Brachydactyly Type A1

Mendelian MONDO:0007215 Pathograph 23 Show in embeddings browser Limb Development Disorders

Brachydactyly type A1 (BDA1) is the first recorded Mendelian autosomal dominant disorder in humans, originally identified by Farabee in 1903. It is characterized by shortening or absence of the middle phalanges. BDA1 is genetically heterogeneous: pathogenic variants in IHH disrupt Hedgehog signaling, while variants in GDF5 or its preferred receptor BMPR1B perturb BMP-SMAD signaling. These pathways converge on chondrocyte differentiation, endochondral ossification, and specification of phalangeal segments in both hands and feet.

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1
Inheritance
6
Pathophys.
14
Phenotypes
23
Pathograph
3
Genes
4
Medical Actions
4
Subtypes
4
References
1
Deep Research
🏷

Classifications

ISDS Skeletal Nosology
brachydactyly without extraskeletal manifestations
👪

Inheritance

1
Autosomal Dominant HP:0000006
Autosomal dominant inheritance, historically the first recorded Mendelian autosomal dominant trait in humans (Farabee 1903). Some GDF5-associated families show semidominant dosage, with a mild phenotype in heterozygotes and more severe disease in homozygotes.
Autosomal dominant inheritance
Show evidence (2 references)
PMID:11455389 SUPPORT
"first recorded example of a human anomaly with Mendelian autosomal-dominant inheritance"
BDA1 is historically significant as the first documented autosomal dominant human trait.
PMID:20683927 SUPPORT Human Clinical
"This data confirms genetic heterogeneity in BDA1, demonstrates that mutations upstream of IHH can result in BDA1, and shows that BDA1 can result from semidominant mutations in GDF5."
A family study establishes the semidominant GDF5-associated form within the genetically heterogeneous BDA1 spectrum.
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Subtypes

4
IHH-related brachydactyly type A1A
The canonical autosomal dominant IHH-related subtype (OMIM:112500).
Show evidence (1 reference)
PMID:11455389 SUPPORT Human Clinical
"Here we show that mutations in IHH, which encodes Indian hedgehog, cause BDA-1."
Establishes IHH as the causal gene for the canonical BDA1 subtype.
GDF5-related brachydactyly type A1B
GDF5-related, sometimes semidominant BDA1 (OMIM:607004).
Show evidence (1 reference)
PMID:20683927 SUPPORT Human Clinical
"This data confirms genetic heterogeneity in BDA1, demonstrates that mutations upstream of IHH can result in BDA1, and shows that BDA1 can result from semidominant mutations in GDF5."
Establishes the GDF5-associated BDA1 subtype.
BMPR1B-related brachydactyly type A1C
Dominant BMPR1B-related BDA1 (OMIM:615072).
Show evidence (1 reference)
PMID:25758993 SUPPORT Human Clinical
"We report pathogenic variants in BMPR1B that are associated with complex BDA1."
Establishes the BMPR1B-associated BDA1 subtype.
Brachydactyly type A1D, unresolved locus
A mapped BDA1 subtype represented by OMIM:616849; its causal gene is not established in this entry and it is retained separately from the three molecularly defined subtypes.
Show evidence (1 reference)
ORPHA:93388 SUPPORT Other
"OMIM:616849 | Broader"
Orphanet cross-references the unresolved OMIM subtype within the broader BDA1 record.
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Pathophysiology

6
Impaired IHH-Patched1 Receptor Signaling
Indian Hedgehog (IHH) is a secreted signaling molecule that binds the receptor Patched1 (PTC1) to regulate chondrocyte proliferation and differentiation in the growth plate via the Smoothened signaling pathway. BDA1-causing missense mutations cluster in the N-terminal signaling domain and reduce IHH binding to PTC1, diminishing its capacity to induce cellular differentiation during endochondral ossification.
Chondrocyte CL:0000138 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves Chondrocyte (CL:0000138). CL:0000138 is a cell type from the Cell Ontology.
IHH hgnc:5956 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves IHH (hgnc:5956). hgnc:5956 is a gene from the HUGO Gene Nomenclature Committee.
Smoothened Signaling Pathway GO:0007224 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased Smoothened Signaling Pathway (GO:0007224). GO:0007224 is a biological process from the Gene Ontology. ↓ DECREASED Endochondral Ossification GO:0001958 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal Endochondral Ossification (GO:0001958). GO:0001958 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Disrupted Chondrocyte Differentiation and Digit Segment Specification Reduced receptor binding and signaling capacity impair the IHH-dependent differentiation program that patterns phalangeal segments.
Show evidence (3 references)
PMID:11455389 SUPPORT Human Clinical
"mutations in IHH, which encodes Indian hedgehog, cause BDA-1"
Identifies IHH as the causative gene for BDA1.
PMID:11455389 SUPPORT Human Clinical
"three heterozygous missense mutations in the region encoding the amino-terminal signaling domain"
Mutations cluster in the IHH N-terminal signaling domain.
PMID:21537345 SUPPORT In Vitro
"all three mutations affected Hh binding to the receptor Patched1 (PTC1), reducing its capacity to induce cellular differentiation"
Demonstrates that BDA1 mutations impair IHH-Patched1 receptor binding and reduce signaling capacity.
IHH Protein Instability and Trafficking Defects
BDA1 mutations also affect IHH protein stability, intracellular trafficking, and interaction with extracellular matrix components. The E95K and D100E mutations cause temperature-sensitive and calcium-dependent instability of the IHH N-terminal domain, leading to increased lysosomal degradation and reduced secretion. These multi-level effects on Hedgehog signaling compound the receptor binding defect.
IHH hgnc:5956 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves IHH (hgnc:5956). hgnc:5956 is a gene from the HUGO Gene Nomenclature Committee.
Chondrocyte Differentiation GO:0002062 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased Chondrocyte Differentiation (GO:0002062). GO:0002062 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Disrupted Chondrocyte Differentiation and Digit Segment Specification Instability, intracellular degradation, and altered extracellular interactions reduce the effective range of IHH signaling during digit development.
Show evidence (3 references)
PMID:21537345 SUPPORT In Vitro
"E95K and D100E mutations led to a temperature-sensitive and calcium-dependent instability of IhhN"
Shows that specific BDA1 mutations cause protein instability under physiological conditions.
PMID:21537345 SUPPORT In Vitro
"these IHH mutations affect Hh signaling at multiple levels, causing abnormal bone development and abnormal digit formation"
Establishes the multi-level disruption of Hedgehog signaling as the pathogenic mechanism.
PMID:40606564 SUPPORT In Vitro
"Western blotting analysis of HEK293T cells in overexpression transfection experiments revealed that the Leu111del mutation led to an increase in the level of the IHH precursor and a reduction in the level of functional IHH protein compared with those in HEK293T cells expressing wild-type IHH,..."
Directly demonstrates precursor accumulation and loss of mature functional IHH.
Reduced GDF5-BMPR1B Signaling
A second BDA1 mechanism affects the BMP-SMAD arm of digit development. GDF5 is the preferred ligand for BMPR1B and promotes chondrogenesis and digit formation. Semidominant loss-of-function GDF5 variants or dominant-negative BMPR1B variants reduce this signaling input and can produce the same middle-phalanx pattern as IHH-associated BDA1.
Chondrocyte CL:0000138 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves Chondrocyte (CL:0000138). CL:0000138 is a cell type from the Cell Ontology.
GDF5 hgnc:4220 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves GDF5 (hgnc:4220). hgnc:4220 is a gene from the HUGO Gene Nomenclature Committee. BMPR1B hgnc:1077 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves BMPR1B (hgnc:1077). hgnc:1077 is a gene from the HUGO Gene Nomenclature Committee.
BMP Signaling Pathway GO:0030509 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased BMP Signaling Pathway (GO:0030509). GO:0030509 is a biological process from the Gene Ontology. ↓ DECREASED Chondrocyte Differentiation GO:0002062 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased Chondrocyte Differentiation (GO:0002062). GO:0002062 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Disrupted Chondrocyte Differentiation and Digit Segment Specification Reduced GDF5 ligand activity or BMPR1B receptor activity impairs chondrogenesis and digit formation upstream of phalangeal segment specification.
Show evidence (2 references)
PMID:20683927 SUPPORT In Vitro
"Functional analysis demonstrated that although the p.Arg399Cys mutant is able to stimulate chondrogenesis, it is much less effective than wild-type GDF5."
Functional analysis shows that the BDA1-associated GDF5 allele has reduced chondrogenic activity.
PMID:25758993 SUPPORT Human Clinical
"These findings demonstrate that BMPR1B is another gene involved with the pathogenesis of BDA1 and illustrates the continuum of phenotypes between BDA1 and BDA2."
Human genetic findings establish BMPR1B-associated BDA1 and place it in the same BMP-signaling spectrum.
Disrupted Chondrocyte Differentiation and Digit Segment Specification
Hedgehog and BMP-SMAD lesions converge on impaired chondrocyte differentiation and endochondral ossification. The resulting disturbance of embryonic limb morphogenesis preferentially shortens or eliminates middle phalangeal segments, with variable effects on the first digits, metacarpals, and overall skeletal growth.
Chondrocyte CL:0000138 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves Chondrocyte (CL:0000138). CL:0000138 is a cell type from the Cell Ontology.
Chondrocyte Differentiation GO:0002062 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased Chondrocyte Differentiation (GO:0002062). GO:0002062 is a biological process from the Gene Ontology. ↓ DECREASED Endochondral Ossification GO:0001958 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal Endochondral Ossification (GO:0001958). GO:0001958 is a biological process from the Gene Ontology. ⚠ ABNORMAL Embryonic Limb Morphogenesis GO:0030326 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal Embryonic Limb Morphogenesis (GO:0030326). GO:0030326 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Failed Interphalangeal Joint Cavitation from Suppressed Interzone Apoptosis Deregulated IHH levels in the developing interzone block the apoptosis that normally drives joint cavitation.
Serially Homologous Hand and Foot Brachydactyly The same disrupted segment-specification program operates in fore- and hindlimb autopods, producing a coordinated hand-and-foot malformation rather than an isolated digit defect.
Short Stature The growth-plate defect can extend beyond the digits and reduce linear skeletal growth, although expression is variable among families.
Talipes Equinovarus Clubfoot is an occasional broader limb-development outcome; the intermediate steps beyond altered limb morphogenesis are unresolved.
Scoliosis Scoliosis is an occasional broader skeletal association; its precise mechanistic connection to the digit-patterning lesion is unresolved.
Hypoplasia of the Ulna Ulnar hypoplasia extends the growth defect proximally within the upper limb, with incompletely defined intermediates.
Show evidence (3 references)
PMID:21537345 SUPPORT In Vitro
"Taken together, our results suggest that these IHH mutations affect Hh signaling at multiple levels, causing abnormal bone development and abnormal digit formation."
Functional IHH studies connect altered signaling to abnormal bone and digit formation.
PMID:25758993 SUPPORT Other
"Human and mouse genetic perturbations in the BMP-SMAD signaling pathway have been associated with many brachymesophalangies, including BDA1, as causative mutations in IHH and GDF5 have been previously identified."
The BMPR1B study places genetically heterogeneous BDA1 within a shared BMP-SMAD developmental mechanism.
PMID:30651074 SUPPORT In Vitro
"our results demonstrated that the E95K mutant signaling altered Gli1-DNA binding pattern, impaired downstream gene expressions, and leaded to weakened cellular proliferation and migration."
Cell-based functional data connect mutant IHH/Gli1 transcriptional output to impaired proliferation and migration in the convergent developmental mechanism.
Failed Interphalangeal Joint Cavitation from Suppressed Interzone Apoptosis
A 2024 mouse study reframes what the missing middle phalanx in BDA1 actually is. Using an Ihh E95K knock-in, it shows the bone is not simply too small: the developing interphalangeal joint fails to cavitate, so two segments stay fused as one and the middle phalanx never appears as a separate element. The proximate cause is loss of apoptosis in the joint interzone. The proposed mechanism inverts the usual reading of a hedgehog signalling defect - IHH is normally kept *low* at the centre of the interzone, which leaves its interacting partners CDON and GAS1 ligand-free to act as dependence receptors and trigger the cell death that separates the segments. A BDA1 allele raises IHH there, the dependence receptors are occupied, apoptosis is suppressed, and cavitation fails. This is why BDA1 pairs shortening with symphalangism rather than producing one or the other, and it makes the node mechanistically continuous with the joint-formation failures of the other brachydactylies (NOG in type B2, GDF5 at the interzone in type C).
Chondrocyte CL:0000138 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves Chondrocyte (CL:0000138). CL:0000138 is a cell type from the Cell Ontology.
IHH hgnc:5956 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves IHH (hgnc:5956). hgnc:5956 is a gene from the HUGO Gene Nomenclature Committee.
Apoptotic Process GO:0006915 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased Apoptotic Process (GO:0006915). GO:0006915 is a biological process from the Gene Ontology. ↓ DECREASED Embryonic Skeletal Limb Joint Morphogenesis GO:0036023 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal Embryonic Skeletal Limb Joint Morphogenesis (GO:0036023). GO:0036023 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Serially Homologous Hand and Foot Brachydactyly Segments that fail to separate present as shortened or absent middle phalanges with symphalangism across the homologous digits.
Distal Finger Symphalangism An uncavitated interzone is, by definition, a fused joint.
Show evidence (3 references)
PMID:38472182 SUPPORT Model Organism
"Here, we use a mouse model of brachydactyly type A1 (BDA1) with an IhhE95K mutation, to show that a missing middle phalangeal bone is due to the failure of the developing joint to cavitate, associated with reduced apoptosis, and a joint is not formed."
Reframes the missing middle phalanx as a joint-cavitation failure driven by reduced interzone apoptosis, in a knock-in model of a human BDA1 allele.
PMID:38472182 SUPPORT Model Organism
"We showed an intricate relationship between IHH and interacting partners, CDON and GAS1, in the interzone that regulates apoptosis."
Identifies CDON and GAS1 as the IHH partners mediating interzone apoptosis.
PMID:38472182 SUPPORT Model Organism
"We propose a model in which CDON/GAS1 may act as dependence receptors in this context."
Supports the CDON/GAS1 dependence-receptor account of interzone apoptosis, with the caveat that the authors offer it as a proposed model rather than a demonstrated one.
Serially Homologous Hand and Foot Brachydactyly
Because the autopod developmental program is reused in the hands and feet, disrupted IHH or GDF5-BMPR1B signaling produces shortening or absence of homologous phalangeal segments across both sets of digits. Variable expressivity also produces symphalangism, clinodactyly, abnormal epiphyses, and metacarpal changes.
Chondrocyte CL:0000138 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves Chondrocyte (CL:0000138). CL:0000138 is a cell type from the Cell Ontology.
Limb Morphogenesis GO:0035108 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal Limb Morphogenesis (GO:0035108). GO:0035108 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Type A1 Brachydactyly
Short Middle Phalanx of Finger
Short Middle Phalanx of Toe
Short Foot
Short Thumb
Short Hallux
Cone-Shaped Epiphysis
Distal Finger Symphalangism
Clinodactyly of Fifth Finger
Broad Metacarpals
Show evidence (1 reference)
PMID:18629882 SUPPORT Human Clinical
"Brachydactyly type A1 is a limb malformation characterized by a uniform shortening of the middle phalanges in all digits."
Human genetic evidence supports a serially repeated middle-phalanx phenotype across the digits.
⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Brachydactyly Type A1 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

