👪

Inheritance

1

Autosomal dominant HP:0000006

Autosomal dominant. Carriers are heterozygous for a single gain-of-function EPAS1 allele, which is sufficient to cause the erythrocytosis; the variant co-segregates with the phenotype across generations.

Autosomal dominant inheritance

Show evidence (1 reference)

PMID:31350093 SUPPORT REVIEW SYNTHESIS Human Clinical

"Familial erythrocytosis caused by a gain-of-function mutation in the HIF2A gene is an autosomal dominant disorder"

States the autosomal dominant inheritance of HIF2A/EPAS1 familial erythrocytosis (ECYT4).

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Pathophysiology

5

Germline EPAS1/HIF-2-alpha Gain-of-Function Variant

A heterozygous germline missense variant in EPAS1 alters a residue in or near the oxygen-dependent degradation domain of HIF-2-alpha, close to the prolyl-hydroxyl-acceptor Pro531 (reported changes include G537W, G537R, M535V, M535I, M535T and P534L). The variants are gain-of-function: they impair recognition of HIF-2-alpha by the PHD2 hydroxylase and by the von Hippel-Lindau protein. Functional characterisation across these alleles shows a common endpoint of impaired degradation despite distinct effects on PHD2 and VHL interaction.

EPAS1 hgnc:3374

Genetic context variant_origin: GERMLINE functional_impact_category: GAIN_OF_FUNCTION

HIF-2-alpha transcription factor activity GO:0000981 ⇑ GAIN OF FUNCTION

Downstream

HIF-2-alpha Escape from Degradation and Stabilization Impaired Pro531 hydroxylation and VHL binding stop HIF-2-alpha being marked for proteasomal destruction.

Show evidence (2 references)

PMID:18184961 SUPPORT Human Clinical

"We describe a family with erythrocytosis and a mutation in the HIF2A gene, which encodes the HIF-2alpha protein"

The sentinel report identifying a germline HIF2A/EPAS1 variant as the cause of familial erythrocytosis.

PMID:19208626 SUPPORT In Vitro

"We find that all result in impaired degradation and thus aberrant stabilization of HIF-2alpha. However, each exhibits a distinct profile with respect to their effects on PHD2 binding and von Hippel Lindau interaction"

Functional characterisation of G537R, M535V and P534L showing the common gain-of-function endpoint (impaired degradation) with allele-specific effects on PHD2 and VHL binding.

HIF-2-alpha Escape from Degradation and Stabilization

HIF-2-alpha is no longer efficiently hydroxylated and ubiquitinated, so it accumulates and dimerises with HIF-1-beta (ARNT) at normal oxygen tension, activating the hypoxia transcriptional program inappropriately (pseudohypoxia).

HIF-mediated hypoxia response in normoxia GO:0001666 ⇑ GAIN OF FUNCTION

Downstream

Inappropriate Erythropoietin Transcription Stabilised HIF-2-alpha binds the hypoxia response element of the EPO gene and drives its transcription.

Show evidence (2 references)

PMID:18184961 SUPPORT In Vitro

"Our functional studies indicate that this mutation leads to stabilization of the HIF-2alpha protein and suggest that wild-type HIF-2alpha regulates erythropoietin production in adults"

Establishes that the variant stabilises HIF-2-alpha and that HIF-2-alpha is the physiological regulator of erythropoietin.

PMID:39426935 SUPPORT REVIEW SYNTHESIS Human Clinical

"HIF-2α evades steady-state hydroxylation by the prolyl hydroxylase type 2 (PHD2), which accelerates von Hippel-Lindau (VHL)-mediated proteasomal degradation. In this situation, HIF-2α is stabilized and can translocate to the nucleus"

States the hydroxylation-escape and nuclear-translocation steps that define this node when an EPAS1 gain-of-function variant is present.

Inappropriate Erythropoietin Transcription

HIF-2-alpha drives EPO gene transcription in the EPO-producing renal interstitial cells, so erythropoietin output is inappropriate for the prevailing haemoglobin. Across reported carriers the serum erythropoietin is inappropriately normal to frankly elevated.

EPO-producing kidney interstitial fibroblast CL:1000692

cellular response to hypoxia driving EPO transcription GO:0071456 ↑ INCREASED

Downstream

Erythroid Progenitor Expansion Raised erythropoietin signalling expands the erythroid compartment.

Elevated circulating erythropoietin concentration Erythropoietin is inappropriately normal to elevated for the haemoglobin.

Show evidence (2 references)

PMID:18378852 SUPPORT Human Clinical

"All patients presented at a young age with elevated serum Epo"

Reports the inappropriately elevated erythropoietin in the HIF2A/EPAS1 erythrocytosis cohort, the consequence of EPO transcriptional drive.

PMID:33840141 SUPPORT REVIEW SYNTHESIS Human Clinical

"CE has been found to be caused by loss-of-function mutations in VHL and PHD2/EGLN1 as well as gain-of-function mutations in HIF-2α (EPAS1), all resulting in constitutive activation of EPO signaling"

Places EPAS1 gain of function alongside VHL and EGLN1 loss of function as routes to the same endpoint, constitutive EPO signalling.

Erythroid Progenitor Expansion

Erythropoietin-driven expansion of the erythroid compartment raises the red cell mass. The white cell and platelet lineages are not expanded, which distinguishes this secondary erythrocytosis from a myeloproliferative polycythemia. A cell-intrinsic contribution of HIF-2-alpha within erythroid progenitors has also been proposed.

erythroid progenitor cell CL:0000038

erythrocyte differentiation GO:0030218 ↑ INCREASED

Animal models

Hif2a G536W knock-in mouse Mouse recapitulates The G536W knock-in allele, corresponding to the first human variant G537W, produces erythrocytosis in a mutation dose-dependent manner with high penetrance, formally establishing missense HIF-2-alpha variants as a cause of erythrocytosis.

Downstream

Polycythemia Erythroid expansion raises the red cell mass.

Increased circulating hemoglobin concentration Erythroid expansion raises haemoglobin.

Increased hematocrit Erythroid expansion raises haematocrit, the usual presenting abnormality.

Thromboembolism Erythrocytosis and hyperviscosity contribute to an increased risk of arterial and venous thrombosis.

Show evidence (1 reference)

PMID:23716564 SUPPORT In Vitro

"The erythroid precursors obtained from the peripheral blood of patients showed an altered phenotype, including an increased rate of growth and a modified expression of some HIF-2α target genes"

Documents the expanded, altered erythroid progenitor phenotype in HIF2A/EPAS1 carriers, including a possible cell-intrinsic effect.

Pulmonary Vascular Remodeling and Hypertension

HIF-2-alpha dysregulation raises pulmonary vascular tone and promotes pulmonary vascular remodeling, producing pulmonary hypertension independent of the red cell mass. In humans with HIF-2-alpha gain-of-function, pulmonary hypertension is accompanied by increased cardiac output, heart rate and ventilation; in the knock-in mouse modeling the first human variant, pulmonary hypertension develops with high penetrance alongside the erythrocytosis.

pulmonary artery smooth muscle cell CL:0002591

pulmonary vasoconstriction GO:0042310 ↑ INCREASED

pulmonary artery UBERON:0002012

Animal models

Hif2a G536W knock-in mouse Mouse recapitulates The same knock-in mice develop pulmonary hypertension with the erythrocytosis, in a mutation dose-dependent manner.

Downstream

Pulmonary arterial hypertension Elevated pulmonary vascular tone and remodeling raise pulmonary artery pressure.

Show evidence (2 references)

PMID:21389259 SUPPORT Human Clinical

"HIF-2α gain-of-function mutations were associated with pulmonary hypertension, increased cardiac output, increased heart rate, and increased pulmonary ventilation relative to metabolism"

Direct human physiological evidence that HIF-2-alpha gain-of-function produces pulmonary hypertension and a hyperdynamic cardiopulmonary state.

PMID:23640890 SUPPORT Model Organism

"these mice display, in a mutation dose-dependent manner, erythrocytosis and pulmonary hypertension with a high degree of penetrance"

The G536W knock-in mouse (modeling human G537W) develops pulmonary hypertension with the erythrocytosis, establishing causality.

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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.

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Phenotypes

7

Blood 4

Polycythemia HP:0001901

Show evidence (1 reference)

PMID:18184961 SUPPORT Human Clinical

"We describe a family with erythrocytosis and a mutation in the HIF2A gene, which encodes the HIF-2alpha protein"

The defining phenotype, erythrocytosis, co-occurring with the EPAS1/HIF2A variant in the index family.

Increased circulating hemoglobin concentration HP:0001900

Show evidence (1 reference)

PMID:18711622 SUPPORT Human Clinical

"The propositus presented with erythrocytosis at the age of 23 years exhibiting an elevated haemoglobin level of 21.7 g/dL and a haematocrit of 0.64"

Records the elevated haemoglobin at presentation in the index ECYT4 case.

Increased hematocrit HP:0001899

Show evidence (1 reference)

PMID:18711622 SUPPORT Human Clinical

"The propositus presented with erythrocytosis at the age of 23 years exhibiting an elevated haemoglobin level of 21.7 g/dL and a haematocrit of 0.64"

Records the elevated haematocrit at presentation in the index ECYT4 case.

Thromboembolism OCCASIONAL HP:0001907

Sequelae: Ischemic stroke

Show evidence (1 reference)

PMID:36700397 SUPPORT Human Clinical

"A history of thrombosis was reported in six patients and six relatives which mainly were of the stroke type"

Documents the thrombotic risk and its predominantly arterial (stroke) character in the EPAS1 erythrocytosis cohort.

Cardiovascular 2

Pulmonary arterial hypertension OCCASIONAL HP:0002092

Show evidence (1 reference)

PMID:36700397 SUPPORT Human Clinical

"Pulmonary arterial hypertension (PAH) was developed by two patients and ocular lesions have been detected in three patients as previously reported in association with EPAS1 mutations"

Reports pulmonary arterial hypertension among the EPAS1 erythrocytosis cohort, as an occasional manifestation.

Ischemic stroke OCCASIONAL HP:0002140

Show evidence (2 references)

PMID:40879399 SUPPORT Human Clinical

"We describe the case of an 11-year-old female with significant polycythemia and pulmonary hypertension with resultant ischemic stroke"

Documents ischemic stroke in a child with germline EPAS1-associated polycythemia and pulmonary hypertension.

PMID:36700397 SUPPORT Human Clinical

"A history of thrombosis was reported in six patients and six relatives which mainly were of the stroke type"

Establishes stroke as the predominant thrombotic event type across the EPAS1 erythrocytosis cohort.

Endocrine 1

Elevated circulating erythropoietin concentration HP:0033644

Show evidence (1 reference)

PMID:18378852 SUPPORT Human Clinical

"All patients presented at a young age with elevated serum Epo"

Documents the elevated serum erythropoietin across the HIF2A/EPAS1 erythrocytosis cohort.

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Genetic Associations

1

EPAS1 germline gain-of-function

Gene: EPAS1 hgnc:3374 gene page relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (6 references)

PMID:18378852 SUPPORT Human Clinical

"identify 4 additional patients with novel heterozygous Met535Val and Gly537Arg mutations"

Reports the recurrent exon 12 EPAS1 missense variants (M535V, G537R) in an erythrocytosis cohort.

PMID:36700397 SUPPORT Human Clinical

"Patients described with a mutation in the EPAS1 gene developed erythrocytosis frequently associated with high EPO levels and are at increased risk of pulmonary hypertension and thrombotic events (thrombosis, infarction, pulmonary embolism)"

The European collection summarising the EPAS1 germline genotype and its clinical consequences.

PMID:36700397 SUPPORT Human Clinical

"We described four new mutations (D525G, L526F, G527K, A530S) close to the key proline P531, which broadens the spectrum of mutations involved in erythrocytosis"

Extends the causal variant spectrum N-terminally from the exon-12 Gly537 cluster while keeping it adjacent to Pro531.

+ 3 more references

Variants (5)

G537W

The first identified ECYT4 allele (c.1609G>T), replacing the small conserved glycine with a bulky tryptophan in the oxygen-dependent degradation domain.

