Overview and History

The MPDs are a group of clonal bone marrow stem cell disorders characterized by the proliferation of differentiated hematopoietic cells resulting in excess production of red cells, platelets, and/or white cells. Other features typical of the MPDs include (a) propensity to exhibit extramedullary hematopoiesis; (b) reactive proliferation of bone marrow fibroblasts; (c) tendency to transform to acute leukemia; and (d) absence of significant dysplasia in blood or bone marrow cells. Except for CML, objectively definable by the translocation between the bcr gene on chromosome 22 and the abl gene on chromosome 9,1 the other MPDs are syndromes characterized by a stereotypical, but often overlapping, set of pathological and clinical features.

The term myeloproliferative disorder was first applied in 1951 by Dameshek who referred to a group of diseases that included CML, polycythemia vera (PCV), agnogenic myeloid metaplasia (AMM), essential (idiopathic) thrombocythemia (ET), and the diGuglielmo-erythroleukemia syndrome.2 He postulated that these disorders were closely related because all manifestations could be explained by neoplastic proliferation of pluripotent bone marrow stem cell or its progeny. In 1971, Ward and Block3 suggested that CML and erythroleukemia behaved in a similar fashion to acute myeloid leukemia (AML), and should be con sidered distinct from AMM, PCV, and ET, each characterized by a benign proliferation of hematopoietic cells. In the French-American-British (FAB) classification schema, erythroleukemia is considered to be an AML subtype4 (defined by the finding of > 30 percent myeloblasts among nonerythroid cells in marrows displaying > 50 percent erythroblasts). The FAB classification of acute leukemia itself may be supplanted by the proposed WHO scheme in which the inherent molecular, cytogenetic, and biological aspects of the diseases are emphasized.5 The MPDs are distinct from myelodysplastic syndromes (MDS), which typically also present with hypercellular marrows; however, MDS are characterized by dysplastic maturation of hematopoietic cells and peripheral cytopenias.6 MDS subgroups have been delineated by an FAB classification scheme as well as by the more biologically based WHO format.5,7 However, one such entity, chronic myelomonocytic leukemia, might better be classified as an MPD due to the frequency of leukocytosis (with an elevated monocyte count) especially if the white blood cell count is elevated.9,10 The Polycythemia Vera Study Group (PVSG) has promulgated useful guidelines11 for the classification of PCV, AMM, and ET. Increased molecular biological understanding of the pathophysiology of these diseases will allow a more biologically based classification system, potentially supplementing the morphological and clinical analysis currently of primary importance for diagnosis and classification of the (non-CML) MPDs.

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