Lack of nucleophosmin mutation in patients with myelodysplastic syndrome and acute myeloid leukemia with chromosome 5 abnormalities

NPM1-Mutated Myeloid Neoplasms with <20% Blasts: A Really Distinct Clinico-Pathologic Entity?

Nucleophosmin (NPM1) gene mutations rarely occur in non-acute myeloid neoplasms (MNs) with <20% blasts. Among nearly 10,000 patients investigated so far, molecular analyses documented NPM1 mutations in around 2% of myelodysplastic syndrome (MDS) cases, mainly belonging to MDS with excess of blasts, and 3% of myelodysplastic/myeloproliferative neoplasm (MDS/MPN) cases, prevalently classified as chronic myelomonocytic leukemia. These uncommon malignancies are associated with an aggressive clinical course, relatively rapid progression to overt acute myeloid leukemia (AML) and poor survival outcomes, raising controversies on their classification as distinct clinico-pathologic entities. Furthermore, fit patients with NPM1-mutated MNs with <20% blasts could benefit most from upfront intensive chemotherapy for AML rather than from moderate intensity MDS-directed therapies, although no firm conclusion can currently be drawn on best therapeutic approaches, due to the limited available data, obtained from small and mainly retrospective series. Caution is also suggested in definitely diagnosing NPM1-mutated MNs with blast count <20%, since NPM1-mutated AML cases frequently present dysplastic features and multilineage bone marrow cells showing abnormal cytoplasmic NPM1 protein delocalization by immunohistochemical staining, therefore belonging to NPM1-mutated clone regardless of blast morphology. Further prospective studies are warranted to definitely assess whether NPM1 mutations may become sufficient to diagnose AML, irrespective of blast percentage.

Clinicopathologic analysis of acute myeloid leukemia arising from chronic myelomonocytic leukemia

Acute myeloid leukemia arising from chronic myelomonocytic leukemia is currently classified as acute myeloid leukemia with myelodysplasia-related changes, a high-risk subtype. However, the specific features of these cases have not been well described. We studied 38 patients with chronic myelomonocytic leukemia who progressed to acute myeloid leukemia. We compared the clinicopathologic and genetic features of these cases with 180 patients with de novo acute myeloid leukemia and 34 patients with acute myeloid leukemia following myelodysplastic syndromes. We also examined features associated with progression from chronic myelomonocytic leukemia to acute myeloid leukemia by comparing the progressed chronic myelomonocytic leukemia cases with a cohort of chronic myelomonocytic leukemia cases that did not transform to acute myeloid leukemia. Higher white blood cell count, marrow cellularity, karyotype risk score, and Revised International Prognostic Scoring System score were associated with more rapid progression from chronic myelomonocytic leukemia to acute myeloid leukemia. Patients with acute myeloid leukemia ex chronic myelomonocytic leukemia were older (P<0.01) and less likely to receive aggressive treatment (P=0.02) than de novo acute myeloid leukemia patients. Most cases showed monocytic differentiation and fell into the intermediate acute myeloid leukemia karyotype risk group; 55% had normal karyotype and 17% had NPM1 mutation. Median overall survival was 6 months, which was inferior to de novo acute myeloid leukemia (17 months, P=0.002) but similar to post myelodysplastic syndrome acute myeloid leukemia. On multivariate analysis of all acute myeloid leukemia patients, only age and karyotype were independent prognostic variables for overall survival. Our findings indicate that acute myeloid leukemia following chronic myelomonocytic leukemia displays aggressive behavior and support placement of these cases within the category of acute myeloid leukemia with myelodysplasia-related changes. The poor prognosis of these patients may be related to an older population and lack of favorable-prognosis karyotypes that characterize many de novo acute myeloid leukemia cases.