14
Limbs 7
Type A1 Brachydactyly HP:0009371 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Type A1 brachydactyly (HP:0009371). HP:0009371 is a phenotype from the Human Phenotype Ontology.
Show evidence (3 references)
PMID:11455389 SUPPORT
"Brachydactyly type A-1 (BDA-1; MIM 112500) is characterized by shortening or missing of the middle phalanges"
Defines the cardinal phenotype of BDA1.
PMID:21537345 SUPPORT
"Brachydactyly type A1 (BDA1), the first recorded Mendelian autosomal dominant disorder in humans, is characterized by a shortening or absence of the middle phalanges"
Confirms middle phalanx shortening/absence as the defining feature.
PMID:32209048 SUPPORT
"The variant co-segregated with BDA-1 in the pedigree, showed 100% penetrance for phalange phenotype with variable expressivity."
A five-generation family showed fully penetrant but variably severe phalangeal involvement.
Short Middle Phalanx of Finger HP:0005819 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Short middle phalanx of finger (HP:0005819). HP:0005819 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:34315464 SUPPORT
"Brachydactyly type A1 (BDA1) is an autosomal dominant disorder characterized by uniform shortening of the middle phalanges in all digits."
Directly supports uniform shortening of the finger middle phalanges in a molecularly confirmed BDA1 family.
PMID:18629882 SUPPORT
"Brachydactyly type A1 is a limb malformation characterized by a uniform shortening of the middle phalanges in all digits."
Independent family report confirms uniform middle-phalanx shortening as the key digital abnormality.
Short Foot VERY_FREQUENT HP:0001773 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Short foot (HP:0001773). HP:0001773 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:93388 SUPPORT Other
"HP:0001773 | Short foot | Very frequent (99-80%)"
Orphanet lists short foot as a very frequent feature of brachydactyly type A1, reflecting the bilateral hand and foot involvement of BDA1.
Short Thumb VERY_FREQUENT HP:0009778 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Short thumb (HP:0009778). HP:0009778 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:93388 SUPPORT Other
"HP:0009778 | Short thumb | Very frequent (99-80%)"
Orphanet lists short thumb as a very frequent feature of brachydactyly type A1. The thumb is affected through shortening of the proximal phalanx.
Short Hallux VERY_FREQUENT HP:0010109 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Short hallux (HP:0010109). HP:0010109 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:93388 SUPPORT Other
"HP:0010109 | Short hallux | Very frequent (99-80%)"
Orphanet lists short hallux as a very frequent feature of brachydactyly type A1, reflecting the foot involvement mirroring the thumb changes.
Talipes Equinovarus OCCASIONAL HP:0001762 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Talipes equinovarus (HP:0001762). HP:0001762 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:93388 SUPPORT Other
"HP:0001762 | Talipes equinovarus | Occasional (29-5%)"
Orphanet lists talipes equinovarus as an occasional feature of brachydactyly type A1.
Hypoplasia of the Ulna OCCASIONAL HP:0003022 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypoplasia of the ulna (HP:0003022). HP:0003022 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:93388 SUPPORT Other
"HP:0003022 | Hypoplasia of the ulna | Occasional (29-5%)"
Orphanet lists hypoplasia of the ulna as an occasional feature of brachydactyly type A1.
Musculoskeletal 1
Scoliosis OCCASIONAL HP:0002650 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Scoliosis (HP:0002650). HP:0002650 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:93388 SUPPORT Other
"HP:0002650 | Scoliosis | Occasional (29-5%)"
Orphanet lists scoliosis as an occasional feature of brachydactyly type A1.
Growth 1
Short Stature VERY_FREQUENT HP:0004322 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Short stature (HP:0004322). HP:0004322 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
ORPHA:93388 SUPPORT Other
"HP:0004322 | Short stature | Very frequent (99-80%)"
Orphanet lists short stature as a very frequent feature of brachydactyly type A1, consistent with IHH's role in skeletal growth regulation.
PMID:34315464 SUPPORT
"The proband, a 9-year-old boy, his siblings, and his father had shortened digits and a short stature of variable severity."
Supports short stature as a variable associated manifestation in a molecularly confirmed BDA1 family.
Other 5
Short Middle Phalanx of Toe HP:0003795 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Short middle phalanx of toe (HP:0003795). HP:0003795 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:30651074 SUPPORT In Vitro
"Brachydactyly type A1 (BDA1, OMIM 112500) is a rare inherited malformation characterized primarily by shortness or absence of middle bones of fingers and toes."
Supports toe middle-phalanx involvement as part of the core BDA1 malformation pattern.
PMID:40606564 SUPPORT Human Clinical
"Brachydactyly A1 (BDA1) is a rare disorder characterized by the disproportionate shortening of fingers and/or toes with or without symphalangism."
Recent prenatal case report independently supports toe shortening within the BDA1 phenotype spectrum.
Cone-Shaped Epiphysis FREQUENT HP:0010579 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cone-shaped epiphysis (HP:0010579). HP:0010579 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:93388 SUPPORT Other
"HP:0010579 | Cone-shaped epiphysis | Frequent (79-30%)"
Orphanet lists cone-shaped epiphysis as a frequent radiographic feature of brachydactyly type A1.
Distal Finger Symphalangism OCCASIONAL HP:0001204 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Distal symphalangism of hands, annotated with Distal finger symphalangism (HP:0001204). HP:0001204 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:93388 SUPPORT Other
"HP:0001204 | Distal symphalangism (hands) | Occasional (29-5%)"
Orphanet lists distal symphalangism of hands as an occasional feature of brachydactyly type A1.
Clinodactyly of Fifth Finger OCCASIONAL Clinodactyly of the 5th finger HP:0004209 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Clinodactyly of the 5th finger (HP:0004209). HP:0004209 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:93388 SUPPORT Other
"HP:0004209 | Clinodactyly of the 5th finger | Occasional (29-5%)"
Orphanet lists clinodactyly of the 5th finger as an occasional feature of brachydactyly type A1.
Broad Metacarpals OCCASIONAL HP:0001230 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Broad metacarpals (HP:0001230). HP:0001230 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:93388 SUPPORT Other
"HP:0001230 | Broad metacarpals | Occasional (29-5%)"
Orphanet lists broad metacarpals as an occasional feature of brachydactyly type A1.
🧬

Genetic Associations

3
IHH Pathogenic Variants (CAUSAL)
Gene: IHH hgnc:5956 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is IHH (hgnc:5956). hgnc:5956 is a gene from the HUGO Gene Nomenclature Committee.
Show evidence (6 references)
PMID:11455389 SUPPORT Human Clinical
"three heterozygous missense mutations in the region encoding the amino-terminal signaling domain in all affected members of three large, unrelated families"
Three independent families with distinct IHH missense mutations establish genotype-phenotype relationship.
PMID:11455389 SUPPORT Human Clinical
"The three mutant amino acids, which are conserved across all vertebrates and invertebrates studied so far, are predicted to be adjacent on the surface of IHH"
Deep evolutionary conservation and surface clustering suggest a critical functional domain.
PMID:21537345 SUPPORT In Vitro
"Heterozygous missense mutations in the Indian Hedgehog (IHH) gene have been identified as a cause of BDA1"
Confirms IHH missense mutations as the established cause of BDA1.
+ 3 more references
GDF5 Pathogenic Variants (CAUSAL)
Gene: GDF5 hgnc:4220 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is GDF5 (hgnc:4220). hgnc:4220 is a gene from the HUGO Gene Nomenclature Committee.
Show evidence (3 references)
PMID:20683927 SUPPORT Human Clinical
"Sequencing of the GDF5 coding region revealed that a mildly affected individual in the family was heterozygous, and that all of the severely affected individuals were homozygous for a novel missense c.1195C>T mutation that predicts a p.Arg399Cys substitution at a highly conserved amino acid."
Segregation in a consanguineous family demonstrates dosage-dependent GDF5-associated BDA1.
PMID:20683927 SUPPORT In Vitro
"Functional analysis demonstrated that although the p.Arg399Cys mutant is able to stimulate chondrogenesis, it is much less effective than wild-type GDF5."
Functional assays show reduced chondrogenic activity of the BDA1-associated GDF5 allele.
PMID:38222807 SUPPORT Human Clinical
"The family was found to have an autosomal-dominantly inherited combination of BDA1 and SYNS2 caused by the S475N variant in the GDF5 gene."
A single GDF5 allele producing BDA1 together with multiple-synostoses syndrome 2, reinforcing that GDF5 phenotype depends on the allele rather than the gene.
BMPR1B Pathogenic Variants (CAUSAL)
Gene: BMPR1B hgnc:1077 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is BMPR1B (hgnc:1077). hgnc:1077 is a gene from the HUGO Gene Nomenclature Committee.
Show evidence (2 references)
PMID:25758993 SUPPORT Human Clinical
"We report pathogenic variants in BMPR1B that are associated with complex BDA1."
The human genetic study identifies BMPR1B variants in individuals with complex BDA1.
PMID:25758993 SUPPORT Other
"Both mutations are most likely to act in a dominant-negative manner, similar to the effects observed in BMPR1B mutations that cause BDA2."
The authors interpret the two BDA1-associated BMPR1B variants as dominant-negative alleles.
💊

Medical Actions

4
Genetic Counseling
Action: Genetic CounselingNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Genetic Counseling (NCIT:C15240). NCIT:C15240 is a clinical intervention from the NCI Thesaurus. NCIT:C15240
Genetic counseling for affected families given autosomal dominant inheritance and the historical significance of BDA1 as the first recorded Mendelian trait.
Show evidence (1 reference)
PMID:18554391 SUPPORT Other
"The nature of genetic counseling depends both on the pattern of inheritance of the type of brachydactyly present in the family and on the presence or absence of accompanying symptoms."
The clinical review directly supports inheritance- and phenotype-specific counseling.
Hand Function Rehabilitation
Action: physical therapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is physical therapy (NCIT:C15302). NCIT:C15302 is a clinical intervention from the NCI Thesaurus. Ontology label: Physical Therapy NCIT:C15302
Physical and occupational therapy may improve hand function when functional limitation is present.
Show evidence (1 reference)
PMID:18554391 SUPPORT Other
"Physical therapy and ergotherapy may ameliorate hand function."
The management review supports function-directed rehabilitation.
Function- or Cosmesis-Directed Hand Surgery
Action: surgical procedureNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is surgical procedure (NCIT:C15329). NCIT:C15329 is a clinical intervention from the NCI Thesaurus. Ontology label: Surgical Procedure NCIT:C15329
Surgery is reserved for meaningful hand-function impairment or selected cosmetic indications and is usually unnecessary.
Show evidence (1 reference)
PMID:18554391 SUPPORT Other
"Plastic surgery is only indicated if the brachydactyly affects hand function or for cosmetic reasons, but is typically not needed."
The review defines the limited indications for surgery.
🔬

Diagnosis

1
Clinical-radiographic and molecular diagnosis
Diagnosis is supported by the characteristic clinical and radiographic pattern of shortened or absent middle phalanges and confirmed by identifying a pathogenic variant in IHH, GDF5, or BMPR1B when molecularly solved.
genetic testing NCIT:C15709 NCI Thesaurus (NCIT)
Results: Characteristic middle-phalanx shortening or absence, with detection of a causal IHH, GDF5, or BMPR1B variant when present.
Show evidence (4 references)
PMID:11455389 SUPPORT Human Clinical
"Brachydactyly type A-1 (BDA-1; MIM 112500) is characterized by shortening or missing of the middle phalanges"
The defining skeletal pattern provides the clinical-radiographic basis for diagnosis.
PMID:11455389 SUPPORT Human Clinical
"Here we show that mutations in IHH, which encodes Indian hedgehog, cause BDA-1."
This supports IHH molecular testing as confirmatory for BDA1.
PMID:20683927 SUPPORT Human Clinical
"This data confirms genetic heterogeneity in BDA1, demonstrates that mutations upstream of IHH can result in BDA1, and shows that BDA1 can result from semidominant mutations in GDF5."
Supports including GDF5 in molecular confirmation of genetically heterogeneous BDA1.
+ 1 more reference
📊

Prevalence

1
Reported pedigrees worldwide
Unknown
Population prevalence has not been established. Brachydactyly type A1 is documented mainly through multigenerational pedigrees and small molecularly characterized family series rather than formal epidemiologic studies.
Show evidence (1 reference)
PMID:19464397 SUPPORT Human Clinical
"We investigated six affected members of a large Swedish family segregating autosomal dominant brachymesophalangia."
This molecular report illustrates that published BDA1 data are largely family based rather than population based.
{ }

