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To elucidate the role of thrombocytosis, alone or in combination with standard (age, previous cardiovascular events) and novel (leukocytosis, JAK2(V617F) mutational status) risk factors, in the cardiovascular events of essential thrombocythemia (ET), we analyzed a cohort of 1063 patients. We found that a platelet count at diagnosis greater than 1000 x 10(9)/L was associated with significantly lower rate of thrombosis in multivariable analysis and, if combined with leukocytes less than 11 x 10(9)/L, pointed to a "low-risk" category with a rate of thrombosis of 1.59% of patients/year. On the contrary, the highest risk category (thrombosis rate, 2.95% of patients/year) was constituted of patients with leukocytosis, lower platelet count, and a JAK2(V617F) mutated genotype in most cases (77% vs 26% in the low-risk group), independently from standard risk factors. These data challenge the theory that elevated platelet count increases thrombosis risk in ET and suggest prospective clinical trials to support this hypothesis.

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.

JAK2 617V>F mutation occurs in a homozygous state in 25% to 30% of patients with polycythemia vera (PV) and 2% to 4% with essential thrombocythemia (ET). Whether homozygosity associates with distinct clinical phenotypes is still under debate. This retrospective multicenter study considered 118 JAK2 617V>F homozygous patients (104 PV, 14 ET) whose clinical characteristics were compared with those of 587 heterozygous and 257 wild-type patients. Irrespective of their clinical diagnosis, homozygous patients were older, displayed a higher leukocyte count and hematocrit value at diagnosis, and presented larger spleen volume. Aquagenic pruritus was significantly more common among homozygous PV patients. JAK2 617V>F homozygosity associated with more frequent evolution into secondary myelofibrosis in both PV and ET. After adjustment for sex, age, leukocyte count, and previous thrombosis in a multivariate analysis, homozygous ET patients displayed a significantly higher risk of cardiovascular events (hazard ratio [HR] 3.97, 95% confidence interval [CI] 1.34-11.7; P = .013) than wild-type (HR = 1.0) or heterozygous patients (HR = 1.49). No significant association of JAK2 617V>F homozygosity with thrombosis risk was observed in PV. Finally, JAK2 617V>F homozygous patients were more likely to receive chemotherapy for control of disease. We conclude that JAK2 617V>F homozygosity identifies PV or ET patients with a more symptomatic myeloproliferative disorder and is associated with a higher risk of major cardiovascular events in patients with ET.

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.