regions of H2 have a protective role in preventing aggregation. Our findings and those in the literature suggest that the helical structures present in the CTD are important in preventing harmful aggregation. These findings could elucidate the pathogenesis of acute myeloid leukemia (AML) caused by NPM1 mutants. Because the CTD is not properly folded in these mutants, we hypothesize that the aggregation propensity of this NPM1 region is involved in the pathogenesis of AML. Preliminary assays on NPM1-Cter-MutA, the most frequent AML-CTD mutation, revealed its significant propensity for aggregation. Thus, the aggregation phenomena should be seriously considered in studies aimed at unveiling the molecular mechanisms of this pathology.

Diagnosing and following adult patients with acute myeloid leukaemia in the genomic age

The diagnosis and follow-up process of adult patients with acute myeloid leukaemia (AML) is challenging to clinicians and laboratory staff alike. While several sets of recommendations have been published over the years, the development of high throughput screening and characterization for both genetic and epigenetic events have evolved with astonishing speed. Here we attempt to provide a practical guide to diagnose and follow adult AML patients with a focus on how to balance the wealth of information on the one hand, with the restriction put on these processes in terms of time, feasibility and economy when caring for these patients, on the other.

C/EBPα is an essential collaborator in Hoxa9/Meis1-mediated leukemogenesis

Homeobox A9 (HOXA9) is a homeodomain-containing transcription factor that plays a key role in hematopoietic stem cell expansion and is commonly deregulated in human acute leukemias. A variety of upstream genetic alterations in acute myeloid leukemia (AML) lead to overexpression of HOXA9, almost always in association with overexpression of its cofactor meis homeobox 1 (MEIS1) . A wide range of data suggests that HOXA9 and MEIS1 play a synergistic causative role in AML, although the molecular mechanisms leading to transformation by HOXA9 and MEIS1 remain elusive. In this study, we identify CCAAT/enhancer binding protein alpha (C/EBPα) as a critical collaborator required for Hoxa9/Meis1-mediated leukemogenesis. We show that C/EBPα is required for the proliferation of Hoxa9/Meis1-transformed cells in culture and that loss of C/EBPα greatly improves survival in both primary and secondary murine models of Hoxa9/Meis1-induced leukemia. Over 50% of Hoxa9 genome-wide binding sites are cobound by C/EBPα, which coregulates a number of downstream target genes involved in the regulation of cell proliferation and differentiation. Finally, we show that Hoxa9 represses the locus of the cyclin-dependent kinase inhibitors Cdkn2a/b in concert with C/EBPα to overcome a block in G1 cell cycle progression. Together, our results suggest a previously unidentified role for C/EBPα in maintaining the proliferation required for Hoxa9/Meis1-mediated leukemogenesis.

Molecular response assessment by quantitative real-time polymerase chain reaction after induction therapy in NPM1-mutated patients identifies those at high risk of relapse

Monitoring minimal residual disease is an important way to identify patients with acute myeloid leukemia at high risk of relapse. In this study we investigated the prognostic potential of minimal residual disease monitoring by quantitative real-time polymerase chain reaction analysis of NPM1 mutations in patients treated in the AMLCG 1999, 2004 and 2008 trials. Minimal residual disease was monitored - in aplasia, after induction therapy, after consolidation therapy, and during follow-up - in 588 samples from 158 patients positive for NPM1 mutations A, B and D (with a sensitivity of 10(-6)). One hundred and twenty-seven patients (80.4%) achieved complete remission after induction therapy and, of these, 56 patients (44.1%) relapsed. At each checkpoint, minimal residual disease cut-offs were calculated. After induction therapy a cut-off NPM1 mutation ratio of 0.01 was associated with a high hazard ratio of 4.26 and the highest sensitivity of 76% for the prediction of relapse. This was reflected in a cumulative incidence of relapse after 2 years of 77.8% for patients with ratios above the cut-off versus 26.4% for those with ratios below the cut-off. In the favorable subgroup according to European LeukemiaNet, the cut-off after induction therapy also separated the cohort into two prognostic groups with a cumulative incidence of relapse of 76% versus 6% after 2 years. Our data demonstrate that in addition to pre-therapeutic factors, the course of minimal residual disease in an individual is an important prognostic factor and could be included in clinical trials for the guidance of post-remission therapy. The trials from which data were obtained were registered at www.clinicaltrials.gov (#NCT01382147, #NCT00266136) and at the European Leukemia Trial Registry (#LN_AMLINT2004_230).