G537R

Recurrent exon 12 missense variant at Gly537.

M535V

Exon 12 missense variant near Pro531.

D525G

One of four variants described close to Pro531 in the European EPAS1 erythrocytosis collection, broadening the causal span N-terminally.

A530S

Reported as a germline variant causing erythrocytosis alone, having previously been identified at somatic state in tumours.

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Medical Actions

3

Therapeutic phlebotomy

Action: phlebotomy Ontology label: Phlebotomy NCIT:C28221

Platform: Other

Phlebotomy on demand reduces the red cell mass and relieves hyperviscosity-related symptoms. It does not address the underlying HIF-2-alpha dysregulation. Current guidance positions it as symptom control rather than as treatment of the mechanism, and explicitly advises against cytoreductive therapy in this setting.

Mechanism Target:

INHIBITS Erythroid Progenitor Expansion — Phlebotomy removes red cell mass to relieve symptoms without affecting the HIF-2-alpha drive.

Show evidence (2 references)

PMID:27292716 SUPPORT Human Clinical

"close clinical follow-up with phlebotomies on demand and regular intake of low doses of anticoagulants seem to prevent from serious complications such as thrombembolic events and pulmonary hypertension"

Describes phlebotomy on demand as part of the supportive management of ECYT4 carriers.

PMID:41123216 SUPPORT REVIEW SYNTHESIS Human Clinical

"Cytoreductive therapy should be avoided. Phlebotomy should be considered for symptom control."

Current guidance for JAK2-unmutated erythrocytosis, including the germline oxygen-sensing forms: phlebotomy for symptom control, and no cytoreduction.

Low-dose aspirin and antithrombotic prophylaxis

Action: Pharmacotherapy NCIT:C15986

Agent: aspirin CHEBI:15365

Platform: Small molecule

Low-dose aspirin or anticoagulation is used to reduce the increased arterial and venous thrombotic risk, by analogy with other erythrocytoses.

Mechanism Target:

INHIBITS Thromboembolism — Antiplatelet/anticoagulant prophylaxis against thrombotic events.

Show evidence (2 references)

PMID:27292716 SUPPORT Human Clinical

"close clinical follow-up with phlebotomies on demand and regular intake of low doses of anticoagulants seem to prevent from serious complications such as thrombembolic events and pulmonary hypertension"

Supports low-dose anticoagulation/antiplatelet prophylaxis to prevent thromboembolic complications in ECYT4.

PMID:41123216 SUPPORT REVIEW SYNTHESIS Human Clinical

"Cardiovascular risk optimization and low-dose aspirin are advised"

Current guidance advising low-dose aspirin alongside cardiovascular risk optimisation in JAK2-unmutated erythrocytosis.

HIF-2-alpha inhibition (belzutifan)

Action: Pharmacotherapy NCIT:C15986

Agent: belzutifan CHEBI:757730

Platform: Small molecule

HIF-2-alpha is the direct effector of the disease, so HIF-2-alpha inhibition is a mechanistically targeted therapy. Belzutifan, a HIF-2-alpha inhibitor, has been reported to normalise the haemoglobin in EPAS1-associated polycythemia without further phlebotomy. Carriers with a causal EPAS1 variant in the hydroxylation region are candidates for this treatment. The evidence is case-level: the 2026 management review of JAK2-unmutated erythrocytosis states that the role of HIF2A inhibitors remains unclear, so this is a mechanistically rational therapy whose place in care is not yet settled.

Mechanism Target:

INHIBITS HIF-2-alpha Escape from Degradation and Stabilization — Belzutifan blocks HIF-2-alpha, the stabilised effector driving the EPO transcription and erythrocytosis.

Show evidence (3 references)

PMID:40879399 SUPPORT Human Clinical

"The therapy has been well tolerated, and her hemoglobin was restored to normal levels without the need for additional phlebotomy"

First reported use of belzutifan in EPAS1-associated congenital erythrocytosis, normalising the haemoglobin.

PMID:36700397 SUPPORT Human Clinical

"all the patients carrying a causal mutation in EPAS1, located between amino acids 525 to 537, are theoretically eligible for HIF-2α inhibitor treatment"

Supports HIF-2-alpha inhibition as a rational targeted therapy for the ECYT4 variant spectrum.

PMID:41123216 SUPPORT REVIEW SYNTHESIS Human Clinical

"the role of HIF2A inhibitors remains unclear"

Supports the stated limit on this treatment: a 2026 management review holds the place of HIF2A inhibitors in care to be unsettled, against which the single case report should be read.

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Diagnosis

3

JAK2 mutation screening to exclude polycythemia vera

The first step in an erythrocytosis workup is excluding polycythemia vera by JAK2 mutation screening across exons 12 to 15. A longstanding erythrocytosis with a positive family history is what moves a hereditary cause, including the oxygen-sensing-pathway genes, up the differential.

JAK2 mutation screening NCIT:C15709

Show evidence (2 references)

PMID:41123216 SUPPORT REVIEW SYNTHESIS Human Clinical

"The foremost step is excluding polycythemia vera (PV) with JAK2 mutation screening (exons 12-15)"

Establishes JAK2 screening as the first diagnostic step before a hereditary erythrocytosis is pursued.

PMID:41123216 SUPPORT REVIEW SYNTHESIS Human Clinical

"Hereditary erythrocytosis should be considered in longstanding erythrocytosis with a positive family history; causes include EPOR mutations (subnormal Epo), high oxygen affinity hemoglobin variants, PIEZO1 mutations, 2,3-bisphosphoglycerate deficiency, methemoglobinemia, and germline oxygen..."

The differential within which ECYT4 sits, and the clinical features that prompt germline oxygen-sensing-pathway testing.

Serum erythropoietin measurement

Serum erythropoietin in the workup of the erythrocytosis; it is inappropriately normal to elevated for the haemoglobin, so a normal result does not exclude the diagnosis.

serum erythropoietin measurement NCIT:C74855

Show evidence (1 reference)

PMID:18378852 SUPPORT Human Clinical

"All patients presented at a young age with elevated serum Epo"

Serum erythropoietin is the key laboratory discriminator and is elevated or inappropriately normal in ECYT4.

Germline gene-panel sequencing

EPAS1 is one of the oxygen-sensing-pathway genes (alongside VHL, EGLN1, EPOR) on congenital erythrocytosis gene panels; identifying the heterozygous EPAS1 variant establishes the diagnosis.

germline gene-panel testing NCIT:C15709

Show evidence (1 reference)

PMID:27292716 SUPPORT Human Clinical

"We applied a genetic approach including whole exome sequencing and Sanger sequencing for the identification of the causative germline mutation in a Bulgarian family with congential erythrocytosis"

Illustrates the sequencing-based diagnosis of the causative germline EPAS1 variant.

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Animal Models

1

Hif2a G536W knock-in mouse

Species

Mouse

Genotype

Hif2a G536W knock-in (models human HIF-2-alpha G537W)

Publication

PMID:23640890

Pathograph links

Erythroid Progenitor Expansion recapitulates fidelity: high observes: organism The G536W knock-in allele, corresponding to the first human variant G537W, produces erythrocytosis in a mutation dose-dependent manner with high penetrance, formally establishing missense HIF-2-alpha variants as a cause of erythrocytosis.

Show evidence (1 reference)

PMID:23640890 SUPPORT Model Organism

"These findings firmly establish missense mutations in HIF-2α as a cause of erythrocytosis, highlight the importance of this HIF-α isoform in erythropoiesis"

Establishes this knock-in as a faithful model of the human erythroid phenotype and its genetic cause.

Pulmonary Vascular Remodeling and Hypertension recapitulates fidelity: moderate observes: organism The same knock-in mice develop pulmonary hypertension with the erythrocytosis, in a mutation dose-dependent manner.

Show evidence (1 reference)

PMID:23640890 SUPPORT Model Organism

"these mice display, in a mutation dose-dependent manner, erythrocytosis and pulmonary hypertension with a high degree of penetrance"

The knock-in reproduces the pulmonary hypertension arm of the human disorder.

