FGFR2-Related Bent Bone Dysplasia

Mendelian MONDO:0013815 Pathograph 14 Show in embeddings browser FGFR2-related skeletal dysplasia Perinatal lethal skeletal dysplasia

Bent bone dysplasia syndrome (BBDS) is an ultra-rare, usually perinatally lethal skeletal dysplasia caused by de novo heterozygous FGFR2 variants that place a polar residue in the hydrophobic transmembrane domain. It combines bent long bones with poor calvarial mineralization, craniosynostosis, prenatal teeth, hypoplastic pubis and clavicles, osteopenia, and a distinctive radiographic signature — moustache-shaped small clavicles and angel-shaped metacarpals and phalanges. BBDS is an FGFR2 disorder that is deliberately NOT curated as a member of the FGFR-Related Skeletal Dysplasias grouping, and the reason is the point of the entry. Every member of that group is defined by a constitutively active or ligand-hypersensitive receptor. The BBDS alleles do the opposite at the plasma membrane: they selectively reduce membrane FGFR2 and markedly diminish the receptor's responsiveness to extracellular FGF. The original delineation states this in its title — "deficient canonical FGF signaling" — and concludes the disorder is separate from previously characterized FGFR2 disorders. What is gained is not receptor activity but receptor *location*. The same mutations amplify nucleolar targeting of FGFR2, where it occupies the ribosomal DNA promoter and activates rDNA transcription through FGF2 and UBF1 by de-repressing RUNX2. The consequence is a ribosome-biogenesis disorder of the osteoprogenitor: elevated rRNA, increased proliferation, delayed differentiation, and — via epigenetic activation of normally silent rDNA — nucleolar disorganization that triggers the Rpl11-MDM2-p53 nucleolar stress response. Inhibiting p53 rescues the differentiation delay in cells carrying the BBDS mutations, which makes p53 a candidate therapeutic target and is the strongest mechanistic claim available for this disorder. The contrast with the gain-of-function group is sharpest at the residue level. BBDS alleles sit in the transmembrane domain, the same domain as achondroplasia p.Gly380Arg and Beare-Stevenson p.Tyr375Cys, and FGFR2 Tyr-381 substitutions have been reported to cause either Crouzon syndrome or bent bone dysplasia depending on the substituting residue. Same gene, same domain, in one case the same codon — opposite consequence for the receptor.

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1
Inheritance
6
Pathophys.
13
Phenotypes
2
Gaps
14
Pathograph
1
Genes
1
Medical Actions
1
Differentials
🏷

Classifications

Harrison's Part
GENETICS ENVIRONMENT DISEASE
ISDS Skeletal Nosology
bent bone dysplasia
👪

Inheritance

1
Autosomal dominant HP:0000006
Autosomal dominant, arising de novo. All reported cases have been sporadic, occurring in a single affected individual of unaffected parents, which is expected given that the disorder is usually lethal before or shortly after birth.
Autosomal dominant inheritance
Show evidence (1 reference)
PMID:22387015 SUPPORT Human Clinical
"We have identified de novo FGFR2 mutations in a sporadically occurring perinatal lethal skeletal dysplasia characterized by poor mineralization of the calvarium, craniosynostosis, dysmorphic facial features, prenatal teeth, hypoplastic pubis and clavicles, osteopenia, and bent long bones."
The delineating paper establishes de novo occurrence and gives the full core phenotype in one sentence.
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Discussions and Knowledge Gaps

2
Does p53 inhibition, which rescues delayed osteoblast differentiation in cells carrying the BBDS FGFR2 mutations, represent a viable therapeutic direction for a disorder whose damage is largely done before birth?
HUMAN MODEL MISMATCH OPEN gap_bbds_p53_inhibition_translation
Attached to
pathophysiology#Nucleolar Disorganization and Rpl11-MDM2-p53 Stress Response pathophysiology#Osteoprogenitor Proliferation with Delayed Differentiation
This is the only therapeutic lead BBDS has, and it is a genuine mechanistic result rather than a correlation: inhibiting p53 in cells expressing the BBDS mutations rescues the differentiation delay, which is what licenses calling p53 activation essential rather than incidental. Three problems stand between that and a treatment. First, the rescue is in cells; no animal model of BBDS has been reported to test whether systemic p53 inhibition rescues the skeletal phenotype, and the entry should not be read as claiming otherwise. Second, the timing is adverse in a way that distinguishes this from most therapeutic gaps: bent long bones, hypoplastic clavicles and poor calvarial mineralization are established in utero, so a postnatal intervention would be addressing a developmental lesion after it has formed, and the window in which p53 inhibition could matter may close before the diagnosis is made. Third, p53 is a tumour suppressor and inhibiting it systemically in a developing child carries an obvious and unquantified oncogenic hazard; no dose, route, duration or selectivity has been proposed. Nothing here should be curated as an emerging treatment.
Proposed experiments
p53 inhibition in an animal model of BBDS
exp_bbds_mouse_model_p53_rescue
Generate a knock-in animal carrying a BBDS transmembrane allele and test whether genetic or pharmacological p53 reduction rescues bone mineralization and long-bone bowing in vivo, which is the step between the existing cell result and any claim about the skeleton.
Developmental window for differentiation rescue
exp_bbds_developmental_window
Establish the latest developmental stage at which reducing p53 still restores osteoblast differentiation, since a window closing in utero would make postnatal treatment futile regardless of efficacy.
Show evidence (2 references)
PMID:28595297 SUPPORT In Vitro
"Inhibition of p53 in cells expressing the FGFR2 mutations in BBDS rescues delayed osteoblast differentiation, suggesting that p53 activation is an essential pathogenic factor in, and potential therapeutic target for, BBDS."
The rescue result and the authors' therapeutic framing, which is a cell-based finding offered as a prospect.
PMID:27240702 SUPPORT Human Clinical
"The prenatal phenotype included stillbirth, bending of the femora, and a high incidence of polyhydramnios, prematurity, and perinatal death in three of 11 patients in the series."
Establishes that the defining skeletal lesions are present prenatally, which is the timing problem any therapy has to solve.
Why do different substitutions at the same FGFR2 transmembrane residue produce either Crouzon syndrome, a canonical gain-of-function craniosynostosis, or bent bone dysplasia, a disorder of reduced membrane signalling?
KNOWLEDGE GAP OPEN gap_bbds_same_residue_opposite_disease
Attached to
pathophysiology#FGFR2 Transmembrane Polar Substitution pathophysiology#Reduced Plasma-Membrane FGFR2 and Deficient Canonical FGF Signaling
This is the cleanest available natural experiment on how FGFR2 transmembrane substitutions are read out, and it is unresolved. Substitutions at FGFR2 Tyr-381 have been reported to cause either Crouzon syndrome or bent bone dysplasia depending on which residue is introduced, so position alone does not determine the disease. The plausible discriminator is the biophysical character of the substituting residue — a cysteine enabling intermolecular disulfide bonding and constitutive dimerization versus a polar residue disrupting membrane insertion and trafficking — but no study cited here compares the two classes side by side in one system, and the nucleolar relocalization arm has not been tested in the gain-of-function alleles at all. It matters for the knowledge base as well as for biology: the FGFR-Related Skeletal Dysplasias grouping admits members on constitutive receptor activation, so whether a given transmembrane allele belongs is a mechanistic question that cannot be settled from the domain it sits in.
Proposed experiments
Side-by-side comparison of FGFR2 transmembrane allele classes
exp_fgfr2_tm_allele_series_side_by_side
Express Crouzon-type and BBDS-type substitutions at the same FGFR2 transmembrane residue in one cell system and measure membrane localization, ligand-dependent and ligand-independent kinase activity, and nucleolar occupancy together, so the discriminating property is measured rather than inferred across papers.
Nucleolar FGFR2 occupancy in gain-of-function craniosynostosis alleles
exp_nucleolar_fgfr2_in_gof_alleles
Test whether Apert, Crouzon and Beare-Stevenson FGFR2 alleles also alter nucleolar rDNA occupancy. A negative result would make the nucleolar arm genuinely specific to BBDS; a positive one would mean the two mechanisms are not mutually exclusive.
Show evidence (2 references)
PMID:22387015 SUPPORT In Vitro
"Using diseased chondrocytes and a cell-based assay, we determined that these mutations selectively reduced plasma-membrane levels of FGFR2 and markedly diminished the receptor's responsiveness to extracellular FGF."
The BBDS half of the contrast — reduced membrane receptor and FGF response.
PMID:24908667 SUPPORT In Vitro
"In this study, we find that these mutations, despite reducing canonical signaling, enhance nucleolar occupancy of FGFR2 at the ribosomal DNA (rDNA) promoter."
The relocalization arm, which has not been examined in the gain-of-function transmembrane alleles and so cannot currently be called BBDS-specific.
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Pathophysiology

