Genetically heterogeneous origins of idiopathic erythrocytosis

Differenzialdiagnose der Erythrozytose – Ursachen und klinische Bedeutung

Aufgrund ihres seltenen Auftretens stellt die Erythrozytose häufig eine Herausforderung für die behandelnden Ärzte dar. Die Erythropoese (= Produktion von Erythrozyten) ist im Knochenmark angesiedelt, und das Hormon Erythropoetin (EPO) übernimmt die Kontrolle über ihre Regulation. Daher ist die Messung von EPO im Serum einer der wichtigsten diagnostischen Schritte. Bei der Erythrozytose muss zwischen angeborenen und erworbenen Ursachen unterschieden werden. Darüber hinaus gibt es primäre und sekundäre Formen. Angeborene Ursachen von Erythrozytosen treten sehr selten auf, werden meist in jungen Jahren diagnostiziert und sollten in spezialisierten Zentren behandelt werden. Die Polycythaemia vera (PV), eine klonale Störung und eine der wichtigsten myeloproliferativen Neoplasien (neben der essenziellen Thrombozythämie und der primären Myelofibrose), stellt die häufigste primär erworbene Ursache für Erythrozytosen dar. Klinisch treten eine erhöhte Thrombophilie und mikrovaskuläre Störungen auf. Die Initialtherapie bei Patienten mit PV umfasst die Verabreichung von Aspirin und Aderlass-Therapie. Sekundär erworbene Formen der Erythrozytose treten vor allem aufgrund einer durch Nikotinabusus oder chronische Herz- und Lungenerkrankungen ausgelösten Hypoxie auf. Als weitere Differenzialdiagnosen müssen eine tumorbedingte EPO-Produktion, Nierenerkrankungen oder eine exogene Zufuhr von EPO (= EPO-Doping) in Betracht gezogen werden.

Differenzialdiagnose der Erythrozytose – Ursachen und klinische Bedeutung

Due to its rare incidence, erythrocytosis frequently represents a challenge for the treating doctors. The erythropoiesis (= production of erythrocytes) is located in the bone marrow, and the hormone erythropoietin (EPO) takes control in its regulation. Therefore, measurement of EPO in serum is one of the main diagnostic steps. In erythrocytosis, congenital causes have to be distinguished from acquired ones. Furthermore, there are primary and secondary forms. Congenital causes of erythrocytoses occur very infrequently, are mainly diagnosed in young age and should be treated in specialized centers. Polycythemia vera (PV), a clonal disorder and one of the main myeloproliferative neoplasms (beside essential thrombocythemia and primary myelofibrosis), represents the most frequent primary acquired cause of erythrocytosis. Clinically, increased thrombophilia and microvascular disturbance occur. The first-line treatment in patients with PV includes administration of aspirin and phlebotomies. Secondary acquired forms of erythrocytosis mainly occur due to hypoxia triggered by nicotine abuse or chronic heart and lung diseases. Regarding other differential diagnoses, a cancer-associated EPO production, kidney diseases or exogenous supply with EPO (= EPO doping) have to be considered.

Cytokine Receptor Endocytosis: New Kinase Activity-Dependent and -Independent Roles of PI3K

Type I and II cytokine receptors are cell surface sensors that bind cytokines in the extracellular environment and initiate intracellular signaling to control processes such as hematopoiesis, immune function, and cellular growth and development. One key mechanism that regulates signaling from cytokine receptors is through receptor endocytosis. In this mini-review, we describe recent advances in endocytic regulations of cytokine receptors, focusing on new paradigms by which PI3K controls receptor endocytosis through both kinase activity-dependent and -independent mechanisms. These advances underscore the notion that the p85 regulatory subunit of PI3K has functions beyond regulating PI3K kinase activity, and that PI3K plays both positive and negative roles in receptor signaling. On the one hand, the PI3K/Akt pathway controls various aspects downstream of cytokine receptors. On the other hand, it stimulates receptor endocytosis and downregulation, thus contributing to signaling attenuation.

