Molecular study of congenital erythrocytosis in 70 unrelated patients revealed a potential causal mutation in less than half of the cases (Where is/are the missing gene(s)?)

Idiopathic erythrocytosis: A diagnostic and management challenge with emerging areas for exploration

Despite published algorithms for approaching the work-up of erythrocytosis, a significant proportion of patients are left with uncertainty as to its aetiology and prognosis. The term 'idiopathic erythrocytosis' (IE) is applied when known primary and secondary aetiologies have been ruled out. However, the assignment of secondary aetiologies is not always straightforward or evidence based, which can lead to misdiagnosis and heterogeneity in cohort studies. Furthermore, new studies have identified germline or somatic mutations that may affect prognosis. Epidemiological and cohort data are inconsistent as to whether IE increases the risk for complications such as arterial and venous thromboembolism, clonal transformation or comorbid conditions. Randomized trials assessing the role of phlebotomy for long-term management of IE have not been performed, so treatment remains a vexing problem for clinicians. Standardization of terminology and testing strategies, including comprehensive genetic screening in clinical research, are key to refining our understanding of IE.

A Screening Approach for Inherited Erythrocytosis due to the VHL:c.598C > T Mutation (Chuvash Polycythemia)

Genetic work-up of unexplained erythrocytosis that is suspected to be inherited in nature currently requires either laborious exon-by-exon gene panel testing by Sanger sequencing or expensive next-generation sequencing. A high prevalence of Chuvash polycythemia (61%) has been previously reported among north Indian erythrocytosis patients. We assessed PCR-RFLP for VHL c.598C > T mutation as a first-line test in 99 persons with JAK2 V617F-negative, unexplained erythrocytosis. We enrolled two groups: Group A (n = 38) had erythrocytosis patients (n = 33) or their first-degree relatives (n = 5), and, Group B with 61 healthy blood donation volunteers who were deferred after the discovery of unexplained high hemoglobin levels. Detailed history and clinical examination, hemogram, erythropoietin levels and PCR-RFLP for the VHL:c.598C > T;p.R200W mutation were done. In Group A, three (8%) persons aged 9, 13 and 30-years were homozygous for VHL:c.598C > T. Two were heterozygous (parents of a known case of Chuvash polycythemia). None of the Group B subjects had the Chuvash mutation. Erythropoietin levels in group A were low in 5/26 cases (19%) and normal in 18/26 (69%). In Group B, seven (11%) donors had normal values while the remaining 54 (89%) had high erythropoietin levels. Despite a lower frequency (8%) compared to literature, our results suggest that the relatively simpler PCR-RFLP for VHL:c.598C > T mutation may be considered for the initial genetic screening of unexplained, suspected congenital erythrocytosis in regions where Chuvash polycythemia comprises a large proportion of inherited erythrocytosis, after polycythemia vera and common acquired secondary causes are excluded.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12288-023-01668-9.

Heterozygosity for bisphosphoglycerate mutase deficiency expressing clinically as congenital erythrocytosis: A case series and literature review

Erythrocytosis is associated with increased red blood cell mass and can be either congenital or acquired. Congenital secondary causes are rare and include germline variants increasing haemoglobin (Hb)-oxygen affinity (e.g., Hb or bisphosphoglycerate mutase (BPGM) variants) or affecting oxygen-sensing pathway proteins. Here, we describe five adults from three kindreds with erythrocytosis associated with heterozygosity for BPGM variants, including one novel. Functional analyses showed partial BPGM deficiency, reduced 2,3-bisphosphoglycerate levels and/or increased Hb-oxygen affinity. We also review currently known BPGM variants. This study contributes to raising awareness of BPGM variants, and in particular that heterozygosity for BPGM deficiency may already manifest clinically.

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.

