Spondylo-megaepiphyseal-metaphyseal Dysplasia

Mendelian MONDO:0013228 Pathograph 21 Show in embeddings browser Spondyloepiphyseal Dysplasia

SMMD is a rare autosomal recessive skeletal dysplasia caused by biallelic inactivating variants in NKX3-2 (also called BAPX1), a homeobox transcription factor on chromosome 4p15.33. Affected individuals have disproportionate short stature with a short, stiff neck and trunk, while the limbs appear relatively long. The radiographic picture is the diagnostic one and it is unusual in combining two opposite failures: delayed and impaired ossification of the vertebral bodies alongside abnormally *large* epiphyseal ossification centres and wide growth plates in the long bones, plus numerous pseudoepiphyses in the short tubular bones of hands and feet. Deficient ossification in the axial skeleton and excessive cartilage in the appendicular skeleton are the same lesion seen in two places. The mechanism explains why. NKX3-2 acts in the proliferative zone of the growth plate as a transcriptional repressor that holds chondrocytes immature - it represses Runx2, the chondrocyte maturation factor, and is normally switched off as maturation begins. It sits downstream of PTHrP signalling. So the protein's job is to *restrain* maturation, and losing it deranges the timing of endochondral ossification rather than simply reducing it: bone that should form does not, and cartilage that should be replaced persists and overgrows. A zebrafish model added a second, later role. Adult viable nkx3.2 mutant fish show cartilage overgrowth and spinal dysmorphology, with mutant chondrocytes showing increased proliferation and upregulated stress-response pathways. That points to a post-embryonic function in damping proliferation and buffering stress in joint-associated chondrocytes, which is the part of the human phenotype that embryonic-lethal models could not show. Severity spans a wide range. At the severe end a newborn had total absence of vertebral body, pubic and ischial ossification and died on day 14, with the clinical picture suggesting very early cervical cord compression.

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1
Inheritance
6
Pathophys.
13
Phenotypes
1
Gaps
21
Pathograph
2
Genes
2
Medical Actions
2
Models
5
References
1
Deep Research
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Classifications

Harrison's Part
GENETICS ENVIRONMENT DISEASE
👪

Inheritance

1
Autosomal recessive HP:0000007
Biallelic inactivating NKX3-2 variants. The three founding families were consanguineous and each proband was homozygous for a different inactivating mutation, which is strong evidence for allelic heterogeneity converging on loss of function rather than a founder effect.
Autosomal recessive inheritance
Show evidence (2 references)
PMID:20004766 SUPPORT Human Clinical
"Each proband was homozygous for a different inactivating mutation in NKX3-2, a homeobox-containing gene located on chromosome 4p15.33."
Three unrelated consanguineous probands, three different inactivating alleles, all homozygous, which establishes recessive loss of function.
PMID:20004766 SUPPORT Human Clinical
"it defines SMMD as yet another skeletal dysplasia with autosomal-recessive inheritance and a distinct phenotype"
The authors' own statement of the inheritance mode and phenotypic distinctness.
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Discussions and Knowledge Gaps

1
Does the skeletal overgrowth of SMMD have a mammalian model, and can the zebrafish chondrocyte proliferation and stress-response mechanism be assumed to hold in human growth plates?
HUMAN MODEL MISMATCH smmd_overgrowth_unmodelled_in_mammals
Attached to
pathophysiology#Loss of Post-Embryonic Restraint on Joint Chondrocyte Proliferation animal_models#Mouse
Evidence for the overgrowth arm exists, but only in fish. Mouse Nkx3-2 mutants show skeletal *reductions* and nkx3.2 knockdown zebrafish show embryonic lethal jaw joint fusions; both lack the skeletal overgrowth seen in SMMD patients. Only the adult viable zebrafish mutant reproduces it, and the proposed mechanism - increased chondrocyte proliferation with upregulated stress-induced pathways including prostaglandin synthases - rests on single-cell RNA-sequencing in that fish. This is a translational-validity question rather than an absence of evidence, which is why it is recorded as HUMAN_MODEL_MISMATCH: the mammalian models diverge from the human phenotype in the specific direction that matters, and the mouse additionally shows asplenia that SMMD patients do not. The relevant node is marked PROVISIONAL for this reason. Resolving it would need chondrocyte proliferation and prostaglandin-pathway activity measured in human SMMD growth-plate or joint cartilage, or a conditional mouse allele that survives past the perinatal period.
Show evidence (2 references)
PMID:33462117 SUPPORT Model Organism
"Whereas nkx3.2 knockdown zebrafish and mouse Nkx3.2 mutants display embryonic lethal jaw joint fusions and skeletal reductions, respectively, they lack the skeletal overgrowth seen in SMMD patients."
States the mismatch explicitly: two model systems fail to reproduce the human overgrowth phenotype, which is the gap this discussion records.
PMID:20004766 SUPPORT Model Organism
"The absence of the latter anomalies in the murine model may be due to the perinatal death of the affected animals."
The proposed explanation for the mouse gap, offered by the authors as a hypothesis ("may be due to") rather than a demonstrated cause.
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Pathophysiology