6
FGFR2 Transmembrane Polar Substitution
A de novo heterozygous FGFR2 missense variant introducing a polar amino acid into the hydrophobic transmembrane domain — the reported alleles include p.Tyr381Asp and p.Met391Arg. This is the same domain in which achondroplasia (FGFR3 p.Gly380Arg) and Beare-Stevenson syndrome (FGFR2 p.Tyr375Cys) alleles sit, and FGFR2 Tyr-381 substitutions have been reported to give either Crouzon syndrome or bent bone dysplasia depending on which residue is substituted in. Despite that adjacency, the consequence here is not constitutive activation, which is why this entry carries no conforms_to anchor against the FGFR gain-of-function module.
FGFR2 hgnc:3689 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves FGFR2 (hgnc:3689). hgnc:3689 is a gene from the HUGO Gene Nomenclature Committee.
Downstream
Reduced Plasma-Membrane FGFR2 and Deficient Canonical FGF Signaling The polar transmembrane residue impairs normal receptor trafficking to and retention at the plasma membrane.
Enhanced Nucleolar FGFR2 Occupancy at rDNA The same substitutions amplify nucleolar targeting of the receptor, redistributing it rather than simply reducing it.
Show evidence (2 references)
PMID:22387015 SUPPORT Human Clinical
"Four unrelated affected individuals were found to be heterozygous for missense mutations that introduce a polar amino acid into the hydrophobic transmembrane domain of FGFR2."
Defines the allele class and its position in the receptor.
PMID:41186107 SUPPORT Human Clinical
"We report the clinical trajectory of a 4-year-old male with BBD, FGFR2-related (FGFR2, c.1141T>G), who initially presented neonatally with respiratory insufficiency requiring intubation, facial dysmorphism, and skeletal differences."
A specific reported allele in the longest-surviving individual.
Reduced Plasma-Membrane FGFR2 and Deficient Canonical FGF Signaling
The arm that disqualifies BBDS from the FGFR gain-of-function grouping. Membrane FGFR2 levels are selectively reduced and the receptor's responsiveness to extracellular FGF is markedly diminished, demonstrated both in patient-derived chondrocytes and in a cell-based assay. This is loss of canonical signaling, not gain — the mechanistic inverse of the constitutive, ligand-independent activation that characterizes Apert, Crouzon, Pfeiffer, Jackson-Weiss and Beare-Stevenson syndrome.
fibroblast growth factor receptor signaling pathway GO:0008543 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased fibroblast growth factor receptor signaling pathway (GO:0008543). GO:0008543 is a biological process from the Gene Ontology. ↓ DECREASED
transmembrane receptor protein tyrosine kinase activity GO:0004714 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased transmembrane receptor protein tyrosine kinase activity (GO:0004714). GO:0004714 is a molecular function from the Gene Ontology. ↓ DECREASED
Show evidence (2 references)
PMID:22387015 SUPPORT In Vitro
"Using diseased chondrocytes and a cell-based assay, we determined that these mutations selectively reduced plasma-membrane levels of FGFR2 and markedly diminished the receptor's responsiveness to extracellular FGF."
The direct demonstration that canonical membrane signaling is reduced, which is why this entry is not a member of the FGFR gain-of-function grouping.
PMID:22387015 SUPPORT Human Clinical
"All together, these clinical and molecular findings are separate from previously characterized FGFR2 disorders and represent a distinct skeletal dysplasia."
The delineating authors' own conclusion that BBDS is mechanistically separate from the other FGFR2 disorders.
Enhanced Nucleolar FGFR2 Occupancy at rDNA
The gain in BBDS is of receptor location, not receptor activity. The mutations amplify nucleolar targeting of FGFR2, where it occupies the ribosomal DNA promoter and activates rDNA transcription through interactions with FGF2 and UBF1, de-repressing RUNX2. A second study showed the mutant receptor also augments recruitment of histone-remodeling factors that epigenetically activate rDNA that is normally transcriptionally silent — so the effect is not only more transcription at active loci but conversion of silent loci to euchromatin.
rRNA transcription GO:0009303 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased rRNA transcription (GO:0009303). GO:0009303 is a biological process from the Gene Ontology. ↑ INCREASED
nucleolus GO:0005730 Gene Ontology (GO) Relation: this pathophysiological event involves this cellular component This pathophysiological event involves nucleolus (GO:0005730). GO:0005730 is a cellular component from the Gene Ontology.
Downstream
Osteoprogenitor Proliferation with Delayed Differentiation
Nucleolar Disorganization and Rpl11-MDM2-p53 Stress Response
Show evidence (3 references)
PMID:24908667 SUPPORT In Vitro
"In this study, we find that these mutations, despite reducing canonical signaling, enhance nucleolar occupancy of FGFR2 at the ribosomal DNA (rDNA) promoter."
States the relocalization gain explicitly against the background of reduced canonical signaling — the single most important sentence for this entry.
PMID:24908667 SUPPORT In Vitro
"Nucleolar FGFR2 activates rDNA transcription via interactions with FGF2 and UBF1 by de-repressing RUNX2."
The molecular route from nucleolar receptor to rDNA transcription.
PMID:28595297 SUPPORT In Vitro
"Here we find that the BBDS mutations augment the ability of FGFR2 to recruit histone-remodeling factors that epigenetically activate transcriptionally silent rDNA."
Extends the mechanism to epigenetic activation of normally silent rDNA loci.
Nucleolar Disorganization and Rpl11-MDM2-p53 Stress Response
Nucleolar morphology is governed by chromatin structure, so the high levels of euchromatic rDNA produced by the BBDS mutations disorganize the nucleolus and alter ribosome biogenesis. That in turn activates the Rpl11-MDM2-p53 nucleolar stress pathway. This is the node with therapeutic relevance: inhibiting p53 in cells expressing the BBDS mutations rescues the delayed osteoblast differentiation, which identifies p53 activation as an essential pathogenic step rather than a bystander.
ribosome biogenesis GO:0042254 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves dysregulated ribosome biogenesis (GO:0042254). GO:0042254 is a biological process from the Gene Ontology. ↕ DYSREGULATED DNA damage response, signal transduction by p53 class mediator GO:0030330 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased DNA damage response, signal transduction by p53 class mediator (GO:0030330). GO:0030330 is a biological process from the Gene Ontology. ↑ INCREASED
Downstream
Osteoprogenitor Proliferation with Delayed Differentiation
Show evidence (2 references)
PMID:28595297 SUPPORT In Vitro
"Nucleolar morphology is controlled by chromatin structure, and the high levels of euchromatic rDNA induced by the BBDS mutations direct nucleolar disorganization, alter ribosome biogenesis, and activate the Rpl11-Mdm2-p53 nucleolar stress response pathway."
The full route from euchromatic rDNA to the p53 nucleolar stress response.
PMID:28595297 SUPPORT In Vitro
"Inhibition of p53 in cells expressing the FGFR2 mutations in BBDS rescues delayed osteoblast differentiation, suggesting that p53 activation is an essential pathogenic factor in, and potential therapeutic target for, BBDS."
The rescue experiment establishing p53 as pathogenic rather than incidental, and the only therapeutic lead this disorder has.
Osteoprogenitor Proliferation with Delayed Differentiation
The convergent cellular lesion. Elevated ribosomal RNA in developing bone promotes osteoprogenitor proliferation while delaying differentiation into mature matrix-depositing osteoblasts. The histological correlate in patient long bones is a growth plate containing smaller hypertrophic chondrocytes together with a thickened, hypercellular periosteum — more cells, less differentiation.
Preosteoblast CL:0007010 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves Preosteoblast (CL:0007010). CL:0007010 is a cell type from the Cell Ontology.
osteoblast differentiation GO:0001649 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased osteoblast differentiation (GO:0001649). GO:0001649 is a biological process from the Gene Ontology. ↓ DECREASED cell population proliferation GO:0008283 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased cell population proliferation (GO:0008283). GO:0008283 is a biological process from the Gene Ontology. ↑ INCREASED
Downstream
Impaired Bone Mineralization and Structural Weakness
Show evidence (2 references)
PMID:24908667 SUPPORT In Vitro
"An increase in the nucleolar activity of FGFR2 in BBDS elevates levels of ribosomal RNA in the developing bone, consequently promoting osteoprogenitor cell proliferation and decreasing differentiation."
The proliferation-up, differentiation-down cellular phenotype in developing bone.
PMID:22387015 SUPPORT Human Clinical
"Histological analysis of the long bones revealed that the growth plate contained smaller hypertrophic chondrocytes and a thickened hypercellular periosteum."
The histological correlate in patient tissue.
Impaired Bone Mineralization and Structural Weakness
Osteoprogenitors that proliferate without differentiating do not deposit and mineralize matrix normally. The clinical readout is poor calvarial mineralization, reduced bone mineral density and osteopenia, and long bones weak enough to bend in utero — the feature the disorder is named for. Note this is a mineralization failure secondary to a cell-fate defect: it is not the calciopenic, phosphopenic or mineralization-inhibitor-excess mechanism of the defective_skeletal_mineralization module, so no conformance is declared there either.
biomineral tissue development GO:0031214 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased biomineral tissue development (GO:0031214). GO:0031214 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Bowing of the long bones
Decreased calvarial ossification
Reduced bone mineral density
Osteopenia
Craniosynostosis
Show evidence (1 reference)
PMID:41186107 SUPPORT Human Clinical
"Bent Bone Dysplasia (BBD), FGFR2-related (MIM# 614592), is a rare severe skeletal dysplasia characterized by craniofacial differences, reduced bone mineral density, and bowed long bones."
Links reduced bone mineral density to the bowing that names the disorder.
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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for FGFR2-Related Bent Bone Dysplasia Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
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Phenotypes

