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In patients with polycythemia vera (PV), an elevated JAK2 p.V617F allele burden is associated with indicators of more severe disease (e.g., leukocytosis, splenomegaly, and increased thrombosis risk); however, correlations between allele burden reductions and clinical benefit in patients with PV have not been extensively evaluated in a randomized trial. This exploratory analysis from the multicenter, open-label, phase 3 Randomized Study of Efficacy and Safety in Polycythemia Vera With JAK Inhibitor INCB018424 Versus Best Supportive Care trial evaluated the long-term effect of ruxolitinib treatment on JAK2 p.V617F allele burden in patients with PV. Evaluable JAK2 p.V617F-positive patients randomized to ruxolitinib ( n  = 107) or best available therapy (BAT) who crossed over to ruxolitinib at week 32 ( n  = 97) had consistent JAK2 p.V617F allele burden reductions throughout the study. At all time points measured (up to weeks 208 [ruxolitinib-randomized] and 176 [ruxolitinib crossover]), mean changes from baseline over time in JAK2 p.V617F allele burden ranged from −12.2 to −40.0% (ruxolitinib-randomized) and −6.3 to −17.8% (ruxolitinib crossover). Complete or partial molecular response was observed in 3 patients (ruxolitinib-randomized, n  = 2; ruxolitinib crossover, n  = 1) and 54 patients (ruxolitinib-randomized, n  = 33; ruxolitinib crossover, n  = 20; BAT, n  = 1), respectively. Among patients treated with interferon as BAT ( n  = 13), the mean maximal reduction in allele burden from baseline was 25.6% after crossover to ruxolitinib versus 6.6% before crossover. Collectively, the data from this exploratory analysis suggest that ruxolitinib treatment for up to 4 years provides progressive reductions in JAK2 p.V617F allele burden in patients with PV who are resistant to or intolerant of hydroxyurea. The relationship between allele burden changes and clinical outcomes in patients with PV remains unclear.

Nick Cross and colleagues report that the JAK2 V617F somatic mutation that drives the development of chronic myeloproliferative neoplasms is associated with the presence of a specific inherited haplotype in JAK2 . Chronic myeloproliferative neoplasms (MPNs) are a group of related conditions characterized by the overproduction of cells from one or more myeloid lineages. More than 95% of cases of polycythemia vera, and roughly half of essential thrombocythemia and primary myelofibrosis acquire a unique somatic 1849G>T JAK2 mutation (encoding V617F) that is believed to be a critical driver of excess proliferation 1 , 2 , 3 , 4 . We report here that JAK2 V617F -associated disease is strongly associated with a specific constitutional JAK2 haplotype, designated 46/1, in all three disease entities compared to healthy controls (polycythemia vera, n = 192, P = 2.9 × 10 −16 ; essential thrombocythemia, n = 78, P = 8.2 × 10 −9 and myelofibrosis, n = 41, P = 8.0 × 10 −5 ). Furthermore, JAK2 V617F specifically arises on the 46/1 allele in most cases. The 46/1 JAK2 haplotype thus predisposes to the development of JAK2 V617F -associated MPNs (OR = 3.7; 95% CI = 3.1–4.3) and provides a model whereby a constitutional genetic factor is associated with an increased risk of acquiring a specific somatic mutation.

Robert Kralovics and colleagues report that the JAK2 V617F somatic mutation that drives the development of chronic myeloproliferative neoplasms is associated with the presence of a specific inherited haplotype in JAK2 . Genome-wide association studies have identified a number of new disease susceptibility loci that represent haplotypes defined by numerous SNPs. SNPs within a disease-associated haplotype are thought to influence either the expression of genes or the sequence of the proteins they encode. In a series of investigations of the JAK2 gene in myeloproliferative neoplasms, we uncovered a new property of haplotypes that can explain their disease association. We observed a nonrandom distribution of the somatic JAK2 V617F oncogenic mutation between two parental alleles of the JAK2 gene. We identified a haplotype that preferentially acquires JAK2 V617F and confers susceptibility to myeloproliferative neoplasms. One interpretation of our results is that a certain combination of SNPs may render haplotypes differentially susceptible to somatic mutagenesis. Thus, disease susceptibility loci may harbor somatic mutations that have a role in disease pathogenesis.