6
NKX3-2 Loss of Function
Biallelic inactivating NKX3-2 variants abolish the homeobox transcription factor. Reported alleles include three distinct inactivating changes in the founding consanguineous families and, in a perinatally lethal case, the frameshift c.507-508delCA (p.Gly171Cysfs*55) in exon 2.
NKX3-2 hgnc:951 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves NKX3-2 (hgnc:951). hgnc:951 is a gene from the HUGO Gene Nomenclature Committee.
NKX3-2 transcriptional repressor activity GO:0001227 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves NKX3-2 transcriptional repressor activity, annotated with DNA-binding transcription repressor activity, RNA polymerase II-specific (GO:0001227), qualified as loss of function. GO:0001227 is a molecular function from the Gene Ontology. ⇓ LOSS OF FUNCTION
Downstream
Derepression of the Chondrocyte Maturation Program
Loss of Post-Embryonic Restraint on Joint Chondrocyte Proliferation
Loss of Chondrocyte Survival Signalling
Show evidence (2 references)
PMID:20004766 SUPPORT Human Clinical
"Genome wide homozygosity mapping followed by a candidate gene approach resulted in the elucidation of the genetic cause in three new consanguineous families with SMMD."
The gene-discovery result establishing NKX3-2 as the SMMD gene.
PMID:29704686 SUPPORT Human Clinical
"Molecular analysis revealed homozygosity for a novel mutation, c.507-508delCA (p.Gly171Cysfs*55) in exon 2 of NKX3-2."
A frameshift allele at the severe end of the reported spectrum.
Derepression of the Chondrocyte Maturation Program
NKX3-2 normally restrains chondrocyte maturation, acting as a transcriptional repressor of Runx2 in the proliferative zone and being switched off as maturation begins. The direction of the mechanism is established by a construct experiment: wild-type Nkx3.2 blocks maturation, while a reverse-function mutant converted into a transcriptional activator accelerates it, and Runx2 mis-expression rescues the Nkx3.2-induced block. Losing NKX3-2 therefore removes a brake on maturation timing rather than removing a positive signal.
growth plate chondrocyte CL:0000138 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves growth plate chondrocyte, annotated with chondrocyte (CL:0000138). CL:0000138 is a cell type from the Cell Ontology.
negative regulation of chondrocyte differentiation GO:0032331 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased negative regulation of chondrocyte differentiation (GO:0032331). GO:0032331 is a biological process from the Gene Ontology. ↓ DECREASED
growth plate UBERON:0002516 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in growth plate, annotated with epiphyseal plate (UBERON:0002516). UBERON:0002516 is an anatomical location from the Uberon multi-species anatomy ontology.
Downstream
Deranged Endochondral Ossification
Show evidence (3 references)
PMID:16421188 SUPPORT Model Organism
"Although wild-type Nkx3.2 blocks chondrocyte maturation by acting as a transcriptional repressor, a ;reverse function' mutant of Nkx3.2 that has been converted into a transcriptional activator conversely accelerates chondrocyte maturation."
Establishes the direction of the mechanism by reversing it. Quoted verbatim including the source's mangled quotation mark in ";reverse function'".
PMID:16421188 SUPPORT Model Organism
"Nkx3.2 represses expression of the chondrocyte maturation factor Runx2, and Runx2 mis-expression can rescue the Nkx3.2-induced blockade of chondrocyte maturation."
Names the transcriptional target and shows the block is relieved by restoring it, placing Runx2 downstream of NKX3-2 in this pathway.
PMID:16421188 SUPPORT Model Organism
"In chick and mouse, expression of Nkx3.2/Bapx1 in the growth plate is restricted to the proliferative zone and is down regulated as chondrocyte maturation begins."
Locates the factor spatially and temporally in the growth plate, which is what makes its loss a timing defect rather than a global one.
Loss of Chondrocyte Survival Signalling