13
Head and Neck 1
Craniosynostosis HP:0001363 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Craniosynostosis (HP:0001363). HP:0001363 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:22387015 SUPPORT Human Clinical
"We have identified de novo FGFR2 mutations in a sporadically occurring perinatal lethal skeletal dysplasia characterized by poor mineralization of the calvarium, craniosynostosis, dysmorphic facial features, prenatal teeth, hypoplastic pubis and clavicles, osteopenia, and bent long bones."
Craniosynostosis named in the core phenotype.
Integument 1
Hirsutism HP:0001007 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hirsutism (HP:0001007). HP:0001007 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27240702 SUPPORT Human Clinical
"Craniofacial abnormalities, hirsutism, hepatic abnormalities, and genitourinary abnormalities were noted as well."
Hirsutism reported among the additional features.
Limbs 1
Bowing of the long bones HP:0006487 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Bowing of the long bones (HP:0006487). HP:0006487 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27240702 SUPPORT Human Clinical
"The prenatal phenotype included stillbirth, bending of the femora, and a high incidence of polyhydramnios, prematurity, and perinatal death in three of 11 patients in the series."
Documents femoral bending as part of the prenatal presentation.
Musculoskeletal 2
Osteopenia HP:0000938 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Osteopenia (HP:0000938). HP:0000938 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:22387015 SUPPORT Human Clinical
"prenatal teeth, hypoplastic pubis and clavicles, osteopenia, and bent long bones"
Osteopenia named in the core phenotype.
Reduced bone mineral density HP:0004349 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Reduced bone mineral density (HP:0004349). HP:0004349 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:41186107 SUPPORT Human Clinical
"Bent Bone Dysplasia (BBD), FGFR2-related (MIM# 614592), is a rare severe skeletal dysplasia characterized by craniofacial differences, reduced bone mineral density, and bowed long bones."
Names reduced bone mineral density as a core feature.
Prenatal and Birth 2
Polyhydramnios HP:0001561 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Polyhydramnios (HP:0001561). HP:0001561 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27240702 SUPPORT Human Clinical
"The prenatal phenotype included stillbirth, bending of the femora, and a high incidence of polyhydramnios, prematurity, and perinatal death in three of 11 patients in the series."
Polyhydramnios reported at high incidence prenatally.
Premature birth HP:0001622 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Premature birth (HP:0001622). HP:0001622 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27240702 SUPPORT Human Clinical
"The prenatal phenotype included stillbirth, bending of the femora, and a high incidence of polyhydramnios, prematurity, and perinatal death in three of 11 patients in the series."
Prematurity reported at high incidence.
Respiratory 1
Respiratory insufficiency HP:0002093 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Respiratory insufficiency (HP:0002093). HP:0002093 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:27240702 SUPPORT Human Clinical
"Longer-term survivors all needed ventilator support."
Establishes ventilator dependence in every reported survivor.
PMID:41186107 SUPPORT Human Clinical
"who initially presented neonatally with respiratory insufficiency requiring intubation, facial dysmorphism, and skeletal differences"
The neonatal respiratory presentation in the longest-surviving individual.
Other 5
Decreased calvarial ossification HP:0005474 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Decreased calvarial ossification (HP:0005474). HP:0005474 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27240702 SUPPORT Human Clinical
"The survivors presented with characteristic radiographic findings that were observed among those with lethality, including bent bones, distinctive (moustache-shaped) small clavicles, angel-shaped metacarpals and phalanges, poor mineralization of the calvarium, and craniosynostosis."
Lists poor calvarial mineralization among the findings shared by survivors and lethal cases.
Angel-shaped phalanx HP:0032078 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Angel-shaped phalanx (HP:0032078). HP:0032078 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27240702 SUPPORT Human Clinical
"The survivors presented with characteristic radiographic findings that were observed among those with lethality, including bent bones, distinctive (moustache-shaped) small clavicles, angel-shaped metacarpals and phalanges, poor mineralization of the calvarium, and craniosynostosis."
The radiographic signature including angel-shaped metacarpals and phalanges.
Short clavicles HP:0000894 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Short clavicles (HP:0000894). HP:0000894 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:22387015 SUPPORT Human Clinical
"prenatal teeth, hypoplastic pubis and clavicles, osteopenia, and bent long bones"
Hypoplastic clavicles named in the core phenotype.
Hypoplastic pubic bone HP:0003173 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypoplastic pubic bone (HP:0003173). HP:0003173 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:22387015 SUPPORT Human Clinical
"prenatal teeth, hypoplastic pubis and clavicles, osteopenia, and bent long bones"
Hypoplastic pubis named in the core phenotype.
Natal tooth HP:0000695 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Natal tooth (HP:0000695). HP:0000695 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:22387015 SUPPORT Human Clinical
"poor mineralization of the calvarium, craniosynostosis, dysmorphic facial features, prenatal teeth, hypoplastic pubis and clavicles"
Prenatal teeth named in the core phenotype.
🧬

Genetic Associations

1
FGFR2 transmembrane-domain variants (Causative)
Gene: FGFR2 hgnc:3689 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is FGFR2 (hgnc:3689). hgnc:3689 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (2 references)
PMID:22387015 SUPPORT Human Clinical
"Four unrelated affected individuals were found to be heterozygous for missense mutations that introduce a polar amino acid into the hydrophobic transmembrane domain of FGFR2."
The defining allele class.
PMID:27240702 SUPPORT Human Clinical
"Heterozygosity for mutations in the gene that encodes Fibroblast Growth Factor Receptor 2 (FGFR2) was identified in the nine individuals with available DNA."
Molecular confirmation across the assembled series.
💊

Medical Actions

1
Respiratory support and tracheostomy
Category: Therapeutic Action: TracheostomyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Tracheostomy, annotated with Tracheotomy (NCIT:C15341). NCIT:C15341 is a clinical intervention from the NCI Thesaurus. Ontology label: Tracheotomy NCIT:C15341
There is no disease-modifying therapy. Management is supportive and dominated by the airway: every reported longer-term survivor required ventilator support, and the longest-surviving individual is maintained on nocturnal mechanical ventilation via tracheostomy together with gastrostomy feeding. Because survival past the neonatal period is possible, the decision to institute long-term respiratory support is a real clinical choice rather than a foregone one.
Target Phenotypes: Respiratory insufficiency HP:0002093 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Respiratory insufficiency (HP:0002093). HP:0002093 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:27240702 SUPPORT Human Clinical
"Longer-term survivors all needed ventilator support."
Establishes ventilator support as universal among survivors.
PMID:41186107 SUPPORT Human Clinical
"He is able to walk, is interactive, has a tracheostomy for mechanical ventilation at night and a gastrostomy tube, and continues to make steady developmental progress."
The supportive regimen in the longest-surviving reported individual.
📈

Progression

2
Prenatal and perinatal
Age: In utero through the neonatal period
The period in which most affected individuals die. The prenatal presentation includes femoral bending detectable on ultrasound, polyhydramnios, prematurity and stillbirth, and perinatal death occurred in three of eleven patients in the largest series. The disorder was originally described as perinatal lethal.
Show evidence (2 references)
PMID:27240702 SUPPORT Human Clinical
"The prenatal phenotype included stillbirth, bending of the femora, and a high incidence of polyhydramnios, prematurity, and perinatal death in three of 11 patients in the series."
The prenatal and perinatal outcome data from the largest series.
PMID:22387015 SUPPORT Human Clinical
"We have identified de novo FGFR2 mutations in a sporadically occurring perinatal lethal skeletal dysplasia"
The original characterization as perinatal lethal.
Childhood in survivors
Age: Beyond the neonatal period
Survival past the neonatal period is possible and the disorder should not be counted as invariably lethal. The longest reported survivor is a 4-year-old who walks, is interactive, and continues to make developmental progress, supported by a tracheostomy for nocturnal ventilation and a gastrostomy tube. This matters for counselling: the published series are weighted toward lethal presentations by ascertainment, so the surviving phenotype is systematically under-described.
Show evidence (2 references)
PMID:41186107 SUPPORT Human Clinical
"He is able to walk, is interactive, has a tracheostomy for mechanical ventilation at night and a gastrostomy tube, and continues to make steady developmental progress."
The functional status of the longest reported survivor.
PMID:41186107 SUPPORT Human Clinical
"Our findings suggest that BBD, FGFR2-related is not definitively lethal perinatally. While the risk for complications and early postnatal death remains high, as demonstrated in previous literature, survival beyond the early postnatal period is possible."
The explicit revision of the perinatal-lethal characterization.
📊