Somatic mutations in Janus kinase 2 (JAK2), including JAK2 V617F, result in dysregulated JAK-signal transducer and activator transcription (STAT) signaling, which is implicated in myeloproliferative neoplasm (MPN) pathogenesis. CYT387 is an ATP-competitive small molecule that potently inhibits JAK1/JAK2 kinases (IC 50 =11 and 18 n M , respectively), with significantly less activity against other kinases, including JAK3 (IC 50 =155 n M ). CYT387 inhibits growth of Ba/F3- JAK2 V617F and human erythroleukemia (HEL) cells (IC 50 ∼1500 n M ) or Ba/F3- MPL W515L cells (IC 50 =200 n M ), but has considerably less activity against BCR–ABL harboring K562 cells (IC=58 000 n M ). Cell lines harboring mutated JAK2 alleles (CHRF-288-11 or Ba/F3- TEL-JAK2 ) were inhibited more potently than the corresponding pair harboring mutated JAK3 alleles (CMK or Ba/F3- TEL-JAK3 ), and STAT-5 phosphorylation was inhibited in HEL cells with an IC 50 =400 n M . Furthermore, CYT387 selectively suppressed the in vitro growth of erythroid colonies harboring JAK2V617F from polycythemia vera (PV) patients, an effect that was attenuated by exogenous erythropoietin. Overall, our data indicate that the JAK1/JAK2 selective inhibitor CYT387 has potential for efficacious treatment of MPN harboring mutated JAK2 and MPL alleles.

Protein kinases, which serve critical functions in signaling pathways in all cells, are popular therapeutic targets. At present, eight kinase inhibitors have been approved in the United States, each of which shows nanomolar potency. Although the initial goal was to generate inhibitors with a high degree of selectivity, recent experience has revealed that many of these approved compounds target more than one kinase. Surprisingly, this promiscuity is less problematic than one would have imagined; indeed, it opens new therapeutic opportunities. In this Perspective, we discuss the present status of Janus kinase inhibitors—a new class of immunosuppressive drugs—and the advantages and disadvantages of selectively inhibiting this class of kinase.

Patients with polycythemia vera (PV), a myeloproliferative neoplasm characterized by an elevated red blood cell mass, are at high risk of vascular and thrombotic complications and have reduced quality of life due to a substantial symptom burden that includes pruritus, fatigue, constitutional symptoms, microvascular disturbances, and bleeding. Conventional therapeutic options aim at reducing vascular and thrombotic risk, with low-dose aspirin and phlebotomy as first-line recommendations for patients at low risk of thrombotic events and cytoreductive therapy (usually hydroxyurea or interferon alpha) recommended for high-risk patients. However, long-term effective and well-tolerated treatments are still lacking. The discovery of mutations in Janus kinase 2 ( JAK2 ) as the underlying molecular basis of PV has led to the development of several targeted therapies, including JAK inhibitors, and results from the first phase 3 clinical trial with a JAK inhibitor in PV are now available. Here, we review the current treatment landscape in PV, as well as therapies currently in development.

