Antley-Bixler Syndrome, FGFR2-Related

Mendelian MONDO:0020667 Pathograph 11 Show in embeddings browser FGFR2-related craniosynostosis Craniosynostosis syndrome

Antley-Bixler syndrome is an exceptionally rare craniosynostosis syndrome defined by radiohumeral (humeroradial) synostosis present from the perinatal period, together with midface hypoplasia, choanal stenosis or atresia, multiple joint contractures, and visceral anomalies. This entry covers the FGFR2-related form specifically — the recurrent p.Ser351Cys substitution in the IgIII domain, reported predominantly in patients with normal genitalia and normal steroidogenesis. Two things about this entry need stating before anything else, because both are traps. First, the name denotes two mechanistically unrelated disorders. "Antley-Bixler syndrome" in the literature is genetically heterogeneous: the form curated here, and a distinct autosomal recessive disorder caused by severe mutations in POR (cytochrome P450 oxidoreductase), which presents with disordered steroidogenesis and ambiguous genitalia and is a disorder of steroid synthesis with no FGFR lesion at all. The POR form dominates the modern literature by a wide margin. The two must never be merged, and a source discussing "Antley-Bixler syndrome" cannot be assumed to be about this entry. Second, and less widely appreciated: whether the FGFR2 form is a distinct entity at all is contested. The original FGFR2 report drew two published rebuttals arguing the patient did not have Antley-Bixler syndrome; a subsequent analysis concluded that ABS without disordered steroidogenesis "appears to be a variant of the autosomal dominant fibroblast growth factor receptor (FGFR)-related craniosynostosis syndromes", and argued the ABS name should be reserved for the POR form — the opposite of the convention this MONDO class encodes. The same FGFR2 p.Ser351Cys allele is also reported in severe Pfeiffer syndrome, so genotype does not separate them either. The entry is curated because MONDO maintains the class and the mechanism is well defined, but its independence is recorded as an open question rather than asserted; see gap_abs_fgfr2_distinct_entity_status. Mechanistically it is unambiguous, and that is why it belongs to the FGFR-Related Skeletal Dysplasias grouping while bent bone dysplasia syndrome does not. p.Ser351Cys creates an unpaired cysteine in the IgIII domain — the same ligand-independent disulfide-dimerization activation class as Beare-Stevenson p.Tyr375Cys and the extracellular-cysteine alleles of Crouzon syndrome and thanatophoric dysplasia type 1. The contest here is over what to call the phenotype, not over how the receptor behaves.

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Classifications

ISDS Skeletal Nosology

👪

Inheritance

Autosomal dominant HP:0000006

Autosomal dominant, reported as a de novo variant. This is itself a point of historical interest: Antley-Bixler syndrome had been assumed autosomal recessive on the basis of sib recurrence and consanguineous parents, and the first FGFR2 case was explicitly reported as an apparent dominant de novo exception. That apparent contradiction is now understood as the two-disorder split — the recessive pedigrees belong to the POR form.

Autosomal dominant inheritance

Show evidence (1 reference)

PMID:9605588 SUPPORT Human Clinical

"The inheritance has been suggested to be autosomal recessive based on two families with sib recurrence with both sexes being affected, and two cases born to consanguineous parents. We report the first case of ABS associated with an apparent dominant de novo mutation in the fibroblast growth..."

Records both the prior recessive assumption and the dominant de novo FGFR2 case that first separated the two forms.

Discussions and Knowledge Gaps

Is FGFR2-related Antley-Bixler syndrome a distinct disease entity, or a severe variant of the FGFR2 craniosynostosis syndromes that happens to carry a separate eponym?

KNOWLEDGE GAP OPEN gap_abs_fgfr2_distinct_entity_status

Attached to

pathophysiology#FGFR2 IgIII Cysteine-Creating Activation pathophysiology#Cranial Suture and Joint Osteogenic Dysregulation

The entity status is disputed in the published record and this entry does not resolve it. The first FGFR2 case (Chun et al. 1998) drew two rebuttals in the same journal arguing the patient did not have Antley-Bixler syndrome — "Patient described by Chun et al. may not present Antley-Bixler syndrome" (PMID:10076886) and "Not Antley-Bixler syndrome" (PMID:10076887); neither caches with an abstract, so neither can be quoted here, and both are cited in prose only. Cragun et al. (PMID:16145814) subsequently concluded that ABS without disordered steroidogenesis appears to be a variant of the FGFR-related craniosynostosis syndromes, and argued the ABS name should be reserved for the POR form — the opposite of the convention MONDO:0020667 encodes and that this entry follows. Compounding it, the same FGFR2 p.Ser351Cys allele is reported in severe Pfeiffer syndrome, so genotype does not discriminate. Three things would be needed to settle this and none exists: a genotype-stratified phenotype comparison of p.Ser351Cys carriers labelled Antley-Bixler against those labelled Pfeiffer; agreement on whether humeroradial synostosis is a discriminating feature or a severity marker within the FGFR2 spectrum; and a nomenclature decision that the field has not made. Note what is NOT in dispute: the receptor mechanism. A future decision that this is not a separate entity would be a lump into Pfeiffer syndrome or a broader FGFR2 craniosynostosis concept — it would not remove the disorder from the FGFR gain-of-function grouping.

Proposed experiments

Genotype-stratified comparison of FGFR2 p.Ser351Cys carriers

exp_abs_fgfr2_vs_pfeiffer_phenotype_comparison

Assemble all reported p.Ser351Cys carriers regardless of the syndrome label applied and compare skeletal, craniofacial and joint features. If carriers labelled Antley-Bixler and carriers labelled severe Pfeiffer are phenotypically indistinguishable, the eponym is not tracking biology.