Prevalence

1
Worldwide, published-case literature
Cases In Literature <1 in 1,000,000
No population estimate exists. Fourteen individuals had been reported as of 2025, following an 11-patient series in 2016 that included the original four. Most died prenatally or neonatally, so the reported series are weighted toward lethal presentations and the surviving phenotype is correspondingly under-described. This case count is the ceiling on every frequency claim in this entry, which is why no phenotype below carries a frequency band.
Show evidence (2 references)
PMID:41186107 SUPPORT Human Clinical
"Only 14 individuals have been reported, of which the majority died prenatally or neonatally."
The 2025 published-case count and the ascertainment skew toward lethal cases.
PMID:27240702 SUPPORT Human Clinical
"Here we report on 11 individuals, including the original four patients plus seven new individuals with three longer-term survivors."
The largest assembled series, and the source of the surviving-phenotype description.
🔀

Differential Diagnoses

1

Conditions with similar clinical presentations that must be differentiated from FGFR2-Related Bent Bone Dysplasia:

{ }

Source YAML

click to show 
name: FGFR2-Related Bent Bone Dysplasia
synonyms:
- BBDS
- BBDS1
- Bent Bone Dysplasia Syndrome
- Bent Bone Dysplasia Syndrome 1
- bent bone dysplasia-FGFR2 type
- bent bone dysplasia with distinctive clavicles and angel-shaped phalanges
- FGFR2-related bent bone dysplasia
creation_date: "2026-08-22T00:00:00Z"
category: Mendelian
description: >
  Bent bone dysplasia syndrome (BBDS) is an ultra-rare, usually perinatally lethal
  skeletal dysplasia caused by de novo heterozygous FGFR2 variants that place a polar
  residue in the hydrophobic transmembrane domain. It combines bent long bones with
  poor calvarial mineralization, craniosynostosis, prenatal teeth, hypoplastic pubis and
  clavicles, osteopenia, and a distinctive radiographic signature — moustache-shaped
  small clavicles and angel-shaped metacarpals and phalanges.

  BBDS is an FGFR2 disorder that is deliberately NOT curated as a member of the
  FGFR-Related Skeletal Dysplasias grouping, and the reason is the point of the entry.
  Every member of that group is defined by a constitutively active or ligand-hypersensitive
  receptor. The BBDS alleles do the opposite at the plasma membrane: they selectively
  reduce membrane FGFR2 and markedly diminish the receptor's responsiveness to
  extracellular FGF. The original delineation states this in its title — "deficient
  canonical FGF signaling" — and concludes the disorder is separate from previously
  characterized FGFR2 disorders.

  What is gained is not receptor activity but receptor *location*. The same mutations
  amplify nucleolar targeting of FGFR2, where it occupies the ribosomal DNA promoter and
  activates rDNA transcription through FGF2 and UBF1 by de-repressing RUNX2. The
  consequence is a ribosome-biogenesis disorder of the osteoprogenitor: elevated rRNA,
  increased proliferation, delayed differentiation, and — via epigenetic activation of
  normally silent rDNA — nucleolar disorganization that triggers the Rpl11-MDM2-p53
  nucleolar stress response. Inhibiting p53 rescues the differentiation delay in cells
  carrying the BBDS mutations, which makes p53 a candidate therapeutic target and is the
  strongest mechanistic claim available for this disorder.

  The contrast with the gain-of-function group is sharpest at the residue level. BBDS
  alleles sit in the transmembrane domain, the same domain as achondroplasia p.Gly380Arg
  and Beare-Stevenson p.Tyr375Cys, and FGFR2 Tyr-381 substitutions have been reported to
  cause either Crouzon syndrome or bent bone dysplasia depending on the substituting
  residue. Same gene, same domain, in one case the same codon — opposite consequence for
  the receptor.
disease_term:
  preferred_term: bent bone dysplasia syndrome 1
  term:
    id: MONDO:0013815
    label: bent bone dysplasia syndrome 1
parents:
- FGFR2-related skeletal dysplasia
- Perinatal lethal skeletal dysplasia
classifications:
  harrisons_chapter:
  - classification_value: GENETICS_ENVIRONMENT_DISEASE
    notes: >-
      Mendelian, de novo dominant skeletal dysplasia (ISDS bent-bone group).
      Curated under the genetics/heritable-disease Part rather than an
      organ-system Part despite the perinatal respiratory failure, which is a
      consequence of the skeletal phenotype.
  isds_skeletal_category:
  - classification_value: bent_bone_dysplasia
    notes: >-
      ISDS Nosology of Genetic Skeletal Disorders, 2023 revision (11th edition;
      Unger et al., PMID:36779427), group 20 "Bent bones dysplasia group",
      entry NOS 20-0050, listed there as "Bent bone dysplasia, FGFR2-related"
      (AD, FGFR2, MIM 614592). Note the committee places it with the bent-bone
      dysplasias rather than with the craniosynostosis syndromes, which is
      consistent with its distinct receptor mechanism — the BBDS alleles reduce
      canonical FGF signaling, where the FGFR2 craniosynostoses are
      gain-of-function.

      Group 20 is a radiographic grouping, and the committee says so in the
      table footnote: "Bent bones is an unspecific finding, particularly in a
      prenatal setting, that can be observed in numerous other conditions, such
      as those with bone fragility". The placement therefore records the
      radiographic sign, not a shared mechanism with the other five members.
inheritance:
- name: Autosomal dominant
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  description: >
    Autosomal dominant, arising de novo. All reported cases have been sporadic, occurring
    in a single affected individual of unaffected parents, which is expected given that
    the disorder is usually lethal before or shortly after birth.
  evidence:
  - reference: PMID:22387015
    reference_title: "Bent bone dysplasia-FGFR2 type, a distinct skeletal disorder, has deficient canonical FGF signaling."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We have identified de novo FGFR2 mutations in a sporadically occurring perinatal lethal skeletal dysplasia characterized by poor mineralization of the calvarium, craniosynostosis, dysmorphic facial features, prenatal teeth, hypoplastic pubis and clavicles, osteopenia, and bent long bones."
    explanation: >-
      The delineating paper establishes de novo occurrence and gives the full core
      phenotype in one sentence.
prevalence:
- population: Worldwide, published-case literature
  measure_type: CASES_IN_LITERATURE
  prevalence_class: BELOW_1_IN_1000000
  notes: >-
    No population estimate exists. Fourteen individuals had been reported as of 2025,
    following an 11-patient series in 2016 that included the original four. Most died
    prenatally or neonatally, so the reported series are weighted toward lethal
    presentations and the surviving phenotype is correspondingly under-described. This
    case count is the ceiling on every frequency claim in this entry, which is why no
    phenotype below carries a frequency band.
  evidence:
  - reference: PMID:41186107
    reference_title: "Clinical Presentation of the Longest Reported Living Individual With Bent Bone Dysplasia-FGFR2-Related."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Only 14 individuals have been reported, of which the majority died prenatally or neonatally."
    explanation: The 2025 published-case count and the ascertainment skew toward lethal cases.
  - reference: PMID:27240702
    reference_title: "Clinical and radiographic delineation of Bent Bone Dysplasia-FGFR2 type or Bent Bone Dysplasia with Distinctive Clavicles and Angel-shaped Phalanges."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here we report on 11 individuals, including the original four patients plus seven new individuals with three longer-term survivors."
    explanation: The largest assembled series, and the source of the surviving-phenotype description.
pathophysiology:
- name: FGFR2 Transmembrane Polar Substitution
  role: trigger
  biological_scale: MOLECULAR
  description: >
    A de novo heterozygous FGFR2 missense variant introducing a polar amino acid into the
    hydrophobic transmembrane domain — the reported alleles include p.Tyr381Asp and
    p.Met391Arg. This is the same domain in which achondroplasia (FGFR3 p.Gly380Arg) and
    Beare-Stevenson syndrome (FGFR2 p.Tyr375Cys) alleles sit, and FGFR2 Tyr-381
    substitutions have been reported to give either Crouzon syndrome or bent bone
    dysplasia depending on which residue is substituted in. Despite that adjacency, the
    consequence here is not constitutive activation, which is why this entry carries no
    conforms_to anchor against the FGFR gain-of-function module.
  gene:
    preferred_term: FGFR2
    description: >-
      Fibroblast growth factor receptor 2. In BBDS the variant alters receptor
      trafficking and subcellular distribution rather than conferring constitutive
      kinase activity.
    term:
      id: hgnc:3689
      label: FGFR2
  evidence:
  - reference: PMID:22387015
    reference_title: "Bent bone dysplasia-FGFR2 type, a distinct skeletal disorder, has deficient canonical FGF signaling."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Four unrelated affected individuals were found to be heterozygous for missense mutations that introduce a polar amino acid into the hydrophobic transmembrane domain of FGFR2."
    explanation: Defines the allele class and its position in the receptor.
  - reference: PMID:41186107
    reference_title: "Clinical Presentation of the Longest Reported Living Individual With Bent Bone Dysplasia-FGFR2-Related."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report the clinical trajectory of a 4-year-old male with BBD, FGFR2-related (FGFR2, c.1141T>G), who initially presented neonatally with respiratory insufficiency requiring intubation, facial dysmorphism, and skeletal differences."
    explanation: A specific reported allele in the longest-surviving individual.
  downstream:
  - target: Reduced Plasma-Membrane FGFR2 and Deficient Canonical FGF Signaling
    description: >
      The polar transmembrane residue impairs normal receptor trafficking to and
      retention at the plasma membrane.
    causal_link_type: DIRECT
  - target: Enhanced Nucleolar FGFR2 Occupancy at rDNA
    description: >
      The same substitutions amplify nucleolar targeting of the receptor, redistributing
      it rather than simply reducing it.
    causal_link_type: DIRECT

