Impact of isolated germline JAK2V617I mutation on human hematopoiesis. Blood. 2013;121:4156-4165. [PMID: 23535062 DOI: 10.1182/blood-2012-05-430926]

Stem cell concepts in myelodysplastic syndromes: lessons and challenges

According to the cancer stem cell (CSC) hypothesis, CSCs are the only cancer cells that can give rise to and sustain all cells that constitute a cancer as they possess inherent or acquired self-renewal potential, and their elimination is required and potentially sufficient to achieve a cure. Whilst establishing CSC identity remains challenging in most cancers, studies of low-intermediate risk myelodysplastic syndromes (MDS), other chronic myeloid malignancies and clonal haematopoiesis of indeterminant potential (CHIP) strongly support that the primary target cell usually resides in the rare haematopoietic stem cell (HSC) compartment. This probably reflects the unique self-renewal potential of HSCs in normal human haematopoiesis, combined with the somatic initiating genomic driver lesion not conferring extensive self-renewal potential to downstream progenitor cells. Mutational 'fate mapping' further supports that HSCs are the only disease-propagating cells in low-intermediate risk MDS, but that MDS-propagating potential might be extended to progenitors upon disease progression. The clinical importance of MDS stem cells has been highlighted through the demonstration of selective persistence of MDS stem cells in patients at complete remission in response to therapy. This implies that MDS stem cells might possess unique resistance mechanisms responsible for relapses following otherwise efficient treatments. Specific surveillance of MDS stem cells should be considered to assess the efficiency of therapies and as an early indicator of emerging relapses in patients in clinical remission. Moreover, further molecular characterization of purified MDS stem cells should facilitate identification and validation of improved and more stem cell-specific therapies for MDS.

Epidemiology of the Philadelphia Chromosome-Negative Classical Myeloproliferative Neoplasms

Polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF) comprise the BCR-ABL-negative classical myeloproliferative neoplasms (MPNs). These clonal myeloid diseases are principally driven by well-described molecular events; however, factors leading to their acquisition are not well understood. Beyond increasing age, male sex, and race/ethnicity differences, few consistent risk factors for the MPNs are known. PV and ET have an incidence of 0.5 to 4.0 and 1.1 to 2.0 cases per 100,000 person-years, respectively, and predict similar survival. PMF, which has an incidence of about 0.3 to 2.0 cases per 100,000 person-years, is associated with the shortest survival of the MPNs.

Germline genetic factors in the pathogenesis of myeloproliferative neoplasms

Myeloproliferative neoplasms (MPN) are clonal hematological malignancies that lead to overproduction of mature myeloid cells. They are due to acquired mutations in genes encoding for AK2, MPL and CALR that result in the activation of the cytokine receptor/JAK2 signaling pathway. In addition, it exists germline variants that can favor the initiation of the disease or may affect its phenotype. First, they can be common risk alleles, which correspond to frequent single nucleotide variants present in control population and that contribute to the development of either sporadic or familial MPN. Second, some variants predispose to the onset of MPN with a higher penetrance and lead to familial clustering of MPN. Finally, some extremely rare genetic variants can induce MPN-like hereditary disease. We will review these different subtypes of germline genetic variants and discuss how they impact the initiation and/or development of the MPN disease.

Epidemiology of the classical myeloproliferative neoplasms: The four corners of an expansive and complex map

The classical myeloproliferative neoplasms (MPNs), specifically chronic myeloid leukemia (CML), polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF), represent clonal myeloid disorders whose pathogenesis is driven by well-defined molecular abnormalities. In this comprehensive review, we summarize the epidemiological literature and present our own analysis of the most recent the Surveillance, Epidemiology, and End Results (SEER) program data through 2016. Older age and male gender are known risk factors for MPNs, but the potential etiological role of other variables is less established. The incidences of CML, PV, and ET are relatively similar at 1.0-2.0 per 100,000 person-years in the United States, while PMF is rarer with an incidence of 0.3 per 100,000 person-years. The availability of tyrosine kinase inhibitor therapy has dramatically improved CML patient outcomes and yield a life expectancy similar to the general population. Patients with PV or ET have better survival than PMF patients.