Frequency of humeroradial synostosis across FGFR2 craniosynostosis genotypes

exp_abs_humeroradial_synostosis_specificity

Determine how often elbow fusion occurs in FGFR2 craniosynostosis syndromes other than Antley-Bixler. If it is a severity marker rather than a discriminating feature, the case for a separate entity weakens further.

Show evidence (2 references)

PMID:16145814 SUPPORT Other

"in the scientific literature is genetically heterogeneous with at least two distinct disorders, (1) ABS without disordered steroidogenesis, which appears to be a variant of the autosomal dominant fibroblast growth factor receptor (FGFR)"

The central claim against separate-entity status: the FGFR2 form is characterized as a variant of the FGFR craniosynostosis syndromes.

PMID:16145814 SUPPORT Other

"We believe it would be better to reserve the use of the term "ABS" for patients with POR deficiency and clinically significant craniosynostosis and/or radiohumoral synostosis."

The proposed nomenclature, which would assign the ABS name to the POR form and leave this entry's concept without one — directly contrary to the MONDO class curated here.

How should a knowledge base represent a disease name that denotes two mechanistically unrelated disorders, when the field has not agreed which one owns the name?

KNOWLEDGE GAP OPEN gap_abs_two_disorders_one_name

Attached to

pathophysiology#FGFR2 IgIII Cysteine-Creating Activation

This is a curation-practice gap as much as a biological one, recorded because it will recur. "Antley-Bixler syndrome" covers this FGFR2 craniosynostosis and a distinct autosomal recessive POR (cytochrome P450 oxidoreductase) deficiency presenting with disordered steroidogenesis and ambiguous genitalia — a disorder of steroid synthesis with no FGFR involvement whatever. MONDO splits them (MONDO:0020667 here, MONDO:0008726 for the POR form), but the literature does not: a PubMed sweep on the syndrome name returns an overwhelmingly POR-dominated result set, so any automated literature harvest keyed on the name will import the wrong disorder into this entry. The published nomenclature proposals conflict, with at least one arguing the name should go to the POR form. Until the field settles, the practical protections are the ones applied here: scope by genotype rather than by name, quote only sources whose subject can be identified from the quoted text, and state the hazard in the entry so it is not re-inherited. The dismech NEC preflight (`just preflight-dr`) is the relevant tooling, and this disorder is a good test case for it.

Proposed experiments

Measure precision of name-keyed literature retrieval for split eponyms

exp_abs_name_keyed_literature_precision

Quantify what fraction of "Antley-Bixler syndrome" publications concern each genotype. A precision figure would let the NEC preflight flag name-keyed harvests for split eponyms generally, rather than relying on a curator recognizing the hazard case by case.

Show evidence (2 references)

PMID:20818252 SUPPORT Human Clinical

"The condition of ABS is curious in that mutations of 2 separate genes have been identified and that there seem to be subtle phenotypic differences between the 2 genotypes."

States the two-gene problem and that the phenotypic separation is subtle.

PMID:16145814 SUPPORT Other

Confirms the name covers two distinct disorders in the literature.

⚙

Pathophysiology

FGFR2 IgIII Cysteine-Creating Activation

A de novo heterozygous FGFR2 p.Ser351Cys substitution in the third immunoglobulin-like (IgIII) domain. Introducing an unpaired cysteine into the extracellular domain permits intermolecular disulfide bonding between receptor monomers and hence ligand-independent constitutive activation — the same activation class as Beare-Stevenson p.Tyr375Cys and the extracellular-cysteine alleles of Crouzon syndrome and thanatophoric dysplasia type 1. This is what qualifies the disorder for the FGFR gain-of-function grouping, and it is the one thing about this entry that is not in dispute.

FGFR2 hgnc:3689 ⇑ GAIN OF FUNCTION

transmembrane receptor protein tyrosine kinase activity GO:0004714 ↑ INCREASED

Downstream

Cranial Suture and Joint Osteogenic Dysregulation Constitutive receptor signalling dysregulates osteogenic differentiation at both cranial sutures and developing limb joints.

Show evidence (3 references)

PMID:9605588 SUPPORT Human Clinical

"The patient was found to be heterozygous for a C-->G transversion at nucleotide 1064, which predicts a Ser351Cys amino acid substitution in the IgIII domain of FGFR2."

The defining allele and its domain position.

PMID:20818252 SUPPORT Human Clinical

"the S351C mutation of the fibroblast growth factor receptor 2 gene has been reported predominantly in those patients with normal genitalia and steroidogenesis"

Ties the FGFR2 allele specifically to the arm of the ABS spectrum without steroidogenic involvement, which is the concept this entry curates.

PMID:29230096 SUPPORT Other

"The majority of these mutations are missense mutations that result in constitutive activation of the receptor and downstream molecular pathways."

Places Antley-Bixler among the FGFR2 syndromic craniosynostoses whose alleles act by constitutive receptor activation.

Cranial Suture and Joint Osteogenic Dysregulation

Accelerated osteogenic differentiation in suture mesenchyme, together with abnormal ossification at developing joints. The joint arm is what distinguishes this phenotype from the other FGFR2 craniosynostoses: the same receptor lesion that fuses cranial sutures also produces bony fusion across the elbow, so synostosis of cranial sutures and of the elbow joint were together the original minimal diagnostic criteria for the syndrome.

Osteoblast CL:0000062

osteoblast differentiation GO:0001649 ↑ INCREASED

Downstream

Premature Cranial Suture Fusion

Humeroradial synostosis

Multiple joint contractures

Show evidence (1 reference)

PMID:9605588 SUPPORT Human Clinical

"The Antley-Bixler syndrome (ABS) is a rare syndrome with synostosis of cranial sutures and elbow joints as minimal diagnostic criteria."

