1 That classification reflected a paradigm shift from previous schemes in that, for the first time, genetic information was incorporated with morphologic, cytochemical, immunophenotypic, and clinical information into diagnostic algorithms for the myeloid neoplasms. The 2001WHO classification was prefaced with a comment predicting that future revisions would be necessary because of rapidly emerging genetic and biologic information. Recently, a revised classification has been published as part of the 4th edition of the WHO monograph series. 2 The aim of the revision was to incorporate new scientific and clinical information to refine diagnostic criteria for previously described neoplasms and to introduce newly recognized disease entities. Our purpose in this communication is to highlight major changes in the revised WHO classification of myeloid neoplasms and acute leukemia and to provide the rationale for those changes.

Polycythemia vera (PV), essential thrombocythemia (ET), and myeloid metaplasia with myelofibrosis (MMM) are clonal disorders arising from hematopoietic progenitors. An internet-based protocol was used to collect clinical information and biological specimens from patients with these diseases. High-throughput DNA resequencing identified a recurrent somatic missense mutation JAK2V617F in granulocyte DNA samples of 121 of 164 PV patients, of which 41 had homozygous and 80 had heterozygous mutations. Molecular and cytogenetic analyses demonstrated that homozygous mutations were due to duplication of the mutant allele. JAK2V617F was also identified in granulocyte DNA samples from 37 of 115 ET and 16 of 46 MMM patients, but was not observed in 269 normal individuals. In vitro analysis demonstrated that JAK2V617F is a constitutively active tyrosine kinase.

The hypereosinophilic syndrome may result from a novel fusion tyrosine kinase - FIP1L1-PDGFRalpha - that is a consequence of an interstitial chromosomal deletion. The acquisition of a T674I resistance mutation at the time of relapse demonstrates that FIP1L1-PDGFRalpha is the target of imatinib. Our data indicate that the deletion of genetic material may result in gain-of-function fusion proteins.

Therapeutic decision-making in primary myelofibrosis (PMF) is becoming more challenging because of the increasing use of allogeneic stem cell transplantation and new investigational drugs. To enhance this process by developing a highly discriminative prognostic system, 1054 patients consecutively diagnosed with PMF at 7 centers were studied. Overall median survival was 69 months (95% confidence interval [CI]: 61-76). Multivariate analysis of parameters obtained at disease diagnosis identified age greater than 65 years, presence of constitutional symptoms, hemoglobin level less than 10 g/dL, leukocyte count greater than 25 ؋ 10 9 /L, and circulating blast cells 1% or greater as predictors of shortened survival. Based on the presence of 0 (low risk), 1 (intermediate risk-1), 2 (intermediate risk-2) or greater than or equal to 3 (high risk) of these variables, 4 risk groups with no overlapping in their survival curves were delineated; respective median survivals were 135, 95, 48, and 27 months (P < .001). Compared with prior prognostic models, the new risk stratification system displayed higher predictive accuracy, replicability, and discriminating power. In 409 patients with assessable metaphases, cytogenetic abnormalities were associated with shorter survival, but their independent contribution to prognosis was restricted to patients in the intermediate-risk groups. JAK2V617F did not cluster with a specific risk group or affect survival. IntroductionPrimary myelofibrosis (PMF) 1 is classified as a chronic myeloproliferative disorder and characterized by variable degrees of cytopenia(s) and/or cytosis, a leukoerythroblastic blood picture, bone marrow fibrosis, and extramedullary hematopoiesis often resulting in hepatosplenomegaly. 2 From a pathogenesis standpoint, the disease features clonal proliferation involving pluripotent hematopoietic stem cells, 3,4 and clonal cell-derived cytokines are implicated for some of the disease aspects such as bone marrow fibrosis and extramedullary hematopoiesis. 2 Most recently, JAK2 5-7 and MPL 8-10 mutations were described in approximately 50% and 10% of patients with PMF, respectively. However, the precise pathogenetic contribution of these mutations is currently not well defined.PMF usually affects subjects with advanced age, 11 but young people are not necessarily spared. 12 Reported median survivals are variable and in the range of 4-7 years. 13,14 Previous studies have identified several adverse prognostic factors for survival, including advanced age, [15][16][17][18][19] marked anemia, 13-22 leukocytosis or leukopenia, 14,16,18,22 abnormal karyotype, 18,23-25 constitutional symptoms, 13,14,17,22 and presence of circulating blasts. 13,14,22 Based on some of these variables, several prognostic scoring systems have been proposed. 12,13,18,19,22,26 More recently, the prognostic value of blood CD34 ϩ cell count 27,28 and JAK2 mutational status [29][30][31] has also been evaluated.Current drug therapy for PMF has not been shown to influence survival and is often used for pal...