Diagnostic workflow for hereditary erythrocytosis and thrombocytosis

In the patient presenting with an elevated blood count who does not have an acquired clonal disorder causing a myeloproliferative neoplasm, hereditary erythrocytosis or hereditary thrombocytosis needs to be considered as a possible explanation. A young patient and/or those with a family history of myeloproliferative neoplasm should specifically raise this possibility. Among the causes of hereditary erythrocytosis are mutations in the genes in the oxygen sensing pathway and high-affinity hemoglobins. Hereditary thrombocytosis has been shown to be accounted for by mutations in THPO, MPL, and JAK2 genes. In those who have a possible hereditary erythrocytosis or thrombocytosis, the investigative pathway includes specific investigation to rule out the more common acquired clonal disorders, and, if indicated, other secondary causes, measurement of specific cytokines as indicated, and search for specific identified molecular lesions that have been shown to cause these hereditary disorders. There remain individuals who appear to have a hereditary disorder in whom a genetic lesion cannot currently be identified.

The JAK2V617F Point Mutation Increases the Osteoclast Forming Ability of Monocytes in Patients with Chronic Myeloproliferative Neoplasms and Makes their Osteoclasts more Susceptible to JAK2 Inhibition

JAK2V617F is a gain of function point mutation that occurs in Myeloproliferative Neoplasm (MPN) patients and deranges their hemopoiesis at cellular level. We speculate that hyperfunctioning JAK2 can modify osteoclast (OCL) homeostasis in MPN patients. We studied 18 newly diagnosed MPN patients and four age-matched normal donors (ND). Osteoclast forming assays started from selected monocytes also and under titrated concentrations of the JAK2 Inhibitor AG-490 (Tyrphostin). Genomic DNA was extracted from the formed osteoclasts, and the JAK2V617F/JAK2WT genomic DNA ratio was calculated. OCLs formed from monocytes derived from heterozygous (Het) for the JAK2V617F mutation MPN patients, were three times more compared to those from JAK2 wild type (WT) MPN patients (p=0,05) and from ND as well (p=0,03). The ratio of JAK2V617F/JAK2WT genomic DNA was increased in OCLs compared to the input monocyte cells showing a survival advantage of the mutated clone. In comparison to ND and JAK2 WT MPN patients, OCLs from patients JAK2V617F (Het) were more susceptible to JAK2 inhibition. These alterations in osteoclast homeostasis, attributed to mutated JAK2, can deregulate the hemopoietic stem cell niche in MPN patients.

Myeloproliferative neoplasms: from origins to outcomes

Substantial progress has been made in our understanding of the pathogenetic basis of myeloproliferative neoplasms. The discovery of mutations in JAK2 over a decade ago heralded a new age for patient care as a consequence of improved diagnosis and the development of therapeutic JAK inhibitors. The more recent identification of mutations in calreticulin brought with it a sense of completeness, with most patients with myeloproliferative neoplasm now having a biological basis for their excessive myeloproliferation. We are also beginning to understand the processes that lead to acquisition of somatic mutations and the factors that influence subsequent clonal expansion and emergence of disease. Extended genomic profiling has established a multitude of additional acquired mutations, particularly prevalent in myelofibrosis, where their presence carries prognostic implications. A major goal is to integrate genetic, clinical, and laboratory features to identify patients who share disease biology and clinical outcome, such that therapies, both existing and novel, can be better targeted.

Myeloproliferative neoplasms: from origins to outcomes

Substantial progress has been made in our understanding of the pathogenetic basis of myeloproliferative neoplasms. The discovery of mutations in JAK2 over a decade ago heralded a new age for patient care as a consequence of improved diagnosis and the development of therapeutic JAK inhibitors. The more recent identification of mutations in calreticulin brought with it a sense of completeness, with most patients with myeloproliferative neoplasm now having a biological basis for their excessive myeloproliferation. We are also beginning to understand the processes that lead to acquisition of somatic mutations and the factors that influence subsequent clonal expansion and emergence of disease. Extended genomic profiling has established a multitude of additional acquired mutations, particularly prevalent in myelofibrosis, where their presence carries prognostic implications. A major goal is to integrate genetic, clinical, and laboratory features to identify patients who share disease biology and clinical outcome, such that therapies, both existing and novel, can be better targeted.