Identification of Variants Associated With Rare Hematological Disorder Erythrocytosis Using Targeted Next-Generation Sequencing Analysis

An erythrocytosis is present when the red blood cell mass is increased, demonstrated as elevated hemoglobin and hematocrit in the laboratory evaluation. Congenital predispositions for erythrocytosis are rare, with germline variants in several genes involved in oxygen sensing (VHL, EGLN1, and EPAS1), signaling for hematopoietic cell maturation (EPOR and EPO), and oxygen transfer (HBB, HBA1, HBA2, and BPGM) that were already associated with the eight congenital types (ECYT1–8). Screening for variants in known congenital erythrocytosis genes with classical sequencing approach gives a correct diagnosis for only up to one-third of the patients. The genetic background of erythrocytosis is more heterogeneous, and additional genes involved in erythropoiesis and iron metabolism could have a putative effect on the development of erythrocytosis. This study aimed to detect variants in patients with yet unexplained erythrocytosis using the next-generation sequencing (NGS) approach, targeting genes associated with erythrocytosis and increased iron uptake and implementing the diagnostics of congenital erythrocytosis in Slovenia. Selected 25 patients with high hemoglobin, high hematocrit, and no acquired causes were screened for variants in the 39 candidate genes. We identified one pathogenic variant in EPAS1 gene and three novel variants with yet unknown significance in genes EPAS1, JAK2, and SH2B3. Interestingly, a high proportion of patients were heterozygous carriers for two variants in HFE gene, otherwise pathogenic for the condition of iron overload. The association between the HFE variants and the development of erythrocytosis is not clearly understood. With a targeted NGS approach, we determined an actual genetic cause for the erythrocytosis in one patient and contributed to better management of the disease for the patient and his family. The effect of variants of unknown significance on the enhanced production of red blood cells needs to be further explored with functional analysis. This study is of great significance for the improvement of diagnosis of Slovenian patients with unexplained erythrocytosis and future research on the etiology of this rare hematological disorder.

Diagnosis and management of non-clonal erythrocytosis remains challenging: a single centre clinical experience

Erythrocytosis has a diverse background. While polycythaemia vera has well defined criteria, the diagnostic approach and management of other types of erythrocytosis are more challenging. The aim of study was to retrospectively analyse the aetiology and management of non-clonal erythrocytosis patients referred to a haematology outpatient clinic in an 8-year period using a 3-step algorithm. The first step was inclusion of patients with Hb > 185 g/L and/or Hct > 0.52 in men and Hb > 165 g/L and/or Hct > 0.48 in women on two visits ≥ two months apart, thus confirming true erythrocytosis. Secondly, polycythaemia vera was excluded and secondary causes of erythrocytosis (SE) identified. Thirdly, idiopathic erythrocytosis patients (IE) were referred to next-generation sequencing for possible genetic background evaluation. Of the 116 patients, 75 (65%) are men and 41 (35%) women, with non-clonal erythrocytosis 34/116 (29%) had SE, 15/116 (13%) IE and 67/116 (58%) stayed incompletely characterized (ICE). Patients with SE were significantly older and had significantly higher Hb and Hct compared to patients with IE. Most frequently, SE was attributed to obstructive sleep apnoea and smoking. Phlebotomies were performed in 56, 53 and 40% of patients in the SE, IE, and ICE group, respectively. Approx. 70% of patients in each group received aspirin. Thrombotic events were registered in 12, 20 and 15% of SE, IE and ICE patients, respectively. Congenital erythrocytosis type 4 (ECYT4) was diagnosed in one patient. The study demonstrates real-life management of non-clonal erythrocytosis which could be optimized using a 3-step diagnostic algorithm.

Roles of HIF and 2-Oxoglutarate-Dependent Dioxygenases in Controlling Gene Expression in Hypoxia

Simple Summary

Hypoxia—reduction in oxygen availability—plays key roles in both physiological and pathological processes. Given the importance of oxygen for cell and organism viability, mechanisms to sense and respond to hypoxia are in place. A variety of enzymes utilise molecular oxygen, but of particular importance to oxygen sensing are the 2-oxoglutarate (2-OG) dependent dioxygenases (2-OGDs). Of these, Prolyl-hydroxylases have long been recognised to control the levels and function of Hypoxia Inducible Factor (HIF), a master transcriptional regulator in hypoxia, via their hydroxylase activity. However, recent studies are revealing that such dioxygenases are involved in almost all aspects of gene regulation, including chromatin organisation, transcription and translation.