- name: Reduced Plasma-Membrane FGFR2 and Deficient Canonical FGF Signaling
  biological_scale: MOLECULAR
  description: >
    The arm that disqualifies BBDS from the FGFR gain-of-function grouping. Membrane
    FGFR2 levels are selectively reduced and the receptor's responsiveness to
    extracellular FGF is markedly diminished, demonstrated both in patient-derived
    chondrocytes and in a cell-based assay. This is loss of canonical signaling, not gain
    — the mechanistic inverse of the constitutive, ligand-independent activation that
    characterizes Apert, Crouzon, Pfeiffer, Jackson-Weiss and Beare-Stevenson syndrome.
  molecular_functions:
  - preferred_term: transmembrane receptor protein tyrosine kinase activity
    term:
      id: GO:0004714
      label: transmembrane receptor protein tyrosine kinase activity
    modifier: DECREASED
  biological_processes:
  - preferred_term: fibroblast growth factor receptor signaling pathway
    term:
      id: GO:0008543
      label: fibroblast growth factor receptor signaling pathway
    modifier: DECREASED
  evidence:
  - reference: PMID:22387015
    reference_title: "Bent bone dysplasia-FGFR2 type, a distinct skeletal disorder, has deficient canonical FGF signaling."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Using diseased chondrocytes and a cell-based assay, we determined that these mutations selectively reduced plasma-membrane levels of FGFR2 and markedly diminished the receptor's responsiveness to extracellular FGF."
    explanation: >-
      The direct demonstration that canonical membrane signaling is reduced, which is why
      this entry is not a member of the FGFR gain-of-function grouping.
  - reference: PMID:22387015
    reference_title: "Bent bone dysplasia-FGFR2 type, a distinct skeletal disorder, has deficient canonical FGF signaling."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All together, these clinical and molecular findings are separate from previously characterized FGFR2 disorders and represent a distinct skeletal dysplasia."
    explanation: >-
      The delineating authors' own conclusion that BBDS is mechanistically separate from
      the other FGFR2 disorders.

- name: Enhanced Nucleolar FGFR2 Occupancy at rDNA
  biological_scale: MOLECULAR
  description: >
    The gain in BBDS is of receptor location, not receptor activity. The mutations amplify
    nucleolar targeting of FGFR2, where it occupies the ribosomal DNA promoter and
    activates rDNA transcription through interactions with FGF2 and UBF1, de-repressing
    RUNX2. A second study showed the mutant receptor also augments recruitment of
    histone-remodeling factors that epigenetically activate rDNA that is normally
    transcriptionally silent — so the effect is not only more transcription at active
    loci but conversion of silent loci to euchromatin.
  cellular_components:
  - preferred_term: nucleolus
    term:
      id: GO:0005730
      label: nucleolus
  biological_processes:
  - preferred_term: rRNA transcription
    term:
      id: GO:0009303
      label: rRNA transcription
    modifier: INCREASED
  evidence:
  - reference: PMID:24908667
    reference_title: "Bent bone dysplasia syndrome reveals nucleolar activity for FGFR2 in ribosomal DNA transcription."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "In this study, we find that these mutations, despite reducing canonical signaling, enhance nucleolar occupancy of FGFR2 at the ribosomal DNA (rDNA) promoter."
    explanation: >-
      States the relocalization gain explicitly against the background of reduced
      canonical signaling — the single most important sentence for this entry.
  - reference: PMID:24908667
    reference_title: "Bent bone dysplasia syndrome reveals nucleolar activity for FGFR2 in ribosomal DNA transcription."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Nucleolar FGFR2 activates rDNA transcription via interactions with FGF2 and UBF1 by de-repressing RUNX2."
    explanation: The molecular route from nucleolar receptor to rDNA transcription.
  - reference: PMID:28595297
    reference_title: "FGFR2 mutations in bent bone dysplasia syndrome activate nucleolar stress and perturb cell fate determination."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Here we find that the BBDS mutations augment the ability of FGFR2 to recruit histone-remodeling factors that epigenetically activate transcriptionally silent rDNA."
    explanation: Extends the mechanism to epigenetic activation of normally silent rDNA loci.
  downstream:
  - target: Osteoprogenitor Proliferation with Delayed Differentiation
    causal_link_type: DIRECT
  - target: Nucleolar Disorganization and Rpl11-MDM2-p53 Stress Response
    causal_link_type: DIRECT

- name: Nucleolar Disorganization and Rpl11-MDM2-p53 Stress Response
  biological_scale: CELLULAR
  description: >
    Nucleolar morphology is governed by chromatin structure, so the high levels of
    euchromatic rDNA produced by the BBDS mutations disorganize the nucleolus and alter
    ribosome biogenesis. That in turn activates the Rpl11-MDM2-p53 nucleolar stress
    pathway. This is the node with therapeutic relevance: inhibiting p53 in cells
    expressing the BBDS mutations rescues the delayed osteoblast differentiation, which
    identifies p53 activation as an essential pathogenic step rather than a bystander.
  biological_processes:
  - preferred_term: ribosome biogenesis
    term:
      id: GO:0042254
      label: ribosome biogenesis
    modifier: DYSREGULATED
  - preferred_term: DNA damage response, signal transduction by p53 class mediator
    term:
      id: GO:0030330
      label: DNA damage response, signal transduction by p53 class mediator
    modifier: INCREASED
  evidence:
  - reference: PMID:28595297
    reference_title: "FGFR2 mutations in bent bone dysplasia syndrome activate nucleolar stress and perturb cell fate determination."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Nucleolar morphology is controlled by chromatin structure, and the high levels of euchromatic rDNA induced by the BBDS mutations direct nucleolar disorganization, alter ribosome biogenesis, and activate the Rpl11-Mdm2-p53 nucleolar stress response pathway."
    explanation: The full route from euchromatic rDNA to the p53 nucleolar stress response.
  - reference: PMID:28595297
    reference_title: "FGFR2 mutations in bent bone dysplasia syndrome activate nucleolar stress and perturb cell fate determination."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Inhibition of p53 in cells expressing the FGFR2 mutations in BBDS rescues delayed osteoblast differentiation, suggesting that p53 activation is an essential pathogenic factor in, and potential therapeutic target for, BBDS."
    explanation: >-
      The rescue experiment establishing p53 as pathogenic rather than incidental, and
      the only therapeutic lead this disorder has.
  downstream:
  - target: Osteoprogenitor Proliferation with Delayed Differentiation
    causal_link_type: DIRECT

- name: Osteoprogenitor Proliferation with Delayed Differentiation
  biological_scale: CELLULAR
  description: >
    The convergent cellular lesion. Elevated ribosomal RNA in developing bone promotes
    osteoprogenitor proliferation while delaying differentiation into mature
    matrix-depositing osteoblasts. The histological correlate in patient long bones is a
    growth plate containing smaller hypertrophic chondrocytes together with a thickened,
    hypercellular periosteum — more cells, less differentiation.
  cell_types:
  - preferred_term: Preosteoblast
    term:
      id: CL:0007010
      label: preosteoblast
  biological_processes:
  - preferred_term: osteoblast differentiation
    term:
      id: GO:0001649
      label: osteoblast differentiation
    modifier: DECREASED
  - preferred_term: cell population proliferation
    term:
      id: GO:0008283
      label: cell population proliferation
    modifier: INCREASED
  evidence:
  - reference: PMID:24908667
    reference_title: "Bent bone dysplasia syndrome reveals nucleolar activity for FGFR2 in ribosomal DNA transcription."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "An increase in the nucleolar activity of FGFR2 in BBDS elevates levels of ribosomal RNA in the developing bone, consequently promoting osteoprogenitor cell proliferation and decreasing differentiation."
    explanation: The proliferation-up, differentiation-down cellular phenotype in developing bone.
  - reference: PMID:22387015
    reference_title: "Bent bone dysplasia-FGFR2 type, a distinct skeletal disorder, has deficient canonical FGF signaling."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Histological analysis of the long bones revealed that the growth plate contained smaller hypertrophic chondrocytes and a thickened hypercellular periosteum."
    explanation: The histological correlate in patient tissue.
  downstream:
  - target: Impaired Bone Mineralization and Structural Weakness
    causal_link_type: DIRECT