Extension of 2016 World Health Organization (WHO) Classification into a New Set of Clinical, Laboratory, Molecular, and Pathological Criteria for the Diagnosis of Myeloproliferative Neoplasms: From Dameshek to Vainchenker, Green, and Kralovics

Improved Clinical, Laboratory, Molecular, and Pathological (CLMP) 2017 criteria for myeloproliferative neoplasms (MPN) define the JAK2V617F trilinear MPNs as a broad continuum of essential thrombocythaemia (ET), polycythaemia vera (PV), masked PV, and post-ET or post-PV myelofibrosis (MF). Normal versus increased erythrocyte counts (5.8×1012/L) on top of bone marrow histology separate JAK2V617F ET and prodromal PV from early and classical PV. Bone marrow histology of the JAK2V617F trilinear MPNs show variable degrees of normocellular megakaryocytic, erythrocytic megakaryocytic and erythrocytic megakaryocytic granulocytic (EMG) myeloproliferation, peripheral cytoses, and splenomegaly related to JAK2V617F allele burden. MPL515 thrombocythaemia displays predominantly normocellular megakaryocytic proliferation. CALR thrombocythaemia intially presents with megakaryocytic followed by dual granulocytic and megakaryocytic myeloproliferation without features of PV. The megakaryocytes are large, mature, and pleomorphic with hyperlobulated nuclei in JAK2V617F ET and prodromal, classical, and masked PV. The megakaryocytes are large to giant with hyperlobulated staghorn-like nuclei in MPL515 thrombocythaemia. The megakaryocytes are densely clustered, large, and immature dysmorphic with bulky (bulbous) hyperchromatic nuclei in CALR thrombocythaemia and MF.

Genomics of Myeloproliferative Neoplasms

Myeloproliferative neoplasms (MPNs) are a group of related clonal hematologic disorders characterized by excess accumulation of one or more myeloid cell lineages and a tendency to transform to acute myeloid leukemia. Deregulated JAK2 signaling has emerged as the central phenotypic driver of BCR -ABL1-negative MPNs and a unifying therapeutic target. In addition, MPNs show unexpected layers of genetic complexity, with multiple abnormalities associated with disease progression, interactions between inherited factors and phenotype driver mutations, and effects related to the order in which mutations are acquired. Although morphology and clinical laboratory analysis continue to play an important role in defining these conditions, genomic analysis is providing a platform for better disease definition, more accurate diagnosis, direction of therapy, and refined prognostication. There is an emerging consensus with regard to many prognostic factors, but there is a clear need to synthesize genomic findings into robust, clinically actionable and widely accepted scoring systems as well as the need to standardize the laboratory methodologies that are used.

An incomplete trafficking defect to the cell-surface leads to paradoxical thrombocytosis for human and murine MPL P106L

The mechanisms behind the hereditary thrombocytosis induced by the thrombopoietin (THPO) receptor MPL P106L mutant remain unknown. A complete trafficking defect to the cell surface has been reported, suggesting either weak constitutive activity or nonconventional THPO-dependent mechanisms. Here, we report that the thrombocytosis phenotype induced by MPL P106L belongs to the paradoxical group, where low MPL levels on platelets and mature megakaryocytes (MKs) lead to high serum THPO levels, whereas weak but not absent MPL cell-surface localization in earlier MK progenitors allows response to THPO by signaling and amplification of the platelet lineage. MK progenitors from patients showed no spontaneous growth and responded to THPO, and MKs expressed MPL on their cell surface at low levels, whereas their platelets did not respond to THPO. Transduction of MPL P106L in CD34⁺ cells showed that this receptor was more efficiently localized at the cell surface on immature than on mature MKs, explaining a proliferative response to THPO of immature cells and a defect in THPO clearance in mature cells. In a retroviral mouse model performed in Mpl^-/- mice, MPL P106L could induce a thrombocytosis phenotype with high circulating THPO levels. Furthermore, we could select THPO-dependent cell lines with more cell-surface MPL P106L localization that was detected by flow cytometry and [¹²⁵I]-THPO binding. Altogether, these results demonstrate that MPL P106L is a receptor with an incomplete defect in trafficking, which induces a low but not absent localization of the receptor on cell surface and a response to THPO in immature MK cells.

Germline genetic variation in JAK2 as a prognostic marker in castration-resistant prostate cancer

OBJECTIVES

To evaluate the prognostic significance of germline variation in candidate genes in patients with castration-resistant prostate cancer (CRPC).

METHODS

Germline DNA was extracted from peripheral blood mononuclear cells of patients with CRPC enrolled in a clinically annotated registry. Fourteen candidate genes implicated in either initiation or progression of prostate cancer were tagged using single nucleotide polymorphisms (SNPs) from HapMap with a minor allele frequency of >5%. The primary endpoint was overall survival (OS), defined as time from development of CRPC to death. Principal component analysis was used for gene levels tests of significance. For SNP-level results the per allele hazard ratios (HRs) and 95% confidence intervals (CIs) under the additive allele model were estimated using Cox regression, adjusted for age at CRPC and Gleason score (GS).