Translocation t(5;18)(q35;q21) as a rare nonrandom abnormality in acute myeloid leukemia

Cytogenetic abnormalities play an important role in diagnosis, classification and prognosis in acute myeloid leukemia (AML). Nevertheless, several chromosome abnormalities have not been completely determined, and their prognostic significance is currently unknown due to their low incidence and the sporadic limited data. We report a case of AML-M2 with a novel, nonrandom translocation t(5;18)(q35;q21) in order to clarify the clinical features and outcome of these patients which could be advisable for prognostic and therapeutic purposes. This translocation has been reported only once in AML. Our patient received intensive chemotherapy, but he achieved a complete remission only initially. Eighteen months post diagnosis, t(3;12)(p23;p13) was detected as a secondary abnormality to t(5;18)(q35;q21) in the progression of the disease. FISH studies confirmed the reciprocal t(5;18)(q35;q21) and demonstrated a rearrangement of ETV6 gene as a consequence of t(3;12)(p23;p13). The patient died a few days later. In conclusion, t(5;18)(q35;q21) is a rare but nonrandom abnormality in AML, found in FAB M2 subtype, possibly associated with a rather poor prognosis, while t(3;12)(p23;p13) seems to contribute to the progression of the disease. The publication of rare, nonrandom chromosome abnormalities such as t(5;18)(q35;q21) contribute to the identification of the whole spectrum of cytogenetic abnormalities in AML and their prognostic significance.

Development of diffuse large B-cell lymphoma in a patient with Waldenström's macroglobulinemia/lymphoplasmacytic lymphoma: clonal identity between two B-cell neoplasms

Waldenström's macroglobulinemia (WM)/ lymphoplasmacytic lymphoma (LPL) is an indolent mature B-cell neoplasm. In rare cases of WM/LPL, diffuse large B-cell lymphoma (DLBCL) develops as a result of histologic transformation. In this report, we present a case of DLBCL developing in a patient with WM/LPL. Combination chemotherapy for DLBCL was effective and complete remission was eventually achieved. We attempted to determine the clonal relatedness between WM/LPL and DLBCL in the patient by analyzing complementarity-determining region 3 (CDR3) in the immunoglobulin heavy chain gene. A common CDR3 sequence was found in tumor cells of DLBCL and those of WM/LPL, indicating that tumor cells of DLBCL are clonally identical to those of WM/LPL. Therefore, in the present case, DLBCL is developed from WM/LPL cells by clonal evolution.

Development of diffuse large B-cell lymphoma in a patient with Waldenström's macroglobulinemia/lymphoplasmacytic lymphoma: clonal identity between two B-cell neoplasms

Waldenström's macroglobulinemia (WM)/ lymphoplasmacytic lymphoma (LPL) is an indolent mature B-cell neoplasm. In rare cases of WM/LPL, diffuse large B-cell lymphoma (DLBCL) develops as a result of histologic transformation. In this report, we present a case of DLBCL developing in a patient with WM/LPL. Combination chemotherapy for DLBCL was effective and complete remission was eventually achieved. We attempted to determine the clonal relatedness between WM/LPL and DLBCL in the patient by analyzing complementarity-determining region 3 (CDR3) in the immunoglobulin heavy chain gene. A common CDR3 sequence was found in tumor cells of DLBCL and those of WM/LPL, indicating that tumor cells of DLBCL are clonally identical to those of WM/LPL. Therefore, in the present case, DLBCL is developed from WM/LPL cells by clonal evolution.

Genetic pathways leading to therapy-related myeloid neoplasms

Therapy-related myeloid neoplasm (t-MN) is a distinctive clinical syndrome occurring after exposure to chemotherapy or radiotherapy. t-MN arises in most cases from a multipotential hematopoietic stem cell or, less commonly, in a lineage committed progenitor cell. The prognosis for patients with t-MN is poor, as current forms of therapy are largely ineffective. Cytogenetic analysis, molecular analysis and gene expression profiling analysis of t-MN has revealed that there are distinct subtypes of the disease; however, our understanding of the genetic basis of t-MN is incomplete. Elucidating the genetic pathways and molecular networks that are perturbed in t-MNs, may facilitate the identification of therapeutic targets that can be exploited for the development of urgently-needed targeted therapies.

