JAK2 exon 12 mutations in polycythemia vera or idiopathic erythrocytosis

JAK2 exon 12 mutations were detected in 4 out of 20 polycythemia vera and idiopathic erythrocytosis V617F-negative patients and were only present in the myeloid lineage. Initial hematologic data of these patients differ from those of V617F-positive patients, but there is no difference in thrombotic development and myelofibrotic transformation.     

The incidence of the JAK2 V617F mutation in patients with idiopathic erythrocytosis

Sixty-three patients with erythrocytosis exhibiting a range of erythropoietin levels were screened for the JAK2 V617F mutation. One patient (1.6%) was found to have this mutation, and has remained stable for 9 years, suggesting that the JAK2 V617F mutation is rare in patients with idiopathic erythrocytosis.     

Genetically heterogeneous origins of idiopathic erythrocytosis

The idiopathic erythrocytosis (IE) group of disorders is defined by an absolute increase in red cell mass and hematocrit without elevation of the megakaryocytic or granulocytic lineages. It is associated with a wide range of serum erythropoietin (Epo) levels and broadly falls into groups of raised/inappropriately normal or low/undetectable Epo levels. A spectrum of molecular defects has been described in association with IE, which reflects the heterogeneity of this disorder. To date the most common identified cause of IE has been mutations in the von Hippel Landau (VHL) protein, which results in aberrant oxygen sensing and dysregulated Epo production. Studying the molecular basis of IE will provide insights into the control of Epo synthesis and Epo-induced signaling pathways.     

HIF pathway mutations and erythrocytosis

Erythrocytosis is present when there is an increase in the red cell mass, usually accompanied by an elevated hemoglobin and hematocrit. This occurs when there is an intrinsic defect in the erythroid component of the bone marrow or for secondary reasons when an increase in erythropoietin production drives red cell production. In normoxic conditions, HIF-α interacts with the other proteins in the HIF pathway and is destroyed, but in hypoxic conditions, HIF-α binds to HIF-β and alters the expression of downstream genes, including the erythropoietin gene. The end result is an increase in erythropoietin production. Mutations in any of the genes in the HIF pathway could lead to changed proteins, abnormalities in the degradation of HIF-α and, ultimately, result in increased erythropoietin levels. A number of mutations in the VHL, PHD2, and HIF2A genes have been identified in individuals. These mutations lead to erythrocytosis. The clinical results of these mutations may include some major thromboembolic events in young patients.     

Phenotypic variations and new mutations in JAK2 V617F-negative polycythemia vera, erythrocytosis, and idiopathic myelofibrosis

OBJECTIVE: The chronic myeloproliferative disorders (MPD), polycythemia vera (PV), essential thrombocytosis, and idiopathic myelofibrosis (IMF), are characterized by a spectrum of clinical features and linked by common genetic lesions in JAK2 and MPL. However, the clinical phenotypes in genetically undefined MPD patients are similar to those patients with JAK2 and MPL lesions. We, therefore, sought to determine whether there were JAK2 or MPL lesions in a well-defined, JAK2 V617F-negative MPD cohort, and to determine if clinical associations could be identified based on variations identified in these genes. METHODS: We examined the JAK2 and MPL genes in JAK2 V617F-negative PV, IMF, and idiopathic erythrocytosis patients for sequence variations. RESULTS: We identified two previously unrecognized JAK2 mutations and three previously unrecognized MPL mutations in JAK2 V617F-negative PV, erythrocytosis, and IMF patients. We identified JAK2 exon 12 lesions in 30% of JAK2 V617F-negative PV patients, and either JAK2 V617F or JAK2 exon 12 lesions in 9% of erythrocytosis patients. In IMF, in addition to the MPL gene mutation, W515K, we identified three additional mutations: 204P and two intervening sequence transitions, IVS 11/12 and 10/11. CONCLUSIONS: While the clinical phenotype of JAK2 exon 12 lesions in the MPD was predominantly erythroid, there was significant disease spectrum overlap between JAK2 V617F and JAK2 exon 12 mutations. By contrast, MPL gene mutations were not associated with erythrocytosis, but segregated primarily with the phenotypes of thrombocytosis, extramedullary disease, myelofibrosis, and osteosclerosis.     

Hereditary gene mutations in Korean patients with isolated erythrocytosis

Most cases of erythrocytosis occur secondary to chronic tissue hypoxia or as a clonal disease such as polycythemia vera with somatic mutations in the Janus kinase 2 (JAK2) gene. Rarely, erythrocytosis is caused by hereditary gene mutations. This study investigated hereditary gene mutations in 38 unrelated Korean patients with isolated erythrocytosis without (1) JAK2 mutation and (2) secondary causes of erythrocytosis other than smoking history. Direct sequencing analyses were performed on six genes associated with hereditary erythrocytosis [HBB, exon 2 and exon 3 of HBA2, VHL, EGLN1 (previously PHD2), exon 12 of EPAS1 (previously HIF2A), and exons 5–8 of EPOR]. As a result, mutations were detected in five patients (three never smokers and two current smokers) out of 38 patients (13.2 %). The mutations detected in those five patients were EPOR:p.W439*, EPOR:p.G212C, HBB:p.H98Q (or conventionally H97Q, Hb Malmö [β 97(FG4) His > Gln]), HBB:p.V138M (V137M), and EGLN1:p.L279Tfs43*, all in heterozygous state. No patient had mutations in HBA2, VHL, or in EPAS1. This study indicates that workup for hereditary gene mutations is needed for isolated erythrocytosis with or without smoking history.     