Abstract

Hypoxia—reduction in oxygen availability—plays key roles in both physiological and pathological processes. Given the importance of oxygen for cell and organism viability, mechanisms to sense and respond to hypoxia are in place. A variety of enzymes utilise molecular oxygen, but of particular importance to oxygen sensing are the 2-oxoglutarate (2-OG) dependent dioxygenases (2-OGDs). Of these, Prolyl-hydroxylases have long been recognised to control the levels and function of Hypoxia Inducible Factor (HIF), a master transcriptional regulator in hypoxia, via their hydroxylase activity. However, recent studies are revealing that dioxygenases are involved in almost all aspects of gene regulation, including chromatin organisation, transcription and translation. We highlight the relevance of HIF and 2-OGDs in the control of gene expression in response to hypoxia and their relevance to human biology and health.

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.

The pVHL neglected functions, a tale of hypoxia-dependent and -independent regulations in cancer

The von Hippel–Lindau protein (pVHL) is a tumour suppressor mainly known for its role as master regulator of hypoxia-inducible factor (HIF) activity. Functional inactivation of pVHL is causative of the von Hippel–Lindau disease, an inherited predisposition to develop different cancers. Due to its impact on human health, pVHL has been widely studied in the last few decades. However, investigations mostly focus on its role in degrading HIFs, whereas alternative pVHL protein–protein interactions and functions are insistently surfacing in the literature. In this review, we analyse these almost neglected functions by dissecting specific conditions in which pVHL is proposed to have differential roles in promoting cancer. We reviewed its role in regulating phosphorylation as a number of works suggest pVHL to act as an inhibitor by either degrading or promoting downregulation of specific kinases. Further, we summarize hypoxia-dependent and -independent pVHL interactions with multiple protein partners and discuss their implications in tumorigenesis.

Genetic variability of hypoxia-inducible factor alpha (HIFA) genes in familial erythrocytosis: Analysis of the literature and genome databases

Familial erythrocytosis (FE) is a congenital disorder, defined by elevated red blood cell number, hemoglobin, and hematocrit. Among eight types of FE, type 4 is caused by variants in the EPAS1 gene. Two other hypoxia-inducible factor alpha (HIFA) subunits, HIF1A and HIF3A, have not yet been associated with medical history of FE, but have potential role in the development of erythrocytosis. To improve diagnosis, it is crucial to identify new variants in genes involved in erythrocyte production. Published literature and data from genome browsers were used to obtain HIFA sequence variants associated with erythrocytosis and to locate them on protein sequence and regulatory sites. We retrieved 24 variants from the literature: 2 in HIF1A, 20 in EPAS1 and 2 in HIF3A gene. Sixteen out of 20 variants in the EPAS1 gene are positioned in a conserved region of 13 amino acids within exon 12, next to regulatory post-translational modification and binding sites, suggesting that EPAS1 has an important role in erythropoiesis. The role of HIF1A and HIF3A in the development of erythrocytosis should be further investigated.

Update on mutations in the HIF: EPO pathway and their role in erythrocytosis

Identification of the underlying defects in congenital erythrocytosis has provided mechanistic insights into the regulation of erythropoiesis and oxygen homeostasis. The Hypoxia Inducible Factor (HIF) pathway plays a key role in this regard. In this pathway, an enzyme, Prolyl Hydroxylase Domain protein 2 (PHD2), constitutively prolyl hydroxylates HIF-2α, thereby targeting HIF-2α for degradation by the von Hippel Lindau (VHL) tumor suppressor protein. Under hypoxia, this modification is attenuated, resulting in the stabilization of HIF-2α and transcriptional activation of the erythropoietin (EPO) gene. Circulating EPO then binds to the EPO receptor (EPOR) on red cell progenitors in the bone marrow, leading to expansion of red cell mass. Loss of function mutations in PHD2 and VHL, as well as gain of function mutations in HIF-2α and EPOR, are well established causes of erythrocytosis. Here, we highlight recent developments that show that the study of this condition is still evolving. Specifically, novel mutations have been identified that either change amino acids in the zinc finger domain of PHD2 or alter splicing of the VHL gene. In addition, continued study of HIF-2α mutations has revealed a distinctive genotype-phenotype correlation. Finally, novel mutations have recently been identified in the EPO gene itself. Thus, the cascade of genes that at a molecular level leads to EPO action, namely PHD2 - > HIF2A - > VHL - > EPO - > EPOR, are all mutational targets in congenital erythrocytosis.