A somatic activating mutation (V617F) in the JAK2 tyrosine kinase was recently discovered in the majority of patients with polycythemia vera (PV), and some with essential thrombocythemia (ET) and chronic idiopathic myelofibrosis. However, the role of mutant JAK2 in disease pathogenesis is unclear. We expressed murine JAK2 WT or V617F via retroviral bone marrow transduction/transplantation in the hematopoietic system of two different inbred mouse strains, Balb/c and C57Bl/6 (B6). In both strains, JAK2 V617F, but not JAK2 WT, induced non-fatal polycythemia characterized by increased hematocrit and hemoglobin, reticulocytosis, splenomegaly, low plasma erythropoietin (Epo), and Epo-independent erythroid colonies. JAK2 V617F also induced leukocytosis and neutrophilia that was much more prominent in Balb/c mice than in B6. Platelet counts were not affected in either strain despite expression of JAK2 V617F in megakaryocytes and markedly prolonged tail bleeding times. The polycythemia tended to resolve after several months, coincident with increased spleen and marrow fibrosis, but was resurrected by transplantation to secondary recipients. Using donor mice with mutations in Lyn, Hck, and Fgr, we demonstrated that the polycythemia was independent of Src kinases. Polycythemia and reticulocytosis responded to treatment with imatinib or a JAK2 inhibitor, but were unresponsive to the Src inhibitor dasatinib. These findings demonstrate that JAK2 V617F induces Epo-independent expansion of the erythroid lineage in vivo. The fact that the central erythroid features of PV are recapitulated by expression of JAK2 V617F argues that it is the primary and direct cause of human PV. The lack of thrombocytosis suggests that additional events may be required for JAK2 V617F to cause ET, but qualitative platelet abnormalities induced by JAK2 V617F may contribute to the hemostatic complications of PV. Despite the role of Src kinases in Epo signaling, our studies predict that Src inhibitors will be ineffective for therapy of PV. However, we provide proof-of-principle that a JAK2 inhibitor should have therapeutic effects on the polycythemia, and perhaps myelofibrosis and hemostatic abnormalities, suffered by MPD patients carrying the JAK2 V617F mutation.

Although a large proportion of patients with polycythemia vera (PV) harbor a valine-to-phenylalanine mutation at amino acid 617 (V617F) in the JAK2 signaling molecule, the stage of hematopoiesis at which the mutation arises is unknown. Here we isolated and characterized hematopoietic stem cells (HSC) and myeloid progenitors from 16 PV patient samples and 14 normal individuals, testing whether the JAK2 mutation could be found at the level of stem or progenitor cells and whether the JAK2 V617F-positive cells had altered differentiation potential. In all PV samples analyzed, there were increased numbers of cells with a HSC phenotype (CD34⁺CD38⁻CD90⁺Lin⁻) compared with normal samples. Hematopoietic progenitor assays demonstrated that the differentiation potential of PV was already skewed toward the erythroid lineage at the HSC level. The JAK2 V617F mutation was detectable within HSC and their progeny in PV. Moreover, the aberrant erythroid potential of PV HSC was potently inhibited with a JAK2 inhibitor, AG490.

Ruxolitinib (Jakavi ® , Jakafi ® ) is an orally administered, first-in-class Janus Kinase (JAK) 1 and 2 inhibitor that was recently approved for the treatment of patients with polycythaemia vera (PV) who have responded inadequately to or are intolerant of hydroxyurea. By inhibiting JAK 1 and 2, ruxolitinib reduces hyperactive JAK-signal transducers and activators of transcription (STAT) signalling that is implicated in the pathogenesis of PV. This article briefly reviews the pharmacology of the drug, focusing on its clinical use in patients with PV. In the phase III RESPONSE trial in PV patients who had an inadequate response to or unacceptable adverse effects from hydroxyurea, ruxolitinib was superior to best available therapy in reducing haematocrit without phlebotomy and reducing spleen size after 32 weeks of treatment. Ruxolitinib was also associated with reducing leukocyte and platelet counts and improving symptoms. Patient follow-up demonstrated that response to ruxolitinib was durable, including preliminary results after up to 80 weeks of treatment. The drug is generally well tolerated, although mild to moderate anaemia, thrombocytopenia and lymphopenia were common in the RESPONSE trial. These effects can usually be managed with dosage modification and did not lead to therapy discontinuation in the RESPONSE trial. Thus, for a subgroup of PV patients for whom few treatment options have existed previously, ruxolitinib provides a valid option.