{ }

Source YAML

click to show

name: Familial Erythrocytosis 4
creation_date: "2026-10-14T15:02:40Z"
category: Mendelian
categories:
- Disorder of Oxygen Sensing
parents:
- Congenital Secondary Erythrocytosis
disease_term:
  preferred_term: erythrocytosis, familial, 4
  term:
    id: MONDO:0012729
    label: erythrocytosis, familial, 4
synonyms:
- ECYT4
- EPAS1 familial polycythemia
- HIF2A-related erythrocytosis
- erythrocytosis, familial, type 4
- familial polycythemia caused by mutation in EPAS1
description: >-
  Familial erythrocytosis 4 (ECYT4) is an autosomal dominant congenital
  secondary erythrocytosis caused by heterozygous germline gain-of-function
  missense variants in EPAS1, the gene encoding hypoxia-inducible factor 2 alpha
  (HIF-2-alpha). The reported variants cluster in exon 12, close to Pro531, the
  prolyl-hydroxyl-acceptor residue of HIF-2-alpha (for example G537W, G537R,
  M535V, M535I, M535T and P534L). In normoxia the prolyl hydroxylase domain
  protein 2 (PHD2) hydroxylates Pro531, which lets the von Hippel-Lindau protein
  recognise HIF-2-alpha and target it for proteasomal degradation; the
  erythrocytosis variants impair that hydroxylation and VHL binding, so
  HIF-2-alpha escapes degradation and is aberrantly stabilised despite adequate
  oxygen (pseudohypoxia). Stabilised HIF-2-alpha drives transcription of the
  erythropoietin gene, so serum erythropoietin is inappropriately normal or
  frankly elevated for the haemoglobin and red cell mass rises. Patients present
  at a young age with an isolated erythrocytosis: haemoglobin and haematocrit are
  raised while white cell and platelet counts are normal. Beyond the red cell
  mass, carriers are at increased risk of pulmonary hypertension and of arterial
  and venous thrombosis (stroke, infarction, pulmonary embolism). Unlike the
  somatic mosaic EPAS1 disorder Pacak-Zhuang syndrome, germline ECYT4 does not
  carry the paraganglioma/pheochromocytoma and somatostatinoma predisposition,
  and unlike Chuvash polycythemia (germline VHL) and EGLN1/PHD2-related
  erythrocytosis, the lesion here is a direct gain of function of HIF-2-alpha
  rather than loss of one of its negative regulators. Management is supportive
  (phlebotomy, low-dose aspirin/anticoagulation); HIF-2-alpha inhibition with
  belzutifan has been reported to normalise the erythrocytosis.
pathophysiology:
- name: Germline EPAS1/HIF-2-alpha Gain-of-Function Variant
  biological_scale: MOLECULAR
  description: >-
    A heterozygous germline missense variant in EPAS1 alters a residue in or near
    the oxygen-dependent degradation domain of HIF-2-alpha, close to the
    prolyl-hydroxyl-acceptor Pro531 (reported changes include G537W, G537R, M535V,
    M535I, M535T and P534L). The variants are gain-of-function: they impair
    recognition of HIF-2-alpha by the PHD2 hydroxylase and by the von Hippel-Lindau
    protein. Functional characterisation across these alleles shows a common
    endpoint of impaired degradation despite distinct effects on PHD2 and VHL
    interaction.
  genes:
  - preferred_term: EPAS1
    term:
      id: hgnc:3374
      label: EPAS1
  molecular_functions:
  - preferred_term: HIF-2-alpha transcription factor activity
    term:
      id: GO:0000981
      label: DNA-binding transcription factor activity, RNA polymerase II-specific
    modifier: GAIN_OF_FUNCTION
  genetic_context:
    variant_origin: GERMLINE
    functional_impact_category: GAIN_OF_FUNCTION
  downstream:
  - target: HIF-2-alpha Escape from Degradation and Stabilization
    description: >-
      Impaired Pro531 hydroxylation and VHL binding stop HIF-2-alpha being marked
      for proteasomal destruction.
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:18184961
    reference_title: A gain-of-function mutation in the HIF2A gene in familial erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We describe a family with erythrocytosis and a mutation in the HIF2A gene, which encodes the HIF-2alpha protein"
    explanation: >-
      The sentinel report identifying a germline HIF2A/EPAS1 variant as the cause
      of familial erythrocytosis.
  - reference: PMID:19208626
    reference_title: Erythrocytosis-associated HIF-2alpha mutations demonstrate a critical role for residues C-terminal to the hydroxylacceptor proline.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "We find that all result in impaired degradation and thus aberrant stabilization of HIF-2alpha. However, each exhibits a distinct profile with respect to their effects on PHD2 binding and von Hippel Lindau interaction"
    explanation: >-
      Functional characterisation of G537R, M535V and P534L showing the common
      gain-of-function endpoint (impaired degradation) with allele-specific
      effects on PHD2 and VHL binding.
- name: HIF-2-alpha Escape from Degradation and Stabilization
  biological_scale: MOLECULAR
  description: >-
    HIF-2-alpha is no longer efficiently hydroxylated and ubiquitinated, so it
    accumulates and dimerises with HIF-1-beta (ARNT) at normal oxygen tension,
    activating the hypoxia transcriptional program inappropriately (pseudohypoxia).
  biological_processes:
  - preferred_term: HIF-mediated hypoxia response in normoxia
    term:
      id: GO:0001666
      label: response to hypoxia
    modifier: GAIN_OF_FUNCTION
  downstream:
  - target: Inappropriate Erythropoietin Transcription
    description: >-
      Stabilised HIF-2-alpha binds the hypoxia response element of the EPO gene
      and drives its transcription.
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:18184961
    reference_title: A gain-of-function mutation in the HIF2A gene in familial erythrocytosis.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Our functional studies indicate that this mutation leads to stabilization of the HIF-2alpha protein and suggest that wild-type HIF-2alpha regulates erythropoietin production in adults"
    explanation: >-
      Establishes that the variant stabilises HIF-2-alpha and that HIF-2-alpha is
      the physiological regulator of erythropoietin.
  - reference: PMID:39426935
    reference_title: Current views on the role of HIF-2α in the pathogenesis and syndromic presentation of pheochromocytoma and paraganglioma.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "HIF-2α evades steady-state hydroxylation by the prolyl hydroxylase type 2 (PHD2), which accelerates von Hippel-Lindau (VHL)-mediated proteasomal degradation. In this situation, HIF-2α is stabilized and can translocate to the nucleus"
    explanation: >-
      States the hydroxylation-escape and nuclear-translocation steps that define
      this node when an EPAS1 gain-of-function variant is present.
- name: Inappropriate Erythropoietin Transcription
  biological_scale: TISSUE
  description: >-
    HIF-2-alpha drives EPO gene transcription in the EPO-producing renal
    interstitial cells, so erythropoietin output is inappropriate for the
    prevailing haemoglobin. Across reported carriers the serum erythropoietin is
    inappropriately normal to frankly elevated.
  cell_types:
  - preferred_term: EPO-producing kidney interstitial fibroblast
    term:
      id: CL:1000692
      label: kidney interstitial fibroblast
  biological_processes:
  - preferred_term: cellular response to hypoxia driving EPO transcription
    term:
      id: GO:0071456
      label: cellular response to hypoxia
    modifier: INCREASED
  downstream:
  - target: Erythroid Progenitor Expansion
    description: Raised erythropoietin signalling expands the erythroid compartment.
    causal_link_type: DIRECT
  - target: Elevated circulating erythropoietin concentration
    description: >-
      Erythropoietin is inappropriately normal to elevated for the haemoglobin.
  evidence:
  - reference: PMID:18378852
    reference_title: Novel exon 12 mutations in the HIF2A gene associated with erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients presented at a young age with elevated serum Epo"
    explanation: >-
      Reports the inappropriately elevated erythropoietin in the HIF2A/EPAS1
      erythrocytosis cohort, the consequence of EPO transcriptional drive.
  - reference: PMID:33840141
    reference_title: Congenital erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "CE has been found to be caused by loss-of-function mutations in VHL and PHD2/EGLN1 as well as gain-of-function mutations in HIF-2α (EPAS1), all resulting in constitutive activation of EPO signaling"
    explanation: >-
      Places EPAS1 gain of function alongside VHL and EGLN1 loss of function as
      routes to the same endpoint, constitutive EPO signalling.
- name: Erythroid Progenitor Expansion
  biological_scale: TISSUE
  description: >-
    Erythropoietin-driven expansion of the erythroid compartment raises the red
    cell mass. The white cell and platelet lineages are not expanded, which
    distinguishes this secondary erythrocytosis from a myeloproliferative
    polycythemia. A cell-intrinsic contribution of HIF-2-alpha within erythroid
    progenitors has also been proposed.
  cell_types:
  - preferred_term: erythroid progenitor cell
    term:
      id: CL:0000038
      label: erythroid progenitor cell
  biological_processes:
  - preferred_term: erythrocyte differentiation
    term:
      id: GO:0030218
      label: erythrocyte differentiation
    modifier: INCREASED
  downstream:
  - target: Polycythemia
    description: Erythroid expansion raises the red cell mass.
  - target: Increased circulating hemoglobin concentration
    description: Erythroid expansion raises haemoglobin.
  - target: Increased hematocrit
    description: Erythroid expansion raises haematocrit, the usual presenting abnormality.
  - target: Thromboembolism
    description: >-
      Erythrocytosis and hyperviscosity contribute to an increased risk of
      arterial and venous thrombosis.
  evidence:
  - reference: PMID:23716564
    reference_title: Congenital erythrocytosis associated with gain-of-function HIF2A gene mutations and erythropoietin levels in the normal range.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "The erythroid precursors obtained from the peripheral blood of patients showed an altered phenotype, including an increased rate of growth and a modified expression of some HIF-2α target genes"
    explanation: >-
      Documents the expanded, altered erythroid progenitor phenotype in
      HIF2A/EPAS1 carriers, including a possible cell-intrinsic effect.
- name: Pulmonary Vascular Remodeling and Hypertension
  biological_scale: TISSUE
  conforms_to: "pulmonary_vascular_remodeling#Pulmonary Artery Smooth Muscle Cell Proliferation and Vasoconstriction"
  description: >-
    HIF-2-alpha dysregulation raises pulmonary vascular tone and promotes
    pulmonary vascular remodeling, producing pulmonary hypertension independent of
    the red cell mass. In humans with HIF-2-alpha gain-of-function, pulmonary
    hypertension is accompanied by increased cardiac output, heart rate and
    ventilation; in the knock-in mouse modeling the first human variant,
    pulmonary hypertension develops with high penetrance alongside the
    erythrocytosis.
  cell_types:
  - preferred_term: pulmonary artery smooth muscle cell
    term:
      id: CL:0002591
      label: smooth muscle cell of the pulmonary artery
  biological_processes:
  - preferred_term: pulmonary vasoconstriction
    term:
      id: GO:0042310
      label: vasoconstriction
    modifier: INCREASED
  locations:
  - preferred_term: pulmonary artery
    term:
      id: UBERON:0002012
      label: pulmonary artery
  downstream:
  - target: Pulmonary arterial hypertension
    description: Elevated pulmonary vascular tone and remodeling raise pulmonary artery pressure.
  evidence:
  - reference: PMID:21389259
    reference_title: "Cardiopulmonary function in two human disorders of the hypoxia-inducible factor (HIF) pathway: von Hippel-Lindau disease and HIF-2alpha gain-of-function mutation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "HIF-2α gain-of-function mutations were associated with pulmonary hypertension, increased cardiac output, increased heart rate, and increased pulmonary ventilation relative to metabolism"
    explanation: >-
      Direct human physiological evidence that HIF-2-alpha gain-of-function
      produces pulmonary hypertension and a hyperdynamic cardiopulmonary state.
  - reference: PMID:23640890
    reference_title: Erythrocytosis and pulmonary hypertension in a mouse model of human HIF2A gain of function mutation.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "these mice display, in a mutation dose-dependent manner, erythrocytosis and pulmonary hypertension with a high degree of penetrance"
    explanation: >-
      The G536W knock-in mouse (modeling human G537W) develops pulmonary
      hypertension with the erythrocytosis, establishing causality.
phenotypes:
- name: Polycythemia
  category: Hematologic
  description: >-
    Erythrocytosis is the dominant and near-constant feature: a congenital
    secondary, EPO-driven erythrocytosis without expansion of the white cell or
    platelet lineages.
  phenotype_term:
    preferred_term: Erythrocytosis
    term:
      id: HP:0001901
      label: Polycythemia
  diagnostic: true
  evidence:
  - reference: PMID:18184961
    reference_title: A gain-of-function mutation in the HIF2A gene in familial erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We describe a family with erythrocytosis and a mutation in the HIF2A gene, which encodes the HIF-2alpha protein"
    explanation: >-
      The defining phenotype, erythrocytosis, co-occurring with the EPAS1/HIF2A
      variant in the index family.
- name: Increased circulating hemoglobin concentration
  category: Hematologic
  phenotype_term:
    preferred_term: Increased circulating hemoglobin concentration
    term:
      id: HP:0001900
      label: Increased circulating hemoglobin concentration
  diagnostic: true
  evidence:
  - reference: PMID:18711622
    reference_title: Familial erythrocytosis arising from a gain-of-function mutation in the HIF2A gene of the oxygen sensing pathway.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The propositus presented with erythrocytosis at the age of 23 years exhibiting an elevated haemoglobin level of 21.7 g/dL and a haematocrit of 0.64"
    explanation: >-
      Records the elevated haemoglobin at presentation in the index ECYT4 case.
- name: Increased hematocrit
  category: Hematologic
  phenotype_term:
    preferred_term: Increased hematocrit
    term:
      id: HP:0001899
      label: Increased hematocrit
  diagnostic: true
  evidence:
  - reference: PMID:18711622
    reference_title: Familial erythrocytosis arising from a gain-of-function mutation in the HIF2A gene of the oxygen sensing pathway.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The propositus presented with erythrocytosis at the age of 23 years exhibiting an elevated haemoglobin level of 21.7 g/dL and a haematocrit of 0.64"
    explanation: >-
      Records the elevated haematocrit at presentation in the index ECYT4 case.
- name: Elevated circulating erythropoietin concentration
  category: Hematologic
  description: >-
    Serum erythropoietin is characteristically inappropriately normal to frankly
    elevated for the haemoglobin. A normal result does not exclude the diagnosis;
    many reported carriers presented with frankly elevated erythropoietin.
  phenotype_term:
    preferred_term: Elevated circulating erythropoietin concentration
    term:
      id: HP:0033644
      label: Elevated circulating erythropoietin concentration
  evidence:
  - reference: PMID:18378852
    reference_title: Novel exon 12 mutations in the HIF2A gene associated with erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients presented at a young age with elevated serum Epo"
    explanation: >-
      Documents the elevated serum erythropoietin across the HIF2A/EPAS1
      erythrocytosis cohort.
- name: Pulmonary arterial hypertension
  category: Cardiovascular
  description: >-
    Pulmonary hypertension is a recognised complication; in the European
    collection of 41 EPAS1 patients, pulmonary arterial hypertension was reported
    in a minority.
  phenotype_term:
    preferred_term: Pulmonary arterial hypertension
    term:
      id: HP:0002092
      label: Pulmonary arterial hypertension
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:36700397
    reference_title: Comprehensive in silico and functional studies for classification of EPAS1/HIF2A genetic variants identified in patients with erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Pulmonary arterial hypertension (PAH) was developed by two patients and ocular lesions have been detected in three patients as previously reported in association with EPAS1 mutations"
    explanation: >-
      Reports pulmonary arterial hypertension among the EPAS1 erythrocytosis
      cohort, as an occasional manifestation.
- name: Thromboembolism
  category: Vascular
  description: >-
    Arterial and venous thrombotic events (stroke, infarction, pulmonary
    embolism) are an increased risk; a history of thrombosis, mainly of the
    stroke type, was reported across the European EPAS1 collection.
  phenotype_term:
    preferred_term: Thromboembolism
    term:
      id: HP:0001907
      label: Thromboembolism
  frequency: OCCASIONAL
  sequelae:
  - target: Ischemic stroke
    description: >-
      The reported thrombotic events in EPAS1 carriers were mainly of the stroke
      type.
  evidence:
  - reference: PMID:36700397
    reference_title: Comprehensive in silico and functional studies for classification of EPAS1/HIF2A genetic variants identified in patients with erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A history of thrombosis was reported in six patients and six relatives which mainly were of the stroke type"
    explanation: >-
      Documents the thrombotic risk and its predominantly arterial (stroke)
      character in the EPAS1 erythrocytosis cohort.
- name: Ischemic stroke
  category: Neurologic
  description: >-
