Absence of polycythemia in a child with a unique erythropoietin receptor mutation in a family with autosomal dominant primary polycythemia

Identification of Two Novel EPOR Gene Variants in Primary Familial Polycythemia: Case Report and Literature Review

Simple Summary

Erythrocytosis can be caused by a wide variety of diseases. Some forms of erythrocytosis have an obvious cause, such as a kidney injury, or it may have an oncological cause, but in some patients, the origin of the disease is not entirely clear, and since the symptoms of an isolated erythrocytosis are not usually cumbersome, sometimes the diagnosis takes several months or years. In the present work, we report a couple of cases of familial erythrocytosis associated with novel variants in the erythropoietin receptor gene. This study serves as a reminder of the clinical and molecular study of this rare disease and expands the list of mutations associated with primary familial polycythemia.

Abstract

Primary familial and congenital polycythemia is a rare disease characterized by an increase in red cell mass that may be due to pathogenic variants in the EPO receptor (EPOR) gene. To date, 33 genetic variants have been reported to be associated. We analyzed the presence of EPOR variants in two patients with polycythemia in whom JAK2 pathogenic variants had been previously discarded. Molecular analysis of the EPOR gene was performed by Sanger sequencing of the coding regions and exon/intron boundaries of exon 8. We performed in vitro culture of erythroid progenitor cells. Segregation studies were done whenever possible. The two patients studied showed hypersensitivity to EPO in in vitro cultures. Analysis of the EPOR gene unveiled two novel pathogenic variants. Genetic testing of asymptomatic relatives could guarantee surveillance and proper management.

JAK2 unmutated erythrocytosis: current diagnostic approach and therapeutic views

JAK2 unmutated or non-polycythemia vera (PV) erythrocytosis encompasses both hereditary and acquired conditions. A systematic diagnostic approach begins with documentation of historical hematocrit (Hct)/hemoglobin (Hgb) measurements and classification of the process as life-long/unknown duration or acquired. Further investigation in both categories is facilitated by determination of serum erythropoietin level (EPO). Workup for hereditary/congenital erythrocytosis requires documentation of family history and laboratory screening for high-oxygen affinity hemoglobin variants, 2, 3 biphosphoglycerate deficiency, and germline mutations that are known to alter cellular oxygen sensing (e.g., PHD2, HIF2A, VHL) or EPO signaling (e.g., EPOR mutations); the latter is uniquely associated with subnormal EPO. Acquired erythrocytosis is often elicited by central or peripheral hypoxia resulting from cardiopulmonary disease/high-altitude dwelling or renal artery stenosis, respectively; EPO in the former instance is often normal (compensated by negative feed-back). Other conditions associated with acquired erythrocytosis include EPO-producing tumors and the use of drugs that promote erythropoiesis (e.g., testosterone, erythropoiesis stimulating agents). "Idiopathic erythrocytosis" loosely refers to an otherwise not explained situation. Historically, management of non-PV erythrocytosis has been conflicted by unfounded concerns regarding thrombosis risk, stemming from limited phenotypic characterization, save for Chuvash polycythemia, well-known for its thrombotic tendency. In general, cytoreductive therapy should be avoided and phlebotomy is seldom warranted where frequency is determined by symptom control rather than Hct threshold. Although not supported by hard evidence, cardiovascular risk optimization and low-dose aspirin use are often advised. Application of modern genetic tests and development of controlled therapeutic intervention trials are needed to advance current clinical practice.

HPO2GO: prediction of human phenotype ontology term associations for proteins using cross ontology annotation co-occurrences

Analysing the relationships between biomolecules and the genetic diseases is a highly active area of research, where the aim is to identify the genes and their products that cause a particular disease due to functional changes originated from mutations. Biological ontologies are frequently employed in these studies, which provides researchers with extensive opportunities for knowledge discovery through computational data analysis. In this study, a novel approach is proposed for the identification of relationships between biomedical entities by automatically mapping phenotypic abnormality defining HPO terms with biomolecular function defining GO terms, where each association indicates the occurrence of the abnormality due to the loss of the biomolecular function expressed by the corresponding GO term. The proposed HPO2GO mappings were extracted by calculating the frequency of the co-annotations of the terms on the same genes/proteins, using already existing curated HPO and GO annotation sets. This was followed by the filtering of the unreliable mappings that could be observed due to chance, by statistical resampling of the co-occurrence similarity distributions. Furthermore, the biological relevance of the finalized mappings were discussed over selected cases, using the literature. The resulting HPO2GO mappings can be employed in different settings to predict and to analyse novel gene/protein—ontology term—disease relations. As an application of the proposed approach, HPO term—protein associations (i.e., HPO2protein) were predicted. In order to test the predictive performance of the method on a quantitative basis, and to compare it with the state-of-the-art, CAFA2 challenge HPO prediction target protein set was employed. The results of the benchmark indicated the potential of the proposed approach, as HPO2GO performance was among the best (Fmax = 0.35). The automated cross ontology mapping approach developed in this work may be extended to other ontologies as well, to identify unexplored relation patterns at the systemic level. The datasets, results and the source code of HPO2GO are available for download at: https://github.com/cansyl/HPO2GO.