A second, parallel arm of NKX3-2 function, distinct from the maturation brake modelled above. In proliferating chondrocytes NKX3-2 constitutively activates RelA by a ligand-independent route - recruiting the RelA-IkappaB-alpha complex into the nucleus by direct protein-protein interaction and degrading IkappaB-alpha there - and that NF-kappaB activity is required for chondrocyte viability. Losing it removes a survival signal from exactly the cells the maturation brake acts on. This makes NKX3-2 a dual-function node rather than a single repressor: a brake on maturation and a survival signal for the cells being braked. The two arms converge, which is consistent with the mouse null showing increased apoptosis alongside failed chondrogenesis. Curated as PROVISIONAL for this disease. The mechanism is established in chick and mouse chondrocytes and has not been shown in a patient with SMMD, so its contribution to the human phenotype is inferred from the gene rather than demonstrated in the disease.
proliferating chondrocyte CL:0000138 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves proliferating chondrocyte, annotated with chondrocyte (CL:0000138). CL:0000138 is a cell type from the Cell Ontology.
canonical NF-kappaB signal transduction GO:0007249 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased canonical NF-kappaB signal transduction (GO:0007249). GO:0007249 is a biological process from the Gene Ontology. ↓ DECREASED negative regulation of apoptotic process GO:0043066 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased negative regulation of apoptotic process (GO:0043066). GO:0043066 is a biological process from the Gene Ontology. ↓ DECREASED
Show evidence (3 references)
PMID:17310243 SUPPORT In Vitro
"Here, we show that the nuclear factor kappa B (NF-kappaB) pathway is required for chondrocyte viability and that Nkx3.2 supports chondrocyte survival by constitutively activating RelA."
The core claim of the survival arm. Graded IN_VITRO: the work is in cultured chondrocytes, with no human disease data.
PMID:17310243 SUPPORT In Vitro
"Nkx3.2 recruits the RelA-IkappaBalpha heteromeric complex into the nucleus by direct protein-protein interactions and activates RelA through proteasome-dependent IkappaBalpha degradation in the nucleus"
The mechanism, and the reason it is ligand-independent. Quoted with the source's ASCII spellings of the Greek letters.
PMID:10572046 SUPPORT Model Organism
"Loss of Bapx1 is accompanied by an increase in apoptotic cell death in affected tissues, although cell cycling rates are unaltered."
The in vivo counterpart: losing the gene raises apoptosis without changing proliferation rate, which is what a lost survival signal predicts and a lost proliferation brake does not. The dissociation is why this arm is modelled separately.
Deranged Endochondral Ossification
The consequence is not uniform under- or over-ossification but a disordered one. In the axial skeleton, vertebral body ossification is delayed and impaired, in the severe case absent altogether. In the appendicular skeleton, epiphyseal ossification centres are large, growth plates are wide, and pseudoepiphyses appear in the short tubular bones. NKX3-2 is therefore required for endochondral ossification of both the axial and appendicular skeleton in humans.
endochondral ossification GO:0001958 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves dysregulated endochondral ossification (GO:0001958). GO:0001958 is a biological process from the Gene Ontology. ↕ DYSREGULATED
Animal models
Nkx3-2 null mouse Mouse partially recapitulates Vertebral ossification defects closely resemble those of SMMD patients; appendicular findings are absent.
Downstream
Delayed vertebral ossification
Delayed pubic bone ossification
Coronal cleft vertebrae
Enlarged epiphyses
Thick growth plates
Pseudoepiphyses of hand bones
Disproportionate short-trunk short stature
Flexion contracture
Cervical Spine Instability and Cord Injury
Show evidence (2 references)
PMID:20004766 SUPPORT Human Clinical
"The most remarkable radiographic features are the delayed and impaired ossification of the vertebral bodies as well as the presence of large epiphyseal ossification centers and wide growth plates in the long tubular bones."
Describes both halves of the ossification derangement, axial deficiency alongside appendicular excess, in one sentence.
PMID:20004766 SUPPORT Human Clinical
"This study illustrates that NKX3-2 plays an important role in endochondral ossification of both the axial and appendicular skeleton in humans."
