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Overview of Myeloproliferative Disorders
The myeloproliferative disorders are abnormal proliferations of bone marrow stem cells, which can manifest as increased platelets, RBCs, or WBCs in the circulation and sometimes as increased fibrosis in the bone marrow. Based on these abnormalities, they are classified as
Essential thrombocythemia (increased platelets)
Primary myelofibrosis (marrow fibrosis or scarring)
Polycythemia vera (increased RBCs)
Chronic myelogenous leukemia (increased WBCs—see Chronic Myelogenous Leukemia (CML))
Essential thrombocythemia, primary myelofibrosis, and polycythemia vera are Philadelphia chromosome–negative myeloproliferative disorders. Myeloproliferative disorders, particularly chronic myelogenous leukemia, sometimes lead to acute leukemia; some hematologists also classify hypereosinophilic syndrome and mastocytosis as myeloproliferative disorders. There are also rare myeloproliferative disorders that overlap with myelodysplastic sydrome (see Myelodysplastic Syndrome).
Each disorder is identified according to its predominant feature or site of proliferation (see Table: Classification of Myeloproliferative Disorders). Despite overlap, each disorder has a somewhat typical constellation of clinical features, laboratory findings, and course. Although proliferation of one cell line may dominate the clinical picture, each disorder is typically caused by clonal proliferation of a pluripotent stem cell, causing varying degrees of abnormal proliferation of RBC, WBC, and platelet progenitors in the bone marrow. This abnormal clone does not, however, produce bone marrow fibroblasts, which can proliferate in polyclonal reactive fashion.
A mutation of the tyrosine kinase gene JAK2 , involved in the bone marrow response to erythropoietin , contributes to the cause of polycythemia vera and is also mutated in a high proportion of cases of essential thrombocythemia and myelofibrosis.
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