Erythropoietin and G-csf Receptors in Human Tumor Cells: Expression and Aspects regarding Functionality

Aims and Background

Recombinant human erythropoietin (Epo) and granulocyte-colony-stimulating factor (G-CSF) are used to stimulate hematopoiesis in patients with malignant diseases. These cytokines transduce their biological signal via the Epo receptor (EpoR) and G-CSF receptor (G-CSF-R) into the cell. We therefore investigated in human tumor cell lines the expression of these receptors in tumor cells as well as their response to Epo and G-CSF.

Methods and Study Design

The expression of EpoR and G-CSF-R mRNA was analyzed with reverse transcription-polymerase chain reaction (RT-PCR). EpoR protein expression was further monitored with Western blot and immunocytochemistry analysis. The cellular response to various concentrations of Epo was evaluated using 3[H]-thymidine uptake, Northern blot of c-fos expression and tyrosine kinase activity assay. The proliferation after G-CSF incubation was analyzed with the MTS assay.

Results

In this study EpoR mRNA and protein were detected in various human tumor cell lines. Treatment with Epo did not influence the proliferation rate of examined EpoR-positive tumor cell lines. Epo did not stimulate the tyrosine kinase activity nor did it affect the c-fos mRNA in these cell lines. G-CSF-R mRNA was only detected in two myeloid cell lines. Treatment with G-CSF did not increase the proliferation of these cells.

Conclusions

These results demonstrate that Epo and G-CSF did not modulate the growth rate of examined receptor-positive tumor cell lines; the presence of the Epo receptor seems not essential for cell growth of these tumor cells in cell culture.

New pathogenic mechanisms induced by germline erythropoietin receptor mutations in primary erythrocytosis

Primary familial and congenital polycythemia is characterized by erythropoietin hypersensitivity of erythroid progenitors due to germline nonsense or frameshift mutations in the erythropoietin receptor gene. All mutations so far described lead to the truncation of the C-terminal receptor sequence that contains negative regulatory domains. Their removal is presented as sufficient to cause the erythropoietin hypersensitivity phenotype. Here we provide evidence for a new mechanism whereby the presence of novel sequences generated by frameshift mutations is required for the phenotype rather than just extensive truncation resulting from nonsense mutations. We show that the erythropoietin hypersensitivity induced by a new erythropoietin receptor mutant, p.Gln434Profs*11, could not be explained by the loss of negative signaling and of the internalization domains, but rather by the appearance of a new C-terminal tail. The latter, by increasing erythropoietin receptor dimerization, stability and cell-surface localization, causes pre-activation of erythropoietin receptor and JAK2, constitutive signaling and hypersensitivity to erythropoietin. Similar results were obtained with another mutant, p.Pro438Metfs*6, which shares the same last five amino acid residues (MDTVP) with erythropoietin receptor p.Gln434Profs*11, confirming the involvement of the new peptide sequence in the erythropoietin hypersensitivity phenotype. These results suggest a new mechanism that might be common to erythropoietin receptor frameshift mutations. In summary, we show that primary familial and congenital polycythemia is more complex than expected since distinct mechanisms are involved in the erythropoietin hypersensitivity phenotype, according to the type of erythropoietin receptor mutation.

Genetic basis of congenital erythrocytosis: mutation update and online databases

Congenital erythrocytosis (CE), or congenital polycythemia, represents a rare and heterogeneous clinical entity. It is caused by deregulated red blood cell production where erythrocyte overproduction results in elevated hemoglobin and hematocrit levels. Primary congenital familial erythrocytosis is associated with low erythropoietin (Epo) levels and results from mutations in the Epo receptor gene (EPOR). Secondary CE arises from conditions causing tissue hypoxia and results in increased Epo production. These include hemoglobin variants with increased affinity for oxygen (HBB, HBA mutations), decreased production of 2,3-bisphosphoglycerate due to BPGM mutations, or mutations in the genes involved in the hypoxia sensing pathway (VHL, EPAS1, and EGLN1). Depending on the affected gene, CE can be inherited either in an autosomal dominant or recessive mode, with sporadic cases arising de novo. Despite recent important discoveries in the molecular pathogenesis of CE, the molecular causes remain to be identified in about 70% of the patients. With the objective of collecting all the published and unpublished cases of CE the COST action MPN&MPNr-Euronet developed a comprehensive Internet-based database focusing on the registration of clinical history, hematological, biochemical, and molecular data (http://www.erythrocytosis.org/). In addition, unreported mutations are also curated in the corresponding Leiden Open Variation Database.

