Essential Thrombocythemia

Essential thrombocythemia is a myeloproliferative neoplasm in which clonal overproduction of platelets by the bone marrow megakaryocytes raises the platelet count, predisposing to both thrombosis and, at very high counts, bleeding. Most cases carry a JAK2, CALR, or MPL driver mutation, and the disorder generally follows an indolent course with a low but real risk of progression to myelofibrosis or leukemia.

Definition

Essential thrombocythemia is a Philadelphia-chromosome-negative myeloproliferative neoplasm characterized by sustained clonal overproduction of platelets, marrow megakaryocytic proliferation, and a driver mutation in JAK2, CALR, or MPL in most cases, carrying an increased risk of thrombosis and a smaller risk of hemorrhage and of transformation to myelofibrosis or acute leukemia.

Scope

This entry describes essential thrombocythemia as a disease: its driver mutations, the clinical consequences of a high platelet count, its classification, and how it is distinguished from reactive thrombocytosis. It is a reference and educational topic, not a guide to therapy or to platelet-count targets for individuals.

Key concepts

• Clonal thrombocytosis
• Megakaryocytic proliferation
• Driver mutations (JAK2 V617F, CALR, MPL)
• Thrombosis and hemorrhage risk
• Acquired von Willebrand syndrome at very high platelet counts
• Distinction from reactive (secondary) thrombocytosis
• Transformation to post-ET myelofibrosis

Mechanisms

Essential thrombocythemia arises from a clonal stem-cell mutation that activates thrombopoietin-receptor signalling. About half of patients carry the JAK2 V617F mutation, a smaller group carry mutations in the calreticulin gene CALR, and a minority carry MPL mutations; together these account for most cases, and a remainder are triple-negative (Kralovics, 2005; Klampfl, 2013). The resulting constitutive signalling drives megakaryocyte proliferation and platelet overproduction. The high platelet count predisposes to thrombosis, while at extreme counts an acquired von Willebrand syndrome can paradoxically cause bleeding. The clone retains the potential to progress to a fibrotic phase or, less often, to acute leukemia (Arber, 2016).

Clinical relevance

Essential thrombocythemia is a principal cause of clonal thrombocytosis and a key consideration when a persistently high platelet count is found without an obvious reactive cause. This entry explains the disease and its driver biology for reference; it does not provide platelet-count thresholds or treatment recommendations for individual patients.

Epidemiology

Essential thrombocythemia is among the more common myeloproliferative neoplasms, with an incidence on the order of one to two cases per hundred thousand people per year. It can present across adulthood, including in younger women, and overall survival is often near-normal, with thrombosis and progression to myelofibrosis being the main long-term concerns.

Evidence & guidelines

Diagnosis rests on World Health Organization criteria that combine a sustained platelet elevation, characteristic marrow megakaryocyte morphology, a clonal driver mutation, and exclusion of other myeloid neoplasms and of reactive causes (Arber, 2016). The discovery of CALR mutations clarified the genetic basis of many JAK2-negative cases and improved diagnostic certainty (Klampfl, 2013). A randomized trial comparing cytoreductive agents in high-risk disease informed the evidence base for management of patients judged to be at higher thrombotic risk (Harrison, 2005).

History

Essential thrombocythemia was long defined clinically as a persistent, unexplained thrombocytosis once reactive causes were excluded. Its molecular understanding advanced with the 2005 identification of JAK2 V617F in a subset of cases and then with the 2013 discovery of CALR mutations, which explained most of the remaining JAK2-negative patients and sharpened diagnosis (Kralovics, 2005; Klampfl, 2013). The disorder is now defined within the World Health Organization framework of myeloproliferative neoplasms (Arber, 2016).

Debates

How does driver mutation status affect prognosis?

JAK2-mutated and CALR-mutated essential thrombocythemia differ in platelet counts, thrombotic risk, and outcome, raising the question of how mutation type should inform risk assessment alongside age and prior thrombosis.

Key figures

• Robert Kralovics
• Thorsten Klampfl
• Robert Skoda
• Claire Harrison
• Daniel Arber

Seminal works

• kralovics-2005
• klampfl-2013
• harrison-2005

Frequently asked questions

How is essential thrombocythemia distinguished from a reactive high platelet count?

Reactive thrombocytosis follows another stimulus such as infection, inflammation, iron deficiency, or splenectomy, whereas essential thrombocythemia is clonal and usually carries a JAK2, CALR, or MPL mutation with characteristic marrow megakaryocyte changes.

Can a very high platelet count cause bleeding rather than clotting?

Yes. At very high platelet counts an acquired von Willebrand syndrome can develop, in which large von Willebrand multimers are lost, and this can paradoxically lead to bleeding.