Source YAML

click to show 
name: Brachydactyly Type A1
synonyms:
- Brachydactyly type A1, IHH-related
creation_date: '2026-02-13T00:31:42Z'
category: Mendelian
description: >
  Brachydactyly type A1 (BDA1) is the first recorded Mendelian autosomal dominant
  disorder in humans, originally identified by Farabee in 1903. It is characterized
  by shortening or absence of the middle phalanges. BDA1 is genetically
  heterogeneous: pathogenic variants in IHH disrupt Hedgehog signaling, while
  variants in GDF5 or its preferred receptor BMPR1B perturb BMP-SMAD signaling.
  These pathways converge on chondrocyte differentiation, endochondral
  ossification, and specification of phalangeal segments in both hands and feet.
disease_term:
  preferred_term: brachydactyly type A1
  term:
    id: MONDO:0007215
    label: brachydactyly type A1
parents:
- Limb Development Disorders
has_subtypes:
- name: Brachydactyly type A1A
  display_name: IHH-related brachydactyly type A1A
  description: The canonical autosomal dominant IHH-related subtype (OMIM:112500).
  evidence:
  - reference: PMID:11455389
    reference_title: "Mutations in IHH, encoding Indian hedgehog, cause brachydactyly type A-1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here we show that mutations in IHH, which encodes Indian hedgehog, cause BDA-1."
    explanation: Establishes IHH as the causal gene for the canonical BDA1 subtype.
- name: Brachydactyly type A1B
  display_name: GDF5-related brachydactyly type A1B
  description: GDF5-related, sometimes semidominant BDA1 (OMIM:607004).
  evidence:
  - reference: PMID:20683927
    reference_title: "Mutations in GDF5 presenting as semidominant brachydactyly A1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This data confirms genetic heterogeneity in BDA1, demonstrates that mutations upstream of IHH can result in BDA1, and shows that BDA1 can result from semidominant mutations in GDF5."
    explanation: Establishes the GDF5-associated BDA1 subtype.
- name: Brachydactyly type A1C
  display_name: BMPR1B-related brachydactyly type A1C
  description: Dominant BMPR1B-related BDA1 (OMIM:615072).
  evidence:
  - reference: PMID:25758993
    reference_title: "Two novel disease-causing variants in BMPR1B are associated with brachydactyly type A1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report pathogenic variants in BMPR1B that are associated with complex BDA1."
    explanation: Establishes the BMPR1B-associated BDA1 subtype.
- name: Brachydactyly type A1D
  display_name: Brachydactyly type A1D, unresolved locus
  description: >-
    A mapped BDA1 subtype represented by OMIM:616849; its causal gene is not
    established in this entry and it is retained separately from the three
    molecularly defined subtypes.
  evidence:
  - reference: ORPHA:93388
    reference_title: "Brachydactyly type A1"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "OMIM:616849 | Broader"
    explanation: Orphanet cross-references the unresolved OMIM subtype within the broader BDA1 record.
inheritance:
- name: Autosomal Dominant
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  description: >
    Autosomal dominant inheritance, historically the first recorded
    Mendelian autosomal dominant trait in humans (Farabee 1903). Some
    GDF5-associated families show semidominant dosage, with a mild phenotype in
    heterozygotes and more severe disease in homozygotes.
  evidence:
  - reference: PMID:11455389
    reference_title: "Mutations in IHH, encoding Indian hedgehog, cause brachydactyly type A-1."
    supports: SUPPORT
    snippet: "first recorded example of a human anomaly with Mendelian autosomal-dominant inheritance"
    explanation: "BDA1 is historically significant as the first documented autosomal dominant human trait."
  - reference: PMID:20683927
    reference_title: "Mutations in GDF5 presenting as semidominant brachydactyly A1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This data confirms genetic heterogeneity in BDA1, demonstrates that mutations upstream of IHH can result in BDA1, and shows that BDA1 can result from semidominant mutations in GDF5."
    explanation: >-
      A family study establishes the semidominant GDF5-associated form within
      the genetically heterogeneous BDA1 spectrum.
classifications:
  isds_skeletal_category:
  - classification_value: brachydactyly_without_extraskeletal_manifestations
    notes: >-
      ISDS Nosology and Classification of Genetic Skeletal Disorders, 2019
      revision (Mortier et al., PMID:31633310), Table 1 group 37 "Brachydactylies
      (without extraskeletal manifestations)"; listed as "Brachydactyly type A1".
prevalence:
- population: Reported pedigrees worldwide
  prevalence_class: UNKNOWN
  notes: >-
    Population prevalence has not been established. Brachydactyly type A1 is
    documented mainly through multigenerational pedigrees and small molecularly
    characterized family series rather than formal epidemiologic studies.
  evidence:
  - reference: PMID:19464397
    reference_title: "Brachydactyly type A1 associated with unusual radiological findings and a novel Arg158Cys mutation in the Indian hedgehog (IHH) gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We investigated six affected members of a large Swedish family segregating autosomal dominant brachymesophalangia."
    explanation: >-
      This molecular report illustrates that published BDA1 data are largely
      family based rather than population based.
pathophysiology:
- name: Impaired IHH-Patched1 Receptor Signaling
  conforms_to: "limb_digit_patterning_serial_homology#Limb Patterning Signal Perturbation"
  role: trigger
  description: >
    Indian Hedgehog (IHH) is a secreted signaling molecule that binds the
    receptor Patched1 (PTC1) to regulate chondrocyte proliferation and
    differentiation in the growth plate via the Smoothened signaling pathway.
    BDA1-causing missense mutations cluster in the N-terminal signaling
    domain and reduce IHH binding to PTC1, diminishing its capacity to
    induce cellular differentiation during endochondral ossification.
  gene:
    preferred_term: IHH
    term:
      id: hgnc:5956
      label: IHH
  biological_processes:
  - preferred_term: Smoothened Signaling Pathway
    term:
      id: GO:0007224
      label: smoothened signaling pathway
    modifier: DECREASED
  - preferred_term: Endochondral Ossification
    term:
      id: GO:0001958
      label: endochondral ossification
    modifier: ABNORMAL
  cell_types:
  - preferred_term: Chondrocyte
    term:
      id: CL:0000138
      label: chondrocyte
  evidence:
  - reference: PMID:11455389
    reference_title: "Mutations in IHH, encoding Indian hedgehog, cause brachydactyly type A-1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "mutations in IHH, which encodes Indian hedgehog, cause BDA-1"
    explanation: "Identifies IHH as the causative gene for BDA1."
  - reference: PMID:11455389
    reference_title: "Mutations in IHH, encoding Indian hedgehog, cause brachydactyly type A-1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "three heterozygous missense mutations in the region encoding the amino-terminal signaling domain"
    explanation: "Mutations cluster in the IHH N-terminal signaling domain."
  - reference: PMID:21537345
    reference_title: "Indian hedgehog mutations causing brachydactyly type A1 impair Hedgehog signal transduction at multiple levels."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "all three mutations affected Hh binding to the receptor Patched1 (PTC1), reducing its capacity to induce cellular differentiation"
    explanation: "Demonstrates that BDA1 mutations impair IHH-Patched1 receptor binding and reduce signaling capacity."
  downstream:
  - target: Disrupted Chondrocyte Differentiation and Digit Segment Specification
    description: >-
      Reduced receptor binding and signaling capacity impair the
      IHH-dependent differentiation program that patterns phalangeal segments.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - reduced Patched1 binding and Smoothened-pathway output
    evidence:
    - reference: PMID:21537345
      reference_title: "Indian hedgehog mutations causing brachydactyly type A1 impair Hedgehog signal transduction at multiple levels."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "all three mutations affected Hh binding to the receptor Patched1 (PTC1), reducing its capacity to induce cellular differentiation"
      explanation: >-
        Functional assays connect defective IHH-Patched1 binding to reduced
        cellular differentiation.
- name: IHH Protein Instability and Trafficking Defects
  role: trigger
  description: >
    BDA1 mutations also affect IHH protein stability, intracellular
    trafficking, and interaction with extracellular matrix components.
    The E95K and D100E mutations cause temperature-sensitive and
    calcium-dependent instability of the IHH N-terminal domain, leading
    to increased lysosomal degradation and reduced secretion. These
    multi-level effects on Hedgehog signaling compound the receptor
    binding defect.
  gene:
    preferred_term: IHH
    term:
      id: hgnc:5956
      label: IHH
  biological_processes:
  - preferred_term: Chondrocyte Differentiation
    term:
      id: GO:0002062
      label: chondrocyte differentiation
    modifier: DECREASED
  evidence:
  - reference: PMID:21537345
    reference_title: "Indian hedgehog mutations causing brachydactyly type A1 impair Hedgehog signal transduction at multiple levels."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "E95K and D100E mutations led to a temperature-sensitive and calcium-dependent instability of IhhN"
    explanation: "Shows that specific BDA1 mutations cause protein instability under physiological conditions."
  - reference: PMID:21537345
    reference_title: "Indian hedgehog mutations causing brachydactyly type A1 impair Hedgehog signal transduction at multiple levels."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "these IHH mutations affect Hh signaling at multiple levels, causing abnormal bone development and abnormal digit formation"
    explanation: "Establishes the multi-level disruption of Hedgehog signaling as the pathogenic mechanism."
  - reference: PMID:40606564
    reference_title: "A Novel Heterozygous IHH c.331_333del Mutation Identified in a Fetus with Brachydactyly Type A1 Causes IHH Protein Maturation Failure in HEK293T Cells."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Western blotting analysis of HEK293T cells in overexpression transfection
      experiments revealed that the Leu111del mutation led to an increase in the
      level of the IHH precursor and a reduction in the level of functional IHH
      protein compared with those in HEK293T cells expressing wild-type IHH,
      indicating that this mutation might cause IHH protein dysmaturity.
    explanation: Directly demonstrates precursor accumulation and loss of mature functional IHH.
  downstream:
  - target: Disrupted Chondrocyte Differentiation and Digit Segment Specification
    description: >-
      Instability, intracellular degradation, and altered extracellular
      interactions reduce the effective range of IHH signaling during digit
      development.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - mutation-dependent loss of stable and correctly trafficked IHH signaling protein
    evidence:
    - reference: PMID:21537345
      reference_title: "Indian hedgehog mutations causing brachydactyly type A1 impair Hedgehog signal transduction at multiple levels."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "The combination of these features alters signaling capacity and range, but the impact is likely to be variable and mutation-dependent."
      explanation: >-
        The experimental study links altered IHH stability, fate, and
        extracellular interactions to reduced signaling capacity and range.
- name: Reduced GDF5-BMPR1B Signaling
  conforms_to: "limb_digit_patterning_serial_homology#Limb Patterning Signal Perturbation"
  role: trigger
  description: >-
    A second BDA1 mechanism affects the BMP-SMAD arm of digit development.
    GDF5 is the preferred ligand for BMPR1B and promotes chondrogenesis and
    digit formation. Semidominant loss-of-function GDF5 variants or
    dominant-negative BMPR1B variants reduce this signaling input and can
    produce the same middle-phalanx pattern as IHH-associated BDA1.
  genes:
  - preferred_term: GDF5
    term:
      id: hgnc:4220
      label: GDF5
  - preferred_term: BMPR1B
    term:
      id: hgnc:1077
      label: BMPR1B
  biological_processes:
  - preferred_term: BMP Signaling Pathway
    term:
      id: GO:0030509
      label: BMP signaling pathway
    modifier: DECREASED
  - preferred_term: Chondrocyte Differentiation
    term:
      id: GO:0002062
      label: chondrocyte differentiation
    modifier: DECREASED
  cell_types:
  - preferred_term: Chondrocyte
    term:
      id: CL:0000138
      label: chondrocyte
  evidence:
  - reference: PMID:20683927
    reference_title: "Mutations in GDF5 presenting as semidominant brachydactyly A1."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Functional analysis demonstrated that although the p.Arg399Cys mutant is able to stimulate chondrogenesis, it is much less effective than wild-type GDF5."
    explanation: >-
      Functional analysis shows that the BDA1-associated GDF5 allele has
      reduced chondrogenic activity.
  - reference: PMID:25758993
    reference_title: "Two novel disease-causing variants in BMPR1B are associated with brachydactyly type A1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These findings demonstrate that BMPR1B is another gene involved with the pathogenesis of BDA1 and illustrates the continuum of phenotypes between BDA1 and BDA2."
    explanation: >-
      Human genetic findings establish BMPR1B-associated BDA1 and place it in
      the same BMP-signaling spectrum.
  downstream:
  - target: Disrupted Chondrocyte Differentiation and Digit Segment Specification
    description: >-
      Reduced GDF5 ligand activity or BMPR1B receptor activity impairs
      chondrogenesis and digit formation upstream of phalangeal segment
      specification.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - reduced BMP-SMAD signaling during chondrogenesis
    evidence:
    - reference: PMID:25758993
      reference_title: "Two novel disease-causing variants in BMPR1B are associated with brachydactyly type A1."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "GDF5 interacts directly as the preferred ligand for the BMP type-1 receptor BMPR1B and is important for both chondrogenesis and digit formation."
      explanation: >-
        The study identifies the ligand-receptor relationship and the shared
        developmental processes connecting GDF5-BMPR1B dysfunction to BDA1.
- name: Disrupted Chondrocyte Differentiation and Digit Segment Specification
  conforms_to: "limb_digit_patterning_serial_homology#Disrupted Digit Number and Identity Specification"
  role: central_effector
  description: >-
    Hedgehog and BMP-SMAD lesions converge on impaired chondrocyte
    differentiation and endochondral ossification. The resulting disturbance
    of embryonic limb morphogenesis preferentially shortens or eliminates
    middle phalangeal segments, with variable effects on the first digits,
    metacarpals, and overall skeletal growth.
  cell_types:
  - preferred_term: Chondrocyte
    term:
      id: CL:0000138
      label: chondrocyte
  biological_processes:
  - preferred_term: Chondrocyte Differentiation
    term:
      id: GO:0002062
      label: chondrocyte differentiation
    modifier: DECREASED
  - preferred_term: Endochondral Ossification
    term:
      id: GO:0001958
      label: endochondral ossification
    modifier: ABNORMAL
  - preferred_term: Embryonic Limb Morphogenesis
    term:
      id: GO:0030326
      label: embryonic limb morphogenesis
    modifier: ABNORMAL
  evidence:
  - reference: PMID:21537345
    reference_title: "Indian hedgehog mutations causing brachydactyly type A1 impair Hedgehog signal transduction at multiple levels."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Taken together, our results suggest that these IHH mutations affect Hh signaling at multiple levels, causing abnormal bone development and abnormal digit formation."
    explanation: >-
      Functional IHH studies connect altered signaling to abnormal bone and
      digit formation.
  - reference: PMID:25758993
    reference_title: "Two novel disease-causing variants in BMPR1B are associated with brachydactyly type A1."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Human and mouse genetic perturbations in the BMP-SMAD signaling pathway have been associated with many brachymesophalangies, including BDA1, as causative mutations in IHH and GDF5 have been previously identified."
    explanation: >-
      The BMPR1B study places genetically heterogeneous BDA1 within a shared
      BMP-SMAD developmental mechanism.
  - reference: PMID:30651074
    reference_title: "p.E95K mutation in Indian hedgehog causing brachydactyly type A1 impairs IHH/Gli1 downstream transcriptional regulation."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      our results demonstrated that the E95K mutant signaling altered Gli1-DNA
      binding pattern, impaired downstream gene expressions, and leaded to
      weakened cellular proliferation and migration.
    explanation: >-
      Cell-based functional data connect mutant IHH/Gli1 transcriptional output
      to impaired proliferation and migration in the convergent developmental mechanism.
  downstream:
  - target: Failed Interphalangeal Joint Cavitation from Suppressed Interzone Apoptosis
    description: >-
      Deregulated IHH levels in the developing interzone block the apoptosis
      that normally drives joint cavitation.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:38472182
      reference_title: Suppression of apoptosis impairs phalangeal joint formation in the pathogenesis of brachydactyly type A1.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "In BDA1, a high concentration of IHH suppresses apoptosis."
      explanation: Connects the BDA1 IHH lesion to failure of interzone apoptosis.
  - target: Serially Homologous Hand and Foot Brachydactyly
    description: >-
      The same disrupted segment-specification program operates in fore- and
      hindlimb autopods, producing a coordinated hand-and-foot malformation
      rather than an isolated digit defect.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - abnormal phalangeal cartilage-template growth and endochondral ossification
    evidence:
    - reference: PMID:18629882
      reference_title: "Deletion of 1 amino acid in Indian hedgehog leads to brachydactylyA1."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Brachydactyly type A1 is a limb malformation characterized by a uniform shortening of the middle phalanges in all digits."
      explanation: >-
        Uniform involvement across all digits documents the serially
        homologous hand-and-foot output of the developmental lesion.
  - target: Short Stature
    description: >-
      The growth-plate defect can extend beyond the digits and reduce linear
      skeletal growth, although expression is variable among families.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:34315464
      reference_title: "Deletion of 2 amino acids in IHH in a Japanese family with brachydactyly type A1."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The proband, a 9-year-old boy, his siblings, and his father had shortened digits and a short stature of variable severity."
      explanation: >-
        A molecularly confirmed family links the IHH-associated digit
        phenotype with variably expressed short stature.
  - target: Talipes Equinovarus
    description: >-
      Clubfoot is an occasional broader limb-development outcome; the
      intermediate steps beyond altered limb morphogenesis are unresolved.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Scoliosis
    description: >-
      Scoliosis is an occasional broader skeletal association; its precise
      mechanistic connection to the digit-patterning lesion is unresolved.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Hypoplasia of the Ulna
    description: >-
      Ulnar hypoplasia extends the growth defect proximally within the upper
      limb, with incompletely defined intermediates.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:19464397
      reference_title: "Brachydactyly type A1 associated with unusual radiological findings and a novel Arg158Cys mutation in the Indian hedgehog (IHH) gene."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Affected individuals show hypoplasia of the ulnar styloid processes, ulna minus, osteoarthritis, normal length of all distal phalanges and shortening or absence of the middle phalanges."
      explanation: >-
        A molecularly confirmed IHH family documents proximal ulnar
        involvement together with the defining phalangeal phenotype.
- name: Failed Interphalangeal Joint Cavitation from Suppressed Interzone Apoptosis
  role: central_effector
  biological_scale: CELLULAR
  description: >
    A 2024 mouse study reframes what the missing middle phalanx in BDA1 actually
    is. Using an Ihh E95K knock-in, it shows the bone is not simply too small:
    the developing interphalangeal joint fails to cavitate, so two segments stay
    fused as one and the middle phalanx never appears as a separate element.
    The proximate cause is loss of apoptosis in the joint interzone. The proposed
    mechanism inverts the usual reading of a hedgehog signalling defect - IHH is
    normally kept *low* at the centre of the interzone, which leaves its
    interacting partners CDON and GAS1 ligand-free to act as dependence
    receptors and trigger the cell death that separates the segments. A BDA1
    allele raises IHH there, the dependence receptors are occupied, apoptosis is
    suppressed, and cavitation fails.