Elevated nuclear factor erythroid-2 levels promote epo-independent erythroid maturation and recapitulate the hematopoietic stem cell and common myeloid progenitor expansion observed in polycythemia vera patients

The molecular etiology of polycythemia vera (PV) remains incompletely understood. Patients harbor increased numbers of hematopoietic stem cells and display Epo-independent erythroid maturation. However, the molecular mechanism underlying Epo hypersensitivity and stem cell expansion is unclear. We have previously shown that the transcription factor nuclear factor erythroid-2 (NF-E2) is overexpressed in the majority of PV patients. Here we demonstrated that elevation of NF-E2 expression in healthy CD34(+) cells to levels observed in PV caused Epo-independent erythroid maturation and expansion of hematopoietic stem cell (HSC) and common myeloid progenitor (CMP) cell numbers. Silencing NF-E2 in PV patients reverted both aberrancies, demonstrating for the first time that NF-E2 overexpression is both required and sufficient for Epo independence and HSC/CMP expansion in PV.

A novel EPAS1/HIF2A germline mutation in a congenital polycythemia with paraganglioma

Congenital polycythemias have diverse etiologies, including mutations in the hypoxia sensing pathway. These include HIF2A at exon 12, VHL gene (Chuvash polycythemia), and PHD2 mutations, which in one family was also associated with recurrent pheochromocytoma/paraganglioma (PHEO/PGL). Over the past two decades, we have studied seven unrelated patients with sporadic congenital polycythemia who subsequently developed PHEO/PGL with, until now, no discernible molecular basis. We now report a polycythemic patient with a novel germline HIF2A (F374Y) (exon 9) mutation, inherited from his mother, who developed PHEO/PGL. We show that this is a gain-of-function mutation and demonstrate no loss-of-heterozygosity or additional somatic mutation of HIF2A in the tumor, indicating HIF2A (F374Y) may be predisposing rather than causative of PHEO/PGL. This report, in view of two other concomitantly reported PHEO/PGL patients with somatic mutations of HIF2A and polycythemia, underscores the PHEO/PGL-promoting potential of mutations of HIF2A that alone are not sufficient for PHEO/PGL development.

EPO receptor gain-of-function causes hereditary polycythemia, alters CD34 cell differentiation and increases circulating endothelial precursors

Background

Gain-of-function of erythropoietin receptor (EPOR) mutations represent the major cause of primary hereditary polycythemia. EPOR is also found in non-erythroid tissues, although its physiological role is still undefined.

Methodology/Principal Findings

We describe a family with polycythemia due to a heterozygous mutation of the EPOR gene that causes a G→T change at nucleotide 1251 of exon 8. The novel EPOR G1251T mutation results in the replacement of a glutamate residue by a stop codon at amino acid 393. Differently from polycythemia vera, EPOR G1251T CD34⁺ cells proliferate and differentiate towards the erythroid phenotype in the presence of minimal amounts of EPO. Moreover, the affected individuals show a 20-fold increase of circulating endothelial precursors. The analysis of erythroid precursor membranes demonstrates a heretofore undescribed accumulation of the truncated EPOR, probably due to the absence of residues involved in the EPO-dependent receptor internalization and degradation. Mutated receptor expression in EPOR-negative cells results in EPOR and Stat5 phosphorylation. Moreover, patient erythroid precursors present an increased activation of EPOR and its effectors, including Stat5 and Erk1/2 pathway.

Conclusions/Significance

Our data provide an unanticipated mechanism for autosomal dominant inherited polycythemia due to a heterozygous EPOR mutation and suggest a regulatory role of EPO/EPOR pathway in human circulating endothelial precursors homeostasis.

Advances in understanding the pathogenesis of primary familial and congenital polycythaemia

Primary familial and congenital polycythemia (PFCP) is an autosomal-dominant proliferative disorder characterized by erythrocytosis and hypersensitivity of erythroid progenitors to erythropoietin (Epo). Several lines of evidence suggest a causal role of truncated erythropoietin receptor (EpoR) in this disease. In this review, we discuss PFCP in the context of erythrocytosis and EpoR signalling. We focus on recent studies describing mechanisms underlying Epo-dependent EpoR down-regulation. One mechanism depends on internalization mediated through the p85 regulatory subunit of the Phosphoinositide 3-Kinase, and the other utilizes ubiquitin-based proteasomal degradation. Truncated PFCP EpoRs are not properly down-regulated upon stimulation, underscoring the importance of these mechanisms in the pathogenesis of PFCP.

