Diagnosis and Classification of the BCR-ABL1-Negative Myeloproliferative Neoplasms

The myeloproliferative neoplasms (MPNs) are clonal hematopoietic stem cell disorders characterized by dysregulated proliferation and expansion of one or more of the myeloid lineages (erythroid, granulocytic, megakaryocytic, monocytic/macrophage, or mast cell). This dysregulation is thought to be a consequence of genetic abnormalities at the level of stem/progenitor cells. Most of the cases are initially diagnosed in a proliferative phase when maturation of the neoplastic cells in the bone marrow is effective and numbers of granulocytes, erythrocytes, and/or platelets in the peripheral blood are increased. Although their onset is often insidious, each MPN has the potential to progress to bone marrow failure due to myelofibrosis, ineffective hematopoiesis, and/or transformation to acute leukemia. Acute leukemia is defined as 20% or more blasts in the peripheral blood and/or bone marrow or by the appearance of a myeloid sarcoma in extramedullary tissue. In some patients, the initial proliferative stage is not apparent or is of very short duration, and the MPN is first recognized in a progressed stage. At diagnosis, splenomegaly and hepatomegaly are common and often become more prominent during the disease course. The organomegaly is caused by sequestration of excess blood cells in the spleen and liver, extramedullary hematopoiesis, or both.