RESULTS

A total of 240 patients with CRPC were genotyped (14 genes; 84 SNPs). The median (range) age of the cohort was 69 (43-93) years. The GS distribution was 55% with GS ≥8, 32% with GS = 7 and 13% with GS <7 or unknown. The median (interquartile range) time from castration resistance to death for the cohort was 2.67 (1.6-4.07) years (144 deaths). At the gene level, a single gene, JAK2 was associated with OS (P < 0.01), and 11 of 18 JAK2 SNPs were individually associated with OS after adjustment for age and GS. A multivariate model consisting of age, GS, rs2149556 (HR 0.67; 95% CI 0.38-1.18) and rs4372063 (HR 2.17; 95% CI 1.25-3.76) was constructed to predict survival in patients with CRPC (concordance of 0.69, P < 3.2 × 10^-9 ).

CONCLUSIONS

Germline variation in the JAK2 gene was associated with survival in patients with CRPC and warrants further validation as a potential prognostic biomarker.

Cooperation of germ line JAK2 mutations E846D and R1063H in hereditary erythrocytosis with megakaryocytic atypia

The role of somatic JAK2 mutations in clonal myeloproliferative neoplasms (MPNs) is well established. Recently, germ line JAK2 mutations were associated with polyclonal hereditary thrombocytosis and triple-negative MPNs. We studied a patient who inherited 2 heterozygous JAK2 mutations, E846D from the mother and R1063H from the father, and exhibited erythrocytosis and megakaryocytic atypia but normal platelet number. Culture of erythroid progenitors from the patient and his parents revealed hypersensitivity to erythropoietin (EPO). Using cellular models, we show that both E846D and R1063H variants lead to constitutive signaling (albeit much weaker than JAK2 V617F), and both weakly hyperactivate JAK2/STAT5 signaling only in the specific context of the EPO receptor (EPOR). JAK2 E846D exhibited slightly stronger effects than JAK2 R1063H and caused prolonged EPO-induced phosphorylation of JAK2/STAT5 via EPOR. We propose that JAK2 E846D predominantly contributes to erythrocytosis, but is not sufficient for the full pathological phenotype to develop. JAK2 R1063H, with very weak effect on JAK2/STAT5 signaling, is necessary to augment JAK2 activity caused by E846D above a threshold level leading to erythrocytosis with megakaryocyte abnormalities. Both mutations were detected in the germ line of rare polycythemia vera, as well as certain leukemia patients, suggesting that they might predispose to hematological malignancy.

JAK2(V617I) results in cytokine hypersensitivity without causing an overt myeloproliferative disorder in a mouse transduction-transplantation model

A germline JAK2(V617I) point mutation results in hereditary thrombocytosis and shares some phenotypic features with myeloproliferative neoplasm, a hematologic malignancy associated with a somatically acquired JAK2(V617F) mutation. We established a mouse transduction-transplantation model of JAK2(V617I) that recapitulated the phenotype of humans with germline JAK2(V617I). We directly compared the phenotypes of JAK2(V617I) and JAK2(V617F) mice. The JAK2(V617I) mice had increased marrow cellularity with expanded myeloid progenitor and megakaryocyte populations, but this phenotype was less severe than that of JAK2(V617F) mice. JAK2(V617I) resulted in cytokine hyperresponsiveness without constitutive activation in the absence of ligand, whereas JAK2(V617F) resulted in constitutive activation. This may explain why JAK2(V617I) produces a mild myeloproliferative phenotype in the mouse model, as well as in humans with germline JAK2(V617I) mutations.