    Ischemic stroke is the most frequently reported thrombotic event in EPAS1
    carriers and has been described as a presenting complication in severe
    germline disease.
  phenotype_term:
    preferred_term: Ischemic stroke
    term:
      id: HP:0002140
      label: Ischemic stroke
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:40879399
    reference_title: Successful Use of Targeted HIF-2α Inhibition in EPAS1-Mutated Congenital Erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We describe the case of an 11-year-old female with significant polycythemia and pulmonary hypertension with resultant ischemic stroke"
    explanation: >-
      Documents ischemic stroke in a child with germline EPAS1-associated
      polycythemia and pulmonary hypertension.
  - reference: PMID:36700397
    reference_title: Comprehensive in silico and functional studies for classification of EPAS1/HIF2A genetic variants identified in patients with erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A history of thrombosis was reported in six patients and six relatives which mainly were of the stroke type"
    explanation: >-
      Establishes stroke as the predominant thrombotic event type across the
      EPAS1 erythrocytosis cohort.
genetic:
- name: EPAS1 germline gain-of-function
  gene_term:
    preferred_term: EPAS1
    term:
      id: hgnc:3374
      label: EPAS1
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  features: >-
    Heterozygous germline missense variants in EPAS1 (HIF2A), clustered in exon
    12 close to the prolyl-hydroxyl-acceptor Pro531. Reported alleles include
    G537W (the first identified), G537R, M535V, M535I, M535T, P534L, and the
    more N-terminal D525G, L526F, G527K and A530S; the described causal span runs
    roughly from amino acid 525 to 537. The mechanism is gain of function: the
    variants impair PHD2-mediated hydroxylation and VHL binding, stabilising
    HIF-2-alpha. Serum erythropoietin is inappropriately normal to elevated.
  notes: >-
    Two points of granularity. Some residues (A530S, Y532C) have been reported
    both as germline variants causing erythrocytosis alone and at somatic state
    in tumours, so residue identity alone does not predict which presentation a
    carrier has. And although germline ECYT4 is characteristically
    erythrocytosis-only, one germline exon-9 variant (F374Y, inherited from the
    proband's mother) has been reported in a polycythemic patient who developed
    pheochromocytoma/paraganglioma; the reporting authors concluded the variant
    was predisposing rather than causative for the tumour, and no loss of
    heterozygosity or second somatic hit was found. No tumour phenotype is
    curated on this entry on the strength of that single case; the chromaffin
    tumour spectrum belongs to the somatic mosaic entity, Pacak-Zhuang syndrome.
  variants:
  - name: G537W
    description: >-
      The first identified ECYT4 allele (c.1609G>T), replacing the small conserved
      glycine with a bulky tryptophan in the oxygen-dependent degradation domain.
  - name: G537R
    description: Recurrent exon 12 missense variant at Gly537.
  - name: M535V
    description: Exon 12 missense variant near Pro531.
  - name: D525G
    description: >-
      One of four variants described close to Pro531 in the European EPAS1
      erythrocytosis collection, broadening the causal span N-terminally.
  - name: A530S
    description: >-
      Reported as a germline variant causing erythrocytosis alone, having
      previously been identified at somatic state in tumours.
  evidence:
  - reference: PMID:18378852
    reference_title: Novel exon 12 mutations in the HIF2A gene associated with erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "identify 4 additional patients with novel heterozygous Met535Val and Gly537Arg mutations"
    explanation: >-
      Reports the recurrent exon 12 EPAS1 missense variants (M535V, G537R) in an
      erythrocytosis cohort.
  - reference: PMID:36700397
    reference_title: Comprehensive in silico and functional studies for classification of EPAS1/HIF2A genetic variants identified in patients with erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients described with a mutation in the EPAS1 gene developed erythrocytosis frequently associated with high EPO levels and are at increased risk of pulmonary hypertension and thrombotic events (thrombosis, infarction, pulmonary embolism)"
    explanation: >-
      The European collection summarising the EPAS1 germline genotype and its
      clinical consequences.
  - reference: PMID:36700397
    reference_title: Comprehensive in silico and functional studies for classification of EPAS1/HIF2A genetic variants identified in patients with erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We described four new mutations (D525G, L526F, G527K, A530S) close to the key proline P531, which broadens the spectrum of mutations involved in erythrocytosis"
    explanation: >-
      Extends the causal variant spectrum N-terminally from the exon-12 Gly537
      cluster while keeping it adjacent to Pro531.
  - reference: PMID:36700397
    reference_title: Comprehensive in silico and functional studies for classification of EPAS1/HIF2A genetic variants identified in patients with erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "we identified patients with only erythrocytosis associated with germline mutations A530S and Y532C previously identified at somatic state in tumors, thereby raising the complexity of the genotype/phenotype correlations"
    explanation: >-
      Supports the notes: the same residues occur as germline erythrocytosis-only
      alleles and as somatic tumour alleles, so residue identity alone does not
      predict the presentation.
  - reference: PMID:23090011
    reference_title: A novel EPAS1/HIF2A germline mutation in a congenital polycythemia with paraganglioma.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "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"
    explanation: >-
      The single reported germline exon-9 EPAS1 gain-of-function variant, with
      transmission from a parent, in a polycythemic patient who also developed a
      chromaffin tumour.
  - reference: PMID:23090011
    reference_title: A novel EPAS1/HIF2A germline mutation in a congenital polycythemia with paraganglioma.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "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"
    explanation: >-
      Why no tumour phenotype is curated here from that case: the authors read
      the germline variant as predisposing rather than causative, with no second
      hit demonstrated.
inheritance:
- name: Autosomal dominant
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  description: >-
    Autosomal dominant. Carriers are heterozygous for a single gain-of-function
    EPAS1 allele, which is sufficient to cause the erythrocytosis; the variant
    co-segregates with the phenotype across generations.
  evidence:
  - reference: PMID:31350093
    reference_title: "Update on mutations in the HIF: EPO pathway and their role in erythrocytosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "Familial erythrocytosis caused by a gain-of-function mutation in the HIF2A gene is an autosomal dominant disorder"
    explanation: >-
      States the autosomal dominant inheritance of HIF2A/EPAS1 familial
      erythrocytosis (ECYT4).
diagnosis:
- name: JAK2 mutation screening to exclude polycythemia vera
  description: >-
    The first step in an erythrocytosis workup is excluding polycythemia vera by
    JAK2 mutation screening across exons 12 to 15. A longstanding erythrocytosis
    with a positive family history is what moves a hereditary cause, including
    the oxygen-sensing-pathway genes, up the differential.
  diagnosis_term:
    preferred_term: JAK2 mutation screening
    term:
      id: NCIT:C15709
      label: Genetic Testing
  evidence:
  - reference: PMID:41123216
    reference_title: "JAK2 Unmutated Erythrocytosis: 2026 Update on Diagnosis and Management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "The foremost step is excluding polycythemia vera (PV) with JAK2 mutation screening (exons 12-15)"
    explanation: >-
      Establishes JAK2 screening as the first diagnostic step before a hereditary
      erythrocytosis is pursued.
  - reference: PMID:41123216
    reference_title: "JAK2 Unmutated Erythrocytosis: 2026 Update on Diagnosis and Management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "Hereditary erythrocytosis should be considered in longstanding erythrocytosis with a positive family history; causes include EPOR mutations (subnormal Epo), high oxygen affinity hemoglobin variants, PIEZO1 mutations, 2,3-bisphosphoglycerate deficiency, methemoglobinemia, and germline oxygen sensing pathway mutations (HIF2A-PHD2-VHL)"
    explanation: >-
      The differential within which ECYT4 sits, and the clinical features that
      prompt germline oxygen-sensing-pathway testing.
- name: Serum erythropoietin measurement
  description: >-
    Serum erythropoietin in the workup of the erythrocytosis; it is
    inappropriately normal to elevated for the haemoglobin, so a normal result
    does not exclude the diagnosis.
  diagnosis_term:
    preferred_term: serum erythropoietin measurement
    term:
      id: NCIT:C74855
      label: Erythropoietin Measurement
  evidence:
  - reference: PMID:18378852
    reference_title: Novel exon 12 mutations in the HIF2A gene associated with erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients presented at a young age with elevated serum Epo"
    explanation: >-
      Serum erythropoietin is the key laboratory discriminator and is elevated or
      inappropriately normal in ECYT4.
- name: Germline gene-panel sequencing
  description: >-
    EPAS1 is one of the oxygen-sensing-pathway genes (alongside VHL, EGLN1, EPOR)
    on congenital erythrocytosis gene panels; identifying the heterozygous EPAS1
    variant establishes the diagnosis.
  diagnosis_term:
    preferred_term: germline gene-panel testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  evidence:
  - reference: PMID:27292716
    reference_title: EPAS1 p.M535T mutation in a Bulgarian family with congenital erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We applied a genetic approach including whole exome sequencing and Sanger sequencing for the identification of the causative germline mutation in a Bulgarian family with congential erythrocytosis"
    explanation: >-
      Illustrates the sequencing-based diagnosis of the causative germline EPAS1
      variant.
treatments:
- name: Therapeutic phlebotomy
  description: >-
    Phlebotomy on demand reduces the red cell mass and relieves
    hyperviscosity-related symptoms. It does not address the underlying HIF-2-alpha
    dysregulation. Current guidance positions it as symptom control rather than
    as treatment of the mechanism, and explicitly advises against cytoreductive
    therapy in this setting.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: phlebotomy
    term:
      id: NCIT:C28221
      label: Phlebotomy
  target_mechanisms:
  - target: Erythroid Progenitor Expansion
    treatment_effect: INHIBITS
    description: >-
      Phlebotomy removes red cell mass to relieve symptoms without affecting the
      HIF-2-alpha drive.
  evidence:
  - reference: PMID:27292716
    reference_title: EPAS1 p.M535T mutation in a Bulgarian family with congenital erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "close clinical follow-up with phlebotomies on demand and regular intake of low doses of anticoagulants seem to prevent from serious complications such as thrombembolic events and pulmonary hypertension"
    explanation: >-
      Describes phlebotomy on demand as part of the supportive management of
      ECYT4 carriers.
  - reference: PMID:41123216
    reference_title: "JAK2 Unmutated Erythrocytosis: 2026 Update on Diagnosis and Management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "Cytoreductive therapy should be avoided. Phlebotomy should be considered for symptom control."
    explanation: >-
      Current guidance for JAK2-unmutated erythrocytosis, including the germline
      oxygen-sensing forms: phlebotomy for symptom control, and no cytoreduction.
- name: Low-dose aspirin and antithrombotic prophylaxis
  description: >-
    Low-dose aspirin or anticoagulation is used to reduce the increased arterial
    and venous thrombotic risk, by analogy with other erythrocytoses.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: aspirin
      term:
        id: CHEBI:15365
        label: acetylsalicylic acid
  target_mechanisms:
  - target: Thromboembolism
    treatment_effect: INHIBITS
    description: Antiplatelet/anticoagulant prophylaxis against thrombotic events.
  evidence:
  - reference: PMID:27292716
    reference_title: EPAS1 p.M535T mutation in a Bulgarian family with congenital erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "close clinical follow-up with phlebotomies on demand and regular intake of low doses of anticoagulants seem to prevent from serious complications such as thrombembolic events and pulmonary hypertension"
    explanation: >-
      Supports low-dose anticoagulation/antiplatelet prophylaxis to prevent
      thromboembolic complications in ECYT4.
  - reference: PMID:41123216
    reference_title: "JAK2 Unmutated Erythrocytosis: 2026 Update on Diagnosis and Management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "Cardiovascular risk optimization and low-dose aspirin are advised"
    explanation: >-
      Current guidance advising low-dose aspirin alongside cardiovascular risk
      optimisation in JAK2-unmutated erythrocytosis.
- name: HIF-2-alpha inhibition (belzutifan)
  description: >-
    HIF-2-alpha is the direct effector of the disease, so HIF-2-alpha inhibition is
    a mechanistically targeted therapy. Belzutifan, a HIF-2-alpha inhibitor, has
    been reported to normalise the haemoglobin in EPAS1-associated polycythemia
    without further phlebotomy. Carriers with a causal EPAS1 variant in the
    hydroxylation region are candidates for this treatment. The evidence is
    case-level: the 2026 management review of JAK2-unmutated erythrocytosis
    states that the role of HIF2A inhibitors remains unclear, so this is a
    mechanistically rational therapy whose place in care is not yet settled.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: belzutifan
      term:
        id: CHEBI:757730
        label: belzutifan
  target_mechanisms:
  - target: HIF-2-alpha Escape from Degradation and Stabilization
    treatment_effect: INHIBITS
    description: >-
      Belzutifan blocks HIF-2-alpha, the stabilised effector driving the EPO
      transcription and erythrocytosis.
  evidence:
  - reference: PMID:40879399
    reference_title: Successful Use of Targeted HIF-2α Inhibition in EPAS1-Mutated Congenital Erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The therapy has been well tolerated, and her hemoglobin was restored to normal levels without the need for additional phlebotomy"
    explanation: >-
      First reported use of belzutifan in EPAS1-associated congenital
      erythrocytosis, normalising the haemoglobin.
  - reference: PMID:36700397
    reference_title: Comprehensive in silico and functional studies for classification of EPAS1/HIF2A genetic variants identified in patients with erythrocytosis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "all the patients carrying a causal mutation in EPAS1, located between amino acids 525 to 537, are theoretically eligible for HIF-2α inhibitor treatment"
    explanation: >-
      Supports HIF-2-alpha inhibition as a rational targeted therapy for the ECYT4
      variant spectrum.
  - reference: PMID:41123216
    reference_title: "JAK2 Unmutated Erythrocytosis: 2026 Update on Diagnosis and Management."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "the role of HIF2A inhibitors remains unclear"
    explanation: >-
      Supports the stated limit on this treatment: a 2026 management review holds
      the place of HIF2A inhibitors in care to be unsettled, against which the
      single case report should be read.
animal_models:
- name: Hif2a G536W knock-in mouse
  species: Mouse
  genotype: Hif2a G536W knock-in (models human HIF-2-alpha G537W)
  publication: PMID:23640890
  modeled_mechanisms:
  - target: Erythroid Progenitor Expansion
    relationship: RECAPITULATES
    fidelity: HIGH
    model_scale: ORGANISM
    description: >-
      The G536W knock-in allele, corresponding to the first human variant G537W,
      produces erythrocytosis in a mutation dose-dependent manner with high
      penetrance, formally establishing missense HIF-2-alpha variants as a cause
      of erythrocytosis.
    evidence:
    - reference: PMID:23640890
      reference_title: Erythrocytosis and pulmonary hypertension in a mouse model of human HIF2A gain of function mutation.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "These findings firmly establish missense mutations in HIF-2α as a cause of erythrocytosis, highlight the importance of this HIF-α isoform in erythropoiesis"
      explanation: >-
        Establishes this knock-in as a faithful model of the human erythroid
        phenotype and its genetic cause.
  - target: Pulmonary Vascular Remodeling and Hypertension
    relationship: RECAPITULATES
    fidelity: MODERATE
    model_scale: ORGANISM
    description: >-
      The same knock-in mice develop pulmonary hypertension with the
      erythrocytosis, in a mutation dose-dependent manner.
    evidence:
    - reference: PMID:23640890
      reference_title: Erythrocytosis and pulmonary hypertension in a mouse model of human HIF2A gain of function mutation.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "these mice display, in a mutation dose-dependent manner, erythrocytosis and pulmonary hypertension with a high degree of penetrance"
      explanation: >-
        The knock-in reproduces the pulmonary hypertension arm of the human
        disorder.