Effects of idiopathic erythrocytosis on the left ventricular diastolic functions and the spectrum of genetic mutations: A case control study

Background:We have aimed at exposing left ventricular diastolic functions and the presence of known genetic mutations for familial erythrocytosis, in patients who exhibit idiopathic erythrocytosis.Methods:Sixty-four patients with idiopathic erythrocytosis (mean age, 46.4 ± 2.7 years) and 30 age-matched healthy subjects were prospectively evaluated. The regions of interest of the erythropoietin receptor, hemoglobin beta-globin, von Hippel-Lindau, hypoxia-inducible factor 2 alpha, and Egl-9 family hypoxia-inducible factor genes were amplified by PCR. Left ventricular (LV) mass was measured by M-mode and 2-dimensional echocardiography. LV diastolic functions were assessed by conventional echocardiography and tissue Doppler imaging.Results:As a result of genetic analyses, genetic mutations for familial erythrocytosis were detected in 5 patients. It has been observed in our study that the risk of cardiovascular disorders is higher in patients. Interventricular septum thickness, left atrial diameter, and some diastolic function parameters such as deceleration time and isovolumetric relaxation time have been found to be significantly higher in idiopathic erythrocytosis group than in the controls.Conclusion:This study has shown that LV diastolic functions were impaired in patients with idiopathic erythrocytosis. In this patient group with increased risk of cardiovascular disorders, the frequent genetic mutations have been detected in 5 patients only. Therefore, further clinical investigations are needed as novel genetic mutations may be discovered in patients with idiopathic erythrocytosis because of cardiovascular risk.     

Two new mutations in the HIF2A gene associated with erythrocytosis

Congenital or familial erythrocytosis/polycythemia can have many causes, and an emerging cause is genetic disruption of the oxygen‐sensing pathway that regulates the Erythropoietin (EPO) gene. More specifically, recent studies have identified erythrocytosis‐associated mutations in the HIF2A gene, which encodes for Hypoxia Inducible Factor‐2α (HIF‐2α), as well as in two genes that encode for proteins that regulate it, Prolyl Hydroxylase Domain protein 2 (PHD2) and the von Hippel Lindau tumor suppressor protein (VHL). We report here the identification of two new heterozygous HIF2A missense mutations, M535T, and F540L, both associated with erythrocytosis. Met‐535 has previously been identified as a residue mutated in other patients with erythrocytosis; although, the mutation of this particular residue to Thr has not been reported. In contrast, Phe‐540 has not been reported as a residue mutated in erythrocytosis, and we present evidence here that this mutation impairs interaction of HIF‐2α with both VHL and PHD2. Am. J. Hematol. 2012. © 2012 Wiley Periodicals, Inc.     

Polycythaemia‐inducing mutations in the erythropoietin receptor (EPOR): mechanism and function as elucidated by epidermal growth factor receptor–EPOR chimeras

Primary familial and congenital polycythaemia (PFCP) is a disease characterized by increased red blood cell mass, and can be associated with mutations in the intracellular region of the erythropoietin (EPO) receptor (EPOR). Here we explore the mechanisms by which EPOR mutations induce PFCP, using an experimental system based on chimeric receptors between epidermal growth factor receptor (EGFR) and EPOR. The design of the chimeras enabled EPOR signalling to be triggered by EGF binding. Using this system we analysed three novel EPOR mutations discovered in PFCP patients: a deletion mutation (Del1377‐1411), a nonsense mutation (C1370A) and a missense mutation (G1445A). Three different chimeras, bearing these mutations in the cytosolic, EPOR region were generated; Hence, the differences in the chimera‐related effects are specifically attributed to the mutations. The results show that the different mutations affect various aspects related to the signalling and metabolism of the chimeric receptors. These include slower degradation rate, higher levels of glycan‐mature chimeric receptors, increased sensitivity to low levels of EGF (replacing EPO in this system) and extended signalling cascades. This study provides a novel experimental system to study polycythaemia‐inducing mutations in the EPOR, and sheds new light on underlying mechanisms of EPOR over‐activation in PFCP patients.     

Novel exon 12 mutations in the HIF2A gene associated with erythrocytosis

Erythrocytosis can arise from deregulation of the erythropoietin (Epo) axis resulting from defects in the oxygen-sensing pathway. Epo synthesis is controlled by the hypoxia inducible factor (HIF) complex, composed of an and a β subunit. There are 2 main subunits, HIF-1 and HIF-2. Recently, a HIF-2 Gly537Trp mutation was identified in a family with erythrocytosis. This raises the possibility of HIF2A mutations being associated with other cases of erythrocytosis. We now report a subsequent analysis of HIF2A in a cohort of 75 erythrocytosis patients and identify 4 additional patients with novel heterozygous Met535Val and Gly537Arg mutations. All patients presented at a young age with elevated serum Epo. Mutations at Gly-537 account for 4 of 5 HIF2A mutations associated with erythrocytosis. These findings support the importance of HIF-2 in human Epo regulation and warrant investigation of HIF2A in patients with unexplained erythrocytosis.