Implications of the miR-10 family in chemotherapy response of NPM1-mutated AML

Nucleophosmin-mutated acute myeloid leukemia (NPM1mut-AML) patients have a high rate of complete remission (CR) to induction chemotherapy. However, the mechanisms responsible for such effects are unknown. Because miR-10 family members are expressed at high levels in NPM1mut-AML, we evaluated whether these microRNAs could predict chemotherapy response in AML. We found that high baseline miR-10 family expression in 54 untreated cytogenetically heterogeneous AML patients was associated with achieving CR. However, when we included NPM1 mutation status in the multivariable model, there was a significant interaction effect between miR-10a-5p expression and NPM1 mutation status. Similar results were observed when using a second cohort of 183 cytogenetically normal older (age ≥ 60 years) AML patients. Loss- and gain-of-function experiments using miR-10a-5p in cell lines and primary blasts did not demonstrate any effect in apoptosis or cell proliferation at baseline or after chemotherapy. These data support a bystander role for the miR-10 family in NPM1mut-AML.

Mutated NPM1 in patients with acute myeloid leukemia in remission and relapse

Patients with newly diagnosed AML (n = 360) including 137 (38%) with normal karyotype (NK) were evaluated. Overall, 60 (16.6%) patients, including 46 of the 137 (33.5%) NK patients, had NPM1 mutation at baseline. Thirty-nine patients (30 NK) had available NPM1 status at the time of complete remission (CR) and all (100%) were negative for mutated NPM1. Among the patients with mutated NPM1 at baseline, 10/39 overall (25%) and 7/30 NK (23%) patients relapsed. NPM1 status was available for eight patients (six with NK) at the time of relapse. Among them, 7/8 overall (87%) and 5/6 NK (83%) patients had mutated NPM1, while 1/8 overall (12%) and 1/6 NK (16%) patients remained NPM1 wild type. Among the 300 patients (including 91 with NK) with wild type NPM1 at diagnosis, none acquired a mutated NPM1 clone, either at CR or at relapse. We conclude that mutated NPM1 is a stable and reliable prognostic marker in AML and can be used to assess MRD.

The level of residual disease based on mutant NPM1 is an independent prognostic factor for relapse and survival in AML

NPM1 RT-PCR levels >1% are associated with poor overall and disease-free survival in AML patients treated with chemotherapy. NPM1 MRD levels >10% are associated with poor overall and disease-free survival in AML patients after allogeneic transplantation.

CD34 expression predicts an adverse outcome in patients with NPM1-positive acute myeloid leukemia

Patients with acute myeloid leukemia (AML) harboring an NPM1 mutation exhibit a heterogeneous clinical outcome. Recent studies have shown that the absence of FLT3 internal tandem duplication (FLT3-ITD) mutation confers a favorable prognosis in NPM1-positive AML. However, the prognostic impact of immunophenotypes in this subgroup remains unclear. In this study, FLT3 mutation status and immunophenotypic profile of 85 NPM1-positive patients with de novo AML were retrospectively analyzed and correlated with their clinical features and survival outcomes. Univariate analysis detected 5 markers with prognostic relevance: older age (≥60 years), high white blood cell (WBC) count (>30 × 10(9)/L), FLT3-ITD, CD7, and CD34 expression. Multivariate analysis showed that high WBC count was the only independent predictor of a lower complete remission rate (P = .019). Older age (P = .035), high WBC count (P = .008), FLT3-ITD (P = .012), and CD34 expression (P = .006) were independent predictors of a shorter event-free survival (EFS). High WBC count (P = .014), FLT3-ITD (P = .005), and CD34 expression (P = .047) were independent predictors of a shorter overall survival (OS). Furthermore, based on FLT3-ITD status in NPM1 mutation-positive patients, we showed that both high WBC and CD34 expression conferred a poor EFS (P = .010 and P = .016, respectively) and OS (P = .032 and P = .001, respectively) in the FLT3-ITD-negative group, whereas high WBC predicted a poor EFS (P = .016) and OS (P = .027) in the FLT3-ITD-positive group. Our results confirm the prognostic value of assessing FLT3-ITD mutations in NPM1-positive AML and identify the adverse prognostic impact of high WBC and CD34 expression in this subgroup of AML.

The human NPM1 mutation A perturbs megakaryopoiesis in a conditional mouse model

The NPM1 mutant affects megakaryocytic development in mice. NPMc+ mutant mice mimic some features of human NPM1-mutated AML.