    This is why BDA1 pairs shortening with symphalangism rather than producing
    one or the other, and it makes the node mechanistically continuous with the
    joint-formation failures of the other brachydactylies (NOG in type B2, GDF5
    at the interzone in type C).
  gene:
    preferred_term: IHH
    term:
      id: hgnc:5956
      label: IHH
  biological_processes:
  - preferred_term: Apoptotic Process
    term:
      id: GO:0006915
      label: apoptotic process
    modifier: DECREASED
  - preferred_term: Embryonic Skeletal Limb Joint Morphogenesis
    term:
      id: GO:0036023
      label: embryonic skeletal limb joint morphogenesis
    modifier: ABNORMAL
  cell_types:
  - preferred_term: Chondrocyte
    term:
      id: CL:0000138
      label: chondrocyte
  evidence:
  - reference: PMID:38472182
    reference_title: Suppression of apoptosis impairs phalangeal joint formation in the pathogenesis of brachydactyly type A1.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Here, we use a mouse model of brachydactyly type A1 (BDA1) with an IhhE95K mutation, to show that a missing middle phalangeal bone is due to the failure of the developing joint to cavitate, associated with reduced apoptosis, and a joint is not formed."
    explanation: >-
      Reframes the missing middle phalanx as a joint-cavitation failure driven by
      reduced interzone apoptosis, in a knock-in model of a human BDA1 allele.
  - reference: PMID:38472182
    reference_title: Suppression of apoptosis impairs phalangeal joint formation in the pathogenesis of brachydactyly type A1.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "We showed an intricate relationship between IHH and interacting partners, CDON and GAS1, in the interzone that regulates apoptosis."
    explanation: Identifies CDON and GAS1 as the IHH partners mediating interzone apoptosis.
  - reference: PMID:38472182
    reference_title: Suppression of apoptosis impairs phalangeal joint formation in the pathogenesis of brachydactyly type A1.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "We propose a model in which CDON/GAS1 may act as dependence receptors in this context."
    explanation: >-
      Supports the CDON/GAS1 dependence-receptor account of interzone apoptosis,
      with the caveat that the authors offer it as a proposed model rather than
      a demonstrated one.
  downstream:
  - target: Serially Homologous Hand and Foot Brachydactyly
    description: >-
      Segments that fail to separate present as shortened or absent middle
      phalanges with symphalangism across the homologous digits.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:38472182
      reference_title: Suppression of apoptosis impairs phalangeal joint formation in the pathogenesis of brachydactyly type A1.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "a missing middle phalangeal bone is due to the failure of the developing joint to cavitate"
      explanation: Ties the cavitation failure directly to the missing skeletal element.
  - target: Distal Finger Symphalangism
    description: >-
      An uncavitated interzone is, by definition, a fused joint.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:38472182
      reference_title: Suppression of apoptosis impairs phalangeal joint formation in the pathogenesis of brachydactyly type A1.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Cavitation is the physical separation of the cartilage templates, and failure leads to joint fusions."
      explanation: States the direct relationship between failed cavitation and joint fusion.
- name: Serially Homologous Hand and Foot Brachydactyly
  conforms_to: "limb_digit_patterning_serial_homology#Serially Homologous Autopod Malformation"
  role: consequence
  description: >-
    Because the autopod developmental program is reused in the hands and feet,
    disrupted IHH or GDF5-BMPR1B signaling produces shortening or absence of
    homologous phalangeal segments across both sets of digits. Variable
    expressivity also produces symphalangism, clinodactyly, abnormal epiphyses,
    and metacarpal changes.
  cell_types:
  - preferred_term: Chondrocyte
    term:
      id: CL:0000138
      label: chondrocyte
  biological_processes:
  - preferred_term: Limb Morphogenesis
    term:
      id: GO:0035108
      label: limb morphogenesis
    modifier: ABNORMAL
  evidence:
  - reference: PMID:18629882
    reference_title: "Deletion of 1 amino acid in Indian hedgehog leads to brachydactylyA1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Brachydactyly type A1 is a limb malformation characterized by a uniform shortening of the middle phalanges in all digits."
    explanation: >-
      Human genetic evidence supports a serially repeated middle-phalanx
      phenotype across the digits.
  downstream:
  - target: Type A1 Brachydactyly
    causal_link_type: DIRECT
  - target: Short Middle Phalanx of Finger
    causal_link_type: DIRECT
  - target: Short Middle Phalanx of Toe
    causal_link_type: DIRECT
  - target: Short Foot
    causal_link_type: DIRECT
  - target: Short Thumb
    causal_link_type: DIRECT
  - target: Short Hallux
    causal_link_type: DIRECT
  - target: Cone-Shaped Epiphysis
    causal_link_type: DIRECT
  - target: Distal Finger Symphalangism
    causal_link_type: DIRECT
  - target: Clinodactyly of Fifth Finger
    causal_link_type: DIRECT
  - target: Broad Metacarpals
    causal_link_type: DIRECT
phenotypes:
- name: Type A1 Brachydactyly
  description: >
    The defining digital pattern of BDA1, caused by shortening or absence
    of the middle phalanges with variable expressivity across affected
    families.
  phenotype_term:
    preferred_term: Type A1 brachydactyly
    term:
      id: HP:0009371
      label: Type A1 brachydactyly
  evidence:
  - reference: PMID:11455389
    reference_title: "Mutations in IHH, encoding Indian hedgehog, cause brachydactyly type A-1."
    supports: SUPPORT
    snippet: "Brachydactyly type A-1 (BDA-1; MIM 112500) is characterized by shortening or missing of the middle phalanges"
    explanation: "Defines the cardinal phenotype of BDA1."
  - reference: PMID:21537345
    reference_title: "Indian hedgehog mutations causing brachydactyly type A1 impair Hedgehog signal transduction at multiple levels."
    supports: SUPPORT
    snippet: "Brachydactyly type A1 (BDA1), the first recorded Mendelian autosomal dominant disorder in humans, is characterized by a shortening or absence of the middle phalanges"
    explanation: "Confirms middle phalanx shortening/absence as the defining feature."
  - reference: PMID:32209048
    reference_title: "A novel variant of IHH in a Chinese family with brachydactyly type 1."
    supports: SUPPORT
    snippet: "The variant co-segregated with BDA-1 in the pedigree, showed 100% penetrance for phalange phenotype with variable expressivity."
    explanation: "A five-generation family showed fully penetrant but variably severe phalangeal involvement."
- name: Short Middle Phalanx of Finger
  description: >
    Uniform shortening of the finger middle phalanges is the core skeletal
    abnormality in classic BDA1.
  phenotype_term:
    preferred_term: Short middle phalanx of finger
    term:
      id: HP:0005819
      label: Short middle phalanx of finger
  evidence:
  - reference: PMID:34315464
    reference_title: "Deletion of 2 amino acids in IHH in a Japanese family with brachydactyly type A1."
    supports: SUPPORT
    snippet: "Brachydactyly type A1 (BDA1) is an autosomal dominant disorder characterized by uniform shortening of the middle phalanges in all digits."
    explanation: "Directly supports uniform shortening of the finger middle phalanges in a molecularly confirmed BDA1 family."
  - reference: PMID:18629882
    reference_title: "Deletion of 1 amino acid in Indian hedgehog leads to brachydactylyA1."
    supports: SUPPORT
    snippet: "Brachydactyly type A1 is a limb malformation characterized by a uniform shortening of the middle phalanges in all digits."
    explanation: "Independent family report confirms uniform middle-phalanx shortening as the key digital abnormality."
- name: Short Middle Phalanx of Toe
  description: >
    Toe middle phalanges can also be shortened or absent in BDA1, showing
    that the malformation affects both hands and feet.
  phenotype_term:
    preferred_term: Short middle phalanx of toe
    term:
      id: HP:0003795
      label: Short middle phalanx of toe
  evidence:
  - reference: PMID:30651074
    reference_title: "p.E95K mutation in Indian hedgehog causing brachydactyly type A1 impairs IHH/Gli1 downstream transcriptional regulation."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Brachydactyly type A1 (BDA1, OMIM 112500) is a rare inherited malformation characterized primarily by shortness or absence of middle bones of fingers and toes."
    explanation: "Supports toe middle-phalanx involvement as part of the core BDA1 malformation pattern."
  - reference: PMID:40606564
    reference_title: "A Novel Heterozygous IHH c.331_333del Mutation Identified in a Fetus with Brachydactyly Type A1 Causes IHH Protein Maturation Failure in HEK293T Cells."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Brachydactyly A1 (BDA1) is a rare disorder characterized by the disproportionate shortening of fingers and/or toes with or without symphalangism."
    explanation: "Recent prenatal case report independently supports toe shortening within the BDA1 phenotype spectrum."
- name: Short Stature
  description: >
    Short stature has been reported in some molecularly confirmed BDA1
    families and appears variable rather than obligatory.
  phenotype_term:
    preferred_term: Short stature
    term:
      id: HP:0004322
      label: Short stature
  frequency: VERY_FREQUENT
  evidence:
  - reference: ORPHA:93388
    reference_title: "Brachydactyly type A1"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0004322 | Short stature | Very frequent (99-80%)"
    explanation: >-
      Orphanet lists short stature as a very frequent feature of brachydactyly
      type A1, consistent with IHH's role in skeletal growth regulation.
  - reference: PMID:34315464
    reference_title: "Deletion of 2 amino acids in IHH in a Japanese family with brachydactyly type A1."
    supports: SUPPORT
    snippet: "The proband, a 9-year-old boy, his siblings, and his father had shortened digits and a short stature of variable severity."
    explanation: "Supports short stature as a variable associated manifestation in a molecularly confirmed BDA1 family."
- name: Short Foot
  description: >
    Shortening of the foot, reflecting the bilateral lower-limb involvement
    of phalangeal and metatarsal shortening seen in BDA1.
  phenotype_term:
    preferred_term: Short foot
    term:
      id: HP:0001773
      label: Short foot
  frequency: VERY_FREQUENT
  evidence:
  - reference: ORPHA:93388
    reference_title: "Brachydactyly type A1"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001773 | Short foot | Very frequent (99-80%)"
    explanation: >-
      Orphanet lists short foot as a very frequent feature of brachydactyly
      type A1, reflecting the bilateral hand and foot involvement of BDA1.
- name: Short Thumb
  description: >
    The proximal phalanx of the thumb is shortened or absent in BDA1,
    contributing to the characteristic overall hand appearance and
    reflecting the role of IHH in proximal phalanx formation.
  phenotype_term:
    preferred_term: Short thumb
    term:
      id: HP:0009778
      label: Short thumb
  frequency: VERY_FREQUENT
  evidence:
  - reference: ORPHA:93388
    reference_title: "Brachydactyly type A1"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0009778 | Short thumb | Very frequent (99-80%)"
    explanation: >-
      Orphanet lists short thumb as a very frequent feature of brachydactyly
      type A1. The thumb is affected through shortening of the proximal phalanx.
- name: Short Hallux
  description: >
    Shortening of the great toe (hallux) proximal phalanx occurs in BDA1
    in parallel to the thumb involvement of the hands.
  phenotype_term:
    preferred_term: Short hallux
    term:
      id: HP:0010109
      label: Short hallux
  frequency: VERY_FREQUENT
  evidence:
  - reference: ORPHA:93388
    reference_title: "Brachydactyly type A1"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0010109 | Short hallux | Very frequent (99-80%)"
    explanation: >-
      Orphanet lists short hallux as a very frequent feature of brachydactyly
      type A1, reflecting the foot involvement mirroring the thumb changes.
- name: Cone-Shaped Epiphysis
  description: >
    Cone-shaped epiphyses of the middle phalanges are a characteristic
    radiographic finding in BDA1, arising from the abnormal endochondral
    ossification due to impaired IHH-Patched1 signaling.
  phenotype_term:
    preferred_term: Cone-shaped epiphysis
    term:
      id: HP:0010579
      label: Cone-shaped epiphysis
  frequency: FREQUENT
  evidence:
  - reference: ORPHA:93388
    reference_title: "Brachydactyly type A1"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0010579 | Cone-shaped epiphysis | Frequent (79-30%)"
    explanation: >-
      Orphanet lists cone-shaped epiphysis as a frequent radiographic feature
      of brachydactyly type A1.
- name: Distal Finger Symphalangism
  description: >
    Fusion of the distal interphalangeal joints (symphalangism) is an
    occasional finding in BDA1, reflecting the variable extent of phalanx
    fusion seen in this disorder.
  phenotype_term:
    preferred_term: Distal symphalangism of hands
    term:
      id: HP:0001204
      label: Distal finger symphalangism
  frequency: OCCASIONAL
  evidence:
  - reference: ORPHA:93388
    reference_title: "Brachydactyly type A1"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001204 | Distal symphalangism (hands) | Occasional (29-5%)"
    explanation: >-
      Orphanet lists distal symphalangism of hands as an occasional feature
      of brachydactyly type A1.
- name: Clinodactyly of Fifth Finger
  description: >
    Lateral deviation of the fifth finger (clinodactyly) is an occasional
    digital feature in BDA1, secondary to asymmetric shortening of the
    middle phalanx.
  phenotype_term:
    preferred_term: Clinodactyly of the 5th finger
    term:
      id: HP:0004209
      label: Clinodactyly of the 5th finger
  frequency: OCCASIONAL
  evidence:
  - reference: ORPHA:93388
    reference_title: "Brachydactyly type A1"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0004209 | Clinodactyly of the 5th finger | Occasional (29-5%)"
    explanation: >-
      Orphanet lists clinodactyly of the 5th finger as an occasional feature
      of brachydactyly type A1.
- name: Broad Metacarpals
  description: >
    Broadening of the metacarpals is an occasional skeletal feature recorded in
    the Orphanet phenotype profile for BDA1.
  phenotype_term:
    preferred_term: Broad metacarpals
    term:
      id: HP:0001230
      label: Broad metacarpals
  frequency: OCCASIONAL
  evidence:
  - reference: ORPHA:93388
    reference_title: "Brachydactyly type A1"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001230 | Broad metacarpals | Occasional (29-5%)"
    explanation: Orphanet lists broad metacarpals as an occasional feature of brachydactyly type A1.
- name: Talipes Equinovarus
  description: >
    Talipes equinovarus is an occasional lower-limb skeletal feature recorded
    for BDA1 in Orphanet.
  phenotype_term:
    preferred_term: Talipes equinovarus
    term:
      id: HP:0001762
      label: Talipes equinovarus
  frequency: OCCASIONAL
  evidence:
  - reference: ORPHA:93388
    reference_title: "Brachydactyly type A1"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001762 | Talipes equinovarus | Occasional (29-5%)"
    explanation: Orphanet lists talipes equinovarus as an occasional feature of brachydactyly type A1.
- name: Scoliosis
  description: >
    Scoliosis is an occasional axial skeletal feature recorded for BDA1 in
    Orphanet.
  phenotype_term:
    preferred_term: Scoliosis
    term:
      id: HP:0002650
      label: Scoliosis
  frequency: OCCASIONAL
  evidence:
  - reference: ORPHA:93388
    reference_title: "Brachydactyly type A1"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0002650 | Scoliosis | Occasional (29-5%)"
    explanation: Orphanet lists scoliosis as an occasional feature of brachydactyly type A1.
- name: Hypoplasia of the Ulna
  description: >
    Ulnar hypoplasia is an occasional upper-limb feature recorded for BDA1 in
    Orphanet.
  phenotype_term:
    preferred_term: Hypoplasia of the ulna
    term:
      id: HP:0003022
      label: Hypoplasia of the ulna
  frequency: OCCASIONAL
  evidence:
  - reference: ORPHA:93388
    reference_title: "Brachydactyly type A1"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0003022 | Hypoplasia of the ulna | Occasional (29-5%)"
    explanation: Orphanet lists hypoplasia of the ulna as an occasional feature of brachydactyly type A1.
genetic:
- name: IHH Pathogenic Variants
  gene_term:
    preferred_term: IHH
    term:
      id: hgnc:5956
      label: IHH
  association: CAUSAL
  notes: >
    Heterozygous pathogenic IHH variants are the canonical cause of BDA1.
    Classic alleles are missense substitutions in the N-terminal signaling
    domain (including E95K, D100E, and E131K), but pathogenic in-frame
    deletions have also been demonstrated. Effects include altered stability,
    receptor binding, trafficking, and extracellular interactions.
  evidence:
  - reference: PMID:11455389
    reference_title: "Mutations in IHH, encoding Indian hedgehog, cause brachydactyly type A-1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "three heterozygous missense mutations in the region encoding the amino-terminal signaling domain in all affected members of three large, unrelated families"
    explanation: "Three independent families with distinct IHH missense mutations establish genotype-phenotype relationship."
  - reference: PMID:11455389
    reference_title: "Mutations in IHH, encoding Indian hedgehog, cause brachydactyly type A-1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The three mutant amino acids, which are conserved across all vertebrates and invertebrates studied so far, are predicted to be adjacent on the surface of IHH"
    explanation: "Deep evolutionary conservation and surface clustering suggest a critical functional domain."
  - reference: PMID:21537345
    reference_title: "Indian hedgehog mutations causing brachydactyly type A1 impair Hedgehog signal transduction at multiple levels."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Heterozygous missense mutations in the Indian Hedgehog (IHH) gene have been identified as a cause of BDA1"
    explanation: "Confirms IHH missense mutations as the established cause of BDA1."
  - reference: PMID:18629882
    reference_title: "Deletion of 1 amino acid in Indian hedgehog leads to brachydactylyA1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report here on the first deletion in IHH, p.delE95, causing mild BrachydactylyA1 in a small Dutch family."
    explanation: >-
      Expands the established pathogenic spectrum beyond missense variants to
      an in-frame IHH deletion.
  - reference: PMID:38917024
    reference_title: Variants in both the N- or C-terminal domains of IHH lead to defective secretion causing short stature and skeletal defects.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Many are located in the C-terminal domain of IHH (IHH-C), which lacks signaling activity but is critical for auto-cleavage and activation of the N-terminal (IHH-N) peptide."
    explanation: >-
      Extends the pathogenic territory beyond the N-terminal signalling domain
      to the C-terminal domain that processes it - relevant because IHH-C
      variants were previously being called variants of uncertain significance.
  - reference: PMID:38917024
    reference_title: Variants in both the N- or C-terminal domains of IHH lead to defective secretion causing short stature and skeletal defects.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "In vitro studies were performed for 9 IHH heterozygous variants, to test their effect on secretion and IHH intracellular processing by western blot of cells expressing each variant."
    explanation: Describes the functional assay used to reclassify IHH variants of uncertain significance.
- name: GDF5 Pathogenic Variants
  gene_term:
    preferred_term: GDF5
    term:
      id: hgnc:4220
      label: GDF5
  association: CAUSAL
  notes: >-
    Semidominant GDF5 variants can produce mild BDA1 in heterozygotes and a
    more severe phenotype in homozygotes. Reduced ligand activity impairs
    chondrogenesis upstream of IHH.
  evidence:
  - reference: PMID:20683927
    reference_title: "Mutations in GDF5 presenting as semidominant brachydactyly A1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Sequencing of the GDF5 coding region revealed that a mildly affected individual in the family was heterozygous, and that all of the severely affected individuals were homozygous for a novel missense c.1195C>T mutation that predicts a p.Arg399Cys substitution at a highly conserved amino acid."
    explanation: >-
      Segregation in a consanguineous family demonstrates dosage-dependent
      GDF5-associated BDA1.
  - reference: PMID:20683927
    reference_title: "Mutations in GDF5 presenting as semidominant brachydactyly A1."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Functional analysis demonstrated that although the p.Arg399Cys mutant is able to stimulate chondrogenesis, it is much less effective than wild-type GDF5."
    explanation: >-
      Functional assays show reduced chondrogenic activity of the
      BDA1-associated GDF5 allele.
  - reference: PMID:38222807
    reference_title: A missense GDF5 variant causes brachydactyly type A1 and multiple-synostoses syndrome 2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The family was found to have an autosomal-dominantly inherited combination of BDA1 and SYNS2 caused by the S475N variant in the GDF5 gene."
    explanation: >-
      A single GDF5 allele producing BDA1 together with multiple-synostoses
      syndrome 2, reinforcing that GDF5 phenotype depends on the allele rather
      than the gene.
- name: BMPR1B Pathogenic Variants
  gene_term:
    preferred_term: BMPR1B
    term:
      id: hgnc:1077
      label: BMPR1B
  association: CAUSAL
  notes: >-
    Heterozygous missense and splice-site BMPR1B variants have been reported
    in complex BDA1 and are predicted to act through a dominant-negative
    mechanism, illustrating phenotypic continuity between BDA1 and BDA2.
  evidence:
  - reference: PMID:25758993
    reference_title: "Two novel disease-causing variants in BMPR1B are associated with brachydactyly type A1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report pathogenic variants in BMPR1B that are associated with complex BDA1."
    explanation: >-
      The human genetic study identifies BMPR1B variants in individuals with
      complex BDA1.
  - reference: PMID:25758993
    reference_title: "Two novel disease-causing variants in BMPR1B are associated with brachydactyly type A1."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Both mutations are most likely to act in a dominant-negative manner, similar to the effects observed in BMPR1B mutations that cause BDA2."
    explanation: >-
      The authors interpret the two BDA1-associated BMPR1B variants as
      dominant-negative alleles.
diagnosis:
- name: Clinical-radiographic and molecular diagnosis
  description: >-
    Diagnosis is supported by the characteristic clinical and radiographic
    pattern of shortened or absent middle phalanges and confirmed by
    identifying a pathogenic variant in IHH, GDF5, or BMPR1B when molecularly solved.
  diagnosis_term:
    preferred_term: genetic testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  results: Characteristic middle-phalanx shortening or absence, with detection of a causal IHH, GDF5, or BMPR1B variant when present.
  evidence:
  - reference: PMID:11455389
    reference_title: "Mutations in IHH, encoding Indian hedgehog, cause brachydactyly type A-1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Brachydactyly type A-1 (BDA-1; MIM 112500) is characterized by shortening or missing of the middle phalanges"
    explanation: The defining skeletal pattern provides the clinical-radiographic basis for diagnosis.
  - reference: PMID:11455389
    reference_title: "Mutations in IHH, encoding Indian hedgehog, cause brachydactyly type A-1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here we show that mutations in IHH, which encodes Indian hedgehog, cause BDA-1."
    explanation: This supports IHH molecular testing as confirmatory for BDA1.
  - reference: PMID:20683927
    reference_title: "Mutations in GDF5 presenting as semidominant brachydactyly A1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This data confirms genetic heterogeneity in BDA1, demonstrates that mutations upstream of IHH can result in BDA1, and shows that BDA1 can result from semidominant mutations in GDF5."
    explanation: Supports including GDF5 in molecular confirmation of genetically heterogeneous BDA1.
  - reference: PMID:25758993
    reference_title: "Two novel disease-causing variants in BMPR1B are associated with brachydactyly type A1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report pathogenic variants in BMPR1B that are associated with complex BDA1."
    explanation: Supports including BMPR1B in molecular confirmation of BDA1.
treatments:
- name: Genetic Counseling
  description: >
    Genetic counseling for affected families given autosomal dominant
    inheritance and the historical significance of BDA1 as the first
    recorded Mendelian trait.
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:18554391
    reference_title: Brachydactyly.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      The nature of genetic counseling depends both on the pattern of inheritance
      of the type of brachydactyly present in the family and on the presence or
      absence of accompanying symptoms.
    explanation: The clinical review directly supports inheritance- and phenotype-specific counseling.
- name: Hand Function Rehabilitation
  description: Physical and occupational therapy may improve hand function when functional limitation is present.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: physical therapy
    term:
      id: NCIT:C15302
      label: Physical Therapy
  evidence:
  - reference: PMID:18554391
    reference_title: Brachydactyly.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Physical therapy and ergotherapy may ameliorate hand function."
    explanation: The management review supports function-directed rehabilitation.
- name: Function- or Cosmesis-Directed Hand Surgery
  description: Surgery is reserved for meaningful hand-function impairment or selected cosmetic indications and is usually unnecessary.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: surgical procedure
    term:
      id: NCIT:C15329
      label: Surgical Procedure
  evidence:
  - reference: PMID:18554391
    reference_title: Brachydactyly.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Plastic surgery is only indicated if the brachydactyly affects hand function or for cosmetic reasons, but is typically not needed."
    explanation: The review defines the limited indications for surgery.
- name: Recombinant Human Growth Hormone for Selected IHH-Related Short Stature
  description: >-
    rhGH may be considered for clinically significant short stature in selected
    IHH-related cases; evidence is limited to a two-sibling case report and does
    not show correction of the hand malformation.
  therapeutic_modality: PROTEIN_REPLACEMENT
  treatment_term:
    preferred_term: growth hormone therapy
    term:
      id: NCIT:C15445
      label: Hormone Therapy
  evidence:
  - reference: PMID:38840672
    reference_title: "Short stature with brachydactyly caused by a novel mutation in the IHH gene and response to 4-year growth hormone therapy: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      After treatment, the siblings' height improved significantly, and they
      acquired a significant increase in the height standard deviation score
      (SDS) (the boy: +2.54, the girl: +1.86) during the 4-year therapy. No
      noticeable adverse effect was observed during rhGH treatment.
    explanation: Two siblings improved over four years, but uncontrolled case-level evidence limits generalization.
references:
- reference: DOI:10.1002/ajmg.a.33761
  title: Brachydactyly type A1 with short humerus and associated skeletal features
  found_in:
  - Brachydactyly_Type_A1-deep-research-falcon.md
  findings:
  - statement: We report on a three‐generation family affected with an osteochondrodysplasia transmitted as an autosomal dominant trait.
    supporting_text: We report on a three‐generation family affected with an osteochondrodysplasia transmitted as an autosomal dominant trait.
    evidence:
    - reference: DOI:10.1002/ajmg.a.33761
      reference_title: Brachydactyly type A1 with short humerus and associated skeletal features
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: We report on a three‐generation family affected with an osteochondrodysplasia transmitted as an autosomal dominant trait.
      explanation: Deep research cited this publication as relevant literature for Brachydactyly Type A1.
- reference: DOI:10.1177/1753193413514363
  title: Embryology of familial (non-syndromic) brachydactyly of the hand
  found_in:
  - Brachydactyly_Type_A1-deep-research-falcon.md
  findings:
  - statement: Embryology of familial (non-syndromic) brachydactyly of the hand
    supporting_text: Isolated familial non-syndromic brachydactyly is interesting from the embryological point of view because the phenotypes of isolated brachydactyly are frequently overlapping, yet they are caused by different gene mutations and the ring finger is frequently relatively preserved.
    evidence:
    - reference: DOI:10.1177/1753193413514363
      reference_title: Embryology of familial (non-syndromic) brachydactyly of the hand
      supports: SUPPORT
      evidence_source: OTHER
      snippet: Isolated familial non-syndromic brachydactyly is interesting from the embryological point of view because the phenotypes of isolated brachydactyly are frequently overlapping, yet they are caused by different gene mutations and the ring finger is frequently relatively preserved.
      explanation: Deep research cited this publication as relevant literature for Brachydactyly Type A1.
- reference: DOI:10.20381/ruor-2868
  title: 'The Genetic Heterogeneity of Brachydactyly Type A1: Identifying the Molecular Pathways'
  found_in:
  - Brachydactyly_Type_A1-deep-research-falcon.md
  findings:
  - statement: Brachydactyly type A1 (BDA1) is a rare autosomal dominant trait characterized by the shortening of the middle phalanges of digits 2-5 and of the proximal phalange of digit 1 in both hands and feet.
    supporting_text: Brachydactyly type A1 (BDA1) is a rare autosomal dominant trait characterized by the shortening of the middle phalanges of digits 2-5 and of the proximal phalange of digit 1 in both hands and feet.
    evidence:
    - reference: DOI:10.20381/ruor-2868
      reference_title: 'The Genetic Heterogeneity of Brachydactyly Type A1: Identifying the Molecular Pathways'
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Brachydactyly type A1 (BDA1) is a rare autosomal dominant trait characterized by the shortening of the middle phalanges of digits 2-5 and of the proximal phalange of digit 1 in both hands and feet.
      explanation: Deep research cited this publication as relevant literature for Brachydactyly Type A1.
- reference: DOI:10.3389/fgene.2022.814786
  title: 'Case Report: Brachydactyly Type A1 Induced by a Novel Variant of in-Frame Insertion in the IHH Gene'
  found_in:
  - Brachydactyly_Type_A1-deep-research-falcon.md
  findings:
  - statement: Brachydactyly type A1 (BDA1) is an autosomal dominant inherited disease characterized by the shortness/absence of the middle phalanges, which can be induced by mutations in the Indian hedgehog gene (IHH).
    supporting_text: Brachydactyly type A1 (BDA1) is an autosomal dominant inherited disease characterized by the shortness/absence of the middle phalanges, which can be induced by mutations in the Indian hedgehog gene (IHH).
    evidence:
    - reference: DOI:10.3389/fgene.2022.814786
      reference_title: 'Case Report: Brachydactyly Type A1 Induced by a Novel Variant of in-Frame Insertion in the IHH Gene'
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Brachydactyly type A1 (BDA1) is an autosomal dominant inherited disease characterized by the shortness/absence of the middle phalanges, which can be induced by mutations in the Indian hedgehog gene (IHH).
      explanation: Deep research cited this publication as relevant literature for Brachydactyly Type A1.
📚

