Polycythaemia Vera
A clonal myeloproliferative disorder, usually driven by a JAK2 mutation that makes marrow precursors hypersensitive to growth signals, so red cells (and often white cells and platelets) are overproduced independent of erythropoietin, raising blood viscosity and thrombosis risk while erythropoietin itself is suppressed.
Definition
Polycythaemia vera is a myeloproliferative neoplasm in which clonal proliferation of bone marrow cells, driven in almost all cases by a JAK2 mutation, causes an increased red cell mass, often with raised white cells and platelets. It carries a high risk of thrombosis.
First principles
A mutation that switches on the growth signal explains why production escapes normal control
The JAK2 V617F mutation, or related JAK2 exon 12 mutations, constitutively activates the JAK-STAT signalling pathway downstream of the erythropoietin receptor, so marrow precursors proliferate as if erythropoietin were constantly present. Production becomes independent of, and unresponsive to, the normal feedback loop that should switch it off once enough red cells have been made.
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