Leukocytes contribute to the pathogenesis of thrombosis in essential thrombocythemia (ET) through recently discovered mechanisms of activation and interaction with platelets and endothelial cells. To evaluate whether an increased leukocyte count was associated with thrombosis and whether this effect can be modulated by therapy, we analyzed the clinical course of 439 patients with ET followed at the Ospedali Riuniti di Bergamo. The strength of the association was measured at diagnosis or before thrombotic events by multivariable analyses carried out using data at baseline as well as time-varying covariates. The results showed that (1) an increased leukocyte count at diagnosis was associated with thrombosis during follow-up ("baseline analysis," relative risk [RR] 2.3, 95% confidence interval [CI] 1.4-3.9, P ‫؍‬ .001); (2) hydroxyurea (HU) lowered leukocytosis and reduced the strength of the association between leukocytosis and thrombosis ("time-dependent analysis," RR 1.6, 95% CI 0.9-2.0, not significant [NS]); (3) the association of leukocytosis and thrombosis was more evident in untreated low-risk patients (RR 2.7, 95% CI 1.2-6.4, P ‫؍‬ .01) compared with HU-treated high-risk patients (RR 1.6, 95% CI 0.8-3.2, NS); and (4) the presence of JAK2 V617F was not identified as a risk factor for thrombosis during follow-up despite a significant association between the mutation and leukocytosis. We suggest validation of these findings in prospective clinical studies. (Blood.

Among 994 patients with essential thrombocythemia (ET) who were genotyped for the MPLW515L/K mutation, 30 patients carrying the mutation were identified (3.0%), 8 of whom also displayed the JAK2V671F mutation. MPLW515L/K patients presented lower hemoglobin levels and higher platelet counts than did wild type (wt) MPL; these differences were highly significant compared with MPLwt/ JAK2V617F-positive patients. Reduced hemoglobin and increased platelet levels were preferentially associated with the W515L and W515K alleles, respectively. MPL mutation was a significant risk factor for microvessel disturbances, suggesting platelet hyperreactivity associated with constitutively active MPL; arterial thromboses were increased only in comparison to MPLwt/JAK2wt patients. MPLW515L/K patients presented reduced total and erythroid bone marrow cellularity, whereas the numbers of megakaryocytes, megakaryocytic clusters, and small-sized megakaryocytes were all significantly increased. These data indicate that MPLW515L/K mutations do not define a distinct phenotype in ET, although some differences depended on the JAK2V617F mutational status of the counterpart. (Blood. 2008;112:844-847)

Key Points Differential diagnosis of pre-PMF and overt PMF by 2016 WHO criteria underscores uniqueness in disease presentation and outcome. Patterns of driver and nondriver myeloid gene mutations contribute to prognosis in both pre-PMF and overt PMF.

A total of 186 patients with primary myelofibrosis (PMF) were genotyped for JAK2V617F at diagnosis aimed at analyzing the correlation of mutational status and mutated allele burden with outcome variables, including time to anemia, leukocytosis, leukopenia, thrombocytopenia, massive splenomegaly, leukemia, and with overall survival. A total of 127 JAK2V617F-mutated patients (68% of whole series) were divided in quartiles of V617F allele burden. After a median follow-up of 17.2 months, 23 patients died, 15 because of leukemia. A JAK2V617F mutated status did not impact on the rate of leukemia transformation or overall survival. Patients in the lower quartile had shorter time to anemia and leukopenia and did not progress to large splenomegaly. Furthermore, survival was significantly reduced in the lower quartile compared with upper quartiles and JAK2 wild-type patients. In multivariate analysis, factors associated with reduced survival were age, a blast count more than 1%, and a JAK2V617F burden within first quartile. Causes of death in the lower quartile were represented mainly by systemic infections. We conclude that a low JAK2V617F allele burden at diagnosis is preferentially associated with a myelodepletive rather than myeloproliferative phenotype and represents an independent factor associated with shortened survival in patients with PMF. (Blood. 2009;114:1477-1483) IntroductionSixty percent of patients with primary myelofibrosis (PMF) present an acquired recurrent mutation located in exon 14 of JAK2 (JAK2V617F) [1][2][3][4] ; an additional 8% to 10% harbor a mutation at codon 515 in MPL, ie, W515L/K/A. 5-7 These mutations represent a major criterion for diagnosis according to the 2008 World Health Organization classification of myeloid neoplasms 8 ; however, they are not specific of this disorder and are shared by other Philadelphia chromosome-negative myeloproliferative neoplasms, which include polycythemia vera (PV), essential thrombocythemia (ET), and refractory anemia with ring sideroblasts and thrombocytosis.A JAK2V617F mutated genotype as well as different V617F allele burden have been variably associated with unique hematologic and clinical characteristics in patients with PV or ET. 9 In particular, it has been shown that increasing amounts of V617F allele corresponded to a more pronounced myeloproliferative phenotype favoring higher hemoglobin level and leukocyte count, delineating a "continuum" of the 2 disorders. 10 Furthermore, the higher the mutational burden, the higher the risk of presenting aquagenic pruritus, developing splenomegaly, having thrombosis, requiring cytotoxic therapy, or transforming to post-PV/post-ET myelofibrosis. 9 On the contrary, information in the setting of PMF is limited to a few studies that generally included patients genotyped at different times from diagnosis and used qualitative or semiquantitative assays [11][12][13] ; therefore, biases resulting from disease duration and previous therapy on hematologic and clinical phenotype and on patients' outcome, as well as the ...