LAUSR

Myeloproliferative neoplasms (MPN) arise from the acquisition of genetic mutations in hematopoietic stem cells. Currently, patients receive treatment with cytotoxic agents, the JAK1/2 inhibitor, ruxolitinib, or pegylated forms of interferon alpha (IFN-α). Ruxolitinib is highly effective in treating constitutional symptoms and splenomegaly, however, MPN patients rarely achieve molecular remissions or cure. In contrast, treatment with IFN-α can induce deep molecular remissions, as measured by allelic burden of the JAK2V617F driver-mutation, but treatment may be limited by side effects. There are now clinical trials underway investigating clinical synergy of ruxolitinib and IFN-α in MPN patients. However, the clinical benefits of this combination are currently unknown and deserve preclinical evaluation. We used a preclinical murine model of Jak2V617F-driven MPN to investigate the mechanisms of action of ruxolitinib and IFN-α, in particular, the effects on haematopoietic stem and progenitor cell populations. We demonstrate that ruxolitinib targets progenitor populations but does not deplete MPN disease initiating long-term hematopoietic stem cells (HSCs). Jak2V617F/+ long-term HSCs show hypersensitivity to IFN-α treatment through enhanced STAT1 signaling. Using a novel murine pegylated IFN-α, we show specific targeting of Jak2V617F/+ long-term HSCs in vivo. This was mediated through the preferential induction of reactive oxygen species and DNA damage in Jak2V617F/+ HSCs compared to wild-type HSCs after chronic treatment, offering a novel mechanistic explanation for IFN-α induced reduction of JAKV617F allelic burden in MPN patients. Interestingly, we demonstrate that the target cell specificity of ruxolitinib vs. IFN-α avoids potential antagonism in vivo, a finding with clinical relevance to future efforts to combine these two agents in patients with MPNs.