The genetic basis of myeloproliferative disorders

For many decades, myeloproliferative disorders (MPD) were largely neglected orphan diseases. The conceptual work of William Dameshek in 1951 provided the basis for understanding MPD as a continuum of related syndromes, possibly with a common pathogenetic cause. Recognition of the clonal origin of peripheral blood cells in MPD in 1976 and the ability to grow erythroid colonies in vitro in the absence of added growth factors in 1974 initiated the search for genetic alterations that might be responsible for myeloproliferation. Mutations in the genes for the erythropoietin receptor, thrombopoietin and the von Hippel-Lindau protein were found to cause familial syndromes resembling MPD, but despite their phenotypic similarities, none of these mutations were later found in patients with the sporadic form of MPD. The discovery of activating mutations in the Janus kinase 2 (JAK2) in most patients with MPD has fully transformed and energized the MPD field. Sensitive assays for detecting the JAK2-V617F mutation have become an essential part of the diagnostic work-up, and JAK2 now constitutes a prime target for developing specific inhibitors for the treatment of patients with MPD. Despite this progress, many questions remain unsolved, including how a single JAK2 mutation causes three different MPD phenotypes, what other genes might be involved in the pathogenesis, and what are the factors determining the progression to acute leukemia.

Lessons from familial myeloproliferative disorders

By definition, myeloproliferative disorders (MPDs) are caused by an acquired somatic mutation of a hematopoietic progenitor/stem cell and have sporadic occurrence. However, well-documented families exist with first-degree relatives acquiring one or several MPDs. It is reasonable to assume that the germ-line mutation(s) or genetic background must facilitate or predispose for one or several somatic mutation(s) that lead to the MPD that is indistinguishable from the sporadic form. This is best documented in familial polycythemia vera (PV), which appears to be inherited as an autosomal dominant disorder with incomplete penetrance. However, there are also families wherein members develop any combination of MPDs, including PV, essential thrombocythemia (ET), chronic myelocytic leukemia (CML), and idiopathic myelofibrosis (IMF). A separate group of familial diseases is the familial thrombocythemias, wherein germ-line mutations in the genes for thrombopoietin or its receptor, MPL, cause polyclonal hereditary thrombocythemia, which may be clinically indistinguishable from ET. Patients with the congenital polycythemic condition "primary familial and congenital polycythemia" (PFCP) have characteristically decreased erythropoietin (Epo) levels similar to PV, hypersensitive erythroid progenitors, and low Epo levels; as such, this condition is often confused with PV. Therefore, PFCP will also be discussed here, while other congenital polycythemic states such as the Chuvash polycythemia that have elevated or inappropriately normal Epo levels will be omitted from this review in view of their distinct phenotype and unique laboratory features.

Erythropoietin hypersensitivity in primary familial and congenital polycythemia: Role of tyrosines Y285 and Y344 in erythropoietin receptor cytoplasmic domain

Erythropoietin receptor (EPOR) gene mutations leading to truncations of the cytoplasmic, carboxy-terminal region of EPOR have been described in some patients with primary familial and congenital polycythemia (PFCP), a disorder characterized by isolated erythrocytosis and increased sensitivity of erythroid progenitors to Epo. We studied the role of EPOR in the pathogenesis of PFCP and the requirement for intracytoplasmic tyrosine residues Y285 and Y344 in generation of Epo hypersensitivity phenotype. Interleukin-3-dependent hematopoietic cells were engineered to express variant human EPORs using retrovirus-mediated gene transfer. We introduced tyrosine to phenylalanine substitutions in EPOR-ME, a naturally occurring, mutant human EPOR (G5881T), truncated by 110 carboxy-terminal amino acids and associated with autosomal dominantly inherited PFCP. Cells expressing EPOR-ME exhibited increased Epo sensitivity compared to cells expressing wild type EPOR. Mutation of Y285 alone had a relatively minor effect on Epo hypersensitivity whereas mutation of Y344 resulted in loss of increased Epo sensitivity. Expression of a tyrosine-null truncated EPOR conferred further decrease of Epo-mediated proliferation suggesting that both Y285 and Y344 may contribute to proliferation signals. In the context of EPOR-ME, Y344 was required for Epo-induced Stat5 tyrosine phosphorylation. The positive effect of either Y285 or Y344 on cellular proliferation was associated with Epo-induced tyrosine phosphorylation of Stat1. These findings suggest that both tyrosine residues Y285 and Y344 in the cytoplasmic domain of EPOR-ME may contribute to increased Epo sensitivity that is characteristic of PFCP phenotype.