Investigation of genetic disturbances in oxygen sensing and erythropoietin signaling pathways in cases of idiopathic erythrocytosis

Background. Idiopathic erythrocytosis is the term reserved for cases with unexplained origins of abnormally increased hemoglobin after initial investigation. Extensive molecular investigation of genes associated with oxygen sensing and erythropoietin signaling pathways, in those cases, usually involves sequencing all of their exons and it may be time consuming. Aim. To perform a strategy for molecular investigation of patients with idiopathic erythrocytosis regarding oxygen sensing and erythropoietin signaling pathways. Methods. Samples of patients with idiopathic erythrocytosis were evaluated for the EPOR, VHL, PHD2, and HIF-2α genes using bidirectional sequencing of their hotspots. Results. One case was associated with HIF-2α mutation. Sequencing did not identify any pathogenic mutation in 4 of 5 cases studied in any of the studied genes. Three known nonpathogenic polymorphisms were found (VHL p.P25L, rs35460768; HIF-2α p.N636N, rs35606117; HIF-2α p.P579P, rs184760160). Conclusion. Extensive molecular investigation of cases considered as idiopathic erythrocytosis does not frequently change the treatment of the patient. However, we propose a complementary molecular investigation of those cases comprising genes associated with erythrocytosis phenotype to meet both academic and genetic counseling purposes.

Diagnosis and management of congenital and idiopathic erythrocytosis

An erythrocytosis occurs when there is an increased red-cell mass. The causes of erythrocytosis are divided into primary, when there is an intrinsic defect in the erythroid cell, and secondary, when the cause is extrinsic to the erythroid cell. An idiopathic erythrocytosis occurs when the increased red-cell mass has no identifiable cause. Primary and secondary defects can be further classified as either congenital or acquired causes. The diagnostic pathway starts with a careful history and examination followed by measurement of the erythropoietin (EPO) levels. This allows a division of those patients with a low EPO level, who can then be investigated for primary causes of erythrocytosis, and those with a normal or high EPO level, where the oxygen-sensing pathway needs to be explored further. Physiological studies in those with congenital defects in the oxygen-sensing pathway show many changes in the downstream metabolism adapting to the defect, which has a bearing on the management of the disorders. Low-dose aspirin and venesection to an achievable target are the main therapeutic options that can be considered in the management of erythrocytosis. Specific guidance on venesection options should be considered with certain causes such as high oxygen-affinity hemoglobins.

Isolated erythrocytosis: study of 67 patients and identification of three novel germ-line mutations in the prolyl hydroxylase domain protein 2 (PHD2) gene

The oxygen sensing pathway modulates erythropoietin expression. In normal cells, intracellular oxygen tensions are directly sensed by prolyl hydroxylase domain (PHD)-containing proteins. PHD2 isozyme has a key role in tagging hypoxia-inducible factor (HIF)-α subunits for polyubiquitination and proteasomal degradation. Erythrocytosis-associated PHD2 mutations reduce hydroxylation of HIF-α. The investigation of 67 patients with isolated erythrocytosis, either sporadic or familial, allowed the identification of three novel mutations in the catalytic domain of the PHD2 protein. All new mutations are germ-line, heterozygous and missense, and code for a predicted full length mutant PHD2 protein. Identification of the disease-causing genes will be of critical importance for a better classification of familial and acquired erythrocytosis, offering additional insight into the erythropoietin regulating oxygen sensing pathway.

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.