The authors' conclusion that the requirement spans both skeletal compartments.
Cervical Spine Instability and Cord Injury
The clinically decisive consequence of the axial ossification failure, and the reason this disorder is dangerous rather than merely deforming. Reduced or absent ossification of the cervical vertebrae leaves the cervical spine mechanically unsupported, producing instability with anterior or posterior kinking - the swan-neck deformity also called kyknodysostosis - and from there cord injury presenting as limb spasticity. Two things make this node worth separating from the general ossification derangement. It hit five of the six patients in the only series to look at it, so within that series it is the rule rather than a complication. And the authors state that the high incidence of cervical spine deformation is unique among skeletal dysplasias, which means it is not a generic consequence of poor vertebral ossification that could be inherited from a sibling disorder. It is also the only point in this disease where an intervention changes the outcome: the lesion upstream is embryonic and inaccessible, while cervical instability is detectable and stabilisable before the cord is injured.
Downstream
Cervical instability
Spasticity
Show evidence (2 references)
PMID:22791571 SUPPORT Human Clinical
"Reduced or absent ossification of the cervical vertebrae leads to cervical instability with anterior or posterior kinking of the cervical spine (swan neck-like deformity, kyknodysostosis)."
The mechanical step from the ossification defect to the instability, which is why this node hangs off Deranged Endochondral Ossification rather than standing alone.
PMID:22791571 SUPPORT Human Clinical
"As a result of the cervical spine instability or deformation, five of six patients in our series suffered cervical cord injury that manifested clinically as limb spasticity."
The step from instability to cord injury and its clinical readout, with the authors' attribution ("as a result of") rather than a co-occurrence. Six patients, so the proportion is fragile even though the direction is not.
Loss of Post-Embryonic Restraint on Joint Chondrocyte Proliferation
A distinct, later-acting arm identified in an adult viable zebrafish mutant: chondrocytes show increased proliferation and upregulation of stress-induced pathways, including prostaglandin synthases, and the animals develop cartilage overgrowth and severe spine and craniofacial dysmorphology. This arm is what accounts for the skeletal *overgrowth* of SMMD rather than its ossification deficit, and it is post-embryonic - the same overgrowth is absent in knockdown animals that lack jaw joints, which is the internal control separating the embryonic from the later role.
joint-associated chondrocyte CL:0000138 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves joint-associated chondrocyte, annotated with chondrocyte (CL:0000138). CL:0000138 is a cell type from the Cell Ontology.
chondrocyte proliferation GO:0035988 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased chondrocyte proliferation (GO:0035988). GO:0035988 is a biological process from the Gene Ontology. ↑ INCREASED
Animal models
Adult viable nkx3.2 mutant zebrafish Zebrafish recapitulates Generated specifically to model the skeletal overgrowth defects of SMMD, which prior models could not show, and identifies increased chondrocyte proliferation and a stress response as the cellular basis.
Downstream
Scoliosis
Show evidence (2 references)
PMID:33462117 SUPPORT Model Organism
"Single-cell RNA-sequencing and in vivo validation reveal increased proliferation and upregulation of stress-induced pathways, including prostaglandin synthases, in mutant chondrocytes."
The cellular and transcriptional phenotype defining this arm.
PMID:33462117 SUPPORT Model Organism
"cartilage overgrowth and scoliosis are absent in rare viable nkx3.2 knockdown animals that lack jaw joints, supporting post-embryonic roles for Nkx3.2"
The internal control that makes this a post-embryonic role: animals with the embryonic joint defect but without the later one do not overgrow.
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Pathograph