Molecular pathogenesis of Philadelphia chromosome negative myeloproliferative disorders

We summarize the current knowledge on molecular alterations in myeloproliferative disorders (MPD), in particular altered in vitro responses of progenitor cells, cytokine signaling, gene expression patterns and genetic lesions. Newly characterized markers, such as altered expression of polycythemia rubra vera-1 (PRV-1) and the thrombopoietin receptor (c-MPL) as well as deletions on chromosome 20q (del20q) and loss of heterozygosity on chromosome 9p (9pLOH) provide an opportunity to diagnose and identify subpopulations of MPD patients. Furthermore, we review familial syndromes that share phenotypic features with sporadic MPD. In some of these families, mutations in the genes for thrombopoietin (TPO), c-MPL, EPO-receptor and the von Hippel-Lindau (VHL) gene have been shown to cause the disease. However, in the majority of familial cases the molecular causes remain unknown. Some of these families display clonal hematopoiesis and other features previously only found in sporadic MPD. Elucidating the molecular defect(s) in these pedigrees will likely be relevant for understanding sporadic MPD pathogenesis.

Childhood polycythemias/erythrocytoses: classification, diagnosis, clinical presentation, and treatment

Polycythemias or erythrocytoses in childhood and adolescence are very rare. Systematic data on the clinical presentation and laboratory evaluations as well as on treatment regimens are sparse. The diagnostic program in absolute erythrocytosis includes extensive clinical, hematological, biochemical, and molecular biological examinations which should be applied following a stepwise algorithm. Absolute erythrocytoses are usually subdivided into primary and secondary forms. Primary erythrocytosis is a condition in which the erythropoietic compartment is expanding independently of extrinsic influences or by responding inadequately to them. Primary erythrocytoses include primary familial and congenital polycythemia (PFCP) due to mutations of the erythropoietin (Epo) receptor gene and the myeloproliferative disorder polycythemia vera. Secondary erythrocytoses are driven by hormonal factors (predominantly by Epo) extrinsic to the erythroid compartment. The increased Epo secretion may represent either a physiologic response to tissue hypoxia, an abnormal autonomous Epo production, or a dysregulation of the oxygen-dependent Epo synthesis. Congenital secondary erythrocytoses are caused, e.g., by hemoglobin variants with increased oxygen affinity, by 2,3-bisphosphoglycerate deficiency, or by mutations in the von Hippel-Lindau gene associated with a disturbed oxygen-dependent regulation of Epo synthesis.

Familial and congenital polycythemias: a diagnostic approach

The rare absolute polycythemias with an innate and hereditary character can be grouped together under the heading "familial and congenital polycythemias" (FCPs). Primary forms, due to an intrinsic defect in the erythroid progenitor cells, and secondary forms, resulting from extrinsic factors such as an elevated erythropoietin level, have both been reported. Despite the widely divergent characteristics of the different FCPs, the range of possible diagnoses is much more restricted and the distribution of disorders markedly different compared with polycythemias in general. Therefore, in FCP, one can argue against following the algorithm of the Polycythemia Vera Study Group for the evaluation of an elevated hematocrit level, following instead a more specific algorithm. In this article the authors describe a child with primary FCP, review the different FCPs, and propose an adapted work-up scheme.

Possible primary familial and congenital polycythemia locus at 7q22.1-7q22.2

Primary familial and congenital polycythemia (PFCP), inherited as an autosomal dominant trait, has been reported to be associated with mutations in the gene encoding the erythropoietin receptor (EpoR). The clinical features include the presence of isolated erythrocytosis, low erythropoietin (Epo) levels, normal hemoglobin-oxygen dissociation curve, hypersensitivity of erythroid progenitors to exogenous Epo in vitro and no progression to leukemia or myelodysplastic syndrome. Less than 15% of PFCP families have an identifiable EPOR mutation. Abnormalities of other genes are therefore likely responsible for the phenotype of the majority PFCP patients. In this study we report a family segregating PFCP with an autosomal dominant pattern of inheritance, where 7 of 14 members of the family were affected in four generations. This family was studied previously and an EPOR mutation was ruled out by sequencing and by genetic means. Here, we confirmed by linkage analysis that the disease phenotype was not linked to the Epo and EPOR genes. We then performed a genomewide screen with 410 polymorphic markers at average spacing 7.67 cM to locate the chromosomal region responsible for PFCP. We identified a region in 7q22.1-7q22.2 with a suggestive LOD score of 1.84, from our data this is the most likely location of a candidate region responsible for PFCP in this family.