Identification of two novel NPM1 mutations in patients with acute myeloid leukemia

Background

Genetic abnormalities in adult AML are caused most frequently by somatic mutations in exon 12 of the NPM1 gene, which is observed in approximately 35% of AML patients and up to 60% of patients with cytogenetically normal AML (CN-AML).

Methods

We performed mutational analysis, including fragment analysis and direct sequencing of exon 12 of the NPM1 gene, on 83 AML patients to characterize the NPM1 mutations completely.

Results

In this study, NPM1 mutations were identified in 19 (22.9%) of the 83 AML patients and in 12 (42.9%) of the 28 CN-AML patients. Among the 19 patients with NPM1 mutations, type A NPM1 mutations were identified in 16 (84.2%) patients, whereas non-A type NPM1 mutations were observed in 3 (15.8%) patients. Two of the 3 non-A type NPM1 mutations were novel: c.867_868insAAAC and c.869_873indelCTTTAGCCC. These 2 novel mutant proteins display a nuclear export signal motif (L-xxx-L-xx-V-x-L) less frequently and exhibit a mutation at tryptophan 290 that disrupts the nucleolar localization signal.

Conclusions

This study suggests that novel NPM1 mutations may be non-rare and that supplementary sequence analysis is needed along with conventional targeted mutational analysis to detect non-A types of NPM1 mutations.

Preclinical activity of a novel CRM1 inhibitor in acute myeloid leukemia

Chromosome maintenance protein 1 (CRM1) is a nuclear export receptor involved in the active transport of tumor suppressors (e.g., p53 and nucleophosmin) whose function is altered in cancer because of increased expression and overactive transport. Blocking CRM1-mediated nuclear export of such proteins is a novel therapeutic strategy to restore tumor suppressor function. Orally bioavailable selective inhibitors of nuclear export (SINE) that irreversibly bind to CRM1 and block the function of this protein have been recently developed. Here we investigated the antileukemic activity of KPT-SINE (KPT-185 and KPT-276) in vitro and in vivo in acute myeloid leukemia (AML). KPT-185 displayed potent antiproliferative properties at submicromolar concentrations (IC50 values; 100-500 nM), induced apoptosis (average 5-fold increase), cell-cycle arrest, and myeloid differentiation in AML cell lines and patient blasts. A strong down-regulation of the oncogene FLT3 after KPT treatment in both FLT3-ITD and wild-type cell lines was observed. Finally, using the FLT3-ITD-positive MV4-11 xenograft murine model, we show that treatment of mice with oral KPT-276 (analog of KPT-185 for in vivo studies) significantly prolongs survival of leukemic mice (P < .01). In summary, KPT-SINE are highly potent in vitro and in vivo in AML. The preclinical results reported here support clinical trials of KPT-SINE in AML.

Prognostic impact of NPM1 mutations in Serbian adult patients with acute myeloid leukemia

Based on current findings, the presence of NPM1 mutations in acute myeloid leukemia (AML) patients is associated with an increased probability of complete remission (CR) and better overall survival (OS). We determined the incidence and prognostic relevance of NPM1 mutations, their association with FLT3 and IDH mutations, and other clinical characteristics in Serbian adult AML patients. Samples from 111 adult de novo AML patients, including 73 AML cases with a normal karyotype (NK-AML), were studied. NPM1, FLT3, and IDH mutations were detected by PCR and direct sequencing. NPM1 mutations were detected in 22.5% of patients. The presence of NPM1 mutations predicted a low CR rate and shorter OS. NPM1 mutations showed an association with both FLT3 and IDH mutations. Survival analysis based on NPM1/FLT3 mutational status revealed a lower OS for NPM1(+)/FLT3(-) compared to the NPM1(-)/FLT3(-) group in NK-AML patients. The lack of impact or unfavorable prognostic effect of NPM1 mutations found in this study can be assigned to a small cohort of analyzed AML patients, as can the presence of FLT3 and IDH mutations or other genetic lesions that cooperate with NPM1 mutations influencing prognosis.