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

13
Digestive 1
Duodenal atresia VERY_RARE HP:0002247 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Duodenal atresia (HP:0002247). HP:0002247 is a phenotype from the Human Phenotype Ontology.
Deliberately left unconnected in the pathograph. No mechanism links NKX3-2 loss to foregut atresia in any source read here, and NKX3-2 has a known role in gut and spleen patterning in mouse, so a plausible-sounding edge would be speculation rather than curation. It stays a recorded observation until somebody can source the link.
Show evidence (1 reference)
PMID:29704686 SUPPORT Human Clinical
"The patient required immediate intubation at the delivery room and duodenal atresia was detected during his course in neonatal intensive care unit."
Single-patient observation. Recorded because it drove management and mortality in that case, not because it is established as part of the syndrome.
Musculoskeletal 5
Coronal cleft vertebrae VERY_FREQUENT HP:0003417 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Sagittal and coronal vertebral clefts, annotated with Coronal cleft vertebrae (HP:0003417). HP:0003417 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:22791571 SUPPORT Human Clinical
"Radiographs show a severe ossification delay of the vertebral bodies with sagittal and coronal clefts"
HPO codes the coronal cleft; preferred_term keeps the sagittal component the source reports alongside it.
Enlarged epiphyses VERY_FREQUENT HP:0010580 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Enlarged epiphyses (HP:0010580). HP:0010580 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20004766 SUPPORT Human Clinical
"the presence of large epiphyseal ossification centers and wide growth plates in the long tubular bones"
The epiphyseal half of the appendicular signature. The same sentence supports the growth-plate phenotype below, which HPO codes separately.
Scoliosis 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)
PMID:33462117 SUPPORT Other
"which is characterized by skeletal defects including scoliosis, large epiphyses, wide growth plates and supernumerary distal limb joints"
Lists scoliosis among the defining human skeletal defects. Graded OTHER rather than HUMAN_CLINICAL because this publication presents no human data - the sentence is its background characterisation of the disease, not an observation it made.
Flexion contracture OCCASIONAL HP:0001371 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Distal joint flexion contractures, annotated with Flexion contracture (HP:0001371). HP:0001371 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20004766 SUPPORT Human Clinical
"The limbs appear relatively long and may show flexion contractures of the distal joints."
Reports the contractures with the source's own hedge ("may show"), which is why the frequency is recorded as occasional.
Spasticity VERY_FREQUENT HP:0001257 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Limb spasticity, annotated with Spasticity (HP:0001257). HP:0001257 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:22791571 SUPPORT Human Clinical
"five of six patients in our series suffered cervical cord injury that manifested clinically as limb spasticity"
The frequency and the causal attribution. 5/6 is 83%, in the VERY_FREQUENT band, but on a denominator of six from a single series - the band is what the number supports and the confidence in it is lower than the band alone conveys.
Growth 1
Disproportionate short-trunk short stature OBLIGATE HP:0003521 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Disproportionate short stature with short, stiff neck and trunk, annotated with Disproportionate short-trunk short stature (HP:0003521). HP:0003521 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20004766 SUPPORT Human Clinical
"Affected individuals have a disproportionate short stature with a short and stiff neck and trunk."
The core clinical description of the disorder.
Other 6
Delayed vertebral ossification VERY_FREQUENT HP:0031096 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Delayed and impaired vertebral body ossification, annotated with Delayed vertebral ossification (HP:0031096). HP:0031096 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:20004766 SUPPORT Human Clinical
"the delayed and impaired ossification of the vertebral bodies"
The axial ossification defect as a defining radiographic feature.
PMID:29704686 SUPPORT Human Clinical
"Skeletal survey revealed total absence of the ossification of the vertebral bodies, pubis, and ischia."
The severe end of the same defect, showing it is graded rather than all-or-none.
Delayed pubic bone ossification VERY_FREQUENT HP:0008788 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Absent pubic and ischial ossification, annotated with Delayed pubic bone ossification (HP:0008788). HP:0008788 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:29704686 SUPPORT Human Clinical
"Skeletal survey revealed total absence of the ossification of the vertebral bodies, pubis, and ischia"
The perinatally lethal case, where the deficit is total. preferred_term names the ischia as well because the source does; HPO codes the pubis only.
PMID:22791571 SUPPORT Human Clinical
"missing ossification of the pubic bones"
Independent confirmation in the six-patient series, where it is listed among the constant radiographic features rather than as a finding of the severe end.
Thick growth plates VERY_FREQUENT HP:0025369 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Thick growth plates (HP:0025369). HP:0025369 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20004766 SUPPORT Human Clinical
"the presence of large epiphyseal ossification centers and wide growth plates in the long tubular bones"
Curated separately from the epiphyseal finding because HPO codes them distinctly and because the growth plate is the metaphyseal compartment - without this the "metaphyseal" third of the disease name has no phenotype behind it.
Pseudoepiphyses of hand bones VERY_FREQUENT HP:0004288 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Pseudoepiphyses of hand and foot bones, annotated with Pseudoepiphyses of hand bones (HP:0004288). HP:0004288 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20004766 SUPPORT Human Clinical
"Numerous pseudoepiphyses of the short tubular bones in hands and feet are another remarkable feature of the disorder."
The source calls this "another remarkable feature", which is why it is its own phenotype rather than folded into an epiphyseal grouping term. preferred_term extends to the feet because the source does; HPO codes the hand bones only.
Supernumerary distal limb joints Abnormal digit morphology HP:0011297 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Supernumerary distal limb joints, annotated with Abnormal digit morphology (HP:0011297). HP:0011297 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:33462117 SUPPORT Other
"which is characterized by skeletal defects including scoliosis, large epiphyses, wide growth plates and supernumerary distal limb joints"
The same background sentence, graded OTHER for the same reason. HPO has no term for a supernumerary limb joint, so the binding is the digit-morphology parent and the finding itself is carried in preferred_term.
Cervical instability VERY_FREQUENT HP:0008462 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cervical instability (HP:0008462). HP:0008462 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:22791571 SUPPORT Human Clinical
"Although the number of individuals observed is small, the high incidence of cervical spine deformation in SMMD is unique among skeletal dysplasias."
The frequency claim and its own caveat in one sentence. Graded VERY_FREQUENT on "high incidence" together with the 5-of-6 cord injury rate, but note the authors themselves flag the small denominator, and this remains the only series to have looked.
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Genetic Associations