[Inherited polycythemia]

PURPOSE

The majority of polycythemias occurs sporadically without any other familial case. Very occasionally polycythemia finds a familial support. This work is a review of the actual knowledge about inherited polycythemias.

CURRENT KNOWLEDGE AND KEY POINTS

Although polycythemias linked to a anomaly of the haemoglobin affinity for oxygen are well understood (haemoglobin mutants with high oxygen affinity, 2-3 Diphosphoglycerate deficiency and methemoglobinemia), so called primary polycythemias (above all primary familial and congenital polycythemias) just begin to find an explanation for ten years (erythropoietin receptor gene mutation).

FUTURE PROSPECTS AND PROJECTS

Progressively, the part of really idiopathic polycythemias is smaller and smaller. Although most of mechanisms to explain congenital polycythemias are understood, some of them are still unresolved (Chuvash polycythemia, the majority of primary familial and congenital polycythemias).

Discrimination of polycythemias and thrombocytoses by novel, simple, accurate clonality assays and comparison with PRV-1 expression and BFU-E response to erythropoietin

Essential thrombocythemia (ET) and polycythemia vera (PV) are clonal myeloproliferative disorders that are often difficult to distinguish from other causes of elevated blood cell counts. Assays that could reliably detect clonal hematopoiesis would therefore be extremely valuable for diagnosis. We previously reported 3 X-chromosome transcription-based clonality assays (TCAs) involving the G6PD, IDS, and MPP1 genes, which together were informative in about 65% of female subjects. To increase our ability to detect clonality, we developed simple TCA for detecting the transcripts of 2 additional X-chromosome genes: Bruton tyrosine kinase (BTK) and 4-and-a-half LIM domain 1 (FHL1). The combination of TCA established the presence or absence of clonal hematopoiesis in about 90% of female subjects. We show that both genes are subject to X-chromosome inactivation and are polymorphic in all major US ethnic groups. The 5 TCAs were used to examine clonality in 46 female patients along with assays for erythropoietin-independent erythroid colonies (EECs) and granulocyte PRV-1 mRNA levels to discriminate polycythemias and thrombocytoses. Of these, all 19 patients with familial polycythemia or thrombocytosis had polyclonal hematopoiesis, whereas 22 of 26 patients with clinical evidence of myeloproliferative disorder and 1 patient with clinically obscure polycythemia were clonal. Interestingly, interferon alpha therapy in 2 patients with PV was associated with reversion of clonal to polyclonal hematopoiesis. EECs were observed in 14 of 14 patients with PV and 4 of 12 with ET, and increased granulocyte PRV-1 mRNA levels were found in 9 of 13 patients with PV and 2 of 12 with ET. Thus, these novel clonality assays are useful in the diagnosis and follow-up of polycythemic conditions and disorders with increased platelet levels.

Mutations in the VHL gene in sporadic apparently congenital polycythemia

The congenital polycythemic disorders with elevated erythropoietin (Epo) have been until recently an enigma, and abnormality in the hypoxia-sensing pathway has been hypothesized as a possible mechanism. The tumor suppressor von Hippel-Lindau (VHL) participates in the hypoxia-sensing pathway, as it binds to the proline-hydroxylated form of the hypoxia-inducible factor 1alpha (HIF-1alpha) and mediates its ubiquitination and proteosomal degradation. The loss of VHL function may result in the accumulation of HIF-1alpha and overproduction of HIF-1 downstream target genes including Epo. VHL syndrome is an autosomal dominant disorder predisposing to the development of tumors, due to inherited mutations in the VHL gene. Some rare patients with VHL syndrome have polycythemia, which has been attributed to Epo production by a tumor. It was recently found that homozygosity for the VHL Arg200Trp mutation is the cause of Chuvash polycythemia, an autosomal recessive polycythemic disorder characterized by elevated serum Epo and hypersensitivity of erythroid cells to Epo. We evaluated the role of VHL in 8 children with a history of polycythemia and an elevated serum Epo level and found 3 different germline VHL mutations in 4 of them. One child was homozygous for the Arg200Trp VHL mutation, and another compound heterozygous for the Arg200Trp and the Val130Leu mutations. Two children (siblings) were heterozygous for an Asp126Tyr mutation, one of them fulfilling some criteria of VHL syndrome. We propose that mutations of the VHL gene represent an important cause of pediatric sporadic polycythemias with an inappropriately high serum Epo concentration.