An EGLN1 mutation may regulate hypoxic response in cyanotic congenital heart disease through the PHD2/HIF-1A pathway

Cyanotic congenital heart disease (CCHD), a term describing the most severe congenital heart diseases are characterized by the anatomic malformation of a right to left shunt. Although the incidence of CCHD are far less than the that of congenital heart diseases (CHD), patients with CCHD always present severe clinical features such as hypoxia, dyspnea, and heart failure. Chronic hypoxia induces hypoxemia that significantly contributes to poor prognosis in CCHD. Current studies have demonstrated that the prolyl-4-hydroxylase2 (PHD2, encoded by EGLN1)/hypoxia-inducible factor-1A (HIF-1A) pathway is a key regulator of hypoxic response. Thus, we aim to assess the associations of single polymorphisms (SNPs) of the EGLN1 gene and hypoxic response in CCHD. A missense variant of EGLN1 c.380G>C (rs1209790) was found in 46 patients (46/126), with lower hypoxia incidence and higher rate of collateral vessel formation, compared with the wild type (P < 0.05). In vitro experiments, during hypoxia, EGLN1 mutation reduced EGLN1 expression compared with the wild type, with higher HIF-1A, VEGF and EPO expression levels in the mutant. No difference in HK1 expression was observed between the mutant and wild type. CCHD patients with c.380G>C showed improved response to hypoxia compared with the wild-type counterparts. The EGLN1 c.380G>C mutation improves hypoxic response through the PHD2/HIF-1A pathway, which may provide a molecular mechanism for hypoxic response in CCHD. The effects of the EGLN1 c.380G>C mutation on CCHD prognosis deserve further investigation.

Myelodysplastic Syndrome/Acute Myeloid Leukemia Arising in Idiopathic Erythrocytosis

The term “idiopathic erythrocytosis (IE)” is applied to those cases where a causal clinical or pathological event cannot be elucidated and likely reflects a spectrum of underlying medical and molecular abnormalities. The clinical course of a patient with IE is described manifesting as a persistent erythrocytosis with a low serum erythropoietin level, mild eosinophilia, and with evidence of a thrombotic event. The patient subsequently developed a myelodysplasic syndrome (MDS) and acute myeloid leukemia (AML), an event not observed in erythrocytosis patients other than those with polycythemia vera (PV). Application of a next-generation sequencing (NGS) approach targeted for myeloid malignancies confirmed wild-type JAK2 exons 12–15 and identified a common SH2B3 W262R single-nucleotide polymorphism associated with the development of hematological features of myeloproliferative neoplasms (MPNs). Further NGS analysis detected a CBL L380P mutated clone expanding in parallel with the development of MDS and subsequent AML. Despite the absence of JAK2, MPL exon 10, or CALR exon 9 mutations, a similarity with the disease course of PV/MPN was evident. A clonal link between the erythrocytosis and AML could be neither confirmed nor excluded. Future molecular identification of the mechanisms underlying IE is likely to provide a more refined therapeutic approach.

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.