Characterization of NPM1-mutated AML with a history of myelodysplastic syndromes or myeloproliferative neoplasms

The role of the nucleophosmin (NPM1) mutations in de novo acute myeloid leukemia (AML) is well analyzed, but the impact in secondary AML (s-AML) following myelodysplastic syndromes (MDS) or transformed myeloproliferative neoplasms (MPNs) remains unclear. We investigated 350 patients-283 s-AML after MDS and 67 transformed MPNs-for NPM1mut. NPM1mut was detected in 43/350 patients (12.3%) at diagnosis of s-AML (transformed MDS: 37/283; 13.1%; transformed MPNs: 6/67; 9.0%). Cytogenetic alterations were present in 12/40 cases (30.0%) with available karyotypes. Additional molecular mutations were found in 23/43 NPM1mut s-AML after MDS (53.5%) and in transformed MPN in 18/37 (48.6%): FLT3-ITD: 14/37 (37.8%); FLT3-TKD: 3/28 (10.7%); NRASmut: 4/37 (10.8%), RUNX1mut: 1/16 (6.3%). In NPM1mut-transformed MPNs, five out of six cases showed 1-2 additional molecular mutations (2 × KITD816V, ETV6-PDGFRB, 2 × JAK2V617F, 2 × FLT3-ITD). Backtracking of nine of these cases by quantitative real time PCR showed the NPM1mut already at diagnosis of MDS/MPN, at variable levels and up to 14 months before diagnosis of AML, and at transformation often being preceded or accompanied by other genetic alterations. Thus, NPM1 mutations are involved in the transformation from MDS to AML or MPN to blast phase in single cases, which should be further confirmed in larger studies.

The frequency of NPM1 mutations in childhood acute myeloid leukemia

BACKGROUND

Mutations in the nucleophosmin (NPM1) gene have been solely associated with childhood acute myeloid leukemia (AML). We evaluated the frequency of NPM1 mutations in childhood AML, their relation to clinical and cytogenetic features and the presence of common FLT3 and RAS mutations.

RESULTS

NPM1 mutations were found in 8% of cases. They involved the typical type 'A' mutation and one novel mutation characterized by two individual base pair substitutions, which resulted in 2 amino acid changes (W290) and (S293) in the NPM protein. FLT3/ITD mutations were observed in 12% of the cases and in one NPM1-mutated case bearing also t(8;21) (q22;q22). No common RAS mutations were identified.

CONCLUSIONS

A relatively consistent NPM1 mutation rate was observed, but with variations in types of mutations. The role of different types of NPM1 mutations, either individually or in the presence of other common gene mutations may be essential for childhood AML prognosis.

Proteomic analysis of childhood de novo acute myeloid leukemia and myelodysplastic syndrome/AML: correlation to molecular and cytogenetic analyses

The aim of this study was to investigate the progression of myelodysplastic syndrome (MDS) to acute myeloid leukemia (AML) and to provide additional data regarding the proteomic analysis of AML. The protein profiles obtained were correlated to cytogenetic and molecular analyses. Bone marrow (BM) and peripheral blood (PB) samples were obtained during MDS diagnosis, at MDS transformation to AML, at de novo AML diagnosis and 3 months following treatment. As controls, non-leukemic pediatric patients were studied. Cytogenetic and molecular analyses were carried out by G banding and polymerase chain reaction followed by sequencing, respectively. Differential proteomic analysis was performed by two-dimensional gel electrophoresis and protein identification by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry. No significant correlations were noted between protein patterns and cytogenetic or molecular analyses. Certain suppressor genes, metabolic enzymes, immunoglobulins and actin-binding proteins were differentially expressed by BM or PB plasma and cell lysates compared to controls. The obtained data showed that vitamin D and gelsolin played contradicting roles in contributing and restraining leukemogenesis, while MOES, EZRI and AIFM1 could be considered as biomarkers for AML.

