Dysregulation Of Bcl2 Family Proteins Induced By JAK2V617F Mutation Contributes To The Abnormal Expansion Of Neoplastic Initiating Cells

The treatment of myeloproliferative neoplasms (MPN) harboring JAK2V617F mutation remains non-curative with the current strategies despite the development of JAK2 inhibitors, which improve the symptoms of the disease but do not effectively reduce the allele burden of the mutation. In addition, it has been shown that, although JAK2 inhibitors can control the expansion of the myeloerythroid precursors that contribute to the clinical phenotype, they fail to eliminate the most primitive stem cells, which are responsible for the initiation of the disease. The differential expression of genes/proteins between the hematopoietic stem cells (HSC) and the more committed bone marrow progenitors may reveal, at least in part, the contribution of each cell type to the pathogenesis of the disease. We have shown that JAK2 inhibition induces down-regulation of the anti-apoptotic protein Bcl-xL and up-regulation of the pro-apoptotic BH3-only protein, Bim, in human cells harboring the JAK2V617F mutation, suggesting a key role of these Bcl-2 family proteins as potential targets for JAK2-mediated apoptosis. Therefore, we aimed to study the contribution of the Bcl-2 family proteins to MPNs by the use of a conditional JAK2V617F knock-in model, particularly focusing on MPN initiating cells. The heterozygous Jak2V617F expressing animals (Jak2^+/VF) develop a lethal MPN characterized by an elevated hematocrit and splenomegaly due to extramedullary hematopoiesis, with a prominent expansion of early (Lin^- CD71⁺ Ter119⁺) and late (Lin^- CD71^- Ter119⁺) erythroid cells in the spleen, and increased pre-megakaryocyte-erythrocyte (Lin^- Sca1^- cKIT^hi CD41^- FcgRII/III^- CD105^- CD150⁺), megakaryocyte (Lin^- Sca1^- cKIT^hi CD41⁺ CD150⁺) and erythroid (Lin^- Sca1^- cKIT^hi CD41^- FcgRII/III^- CD105⁺ CD150⁺) precursors in the bone marrow. HSC numbers (Lin^- Sca1^- cKIT^hi CD105⁺ CD150⁺) are also increased in Jak2^+/VF as compared to the Jak2^+/+ controls. Gene expression analysis revealed that Bcl-xL was upregulated in sorted LSKs (Lin^- Sca-1⁺ cKIT^hi) containing HSCs but not in the more committed progenitors (CMP, GMP, MEP). By contrast, the pro-apoptotic Bim was downregulated in CMPs and MEPs but not at the LSK cells containing HSCs. Expression of Bcl-2 showed no difference between Jak2^+/VF and Jak2^+/+ mice. These data suggest that signals from JAK2V617F mutation may be cell-stage specific, which may explain why JAK2 inhibitors fail to eliminate MPN initiating cells. Modulating the overexpression of Bcl2 antiapoptotic proteins at the MPN initiating cells level could potentially render these cells sensitive to the JAK2 inhibition.