Establishes that suture and joint synostosis together define the syndrome, which is why this node covers both compartments.

Premature Cranial Suture Fusion

Craniosynostosis with the associated craniofacial consequences — midface hypoplasia, depressed nasal bridge, proptosis — and the upper-airway compromise that follows from a restricted midface together with choanal stenosis or atresia.

cranial suture morphogenesis GO:0060363 ↓ DECREASED

Downstream

Craniosynostosis

Midface retrusion

Depressed nasal bridge

Choanal atresia

Show evidence (1 reference)

PMID:20818252 SUPPORT Human Clinical

"There is a wide spectrum of anomalies seen within ABS, and other features include midface hypoplasia; choanal stenosis or atresia; multiple joint contractures; visceral anomalies, particularly of the genitourinary system; and impaired steroidogenesis."

The associated feature spectrum. Note the quoted sentence describes ABS as a whole and therefore includes impaired steroidogenesis, which belongs to the POR form and is deliberately not curated as a phenotype of this entry.

⬡

Pathograph

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

Referential integrity issues (1):

Target 'Depressed nasal bridge' (from 'Premature Cranial Suture Fusion') not found in named elements

●

Phenotypes

Head and Neck 3

Craniosynostosis HP:0001363

Show evidence (1 reference)

PMID:9605588 SUPPORT Human Clinical

"The Antley-Bixler syndrome (ABS) is a rare syndrome with synostosis of cranial sutures and elbow joints as minimal diagnostic criteria."

Craniosynostosis as a minimal diagnostic criterion.

Midface retrusion HP:0011800

Show evidence (1 reference)

PMID:20818252 SUPPORT Human Clinical

"other features include midface hypoplasia; choanal stenosis or atresia; multiple joint contractures"

Midface hypoplasia among the associated features.

Choanal atresia HP:0000453

Show evidence (1 reference)

PMID:20818252 SUPPORT Human Clinical

"other features include midface hypoplasia; choanal stenosis or atresia; multiple joint contractures"

Choanal stenosis or atresia among the associated features.

Nervous System 1

Spinal dysraphism HP:0010301

Show evidence (1 reference)

PMID:9605588 SUPPORT Human Clinical

"Apart from the craniosynostosis and elbow ankylosis, our patient also presented with severe spinal dysraphism, the first report of such a finding in association with ABS."

A single case observation explicitly flagged by its authors as a first report, so marked PARTIAL and not treated as a defining feature.

Other 2

Humeroradial synostosis HP:0003041

Show evidence (2 references)

PMID:20818252 SUPPORT Human Clinical

"Antley-Bixler syndrome (ABS) is an exceptionally rare craniosynostosis syndrome characterized by radiohumeral synostosis present from the perinatal period."

The defining feature and its timing.

PMID:9605588 SUPPORT Human Clinical

"The Antley-Bixler syndrome (ABS) is a rare syndrome with synostosis of cranial sutures and elbow joints as minimal diagnostic criteria."

Elbow synostosis as a minimal diagnostic criterion.

Multiple joint contractures HP:0002828

Show evidence (1 reference)

PMID:20818252 SUPPORT Human Clinical

"other features include midface hypoplasia; choanal stenosis or atresia; multiple joint contractures"

Multiple joint contractures among the associated features.

🧬

Genetic Associations

FGFR2 p.Ser351Cys (Causative)

Gene: FGFR2 hgnc:3689 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (2 references)

PMID:9605588 SUPPORT Human Clinical

"The patient was found to be heterozygous for a C-->G transversion at nucleotide 1064, which predicts a Ser351Cys amino acid substitution in the IgIII domain of FGFR2."

The molecular definition of the allele.

PMID:20818252 SUPPORT Human Clinical

"Mutations of the P450 oxidoreductase gene have been reported in those patients with genital anomalies and/or impaired steroidogenesis, and the S351C mutation of the fibroblast growth factor receptor 2 gene has been reported predominantly in those patients with normal genitalia and steroidogenesis."

The genotype-phenotype split between the two disorders that share the name, and the basis on which this entry is scoped.

💊

Medical Actions

Staged craniofacial and airway management

Category: Therapeutic Action: Craniofacial surgery Ontology label: Surgical Procedure NCIT:C15329

No disease-modifying therapy exists. Management follows the pattern of the other severe FGFR2 craniosynostoses — airway securing in the neonate given the combination of midface hypoplasia and choanal narrowing, then staged cranial vault and midface procedures. The fused elbows and joint contractures add an orthopaedic and rehabilitation dimension the purely craniofacial FGFR2 syndromes do not have, and they are not correctable by the craniofacial pathway.

Target Phenotypes: Craniosynostosis HP:0001363 Midface retrusion HP:0011800

Show evidence (1 reference)

PMID:20818252 SUPPORT INDIRECT Human Clinical

"We report a series of 4 patients with ABS and review their main findings and management."

Marked INDIRECT: the series reports management of ABS patients without separating outcomes by genotype, so it does not establish a management standard specific to the FGFR2 form.

📊

Prevalence

Worldwide, published-case literature

Cases In Literature <1 in 1,000,000

No population estimate exists, and the case literature is additionally confounded by the two-disorder problem: many published "Antley-Bixler syndrome" cases are the POR form, so counts drawn from the name rather than the genotype overstate this entry. The FGFR2 form is described in single case reports and small series.