References & Deep Research

References

4
DOI:10.1002/ajmg.a.33761
Brachydactyly type A1 with short humerus and associated skeletal features
1 finding
We report on a three‐generation family affected with an osteochondrodysplasia transmitted as an autosomal dominant trait.
"We report on a three‐generation family affected with an osteochondrodysplasia transmitted as an autosomal dominant trait."
Show evidence (1 reference)
DOI:10.1002/ajmg.a.33761 SUPPORT Human Clinical
"We report on a three‐generation family affected with an osteochondrodysplasia transmitted as an autosomal dominant trait."
Deep research cited this publication as relevant literature for Brachydactyly Type A1.
DOI:10.1177/1753193413514363
Embryology of familial (non-syndromic) brachydactyly of the hand
1 finding
Embryology of familial (non-syndromic) brachydactyly of the hand
"Isolated familial non-syndromic brachydactyly is interesting from the embryological point of view because the phenotypes of isolated brachydactyly are frequently overlapping, yet they are caused by different gene mutations and the ring finger is frequently relatively preserved."
Show evidence (1 reference)
DOI:10.1177/1753193413514363 SUPPORT Other
"Isolated familial non-syndromic brachydactyly is interesting from the embryological point of view because the phenotypes of isolated brachydactyly are frequently overlapping, yet they are caused by different gene mutations and the ring finger is frequently relatively preserved."
Deep research cited this publication as relevant literature for Brachydactyly Type A1.
DOI:10.20381/ruor-2868
The Genetic Heterogeneity of Brachydactyly Type A1: Identifying the Molecular Pathways
1 finding
Brachydactyly type A1 (BDA1) is a rare autosomal dominant trait characterized by the shortening of the middle phalanges of digits 2-5 and of the proximal phalange of digit 1 in both hands and feet.
"Brachydactyly type A1 (BDA1) is a rare autosomal dominant trait characterized by the shortening of the middle phalanges of digits 2-5 and of the proximal phalange of digit 1 in both hands and feet."
Show evidence (1 reference)
DOI:10.20381/ruor-2868 SUPPORT Human Clinical
"Brachydactyly type A1 (BDA1) is a rare autosomal dominant trait characterized by the shortening of the middle phalanges of digits 2-5 and of the proximal phalange of digit 1 in both hands and feet."
Deep research cited this publication as relevant literature for Brachydactyly Type A1.
DOI:10.3389/fgene.2022.814786
Case Report: Brachydactyly Type A1 Induced by a Novel Variant of in-Frame Insertion in the IHH Gene
1 finding
Brachydactyly type A1 (BDA1) is an autosomal dominant inherited disease characterized by the shortness/absence of the middle phalanges, which can be induced by mutations in the Indian hedgehog gene (IHH).
"Brachydactyly type A1 (BDA1) is an autosomal dominant inherited disease characterized by the shortness/absence of the middle phalanges, which can be induced by mutations in the Indian hedgehog gene (IHH)."
Show evidence (1 reference)
DOI:10.3389/fgene.2022.814786 SUPPORT Human Clinical
"Brachydactyly type A1 (BDA1) is an autosomal dominant inherited disease characterized by the shortness/absence of the middle phalanges, which can be induced by mutations in the Indian hedgehog gene (IHH)."
Deep research cited this publication as relevant literature for Brachydactyly Type A1.

Deep Research

1
Falcon ▸
Disease Characteristics Research Template
Edison Scientific Literature 26 citations 2026-05-08T10:32:00.966624

Question: You are an expert researcher providing comprehensive, well-cited information.

Provide detailed information focusing on: 1. Key concepts and definitions with current understanding 2. Recent developments and latest research (prioritize 2023-2024 sources) 3. Current applications and real-world implementations 4. Expert opinions and analysis from authoritative sources 5. Relevant statistics and data from recent studies

Format as a comprehensive research report with proper citations. Include URLs and publication dates where available. Always prioritize recent, authoritative sources and provide specific citations for all major claims.

Disease Characteristics Research Template

Target Disease

  • Disease Name: Brachydactyly Type A1
  • MONDO ID: (if available)
  • Category: Mendelian

Research Objectives

Please provide a comprehensive research report on Brachydactyly Type A1 covering all of the disease characteristics listed below. This report will be used to populate a disease knowledge base entry. Be thorough and cite primary literature (PMID preferred) for all claims.

For each section, suggested databases/resources are listed. These are the first places you should search for information on each topic.

1. Disease Information

Search first: OMIM, Orphanet, ICD-10/ICD-11, MeSH, PubMed

  • What is the disease? Provide a concise overview.
  • What are the key identifiers? (OMIM, Orphanet, ICD-10/ICD-11, MeSH, Mondo)
  • What are the common synonyms and alternative names?
  • Is the information derived from individual patients (e.g., EHR) or aggregated disease-level resources?

2. Etiology

  • Disease Causal Factors: What are the primary causes? (genetic, environmental, infectious, mechanistic)
  • Risk Factors:

    Search first: PubMed, Cochrane Library, UpToDate, clinical guidelines, ClinVar, ClinGen, GWAS Catalog, PheGenI, CTD, CDC, WHO, epidemiological databases

  • Genetic risk factors (causal variants, susceptibility loci, modifier genes)
  • Environmental risk factors (toxins, lifestyle, occupational exposures, age, sex, family history)
  • Protective Factors:

    Search first: PubMed, Cochrane Library, clinical trial databases, GWAS Catalog, gnomAD, WHO, CDC, nutrition databases

  • Genetic protective factors (protective variants, modifier alleles)
  • Environmental protective factors (diet, lifestyle, exposures that reduce risk)
  • Gene-Environment Interactions: How do genetic and environmental factors interact to influence disease?