📚

References & Deep Research

Deep Research

1

Deep research results are used as seeds for research; they do not undergo the same validation as the main records and may contain errors. How we use deep research.

Evaluations and curation notes (1)

Create: Familial Erythrocytosis 4 · 2026-10-14T15:29:18Z · View source

Deep-research report used: research/Familial_Erythrocytosis_4-deep-research-openscientist.md (OpenScientist, 15 references, all resolved, confabulation_rate 0.0, needs_review true). Taken from it and independently verified against the reference cache: the four P531-proximal germline variants D525G, L526F, G527K and A530S plus the germline/somatic residue overlap at A530S and Y532C (PMID:36700397, snippets re-quoted from the cached abstract); the JAK2-exclusion-first diagnostic step, the hereditary-erythrocytosis differential, and the management statements on phlebotomy, cytoreduction avoidance, low-dose aspirin and the unsettled role of HIF2A inhibitors (PMID:41123216, newly fetched); the hydroxylation-escape and nuclear-translocation mechanism sentence (PMID:39426935); the oxygen-sensing nosology sentence placing EPAS1 gain of function beside VHL and EGLN1 loss of function (PMID:33840141); the germline exon-9 F374Y case, recorded in genetic notes with its own evidence (PMID:23090011); and ischemic stroke as a phenotype, wired as a sequela of Thromboembolism (PMID:40879399, PMID:36700397). Rejected from the report: its suggested HP:0003573 for elevated erythropoietin, which is Increased total bilirubin (the report's own term-validation section flags it, and HP:0033644 is the right term, already bound); NCIT:C15843 labelled "Therapeutic Phlebotomy", which is Preventive Intervention, and NCIT:C156989 labelled "Belzutifan", which is a questionnaire item, both flagged by the same section (NCIT:C28221 Phlebotomy and the CHEBI belzutifan binding were kept); obsolete GO:0061418; the off-topic iron-crosstalk reference PMID:35618957 and the iron-axis material from PMID:28263291, which is about physiological EPO regulation rather than this disease; a headache phenotype, which the report tabulates with no citation supporting it in ECYT4; and the whole Pacak-Zhuang/mosaic branch (PMID:37450881, 31091718, 31185588, 31876943, 41518596, 22931260, 27659016), which belongs to the separate mosaic entity. The report's flagged unsupported quote, "role remains unclear" from PMID:41123216, is in fact present in that abstract as "the role of HIF2A inhibitors remains unclear" and verifies against the local cache, so the flag is a false negative in the report's own quote matcher. Disease identity was checked by hand rather than with just preflight-dr, which refuses without the 1.3 GB local MONDO build: the report names EPAS1/HIF2A throughout, cites MONDO:0012729 and OMIM 611783, and names no rival causal gene. Also added in this pass: GO:0000981 as the HIF-2-alpha molecular function on the gene-bearing node (a real EPAS1 annotation, checked against QuickGO for UniProt Q99814), which clears this entry from check-gene-activity-grounding. De novo curation of ECYT4 (autosomal dominant congenital secondary erythrocytosis from germline gain-of-function EPAS1/HIF2A missense variants near Pro531). Curated from primary literature: Percy 2008 NEJM (PMID:18184961, first HIF2A GOF variant G537W), Percy 2008 Blood exon-12 cohort (PMID:18378852, M535V/G537R, elevated EPO), Percy 2008 Ulster Med J (PMID:18711622, index proband Hb/Hct), Furlow 2009 JBC (PMID:19208626, impaired degradation/stabilization across alleles), van Wijk 2013 (PMID:23716564, erythroid progenitor phenotype), Formenti 2011 (PMID:21389259, human HIF-2a GOF pulmonary hypertension), Tan 2013 (PMID:23640890, G536W knock-in mouse erythrocytosis+PH), Gharib/European collection 2023 (PMID:36700397, 41-patient cohort: PAH, thrombosis, belzutifan eligibility), Giani/Bulgarian family (PMID:27292716, phlebotomy+anticoagulant management), Lee 2019 review (PMID:31350093, autosomal dominant/ECYT4), belzutifan case (PMID:40879399). Five pathophysiology nodes: the EPAS1 variant, HIF-2a stabilization, inappropriate EPO transcription and erythroid progenitor expansion form the core chain, plus a pulmonary vascular remodeling node conforming to the pulmonary_vascular_remodeling module; all phenotypes are downstream targets. 37/37 snippets verified after the second pass. Rejected/not used: Gale 2008 Blood (PMID:18650473) abstract empty in cache; Pacak-Zhuang/mosaic EPAS1 papers (distinct somatic-mosaic entity, kept separate). Second pass integrated the OpenScientist report as described above; snippet count rose from 24 to 37. Validation: just validate, validate-terms, count-verified-snippets (37/37), check-entity-refs, check-causal-targets, check-duplicate-keys, check-block-scalar-comments, check-qualifier-terms, check-coarse-phenotypes, check-genereviews (NO_CHAPTER, no GeneReviews chapter for germline ECYT4), and validate-disorders all pass.

OpenScientist ▸

Familial Erythrocytosis 4 (ECYT4): A Comprehensive Disease Characteristics Report

openscientist-autonomous 14 citations 2026-10-14T15:40:08.005666

Familial Erythrocytosis 4 (ECYT4): A Comprehensive Disease Characteristics Report

Disease: Familial Erythrocytosis 4 (ECYT4) MONDO ID: MONDO:0012729 | OMIM: 611783 | Gene: EPAS1 (HIF2A) Category: Mendelian, autosomal dominant Report date: 2026-10-14

Summary

Familial Erythrocytosis 4 (ECYT4) is a rare autosomal dominant, secondary (erythropoietin-driven) hereditary erythrocytosis caused by germline gain-of-function missense variants in EPAS1 (which encodes hypoxia-inducible factor 2-alpha, HIF-2α). The causal variants cluster in the oxygen-dependent degradation domain (ODDD) of HIF-2α near the critical hydroxylation target proline 531 (P531). By disrupting this hydroxylation site, the variants prevent prolyl hydroxylase domain protein 2 (PHD2/EGLN1) from tagging HIF-2α for von Hippel–Lindau (pVHL)-mediated proteasomal degradation. HIF-2α is consequently stabilized, accumulates, translocates to the nucleus, and constitutively transactivates its target gene EPO, producing inappropriate erythropoietin secretion and absolute erythrocytosis with an inappropriately normal-to-elevated serum EPO level.

ECYT4 sits within a well-defined nosology of four Mendelian familial erythrocytoses. It is distinguished from ECYT1 (EPOR, primary, low EPO), ECYT2 (VHL/Chuvash, recessive, oxygen-sensing), and ECYT3 (EGLN1/PHD2, oxygen-sensing) as the EPAS1/HIF2A, high-EPO, dominantly inherited oxygen-sensing subtype. The same molecular lesion, when it arises post-zygotically as somatic mosaicism, produces a distinct multisystem disorder — Pacak–Zhuang syndrome (PZS) — characterized by multiple paragangliomas/pheochromocytomas, duodenal somatostatinomas, and polycythemia, along with ocular/retinal vascular abnormalities, pancreatic neuroendocrine tumors, and other developmental anomalies. ECYT4 (germline, heritable) and PZS (mosaic, tumor-predisposing) are therefore two phenotypic expressions of the same gain-of-function biology.

Clinically, ECYT4 is managed supportively with therapeutic phlebotomy, cardiovascular-risk optimization, and low-dose aspirin, while cytoreductive therapy is avoided. The most important recent advance is mechanism-targeted pharmacotherapy: belzutifan, a small-molecule HIF-2α inhibitor, has been used successfully to normalize hemoglobin in germline EPAS1-associated polycythemia without further phlebotomy, directly correcting the underlying pathophysiology. This report integrates nine confirmed findings drawn from 15 primary papers to document the disease across all requested domains, flagging gaps where disease-specific data are absent.

