The Zygosity of JAK2V617F Determines the Disease Entities of Myeloproliferative Neoplasms By Modulating Erythropoiesis but Not Megakaryopoiesis

Philadelphia-chromosome negative myeloproliferative neoplasms (MPNs) consists of different clinical entities that include polycythemia vera (PV) and essential thrombocythemia (ET). Despite of differential clinical features, JAK2V617F mutation is found in both PV and ET, leaving the cause of differential biological phenotypes by an oncogene obscure. Previously, studies have shown that higher allele frequencies or expression of JAK2V617F are associated with PV symptom/features in patients or model animals, respectively, suggesting that the copy number of JAK2V617F modulates hematopoietic cell differentiation and thus exhibits differential clinical features. However, this remained elusive in human hematopoiesis. To examine the impact of the zygosity of JAK2V617F allele on hematopoietic differentiation in human cells, we established induced pluripotent stem cells (iPSCs) harboring heterozygous- and homozygous-JAK2V617F mutation using genome-editing techniques from normal iPSCs. The introduction of JAK2V617F mutation with one or two copies did not alter the pluripotency of iPSCs and their capacity to differentiate into hematopoietic stem/progenitor cells (HSPCs) in vitro. When we induced hematopoietic cell differentiation from HSPCs, factor-independent erythropoiesis and megakaryopoiesis were induced in both heterozygous and homozygous JAK2V617F-HSPCs. Furthermore, homozygous JAK2V617F-HSPCs showed higher potential for erythropoiesis compared to the ones with heterozygous JAK2V617F, while the zygosity of JAK2V617F showed less effect on megakaryopoiesis. To further understand the molecular mechanism of hematopoietic cell differentiation modulated by differential copy number of JAK2V617F, we analyzed the activation of JAK-STAT signal by immunoblot analysis. The activation of JAK-STAT signals was more prominent in HSPCs harboring homozygous JAK2V617F, than those with heterozygous JAK2V617F. This suggested that the level of JAK2 phosphorylation was positively correlated with the copy number of JAK2V617F. These observations implied followings: 1) heterozygous JAK2V617F was sufficient to promote the development of MPN by inducing the factor-independent erythropoiesis and megakaryopoiesis; 2) the zygosity of JAK2V617F determined the disease phenotypes of MPNs by modulating erythropoiesis but not megakaryopoiesis; and 3) the homozygous JAK2V617F increased JAK-STAT signaling in HSPCs, promoting an increased erythropoiesis.