A novel mutation in the erythropoietin receptor gene is associated with familial erythrocytosis

Primary familial erythrocytosis (familial polycythemia) is a rare myeloproliferative disorder with an autosomal dominant mode of inheritance. We studied a new kindred with autosomal dominantly inherited familial erythrocytosis. The molecular basis for the observed phenotype of isolated erythrocytosis is heterozygosity for a novel nonsense mutation affecting codon 399 in exon 8 of the erythropoietin receptor (EPOR) gene, encoding an EpoR peptide that is truncated by 110 amino acids at its C-terminus. The new EPOR gene mutation 5881G>T was found to segregate with isolated erythrocytosis in the affected family and this mutation represents the most extensive EpoR truncation reported to date, associated with familial erythrocytosis. Erythroid progenitors from an affected individual displayed Epo hypersensitivity in in vitro methylcellulose cultures, as indicated by more numerous erythroid burst-forming unit-derived colonies in low Epo concentrations compared to normal controls. Expression of mutant EpoR in interleukin 3-dependent hematopoietic cells was associated with Epo hyperresponsiveness compared to cells expressing wild-type EpoR.

Genetic heterogeneity of primary familial and congenital polycythemia

Primary familial and congenital polycythemia (PFCP) is an inherited disorder of erythroid progenitor cells resulting in elevated erythrocyte mass. Several mutations of the erythropoietin receptor (EPOR) gene have been associated with PFCP, although in a few families the linkage between the EPOR gene and PFCP has been excluded. To examine the role of EPOR mutations in the pathogenesis of PFCP, we studied 43 unrelated PFCP subjects. Erythroid culture data were available in 26 subjects, and in all these subjects, we observed hypersensitivity of erythroid progenitors to erythropoietin (EPO). We screened all EPOR gene exons for mutations using ribonuclease cleavage assay and protein truncation test. We detected five mutations in exon VIII of the EPOR gene, four of which we reported earlier. A new EPOR gene mutation was found (G5959T) that changes codon 425 GAG to a termination codon, resulting in truncation of the EPOR by 84 amino acids. The G5959T mutation was found to segregate with the disease in the affected family and represents another example of a nonsense mutation associated with PFCP. We also report the first intronic mutation (A2706T) of the EPOR gene. The finding of only five disease-causing mutations in our PFCP patient pool of 43 subjects (12%) indicates that EPOR gene mutations are not the major genetic defect associated with PFCP. The hypersensitivity of erythroid progenitors to EPO seen in all examined PFCP subjects suggests a dominant lesion of an as yet unidentified gene either at the level of the EPOR signaling pathway or another erythropoiesis regulating pathway.

Increased cell surface expression of C-terminal truncated erythropoietin receptors in polycythemia

Primary familial and congenital polycythemia (PFCP) is a disorder characterized by an increased number of erythrocytes despite normal blood oxygen pressure and a normal serum erythropoietin (EPO) level. Recent studies revealed that erythroid progenitor cells from certain individuals with PFCP express various forms of EPO receptor (EPOR) truncated at the terminal carboxyl site (EPOR-TTC(PFCP)). EPOR-TTC(PFCP) can transmit EPO-mediated proliferative signals more efficiently than can full-length EPOR (EPOR-F), at least partly because of defective recruitment of SHP-1 phosphatase to these receptors. In agreement with previous studies, Ba/F3 transfectants expressing EPOR-TTC(PFCP) showed higher proliferative responses to EPO. In those transfectants, we found that EPOR-TTC(PFCP) was expressed more abundantly on the cell surface than was EPOR-F. This tendency was confirmed by a transient-expression experiment using COS7 cells. Since expression levels of EPOR protein were not significantly different among these transfectants, differences in cell surface expression were likely dependent on post-translational mechanism(s). In addition to defective recruitment of SHP-1 to EPOR-TTC(PFCP), more efficient transport and expression on the cell surface appear to serve as mechanisms responsible for increased EPO-responsiveness of erythroid progenitor cells in PFCP.

Mouse model of congenital polycythemia: Homologous replacement of murine gene by mutant human erythropoietin receptor gene

Mutations causing truncations of the cytoplasmic domain of the human erythropoietin receptor (EPOR) result in a dominantly inherited disorder-primary familial congenital polycythemia. This disorder is characterized by increased numbers of erythrocytes (polycythemia) and by in vitro hypersensitivity of erythroid precursors to erythropoietin. The consequences of EPOR truncation in nonerythroid tissues are unknown. We replaced the murine EPOR gene with a wild-type human EPOR gene and a mutant human EPOR gene that we initially identified in a patient with polycythemia. This mutation leads to an EPOR truncated after the first tyrosine residue of the intracellular domain. Mice heterozygous for this mutant allele and a wild-type human EPOR allele mimicked the human disorder. Interestingly, mice that were homozygous for the mutant human allele were severely polycythemic but viable. Our results provide a model for functional studies of EPOR-triggered signaling pathways in erythropoiesis. These animals can now be used to investigate the molecular pathophysiology of this gain-of-function EPOR mutation in erythroid tissue and in those nonerythroid tissues that express EPOR.