Erythropoiesis: model systems, molecular regulators, and developmental programs

Human erythropoiesis is a complex multistep developmental process that begins at the level of pluripotent hematopoietic stem cells (HSCs) at bone marrow microenvironment (HSCs niche) and terminates with the production of erythrocytes (RBCs). This review covers the basic and contemporary aspects of erythropoiesis. These include the: (a) cell-lineage restricted pathways of differentiation originated from HSCs and going downward toward the blood cell development; (b) model systems employed to study erythropoiesis in culture (erythroleukemia cell lines and embryonic stem cells) and in vivo (knockout animals: avian, mice, zebrafish, and xenopus); (c) key regulators of erythropoiesis (iron, hypoxia, stress, and growth factors); (d) signaling pathways operating at hematopoietic stem cell niche for homeostatic regulation of self renewal (SCF/c-kit receptor, Wnt, Notch, and Hox) and for erythroid differentiation (HIF and EpoR). Furthermore, this review presents the mechanisms through which transcriptional factors (GATA-1, FOG-1, TAL-1/SCL/MO2/Ldb1/E2A, EKLF, Gfi-1b, and BCL11A) and miRNAs regulate gene pattern expression during erythroid differentiation. New insights regarding the transcriptional regulation of alpha- and beta-globin gene clusters were also presented. Emphasis was also given on (i) the developmental program of erythropoiesis, which consists of commitment to terminal erythroid maturation and hemoglobin production, (two closely coordinated events of erythropoieis) and (ii) the capacity of human embryonic and umbilical cord blood (UCB) stem cells to differentiate and produce RBCs in culture with highly selective media. These most recent developments will eventually permit customized red blood cell production needed for transfusion.

Concurrence of multiple myeloma and idiopathic erythrocytosis

The association of erythrocytosis and multiple myeloma is rare. We encountered a 76-year-old male patient with erythrocytosis followed by the diagnosis of multiple myeloma 8 months later. Related laboratory examinations revealed absolute erythrocytosis, normal oxygen saturation and erythropoietin (EPO) levels, the absence of endogenous erythroid colony (EEC) and JAK2-V617F mutations. The diagnosis of idiopathic erythrocytosis (IE), instead of polycythemic vera (PV), was made. In the literature, about 20 cases of erythrocytosis associated with myeloma can be found. Based on elevated EPO levels, 2 of such cases should be considered secondary erythrocytosis while others are reported as PV. None of them is considered to be have idiopathic erythrocytosis. Our present case is the first one with multiple myeloma developing in a patient with the diagnosis well established by extensive laboratory workup. The pathogenic role of these two entities remains to be established.

Ligand-induced EpoR internalization is mediated by JAK2 and p85 and is impaired by mutations responsible for primary familial and congenital polycythemia

Epo-induced endocytosis of EpoR plays important roles in the down-regulation of EpoR signaling and is the primary means that regulates circulating Epo concentrations. Here we show that cell-surface EpoR is internalized via clathrin-mediated endocytosis. Both JAK2 kinase activity and EpoR cytoplasmic tyrosines are important for ligand-dependent EpoR internalization. Phosphorylated Y429, Y431, and Y479 in the EpoR cytoplasmic domain bind p85 subunit of PI3 kinase on Epo stimulation and individually are sufficient to mediate Epo-dependent EpoR internalization. Knockdown of p85alpha and p85beta or expression of their dominant-negative forms, but not inhibition of PI3 kinase activity, dramatically impaired EpoR internalization, indicating that p85alpha and p85beta may recruit proteins in the endocytic machinery on Epo stimulation. Furthermore, mutated EpoRs from primary familial and congenital polycythemia (PFCP) patients lacking the 3 important tyrosines do not bind p85 or internalize on stimulation. Addition of residues encompassing Y429 and Y431 to these truncated receptors restored p85beta binding and Epo sensitivity. Our results identify a novel PI3 kinase activity-independent function of p85 in EpoR internalization and support a model that defects of internalization in truncated EpoRs from PFCP patients contribute to Epo hypersensitivity and prolonged signaling.