Key Findings

Finding 1 — Causal genetics: germline gain-of-function EPAS1/HIF2A variants clustering near Pro531

ECYT4 is caused by heterozygous germline gain-of-function missense variants in EPAS1, the gene encoding HIF-2α. In a European cohort of 41 patients plus 28 relatives with EPAS1-associated erythrocytosis, 11 mutations were classified as pathogenic across 17 patients and 23 relatives, and four novel mutations — D525G, L526F, G527K, and A530S — were identified clustering close to the key proline residue P531. The functional impact of these variants was validated by luciferase reporter assays demonstrating enhanced HIF-2α-dependent EPO promoter induction. As the authors state, "Gain-of-function mutations in the EPAS1/HIF2A gene have been identified in patients with hereditary erythrocytosis that can be associated with the development of paraganglioma, pheochromocytoma and somatostatinoma" and "We described four new mutations (D525G, L526F, G527K, A530S) close to the key proline P531, which broadens the spectrum of mutations involved in erythrocytosis" (PMID: 36700397).

The P531 residue is the primary hydroxylation acceptor within the ODDD; variants adjacent to it mechanically shield the hydroxylation machinery, which explains the tight genotypic clustering. Variant class: missense (point mutations). Origin: germline (heritable). Functional consequence: gain of function. HGNC: EPAS1.

Finding 2 — Core mechanism: escape from PHD2 hydroxylation and VHL degradation stabilizes HIF-2α and drives EPO

Under normoxia, HIF-α subunits are hydroxylated on specific proline residues by PHD2 (EGLN1), which creates the recognition motif for the pVHL E3 ubiquitin ligase complex and targets HIF-α for proteasomal degradation. Congenital erythrocytosis arises from lesions anywhere along this oxygen-sensing axis: "CE has been found to be caused by loss-of-function mutations in VHL and PHD2/EGLN1 as well as gain-of-function mutations in HIF-2α (EPAS1), all resulting in constitutive activation of EPO signaling" (PMID: 33840141).

In ECYT4 specifically, the gain-of-function variants cause HIF-2α to evade hydroxylation: "HIF-2α evades steady-state hydroxylation by the prolyl hydroxylase type 2 (PHD2), which accelerates von Hippel-Lindau (VHL)-mediated proteasomal degradation. In this situation, HIF-2α is stabilized and can translocate to the nucleus" (PMID: 39426935). Stabilized nuclear HIF-2α dimerizes with HIF-1β (ARNT) and transactivates hypoxia-response genes, foremost among them EPO. This is the central, biochemically demonstrated causal step of the disease.

Finding 3 — HIF-2α is the principal physiologic transcriptional driver of EPO, tuned by oxygen, iron, and 2-oxoglutarate

The reason EPAS1 gain-of-function produces erythrocytosis is that HIF-2α is the dominant physiologic transactivator of EPO: "Hypoxia-inducible factor-2 alpha (HIF-2α), a positive regulator of the EPO gene" (PMID: 28263291). HIF-2α activity is normally tuned by multiple inputs — molecular oxygen and 2-oxoglutarate (co-substrates of PHD2), and iron, which modulates both PHD2 activity and HIF-2α translation. EPO expression and renal HIF-2α are suppressed by iron through an oxidative-stress-dependent pathway, and erythropoietin production is modulated by iron via iron-regulatory-protein/iron-responsive-element control of HIF-2α (PMID: 28263291; PMID: 35618957). In ECYT4, the constitutive stabilization of HIF-2α overrides these physiologic brakes, uncoupling EPO production from true tissue oxygenation.

Finding 4 — Historical discovery and heritability

EPAS1/HIF2A gain-of-function mutations were first reported in 2012. The landmark paper identified "two novel somatic gain-of-function mutations in the gene encoding hypoxia-inducible factor 2α (HIF2A) in two patients, one presenting with paraganglioma and the other with paraganglioma and somatostatinoma, both of whom had polycythemia. The two mutations were associated with increased HIF-2α activity and increased protein half-life" (PMID: 22931260). The heritable, germline nature of the erythrocytosis phenotype was established shortly afterward by the report of a germline F374Y (exon 9) variant: "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" (PMID: 23090011). Notably, there was no loss of heterozygosity in the tumor, consistent with a dominant gain-of-function rather than a classic tumor-suppressor two-hit mechanism.

Finding 5 — The mosaic counterpart: Pacak–Zhuang syndrome

The somatic-mosaic version of ECYT4 biology is Pacak–Zhuang syndrome (PZS): "a new syndrome of multiple paragangliomas, somatostatinomas, and polycythemia caused by early postzygotic mosaic mutations in EPAS1, encoding hypoxia-inducible factor 2 alpha (HIF-2α)" (PMID: 37450881). In PZS, polycythemia precedes tumor development and is only partially corrected by tumor removal, with recurrence shortly after surgery (PMID: 31091718). EPAS1-related pheochromocytomas/paragangliomas (PPGLs) comprise roughly 3–6% of all PPGLs and show a characteristic phenotype: "EPAS1-related PPGLs usually occur in the second or third decade of life, more frequently in females, and are usually multiple, adrenal and extra-adrenal, and norepinephrine-secreting" with metastatic disease in up to 30% (PMID: 39426935). The phenotypic spectrum also includes ocular/retinal and choroidal vascular abnormalities (PMID: 31876943), Chiari malformation type 1 (PMID: 31185588), and pancreatic neuroendocrine tumors (PMID: 41518596).

Finding 6 — Mechanism-targeted therapy: belzutifan

The most consequential therapeutic advance is direct pharmacologic inhibition of HIF-2α. In the first reported use of belzutifan for germline EPAS1-associated polycythemia — an 11-year-old with a likely-pathogenic EPAS1 variant causing polycythemia, pulmonary hypertension, and ischemic stroke — "she started novel therapy with belzutifan, a small molecule inhibitor of HIF-2α, to target her underlying disease pathophysiology. This is the first reported usage of belzutifan for EPAS1-associated polycythemia. The therapy has been well tolerated, and her hemoglobin was restored to normal levels without the need for additional phlebotomy" (PMID: 40879399). This addresses the disease at its causal node rather than merely managing its hematologic consequence.

Finding 7 — Transgenic mouse model recapitulation

A transgenic mouse model expressing a gain-of-function EPAS1/HIF-2α mutation recapitulates the human phenotype. Through the NIH PPGL translational program, "the generation and study of a corresponding disease mouse model ... has provided new insights into the natural history of these developmental anomalies and the pathophysiologic role of HIF-2α" (PMID: 37450881). A specific Epas1^A529V transgenic model reproduced the ocular/retinal and choroidal vascular features seen in patients (PMID: 31876943), and models have been used to study polycythemia, vascular malformations, and neural tube defects (PMID: 31091718).

Finding 8 — Diagnostic framework

Diagnosis requires systematic exclusion of polycythemia vera (PV) and secondary causes. In JAK2-unmutated erythrocytosis, "Hereditary erythrocytosis should be considered in longstanding erythrocytosis with a positive family history; causes include EPOR mutations (subnormal Epo), high oxygen affinity hemoglobin variants, PIEZO1 mutations, 2,3-bisphosphoglycerate deficiency, methemoglobinemia, and germline oxygen sensing pathway mutations (HIF2A-PHD2-VHL)" (PMID: 41123216). Management guidance: "Cytoreductive therapy should be avoided. Phlebotomy should be considered for symptom control. Cardiovascular risk optimization and low-dose aspirin are advised, while the role of HIF2A inhibitors remains unclear" (PMID: 41123216).

Finding 9 — Position within the Mendelian erythrocytosis nosology

ECYT4 is one of four classically numbered familial erythrocytoses, distinguished by causal gene, inheritance, and EPO level (see table in the Mechanistic Model section). It is the EPAS1/HIF2A, high-EPO, autosomal-dominant, oxygen-sensing subtype (PMID: 33840141).

Section-by-Section Report

1. Disease Information

Overview: ECYT4 is a rare inherited absolute erythrocytosis (increased red cell mass) caused by constitutive activation of HIF-2α, leading to inappropriate erythropoietin production and secondary polycythemia. It is a lifelong, congenital-onset condition.
Key identifiers: MONDO:0012729; OMIM 611783; gene EPAS1 (HGNC:3374, OMIM 603349, chromosome 2p21). MeSH: "Familial Erythrocytosis" / "Polycythemia." ICD-10: D75.1 (secondary polycythemia). ICD-11: 3A81 (secondary polycythaemia). Orphanet: within the familial erythrocytosis grouping (e.g., ORPHA:90042 "Autosomal dominant secondary polycythemia"). (Exact Orphanet subtype code for ECYT4 is not firmly established in the reviewed sources — flagged as uncertain.)
Synonyms/alternative names: Familial erythrocytosis 4; ECYT4; erythrocytosis, familial, 4; EPAS1-related/HIF2A-related congenital erythrocytosis; HIF2A erythrocytosis. The germline erythrocytosis-only presentation is distinct from the mosaic "Pacak–Zhuang syndrome" / "EPAS1 gain-of-function mutation syndrome."
Information source: Primarily aggregated disease-level resources (OMIM, cohort studies, case series) complemented by individual case reports; no large EHR-derived datasets were used.

2. Etiology

Primary cause (genetic): Germline heterozygous gain-of-function missense variants in EPAS1/HIF2A (PMID: 36700397; PMID: 23090011).
Genetic risk factors: The causal variants themselves are the risk factor; they cluster in the C-terminal ODDD near codon 531 (e.g., D525G, L526F, G527K, A530S) and in exon 9 (e.g., F374Y). No separate susceptibility loci or validated modifier genes are established for ECYT4 in the reviewed literature.
Environmental risk factors: None established as causing ECYT4. However, HIF-2α activity is physiologically sensitive to hypoxia and iron; environmental hypoxia (altitude) and iron status may plausibly modulate phenotype severity (inferred, not demonstrated for ECYT4).
Protective factors: No genetic protective variants or environmental protective factors specific to ECYT4 are documented. Mechanistically, iron sufficiency suppresses HIF-2α/EPO (PMID: 28263291), but therapeutic phlebotomy deliberately induces iron deficiency, so the net clinical interaction is complex (inferred).
Gene–environment interactions: Not specifically characterized; the oxygen- and iron-responsiveness of the HIF pathway is the plausible substrate for GxE effects (inferred).

3. Phenotypes

The core phenotype is a laboratory abnormality — absolute erythrocytosis — with downstream clinical signs and symptoms from hyperviscosity and the thrombotic tendency of a high hematocrit.

Phenotype	Type	Characteristics	Suggested HPO
Erythrocytosis / increased red cell mass	Lab abnormality	Congenital onset; lifelong; variable severity	HP:0001901 (Polycythemia)
Elevated hemoglobin/hematocrit	Lab abnormality	Congenital; progressive if untreated	HP:0001899 (Increased hemoglobin)
Inappropriately normal/high serum EPO	Lab abnormality	Characteristic; distinguishes from EPOR (ECYT1) low-EPO forms	HP:0003573 (Increased serum EPO)
Headache	Symptom	Hyperviscosity-related; variable	HP:0002315
Thrombosis / thromboembolism	Clinical sign	Risk rises with hematocrit; episodic	HP:0001907
Pulmonary hypertension	Clinical sign	Reported in severe germline cases	HP:0002092
Ischemic stroke	Clinical sign	Reported in severe germline cases	HP:0002140

In germline ECYT4, severe vascular complications (pulmonary hypertension, ischemic stroke) can occur, as in the pediatric case treated with belzutifan (PMID: 40879399). Tumor phenotypes (PPGL, somatostatinoma) and ocular/retinal vascular disease are predominantly features of the mosaic PZS form rather than typical germline erythrocytosis-only ECYT4, though the germline tumor-predisposing variant in PMID: 23090011 shows overlap is possible. Quality-of-life data specific to ECYT4 (EQ-5D, SF-36) are not available in the reviewed literature.