Congenital and inherited polycythemia

Absolute polycythemia is a condition with increased red blood cell mass. There are a number of primary and secondary polycythemic disorders leading to absolute polycythemia. Primary polycythemias are caused by a defect intrinsic to the erythroid progenitor cells. The best characterized primary polycythemia is the autosomal dominant primary familial and congenital polycythemia (PFCP). Familial or childhood occurrence of the myeloproliferative disorder polycythemia vera are also discussed, emphasizing the importance of distinction between polycythemia vera and PFCP. Congenital or familial secondary polycythemic conditions are characterized by increased red cell mass, which is caused by circulating serum factors, typically erythropoietin.

Signal transduction in the erythropoietin receptor system

Events relayed via the single transmembrane receptor for erythropoietin (Epo) are essential for the development of committed erythroid progenitor cells beyond the colony-forming unit-erythroid stage, and this clearly involves Epo's inhibition of programmed cell death (PCD). Less well resolved, however, are issues regarding the precise nature of Epo-dependent antiapoptotic mechanisms, the extent to which Epo might also promote mitogenesis and/or terminal erythroid differentiation, and the essential vs modulatory nature of certain Epo receptor cytoplasmic subdomains, signal transducing factors, and downstream pathways. Accordingly, this review focuses on the following aspects of Epo signal transduction: (1) Epo receptor/Jak2 activation mechanisms; (2) the critical vs dispensable nature of (P)Y sites and SH2 domain-encoding effectors in survival, growth, and differentiation responses; (3) primary mechanisms by which Epo inhibits PCD; (4) the integration of signals relayed by coexpressed and possibly directly interacting cytokine receptors; and (5) predictions regarding effector function which are provided by the association of certain primary and familial polycythemias with mutated human Epo receptor forms.

Abnormalities of cytokine receptor signalling contributing to diseases of red blood cell production

The production of erythroid cells is a dynamic and exquisitely regulated process. The mature red cell is only the final phase of a complex but orderly series of genetic events that are initiated at the time a multipotent stem cell becomes committed to expressing the erythroid programme. Aberrations either in the intrinsic generation and/or amplification of functional erythroid cells or in the regulatory influences of microenvironment or cytokines form the basis for a number of blood diseases. In this review we focus upon abnormalities in red blood cell production and discuss how alterations in cytokine regulation of red blood cell production may contribute to these disease processes. We discuss clinical states in which blood red cell numbers are altered, including primary familial and congenital polycythaemia, the myeloproliferative disorder polycythaemia vera, erythroleukaemia, and Diamond-Blackfan anaemia. These disorders are briefly described and evidence supporting a potential role of specific cytokine receptor signalling defects as contributing to these phenotypes is discussed.

Haematopoietic progenitors and signal transduction in polycythaemia vera and primary thrombocythaemia

While significant progress has been made in understanding the cellular defect and molecular basis of polycythaemia vera (PV), elucidation of the primary mutation leading to PV remains elusive. While clinically useful, the PV diagnostic criteria put forward by the Polycythemia Vera Study Group are not based on the pathophysiology of this disorder and in some instances may lead to false diagnosis or may not be sufficient to diagnose an early PV. In diagnostically unclear situations, clinical and laboratory findings must take into account the acquired nature of PV, its clonality, and the presence of endogenous erythroid colony formation in serum-containing media. It is likely that other simpler assays may be developed based on the rapidly emerging knowledge of the cellular pathology of PV. Several intriguing observations of abnormalities pertaining to the erythroid signal transduction have been recently reported; these remain to be validated in other laboratories and to be proven specific for PV. The clinical concept of primary thrombocythaemia (PT) lags behind what we know about PV. While the diagnosis of PT is still based on the exclusion of other known causes of thrombocytosis, new knowledge is emerging. Recent clonality studies of a large number of PT females show that the majority are clonal. It is our belief that thrombocythaemic subjects who are not found to be clonal are those with secondary thrombocytosis. Multiple in vitro-based assays of megakaryocytic and erythroid progenitors have been developed and conflicting data published. It is likely that standardized assays of megakaryocytic progenitors will soon become available and a reproducible PT specific defect will be found. Such a specific test would be of immense diagnostic value in this most elusive of all myeloproliferative disorders.