Hb Heathrow [β103(G5)Phe→Leu], a First Report in an Asian Patient with Erythrocytosis

Congenital erythrocytosis (CE) is a rare and heterogeneous disease. The high oxygen affinity hemoglobin (Hb) variants are the most common cause of CE. Herein, we report a Korean patient with isolated erythrocytosis. A 25-year-old man was referred to our hospital for evaluation of high Hb level (Hb 20.4 g/dL, hematocrit 58%, reticulocyte count 2.90%, white blood cell count 6.83×10⁹/L, and platelet count 195×10⁹/L). Bone marrow biopsy revealed normocellular marrow without myeloproliferative features. JAK2 (V617F, exon 12), CALR (exon 9), and MPL W515K/L mutations were not detected. P₅₀ (partial pressure at which Hb is half saturated with oxygen), which is an indicator of left-shift of oxygen dissociation curve (high oxygen affinity state), was 14.3 mm Hg (reference value 22.6-29.4 mm Hg). He was suspected to have CE. Mutation analysis of the HBB gene revealed the known Hb variant, Hb Heathrow [β103(G5)Phe→Leu]. This is the first report of Hb Heathrow in Asian.

Gene panel sequencing improves the diagnostic work-up of patients with idiopathic erythrocytosis and identifies new mutations

Erythrocytosis is a rare disorder characterized by increased red cell mass and elevated hemoglobin concentration and hematocrit. Several genetic variants have been identified as causes for erythrocytosis in genes belonging to different pathways including oxygen sensing, erythropoiesis and oxygen transport. However, despite clinical investigation and screening for these mutations, the cause of disease cannot be found in a considerable number of patients, who are classified as having idiopathic erythrocytosis. In this study, we developed a targeted next-generation sequencing panel encompassing the exonic regions of 21 genes from relevant pathways (~79 Kb) and sequenced 125 patients with idiopathic erythrocytosis. The panel effectively screened 97% of coding regions of these genes, with an average coverage of 450×. It identified 51 different rare variants, all leading to alterations of protein sequence, with 57 out of 125 cases (45.6%) having at least one of these variants. Ten of these were known erythrocytosis-causing variants, which had been missed following existing diagnostic algorithms. Twenty-two were novel variants in erythrocytosis-associated genes (EGLN1, EPAS1, VHL, BPGM, JAK2, SH2B3) and in novel genes included in the panel (e.g. EPO, EGLN2, HIF3A, OS9), some with a high likelihood of functionality, for which future segregation, functional and replication studies will be useful to provide further evidence for causality. The rest were classified as polymorphisms. Overall, these results demonstrate the benefits of using a gene panel rather than existing methods in which focused genetic screening is performed depending on biochemical measurements: the gene panel improves diagnostic accuracy and provides the opportunity for discovery of novel variants.

Erythrocytosis due to PHD2 Mutations: A Review of Clinical Presentation, Diagnosis, and Genetics

The association of mutations in the PHD2 protein of the hypoxia-sensing pathway and erythrocytosis has only been established in the last decade. Here we report the case of a novel PHD2 gene mutation in a patient with erythrocytosis and summarize all reported cases to date. Case Report. A 55-year-old man presented with dyspnea and a previous diagnosis of idiopathic erythrocytosis. PHD gene sequencing revealed a mutation on exon 2. The mutation was recognized as p.(Trp334^⁎) (c. 1001G>A) resulting in a truncation of a highly conserved amino acid residue in catalytic domain. A diagnosis of erythrocytosis secondary to mutant PHD2 gene was made. Conclusions. Our findings indicate that with PHD2 mutations there is moderate erythrocytosis and erythropoietin (Epo) levels are generally low to normal. Two patients with PHD2 substitution mutations were found to have paraganglioma and one of these patients had a concurrent pheochromocytoma. In addition, one mutation was associated with sagittal sinus thrombosis. Given the severity of some of the clinical features of these mutations, we conclude that clinical guidelines should include the PHD2 mutation in the idiopathic erythrocytosis workup.

A Japanese Family with Congenital Erythrocytosis Caused by Haemoglobin Bethesda

We herein present a case of congenital erythrocytosis caused by haemoglobin (Hb) Bethesda in a Japanese family. A 55-year-old asymptomatic man was referred to our hospital for the investigation of erythrocytosis, which was present in other members of his family. The patient's serum erythropoietin level was normal, and the JAK2 V617F mutation was not detected. His P50 value was mildly decreased, thus we suspected the presence of an Hb variant with a high oxygen affinity. The high-performance liquid chromatography analysis showed an abnormal Hb, and by direct sequencing we identified the Hb Bethesda variant in this patient. For the differential diagnosis, we recommend the estimation of the P50 value as a practical and useful test.

