The Philadelphia chromosome (Ph)–negative myeloproliferative neoplasms (MPNs) are a group of chronic myeloid blood cancers driven by the acquisition of somatic mutations affecting JAK-STAT signaling in stem cells. These mutations result in overproduction of hematopoietic progenitor cells. The most common mutation is JAK2 V617F, which occurs in all patients with polycythemia vera and approximately two-thirds of patients with essential thrombocythemia and myelofibrosis.1 JAK inhibitors are the mainstay of treatment for myelofibrosis and are also used to treat polycythemia vera. JAK inhibition can be highly effective at reducing symptoms, spleen size, and blood counts. However, currently available JAK inhibitors do not reduce the mutant clone burden, and many patients lose response to treatment over time.2,3
Reasons for the limited efficacy of these treatments are unclear. A likely contributing factor is the inability of currently available JAK inhibitors to discriminate between physiologic and oncogenic signalling. JAK2 is also essential for blood production, which means that achieving complete suppression of JAK2 V617F signalling is not possible without also totally suppressing healthy hematopoiesis. It has also been suggested that, when the malignant clone accumulates concomitant mutations, particularly in advanced MPNs such as myelofibrosis, the MPN clone is no longer completely dependent on JAK2 V617F signalling.4 Therefore, JAK-STAT inhibition alone may be insufficient to induce total disease regression.
To address this key question, Andrew Dunbar, MD, and colleagues developed a new mouse model that uses a dual-recombinase Dre/Cre system to induce reversible JAK2 V617F activation. Mice transplanted with JAK2 V617F-induced stem/progenitor cells showed leukocytosis, an elevated haematocrit, hepatosplenomegaly, and megakaryocyte hyperplasia — consistent with the MPN phenotype also observed in prior JAK2 V617F mouse models.5 Following induction of JAK2 V617F using Dre recombinase and development of the MPN phenotype, they employed the tamoxifen-activated Cre system to delete JAK2 V617F, leaving only a functional wild-type JAK2 allele. Strikingly, reinstatement of JAK2 wild-type status led to a loss of overactive JAK/STAT signalling, normalization of blood counts, normalization of spleen weights, reduction of bone marrow fibrosis, and prolonged survival. These findings confirm that complete ablation of JAK2 V617F signalling in vivo leads to MPN disease remission.
Following these results, Dr. Dunbar and colleagues validated the effects of JAK2 V617F reversal on MPN clone fitness using a competitive transplant system, in which mutant and wild-type cells can be distinguished via expression of CD45 variants. Consistent with their previous observations, the mutant clone burden drastically reduced with time, correlating with the normalization of the hematologic parameters. Bone marrow analysis also revealed a striking reduction of the mutant clone among the different hematopoietic stem and progenitor cell compartments including long-term hematopoietic stem cells, suggesting that JAK2 V617F reversal affects cell fitness across the hematopoietic hierarchy.
The authors then compared the impact of switching off JAK2 V617F genetically to treatment with ruxolitinib on the MPN transcriptome. While ruxolitinib only induced modest JAK/STAT signalling reduction, JAK2V617F genetic reversal had a profound impact on signalling. Consistent with this observation, compared with JAK2 wild type-reinstated mice, ruxolitinib-treated mice showed more modest improvements in hematologic parameters, spleen weights, and mutant cell fraction burden. Importantly, treating mice with CHZ868 (a type II JAK inhibitor with improved potency compared to ruxolitinib) improved the MPN phenotype, albeit not to the same extent as JAK2 V617F deletion, highlighting the potential clinical benefit of more potent and selective JAK inhibitors for patients with MPN.6
Lastly, the authors tested whether switching off the JAK2 V617F mutation would reverse the disease phenotype, even in combination with TET2 loss. TET2 loss is the most common concomitant mutation found in patients with MPN and can precede the acquisition of the JAK2 V617F mutation.7 Loss of TET2 in the presence of JAK2 V617F induced a more aggressive MPN phenotype compared to JAK2 V617F alone, with increased reticulin staining and white blood cell counts. The authors confirmed that JAK2 V617F excision reverted the MPN phenotype regardless of TET2 status, suggesting that specific and potent inhibition of JAK2 V617F can reverse MPN even in the presence of concomitant mutations that act synergistically to the JAK2 V617F driver.
In Brief
Dr. Dunbar and colleagues developed a mouse model of advanced MPN that allows JAK2 V617F induction and subsequent excision in vivo. Using this system, the authors demonstrated that the MPN phenotype is directly associated with the presence of JAK2 V617F and can be fully reversed by solely targeting this mutation, regardless of the existence of concomitant mutations such as TET2. Some questions remain unanswered, such as whether other MPN-associated mutations like CALR or MPL are also essential disease drivers, and whether DNMT3A or high-molecular-risk mutations such as EZH2, ASXL1, or SF3B1 may confer resistance to mutant JAK2 targeting. Regardless, this study highlights the potential benefits of achieving more selective JAK2 V617F inhibition in the clinic. Further, it provides an essential dataset for interrogating the consequences of JAK2 V617F induction and reversal on cell state and is an excellent demonstration of the utility of dual recombinase systems and advanced genetic models to assess oncogenic dependencies.
Competing Interests
Mr. Rodriguez-Romera indicated no relevant conflicts of interest. Dr. Psaila reported consulting activity and research funding from Alethiomics, where she is a co-founder and major shareholder. She also reported consulting/advisory board activity and paid speaking engagements for GSK, consulting activity for Blueprint medicines, advisory board activity and paid speaking engagements for Novartis, and research funding from Incyte.