The complete evaluation of erythrocytosis: congenital and acquired

The approach to a patient with erythrocytosis is greatly simplified by assessing the clonality of the process upfront. In this regard, there has been a dramatic shift toward genetic testing and away from traditional tests, such as measurement of red cell mass. Clonal erythrocytosis is the diagnostic feature of polycythemia vera (PV) and is almost always associated with a JAK2 mutation (JAK2V617F or exon 12). All other scenarios represent non-clonal erythrocytosis, often referred to as secondary erythrocytosis. Serum erythropoietin (Epo) level is usually normal or elevated in secondary erythrocytosis and subnormal in PV. Therefore, in a patient with acquired erythrocytosis, it is reasonable to begin the diagnostic work-up with peripheral blood JAK2 mutation analysis and serum Epo measurement to distinguish PV from secondary erythrocytosis. Conversely, the patient with life-long erythrocytosis is more likely to suffer from congenital polycythemia and should therefore be evaluated for germline mutations that result in enhanced Epo effect (for example, Epo receptor mutations), altered intracellular oxygen sensing (for example, mutations involving the von Hippel-Lindau tumor suppressor gene) or decreased P50 (for example, high-oxygen-affinity hemoglobinopathy). The order of tests in this instance depends on the clinical scenario and serum Epo level.

Genetic origins and clinical phenotype of familial and acquired erythrocytosis and thrombocytosis

Familial and acquired erythrocytosis and thrombocytosis are characterized by myeloid lineage hyperproliferation, which is either single or multi-lineage in origin. The single lineage disorders exhibit Mendelian inheritance with polyclonal hematopoiesis and often arise from a single genetic defect. In contrast, the multi-lineage disorders exhibit complex patterns of inheritance with multi-genetic origins and clonal hematopoiesis. They have the potential to acquire JAK2 somatic mutations, but this is not the primary event. Identification of the disease-causing genes will enable better classification of familial and acquired erythrocytosis and thrombocytosis. Furthermore, it will provide an insight into the mechanisms regulating myeloid cell proliferation.

Idiopathic erythrocytosis: a disappearing entity

Erythrocytosis results when there is an increased red cell mass and thus an increased hemoglobin. The causes can be divided into primary intrinsic defects of the erythroid progenitor cell and secondary defects, where factors external to the erythroid compartment are responsible. Both can then be further divided into congenital and acquired categories. Congenital causes include mutations of the erythropoietin receptor and defects of the oxygen-sensing pathway including VHL, PHD2 and HIF2A mutations. When fully investigated there remain a number of patients in whom no cause can be elucidated who are currently described as having idiopathic erythrocytosis. Investigation should start with a full history and examination. Having eliminated the common entity polycythemia vera, further direction for investigation is guided by the erythropoietin level. Clinical consequences of the various erythrocytoses are not clear, but in some groups thromboembolic events have been described in young patients. Evidence is lacking to define best management, but aspirin and venesection to a target hematocrit should be considered.

The human side of hypoxia-inducible factor

When humans are exposed to hypoxia, systemic and intracellular changes operate together to minimise hypoxic injury and restore adequate oxygenation. Emerging evidence indicates that the hypoxia-inducible factor (HIF) family of transcription factors plays a central regulatory role in these homeostatic changes at both the systemic and cellular levels. HIF was discovered through its action as the transcriptional activator of erythropoietin, and has subsequently been found to control intracellular hypoxic responses throughout the body. HIF is primarily regulated by specific prolyl hydroxylase-domain enzymes (PHDs) that initiate its degradation via the von Hippel-Lindau tumour suppressor protein (VHL). The oxygen and iron dependency of PHD activity accounts for regulation of the pathway by both cellular oxygen and iron status. Recent studies conducted in patients with rare genetic diseases have begun to uncover the wider importance of the PHD-VHL-HIF axis in systems-level human biology. These studies indicate that, in addition to regulating erythropoiesis, the system plays an important role in cardiopulmonary regulation. This article reviews our current understanding of the importance of HIF in human systems-level physiology, and is modelled around the classic physiological response to high-altitude hypoxia.