Molecular mechanisms involved in the progression of myelodysplastic syndrome

Myelodysplastic syndromes (MDS) are a heterogeneous group of diseases characterized by ineffective hematopoiesis presenting with peripheral cytopenias in combination with a hyperplastic bone marrow. MDS patients have an increased risk of disease evolution to acute leukemia. Strong efforts have been made to gain further insights into the pathobiology of MDS. Development and progression of MDS to acute myeloid leukemia is suggested to be a multistep alteration to hematopoietic stem cells consisting of class I and class II alterations: the former targeting genes that are involved in signal transduction (e.g., FLT3, RAS and KIT), whereas the latter affect transcription factors (e.g., RUNX, RARA, EVI1 and WT1). These alterations consist of not only genomic mutations but also epigenetic aberrations, which can lead to reversible gene silencing. However, whether numerical and structural alterations of chromosomes and/or single genes or epigenetic changes represent the initiating event or, more likely, secondary events remains part of the discussion. Accumulation of such defects may finally cause the leukemic transformation of MDS.

Haploinsufficiency of Apc leads to ineffective hematopoiesis

Loss of a whole chromosome 5 or a deletion of the long arm of chromosome 5, -5/del(5q), is a recurring abnormality in myeloid neoplasms. The APC gene is located at chromosome band 5q23, and is deleted in more than 95% of patients with a -5/del(5q), raising the question of whether haploinsufficiency of APC contributes to the development of myeloid neoplasms with loss of 5q. We show that conditional inactivation of a single allele of Apc in mice leads to the development of severe anemia with macrocytosis and monocytosis. Further characterization of the erythroid lineage revealed that erythropoiesis is blocked at the early stages of differentiation. The long-term hematopoietic stem cell (LT-HSC) and short-term HSC (ST-HSC) populations are expanded in Apc-heterozygous mice compared with the control littermates; however, the HSCs have a reduced capacity to regenerate hematopoiesis in vivo in the absence of a single allele of Apc. Apc heterozygous myeloid progenitor cells display an increased frequency of apoptosis, and decreased in vitro colony-forming capacity, recapitulating several characteristic features of myeloid neoplasms with a -5/del(5q). Our results indicate that haploinsufficiency of Apc impairs hematopoiesis, and raise the possibility that loss of function of APC contributes to the development of myelodysplasia.

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.

Nucleophosmin (NPM1) mutations in adult and childhood acute myeloid leukaemia: towards definition of a new leukaemia entity

Nucleophosmin (NPM) is a ubiquitously expressed chaperone protein that shuttles rapidly between the nucleus and cytoplasm, but predominantly resides in the nucleolus. It plays key roles in ribosome biogenesis, centrosome duplication, genomic stability, cell cycle progression and apoptosis. Somatic mutations in exon 12 of the NPM gene (NPM1) are the most frequent genetic abnormality in adult acute myeloid leukaemia (AML), found in approximately 35% of all cases and up to 60% of patients with normal karyotype (NK) AML. In children, NPM1 mutations are far less frequent, occurring in 8-10% of all AML cases, and in approximately 25% of those with a NK. NPM1 mutations lead to aberrant localization of the NPM protein into the cytoplasm, thus the designation, NPMc+ AML. NPMc+ AML is seen predominantly in patients with a NK and is essentially mutually exclusive of recurrent chromosomal translocations. Patients with NPM1 mutations are twice as likely as those who lack an NPM1 mutation to also have a FMS-like tyrosine kinase (FLT3) internal tandem duplication (ITD) mutation. NPMc+ AML is also characterized by a unique gene expression signature and microRNA signature. NPMc+ AML has important prognostic significance, as NPMc+ AML, in the absence of a coexisting FLT3-ITD mutation, is associated with a favourable outcome. NPM1 mutations have also shown great stability during disease evolution, and therefore represent a possible marker for minimal residual disease detection. Given its distinctive biologic and clinical features and its clear clinical relevance, NPMc+ AML is included as a provisional entity in the 2008 WHO classifications. There is still much to be learned about this genetic alteration, including its exact role in leukaemogenesis, how it interacts with other mutations and why it confers a more favourable prognosis. Further, it represents a potential therapeutic target warranting research aimed at identifying novel small molecules with activity in NPMc+ AML.