Show evidence (1 reference)

PMID:20818252 SUPPORT Human Clinical

"Antley-Bixler syndrome (ABS) is an exceptionally rare craniosynostosis syndrome characterized by radiohumeral synostosis present from the perinatal period."

Establishes rarity and the defining skeletal feature.

{ }

Source YAML

click to show

name: Antley-Bixler Syndrome, FGFR2-Related
synonyms:
- ABS without genital anomalies or disordered steroidogenesis
- Antley-Bixler syndrome without genital anomalies or disordered steroidogenesis
- trapezoidocephaly-synostosis syndrome
- FGFR2-related Antley-Bixler syndrome
creation_date: "2026-08-24T00:00:00Z"
category: Mendelian
description: >
  Antley-Bixler syndrome is an exceptionally rare craniosynostosis syndrome defined by
  radiohumeral (humeroradial) synostosis present from the perinatal period, together with
  midface hypoplasia, choanal stenosis or atresia, multiple joint contractures, and
  visceral anomalies. This entry covers the FGFR2-related form specifically — the
  recurrent p.Ser351Cys substitution in the IgIII domain, reported predominantly in
  patients with normal genitalia and normal steroidogenesis.

  Two things about this entry need stating before anything else, because both are traps.

  First, the name denotes two mechanistically unrelated disorders. "Antley-Bixler
  syndrome" in the literature is genetically heterogeneous: the form curated here, and a
  distinct autosomal recessive disorder caused by severe mutations in POR (cytochrome
  P450 oxidoreductase), which presents with disordered steroidogenesis and ambiguous
  genitalia and is a disorder of steroid synthesis with no FGFR lesion at all. The POR
  form dominates the modern literature by a wide margin. The two must never be merged,
  and a source discussing "Antley-Bixler syndrome" cannot be assumed to be about this
  entry.

  Second, and less widely appreciated: whether the FGFR2 form is a distinct entity at all
  is contested. The original FGFR2 report drew two published rebuttals arguing the patient
  did not have Antley-Bixler syndrome; a subsequent analysis concluded that ABS without
  disordered steroidogenesis "appears to be a variant of the autosomal dominant fibroblast
  growth factor receptor (FGFR)-related craniosynostosis syndromes", and argued the ABS
  name should be reserved for the POR form — the opposite of the convention this MONDO
  class encodes. The same FGFR2 p.Ser351Cys allele is also reported in severe Pfeiffer
  syndrome, so genotype does not separate them either. The entry is curated because
  MONDO maintains the class and the mechanism is well defined, but its independence is
  recorded as an open question rather than asserted; see
  gap_abs_fgfr2_distinct_entity_status.

  Mechanistically it is unambiguous, and that is why it belongs to the FGFR-Related
  Skeletal Dysplasias grouping while bent bone dysplasia syndrome does not. p.Ser351Cys
  creates an unpaired cysteine in the IgIII domain — the same ligand-independent
  disulfide-dimerization activation class as Beare-Stevenson p.Tyr375Cys and the
  extracellular-cysteine alleles of Crouzon syndrome and thanatophoric dysplasia type 1.
  The contest here is over what to call the phenotype, not over how the receptor behaves.
disease_term:
  preferred_term: Antley-Bixler syndrome without genital anomalies or disordered steroidogenesis
  term:
    id: MONDO:0020667
    label: Antley-Bixler syndrome without genital anomalies or disordered steroidogenesis
parents:
- FGFR2-related craniosynostosis
- Craniosynostosis syndrome
classifications:
  isds_skeletal_category:
  - classification_value: craniosynostosis_syndromes
    notes: >-
      ISDS Nosology of genetic skeletal disorders, 2023 revision (PMID:36779427), group
      "Craniosynostosis syndromes". Note the ISDS listing does not by itself settle
      whether the FGFR2 form is separable from the other FGFR craniosynostosis syndromes;
      see gap_abs_fgfr2_distinct_entity_status.
inheritance:
- name: Autosomal dominant
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  description: >
    Autosomal dominant, reported as a de novo variant. This is itself a point of
    historical interest: Antley-Bixler syndrome had been assumed autosomal recessive on
    the basis of sib recurrence and consanguineous parents, and the first FGFR2 case was
    explicitly reported as an apparent dominant de novo exception. That apparent
    contradiction is now understood as the two-disorder split — the recessive pedigrees
    belong to the POR form.
  evidence:
  - reference: PMID:9605588
    reference_title: "FGFR2 mutation associated with clinical manifestations consistent with Antley-Bixler syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The inheritance has been suggested to be autosomal recessive based on two families with sib recurrence with both sexes being affected, and two cases born to consanguineous parents. We report the first case of ABS associated with an apparent dominant de novo mutation in the fibroblast growth factor receptor 2 (FGFR2) gene."
    explanation: >-
      Records both the prior recessive assumption and the dominant de novo FGFR2 case
      that first separated the two forms.
prevalence:
- population: Worldwide, published-case literature
  measure_type: CASES_IN_LITERATURE
  prevalence_class: BELOW_1_IN_1000000
  notes: >-
    No population estimate exists, and the case literature is additionally confounded by
    the two-disorder problem: many published "Antley-Bixler syndrome" cases are the POR
    form, so counts drawn from the name rather than the genotype overstate this entry.
    The FGFR2 form is described in single case reports and small series.
  evidence:
  - reference: PMID:20818252
    reference_title: "Spectrum of Antley-Bixler syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Antley-Bixler syndrome (ABS) is an exceptionally rare craniosynostosis syndrome characterized by radiohumeral synostosis present from the perinatal period."
    explanation: Establishes rarity and the defining skeletal feature.
pathophysiology:
- name: FGFR2 IgIII Cysteine-Creating Activation
  role: trigger
  biological_scale: MOLECULAR
  conforms_to: "fgfr_gain_of_function_skeletal_dysplasia#Constitutive FGFR Activation"
  description: >
    A de novo heterozygous FGFR2 p.Ser351Cys substitution in the third immunoglobulin-like
    (IgIII) domain. Introducing an unpaired cysteine into the extracellular domain permits
    intermolecular disulfide bonding between receptor monomers and hence
    ligand-independent constitutive activation — the same activation class as
    Beare-Stevenson p.Tyr375Cys and the extracellular-cysteine alleles of Crouzon syndrome
    and thanatophoric dysplasia type 1. This is what qualifies the disorder for the FGFR
    gain-of-function grouping, and it is the one thing about this entry that is not in
    dispute.
  gene:
    preferred_term: FGFR2
    description: >-
      Fibroblast growth factor receptor 2, constitutively activated by a cysteine-creating
      IgIII-domain substitution.
    modifier: GAIN_OF_FUNCTION
    term:
      id: hgnc:3689
      label: FGFR2
  molecular_functions:
  - preferred_term: transmembrane receptor protein tyrosine kinase activity
    term:
      id: GO:0004714
      label: transmembrane receptor protein tyrosine kinase activity
    modifier: INCREASED
  evidence:
  - reference: PMID:9605588
    reference_title: "FGFR2 mutation associated with clinical manifestations consistent with Antley-Bixler syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The patient was found to be heterozygous for a C-->G transversion at nucleotide 1064, which predicts a Ser351Cys amino acid substitution in the IgIII domain of FGFR2."
    explanation: The defining allele and its domain position.
  - reference: PMID:20818252
    reference_title: "Spectrum of Antley-Bixler syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the S351C mutation of the fibroblast growth factor receptor 2 gene has been reported predominantly in those patients with normal genitalia and steroidogenesis"
    explanation: >-
      Ties the FGFR2 allele specifically to the arm of the ABS spectrum without
      steroidogenic involvement, which is the concept this entry curates.
  - reference: PMID:29230096
    reference_title: "Fibroblast Growth Factor Receptor 2 (FGFR2) Mutation Related Syndromic Craniosynostosis."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The majority of these mutations are missense mutations that result in constitutive activation of the receptor and downstream molecular pathways."
    explanation: >-
      Places Antley-Bixler among the FGFR2 syndromic craniosynostoses whose alleles act
      by constitutive receptor activation.
  downstream:
  - target: Cranial Suture and Joint Osteogenic Dysregulation
    description: >
      Constitutive receptor signalling dysregulates osteogenic differentiation at both
      cranial sutures and developing limb joints.
    causal_link_type: DIRECT