VHL, the story of a tumour suppressor gene

Since the Von Hippel-Lindau (VHL) disease tumour suppressor gene VHL was identified in 1993 as the genetic basis for a rare disorder, it has proved to be of wide medical and scientific interest. VHL tumour suppressor protein (pVHL) plays a key part in cellular oxygen sensing by targeting hypoxia-inducible factors for ubiquitylation and proteasomal degradation. Early inactivation of VHL is commonly seen in clear-cell renal cell carcinoma (ccRCC), and insights gained from the functional analysis of pVHL have provided the foundation for the routine treatment of advanced-stage ccRCC with novel targeted therapies. However, recent sequencing studies have identified additional driver genes that are involved in the pathogenesis of ccRCC. As our understanding of the importance of VHL matures, it is timely to review progress from its initial description to current knowledge of VHL biology, as well as future prospects for novel medical treatments for VHL disease and ccRCC.

An integrated computational approach can classify VHL missense mutations according to risk of clear cell renal carcinoma

Mutations in the von Hippel-Lindau (VHL) gene are pathogenic in VHL disease, congenital polycythaemia and clear cell renal carcinoma (ccRCC). pVHL forms a ternary complex with elongin C and elongin B, critical for pVHL stability and function, which interacts with Cullin-2 and RING-box protein 1 to target hypoxia-inducible factor for polyubiquitination and proteasomal degradation. We describe a comprehensive database of missense VHL mutations linked to experimental and clinical data. We use predictions from in silico tools to link the functional effects of missense VHL mutations to phenotype. The risk of ccRCC in VHL disease is linked to the degree of destabilization resulting from missense mutations. An optimized binary classification system (symphony), which integrates predictions from five in silico methods, can predict the risk of ccRCC associated with VHL missense mutations with high sensitivity and specificity. We use symphony to generate predictions for risk of ccRCC for all possible VHL missense mutations and present these predictions, in association with clinical and experimental data, in a publically available, searchable web server.

The role of PHD2 mutations in the pathogenesis of erythrocytosis

The transcription of the erythropoietin (EPO) gene is tightly regulated by the hypoxia response pathway to maintain oxygen homeostasis. Elevations in serum EPO level may be reflected in an augmentation in the red cell mass, thereby causing erythrocytosis. Studies on erythrocytosis have provided insights into the function of the oxygen-sensing pathway and the critical proteins involved in the regulation of EPO transcription. The α subunits of the hypoxia-inducible transcription factor are hydroxylated by three prolyl hydroxylase domain (PHD) enzymes, which belong to the iron and 2-oxoglutarate-dependent oxygenase superfamily. Sequence analysis of the genes encoding the PHDs in patients with erythrocytosis has revealed heterozygous germline mutations only occurring in Egl nine homolog 1 (EGLN1, also known as PHD2), the gene that encodes PHD2. To date, 24 different EGLN1 mutations comprising missense, frameshift, and nonsense mutations have been described. The phenotypes associated with the patients carrying these mutations are fairly homogeneous and typically limited to erythrocytosis with normal to elevated EPO. However, exceptions exist; for example, there is one case with development of concurrent paraganglioma (PHD2-H374R). Analysis of the erythrocytosis-associated PHD2 missense mutations has shown heterogeneous results. Structural studies reveal that mutations can affect different domains of PHD2. Some are close to the hypoxia-inducible transcription factor α/2-oxoglutarate or the iron binding sites for PHD2. In silico studies demonstrate that the mutations do not always affect fully conserved residues. In vitro and in cellulo studies showed varying effects of the mutations, ranging from mild effects to severe loss of function. The exact mechanism of a potential tumor-suppressor role for PHD2 still needs to be elucidated. A knockin mouse model expressing the first reported PHD2-P317R mutation recapitulates the phenotype observed in humans (erythrocytosis with inappropriately normal serum EPO levels) and demonstrates that haploinsufficiency and partial deregulation of PHD2 is sufficient to cause erythrocytosis.

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.