4. Genetic / Molecular Information

Causal gene: EPAS1 / HIF2A (HGNC:3374; OMIM 603349; chromosome 2p21).
Pathogenic variants: Missense substitutions in the ODDD clustering near P531 (D525G, L526F, G527K, A530S) and exon-9 variants (F374Y); A529V used in mouse modeling (PMID: 36700397; PMID: 23090011; PMID: 31876943).
Classification: Pathogenic / likely pathogenic per ACMG, supported by functional luciferase/EPO-promoter assays (functional evidence, PS3) and recurrent clustering (PM1) (PMID: 36700397).
Variant type/class: Missense point mutations.
Allele frequency: Essentially absent from population databases (gnomAD) given rarity and pathogenicity (consistent with private/de novo and familial transmission; not explicitly quantified in sources).
Somatic vs germline: Both occur — germline (heritable ECYT4) and postzygotic somatic mosaic (PZS). No loss of heterozygosity in associated tumors (PMID: 23090011; PMID: 37450881).
Functional consequence: Gain of function — increased HIF-2α protein half-life and transcriptional activity (PMID: 22931260).
Modifier genes / epigenetics / chromosomal abnormalities: None established for ECYT4.

5. Environmental Information

No environmental, lifestyle, or infectious agents cause ECYT4; it is a monogenic disorder. Physiologic modulators of the HIF-2α/EPO axis (ambient oxygen tension/altitude, iron availability) may theoretically influence phenotype expression but are not established disease determinants (PMID: 28263291; PMID: 35618957). Smoking and other causes of secondary erythrocytosis are relevant only as differential diagnoses to exclude.

6. Mechanism / Pathophysiology

Ordered causal chain (initiating lesion → clinical manifestation):

A germline gain-of-function missense variant in EPAS1 alters the HIF-2α ODDD near P531 → leads to impaired recognition of HIF-2α by PHD2 (EGLN1). (demonstrated — variant clustering + functional assays, PMID: 36700397)
Impaired PHD2-mediated prolyl hydroxylation → results in failure of pVHL E3-ligase recognition and loss of ubiquitin-proteasomal degradation of HIF-2α. (demonstrated, PMID: 39426935)
Escape from degradation → leads to stabilization and nuclear accumulation of HIF-2α (increased protein half-life). (demonstrated, PMID: 22931260; PMID: 39426935)
Nuclear HIF-2α dimerizes with HIF-1β/ARNT and binds hypoxia-response elements → results in constitutive transactivation of EPO. (demonstrated via EPO-promoter luciferase assays, PMID: 36700397; PMID: 28263291)
Inappropriately elevated erythropoietin → leads to increased EPO-receptor signaling in erythroid progenitors and expanded erythropoiesis. (demonstrated, PMID: 33840141)
Expanded red cell mass → results in erythrocytosis, elevated hematocrit, and increased blood viscosity → leads to headache, thrombosis, and (in severe cases) pulmonary hypertension and ischemic stroke. (clinical, PMID: 40879399)

Branch (mosaic lesion): The identical gain-of-function lesion arising post-zygotically → produces mosaic tissue distribution → leads to Pacak–Zhuang syndrome: multiple PPGLs, duodenal somatostatinoma, pancreatic NETs, polycythemia, and developmental vascular/ocular anomalies (PMID: 37450881; PMID: 41518596; PMID: 31876943).

Upstream vs downstream: The mutation, hydroxylation escape, and HIF-2α stabilization are upstream; EPO upregulation is the pivotal intermediate; erythrocytosis and its vascular sequelae are downstream.

Molecular pathway: HIF oxygen-sensing / hypoxia-response pathway (EGLN1/PHD2 → VHL → HIF-2α → EPO). Co-substrates: molecular O₂, 2-oxoglutarate (α-ketoglutarate, CHEBI:30915), ferrous iron (CHEBI:29033) (PMID: 28263291).

Suggested GO terms: cellular response to hypoxia (GO:0071456); regulation of transcription from RNA Pol II promoter in response to hypoxia (GO:0061418); protein hydroxylation (GO:0018126); proteasome-mediated ubiquitin-dependent protein catabolic process (GO:0043161); positive regulation of erythrocyte differentiation (GO:0045648).

Suggested CL terms: EPO-producing renal interstitial fibroblast; erythroid progenitor cell (CL:0000038); erythroblast (CL:0000765); erythrocyte (CL:0000232).

7. Anatomical Structures Affected

Organ level (primary): Kidney (renal interstitial EPO-producing cells; UBERON:0002113) as the EPO source; bone marrow (UBERON:0002371) as the site of expanded erythropoiesis. Secondary/complication: blood/vasculature (UBERON:0000178), lungs (pulmonary hypertension; UBERON:0002048), brain (ischemic stroke; UBERON:0000955).
Body systems: Hematopoietic and cardiovascular systems primarily. In the mosaic PZS branch, additionally the autonomic/adrenal system (adrenal medulla, UBERON:0001235; paraganglia), duodenum/pancreas (UBERON:0002114 / UBERON:0001264), and eye (retina/choroid, UBERON:0000966 / UBERON:0001776).
Tissue/cell level: Erythroid lineage cells; renal peritubular fibroblasts; (mosaic) chromaffin cells, neuroendocrine cells, vascular endothelium.
Subcellular compartments: Nucleus (GO:0005634) — site of HIF-2α transcriptional action; cytoplasm — site of hydroxylation/degradation; proteasome (GO:0000502).
Lateralization: Systemic/bilateral (hematologic); tumors in PZS are frequently multiple and bilateral/multifocal.

8. Temporal Development

Onset: Congenital — erythrocytosis is present from early life, often detected in childhood or young adulthood; family history is typical (PMID: 41123216). In PZS, polycythemia precedes tumors, which arise in the 2nd–3rd decade (PMID: 39426935; PMID: 31091718).
Onset pattern: Chronic/insidious.
Progression: Erythrocytosis is stable-to-progressive and lifelong without treatment; tumor burden (PZS) can be progressive, recurrent, and metastatic (up to ~30%) (PMID: 39426935).
Remission/critical periods: No spontaneous remission of erythrocytosis; it is treatment-controlled. Early detection permits intervention before thrombotic complications.

9. Inheritance and Population

Inheritance: Autosomal dominant (germline ECYT4) (PMID: 33840141; PMID: 23090011).
Penetrance/expressivity: Appears high for erythrocytosis but with variable expressivity for vascular/tumor features; precise penetrance estimates are not established.
Epidemiology: Very rare; ECYT4 is one of the rarest familial erythrocytoses. Precise prevalence/incidence figures are not available. EPAS1 variants account for ~3–6% of PPGLs (the mosaic phenotype context) (PMID: 39426935).
Sex ratio: The EPAS1-related tumor phenotype is more frequent in females (PMID: 39426935); a sex skew for isolated germline erythrocytosis is not established.
Founder effects / consanguinity / anticipation / mosaicism: No founder effect or consanguinity role (dominant). Postzygotic somatic mosaicism is the defining feature of the PZS branch. No genetic anticipation.

10. Diagnostics

Initial workup: Confirm absolute erythrocytosis (elevated Hb/Hct, increased red cell mass); measure serum EPO (inappropriately normal/elevated in ECYT4, contrasting with the subnormal EPO of EPOR-driven ECYT1); exclude PV by JAK2 V617F and exon 12 testing (PMID: 41123216).
Exclude secondary causes: Hypoxia, high-oxygen-affinity hemoglobin variants, smoking, EPO-secreting tumors.
Genetic testing: Targeted EPAS1 sequencing or a hereditary-erythrocytosis gene panel (EPAS1, EGLN1, VHL, EPOR, HBB/HBA, BPGM, PIEZO1). For suspected mosaicism (PZS), deep sequencing of tumor/affected tissue rather than blood may be required, since variants can be absent or low-level in peripheral blood (PMID: 41123216; PMID: 37450881).
Differential diagnosis: PV (JAK2+); other hereditary erythrocytoses (ECYT1/EPOR, ECYT2/VHL-Chuvash, ECYT3/EGLN1); secondary erythrocytosis.
Imaging: Not required for isolated erythrocytosis; in PZS, functional/anatomic imaging for PPGL surveillance.

11. Outcome / Prognosis

With appropriate control of hematocrit, the hematologic prognosis of germline ECYT4 is generally good; the principal threat is thrombosis/hyperviscosity, with severe cases suffering pulmonary hypertension and ischemic stroke (PMID: 40879399). Disease-specific survival/mortality rates are not quantified in the reviewed literature. In the mosaic PZS branch, prognosis is driven by tumor behavior, including metastatic PPGL in up to ~30% (PMID: 39426935). Prognostic factors: hematocrit control, presence of vascular complications, and (in PZS) tumor multiplicity/metastasis.

12. Treatment

Modality	Intervention	Role / evidence	Suggested NCIT
Supportive	Therapeutic phlebotomy	First-line for symptom/hematocrit control (PMID: 41123216)	NCIT:C15843 (Therapeutic Phlebotomy)
Risk reduction	Low-dose aspirin + CV risk optimization	Advised to reduce thrombosis (PMID: 41123216)	NCIT:C287 (Aspirin)
Avoid	Cytoreductive therapy	Should be avoided (PMID: 41123216)	—
Targeted	Belzutifan (HIF-2α inhibitor)	Normalized Hb without phlebotomy; mechanism-targeted; first reported in germline EPAS1 polycythemia (PMID: 40879399)	NCIT:C156989 (Belzutifan)
PZS tumors	Surgery ± systemic therapy	For PPGL/somatostatinoma; polycythemia only partially corrected by tumor removal (PMID: 31091718)	NCIT:C17173 (Surgical Procedure)

Pharmacogenomics / personalized medicine: Belzutifan exemplifies genotype-guided precision therapy — a drug directly targeting the mutated HIF-2α protein. Its routine role in hereditary erythrocytosis is still being defined; guidelines note "the role of HIF2A inhibitors remains unclear" (PMID: 41123216), reflecting the gap between early case-level success and formal evidence.

13. Prevention

As a monogenic disorder, ECYT4 cannot be prevented primarily. Relevant measures are: - Secondary prevention: Early detection via family screening and genetic testing; cascade testing of at-risk relatives given autosomal dominant inheritance. - Tertiary prevention: Hematocrit control (phlebotomy/belzutifan), aspirin, and CV risk-factor management to prevent thrombosis; in PZS, PPGL surveillance (PMID: 41123216; PMID: 40879399). - Counseling: Genetic counseling for affected families; prenatal/preimplantation options are theoretically available for known family variants. No immunization is applicable.

14. Other Species / Natural Disease

Taxonomy / orthologs: Human EPAS1 (NCBI Gene 2034) has a mouse ortholog Epas1 (NCBI Gene 13819). The oxygen-sensing pathway (EGLN1/VHL/HIF-2α/EPO) is deeply conserved across mammals.
Natural disease in other species: No well-characterized naturally occurring EPAS1 gain-of-function erythrocytosis is documented in companion animals or wildlife in the reviewed sources. (EPAS1 is notable in high-altitude adaptation genetics, e.g., Tibetan human populations and some animals, but that is adaptive variation, not disease.)
Zoonotic potential: Not applicable (non-infectious).

15. Model Organisms

Mouse (primary model): Transgenic mice expressing gain-of-function Epas1/HIF-2α (including Epas1^A529V) recapitulate polycythemia, vascular malformations, neural tube defects, and ocular/retinal/choroidal vascular changes (PMID: 31091718; PMID: 31876943; PMID: 37450881).
Model type: Mammalian, transgenic/knock-in gain-of-function.
Phenotype recapitulation: Strong for polycythemia and the developmental/vascular/ocular PZS features; the model has clarified natural history and the pathophysiologic role of HIF-2α.
Limitations: Mouse models of mosaic PZS may not fully reproduce the human tumor spectrum or variant-specific severity; germline erythrocytosis-only phenotypes versus mosaic tumor phenotypes depend on expression timing and tissue distribution.
Resources: MGI for Epas1 alleles; NIH PPGL translational program models.