2
NKX3-2
Gene: NKX3-2 hgnc:951 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is NKX3-2 (hgnc:951). hgnc:951 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE
Show evidence (2 references)
PMID:20004766 SUPPORT Human Clinical
"Each proband was homozygous for a different inactivating mutation in NKX3-2, a homeobox-containing gene located on chromosome 4p15.33."
Identifies the gene, its class and locus, and establishes that the disease alleles are inactivating rather than of mixed consequence.
PMID:16421188 SUPPORT Model Organism
"Nkx3.2/Bapx1 expression is lost in the growth plates of mice engineered to lack PTHrP signaling and, conversely, is maintained by ectopic expression of PTHrP in developing bones."
Places NKX3-2 downstream of PTHrP signalling in the growth plate, in both directions of the manipulation.
NKX3-1
Gene: NKX3-1 hgnc:7838 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is NKX3-1 (hgnc:7838). hgnc:7838 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: MODIFIER
Show evidence (2 references)
PMID:12204261 SUPPORT Model Organism
"Double mutants exhibited enhanced defects of vertebrae compared with Bapx1-deficient animals."
The redundancy result. Enhancement over the single null is what makes this a modifier rather than an independent cause.
PMID:12204261 SUPPORT Model Organism
"In contrast, inactivation of the Nkx3.1 gene causes no apparent skeletal phenotype despite its early expression in sclerotomal cells."
The negative half, and the reason this is a modifier and not a second disease gene: losing NKX3-1 alone does nothing to the skeleton.
💊

Medical Actions

2
Prophylactic Cervical Spine Stabilization
Action: cervical spine stabilizationNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is cervical spine stabilization, annotated with Spinal Fusion (NCIT:C157986). NCIT:C157986 is a clinical intervention from the NCI Thesaurus. Ontology label: Spinal Fusion NCIT:C157986
The one intervention in this disorder that changes the outcome rather than managing it. The upstream lesion is an embryonic transcription-factor failure and is not therapeutically accessible after birth; cervical instability is. Stabilization is prophylactic in the strict sense - performed on the basis of the radiographic pattern before cord injury has occurred, not as treatment for an established myelopathy. The evidence is a proposal rather than a demonstration, and the entry records it that way. The authors write that early diagnosis "might prevent" the neurologic complications; no series has reported outcomes in patients stabilized prophylactically, and with six patients described in total there is no prospect of one soon. This is curated because the reasoning is sound and the stakes are high, not because it has been shown to work.
Mechanism Target:
Cervical Spine Instability and Cord Injury — Mechanically stabilizes the unossified cervical spine, interrupting the step from instability to cord compression. It does not address the ossification defect itself.
Show evidence (1 reference)
PMID:22791571 SUPPORT Human Clinical
"Early diagnosis of SMMD by recognition of the radiographic pattern might prevent of the neurologic complications via prophylactic cervical spine stabilization."
The recommendation, quoted with the authors' hedge ("might prevent") and with their own grammatical slip left in place, since a snippet does not correct its source. This is a proposal from a case series, not an outcome.
Respiratory and Surgical Supportive Care
Action: Supportive CareNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Supportive Care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. NCIT:C15747
There is no disease-modifying treatment. Management is supportive and, at the severe end, intensive: the perinatally lethal case required immediate intubation in the delivery room and surgery for duodenal atresia on day 7, and died of sepsis and respiratory failure on day 14. The clinical picture in that infant suggested very early cervical cord compression, which is the specific risk the axial ossification defect creates and the reason airway and cervical spine assessment matter in this disorder.
Show evidence (2 references)
PMID:29704686 SUPPORT Human Clinical
"The patient required immediate intubation at the delivery room and duodenal atresia was detected during his course in neonatal intensive care unit."
Documents the supportive interventions required at the severe end of the spectrum.
PMID:29704686 SUPPORT Human Clinical
"the findings of clubfoot, neuromuscular respiratory insufficiency requiring invasive mechanical ventilation and downward sloping or tented appearance of the ribs were suggestive of very early cervical cord compression leading to perinatal mortality"
Identifies cervical cord compression as the proposed mechanism of perinatal death, which is what makes airway and cervical spine assessment the priority. Stated by the authors as suggestive rather than demonstrated; no imaging was possible.
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Diagnosis