    Search first: CTD, PubMed, PheGenI, GxE databases

3. Phenotypes

Search first: HPO (Human Phenotype Ontology), OMIM, Orphanet, PubMed, clinicaltrials.gov, MedDRA, SNOMED CT, DECIPHER, LOINC

For each phenotype, provide: - Phenotype type: symptoms, clinical signs, physical manifestations, behavioral changes, or laboratory abnormalities

For symptoms/signs: HPO, OMIM, Orphanet, PubMed For behavioral changes: HPO, DSM, RDoC (Research Domain Criteria), PubMed For laboratory abnormalities: LOINC, SNOMED CT, LabTests Online, PubMed - Phenotype characteristics: Search first: OMIM, Orphanet, HPO, PubMed - Age of symptom onset (neonatal, childhood, adult-onset, late-onset) - Symptom severity (mild, moderate, severe, variable) - Symptom progression (stable, progressive, episodic, fluctuating) - Frequency among affected individuals (percentage or qualitative) - Quality of life impact: Effects on daily functioning and well-being (per-phenotype when possible) Search first: EQ-5D database, SF-36, WHO QOL databases, PubMed - Suggest HPO (Human Phenotype Ontology) terms for each phenotype

4. Genetic/Molecular Information

  • Causal Genes: Gene mutations or chromosomal abnormalities responsible for disease (gene symbols, OMIM IDs)

    Search first: OMIM, ClinVar, HGMD, Ensembl, NCBI Gene

  • Pathogenic Variants:
  • Affected genes (gene symbols, HGNC IDs) > Search first: OMIM, NCBI Gene, Ensembl, HGNC, UniProt, GeneCards
  • Variant classification (pathogenic, likely pathogenic, VUS per ACMG/AMP guidelines) > Search first: ClinVar, ClinGen, ACMG/AMP guidelines, VarSome
  • Variant type/class (missense, frameshift, nonsense, splice-site, structural)
  • Allele frequency in population databases > Search first: gnomAD, 1000 Genomes, ExAC, TOPMed, dbSNP
  • Somatic vs germline origin > Search first: COSMIC (somatic), ClinVar, ICGC, TCGA
  • Functional consequences (loss of function, gain of function, dominant negative)
  • Modifier Genes: Genes that modify disease severity or expression
  • Epigenetic Information: DNA methylation, histone modifications, chromatin changes affecting disease

    Search first: ENCODE, Roadmap Epigenomics, MethBase, DiseaseMeth

  • Chromosomal Abnormalities: Large-scale genetic changes (aneuploidy, translocations, inversions)

    Search first: DECIPHER, ClinVar, ECARUCA, UCSC Genome Browser

5. Environmental Information

  • Environmental Factors: Non-genetic contributing factors (toxins, radiation, pollution, occupational exposure)

    Search first: CTD (Comparative Toxicogenomics Database), TOXNET, PubMed, EPA databases

  • Lifestyle Factors: Behavioral factors (smoking, diet, exercise, alcohol consumption)

    Search first: CDC databases, WHO, PubMed, NHANES

  • Infectious Agents: If applicable, pathogens causing or triggering disease (bacteria, viruses, fungi, parasites)

    Search first: NCBI Taxonomy, ViPR, BV-BRC, MicrobeDB, GIDEON

6. Mechanism / Pathophysiology

  • Molecular Pathways: Specific signaling cascades or biochemical pathways involved (Wnt, MAPK, mTOR, PI3K-AKT, etc.)

    Search first: KEGG, Reactome, WikiPathways, PathBank, BioCyc

  • Cellular Processes: Cell-level mechanisms (apoptosis, autophagy, cell cycle dysregulation, inflammation, etc.)

    Search first: Gene Ontology (GO), Reactome, KEGG, PubMed

  • Protein Dysfunction: How protein structure or function is altered (misfolding, aggregation, loss of function, gain of function)

    Search first: UniProt, PDB (Protein Data Bank), InterPro, Pfam, AlphaFold

  • Metabolic Changes: Alterations in metabolic processes (energy metabolism, lipid metabolism, amino acid metabolism)

    Search first: KEGG, BioCyc, HMDB (Human Metabolome Database), BRENDA

  • Immune System Involvement: Role of immune response (autoimmunity, immunodeficiency, chronic inflammation)

    Search first: ImmPort, Immunome Database, IEDB, Gene Ontology

  • Tissue Damage Mechanisms: How tissues/ are injured (oxidative stress, ischemia, fibrosis, necrosis)

    Search first: PubMed, Gene Ontology, Reactome

  • Biochemical Abnormalities: Specific molecular defects (enzyme deficiencies, receptor dysfunction, ion channel defects)

    Search first: BRENDA, UniProt, KEGG, OMIM, PubMed

  • Epigenetic Changes: DNA methylation, histone modifications affecting gene expression in disease

    Search first: ENCODE, Roadmap Epigenomics, MethBase, DiseaseMeth

  • Molecular Profiling (if available):
  • Transcriptomics/gene expression changes > Search first: GEO (Gene Expression Omnibus), ArrayExpress, GTEx, Human Cell Atlas, SRA
  • Proteomics findings > Search first: PRIDE, ProteomeXchange, Human Protein Atlas, STRING, BioGRID
  • Metabolomics signatures > Search first: MetaboLights, Metabolomics Workbench, HMDB, METLIN
  • Lipidomics alterations > Search first: LIPID MAPS, SwissLipids, LipidHome, Metabolomics Workbench
  • Genomic structural features > Search first: UCSC Genome Browser, Ensembl, NCBI, dbVar, DGV
  • Advanced Technologies (if applicable):
  • Single-cell analysis findings (cell-type specific mechanisms, cellular heterogeneity) > Search first: Human Cell Atlas, Single Cell Portal, GEO, CELLxGENE
  • Spatial transcriptomics findings > Search first: GEO, Spatial Research, Vizgen, 10x Genomics data
  • Multi-omics integration results > Search first: TCGA, ICGC, cBioPortal, LinkedOmics, PubMed
  • Functional genomics screens (CRISPR, RNAi) > Search first: DepMap, GenomeRNAi, PubMed, BioGRID ORCS

For each mechanism, describe: - The causal chain from initial trigger to clinical manifestation - Which mechanisms are upstream vs downstream - What cell types and biological processes are involved - Suggest GO terms for biological processes and CL terms for cell types

7. Anatomical Structures Affected

  • Organ Level:
  • Primary organs directly affected
  • Secondary organ involvement (complications, secondary effects)
  • Body systems involved (cardiovascular, nervous, digestive, respiratory, endocrine, etc.)

    Search first: Uberon, FMA (Foundational Model of Anatomy), OMIM, HPO, ICD-11, MeSH, SNOMED CT

  • Tissue and Cell Level:
  • Specific tissue types affected (epithelial, connective, muscle, nervous)
  • Specific cell populations targeted (with Cell Ontology terms)

    Search first: Uberon, Human Protein Atlas, Cell Ontology, Human Cell Atlas, CellMarker, PanglaoDB

  • Subcellular Level:
  • Cellular compartments involved (mitochondria, nucleus, ER, lysosomes) (with GO Cellular Component terms)

    Search first: Gene Ontology (Cellular Component), UniProt, Human Protein Atlas

  • Localization:
  • Specific anatomical sites (with UBERON terms) > Search first: FMA, Uberon, NeuroNames (for brain), SNOMED CT
  • Lateralization (unilateral, bilateral, asymmetric) > Search first: HPO, clinical literature, imaging databases

8. Temporal Development

  • Onset:
  • Typical age of onset (congenital, pediatric, adult, geriatric)
  • Onset pattern (acute, subacute, chronic, insidious)

    Search first: OMIM, Orphanet, HPO, PubMed

  • Progression:
  • Disease stages (early, intermediate, advanced, end-stage) > Search first: Cancer Staging Manual (AJCC), WHO classifications, PubMed
  • Progression rate (rapid, slow, variable)
  • Disease course pattern (episodic, relapsing-remitting, progressive, stable)
  • Disease duration (self-limited, chronic lifelong)

    Search first: Disease registries, longitudinal cohort databases, natural history studies, PubMed, Orphanet, OMIM

  • Patterns:
  • Remission patterns (spontaneous, treatment-induced) > Search first: Clinical trial databases, disease registries, PubMed
  • Critical periods (time windows of vulnerability or opportunity for intervention) > Search first: PubMed, developmental biology databases, clinical guidelines

9. Inheritance and Population

  • Epidemiology:
  • Prevalence (cases per 100,000 at given time)
  • Incidence (new cases per 100,000 per year)

    Search first: Orphanet, CDC, WHO, GBD (Global Burden of Disease), national registries, SEER, disease registries

  • For Genetic Etiology:
  • Inheritance pattern (AD, AR, X-linked, mitochondrial, multifactorial, polygenic) > Search first: OMIM, Orphanet, ClinVar, GTR (Genetic Testing Registry)
  • Penetrance (complete, incomplete, age-dependent) > Search first: ClinVar, OMIM, PubMed, ClinGen
  • Expressivity (variable, consistent) > Search first: OMIM, ClinVar, PubMed
  • Genetic anticipation (increasing severity in successive generations) > Search first: OMIM, PubMed (especially for repeat expansion disorders)
  • Germline mosaicism > Search first: ClinVar, OMIM, genetic counseling literature, PubMed
  • Founder effects (population-specific mutations) > Search first: gnomAD, population genetics databases, PubMed
  • Consanguinity role > Search first: OMIM, population studies, genetic counseling resources
  • Carrier frequency > Search first: gnomAD, carrier screening databases, GeneReviews, GTR
  • Population Demographics:
  • Affected populations (ethnic or demographic groups with higher prevalence) > Search first: gnomAD, 1000 Genomes, PAGE Study, PubMed, population registries
  • Geographic distribution (endemic areas, regional variation) > Search first: WHO, CDC, GBD, Orphanet, geographic epidemiology databases
  • Geographic distribution of specific variants
  • Sex ratio (male:female) > Search first: Disease registries, OMIM, PubMed, epidemiological databases
  • Age distribution of affected individuals > Search first: CDC, disease registries, SEER, Orphanet

10. Diagnostics

  • Clinical Tests:
  • Laboratory tests (blood, urine, tissue chemistry, specific enzyme assays) > Search first: LOINC, LabTests Online, PubMed
  • Biomarkers (proteins, metabolites, genetic markers, circulating biomarkers) > Search first: FDA Biomarker List, BEST (Biomarkers, EndpointS, and other Tools), PubMed
  • Imaging studies (X-ray, CT, MRI, PET, ultrasound) > Search first: RadLex, DICOM, Radiopaedia, imaging databases
  • Functional tests (pulmonary function, cardiac stress tests) > Search first: LOINC, clinical guidelines, PubMed
  • Electrophysiology (EEG, EMG, ECG, nerve conduction studies) > Search first: LOINC, clinical neurophysiology databases, PubMed
  • Biopsy findings (histopathology, immunohistochemistry) > Search first: SNOMED CT, College of American Pathologists resources, PubMed
  • Pathology findings (microscopic examination) > Search first: SNOMED CT, Digital Pathology databases, PubMed
  • Genetic Testing:

    Search first: GTR (Genetic Testing Registry), GeneReviews, ClinGen

  • Overview of recommended genetic testing approach
  • Whole genome sequencing (WGS) utility > Search first: GTR, ClinVar, GEL (Genomics England), gnomAD
  • Whole exome sequencing (WES) utility > Search first: GTR, ClinVar, OMIM, GeneMatcher
  • Gene panels (which panels, which genes) > Search first: GTR, ClinVar, laboratory-specific databases
  • Single gene testing > Search first: GTR, ClinVar, OMIM, GeneReviews
  • Chromosomal microarray (CMA) > Search first: DECIPHER, ClinVar, dbVar, ECARUCA
  • Karyotyping > Search first: Chromosome Abnormality Database, ClinVar, cytogenetics resources
  • FISH > Search first: ClinVar, cytogenetics databases, PubMed
  • Mitochondrial DNA testing > Search first: MITOMAP, MSeqDR, ClinVar, GTR
  • Repeat expansion testing > Search first: GTR, ClinVar, repeat expansion databases, PubMed
  • Omics-Based Diagnostics (if applicable):
  • RNA sequencing / transcriptomics > Search first: GEO, ArrayExpress, GTEx, RNA-seq databases
  • Proteomics > Search first: PRIDE, ProteomeXchange, FDA Biomarker database
  • Metabolomics > Search first: MetaboLights, Metabolomics Workbench, HMDB
  • Epigenomics > Search first: GEO, ENCODE, Roadmap Epigenomics, MethBase
  • Liquid biopsy > Search first: COSMIC, ClinVar, liquid biopsy databases, PubMed
  • Clinical Criteria:
  • Standardized diagnostic criteria (DSM, ICD, society guidelines) > Search first: DSM-5, ICD-11, clinical society guidelines, UpToDate
  • Differential diagnosis (other conditions to rule out, with distinguishing features) > Search first: DynaMed, UpToDate, clinical decision support systems
  • Screening:
  • Screening methods for asymptomatic individuals (newborn screening, carrier screening, cascade screening) > Search first: ACMG recommendations, CDC newborn screening, GTR

11. Outcome/Prognosis

  • Survival and Mortality:
  • Survival rate (5-year, 10-year, overall) > Search first: SEER, cancer registries, disease-specific registries, PubMed
  • Life expectancy (with and without treatment if applicable) > Search first: Orphanet, disease registries, actuarial databases, PubMed
  • Mortality rate > Search first: CDC, WHO, GBD, national mortality databases
  • Disease-specific mortality (deaths directly attributable to disease) > Search first: Disease registries, CDC Wonder, GBD, PubMed
  • Morbidity and Function:
  • Morbidity (disease-related disability and health impacts) > Search first: GBD, WHO, disability databases, PubMed
  • Disability outcomes (long-term functional impairments) > Search first: ICF (International Classification of Functioning), disability registries
  • Quality of life measures (EQ-5D, SF-36, PROMIS, disease-specific tools) > Search first: EQ-5D database, SF-36, PROMIS, PubMed
  • Disease Course:
  • Complications (secondary problems: infections, organ failure, etc.) > Search first: ICD codes, disease registries, clinical databases, PubMed
  • Recovery potential (likelihood and extent of recovery, with vs without treatment) > Search first: Natural history studies, rehabilitation databases, PubMed
  • Prediction:
  • Prognostic factors (age, disease severity, biomarkers, treatment response) > Search first: Prognostic models databases, clinical calculators, PubMed
  • Prognostic biomarkers (molecular markers predicting disease course) > Search first: FDA Biomarker database, PubMed, cancer prognostic databases

12. Treatment

  • Pharmacotherapy:
  • Pharmacological treatments (drug names, drug classes, mechanisms of action) > Search first: DrugBank, RxNorm, ATC classification, DailyMed, FDA databases
  • Pharmacogenomics (how genetic variants affect drug metabolism, efficacy, toxicity) > Search first: PharmGKB, CPIC (Clinical Pharmacogenetics), FDA Table of PGx Biomarkers
  • Advanced Therapeutics:
  • Gene therapy (viral vectors, CRISPR, gene replacement, gene editing) > Search first: ClinicalTrials.gov, FDA gene therapy database, ASGCT resources
  • Cell therapy (stem cell transplant, CAR-T, cellular therapeutics) > Search first: ClinicalTrials.gov, FDA cell therapy database, FACT standards
  • RNA-based therapies (ASOs, siRNA, mRNA therapies) > Search first: ClinicalTrials.gov, FDA approvals, PubMed
  • Targeted therapies (treatments directed at specific molecular targets) > Search first: My Cancer Genome, OncoKB, ClinicalTrials.gov, FDA approvals
  • Immunotherapies (checkpoint inhibitors, monoclonal antibodies) > Search first: Cancer Immunotherapy Database, FDA approvals, ClinicalTrials.gov
  • Surgical and Interventional:
  • Surgical interventions (types of surgery, timing, outcomes) > Search first: CPT codes, surgical registries, clinical guidelines, PubMed
  • Supportive and Rehabilitative:
  • Supportive care (symptom management, pain control, nutrition) > Search first: Clinical guidelines, Cochrane Library, PubMed
  • Rehabilitation (physical therapy, occupational therapy, speech therapy) > Search first: Rehabilitation medicine databases, clinical guidelines, PubMed
  • Experimental:
  • Experimental treatments in clinical trials (with NCT identifiers if available) > Search first: ClinicalTrials.gov, EU Clinical Trials Register, WHO ICTRP
  • Treatment Outcomes:
  • Treatment response rates > Search first: Clinical trial databases, FDA reviews, systematic reviews, PubMed
  • Side effects and adverse events > Search first: FDA Adverse Event Reporting System (FAERS), MedWatch, PubMed
  • Treatment Strategy:
  • Treatment algorithms (clinical pathways, decision trees) > Search first: Clinical practice guidelines, NCCN Guidelines, UpToDate
  • Combination therapies > Search first: ClinicalTrials.gov, treatment guidelines, PubMed
  • Personalized medicine approaches (genotype-guided treatment) > Search first: My Cancer Genome, CIViC, PharmGKB, precision medicine databases

For each treatment, suggest MAXO (Medical Action Ontology) terms where applicable.

