Molecular Pathogenesis of Polycythemia Induced in Mice by JAK2 V617F.

A somatic mutation (V617F) in the JH2 pseudokinase domain of the JAK2 tyrosine kinase is found in the majority of patients with polycythemia vera (PV), and some patients with essential thrombocythemia (ET) and chronic idiopathic myelofibrosis (CIMF). The JAK2 V617F mutant is constitutively active and induces polycythemia in mice after retroviral expression in transplanted bone marrow (James et al., Nature 2005; 434:1144). We expressed murine JAK2 wild-type (WT) or a JAK2 V617F mutant in bone marrow (BM) using retroviral transduction and transplantation in two different strains of inbred mice, C57Bl/6 (B6) and Balb/c. In both B6 and Balb/c mice, JAK2 V671F induced non-fatal polycythemia characterized by increased hematocrit and hemoglobin, reticulocytosis, and splenomegaly that were manifest by 3 weeks post-transplantation and were sustained for months. JAK2 V617F also induced significant leukocytosis and neutrophilia that was much more pronounced in Balb/c mice than in B6. Platelet counts were not significantly increased in either strain, despite expression of the mutant JAK2 kinase in megakaryocytes. By contrast, peripheral blood counts in recipients of BM transduced with JAK2 WT were normal and not significantly different from recipients of vector-transduced BM. We also employed BM from donor mice with mutations in erythropoiesis signaling pathways to analyze the pathogenesis of PV. Mice lacking the Src family kinase Lyn or with deficiency of both Stat5a and Stat5b have normal hematopoiesis as adults, but exhibit defects in stress and fetal erythropoiesis. JAK2 V617F efficiently induced polycythemia in BM from mice lacking the Src kinases Lyn, Hck and Fgr. By contrast, deficiency of both Stat5a and Stat5b in the donor BM attenuated the polycythemia and reticulocytosis induced by JAK2 V617F. These results demonstrate that the principal effect of expression of JAK2 V617F in hematopoietic stem/progenitor cells is expansion of the erythroid lineage through a Stat5-dependent, Src kinase-independent pathway. The strain-dependent leukocytosis suggests that genetic modifiers affect the hematopoietic phenotype of this activated JAK2 mutation. The lack of thrombocytosis implies that additional events may be required for JAK2 V617F to cause essential thrombocythemia.