- name: Impaired Bone Mineralization and Structural Weakness
  biological_scale: TISSUE
  description: >
    Osteoprogenitors that proliferate without differentiating do not deposit and mineralize
    matrix normally. The clinical readout is poor calvarial mineralization, reduced bone
    mineral density and osteopenia, and long bones weak enough to bend in utero — the
    feature the disorder is named for. Note this is a mineralization failure secondary to
    a cell-fate defect: it is not the calciopenic, phosphopenic or
    mineralization-inhibitor-excess mechanism of the defective_skeletal_mineralization
    module, so no conformance is declared there either.
  biological_processes:
  - preferred_term: biomineral tissue development
    term:
      id: GO:0031214
      label: biomineral tissue development
    modifier: DECREASED
  evidence:
  - reference: PMID:41186107
    reference_title: "Clinical Presentation of the Longest Reported Living Individual With Bent Bone Dysplasia-FGFR2-Related."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Bent Bone Dysplasia (BBD), FGFR2-related (MIM# 614592), is a rare severe skeletal dysplasia characterized by craniofacial differences, reduced bone mineral density, and bowed long bones."
    explanation: Links reduced bone mineral density to the bowing that names the disorder.
  downstream:
  - target: Bowing of the long bones
  - target: Decreased calvarial ossification
  - target: Reduced bone mineral density
  - target: Osteopenia
  - target: Craniosynostosis
phenotypes:
- category: Skeletal
  name: Bowing of the long bones
  description: >
    Bent long bones, most conspicuously the femora, detectable prenatally and the feature
    for which the disorder is named.
  phenotype_term:
    preferred_term: Bowing of the long bones
    term:
      id: HP:0006487
      label: Bowing of the long bones
  evidence:
  - reference: PMID:27240702
    reference_title: "Clinical and radiographic delineation of Bent Bone Dysplasia-FGFR2 type or Bent Bone Dysplasia with Distinctive Clavicles and Angel-shaped Phalanges."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The prenatal phenotype included stillbirth, bending of the femora, and a high incidence of polyhydramnios, prematurity, and perinatal death in three of 11 patients in the series."
    explanation: Documents femoral bending as part of the prenatal presentation.
- category: Craniofacial
  name: Decreased calvarial ossification
  description: >
    Poor mineralization of the skull vault, present in both lethal and surviving
    individuals, and a striking counterpoint to the coexisting craniosynostosis.
  phenotype_term:
    preferred_term: Decreased calvarial ossification
    term:
      id: HP:0005474
      label: Decreased calvarial ossification
  evidence:
  - reference: PMID:27240702
    reference_title: "Clinical and radiographic delineation of Bent Bone Dysplasia-FGFR2 type or Bent Bone Dysplasia with Distinctive Clavicles and Angel-shaped Phalanges."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The survivors presented with characteristic radiographic findings that were observed among those with lethality, including bent bones, distinctive (moustache-shaped) small clavicles, angel-shaped metacarpals and phalanges, poor mineralization of the calvarium, and craniosynostosis."
    explanation: >-
      Lists poor calvarial mineralization among the findings shared by survivors and
      lethal cases.
- category: Craniofacial
  name: Craniosynostosis
  description: >
    Premature suture fusion, present alongside poor calvarial mineralization. Its presence
    here is what makes BBDS look superficially like the FGFR craniosynostosis syndromes
    despite the opposite receptor mechanism.
  phenotype_term:
    preferred_term: Craniosynostosis
    term:
      id: HP:0001363
      label: Craniosynostosis
  evidence:
  - reference: PMID:22387015
    reference_title: "Bent bone dysplasia-FGFR2 type, a distinct skeletal disorder, has deficient canonical FGF signaling."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We have identified de novo FGFR2 mutations in a sporadically occurring perinatal lethal skeletal dysplasia characterized by poor mineralization of the calvarium, craniosynostosis, dysmorphic facial features, prenatal teeth, hypoplastic pubis and clavicles, osteopenia, and bent long bones."
    explanation: Craniosynostosis named in the core phenotype.
- category: Skeletal
  name: Angel-shaped phalanx
  description: >
    A distinctive radiographic sign of the metacarpals and phalanges, part of the
    diagnostic radiographic signature together with the moustache-shaped clavicles.
  phenotype_term:
    preferred_term: Angel-shaped phalanx
    term:
      id: HP:0032078
      label: Angel-shaped phalanx
  evidence:
  - reference: PMID:27240702
    reference_title: "Clinical and radiographic delineation of Bent Bone Dysplasia-FGFR2 type or Bent Bone Dysplasia with Distinctive Clavicles and Angel-shaped Phalanges."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The survivors presented with characteristic radiographic findings that were observed among those with lethality, including bent bones, distinctive (moustache-shaped) small clavicles, angel-shaped metacarpals and phalanges, poor mineralization of the calvarium, and craniosynostosis."
    explanation: The radiographic signature including angel-shaped metacarpals and phalanges.
- category: Skeletal
  name: Short clavicles
  description: >
    Small, hypoplastic clavicles with a characteristic moustache shape on radiographs.
  phenotype_term:
    preferred_term: Short clavicles
    term:
      id: HP:0000894
      label: Short clavicles
  evidence:
  - reference: PMID:22387015
    reference_title: "Bent bone dysplasia-FGFR2 type, a distinct skeletal disorder, has deficient canonical FGF signaling."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "prenatal teeth, hypoplastic pubis and clavicles, osteopenia, and bent long bones"
    explanation: Hypoplastic clavicles named in the core phenotype.
- category: Skeletal
  name: Hypoplastic pubic bone
  description: Underdeveloped pubic bones, part of the original phenotype description.
  phenotype_term:
    preferred_term: Hypoplastic pubic bone
    term:
      id: HP:0003173
      label: Hypoplastic pubic bone
  evidence:
  - reference: PMID:22387015
    reference_title: "Bent bone dysplasia-FGFR2 type, a distinct skeletal disorder, has deficient canonical FGF signaling."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "prenatal teeth, hypoplastic pubis and clavicles, osteopenia, and bent long bones"
    explanation: Hypoplastic pubis named in the core phenotype.
- category: Skeletal
  name: Osteopenia
  description: Generalized reduction in bone density, consistent with the mineralization defect.
  phenotype_term:
    preferred_term: Osteopenia
    term:
      id: HP:0000938
      label: Osteopenia
  evidence:
  - reference: PMID:22387015
    reference_title: "Bent bone dysplasia-FGFR2 type, a distinct skeletal disorder, has deficient canonical FGF signaling."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "prenatal teeth, hypoplastic pubis and clavicles, osteopenia, and bent long bones"
    explanation: Osteopenia named in the core phenotype.
- category: Skeletal
  name: Reduced bone mineral density
  description: >
    Quantitatively reduced mineralization, described as a defining feature alongside the
    craniofacial differences and bowed long bones.
  phenotype_term:
    preferred_term: Reduced bone mineral density
    term:
      id: HP:0004349
      label: Reduced bone mineral density
  evidence:
  - reference: PMID:41186107
    reference_title: "Clinical Presentation of the Longest Reported Living Individual With Bent Bone Dysplasia-FGFR2-Related."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Bent Bone Dysplasia (BBD), FGFR2-related (MIM# 614592), is a rare severe skeletal dysplasia characterized by craniofacial differences, reduced bone mineral density, and bowed long bones."
    explanation: Names reduced bone mineral density as a core feature.
- category: Dental
  name: Natal tooth
  description: >
    Prenatal (natal) teeth, an unusual feature shared with Beare-Stevenson syndrome among
    the FGFR2 disorders.
  phenotype_term:
    preferred_term: Natal tooth
    term:
      id: HP:0000695
      label: Natal tooth
  evidence:
  - reference: PMID:22387015
    reference_title: "Bent bone dysplasia-FGFR2 type, a distinct skeletal disorder, has deficient canonical FGF signaling."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "poor mineralization of the calvarium, craniosynostosis, dysmorphic facial features, prenatal teeth, hypoplastic pubis and clavicles"
    explanation: Prenatal teeth named in the core phenotype.
- category: Prenatal
  name: Polyhydramnios
  description: Excess amniotic fluid, reported at high incidence in the prenatal presentation.
  phenotype_term:
    preferred_term: Polyhydramnios
    term:
      id: HP:0001561
      label: Polyhydramnios
  evidence:
  - reference: PMID:27240702
    reference_title: "Clinical and radiographic delineation of Bent Bone Dysplasia-FGFR2 type or Bent Bone Dysplasia with Distinctive Clavicles and Angel-shaped Phalanges."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The prenatal phenotype included stillbirth, bending of the femora, and a high incidence of polyhydramnios, prematurity, and perinatal death in three of 11 patients in the series."
    explanation: Polyhydramnios reported at high incidence prenatally.
- category: Prenatal
  name: Premature birth
  description: Preterm delivery, reported at high incidence in the series.
  phenotype_term:
    preferred_term: Premature birth
    term:
      id: HP:0001622
      label: Premature birth
  evidence:
  - reference: PMID:27240702
    reference_title: "Clinical and radiographic delineation of Bent Bone Dysplasia-FGFR2 type or Bent Bone Dysplasia with Distinctive Clavicles and Angel-shaped Phalanges."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The prenatal phenotype included stillbirth, bending of the femora, and a high incidence of polyhydramnios, prematurity, and perinatal death in three of 11 patients in the series."
    explanation: Prematurity reported at high incidence.
- category: Respiratory
  name: Respiratory insufficiency
  description: >
    Respiratory failure is the proximate cause of death in most cases and the dominant
    ongoing problem in survivors, all of whom required ventilator support.
  phenotype_term:
    preferred_term: Respiratory insufficiency
    term:
      id: HP:0002093
      label: Respiratory insufficiency
  evidence:
  - reference: PMID:27240702
    reference_title: "Clinical and radiographic delineation of Bent Bone Dysplasia-FGFR2 type or Bent Bone Dysplasia with Distinctive Clavicles and Angel-shaped Phalanges."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Longer-term survivors all needed ventilator support."
    explanation: Establishes ventilator dependence in every reported survivor.
  - reference: PMID:41186107
    reference_title: "Clinical Presentation of the Longest Reported Living Individual With Bent Bone Dysplasia-FGFR2-Related."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "who initially presented neonatally with respiratory insufficiency requiring intubation, facial dysmorphism, and skeletal differences"
    explanation: The neonatal respiratory presentation in the longest-surviving individual.
- category: Dermatologic
  name: Hirsutism
  description: >
    Excess hair growth, reported among the additional features in the assembled series.
  phenotype_term:
    preferred_term: Hirsutism
    term:
      id: HP:0001007
      label: Hirsutism
  evidence:
  - reference: PMID:27240702
    reference_title: "Clinical and radiographic delineation of Bent Bone Dysplasia-FGFR2 type or Bent Bone Dysplasia with Distinctive Clavicles and Angel-shaped Phalanges."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Craniofacial abnormalities, hirsutism, hepatic abnormalities, and genitourinary abnormalities were noted as well."
    explanation: Hirsutism reported among the additional features.
genetic:
- name: FGFR2 transmembrane-domain variants
  gene_term:
    preferred_term: FGFR2
    term:
      id: hgnc:3689
      label: FGFR2
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  notes: >-
    De novo heterozygous missense variants placing a polar residue in the hydrophobic
    transmembrane domain. Reported alleles include p.Tyr381Asp (c.1141T>G) and
    p.Met391Arg. Substitutions at FGFR2 Tyr-381 have been reported to cause either
    Crouzon syndrome or bent bone dysplasia depending on the substituting residue, making
    this codon a clean demonstration that the disorder is set by the biophysical
    consequence of the substitution rather than by its position.
  evidence:
  - reference: PMID:22387015
    reference_title: "Bent bone dysplasia-FGFR2 type, a distinct skeletal disorder, has deficient canonical FGF signaling."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Four unrelated affected individuals were found to be heterozygous for missense mutations that introduce a polar amino acid into the hydrophobic transmembrane domain of FGFR2."
    explanation: The defining allele class.
  - reference: PMID:27240702
    reference_title: "Clinical and radiographic delineation of Bent Bone Dysplasia-FGFR2 type or Bent Bone Dysplasia with Distinctive Clavicles and Angel-shaped Phalanges."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Heterozygosity for mutations in the gene that encodes Fibroblast Growth Factor Receptor 2 (FGFR2) was identified in the nine individuals with available DNA."
