Bone Marrow Features and Natural History of BCR/ABL-Positive Thrombocythemia and Chronic Myeloid Leukemia Compared to BCR/ABL- Negative Thrombocythemia in Essential Thrombocythemia and Polycythemia Vera

The Hannover bone marrow (BM) classification distinguished three phenotypes of BCR/ABL-positive CML: CML of common type (CML.CT), CML with megakaryocyte increase (CML.MI) and CML with megakaryocyte predominance (CML.MP). BCR/ABL-positive essential thrombocythemia (Ph-positive ET) is featured by CML.MP bone marrow picture of small monolobulated megakaryocytes and is part of the CML spectrum as a malignant disease (neoplasia) with an obligate transition into acute leukemia of near to 100% after 10 years follow-up. The Hannover BM classification distinguished three primary prefibrotic BCR/ABL-negative (Ph-negative) myeloproiferative disorders (MPD)s: essential thrombocythemia (ET), polycythemia vera (PV) and chronic or primary megakaryocytic granulocytic myeloproliferation (CMGM/PMGM). The incidence of blasts crisis is low in the Phnegative MPDs ET, PV and CMGM. The risk of myelofibrosis is high in CMGM/PMGM, moderate in PV but low in Ph-negative ET. In BCR/ABL-positive thrombocythemia the platelets are small and indolent (non-reactive) and megakaryocytes are smaller than normal with hypolobulated nuclei caused by BCR/ABL induced maturation defect. BCR/ABL-positive thrombocythemia does not present erythromelalgic thrombotic or bleedings manifestations at increased platelet count in excess of 400 to 1500 × 109/L. The platelets and megakaryocytes in BCR/ABL-negative ET and PV are large due to growth advantage caused by constitutively activated by the JAK2V617F or MPL515 mutation. JAK2 and MPL mutated thrombocythemias are associated with a high risk on aspirin responsive plateletmediated inflammation and thrombosis in the end-arterial circulation (platelet thrombophilia).