2
Clinical and radiological diagnosis, confirmed molecularly
Diagnosis rests on the radiographic pattern: delayed and impaired vertebral body ossification together with large epiphyseal ossification centres, wide growth plates and pseudoepiphyses of the short tubular bones. That combination of axial under-ossification with appendicular excess is what distinguishes SMMD from other spondyloepiphyseal dysplasias. Molecular confirmation is by NKX3-2 sequencing. Because the disorder is so rarely seen, the review describing the perinatally lethal case notes explicitly that the diagnostic features are not yet fully established.
Show evidence (2 references)
PMID:29704686 SUPPORT Human Clinical
"A diagnosis of SMMD was made on clinical and radiological grounds."
States that the diagnosis is made clinically and radiographically.
PMID:29704686 SUPPORT Human Clinical
"Because of the rarity of the disorder, the diagnostic feature has not been fully established yet."
Records the authors' own caveat that the diagnostic criteria are not settled, which is the honest limit on the section above.
Cervical spine surveillance
Distinct from the diagnostic workup above, and with a different purpose: not to establish the diagnosis but to detect instability before the cord is injured. It is the step that makes the one effective intervention in this disorder possible, and it is prompted by recognising the radiographic pattern rather than by neurological symptoms - by the time spasticity appears, the injury has happened.
Show evidence (2 references)
PMID:22791571 SUPPORT Human Clinical
"Early diagnosis of SMMD by recognition of the radiographic pattern might prevent of the neurologic complications via prophylactic cervical spine stabilization."
Ties recognition of the radiographic pattern to the preventive action. The same sentence supports the treatment entry; it is cited in both places because it makes both claims and the surveillance is useless without the intervention it enables.
PMID:22791571 SUPPORT Human Clinical
"Because of the rarity of the condition, its diagnostic features and natural course are not well known."
The caveat that governs this whole section. Recorded because a surveillance recommendation reads as settled practice unless the state of knowledge behind it is stated.
📊

Prevalence

1
Worldwide, reported cases
Cases In Literature Ultra Rare
Ultra-rare, with only a few cases in the literature at the time the gene was identified. No population rate has been estimated and none is asserted here.
Show evidence (1 reference)
PMID:20004766 SUPPORT Human Clinical
"is a rare skeletal dysplasia with only a few cases reported in the literature."
The stated rarity of the disorder in its founding genetic description.
🐁

Animal Models

2
Nkx3-2 null mouse
Reproduces the human vertebral ossification defect with striking similarity, but diverges in two directions: the mice are asplenic, which SMMD patients are not, and they lack the limb radiographic abnormalities seen in patients. The authors attribute the missing limb findings to perinatal death of the mutant animals rather than to a species difference in gene function.
Species
Mouse
Genotype
Nkx3-2 null
Publication
PMID:10572046
Pathograph links
Deranged Endochondral Ossification partially recapitulates fidelity: moderate Vertebral ossification defects closely resemble those of SMMD patients; appendicular findings are absent.
Limitations: Asplenia occurs in the mutant mice but not in SMMD patients, and the limb radiographic abnormalities that are a defining human feature are not seen. The perinatal lethality of the model plausibly explains the missing limb phenotype but has not been shown to, so the appendicular arm of the human disease is effectively unmodelled here.
Show evidence (2 references)
PMID:20004766 SUPPORT Model Organism
"Striking similarities were found when comparing the vertebral ossification defects in SMMD patients with those observed in the Nkx3-2 null mice."
Establishes the axial correspondence between model and human disease.
+ 1 more reference
Show evidence (2 references)
PMID:10572046 SUPPORT Model Organism
"Bapx1 null mice are affected by a perinatal lethal skeletal dysplasia and asplenia, with severe malformation or absence of specific bones of the vertebral column and cranial bones of mesodermal origin"
The primary description of this model, cited directly rather than through the human genetics paper that summarised it.
PMID:10572046 SUPPORT Model Organism
"downregulation of several molecular markers required for normal chondroblast differentiation (&agr; 1(II) collagen, Fgfr3, Osf2, Indian hedgehog, Sox9)"
The molecular readout: the chondrogenic network collapses downstream of the loss, which is what makes this a failure of cartilage development rather than of ossification alone. Quoted with the source's own "&agr;" HTML entity where the character should be alpha - the report's validator flagged this quote as unsupported purely on that encoding difference, and a snippet does not correct its source.
Adult viable nkx3.2 mutant zebrafish
Survives to adulthood, unlike the embryonic-lethal mouse and knockdown models, and is the only model to reproduce the skeletal overgrowth arm of SMMD. Shows cartilage overgrowth in place of a missing jaw joint, severe facial, skullcap and spine dysmorphology.
Species
Zebrafish
Genotype
nkx3.2 mutant (adult viable)
Publication
PMID:33462117
Pathograph links
Loss of Post-Embryonic Restraint on Joint Chondrocyte Proliferation recapitulates fidelity: moderate Generated specifically to model the skeletal overgrowth defects of SMMD, which prior models could not show, and identifies increased chondrocyte proliferation and a stress response as the cellular basis.
Limitations: Zebrafish skeletal anatomy differs substantially from human: the overgrowth phenotype is read out at a jaw joint that has no direct human counterpart, and the fish spine is not loaded as a human spine is. The prostaglandin-synthase upregulation has not been shown in human SMMD tissue.
Show evidence (1 reference)
PMID:33462117 SUPPORT Model Organism
"By generating a zebrafish model for the skeletal overgrowth defects of SMMD, we reveal post-embryonic roles for Nkx3.2 in dampening proliferation and buffering the stress response in joint-associated chondrocytes."
The authors' statement of what the model was built for and what it showed.
{ }

