Conditional Expression of Heterozygous or Homozygous Jak2V617F From Its Endogenous Promoter Induces a Polycythemia Vera-Like Disease.

Abstract 437

A somatic point mutation (V617F) in the JAK2 tyrosine kinase was found in a majority of patients with polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). However, the contribution of JAK2V617F in these three clinically distinct myeloproliferative disorders (MPDs) remained unclear. To investigate the actual role of JAK2V617F in the pathogenesis of MPDs, we generated an inducible Jak2V617F knock-in mouse, in which the expression of Jak2V617F is under control of the endogenous Jak2 promoter. Expression of heterozygous Jak2V617F evoked all major features of human PV, which included marked increase in hemoglobin and hematocrit, increased red blood cells, leukocytosis, thrombocytosis, splenomegaly, reduced serum levels of erythropoietin (Epo) and Epo-independent erythroid colonies. Homozygous Jak2V617F expression resulted in a more severe form of PV associated with markedly elevated leukocytosis, neutrophilia and thrombocytosis, and a majority of these mice succumbed due to massive cardiac thrombosis. Activation of Stat5, Akt and Erk was significantly enhanced in erythroblast cells expressing homozygous Jak2V617F compared to heterozygous Jak2V617F, suggesting that the degree of activation of downstream signaling pathways would be affected by the Jak2V617F gene dosage. This may also partly explain the severe phenotype in mice expressing homozygous Jak2V617F. We conclude that heterozygous Jak2V617F is sufficient to cause PV, whereas homozygous Jak2V617F increases the severity of PV disease and the risk of thrombosis. Our results also provide strong evidence that Jak2V617F gene dosage does not play a defining role in determining PV versus ET phenotype in this mouse model of MPD.