Clinical potential of pacritinib in the treatment of myelofibrosis

Myelofibrosis (MF) is a myeloid disorder caused by a clonal hematopoietic stem-cell proliferation associated with activation of the Janus kinase (JAK) signal transducer and activator of transcription (STAT) signaling pathways. Patients with MF often develop severe splenomegaly, marked symptom burden and significant cytopenias, with a consequent marked negative impact on quality of life and survival. The management of MF patients has dramatically improved with the development of a group of drugs that inhibit JAK signaling. The first of these agents to be approved was ruxolitinib, a JAK1/JAK2 inhibitor, which has been shown to improve both spleen size and symptoms in patients with MF. However, myelotoxicity, particularly of the platelet lineage, significantly limits the patient population who can benefit from this agent. Thus, there is an unmet need for novel agents with limited myelotoxicity to treat MF. Pacritinib, a JAK2 and FMS-like tyrosine kinase 3 (FLT3) inhibitor, has shown promising results in early phase trials with limited myelotoxicity and clinical responses that are comparable with those seen with ruxolitinib, even in patients with severe thrombocytopenia. Currently there are two large phase III clinical trials of pacritinib in MF, including patients with thrombocytopenia, and those previously treated with ruxolitinib. If the encouraging results observed in early phase clinical trials are confirmed, pacritinib will represent a new and exciting treatment option for patients with MF and particularly patients with significant cytopenias.

European vs 2015-World Health Organization clinical molecular and pathological classification of myeloproliferative neoplasms

The BCR/ABL fusion gene or the Ph¹-chromosome in the t(9;22)(q34;q11) exerts a high tyrokinase acticity, which is the cause of chronic myeloid leukemia (CML). The 1990 Hannover Bone Marrow Classification separated CML from the myeloproliferative disorders essential thrombocythemia (ET), polycythemia vera (PV) and chronic megakaryocytic granulocytic myeloproliferation (CMGM). The 2006-2008 European Clinical Molecular and Pathological (ECMP) criteria discovered 3 variants of thrombocythemia: ET with features of PV (prodromal PV), “true” ET and ET associated with CMGM. The 2008 World Health Organization (WHO)-ECMP and 2014 WHO-CMP classifications defined three phenotypes of JAK2^V617F mutated ET: normocellular ET (WHO-ET), hypercelluar ET due to increased erythropoiesis (prodromal PV) and ET with hypercellular megakaryocytic-granulocytic myeloproliferation. The JAK2^V617F mutation load in heterozygous WHO-ET is low and associated with normal life expectance. The hetero/homozygous JAK2^V617F mutation load in PV and myelofibrosis is related to myeloproliferative neoplasm (MPN) disease burden in terms of symptomatic splenomegaly, constitutional symptoms, bone marrow hypercellularity and myelofibrosis. JAK2 exon 12 mutated MPN presents as idiopathic eryhrocythemia and early stage PV. According to 2014 WHO-CMP criteria JAK2 wild type MPL⁵¹⁵ mutated ET is the second distinct thrombocythemia featured by clustered giant megakaryocytes with hyperlobulated stag-horn-like nuclei, in a normocellular bone marrow consistent with the diagnosis of “true” ET. JAK2/MPL wild type, calreticulin mutated hypercellular ET appears to be the third distinct thrombocythemia characterized by clustered larged immature dysmorphic megakaryocytes and bulky (bulbous) hyperchromatic nuclei consistent with CMGM or primary megakaryocytic granulocytic myeloproliferation.

Back to biology: new insights on inheritance in myeloproliferative disorders

The myeloproliferative disorders (MPDs) are a group of hematologic diseases with significant overlap in both clinical phenotype and genetic etiology. While most often caused by acquired somatic mutations in hematopoietic stem cells, the presence of familial clustering in MPD cases suggests that inheritance is an important factor in the etiology of this disease. Though far less common than sporadic disease, inherited MPDs can be clinically indistinguishable from sporadic disease. Recently, germline mutations in Janus kinase 2 (JAK2) and MPL, two genes frequently mutated in sporadic MPD, have been shown to cause inherited thrombocytosis. Study of the function of these mutant proteins has led to a new understanding of the biological mechanisms that produce myeloproliferative disease. In this review, we summarize the data regarding inherited mutations that cause or predispose to MPDs, with a focus on the biological effects of mutant proteins. We propose that defining inherited MPDs in this manner has the potential to simplify diagnosis in a group of disorders that can be difficult to differentiate clinically.

The role of JAK/STAT signalling in the pathogenesis, prognosis and treatment of solid tumours

Aberrant activation of intracellular signalling pathways confers malignant properties on cancer cells. Targeting intracellular signalling pathways has been a productive strategy for drug development, with several drugs acting on signalling pathways already in use and more continually being developed. The JAK/STAT signalling pathway provides an example of this paradigm in haematological malignancies, with the identification of JAK2 mutations in myeloproliferative neoplasms leading to the development of specific clinically effective JAK2 inhibitors, such as ruxolitinib. It is now clear that many solid tumours also show activation of JAK/STAT signalling. In this review, we focus on the role of JAK/STAT signalling in solid tumours, examining the molecular mechanisms that cause inappropriate pathway activation and their cellular consequences. We also discuss the degree to which activated JAK/STAT signalling contributes to oncogenesis. Studies showing the effect of activation of JAK/STAT signalling upon prognosis in several tumour types are summarised. Finally, we discuss the prospects for treating solid tumours using strategies targeting JAK/STAT signalling, including what can be learned from haematological malignancies and the extent to which results in solid tumours might be expected to differ.