Structure of nucleophosmin DNA-binding domain and analysis of its complex with a G-quadruplex sequence from the c-MYC promoter

Nucleophosmin (NPM1) is a nucleocytoplasmic shuttling protein, mainly localized at nucleoli, that plays a key role in several cellular functions, including ribosome maturation and export, centrosome duplication, and response to stress stimuli. More than 50 mutations at the terminal exon of the NPM1 gene have been identified so far in acute myeloid leukemia; the mutated proteins are aberrantly and stably localized in the cytoplasm due to high destabilization of the NPM1 C-terminal domain and the appearance of a new nuclear export signal. We have shown previously that the 70-residue NPM1 C-terminal domain (NPM1-C70) is able to bind with high affinity a specific region at the c-MYC gene promoter characterized by parallel G-quadruplex structure. Here we present the solution structure of the NPM1-C70 domain and NMR analysis of its interaction with a c-MYC-derived G-quadruplex. These data were used to calculate an experimentally restrained molecular docking model for the complex. The NPM1-C70 terminal three-helix bundle binds the G-quadruplex DNA at the interface between helices H1 and H2 through electrostatic interactions with the G-quadruplex phosphate backbone. Furthermore, we show that the 17-residue lysine-rich sequence at the N terminus of the three-helix bundle is disordered and, although necessary, does not participate directly in the contact surface in the complex.

Regulation of subcellular distribution and oncogenic potential of nucleophosmin by plakoglobin

Nucleophosmin (NPM) is a nucleolar phosphoprotein that is involved in many cellular processes and has both oncogenic and growth suppressing activities. NPM is localized primarily in nucleoli but shuttles between the nucleus and the cytoplasm, and sustained cytoplasmic distribution contributes to its tumor promoting activities. Plakoglobin (PG, γ-catenin) is a homolog of β-catenin with dual adhesive and signaling functions. These proteins interact with cadherins and mediate adhesion, while their signaling activities are regulated by association with various intracellular partners. Despite these similarities, β-catenin has a well-defined oncogenic activity, whereas PG acts as a tumor/metastasis suppressor through unknown mechanisms. Comparison of the proteomic profiles of carcinoma cell lines with low- or no PG expression with their PG-expressing transfectants has identified NPM as being upregulated upon PG expression. Here, we examined NPM subcellular distribution and in vitro tumorigenesis/metastasis in the highly invasive and very low PG expressing MDA-MB-231 (MDA-231) breast cancer cells and their transfectants expressing increased PG (MDA-231-PG) or NPM shRNA (MDA-231-NPM-KD) or both (MDA-231-NPM-KD+PG). Increased PG expression increased the levels of nucleolar NPM and coimmunoprecipitation studies showed that NPM interacts with PG. PG expression or NPM knockdown decreased the growth rate of MDA-231 cells substantially and this reduction was decreased further in MDA-231-NPM-KD+PG cells. In in vitro tumorigenesis/metastasis assays, MDA-231-PG cells showed substantially lower and MDA-231-NPM-KD cells substantially higher invasiveness relative to the MDA-231 parental cells, and the co-expression of PG and NPM shRNA led to even further reduction of the invasiveness of MDA-231-PG cells. Furthermore, examination of the levels and localization of PG and NPM in primary biopsies of metastatic infiltrating ductal carcinomas revealed coordinated expression of PG and NPM. Together, the data suggest that PG may regulate NPM subcellular distribution, which may potentially change the function of the NPM protein from oncogenic to tumor suppression.

Frequent loss of RAF kinase inhibitor protein expression in acute myeloid leukemia

RAF kinase inhibitor protein (RKIP) is a negative regulator of the RAS-mitogen-activated protein kinase/extracellular signal-regulated kinase signaling cascade. We investigated its role in acute myeloid leukemia (AML), an aggressive malignancy arising from hematopoietic stem and progenitor cells (HSPCs). Western blot analysis revealed loss of RKIP expression in 19/103 (18%) primary AML samples and 4/17 (24%) AML cell lines but not in 10 CD34+ HSPC specimens. In in-vitro experiments with myeloid cell lines, RKIP overexpression inhibited cellular proliferation and colony formation in soft agar. Analysis of two cohorts with 103 and 285 AML patients, respectively, established a correlation of decreased RKIP expression with monocytic phenotypes. RKIP loss was associated with RAS mutations and in transformation assays, RKIP decreased the oncogenic potential of mutant RAS. Loss of RKIP further related to a significantly longer relapse-free survival and overall survival in uni- and multivariate analyses. Our data show that RKIP is frequently lost in AML and correlates with monocytic phenotypes and mutations in RAS. RKIP inhibits proliferation and transformation of myeloid cells and decreases transformation induced by mutant RAS. Finally, loss of RKIP seems to be a favorable prognostic parameter in patients with AML.

New agents for acute myeloid leukemia: is it time for targeted therapies?

