A somatic point mutation (V617F) in the JAK2 tyrosine kinase was found in most cases of Ph-negative myeloproliferative neoplasms (MPNs) including ∼95% patients with polycythemia vera (PV) and 50–60% patients with essential thrombocythemia (ET) and primary myelofibrosis (PMF). To investigate the contribution of JAK2V617F in MPNs, we generated a conditional Jak2V617F knock-in mouse (Akada et al., Blood 2010; 115: 3589–3597). Expression of Jak2V617F in all hematopoietic compartments including the hematopoietic stem cells (HSC) resulted in a PV-like disease associated with a marked expansion of erythroid progenitors in the bone marrow and spleen. Since Jak2 is essential for normal erythropoiesis and expression of Jak2V617F mutant enhances erythropoiesis, so we asked if erythroid progenitors are actual target cells for Jak2V617F mutation. To address this question, we have specifically expressed Jak2V617F in erythroid progenitors using the EpoR-Cre mice. Expression of heterozygous Jak2V617F in erythroid progenitors resulted in a polycythemia-like phenotype characterized by increase in hematocrit and hemoglobin, increased red blood cells, Epo-independent erythroid colonies, and splenomegaly. Erythroid lineage-specific expression of homozygous Jak2V617F resulted in significantly greater increase in hematocrit, hemoglobin, red blood cells, Epo-independent erythroid colonies, and splenomegaly compared to heterozygous Jak2V617F expression. These results suggest that erythroid lineage-restricted expression of Jak2V617F is sufficient to induce a polycythemia-like disease in a gene-dose dependent manner. However, transplantation of Jak2V617F-expressing erythroid progenitors (c-kithighTer119lowCD71high or c-kitlowTer119highCD71high) from the diseased mice into lethally irradiated recipients could not transfer the disease suggesting that Jak2V617F mutation does not confer self-renewal capacity to erythroid progenitors. We also observed that only Jak2V617F-expressing HSC has the unique capacity to serially transplant the myeloproliferative disease in mice. Taken together, our results suggest that HSCs are the disease-initiating cancer stem cells and erythroid progenitors are the target cells in Jak2V617F-evoked MPN.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.