Cytogenetic and genetic pathways in therapy-related acute myeloid leukemia

Therapy-related myelodysplastic syndrome and acute myeloid leukemia (t-MDS/t-AML) are late complications of cytotoxic therapy used in the treatment of malignant diseases. The most common subtype of t-AML ( approximately 75% of cases) develops after exposure to alkylating agents, and is characterized by loss or deletion of chromosome 5 and/or 7 [-5/del(5q), -7/del(7q)], and a poor outcome (median survival 8 months). In the University of Chicago's series of 386 patients with t-MDS/t-AML, 79 (20%) patients had abnormalities of chromosome 5, 95 (25%) patients had abnormalities of chromosome 7, and 85 (22%) patients had abnormalities of both chromosomes 5 and 7. t-MDS/t-AML with a -5/del(5q) is associated with a complex karyotype, characterized by trisomy 8, as well as loss of 12p, 13q, 16q22, 17p (TP53 locus), chromosome 18, and 20q. In addition, this subtype of t-AML is characterized by a unique expression profile (higher expression of genes) involved in cell cycle control (CCNA2, CCNE2, CDC2), checkpoints (BUB1), or growth (MYC), loss of expression of IRF8, and overexpression of FHL2. Haploinsufficiency of the RPS14, EGR1, APC, NPM1, and CTNNA1 genes on 5q has been implicated in the pathogenesis of MDS/AML. In previous studies, we determined that Egr1 acts by haploinsufficiency and cooperates with mutations induced by alkylating agents to induce myeloid leukemias in the mouse. To identify mutations that cooperate with Egr1 haploinsufficiency, we used retroviral insertional mutagenesis. To date, we have identified two common integration sites involving genes encoding transcription factors that play a critical role in hematopoiesis (Evi1 and Gfi1b loci). Of note is that the EVI1 transcription factor gene is deregulated in human AMLs, particularly those with -7, and abnormalities of 3q. Identifying the genetic pathways leading to t-AML will provide new insights into the underlying biology of this disease, and may facilitate the identification of new therapeutic targets.

AML with mutated NPM1 carrying a normal or aberrant karyotype show overlapping biologic, pathologic, immunophenotypic, and prognostic features

Acute myeloid leukemia (AML) with mutated NPM1 usually carries normal karyotype (NK), but it may harbor chromosomal aberrations whose significance remains unclear. We addressed this question in 631 AML patients with mutated/cytoplasmic NPM1. An abnormal karyotype (AK) was present in 93 of 631 cases (14.7%), the most frequent abnormalities being +8, +4, −Y, del(9q), +21. Chromosome aberrations in NPM1-mutated AML were similar to, but occurred less frequently than additional chromosome changes found in other AML with recurrent cytogenetic abnormalities according to WHO classification. Four of the 31 NPM1-mutated AML patients karyotyped at different time points had NK at diagnosis but AK at relapse: del(9q) (n = 2), t(2;11) (n = 1), inv(12) (n = 1). NPM1-mutated AML with NK or AK showed overlapping morphologic, immunophenotypic (CD34 negativity), and gene expression profile (down-regulation of CD34 and up-regulation of HOX genes). No difference in survival was observed among NPM1-mutated AML patients independently of whether they carried a NK or an AK, the NPM1-mutated/FLT3-ITD negative cases showing the better prognosis. Findings in our patients point to chromosomal aberrations as secondary events, reinforce the concept that NPM1 mutation is a founder genetic lesion, and indicate that NPM1-mutated AML should be clinically handled as one entity, irrespective of the karyotype.