13. Prevention

  • Prevention Levels:
  • Primary prevention (preventing disease occurrence: vaccination, risk factor modification) > Search first: CDC, WHO, USPSTF recommendations, Cochrane Library
  • Secondary prevention (early detection and treatment: screening programs, early intervention) > Search first: USPSTF, CDC screening guidelines, WHO
  • Tertiary prevention (preventing complications in those with disease) > Search first: Clinical guidelines, disease management protocols, PubMed
  • Immunization: Vaccine strategies (if applicable)

    Search first: CDC vaccine schedules, WHO immunization, FDA vaccine database

  • Screening and Early Detection:
  • Screening programs (population-based: newborn screening, cancer screening) > Search first: CDC screening programs, USPSTF, cancer screening databases
  • Genetic screening (carrier screening, preimplantation genetic diagnosis, prenatal testing) > Search first: ACMG recommendations, ACOG guidelines, GTR
  • Risk stratification (identifying high-risk individuals for targeted prevention) > Search first: Risk prediction models, clinical calculators, PubMed
  • Behavioral Interventions: Lifestyle modifications to reduce risk

    Search first: CDC, WHO, behavioral intervention databases, Cochrane Library

  • Counseling: Genetic counseling (risk assessment, family planning guidance)

    Search first: NSGC resources, ACMG guidelines, GeneReviews

  • Public Health:
  • Public health interventions (sanitation, vector control, health education) > Search first: CDC, WHO, public health databases, PubMed
  • Environmental interventions (reducing environmental risk factors) > Search first: EPA databases, WHO environmental health, PubMed
  • Prophylaxis: Preventive medications or procedures

    Search first: Clinical guidelines, FDA approvals, PubMed

14. Other Species / Natural Disease

  • Taxonomy: Species affected (with NCBI Taxon identifiers)

    Search first: NCBI Taxonomy

  • Breed: Specific breeds affected (with VBO identifiers if applicable)

    Search first: VBO (Vertebrate Breed Ontology)

  • Gene: Orthologous genes in other species (with NCBI Gene IDs)

    Search first: NCBI Gene

  • Natural Disease:
  • Naturally occurring disease in other species (companion animals, wildlife) > Search first: OMIA (Online Mendelian Inheritance in Animals), VetCompass, PubMed
  • Veterinary relevance and importance in animal health > Search first: OMIA, veterinary databases, PubMed
  • Comparative Biology:
  • Comparative pathology (similarities and differences across species) > Search first: OMIA, comparative pathology databases, PubMed
  • Evolutionary conservation of disease mechanisms > Search first: HomoloGene, OrthoMCL, Alliance of Genome Resources
  • Transmission (if applicable):
  • Zoonotic potential > Search first: CDC zoonotic diseases, WHO zoonoses, GIDEON
  • Cross-species susceptibility > Search first: NCBI Taxonomy, veterinary databases, PubMed

15. Model Organisms

  • Model Types:
  • Model organism type (mammalian, invertebrate, cellular, in vitro) > Search first: Alliance of Genome Resources, model organism databases
  • Specific model systems (mouse, rat, zebrafish, Drosophila, C. elegans, yeast, cell lines, organoids, iPSCs) > Search first: MGI, RGD, ZFIN, FlyBase, WormBase, SGD, ATCC, Cellosaurus
  • Induced models (drug treatment, surgical intervention, environmental manipulation) > Search first: MGI, model organism databases, PubMed
  • Genetic Models:
  • Types available (knockout, knock-in, transgenic, conditional, humanized) > Search first: MGI, IMPC, KOMP, EuMMCR, IMSR
  • Model Characteristics:
  • Phenotype recapitulation (how well model reproduces human disease features) > Search first: Model organism databases, comparative studies, PubMed
  • Model limitations (aspects of human disease not captured) > Search first: Model organism databases, PubMed, review articles
  • Applications:
  • Research applications (what aspects of disease can be studied) > Search first: Model organism databases, PubMed
  • Resources:
  • Model databases > Search first: MGI, RGD, ZFIN, FlyBase, WormBase, IMSR, EMMA, MMRRC

Citation Requirements

  • Cite primary literature (PMID preferred) for all mechanistic and clinical claims
  • Prioritize recent reviews and landmark papers
  • Include direct quotes from abstracts where possible to support key statements
  • Distinguish evidence source types: human clinical, model organism, in vitro, computational

Output Format

Structure your response as a comprehensive narrative organized by the sections above. For each section, provide: - Factual content with specific details (numbers, percentages, gene names, variant nomenclature) - Ontology term suggestions (HPO, GO, CL, UBERON, CHEBI, MAXO, MONDO) where applicable - Evidence citations with PMIDs - Direct quotes from abstracts to support key claims - Clear indication when information is not available or not applicable for this disease

This report will be used to populate a disease knowledge base entry with: - Pathophysiology descriptions with causal chains - Gene/protein annotations (HGNC, GO terms) - Phenotype associations (HP terms) with frequencies - Cell type involvement (CL terms) - Anatomical locations (UBERON terms) - Chemical entities (CHEBI terms) - Treatment annotations (MAXO terms) - Evidence items with PMIDs and exact abstract quotes - Epidemiology, prognosis, diagnostic, and prevention information - Animal model descriptions with phenotype recapitulation details

Disease Characteristics Research Report: Brachydactyly Type A1 (BDA1)

Executive summary

Brachydactyly type A1 (BDA1) is a rare, typically autosomal-dominant, congenital digital malformation primarily characterized by hypoplasia/aplasia of the middle phalanges of digits 2–5, often with short proximal phalanges of the thumb and big toe and occasional symphalangism. (temtamy2008brachydactyly pages 2-5, david2015isolatedandsyndromic pages 1-3) The best-established causal gene is IHH (Indian hedgehog), with pathogenic variants clustering in the N-terminal signaling fragment (classically codons ~95–154) and recurrent hotspots at residues such as E95, D100, and E131. (racacho2015thegeneticheterogeneity pages 72-76, zeng2022casereportbrachydactyly pages 1-2) Recent case-level work (2024) underscores that some IHH-related families present with mild/non-classical BDA1 plus short stature and may be misclassified as idiopathic short stature; whole-exome sequencing (WES) can resolve diagnosis, and recombinant human growth hormone (rhGH) therapy improved height SDS in a small family case report. (chen2024shortstaturewith pages 1-2, chen2024shortstaturewith pages 2-4)

1. Disease information

1.1 Overview / definition

BDA1 is an isolated brachydactyly subtype in which shortening is mainly confined to the middle phalanges: “middle phalanges of all digits are variably short or rudimentary and are occasionally fused with terminal phalanges,” and “the proximal phalanges of the thumbs and big toes are short.” (temtamy2008brachydactyly pages 2-5) Radiographically, classification relies on the selective pattern of middle phalanx hypoplasia/aplasia on standard postero-anterior (PA) hand radiographs. (temtamy2008brachydactyly pages 2-5, david2015isolatedandsyndromic pages 1-3)

1.2 Key identifiers

  • OMIM (phenotype): 112500 (explicitly stated). (zeng2022casereportbrachydactyly pages 1-2)
  • Causal locus: IHH at 2q35–36; a second locus (“BDA1B”) maps to 5p13.3–p13.2. (temtamy2008brachydactyly pages 2-5)
  • MONDO / Orphanet / ICD-10/ICD-11 / MeSH: not directly retrievable from the provided tool evidence (evidence gap).

1.3 Synonyms / alternative names

  • “Brachydactyly type A1”
  • “Brachymesophalangy” (middle phalanx shortening)
  • “Isolated brachydactyly type A1” (used in radiology/clinical genetics contexts) (temtamy2008brachydactyly pages 2-5, david2015isolatedandsyndromic pages 1-3)

1.4 Evidence source type

The knowledge base content below integrates: * Aggregated disease-level resources/reviews (e.g., Orphanet Journal of Rare Diseases review). (temtamy2008brachydactyly pages 2-5) * Radiology review evidence for diagnostic imaging hallmarks. (david2015isolatedandsyndromic pages 1-3) * Family case reports/series and molecular diagnostics/functional follow-up reports. (zeng2022casereportbrachydactyly pages 1-2, chen2024shortstaturewith pages 2-4, zhu2025anovelheterozygous pages 1-2)

2. Etiology

2.1 Disease causal factors

BDA1 is a Mendelian congenital limb malformation primarily caused by germline pathogenic variants affecting endochondral ossification signaling pathways—most prominently IHH. (temtamy2008brachydactyly pages 2-5, david2015isolatedandsyndromic pages 1-3)

2.2 Risk factors

Genetic risk factors (causal variants): * IHH heterozygous variants are the canonical cause; classic reviews identify IHH mutations at 2q35–36. (temtamy2008brachydactyly pages 2-5, david2015isolatedandsyndromic pages 1-3) * Additional reported causal genes/loci include GDF5 and BMPR1B in genetic synthesis work; these converge on BMP/TGF-β axis biology relevant to digit formation and can phenocopy/overlap BDA1. (racacho2015thegeneticheterogeneity pages 111-116, racacho2015thegeneticheterogeneity pages 98-102)

Environmental risk factors: No environmental, infectious, or lifestyle risk factors are established in the retrieved evidence, consistent with a primary Mendelian etiology.

2.3 Protective factors / gene–environment interaction

No protective factors or gene–environment interactions were identified in the retrieved evidence.

3. Phenotypes

3.1 Core phenotypes and HPO suggestions

Digital skeletal phenotypes * Middle phalanges of digits 2–5 are variably short/absent; can be fused to distal/terminal phalanges (symphalangism). (temtamy2008brachydactyly pages 2-5, david2015isolatedandsyndromic pages 1-3) * Thumb proximal phalanx and big toe proximal phalanx often short. (temtamy2008brachydactyly pages 2-5, david2015isolatedandsyndromic pages 1-3) * Metacarpals can be short with broad epiphyses; distal phalanges may be short; digit 2 and 5 often most affected. (david2015isolatedandsyndromic pages 1-3)

Suggested HPO terms (non-exhaustive) * Brachydactyly — HP:0001156 * Brachydactyly, type A1 — (HPO term exists in many installations; not verified in retrieved evidence) * Symphalangism — HP:0001204 * Clinodactyly — HP:0004209 * Short stature (variable) — HP:0004322 (lacombe2010brachydactylytypea1 pages 3-5, chen2024shortstaturewith pages 2-4)

3.2 Age of onset, severity, progression

  • Onset: congenital (structural). (temtamy2008brachydactyly pages 2-5)
  • Course: generally stable/non-progressive as a malformation; functional impact varies with severity. (temtamy2008brachydactyly pages 2-5)

3.3 Quality of life impact

No validated QoL instrument data (e.g., SF-36/EQ-5D) were found in the retrieved evidence. Functional limitation is typically the driver of intervention decisions. (temtamy2008brachydactyly pages 2-5)

4. Genetic / molecular information

4.1 Causal genes

Primary causal gene: * IHH (Indian hedgehog). (temtamy2008brachydactyly pages 2-5, david2015isolatedandsyndromic pages 1-3)

Additional reported genes/loci (evidence in retrieved context but less canonical): * GDF5, BMPR1B, and a locus at 5p13.3–p13.2 (BDA1B). (temtamy2008brachydactyly pages 2-5, racacho2015thegeneticheterogeneity pages 111-116, racacho2015thegeneticheterogeneity pages 98-102)

4.2 Pathogenic variant spectrum and hotspots

A recurring theme is that BDA1-causing IHH variants cluster in the N-terminal signaling fragment. Genetic synthesis evidence states: “all of the BDA1-causing IHH mutations are missense and are limited to a 59 amino acid region… (codons 95–154).” (racacho2015thegeneticheterogeneity pages 72-76) Hotspots/recurrent residues include codons/residues E95, D100, and E131, among others; case reports add in-frame indels and frameshift examples. (racacho2015thegeneticheterogeneity pages 72-76, zeng2022casereportbrachydactyly pages 1-2, chen2024shortstaturewith pages 2-4) Examples in the retrieved evidence: * In-frame duplication: IHH NM_002181.4 c.383_415dup, p.(R128_H138dup). (zeng2022casereportbrachydactyly pages 1-2) * Frameshift (short stature + non-classical BDA1): IHH c.387_388insC, p.Thr130Hisfs18. (chen2024shortstaturewith pages 1-2) * In-frame deletion (prenatal case; functional maturation defect):* IHH c.331_333delCTG, p.Leu111del. (zhu2025anovelheterozygous pages 1-2)

Allele frequency: population allele-frequency values (gnomAD/1000G) were not available in retrieved tool evidence (gap).

4.3 Functional consequences

A recent functional report highlights the importance of precursor processing and maturation: IHH is synthesized as a precursor and autocatalytically generates an active N-terminal signaling fragment; the p.Leu111del variant led to increased precursor with reduced mature functional IHH in HEK293T assays, consistent with “protein maturation failure.” (zhu2025anovelheterozygous pages 1-2)

5. Environmental information

No specific environmental/lifestyle/toxin/infectious contributors were identified in the retrieved evidence; the condition is primarily genetic.

6. Mechanism / pathophysiology

6.1 Current mechanistic understanding

IHH is central to growth plate biology and endochondral ossification. The 2024 case report explicitly summarizes that IHH “regulates proliferation and differentiation of chondrocytes and is essential for bone formation,” and “coordinates endochondral bone growth and morphogenesis via parathyroid hormone related-protein-dependent and -independent pathways,” referencing the Ihh–PTHrP feedback loop. (chen2024shortstaturewith pages 8-8) Mechanistic chain (high-level): 1. Upstream trigger: germline pathogenic variant in IHH (or related pathway genes). (temtamy2008brachydactyly pages 2-5, zeng2022casereportbrachydactyly pages 1-2) 2. Molecular dysfunction: reduced functional IHH (maturation/secretion/binding defects) or altered hedgehog signaling output. (zhu2025anovelheterozygous pages 1-2) 3. Cellular/tissue consequence: disrupted chondrocyte proliferation/differentiation gradients and growth plate signaling, altering cartilage template growth for phalanges/metacarpals. (chen2024shortstaturewith pages 8-8) 4. Clinical phenotype: congenital shortening/aplasia of middle phalanges ± symphalangism and variable short stature. (temtamy2008brachydactyly pages 2-5, david2015isolatedandsyndromic pages 1-3)

6.2 Suggested GO / CL terms

  • Endochondral ossification — GO:0001958
  • Hedgehog signaling pathway — GO:0007224
  • Chondrocyte differentiation — GO:0002062
  • Chondrocyte (growth plate) — CL:0000138; hypertrophic chondrocyte — CL:0000218 (ontology suggestions derived from mechanism described in retrieved evidence) (chen2024shortstaturewith pages 8-8)

7. Anatomical structures affected

Primary structures: * Middle phalanges (digits 2–5), thumb proximal phalanx, toe phalanges, and sometimes metacarpals/metatarsals. (temtamy2008brachydactyly pages 2-5, david2015isolatedandsyndromic pages 1-3) Growth plate cartilage is the key tissue context. (chen2024shortstaturewith pages 8-8)

Suggested UBERON terms: * Phalanx — UBERON:0006002 * Metacarpal bone — UBERON:0011137 * Metatarsal bone — UBERON:0011138 * Growth plate cartilage — UBERON:0004367

8. Temporal development

Typical onset: congenital. (temtamy2008brachydactyly pages 2-5) Critical periods: embryonic and postnatal growth plate development (implied by IHH growth plate function). (chen2024shortstaturewith pages 8-8) Progression: not a progressive systemic disorder; skeletal proportions are established developmentally and remain stable, though secondary issues (e.g., arthritis) have been reported in some families. (racacho2015thegeneticheterogeneity pages 72-76)

9. Inheritance and population

9.1 Inheritance pattern

Classic descriptions indicate autosomal dominant inheritance (“denoting autosomal dominant inheritance”). (temtamy2008brachydactyly pages 2-5)

9.2 Penetrance and expressivity

Marked phenotypic heterogeneity and variable expressivity across families is documented; one synthesis notes “remarkable phenotypic heterogeneity.” (racacho2015thegeneticheterogeneity pages 72-76)

9.3 Epidemiology

A key knowledge-base limitation is the lack of formal epidemiology: “No epidemiological studies have been reported. It is a rare hand malformation with only few pedigrees reported.” (temtamy2008brachydactyly pages 2-5)

10. Diagnostics

10.1 Clinical and imaging tests

Diagnosis relies on clinical exam + anthropometry + radiographs; “X-ray of hands on postero-anterior (PA) view show the selective distribution of the hypoplasia and aplasia of the middle phalanges.” (temtamy2008brachydactyly pages 2-5) A radiology review emphasizes that standard PA radiographs are first-line to classify subtype and localize shortening, and that BDA1 shows short/absent middle phalanges with occasional fusion and often short metacarpals with broad epiphyses. (david2015isolatedandsyndromic pages 1-3)

10.2 Genetic testing (real-world implementations)

Modern implementation is WES in families with mild skeletal signs and short stature: * In a 2024 family with short stature and non-classical BDA1, WES identified a heterozygous IHH frameshift (c.387_388insC; p.Thr130Hisfs*18) with Sanger confirmation. (chen2024shortstaturewith pages 2-4) Similarly, case reports identify novel in-frame IHH insertions in multi-generation pedigrees. (zeng2022casereportbrachydactyly pages 1-2)

10.3 Differential diagnosis

Differentials reported in a 2022 case report include Robinow syndrome, Feingold syndrome, and Temtamy preaxial brachydactyly syndrome, reflecting the need to distinguish isolated BDA1 from syndromic brachydactylies. (zeng2022casereportbrachydactyly pages 1-2)

11. Outcome / prognosis

BDA1 is generally compatible with normal life expectancy; morbidity is primarily functional/cosmetic. Surgical intervention is typically reserved for function/cosmesis. (temtamy2008brachydactyly pages 2-5) No quantitative mortality/survival statistics were found in the retrieved evidence (gap).