Mechanistic Model / Interpretation

 Germline EPAS1 gain-of-function variant (e.g., D525G/L526F/G527K/A530S near P531; F374Y exon 9)
                   │
                   ▼
 Impaired PHD2 (EGLN1) prolyl-hydroxylation of HIF-2α at P531
                   │
                   ▼
  Loss of pVHL recognition → no ubiquitin-proteasomal degradation
                   │
                   ▼
   HIF-2α stabilized (↑ half-life) → nuclear accumulation
                   │
                   ▼
 HIF-2α:HIF-1β binds HRE → constitutive EPO transactivation
                   │
                   ▼
↑ Serum EPO (inappropriately normal/high) → ↑ erythropoiesis
                   │
                   ▼
      Erythrocytosis / ↑ hematocrit / hyperviscosity
                   │
     ┌─────────────────────┴─────────────────────┐
     ▼                                            ▼
   Headache, thrombosis                     Pulmonary hypertension, ischemic stroke
                                   (severe cases)

   ── Branch: identical lesion arising POST-ZYGOTICALLY (mosaic) ──►
      Pacak–Zhuang syndrome: multiple PPGL + somatostatinoma +
      pancreatic NET + polycythemia + ocular/vascular + Chiari I

Comparative nosology of Mendelian familial erythrocytoses:

Subtype	Gene	Mechanism	Inheritance	Serum EPO
ECYT1	EPOR	Gain-of-function (primary)	AD	Low/subnormal
ECYT2 (Chuvash)	VHL	Loss-of-function (O₂-sensing)	AR	Elevated
ECYT3	EGLN1/PHD2	Loss-of-function (O₂-sensing)	AD	Normal/elevated
ECYT4	EPAS1/HIF2A	Gain-of-function (O₂-sensing)	AD	Normal/elevated

All secondary (O₂-sensing) subtypes converge on constitutive EPO signaling (PMID: 33840141). The key conceptual insight is that ECYT4 is the gain-of-function endpoint of the same pathway whose loss-of-function endpoints (VHL, EGLN1) produce ECYT2/ECYT3 — all four classic subtypes are reducible to "too much EPO signal," reached by different molecular routes.

Evidence Base

PMID	Title (abbrev.)	Contribution
36700397	Comprehensive EPAS1 cohort	Defines causal germline variants clustering near P531; functional validation (F001)
33840141	Congenital erythrocytosis	Places EPAS1 GoF in O₂-sensing nosology; constitutive EPO (F002, F009)
39426935	HIF-2α in PPGL pathogenesis	Molecular stabilization mechanism; tumor demographics (F002, F003)
37450881	Pacak–Zhuang syndrome	Defines mosaic syndrome; mouse model insights (F003, F007)
40879399	Belzutifan in EPAS1 erythrocytosis	First mechanism-targeted therapy success (F004, F006)
22931260	Somatic HIF2A GoF, 2012	Landmark discovery; ↑ half-life/activity (F005)
23090011	Germline EPAS1 F374Y	Proves heritable germline GoF; no tumor LOH (F005)
41123216	JAK2-unmutated erythrocytosis 2026	Diagnostic/management framework (F006)
28263291	Iron suppresses EPO via HIF-2α	HIF-2α as EPO regulator; iron tuning (F008)
35618957	Iron–erythropoiesis crosstalk	IRP/IRE control of HIF-2α (F008)
31876943	Retinal/choroidal changes in PZS	Ocular phenotype; Epas1^A529V mouse (F003, F007)
31091718	Transgenic PZS mouse	Model of mosaic GoF; polycythemia before tumors (F007)
31185588	Chiari I in EPAS1 syndrome	Expands developmental phenotype (F003)
41518596	Pancreatic NETs in PZS	Expands tumor spectrum (F003)
27659016	HIF2A PPGL expression cluster	HIF2A tumors form discrete pseudohypoxic subcluster

Consistency: The 15 papers are mutually reinforcing, with no direct contradictions. The main tension is interpretive: case-level success with belzutifan (PMID: 40879399) versus the 2026 guideline's cautious "role remains unclear" (PMID: 41123216) — reflecting an evidence-maturity gap rather than disagreement.

Supported and Refuted Hypotheses

No hypotheses were formally refuted during the investigation. The following working hypotheses were supported by the evidence:

H1 (supported): ECYT4 is caused by germline gain-of-function EPAS1 variants near P531 → confirmed (F001, PMID: 36700397).
H2 (supported): The mechanism is escape from PHD2/VHL degradation → HIF-2α stabilization → EPO upregulation → confirmed (F002, F003, F008).
H3 (supported): Germline and mosaic forms are two expressions of the same lesion → confirmed (F005, F003).
H4 (supported): HIF-2α inhibition (belzutifan) is a rational, effective therapy → supported by first case report (F004/F006), with the caveat that formal evidence is still limited.

Limitations and Knowledge Gaps

Epidemiology: No reliable prevalence/incidence figures for germline ECYT4; it is simply described as very rare. Penetrance and expressivity estimates are not quantified.
Germline vs mosaic phenotype boundary: Much of the deep phenotyping (tumors, ocular, Chiari) derives from the mosaic PZS form; the degree to which isolated germline ECYT4 patients develop tumors is less well characterized, though overlap exists (PMID: 23090011).
Therapy evidence: Belzutifan data in germline erythrocytosis are limited to case-level reports; no randomized/controlled data, long-term safety, or dosing standards exist for this indication.
Quality-of-life / natural history: No ECYT4-specific QoL instruments or longitudinal cohort outcome data were identified.
Allele frequencies and ACMG population criteria: Precise gnomAD frequencies for individual ECYT4 variants were not extracted.
No primary data analysis: This report is a literature synthesis; no patient-level datasets were available for independent statistical testing.

Proposed Follow-up Experiments / Actions

Belzutifan evidence development: Design a prospective registry or basket trial of HIF-2α inhibition in genetically confirmed germline EPAS1 erythrocytosis, with hematocrit, EPO, thrombotic events, pulmonary pressures, and safety (hypoxia, anemia) as endpoints.
Genotype–phenotype mapping: Correlate specific ODDD variants (P531-proximal vs exon-9) with erythrocytosis severity and tumor risk to refine surveillance recommendations and penetrance estimates.
Tumor-risk stratification in germline carriers: Systematically screen germline ECYT4 families for PPGL/somatostatinoma to quantify the germline tumor penetrance currently inferred mainly from mosaic cases.
Population screening: Query gnomAD/large biobanks for EPAS1 ODDD variants to estimate true allele frequencies and carrier burden.
Mechanistic refinement: Use the Epas1^A529V (and new knock-in) mouse models plus structural/AlphaFold analysis of the HIF-2α ODDD to quantify how each variant perturbs PHD2 docking, informing variant classification (PS3).
Iron-axis modulation: Formally test whether iron status (deliberately lowered by phlebotomy) interacts with HIF-2α stability to modulate ECYT4 severity (PMID: 28263291).

Report compiled from 9 confirmed findings and 15 primary-literature sources across a 5-iteration autonomous investigation. Evidence source types: predominantly human clinical (cohorts/case reports) and model organism (transgenic mouse), supported by in vitro functional assays.

Artifacts

Reference Validation

Checked with linkml-reference-validator 0.3.0.

Outcome	Count
References checked	15
Resolved	15
Unresolved (possible confabulation)	0
Unverifiable	0
Quoted claims checked	1
Quoted claims found in source	0
Quoted claims not found in source	1
References weighed for topical relevance	15
On topic	11
Off topic	1

Quotes not found in the cited source

Searched the abstract, any retrieved full text, and the title. A quote drawn from a part of the paper that was not retrieved will appear here too, so check before treating one as invented:

Every one of these was searched against an abstract alone, with no full text retrieved - marked abstract only below. Where full text can be fetched, re-running with it will settle them; where the source publishes only a summary to PubMed, as GeneReviews chapters do, it will not, and the quote has to be checked by hand against the chapter itself.

PMID:41123216 (abstract only): "role remains unclear"
Text part not found as substring: 'role remains unclear' (note: only abstract available for PMID:41123216, full text may contain this excerpt)

References that may not be about this subject

These identifiers resolve, so they are not fabrications, but the records they resolve to share almost none of this report's vocabulary. That is a clue and not a verdict - a paper can be relevant in ways its title and abstract do not spell out - so read them before deciding:

PMID:35618957 (6 mentions) - The mutual crosstalk between iron and erythropoiesis.
shared terms: none

Weighed against this report's own most characteristic terms: epas1, hif-2, erythrocytosis, ecyt4, germline, epo, variant, tumor, pzs, mosaic, gain-of-function, polycythemia, gene, hif2a, ppgl, phenotype, disease, mutation, belzutifan, vascular.

Term Validation

Checked with linkml-term-validator 0.4.5, through the ols: adapter.

Outcome	Count
Terms checked	36
Resolved	33
Unresolved (possible confabulation)	0
Obsolete	1
Unverifiable	2
Terms whose name was checked	10
Terms named correctly	2
Terms named as a different term	4
Terms whose name is worth a second look	4

Terms the report names something else

These identifiers resolve, so nothing about them looks wrong, and the ontology calls them something unrelated to what the report calls them. That usually means the identifier is not the one the sentence needs:

HP:0002315 (1 mention) - the report calls it "Hyperviscosity-related; variable"; HP calls it Headache
HP:0001907 (1 mention) - the report calls it "Risk rises with hematocrit; episodic"; HP calls it Thromboembolism
NCIT:C15843 (1 mention) - the report calls it "Therapeutic Phlebotomy"; NCIT calls it Preventive Intervention
NCIT:C156989 (1 mention) - the report calls it "Belzutifan"; NCIT calls it Walk or Hike Uphill Question

Obsolete terms

These terms are real but deprecated. Citing one is not a fabrication; it does mean the report is naming something the ontology has retired:

GO:0061418 (obsolete regulation of transcription from RNA polymerase II promoter in response to hypoxia) (1 mention) - replaced by GO:0006357

Terms whose name is worth a second look

The report's name for these is recognisably related to the term's own name without being one of them. A loose paraphrase reads the same way as a citation of the wrong sibling term - and so does a related synonym, which the ontology records precisely because it names something adjacent rather than the same thing - so these are listed rather than judged:

HP:0001899 (1 mention) - the report calls it "Increased hemoglobin"; HP calls it Increased hematocrit
HP:0003573 (1 mention) - the report calls it "Increased serum EPO"; HP calls it Increased total bilirubin, and lists "Increased bilirubin" among its other names
GO:0005634 (1 mention) - the report calls it "Subcellular compartments: Nucleus"; GO calls it nucleus**, and lists "cell nucleus" among its other names
NCIT:C17173 (1 mention) - the report calls it "Surgical Procedure"; NCIT calls it Surgery, and lists "Surgical Profession" among its other names

Prefixes with no resolver

Terms carrying these prefixes were not checked either way, because no configured ontology covers them. An unrecognised prefix may name an ontology this run could not reach as easily as one that does not exist, so nothing here is evidence of fabrication: ORPHA.

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Familial Erythrocytosis 4 (ECYT4): A Comprehensive Disease Characteristics Report

Summary

Key Findings

Finding 1 — Causal genetics: germline gain-of-function EPAS1/HIF2A variants clustering near Pro531

Finding 2 — Core mechanism: escape from PHD2 hydroxylation and VHL degradation stabilizes HIF-2α and drives EPO

Finding 3 — HIF-2α is the principal physiologic transcriptional driver of EPO, tuned by oxygen, iron, and 2-oxoglutarate

Finding 4 — Historical discovery and heritability

Finding 5 — The mosaic counterpart: Pacak–Zhuang syndrome

Finding 6 — Mechanism-targeted therapy: belzutifan

Finding 7 — Transgenic mouse model recapitulation

Finding 8 — Diagnostic framework

Finding 9 — Position within the Mendelian erythrocytosis nosology

Section-by-Section Report

1. Disease Information

2. Etiology

3. Phenotypes

4. Genetic / Molecular Information

5. Environmental Information

6. Mechanism / Pathophysiology

7. Anatomical Structures Affected

8. Temporal Development

9. Inheritance and Population

10. Diagnostics

11. Outcome / Prognosis

12. Treatment

13. Prevention

14. Other Species / Natural Disease

15. Model Organisms

Mechanistic Model / Interpretation

Evidence Base

Supported and Refuted Hypotheses

Limitations and Knowledge Gaps

Proposed Follow-up Experiments / Actions

Artifacts

Reference Validation

Quotes not found in the cited source

References that may not be about this subject

Term Validation

Terms the report names something else

Obsolete terms

Terms whose name is worth a second look

Prefixes with no resolver