- name: Cranial Suture and Joint Osteogenic Dysregulation
  biological_scale: TISSUE
  conforms_to: "fgfr_gain_of_function_skeletal_dysplasia#Cranial Suture Osteogenic Acceleration"
  description: >
    Accelerated osteogenic differentiation in suture mesenchyme, together with abnormal
    ossification at developing joints. The joint arm is what distinguishes this phenotype
    from the other FGFR2 craniosynostoses: the same receptor lesion that fuses cranial
    sutures also produces bony fusion across the elbow, so synostosis of cranial sutures
    and of the elbow joint were together the original minimal diagnostic criteria for the
    syndrome.
  cell_types:
  - preferred_term: Osteoblast
    term:
      id: CL:0000062
      label: osteoblast
  biological_processes:
  - preferred_term: osteoblast differentiation
    term:
      id: GO:0001649
      label: osteoblast differentiation
    modifier: INCREASED
  evidence:
  - reference: PMID:9605588
    reference_title: "FGFR2 mutation associated with clinical manifestations consistent with Antley-Bixler syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The Antley-Bixler syndrome (ABS) is a rare syndrome with synostosis of cranial sutures and elbow joints as minimal diagnostic criteria."
    explanation: >-
      Establishes that suture and joint synostosis together define the syndrome, which is
      why this node covers both compartments.
  downstream:
  - target: Premature Cranial Suture Fusion
    causal_link_type: DIRECT
  - target: Humeroradial synostosis
  - target: Multiple joint contractures