CALR exon 9 mutations are somatically acquired events in familial cases of essential thrombocythemia or primary myelofibrosis

Somatic indels of CALR exon 9 are present in about 20% to 25% of sporadic patients with essential thrombocythemia or primary myelofibrosis. These mutations are found also in familial cases of essential thrombocythemia or primary myelofibrosis as somatically acquired events.

A novel activating, germline JAK2 mutation, JAK2R564Q, causes familial essential thrombocytosis

JAK2R564Q is the first germline JAK2 mutation found to contribute to a familial MPN that involves a residue other than V617. The kinase activity of JAK2R564Q and JAK2V617F are the same, but only V617F is able to escape regulation by SOCS3 and p27.

Germ-line JAK2 mutations in the kinase domain are responsible for hereditary thrombocytosis and are resistant to JAK2 and HSP90 inhibitors

The main molecular basis of essential thrombocythemia and hereditary thrombocytosis is acquired, and germ-line-activating mutations affect the thrombopoietin signaling axis. We have identified 2 families with hereditary thrombocytosis presenting novel heterozygous germ-line mutations of JAK2. One family carries the JAK2 R867Q mutation located in the kinase domain, whereas the other presents 2 JAK2 mutations, S755R/R938Q, located in cis in both the pseudokinase and kinase domains. Expression of Janus kinase 2 (JAK2) R867Q and S755R/R938Q induced spontaneous growth of Ba/F3-thrombopoietin receptor (MPL) but not of Ba/F3-human receptor of erythropoietin cells. Interestingly, both Ba/F3-MPL cells expressing the mutants and platelets from patients displayed thrombopoietin-independent phosphorylation of signal transducer and activator of transcription 1. The JAK2 R867Q and S755R/R938Q proteins had significantly longer half-lives compared with JAK2 V617F. The longer half-lives correlated with increased binding to the heat shock protein 90 (HSP90) chaperone and with higher MPL cell-surface expression. Moreover, these mutants were less sensitive to JAK2 and HSP90 inhibitors than JAK2 V617F. Our results suggest that the mutations in the kinase domain of JAK2 may confer a weak activation of signaling specifically dependent on MPL while inducing a decreased sensitivity to clinically available JAK2 inhibitors.

JAK2 mutations to the fore in hereditary thrombocythemia

Acquired mutations of the gene that encodes the intracellular signalling molecule JAK2 are the most frequently observed disease-driving events of the common myeloproliferative neoplasms. A number of germline JAK2 mutations have recently been described in several kindred with the rare disease of hereditary thrombocythemia, also known as familial thrombocythemia or familial thrombocytosis. These inherited mutations are all located within the pseudo-kinase and kinase domains of JAK2 and have been shown to directly contribute to the thrombocythemic phenotype. Molecular characterisation of the resulting, aberrant signalling signatures may provide insights into genotype-phenotype relationships of both hereditary thrombocythemia and the common myeloproliferative neoplasms.

Activating Janus kinase pseudokinase domain mutations in myeloproliferative and other blood cancers

The discovery of the highly prevalent activating JAK (Janus kinase) 2 V617F mutation in myeloproliferative neoplasms, and of other pseudokinase domain-activating mutations in JAK2, JAK1 and JAK3 in blood cancers, prompted great interest in understanding how pseudokinase domains regulate kinase domains in JAKs. Recent functional and mutagenesis studies identified residues required for the V617F mutation to induce activation. Several X-ray crystal structures of either kinase or pseudokinase domains including the V617F mutant of JAK2 pseudokinase domains are now available, and a picture has emerged whereby the V617F mutation induces a defined conformational change around helix C of JH (JAK homology) 2. Effects of mutations on JAK2 can be extrapolated to JAK1 and TYK2 (tyrosine kinase 2), whereas JAK3 appears to be different. More structural information of the full-length JAK coupled to cytokine receptors might be required in order to define the structural basis of JH1 activation by JH2 mutants and eventually obtain mutant-specific inhibitors.