Median survival in essential thrombocythemia exceeds 20 years and clinical course is usually indolent with a minority of patients experiencing thrombohemorrhagic complications. Leukemic, polycythemic, or fibrotic disease transformation in essential thrombocythemia is an infrequent occurrence with a 15-year cumulative risk of approximately 5% or less in each instance. The major incentives for this review have been the recent description of an activating JAK2 tyrosine kinase mutation (JAK2V617F) in essential thrombocythemia, related myeloproliferative disorders, and the impact on clinical practice from the publication of a major treatment trial.
Several studies have reported on the occurrence of JAK2V617F in approximately 50% of patients with essential thrombocythemia and its presence has been associated with advanced age at diagnosis, higher hemoglobin and leukocyte levels, and increased rate of polycythemic transformation. In contrast, the mutation did not appear to affect the incidence of thrombotic, leukemic, or fibrotic events. There is increasing evidence regarding the thrombogenic role of neutrophils in essential thrombocythemia and this might partly explain the superior overall performance by hydroxyurea, compared with anagrelide, in a recent randomized study.
Although it is in vogue to consider essential thrombocythemia as more than one disease in terms of both molecular phenotype (presence or absence of JAK2V617F) and putative pattern of myelopoiesis (monoclonal versus polyclonal), it is yet to be shown that such differences influence either the natural history of the disease or current therapy. From a treatment standpoint, hydroxyurea is now confirmed to be the drug of choice for high-risk patients with essential thrombocythemia.
Division of Hematology, Department of Internal Medicine, Mayo Clinic, Rochester, Minnesota, USA
Correspondence to Ayalew Tefferi MD, Division of Hematology, Mayo Clinic, 200 First Street SW, Rochester, MN 55905, USA Tel: +1 507 284 3159; fax: +1 507 266 4972; e-mail: firstname.lastname@example.org