Polycythemia vera (PV) is a clonal disorder of unknown etiology involving a multipotent hematopoietic progenitor cell that is characterized by the accumulation of phenotypically normal red blood cells, white blood cells, and platelets in the absence of a definable cause; extramedullary hematopoiesis, marrow
fibrosis, and, in a few patients, transformation to acute
leukemia can also occur. First described in 1892, the cause of the disease remains unknown and no potentially curative
therapy other than
bone marrow transplantation is currently available. It is commonly held that PV is a rare disorder, when in fact with a minimum incidence of 2.6 per 100,000 it is more common than
chronic myelogenous leukemia (CML) and is particularly prevalent in persons of Ashkenazi Jewish ancestry. However, the incidence of PV is not as high as that of
erythrocytosis from other causes collectively, which poses a problem in differential diagnosis when PV presents as isolated
erythrocytosis. Characteristic features of PV are
erythropoietin (Epo)-independent in vitro erythroid colony formation, as well as
hypersensitivity to many other hematopoietic
growth factors. Recently, a remarkable association between PV and a somatic point mutation of the JAK2
tyrosine kinase (JAK2 V617F) was described. Functional assays have revealed that JAK2 V617F is capable of inducing constitutive STAT5-mediated signaling in vitro, as well as
erythrocytosis in vivo in mice. These data suggest that the JAK2 V617F mutation participates in the pathogenesis of PV. In current clinical practice, two different clinical approaches have been used to diagnose PV. One approach requires establishing the presence of absolute
erythrocytosis by directly determining the red cell mass (RCM). A second approach utilizes a RCM-independent diagnostic algorithm based on the serum Epo level and bone marrow histology. Screening for JAK2 V617F can now be added to both diagnostic algorithms. However, it is very clear that some patients with classical PV lack the JAK2 V617F mutation, while some patients with other chronic
myeloproliferative disorders such as
idiopathic myelofibrosis (IMF) and essential
thrombocytosis (ET) also express the JAK2 V617F mutation. Therefore, by necessity, any discussion of PV must take into consideration these companion
myeloproliferative disorders, and since
erythrocytosis is the single clinical feature that sets PV apart from IMF and ET, it is clear that the presence of the JAK2 V617F mutation cannot by itself establish a diagnosis of PV. Phlebotomy remains the mainstay of
therapy for PV. In addition, both
aspirin and cytoreductive
therapy have been employed to control
thrombocytosis and in the case of the latter,
leukocytosis and extramedullary hematopoiesis as well. Despite recent progress in the field, several important issues remain controversial. In this review, we will present the areas of agreement, but also point out where the authors' personal viewpoints differ.