- name: Premature Cranial Suture Fusion
  biological_scale: TISSUE
  conforms_to: "fgfr_gain_of_function_skeletal_dysplasia#Premature Suture Fusion and Craniosynostosis"
  description: >
    Craniosynostosis with the associated craniofacial consequences — midface hypoplasia,
    depressed nasal bridge, proptosis — and the upper-airway compromise that follows from
    a restricted midface together with choanal stenosis or atresia.
  biological_processes:
  - preferred_term: cranial suture morphogenesis
    term:
      id: GO:0060363
      label: cranial suture morphogenesis
    modifier: DECREASED
  evidence:
  - reference: PMID:20818252
    reference_title: "Spectrum of Antley-Bixler syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "There is a wide spectrum of anomalies seen within ABS, and other features include midface hypoplasia; choanal stenosis or atresia; multiple joint contractures; visceral anomalies, particularly of the genitourinary system; and impaired steroidogenesis."
    explanation: >-
      The associated feature spectrum. Note the quoted sentence describes ABS as a whole
      and therefore includes impaired steroidogenesis, which belongs to the POR form and
      is deliberately not curated as a phenotype of this entry.
  downstream:
  - target: Craniosynostosis
  - target: Midface retrusion
  - target: Depressed nasal bridge
  - target: Choanal atresia
phenotypes:
- category: Skeletal
  name: Humeroradial synostosis
  description: >
    Bony fusion between the distal humerus and proximal radius, present from the perinatal
    period. This is the defining feature of the syndrome and the one that separates it
    clinically from the other FGFR2 craniosynostoses, none of which fuse the elbow.
  phenotype_term:
    preferred_term: Humeroradial synostosis
    term:
      id: HP:0003041
      label: Humeroradial synostosis
  evidence:
  - reference: PMID:20818252
    reference_title: "Spectrum of Antley-Bixler syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Antley-Bixler syndrome (ABS) is an exceptionally rare craniosynostosis syndrome characterized by radiohumeral synostosis present from the perinatal period."
    explanation: The defining feature and its timing.
  - reference: PMID:9605588
    reference_title: "FGFR2 mutation associated with clinical manifestations consistent with Antley-Bixler syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The Antley-Bixler syndrome (ABS) is a rare syndrome with synostosis of cranial sutures and elbow joints as minimal diagnostic criteria."
    explanation: Elbow synostosis as a minimal diagnostic criterion.
- category: Craniofacial
  name: Craniosynostosis
  description: >
    Premature fusion of cranial sutures, one of the two minimal diagnostic criteria
    alongside elbow synostosis.
  phenotype_term:
    preferred_term: Craniosynostosis
    term:
      id: HP:0001363
      label: Craniosynostosis
  evidence:
  - reference: PMID:9605588
    reference_title: "FGFR2 mutation associated with clinical manifestations consistent with Antley-Bixler syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The Antley-Bixler syndrome (ABS) is a rare syndrome with synostosis of cranial sutures and elbow joints as minimal diagnostic criteria."
    explanation: Craniosynostosis as a minimal diagnostic criterion.
- category: Craniofacial
  name: Midface retrusion
  description: Midface hypoplasia, contributing with choanal narrowing to airway compromise.
  phenotype_term:
    preferred_term: Midface retrusion
    term:
      id: HP:0011800
      label: Midface retrusion
  evidence:
  - reference: PMID:20818252
    reference_title: "Spectrum of Antley-Bixler syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "other features include midface hypoplasia; choanal stenosis or atresia; multiple joint contractures"
    explanation: Midface hypoplasia among the associated features.
- category: Respiratory
  name: Choanal atresia
  description: >
    Choanal stenosis or atresia, a major contributor to neonatal airway compromise in this
    syndrome and shared with Beare-Stevenson syndrome among the FGFR2 disorders.
  phenotype_term:
    preferred_term: Choanal atresia
    term:
      id: HP:0000453
      label: Choanal atresia
  evidence:
  - reference: PMID:20818252
    reference_title: "Spectrum of Antley-Bixler syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "other features include midface hypoplasia; choanal stenosis or atresia; multiple joint contractures"
    explanation: Choanal stenosis or atresia among the associated features.
- category: Skeletal
  name: Multiple joint contractures
  description: >
    Contractures beyond the fused elbow, part of the broader joint involvement that
    distinguishes this phenotype from the purely craniofacial FGFR2 syndromes.
  phenotype_term:
    preferred_term: Multiple joint contractures
    term:
      id: HP:0002828
      label: Multiple joint contractures
  evidence:
  - reference: PMID:20818252
    reference_title: "Spectrum of Antley-Bixler syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "other features include midface hypoplasia; choanal stenosis or atresia; multiple joint contractures"
    explanation: Multiple joint contractures among the associated features.
- category: Neurologic
  name: Spinal dysraphism
  description: >
    Reported in the index FGFR2 case as severe spinal dysraphism, and noted there as the
    first such report in association with the syndrome. It is a single observation, not an
    established component of the phenotype, and is curated here because the authors used
    it to argue for very early FGFR2 expression during somite formation.
  phenotype_term:
    preferred_term: Spinal dysraphism
    term:
      id: HP:0010301
      label: Spinal dysraphism
  evidence:
  - reference: PMID:9605588
    reference_title: "FGFR2 mutation associated with clinical manifestations consistent with Antley-Bixler syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Apart from the craniosynostosis and elbow ankylosis, our patient also presented with severe spinal dysraphism, the first report of such a finding in association with ABS."
    explanation: >-
      A single case observation explicitly flagged by its authors as a first report, so
      marked PARTIAL and not treated as a defining feature.
genetic:
- name: FGFR2 p.Ser351Cys
  gene_term:
    preferred_term: FGFR2
    term:
      id: hgnc:3689
      label: FGFR2
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  notes: >-
    The recurrent allele of the FGFR2 form, c.1052C>G p.Ser351Cys in the IgIII domain,
    reported predominantly in patients with normal genitalia and normal steroidogenesis.
    The same substitution has been reported in severe Pfeiffer syndrome, which is part of
    why the entity status of the FGFR2 form is disputed. Distinguishing this entry from
    the POR form is a genotype question, not a phenotype one: the two overlap
    substantially in skeletal features, and urinary steroid profiling is what separates
    them clinically.
  evidence:
  - reference: PMID:9605588
    reference_title: "FGFR2 mutation associated with clinical manifestations consistent with Antley-Bixler syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The patient was found to be heterozygous for a C-->G transversion at nucleotide 1064, which predicts a Ser351Cys amino acid substitution in the IgIII domain of FGFR2."
    explanation: The molecular definition of the allele.
  - reference: PMID:20818252
    reference_title: "Spectrum of Antley-Bixler syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mutations of the P450 oxidoreductase gene have been reported in those patients with genital anomalies and/or impaired steroidogenesis, and the S351C mutation of the fibroblast growth factor receptor 2 gene has been reported predominantly in those patients with normal genitalia and steroidogenesis."
    explanation: >-
      The genotype-phenotype split between the two disorders that share the name, and the
      basis on which this entry is scoped.
treatments:
- name: Staged craniofacial and airway management
  action_category: THERAPEUTIC
  therapeutic_modality: SURGERY
  description: >
    No disease-modifying therapy exists. Management follows the pattern of the other
    severe FGFR2 craniosynostoses — airway securing in the neonate given the combination
    of midface hypoplasia and choanal narrowing, then staged cranial vault and midface
    procedures. The fused elbows and joint contractures add an orthopaedic and
    rehabilitation dimension the purely craniofacial FGFR2 syndromes do not have, and