12. Treatment

12.1 Standard management

A commonly cited management approach is conservative: * “Plastic surgery is only indicated if the brachydactyly affects hand function or for cosmetic reasons… Physical therapy and ergotherapy may ameliorate hand function.” (temtamy2008brachydactyly pages 2-5) A 2022 case report similarly notes most patients are treated only if function is affected or for cosmetic reasons. (zeng2022casereportbrachydactyly pages 1-2)

Suggested MAXO terms: * Surgical procedure — MAXO:0000004 * Physical therapy — MAXO:0000011 * Occupational therapy — MAXO:0000014

12.2 Recent developments (2023–2024 priority)

rhGH in selected IHH-related short stature presentations: A 2024 report treated two siblings with rhGH (33 μg/kg/day) for 4 years, observing height SDS changes of +2.54 (boy) and +1.86 (girl), with “No noticeable adverse effect.” (chen2024shortstaturewith pages 1-2) Interpretation: this is not a corrective therapy for brachydactyly per se, but a real-world implementation relevant when BDA1 co-occurs with clinically significant short stature and growth hormone axis abnormalities. (chen2024shortstaturewith pages 2-4)

12.3 Experimental / targeted therapies

No BDA1-specific interventional clinical trials were retrieved; however, mechanistic literature referenced in a 2024 report notes Smoothened agonist (SAG) rescue in Ihh-deficiency models, suggesting pathway-modulation concepts in broader skeletal dysplasia research. (chen2024shortstaturewith pages 8-8)

13. Prevention

No primary prevention is applicable for this Mendelian condition. Prevention focuses on genetic counseling, cascade testing, and reproductive options when a familial variant is known. (temtamy2008brachydactyly pages 2-5) Prenatal phenotyping with ultrasound plus WES has been used in recent fetal IHH-related BDA1 diagnosis. (zhu2025anovelheterozygous pages 1-2)

Suggested MAXO terms: Genetic counseling — MAXO:0000055.

14. Other species / natural disease

No naturally occurring veterinary analogs were identified in the retrieved evidence.

15. Model organisms

Evidence supporting pathway relevance includes: * Bmpr1b-null mice exhibit phalange-restricted brachydactyly, supporting the role of BMP receptor biology in digit development. (racacho2015thegeneticheterogeneity pages 111-116) * Growth-plate models of Ihh deficiency and pharmacologic manipulation (e.g., SAG) are referenced in recent clinical mechanistic summaries. (chen2024shortstaturewith pages 8-8) Limitations: these models are not necessarily allele-matched to the common human BDA1 IHH hotspot variants.

Visual evidence

A schematic classification figure including a BDA1 panel is available in the Temtamy & Aglan review. (temtamy2008brachydactyly media a1b197b0)

Data/statistics extracted from recent studies (2023–2024 priority)

  • Height SDS gain with 4 years rhGH in an IHH-related short stature + non-classical BDA1 family: +2.54 (boy) and +1.86 (girl) at rhGH 33 μg/kg/day, no noticeable adverse effects (case report-level evidence). (chen2024shortstaturewith pages 1-2)
  • Epidemiology: “No epidemiological studies have been reported” for BDA1; rare with few pedigrees (evidence gap for prevalence/incidence). (temtamy2008brachydactyly pages 2-5)

Knowledge-base summary table

Domain Key knowledge-base fields Suggested ontology terms Supporting citation IDs
Disease identifiers / names Disease: Brachydactyly type A1 (BDA1); isolated brachydactyly / brachymesophalangy predominantly affecting middle phalanges; OMIM: 112500; historic alternate locus BDA1B at 5p13.3-p13.2; information here is derived from aggregated disease-level literature/reviews plus family case reports and case series. Suggested MONDO: Brachydactyly type A1 (MONDO not confirmed in retrieved evidence); MeSH/ICD/Orphanet identifiers not directly confirmed in retrieved context. (temtamy2008brachydactyly pages 2-5, zeng2022casereportbrachydactyly pages 1-2, racacho2015thegeneticheterogeneity pages 84-88)
Inheritance / population genetics Usually autosomal dominant with generally high/complete penetrance reported in classic descriptions; marked variable expressivity between and within families; rare evidence for semi-dominant behavior in some GDF5-related BDA1 presentations (milder heterozygotes, more severe homozygotes). Founder and recurrent hotspot mutations have been described for some IHH alleles. HP: Autosomal dominant inheritance HP:0000006; Variable expressivity HP:0003828. (temtamy2008brachydactyly pages 2-5, racacho2015thegeneticheterogeneity pages 84-88, racacho2015thegeneticheterogeneity pages 72-76, racacho2015thegeneticheterogeneity pages 98-102)
Core phenotypes Congenital shortening/aplasia of middle phalanges of digits 2-5, often most severe in digits 2 and 5; short proximal phalanx of thumb and sometimes hallux; occasional fusion of middle and terminal phalanges (symphalangism); short/broad metacarpals; short toes; clinodactyly; absent distal finger creases; variable short stature and mild limb shortening in some families. HP: Brachydactyly HP:0001156; Aplasia/Hypoplasia of middle phalanges of the hand HP:0009843/HP:0009882; Short thumb HP:0009778; Toe brachydactyly HP:0001773; Symphalangism HP:0001204; Clinodactyly HP:0004209; Short stature HP:0004322. (temtamy2008brachydactyly pages 2-5, david2015isolatedandsyndromic pages 1-3, lacombe2010brachydactylytypea1 pages 3-5, zeng2022casereportbrachydactyly pages 1-2, chen2024shortstaturewith pages 2-4)
Radiographic hallmarks Postero-anterior hand radiographs show selective hypoplasia/aplasia of middle phalanges; short or absent middle phalanges, occasional fusion with distal phalanges, short distal phalanges, short metacarpals with broad epiphyses; “chess pawn-shaped” distal bone described in classic BDA1; hand X-rays are central to subtype classification. HP: Abnormality of hand bone morphology HP:0011304; Short middle phalanx of the 2nd finger HP:0009871; Short middle phalanx of the 5th finger HP:0009175. (temtamy2008brachydactyly pages 2-5, david2015isolatedandsyndromic pages 1-3, racacho2015thegeneticheterogeneity pages 72-76, zhu2025anovelheterozygous pages 10-11, temtamy2008brachydactyly media a1b197b0)
Causal genes / loci Primary causal gene: IHH (Indian hedgehog signaling molecule); additional reported BDA1 genes/loci include GDF5, BMPR1B, and a mapped locus at 5p13.3-p13.2. IHH explains a substantial fraction of molecularly solved cases (~40% cited in review-level evidence). HGNC: IHH, GDF5, BMPR1B. (racacho2015thegeneticheterogeneity pages 84-88, racacho2015thegeneticheterogeneity pages 111-116, racacho2015thegeneticheterogeneity pages 98-102, temtamy2008brachydactyly pages 2-5)
Pathogenic variants / hotspots IHH BDA1 variants cluster in the N-terminal active/signaling fragment, especially codons 95-154; recurrent/hotspot residues include E95 (e.g., p.Glu95del / p.Glu95Lys), D100 (p.Asp100Asn/Glu), L111del, R128_H138dup, Thr130Hisfs*18, E131K. Variant classes include mostly missense, plus in-frame insertion/deletion and rare frameshift changes. GDF5 BDA1 variants include cysteine-disrupting missense alleles affecting the mature domain; BMPR1B missense/splice variants also reported. Germline origin is implied/observed. Sequence Ontology suggestions: missense_variant, inframe_insertion, inframe_deletion, frameshift_variant, splice_acceptor_variant. (racacho2015thegeneticheterogeneity pages 72-76, zeng2022casereportbrachydactyly pages 1-2, chen2024shortstaturewith pages 2-4, zhu2025anovelheterozygous pages 1-2, zhu2025anovelheterozygous pages 10-11, racacho2015thegeneticheterogeneity pages 111-116, racacho2015thegeneticheterogeneity pages 98-102)
Mechanism / pathways Disease reflects disturbed endochondral ossification and growth-plate signaling. IHH normally regulates chondrocyte proliferation/differentiation and couples chondrogenesis to osteogenesis through IHH–PTCH1–SMO–GLI signaling and the IHH–PTHrP feedback loop. BDA1-associated variants can reduce mature functional IHH, impair IHH-PTC/PTCH binding, alter secretion/maturation, and downstream perturb digit cartilage template growth. BDA1 genes converge on BMP-SMAD / GDF5-BMPR1B signaling interacting with IHH-PTHrP biology. GO: endochondral ossification GO:0001958; chondrocyte differentiation GO:0002062; regulation of chondrocyte differentiation GO:0032330; hedgehog signaling pathway GO:0007224; ossification GO:0001503. Key nodes: IHH, PTCH1, SMO, GLI1/2/3, PTHLH/PTHrP, PTH1R, GDF5, BMPR1B, SMADs. (chen2024shortstaturewith pages 8-8, zhu2025anovelheterozygous pages 1-2, racacho2015thegeneticheterogeneity pages 84-88, racacho2015thegeneticheterogeneity pages 111-116, racacho2015thegeneticheterogeneity pages 98-102, alqattan2014embryologyoffamilial pages 3-7)
Anatomy / cell types Primarily affects phalanges, metacarpals, metatarsals, and growth-plate cartilage of hands/feet; in some families also short humerus/femur or ulna-related changes. Key cell type is the growth plate chondrocyte. UBERON: phalanx UBERON:0006002; metacarpal bone UBERON:0011137; metatarsal bone UBERON:0011138; growth plate cartilage UBERON:0004367. CL: chondrocyte CL:0000138; hypertrophic chondrocyte CL:0000218. (lacombe2010brachydactylytypea1 pages 3-5, david2015isolatedandsyndromic pages 1-3, chen2024shortstaturewith pages 8-8)
Onset / course / prognosis Typically congenital and usually non-progressive/stable as a structural limb malformation; lifelong but generally compatible with normal lifespan. Functional impact is often mild, though dexterity/cosmetic concerns and short stature may prompt evaluation. Osteoarthritis/arthritis has occasionally been reported in some families. HP: Congenital onset HP:0003577; Abnormal hand morphology HP:0005922. (temtamy2008brachydactyly pages 2-5, racacho2015thegeneticheterogeneity pages 72-76, zeng2022casereportbrachydactyly pages 1-2)
Epidemiology No robust prevalence/incidence estimates for BDA1 were identified in the retrieved evidence. Reviews state brachydactylies are rare overall and specifically note that BDA1 is a rare hand malformation with only a few pedigrees/case series reported; prevalence figures around ~2% apply to types A3 and D, not A1. Rare disease designation appropriate; no disease-specific frequency ontology assignment supported by retrieved evidence. (temtamy2008brachydactyly pages 2-5, david2015isolatedandsyndromic pages 1-3, zeng2022casereportbrachydactyly pages 1-2)
Diagnostics Diagnosis is primarily clinical + radiographic (PA hand/foot X-rays), followed by molecular confirmation. Current real-world testing uses WES with Sanger confirmation; targeted skeletal dysplasia / short stature / brachydactyly panels are reasonable when phenotype is suggestive. Differential diagnosis includes other isolated brachydactylies and syndromic entities such as Robinow syndrome, Feingold syndrome, and Temtamy preaxial brachydactyly syndrome; radiographs help subtype classification. MAXO not applicable here; HPO terms above support phenotyping. Testing methods: WES, Sanger sequencing, multigene panel. (temtamy2008brachydactyly pages 2-5, david2015isolatedandsyndromic pages 1-3, zeng2022casereportbrachydactyly pages 1-2, chen2024shortstaturewith pages 2-4, chen2024shortstaturewith pages 1-2)
Management / treatment No disease-modifying therapy established for classic isolated BDA1. Management is usually conservative, with plastic/orthopedic surgery only if hand function or cosmesis is significantly affected; physical therapy/ergotherapy (occupational therapy) may improve function. In recent IHH-related short stature families, recombinant human growth hormone (rhGH) produced substantial height SDS gains over 4 years without major adverse effects, but evidence is limited to case-level reports and applies mainly to selected short-stature presentations rather than hand malformation correction. MAXO: physical therapy MAXO:0000011; occupational therapy MAXO:0000014; surgical procedure MAXO:0000004; growth hormone therapy MAXO:0010020. (temtamy2008brachydactyly pages 2-5, zeng2022casereportbrachydactyly pages 1-2, chen2024shortstaturewith pages 2-4, chen2024shortstaturewith pages 1-2, chen2024shortstaturewith pages 8-8)
Prevention / counseling No primary environmental prevention known for this Mendelian malformation. Key preventive actions are genetic counseling, family segregation testing, and consideration of prenatal/preimplantation testing when a familial pathogenic variant is known. MAXO: genetic counseling MAXO:0000055; cascade genetic testing MAXO:0000127. (temtamy2008brachydactyly pages 2-5, chen2024shortstaturewith pages 2-4, chen2024shortstaturewith pages 1-2)
Model organism / comparative evidence Mouse and developmental studies support pathway relevance: Bmpr1b-null mice show phalange-restricted brachydactyly; broader skeletal biology literature shows Ihh is essential for growth-plate homeostasis and chondrocyte hypertrophy/differentiation. Model evidence is mechanistically strong but does not perfectly recapitulate the full human BDA1 hand phenotype for every allele. GO as above; model organism: Mus musculus. (racacho2015thegeneticheterogeneity pages 111-116, chen2024shortstaturewith pages 8-8)

Table: This table condenses the main disease-knowledge fields for brachydactyly type A1, including phenotype, genetics, mechanism, diagnosis, and management. It is designed for rapid knowledge-base population and each row links to supporting evidence context IDs.

Key references (with URLs and publication dates)

  • Temtamy SA, Aglan MS. Brachydactyly. Orphanet Journal of Rare Diseases. 2008-06. https://doi.org/10.1186/1750-1172-3-15 (temtamy2008brachydactyly pages 2-5)
  • David A, et al. Isolated and syndromic brachydactylies: Diagnostic value of hand X-rays. Diagnostic and Interventional Imaging. 2015-05. https://doi.org/10.1016/j.diii.2014.12.007 (david2015isolatedandsyndromic pages 1-3)
  • Zeng F, et al. Case Report: Brachydactyly Type A1 Induced by a Novel Variant of in-Frame Insertion in the IHH Gene. Frontiers in Genetics. 2022-05. https://doi.org/10.3389/fgene.2022.814786 (zeng2022casereportbrachydactyly pages 1-2)
  • Chen Y, et al. Short stature with brachydactyly caused by a novel mutation in the IHH gene and response to 4-year growth hormone therapy: a case report. Translational Pediatrics. 2024-05. https://doi.org/10.21037/tp-23-578 (chen2024shortstaturewith pages 1-2)
  • Zhu T, et al. A Novel Heterozygous IHH c.331_333del Mutation… Causes IHH Protein Maturation Failure in HEK293T Cells. Phenomics. 2025-12. https://doi.org/10.1007/s43657-024-00191-9 (zhu2025anovelheterozygous pages 1-2)

Notes on evidence gaps

Within the retrieved tool evidence, the following could not be confirmed and should be filled by targeted database queries (OMIM/Orphanet/MONDO/MeSH/GTR/ClinVar/gnomAD) in a subsequent pass: MONDO ID; Orphanet ORPHA code; ICD-10/ICD-11 codes; MeSH descriptor; ClinVar variant classifications and population allele frequencies; and validated QoL statistics.

References

  1. (temtamy2008brachydactyly pages 2-5): Samia A Temtamy and Mona S Aglan. Brachydactyly. Orphanet Journal of Rare Diseases, Jun 2008. URL: https://doi.org/10.1186/1750-1172-3-15, doi:10.1186/1750-1172-3-15. This article has 223 citations and is from a peer-reviewed journal.

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  3. (racacho2015thegeneticheterogeneity pages 72-76): Lemuel Jean Racacho. The genetic heterogeneity of brachydactyly type a1: identifying the molecular pathways. ArXiv, Mar 2015. URL: https://doi.org/10.20381/ruor-2868, doi:10.20381/ruor-2868. This article has 0 citations.

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