Diagnostic challenges in the myelodysplastic syndromes: the current and future role of genetic and immunophenotypic studies

Myelodysplastic syndromes (MDS) comprise a clinically and pathologically diverse collection of hematopoietic neoplasms, most commonly presenting with peripheral cytopenias typically in the context of bone marrow hypercellularity. Mechanistically, at least in the early phases of the disease, this apparently paradoxical picture is primarily due to ineffective hematopoiesis, which is accompanied by a variety of morphologic abnormalities in hematopoietic cells. The identification of recurrent, clinically relevant cytogenetic defects in MDS has spurred the research of molecular mechanisms that contribute to its inception as well as to the development of heterogeneous subtypes. Although conventional cytogenetic analyses remain a diagnostic mainstay in MDS, the application of contemporary techniques including molecular cytogenetics, microarray technologies and multiparametric flow cytometry may ultimately reveal new diagnostic parameters that are theoretically more objective and sensitive than current morphologic approaches. This review aims to outline the role of genetic and immunophenotypic studies in the evaluation of MDS, including findings that may potentially influence future diagnostic classifications, which could refine prognostication and ultimately facilitate the growth of targeted therapies.

Myelodysplastic syndromes: biology and treatment

Optimal management of patients with myelodysplastic syndromes (MDS) requires an insight into the biology of the disease and the mechanisms of action of the available therapies. This review focuses on low-risk MDS, for which chronic anaemia and eventual progression to acute myeloid leukaemia are the main concerns. We cover the updated World Health Organization classification, the latest prognostic scoring system, and describe novel findings in the pathogenesis of 5q- syndrome. We perform in depth analyses of two of the most widely used treatments, erythropoietin and lenalidomide, discussing mechanisms of action, reasons for treatment failure and influence on survival.

Cytoplasmic nucleophosmin is not detected in blastic plasmacytoid dendritic cell neoplasm

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

Myelodysplastic syndromes: molecular pathogenesis and genomic changes

Myelodysplastic syndromes (MDS) are characterized by ineffective hematopoiesis presenting with peripheral cytopenias in combination with a hyperplastic bone marrow and an increased risk of evolution to acute myeloid leukemia. The classification systems such as the WHO classification mainly rely on morphological criteria and are supplemented by the International Prognostic Scoring System which takes cytogenetical changes into consideration when determining the prognosis of MDS but wide intra-subtype variations do exist. The pathomechanisms causing primary MDS require further work. Development and progression of MDS is suggested to be a multistep alteration to hematopoietic stem cells. Different molecular alterations have been described, affecting genes involved in cell-cycle control, mitotic checkpoints, and growth factor receptors. Secondary signal proteins and transcription factors, which gives the cell a growth advantage over its normal counterpart, may be affected as well. The accumulation of such defects may finally cause the leukemic transformation of MDS.

NPM1 mutations in therapy-related acute myeloid leukemia with uncharacteristic features

Frameshift mutations of the nucleophosmin gene (NPM1) were recently reported as a frequently occurring abnormality in patients with de novo acute myeloid leukemia (AML). To evaluate the frequency of NPM1 mutations in patients with therapy-related myelodysplasia (t-MDS) and therapy-related AML (t-AML), and their possible association to type of previous therapy and to other gene mutations, 140 patients with t-MDS or t-AML were analyzed for mutations of NPM1. NPM1 mutations were observed in 7 of 51 patients presenting as overt t-AML, as compared to only 3 of 89 patients presenting as t-MDS (P=0.037). The mutations were not related to any specific type of previous therapy, but they were significantly associated with a normal karyotype and mutations of FLT3 (P=0.0002 for both comparisons). Only 1 of 10 patients with NPM1 mutations presented chromosome aberrations characteristic of therapy-related disease, and 7q-/-7, the most frequent abnormalities of t-MDS/t-AML, were not observed (P=0.002). This raises the question whether some of the cases presenting NPM1 mutations were in fact cases of de novo leukemia. The close association to class I mutations and the inverse association to class II mutations suggest mutations of NPM1 as representing a class II mutation-like abnormality in AML.