    they are not correctable by the craniofacial pathway.
  treatment_term:
    preferred_term: Craniofacial surgery
    term:
      id: NCIT:C15329
      label: Surgical Procedure
  notes: >-
    treatment_term is left at the generic Surgical Procedure deliberately. This entry
    curates a staged multi-compartment management program - neonatal airway securing,
    then cranial vault procedures, then midface advancement, alongside orthopedic
    management of the synostosed elbows - not a single operation. NCIT:C15214 Craniotomy
    would name only the vault component and would misdescribe the airway, midface and
    limb arms. Splitting this into per-compartment treatments, each separately bound,
    is the right long-term fix and is deferred rather than done here.
  target_phenotypes:
  - preferred_term: Craniosynostosis
    term:
      id: HP:0001363
      label: Craniosynostosis
  - preferred_term: Midface retrusion
    term:
      id: HP:0011800
      label: Midface retrusion
  evidence:
  - reference: PMID:20818252
    reference_title: "Spectrum of Antley-Bixler syndrome."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report a series of 4 patients with ABS and review their main findings and management."
    explanation: >-
      Marked INDIRECT: the series reports management of ABS patients without separating
      outcomes by genotype, so it does not establish a management standard specific to
      the FGFR2 form.
discussions:
- discussion_id: gap_abs_fgfr2_distinct_entity_status
  prompt: >-
    Is FGFR2-related Antley-Bixler syndrome a distinct disease entity, or a severe variant
    of the FGFR2 craniosynostosis syndromes that happens to carry a separate eponym?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#FGFR2 IgIII Cysteine-Creating Activation
  - pathophysiology#Cranial Suture and Joint Osteogenic Dysregulation
  rationale: >-
    The entity status is disputed in the published record and this entry does not resolve
    it. The first FGFR2 case (Chun et al. 1998) drew two rebuttals in the same journal
    arguing the patient did not have Antley-Bixler syndrome — "Patient described by Chun
    et al. may not present Antley-Bixler syndrome" (PMID:10076886) and "Not Antley-Bixler
    syndrome" (PMID:10076887); neither caches with an abstract, so neither can be quoted
    here, and both are cited in prose only. Cragun et al. (PMID:16145814) subsequently
    concluded that ABS without disordered steroidogenesis appears to be a variant of the
    FGFR-related craniosynostosis syndromes, and argued the ABS name should be reserved
    for the POR form — the opposite of the convention MONDO:0020667 encodes and that this
    entry follows. Compounding it, the same FGFR2 p.Ser351Cys allele is reported in severe
    Pfeiffer syndrome, so genotype does not discriminate. Three things would be needed to
    settle this and none exists: a genotype-stratified phenotype comparison of
    p.Ser351Cys carriers labelled Antley-Bixler against those labelled Pfeiffer; agreement
    on whether humeroradial synostosis is a discriminating feature or a severity marker
    within the FGFR2 spectrum; and a nomenclature decision that the field has not made.
    Note what is NOT in dispute: the receptor mechanism. A future decision that this is
    not a separate entity would be a lump into Pfeiffer syndrome or a broader FGFR2
    craniosynostosis concept — it would not remove the disorder from the FGFR
    gain-of-function grouping.
  evidence:
  - reference: PMID:16145814
    reference_title: 'Use of the term "Antley-Bixler syndrome": minimizing confusion.'
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "in the scientific literature is genetically heterogeneous with at least two distinct disorders, (1) ABS without disordered steroidogenesis, which appears to be a variant of the autosomal dominant fibroblast growth factor receptor (FGFR)"
    explanation: >-
      The central claim against separate-entity status: the FGFR2 form is characterized as
      a variant of the FGFR craniosynostosis syndromes.
  - reference: PMID:16145814
    reference_title: 'Use of the term "Antley-Bixler syndrome": minimizing confusion.'
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "We believe it would be better to reserve the use of the term \"ABS\" for patients with POR deficiency and clinically significant craniosynostosis and/or radiohumoral synostosis."
    explanation: >-
      The proposed nomenclature, which would assign the ABS name to the POR form and
      leave this entry's concept without one — directly contrary to the MONDO class
      curated here.
  proposed_experiments:
  - experiment_id: exp_abs_fgfr2_vs_pfeiffer_phenotype_comparison
    name: Genotype-stratified comparison of FGFR2 p.Ser351Cys carriers
    description: >-
      Assemble all reported p.Ser351Cys carriers regardless of the syndrome label applied
      and compare skeletal, craniofacial and joint features. If carriers labelled
      Antley-Bixler and carriers labelled severe Pfeiffer are phenotypically
      indistinguishable, the eponym is not tracking biology.
  - experiment_id: exp_abs_humeroradial_synostosis_specificity
    name: Frequency of humeroradial synostosis across FGFR2 craniosynostosis genotypes
    description: >-
      Determine how often elbow fusion occurs in FGFR2 craniosynostosis syndromes other
      than Antley-Bixler. If it is a severity marker rather than a discriminating feature,
      the case for a separate entity weakens further.
- discussion_id: gap_abs_two_disorders_one_name
  prompt: >-
    How should a knowledge base represent a disease name that denotes two mechanistically
    unrelated disorders, when the field has not agreed which one owns the name?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#FGFR2 IgIII Cysteine-Creating Activation
  rationale: >-
    This is a curation-practice gap as much as a biological one, recorded because it will
    recur. "Antley-Bixler syndrome" covers this FGFR2 craniosynostosis and a distinct
    autosomal recessive POR (cytochrome P450 oxidoreductase) deficiency presenting with
    disordered steroidogenesis and ambiguous genitalia — a disorder of steroid synthesis
    with no FGFR involvement whatever. MONDO splits them (MONDO:0020667 here,
    MONDO:0008726 for the POR form), but the literature does not: a PubMed sweep on the
    syndrome name returns an overwhelmingly POR-dominated result set, so any automated
    literature harvest keyed on the name will import the wrong disorder into this entry.
    The published nomenclature proposals conflict, with at least one arguing the name
    should go to the POR form. Until the field settles, the practical protections are the
    ones applied here: scope by genotype rather than by name, quote only sources whose
    subject can be identified from the quoted text, and state the hazard in the entry so
    it is not re-inherited. The dismech NEC preflight (`just preflight-dr`) is the
    relevant tooling, and this disorder is a good test case for it.
  evidence:
  - reference: PMID:20818252
    reference_title: "Spectrum of Antley-Bixler syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The condition of ABS is curious in that mutations of 2 separate genes have been identified and that there seem to be subtle phenotypic differences between the 2 genotypes."
    explanation: States the two-gene problem and that the phenotypic separation is subtle.
  - reference: PMID:16145814
    reference_title: 'Use of the term "Antley-Bixler syndrome": minimizing confusion.'
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "in the scientific literature is genetically heterogeneous with at least two distinct disorders, (1) ABS without disordered steroidogenesis, which appears to be a variant of the autosomal dominant fibroblast growth factor receptor (FGFR)"
    explanation: Confirms the name covers two distinct disorders in the literature.
  proposed_experiments:
  - experiment_id: exp_abs_name_keyed_literature_precision
    name: Measure precision of name-keyed literature retrieval for split eponyms
    description: >-
      Quantify what fraction of "Antley-Bixler syndrome" publications concern each
      genotype. A precision figure would let the NEC preflight flag name-keyed harvests
      for split eponyms generally, rather than relying on a curator recognizing the
      hazard case by case.
notes: >
  Curated de novo from primary literature under claim #9404, completing the FGFR-Related
  Skeletal Dysplasias grouping — this was its last unfilled seat after Beare-Stevenson
  (#9257) and the bent bone dysplasia retraction (#9384).