    explanation: Molecular confirmation across the assembled series.
progression:
- phase: Prenatal and perinatal
  age_range: In utero through the neonatal period
  notes: >-
    The period in which most affected individuals die. The prenatal presentation includes
    femoral bending detectable on ultrasound, polyhydramnios, prematurity and stillbirth,
    and perinatal death occurred in three of eleven patients in the largest series. The
    disorder was originally described as perinatal lethal.
  evidence:
  - reference: PMID:27240702
    reference_title: "Clinical and radiographic delineation of Bent Bone Dysplasia-FGFR2 type or Bent Bone Dysplasia with Distinctive Clavicles and Angel-shaped Phalanges."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The prenatal phenotype included stillbirth, bending of the femora, and a high incidence of polyhydramnios, prematurity, and perinatal death in three of 11 patients in the series."
    explanation: The prenatal and perinatal outcome data from the largest series.
  - reference: PMID:22387015
    reference_title: "Bent bone dysplasia-FGFR2 type, a distinct skeletal disorder, has deficient canonical FGF signaling."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We have identified de novo FGFR2 mutations in a sporadically occurring perinatal lethal skeletal dysplasia"
    explanation: The original characterization as perinatal lethal.
- phase: Childhood in survivors
  age_range: Beyond the neonatal period
  notes: >-
    Survival past the neonatal period is possible and the disorder should not be counted
    as invariably lethal. The longest reported survivor is a 4-year-old who walks, is
    interactive, and continues to make developmental progress, supported by a tracheostomy
    for nocturnal ventilation and a gastrostomy tube. This matters for counselling: the
    published series are weighted toward lethal presentations by ascertainment, so the
    surviving phenotype is systematically under-described.
  evidence:
  - reference: PMID:41186107
    reference_title: "Clinical Presentation of the Longest Reported Living Individual With Bent Bone Dysplasia-FGFR2-Related."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "He is able to walk, is interactive, has a tracheostomy for mechanical ventilation at night and a gastrostomy tube, and continues to make steady developmental progress."
    explanation: The functional status of the longest reported survivor.
  - reference: PMID:41186107
    reference_title: "Clinical Presentation of the Longest Reported Living Individual With Bent Bone Dysplasia-FGFR2-Related."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our findings suggest that BBD, FGFR2-related is not definitively lethal perinatally. While the risk for complications and early postnatal death remains high, as demonstrated in previous literature, survival beyond the early postnatal period is possible."
    explanation: The explicit revision of the perinatal-lethal characterization.
treatments:
- name: Respiratory support and tracheostomy
  action_category: THERAPEUTIC
  therapeutic_modality: SURGERY
  description: >
    There is no disease-modifying therapy. Management is supportive and dominated by the
    airway: every reported longer-term survivor required ventilator support, and the
    longest-surviving individual is maintained on nocturnal mechanical ventilation via
    tracheostomy together with gastrostomy feeding. Because survival past the neonatal
    period is possible, the decision to institute long-term respiratory support is a real
    clinical choice rather than a foregone one.
  treatment_term:
    preferred_term: Tracheostomy
    term:
      id: NCIT:C15341
      label: Tracheotomy
  target_phenotypes:
  - preferred_term: Respiratory insufficiency
    term:
      id: HP:0002093
      label: Respiratory insufficiency
  evidence:
  - reference: PMID:27240702
    reference_title: "Clinical and radiographic delineation of Bent Bone Dysplasia-FGFR2 type or Bent Bone Dysplasia with Distinctive Clavicles and Angel-shaped Phalanges."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Longer-term survivors all needed ventilator support."
    explanation: Establishes ventilator support as universal among survivors.
  - reference: PMID:41186107
    reference_title: "Clinical Presentation of the Longest Reported Living Individual With Bent Bone Dysplasia-FGFR2-Related."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "He is able to walk, is interactive, has a tracheostomy for mechanical ventilation at night and a gastrostomy tube, and continues to make steady developmental progress."
    explanation: The supportive regimen in the longest-surviving reported individual.
differential_diagnoses:
- name: LAMA5-Related Bent Bone Dysplasia
  disease_term:
    preferred_term: bent bone dysplasia syndrome 2
    term:
      id: MONDO:0859573
      label: bent bone dysplasia syndrome 2
  description: >-
    The other disorder carrying the bent bone dysplasia syndrome name, and
    radiographically the closest mimic: both give bent long bones with a poorly
    mineralized calvarium in a perinatally lethal setting. BBDS2 is autosomal
    recessive rather than dominant de novo, and its mechanism is unrelated — a
    destabilized alpha-5 laminin disrupting a LAMA5-beta1 integrin-PYK2-FYN
    focal adhesion axis in chondrocytes, not an FGFR2 receptor-trafficking and
    ribosome-biogenesis defect. Clinically, BBDS2 adds severe distal limb
    contractures, coronal cleft vertebrae, hypoplastic acetabula and atrial
    septal defects, and lacks the craniosynostosis, prenatal teeth and
    moustache-shaped clavicles of BBDS1.
  distinguishing_features:
  - Causal gene LAMA5 rather than FGFR2
  - Autosomal recessive rather than dominant de novo
  - Arthrogryposis with severe distal limb contractures
  - Coronal cleft vertebrae and hypoplastic acetabula
  - No craniosynostosis and no prenatal teeth
  notes: >-
    Both are listed in ISDS 2023 group 20 (NOS 20-0050 FGFR2, NOS 20-0060
    LAMA5). PMID:33242826 delineates the LAMA5 disorder; it caches only as a
    differential here and contributes no evidence to BBDS1 claims.
discussions:
- discussion_id: gap_bbds_p53_inhibition_translation
  prompt: >-
    Does p53 inhibition, which rescues delayed osteoblast differentiation in cells
    carrying the BBDS FGFR2 mutations, represent a viable therapeutic direction for a
    disorder whose damage is largely done before birth?
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  attaches_to:
  - pathophysiology#Nucleolar Disorganization and Rpl11-MDM2-p53 Stress Response
  - pathophysiology#Osteoprogenitor Proliferation with Delayed Differentiation
  rationale: >-
    This is the only therapeutic lead BBDS has, and it is a genuine mechanistic result
    rather than a correlation: inhibiting p53 in cells expressing the BBDS mutations
    rescues the differentiation delay, which is what licenses calling p53 activation
    essential rather than incidental. Three problems stand between that and a treatment.
    First, the rescue is in cells; no animal model of BBDS has been reported to test
    whether systemic p53 inhibition rescues the skeletal phenotype, and the entry should
    not be read as claiming otherwise. Second, the timing is adverse in a way that
    distinguishes this from most therapeutic gaps: bent long bones, hypoplastic clavicles
    and poor calvarial mineralization are established in utero, so a postnatal
    intervention would be addressing a developmental lesion after it has formed, and the
    window in which p53 inhibition could matter may close before the diagnosis is made.
    Third, p53 is a tumour suppressor and inhibiting it systemically in a developing
    child carries an obvious and unquantified oncogenic hazard; no dose, route, duration
    or selectivity has been proposed. Nothing here should be curated as an emerging
    treatment.
  evidence:
  - reference: PMID:28595297
    reference_title: "FGFR2 mutations in bent bone dysplasia syndrome activate nucleolar stress and perturb cell fate determination."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Inhibition of p53 in cells expressing the FGFR2 mutations in BBDS rescues delayed osteoblast differentiation, suggesting that p53 activation is an essential pathogenic factor in, and potential therapeutic target for, BBDS."
    explanation: >-
      The rescue result and the authors' therapeutic framing, which is a cell-based
      finding offered as a prospect.
  - reference: PMID:27240702
    reference_title: "Clinical and radiographic delineation of Bent Bone Dysplasia-FGFR2 type or Bent Bone Dysplasia with Distinctive Clavicles and Angel-shaped Phalanges."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The prenatal phenotype included stillbirth, bending of the femora, and a high incidence of polyhydramnios, prematurity, and perinatal death in three of 11 patients in the series."
    explanation: >-
      Establishes that the defining skeletal lesions are present prenatally, which is the
      timing problem any therapy has to solve.
  proposed_experiments:
  - experiment_id: exp_bbds_mouse_model_p53_rescue
    name: p53 inhibition in an animal model of BBDS
    description: >-
      Generate a knock-in animal carrying a BBDS transmembrane allele and test whether
      genetic or pharmacological p53 reduction rescues bone mineralization and long-bone
      bowing in vivo, which is the step between the existing cell result and any claim
      about the skeleton.
  - experiment_id: exp_bbds_developmental_window
    name: Developmental window for differentiation rescue
    description: >-
      Establish the latest developmental stage at which reducing p53 still restores
      osteoblast differentiation, since a window closing in utero would make postnatal
      treatment futile regardless of efficacy.
- discussion_id: gap_bbds_same_residue_opposite_disease
  prompt: >-
    Why do different substitutions at the same FGFR2 transmembrane residue produce either
    Crouzon syndrome, a canonical gain-of-function craniosynostosis, or bent bone
    dysplasia, a disorder of reduced membrane signalling?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#FGFR2 Transmembrane Polar Substitution
  - pathophysiology#Reduced Plasma-Membrane FGFR2 and Deficient Canonical FGF Signaling
  rationale: >-
    This is the cleanest available natural experiment on how FGFR2 transmembrane
    substitutions are read out, and it is unresolved. Substitutions at FGFR2 Tyr-381 have
    been reported to cause either Crouzon syndrome or bent bone dysplasia depending on
    which residue is introduced, so position alone does not determine the disease. The
    plausible discriminator is the biophysical character of the substituting residue — a
    cysteine enabling intermolecular disulfide bonding and constitutive dimerization
    versus a polar residue disrupting membrane insertion and trafficking — but no study
    cited here compares the two classes side by side in one system, and the nucleolar
    relocalization arm has not been tested in the gain-of-function alleles at all. It
    matters for the knowledge base as well as for biology: the FGFR-Related Skeletal
    Dysplasias grouping admits members on constitutive receptor activation, so whether a
    given transmembrane allele belongs is a mechanistic question that cannot be settled
    from the domain it sits in.
  evidence:
  - reference: PMID:22387015
    reference_title: "Bent bone dysplasia-FGFR2 type, a distinct skeletal disorder, has deficient canonical FGF signaling."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Using diseased chondrocytes and a cell-based assay, we determined that these mutations selectively reduced plasma-membrane levels of FGFR2 and markedly diminished the receptor's responsiveness to extracellular FGF."
    explanation: The BBDS half of the contrast — reduced membrane receptor and FGF response.
  - reference: PMID:24908667
    reference_title: "Bent bone dysplasia syndrome reveals nucleolar activity for FGFR2 in ribosomal DNA transcription."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "In this study, we find that these mutations, despite reducing canonical signaling, enhance nucleolar occupancy of FGFR2 at the ribosomal DNA (rDNA) promoter."
    explanation: >-
      The relocalization arm, which has not been examined in the gain-of-function
      transmembrane alleles and so cannot currently be called BBDS-specific.
  proposed_experiments:
  - experiment_id: exp_fgfr2_tm_allele_series_side_by_side
    name: Side-by-side comparison of FGFR2 transmembrane allele classes
    description: >-
      Express Crouzon-type and BBDS-type substitutions at the same FGFR2 transmembrane
      residue in one cell system and measure membrane localization, ligand-dependent and
      ligand-independent kinase activity, and nucleolar occupancy together, so the
      discriminating property is measured rather than inferred across papers.
  - experiment_id: exp_nucleolar_fgfr2_in_gof_alleles
    name: Nucleolar FGFR2 occupancy in gain-of-function craniosynostosis alleles
    description: >-
      Test whether Apert, Crouzon and Beare-Stevenson FGFR2 alleles also alter nucleolar
      rDNA occupancy. A negative result would make the nucleolar arm genuinely specific
      to BBDS; a positive one would mean the two mechanisms are not mutually exclusive.
notes: >
  Curated de novo from primary literature under claim #9384, following the FGFR-Related
  Skeletal Dysplasias grouping review (#9257).

