William Vainchenker

Myeloproliferative neoplasms (MPNs) arise following the acquisition of a mutation in a hematopoietic stem cell (HSC) that causes oncogenic cytokine receptor signaling. Mutations in the calreticulin (CALR) gene are the second most common key driver mutations in MPNs, yet the identity of the disease-initiating HSCs and the mechanisms underlying their clonal expansion remain elusive. Knock-in mice bearing CALRdel52 and CALRins5 mutations recapitulate disease phenotypes, with a greater HSC amplification in CALRdel52 mice. Here, we crossed these strains with transgenic reporter mice expressing GFP under the control of the Von willebrand factor (Vwf) promoter, enabling discrimination between different HSC subsets. We show that CALR-mutated MPNs are mainly initiated from the expansion of platelet-biased, Vwf-positive HSCs, without substantially altering their lineage bias. Notably, the selective amplification of Vwf-positive CALRdel52 compared to CALRins5 HSCs is associated with increased signaling of the thrombopoietin receptor MPL and activation of the integrated stress response as evidenced by eIF2a phosphorylation downstream of PERK kinase triggered by endoplasmic reticulum stress. This pathway is also transcriptionally upregulated in CALRdel52-like compared with CALRins5-like patient hematopoietic stem and progenitor cells (HSPCs), as supported by reanalysis of a previous dataset, and is associated with increased phosphorylation of eIF2a. Pharmacological inhibition of PERK markedly reduces the proliferation of mouse CALRdel52 HSCs and impairs the megakaryocytic differentiation of CALRdel52-like HSPCs from patients while sparing cells from healthy donors. These findings identify the PERK/eIF2a pathway as a mechanistic vulnerability in the MPN cell-of-origin and offer a rationale for exploring new treatment approaches.

Janus kinase (JAK) inhibitors have changed the treatment landscape of myeloproliferative neoplasms (MPNs), graft-versus-host disease, and several autoimmune conditions. Although approved JAK inhibitors generally target the JAK2 kinase domain, and several also target the JAK1 kinase domain in active form (type I inhibition), new inhibitors that either exhibit a type II mechanism of inhibition of the kinase domain in an inactive state or that target the pseudokinase domain with potential preference or specificity for the JAK2 V617F mutant have progressed to clinical trials. This is the most prevalent mutation in MPNs. An ideal inhibitor would target persistently activated JAK2 in MPNs, eradicate the clone or induce deep molecular remission in addition to clinical and hematologic remission, and spare wild-type JAK2 that is critical for hematopoiesis and immune response. We discuss perspectives of these and other modes of JAK inhibition, as well as primary and secondary/exploratory study end points in clinical trial design, along with potential biomarker correlates to evaluate the potential efficacy of next-generation vs conventional JAK inhibitors.

JAK (Janus Kinase) inhibitors, such as ruxolitinib, were introduced a decade ago for treatment of myeloproliferative neoplasms (MPN). To evaluate ruxolitinib’s impact on MPN clonal evolution, we interrogate a myelofibrosis patient cohort with longitudinal molecular evaluation and discover that ruxolitinib is associated with clonal outgrowth of RAS pathway mutations. Single-cell DNA sequencing combined with ex vivo treatment of RAS mutated CD34⁺ primary patient cells, demonstrates that ruxolitinib induces RAS clonal selection both in a JAK/STAT wild-type and hyper-activated context. RAS mutations are associated with decreased transformation-free and overall survival only in patients treated with ruxolitinib. In vitro and in vivo competition assays demonstrate increased cellular fitness of RAS-mutated cells under ruxolitinib or JAK2 knock-down, consistent with an on-target effect. MAPK pathway activation is associated with JAK2 downregulation resulting in enhanced oncogenic potential of RAS mutations. Our results prompt screening for pre-existing RAS mutations in JAK inhibitor treated patients with MPN.

Mutations in calreticulin (mutCALR) are the second most common drivers of myeloproliferative neoplasms (MPNs) and yet, the current therapeutic landscape lacks a selective agent for mutCALR-expressing MPNs. Here, we show that the monoclonal antibody INCA033989 selectively targets mutCALR-positive cells. INCA033989 antagonized mutCALR-driven signaling and proliferation in engineered cell lines and primary CD34+ cells from patients with MPN. No antibody binding or functional activity was observed in the cells lacking mutCALR. In a mouse model of mutCALR-driven MPN, treatment with an INCA033989 mouse surrogate antibody effectively prevented the development of thrombocytosis and accumulation of megakaryocytes in the bone marrow. INCA033989 reduced the pathogenic self-renewal of mutCALR-positive disease-initiating cells in both primary and secondary transplantations, illustrating its disease-modifying potential. In summary, we describe a novel mutCALR-targeted therapy for MPNs, a monoclonal antibody that selectively inhibits the oncogenic function of MPN cells without interfering with normal hematopoiesis.

In this issue of Blood, Usart et al1 provide strong experimental evidence that heterozygous and homozygous deletion of DNA methyltransferase 3 alpha (Dnmt3 In this issue of Blood, Usart et al1 provide strong experimental evidence that heterozygous and homozygous deletion of DNA methyltransferase 3 alpha (Dnmt3

Supplementary Data from RETRACTED: In Vivo Monitoring of Polycythemia Vera Development Reveals Carbonic Anhydrase 1 as a Potent Therapeutic Target

Current murine models of myeloproliferative neoplasms (MPNs) cannot examine how MPNs progress from a single bone marrow source to the entire hematopoietic system. Thus, using transplantation of knock-in JAK2V617F hematopoietic cells into a single irradiated leg, we show development of polycythemia vera (PV) from a single anatomic site in immunocompetent mice. Barcode experiments reveal that grafted JAK2V617F stem/progenitor cells migrate from the irradiated leg to nonirradiated organs such as the contralateral leg and spleen, which is strictly required for development of PV. Mutant cells colonizing the nonirradiated leg efficiently induce PV in nonconditioned recipient mice and contain JAK2V617F hematopoietic stem/progenitor cells that express high levels of carbonic anhydrase 1 (CA1), a peculiar feature also found in CD34+ cells from patients with PV. Finally, genetic and pharmacologic inhibition of CA1 efficiently suppresses PV development and progression in mice and decreases PV patients’ erythroid progenitors, strengthening CA1 as a potent therapeutic target for PV.Significance:Follow-up of hematopoietic malignancies from their initiating anatomic site is crucial for understanding their development and discovering new therapeutic avenues. We developed such an approach, used it to characterize PV progression, and identified CA1 as a promising therapeutic target of PV.This article is highlighted in the In This Issue feature, p. 265