  Scope. This entry is scoped by GENOTYPE, not by the syndrome name. It covers only the
  FGFR2 form (MONDO:0020667). The POR form (MONDO:0008726) is a separate autosomal
  recessive disorder of steroidogenesis with no FGFR lesion, has its own stub, and must
  never be merged into this entry. Where a quoted snippet describes "ABS" as a whole and
  therefore mentions impaired steroidogenesis, the explanation says so explicitly and no
  steroidogenic phenotype is curated here.

  Grouping membership. Added as a member, unlike bent bone dysplasia syndrome. The
  distinction is mechanistic and was checked before asserting it: p.Ser351Cys creates an
  unpaired cysteine in the IgIII domain, giving ligand-independent disulfide-mediated
  dimerization and genuine constitutive activation — the grouping's criterion. Bent bone
  dysplasia's transmembrane alleles instead reduce membrane receptor and FGF
  responsiveness, which is why it is excluded. Same gene, opposite receptor behaviour,
  opposite membership.

  Entity status is deliberately left open rather than asserted; see
  gap_abs_fgfr2_distinct_entity_status. If the field later lumps this into Pfeiffer
  syndrome or a broader FGFR2 craniosynostosis concept, the correct response is a
  has_subtypes entry on that parent plus a superseded_by history record — not deletion,
  and not removal from the grouping, since the mechanism claim would survive the renaming.

  Three sources central to the nosology dispute cache with content_type unavailable and
  are therefore cited in attributed prose only, never quoted: PMID:10076886 ("Patient
  described by Chun et al. may not present Antley-Bixler syndrome"), PMID:10076887 ("Not
  Antley-Bixler syndrome") and PMID:11430730 ("Further evidence that fibroblast growth
  factor receptor 2 mutations cause Antley-Bixler syndrome"). If any is later cached with
  an abstract it should be added as evidence to gap_abs_fgfr2_distinct_entity_status,
  where the rebuttals in particular would strengthen the record considerably.

  No phenotype carries a frequency band. The case literature is small and additionally
  contaminated by the two-disorder problem, so name-derived counts would overstate this
  entry.

  GeneReviews: PMID:20301628 (FGFR Craniosynostosis Syndromes Overview) is recorded as the
  baseline chapter. The entity dispute curated in gap_abs_fgfr2_distinct_entity_status is
  about whether the FGFR2 form is separable from Pfeiffer syndrome, not about whether it is
  an FGFR craniosynostosis syndrome - it is one on any reading of the dispute, so the
  overview is in scope. Its cached record is abstract_only and carries only a
  purpose statement, so findings is empty rather than mined.
references:
- reference: PMID:20301628
  title: "FGFR Craniosynostosis Syndromes Overview."
  tags:
  - GeneReviews
  findings: []

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PMID:20301628

FGFR Craniosynostosis Syndromes Overview.

No top-level findings curated for this source.

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