  This entry exists partly to correct that review. It listed bent bone dysplasia syndrome
  among the grouping's "known unfilled seats" — disorders said to belong to the group on
  mechanism but lacking a curated entry. Checking the mechanism before curating showed
  that is wrong. The BBDS transmembrane alleles reduce plasma-membrane FGFR2 and diminish
  FGF responsiveness (PMID:22387015, whose title is "…has deficient canonical FGF
  signaling" and which concludes the disorder is "separate from previously characterized
  FGFR2 disorders"); the gain is relocalization to the nucleolus, not receptor activation
  (PMID:24908667). The grouping's criterion is conformance to
  fgfr_gain_of_function_skeletal_dysplasia#Constitutive FGFR Activation, which BBDS does
  not satisfy. The grouping note has been corrected accordingly and this entry is
  deliberately NOT a member.

  Consequently this entry declares NO conforms_to anchors at all, which is unusual for an
  FGFR skeletal disorder and is the substantive claim rather than an omission. Two module
  non-conformances are stated explicitly in the node descriptions rather than left
  implicit: the FGFR gain-of-function module (receptor mechanism is inverted), and
  defective_skeletal_mineralization (the mineralization failure here is secondary to an
  osteoprogenitor cell-fate defect, not a calciopenic, phosphopenic or
  mineralization-inhibitor-excess arm).

  Stillbirth is reported in the prenatal phenotype and is curated in `progression` rather
  than `phenotypes`, because HP:0003826 sits outside the Phenotypic abnormality subtree
  that the PhenotypeTerm enum draws from — it is an outcome, not a phenotypic abnormality.

  Three on-topic papers were reviewed and not cited because they cache with
  content_type unavailable, so no snippet could be verified: PMID:23808569 (Crouzon and
  bent bone dysplasia at the same FGFR2 Tyr-381 residue), PMID:24300289 (postnatal
  features) and PMID:26573129 (new case and review). The Tyr-381 observation is used in
  prose in the description, genetic notes and gap_bbds_same_residue_opposite_disease
  rationale, where it is attributed as a report rather than quoted; if that paper is
  later cached with an abstract it should be added as evidence to those places.
  PMID:33242826 describes a biallelic LAMA5 bent bone dysplasia — a distinct disorder and
  a differential-diagnosis consideration, not BBDS1 evidence. It is now curated as
  kb/disorders/LAMA5-Related_Bent_Bone_Dysplasia.yaml and cited only from the
  differential_diagnoses entry here.

  RENAME: this entry was named "Bent Bone Dysplasia Syndrome" until the LAMA5 disorder
  was curated. The bare name then read like a parent concept covering both, so it was
  gene-anchored to "FGFR2-Related Bent Bone Dysplasia". The numbered OMIM/MONDO form
  (bent bone dysplasia syndrome 1) is retained as a synonym and remains the
  `disease_term` label; the entry name carries the gene because "bent bone dysplasia
  syndrome" alone is locus-ambiguous. The file was renamed to match, per the repo
  convention that a filename tracks its entry name; the two history records under the
  old slug were moved and carry `target.superseded_by`.

  No phenotype carries a frequency band: fourteen reported individuals, weighted toward
  lethal presentations by ascertainment, will not support one.