INTRODUCTION

The prognosis of acute myeloid leukemia (AML) is improved in the last two decades, even though induction and consolidation chemotherapy has not involved new drugs. The more effective use of well-known agents as well as refinement of supportive care during the inevitable phase of severe pancytopenia following intensive chemotherapy accounts for the reduction of treatment-related death rate. In addition, mortality due to allogeneic and autologous stem cell transplantation has also been reduced, due to adoption of more effective therapies for graft versus host disease and other transplant-related complications.

AREAS COVERED

The multitude of chromosomal and molecular abnormalities makes the treatment of AML a challenging prospect. In addition, genetic aberrations are not mutually exclusive and coexist in the leukemic cells. As a consequence, the clinical development of new biologic agents proceeds slowly. Data for this review were identified from PubMed and references from relevant articles published in English from 2000 to 2011.

EXPERT OPINION

In Phase II studies, different new agents have been found to be active in AML and are currently under investigation in Phase III trials also in combination with conventional chemotherapy. In the near future, we would have more information about the possibility of introducing new drugs into daily practice.

Clinical implications of non-A-type NPM1 and FLT3 mutations in patients with normal karyotype acute myeloid leukemia

Mutations in the nucleophosmin (NPM1) and fms-like tyrosine kinase-3 (FLT3) genes are the most commonly observed mutations in patients with normal-karyotype acute myeloid leukemia (AML-NK). We analyzed the prognostic effects and interactions of these mutations in 201 AML-NK patients. NPM1 and FLT3 mutations were found in 38.3 and 24.9% of AML-NK patients, respectively. NPM1 mutations (NPM1mut), especially in patients without FLT3 mutations (FLT3mut), were associated with a favorable outcome. However, NPM1mut did not affect survival. FLT3mut tended to be associated with a poor survival outcome. FLT3mut showed no prognostic effects in patients with A-type NPM1mut. However, FLT3mut were associated with a significantly worse prognosis in patients with non-A-type NPM1mut. The prognostic interaction between the NPM1 and FLT3 mutations was significant in patients with non-A-type NPM1mut.

Acute myeloid leukemia with mutated nucleophosmin (NPM1): any hope for a targeted therapy?

Acute myeloid leukemia (AML) carrying nucleophosmin (NPM1) mutations displays distinct molecular and clinical-pathological features that led to its inclusion as provisional entity in 2008 WHO classification of myeloid neoplasms. Since NPM1 mutations behave as a founder genetic lesion in AML, they could be an attractive target for therapeutic intervention. Here, we discuss the potential for developing targeted therapies for NPM1-mutated AML with focus on: (i) interfering with the abnormal traffic of the NPM1 leukemic mutant, i.e., its cytoplasmic dislocation; (ii) disrupting the nucleolar structure/function by interfering with residual wild-type nucleophosmin and other nucleolar components acting as hub proteins; and (iii) evaluating the activity of epigenetic drugs (e.g., 5-azacytidine) or agents acting on differentiation and apoptosis. As quantitative assessment of NPM1 mutated transcript copies now provides the means to measure minimal residual disease, we also discuss the potential for intervening in NPM1-mutated AML before overt hematological relapse occurs (so-called pre-emptive therapy).

Targeting levels or oligomerization of nucleophosmin 1 induces differentiation and loss of survival of human AML cells with mutant NPM1

Nucleophosmin 1 (NPM1) is an oligomeric, nucleolar phosphoprotein that functions as a molecular chaperone for both proteins and nucleic acids. NPM1 is mutated in approximately one-third of patients with AML. The mutant NPM1c+ contains a 4-base insert that results in extra C-terminal residues encoding a nuclear export signal, which causes NPM1c+ to be localized in the cytoplasm. Here, we determined the effects of targeting NPM1 in cultured and primary AML cells. Treatment with siRNA to NPM1 induced p53 and p21, decreased the percentage of cells in S-phase of the cell cycle, as well as induced differentiation of the AML OCI-AML3 cells that express both NPMc+ and unmutated NPM1. Notably, knockdown of NPM1 by shRNA abolished lethal AML phenotype induced by OCI-AML3 cells in NOD/SCID mice. Knockdown of NPM1 also sensitized OCI-AML3 to all-trans retinoic acid (ATRA) and cytarabine. Inhibition of NPM1 oligomerization by NSC348884 induced apoptosis and sensitized OCI-AML3 and primary AML cells expressing NPM1c+ to ATRA. This effect was significantly less in AML cells coexpressing FLT3-ITD, or in AML or normal CD34+ progenitor cells expressing wild-type NPM1. Thus, attenuating levels or oligomerization of NPM1 selectively induces apoptosis and sensitizes NPM1c+ expressing AML cells to treatment with ATRA and cytarabine.