Molecular insights into regulation of JAK2 in myeloproliferative neoplasms

Abstract

The critical role of Janus kinase-2 (JAK2) in regulation of myelopoiesis was established 2 decades ago, but identification of mutations in the pseudokinase domain of JAK2 in myeloproliferative neoplasms (MPNs) and in other hematologic malignancies highlighted the role of JAK2 in human disease. These findings have revolutionized the diagnostics of MPNs and led to development of novel JAK2 therapeutics. However, the molecular mechanisms by which mutations in the pseudokinase domain lead to hyperactivation of JAK2 and clinical disease have been unclear.Here, we describe recent advances in the molecular characterization of the JAK2 pseudokinase domain and how pathogenic mutations lead to constitutive activation of JAK2. (Blood. 2015;125(22):3388-3392) IntroductionMyeloproliferative neoplasms (MPNs) are clonal malignancies characterized by overproduction of one or more differentiated myeloid lineages. In 1951, Dameshek recognized the relationship between chronic myelogenous leukemia, polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF), and thus characterized the MPN disease entity. 1 The molecular basis for the most common MPNs (ie, PV, ET, and PMF) remained unknown until 2005, when 4 groups using different strategies all identified a single somatic mutation, V617F, in the pseudokinase domain of Janus kinase-2 (JAK2), a cytoplasmic tyrosine kinase. [2][3][4][5] JAK2 V617F was found to be constitutively active and lead to MPNs in several mouse models. 6 Identification of the JAK2 mutation V617F (exon 14) stimulated research efforts on JAKs in hematologic diseases and has led to a rather complete picture of genetic alterations in MPN. In PV, JAK2 V617F is found in .95% of patients and JAK2 exon 12 mutations in ;4%. In ET and PMF, ;60% of patients harbor JAK2 V617F, 20% to 25% calreticulin mutations, ;5% thrombopoietin receptor (MPL) mutations, and 5% to 10% of patients lack mutations in any of these genes.7-9 These mutations in JAK2, MPL, and calreticulin are driver mutations, and they all activate the JAK2 pathway, but additional recurrent somatic mutations in several genes (TET2, ASXL1, DNMT3A, CBL, LNK, IDH1/2, IKF1, EZH2, TP53, SRSF2), encoding transcriptional and epigenetic regulators and signaling proteins, occur in MPNs.10 These additional mutations modulate disease progression and can also occur as a primary mutation, but it is now convincingly demonstrated that MPNs can be initiated from a single JAK2 V617F hematopoietic stem cell. 11JAK mutations have also emerged in other hematologic diseases, and the majority of the pathogenic mutations in JAK2 (also in JAK1 and JAK3) localize in or near the pseudokinase domain.12 However, the molecular mechanisms by which different mutations in the pseudokinase domain lead to constitutive tyrosine kinase activity and human disease have remained elusive. Regulation of JAK2The JAK tyrosine kinases (JAK1-3, TYK2) mediate signaling of approximately 60 cytokines and hormones. 13 The biological functions of JAKs are determined by their int...

Targeting the Inactive Conformation of JAK2 in Hematological Malignancies

“…In B cell acute lymphoblastic leukemia (B-ALL), mutations in both the pseudokinase domain (R683G/S) and the kinase domain (D873R, P933R) have been linked to the disease. Recent structural data on JAKs show how the pseudokinase domain interacts with the kinase domain to maintain low basal activity (Lupardus et al, 2014, Shan et al, 2014, and most of the pathogenic mutations localize in this interface and are expected to destabilize the inhibitory interaction and lead to constitutive kinase activation (Silvennoinen and Hubbard, 2015).…”

Molecular determinants of pathogenesis and clinical phenotype in myeloproliferative neoplasms

“…The mechanism by which this occurs is increasingly becoming clear. 6 In short, there is evidence that the mutation may reduce the autoinhibitory function of the JH2 domain via changes in JH1-JH2 conformation 7 and adenosine triphosphate (ATP) binding. 8 The expression of JAK2V617F has been shown to allow for JAK2 signaling in the absence of cytokine receptor ligation, 3,4 but the expression of type I cytokine receptors 9 and a functional FERM domain (required for receptor binding) 10 are still required for JAK2 signaling and cytokine-independent growth.…”

Phosphorylation of Janus kinase 1 (JAK1) by AMP-activated protein kinase (AMPK) links energy sensing to anti-inflammatory signaling

“…Analogous to the V617F mutation in JAK2, which is implicated in the etiology of several myeloproliferative neoplasms, this mutation within the JAK1 JH2 pseudokinase domain is thought to relieve an auto-inhibitory interaction with the catalytic JH1 domain, thereby increasing Tyr kinase activity independently of cytokine stimulation (1) (50). In contrast to wildtype JAK1, transient expression of V658F-mutant JAK1 in JAK1-null U4C cells was sufficient to trigger detectable Tyr phosphorylation of both STAT1 and STAT3 in the absence of added cytokine (Fig.…”