Source YAML

click to show 
name: Spondylo-megaepiphyseal-metaphyseal Dysplasia
category: Mendelian
creation_date: "2026-09-04T00:00:00Z"
synonyms:
- SMMD
- spondylo-megaepiphyseal-metaphyseal dysplasia
description: >-
  SMMD is a rare autosomal recessive skeletal dysplasia caused by biallelic inactivating
  variants in NKX3-2 (also called BAPX1), a homeobox transcription factor on chromosome 4p15.33.
  Affected individuals have disproportionate short stature with a short, stiff neck and trunk,
  while the limbs appear relatively long.

  The radiographic picture is the diagnostic one and it is unusual in combining two opposite
  failures: delayed and impaired ossification of the vertebral bodies alongside abnormally
  *large* epiphyseal ossification centres and wide growth plates in the long bones, plus
  numerous pseudoepiphyses in the short tubular bones of hands and feet. Deficient ossification
  in the axial skeleton and excessive cartilage in the appendicular skeleton are the same
  lesion seen in two places.

  The mechanism explains why. NKX3-2 acts in the proliferative zone of the growth plate as a
  transcriptional repressor that holds chondrocytes immature - it represses Runx2, the
  chondrocyte maturation factor, and is normally switched off as maturation begins. It sits
  downstream of PTHrP signalling. So the protein's job is to *restrain* maturation, and losing
  it deranges the timing of endochondral ossification rather than simply reducing it: bone that
  should form does not, and cartilage that should be replaced persists and overgrows.

  A zebrafish model added a second, later role. Adult viable nkx3.2 mutant fish show cartilage
  overgrowth and spinal dysmorphology, with mutant chondrocytes showing increased proliferation
  and upregulated stress-response pathways. That points to a post-embryonic function in damping
  proliferation and buffering stress in joint-associated chondrocytes, which is the part of the
  human phenotype that embryonic-lethal models could not show.

  Severity spans a wide range. At the severe end a newborn had total absence of vertebral body,
  pubic and ischial ossification and died on day 14, with the clinical picture suggesting very
  early cervical cord compression.
disease_term:
  preferred_term: spondylo-megaepiphyseal-metaphyseal dysplasia
  term:
    id: MONDO:0013228
    label: spondylo-megaepiphyseal-metaphyseal dysplasia
parents:
- Spondyloepiphyseal Dysplasia
references:
- reference: PMID:20004766
  title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
- reference: PMID:29704686
  title: "A novel NKX3-2 mutation associated with perinatal lethal phenotype of spondylo-megaepiphyseal-metaphyseal dysplasia in a neonate."
- reference: PMID:33462117
  title: "Zebrafish model for spondylo-megaepiphyseal-metaphyseal dysplasia reveals post-embryonic roles of Nkx3.2 in the skeleton."
- reference: PMID:16421188
  title: "Nkx3.2/Bapx1 acts as a negative regulator of chondrocyte maturation."
- reference: PMID:22791571
  title: Severe neurologic manifestations from cervical spine instability in spondylo-megaepiphyseal-metaphyseal dysplasia.
inheritance:
- name: Autosomal recessive
  description: >-
    Biallelic inactivating NKX3-2 variants. The three founding families were consanguineous and
    each proband was homozygous for a different inactivating mutation, which is strong evidence
    for allelic heterogeneity converging on loss of function rather than a founder effect.
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  evidence:
  - reference: PMID:20004766
    reference_title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Each proband was homozygous for a different inactivating mutation in NKX3-2, a homeobox-containing gene located on chromosome 4p15.33."
    explanation: >-
      Three unrelated consanguineous probands, three different inactivating alleles, all
      homozygous, which establishes recessive loss of function.
  - reference: PMID:20004766
    reference_title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "it defines SMMD as yet another skeletal dysplasia with autosomal-recessive inheritance and a distinct phenotype"
    explanation: The authors' own statement of the inheritance mode and phenotypic distinctness.
pathophysiology:
- name: NKX3-2 Loss of Function
  description: >-
    Biallelic inactivating NKX3-2 variants abolish the homeobox transcription factor. Reported
    alleles include three distinct inactivating changes in the founding consanguineous families
    and, in a perinatally lethal case, the frameshift c.507-508delCA (p.Gly171Cysfs*55) in
    exon 2.
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  genes:
  - preferred_term: NKX3-2
    term:
      id: hgnc:951
      label: NKX3-2
  molecular_functions:
  - preferred_term: NKX3-2 transcriptional repressor activity
    modifier: LOSS_OF_FUNCTION
    term:
      id: GO:0001227
      label: DNA-binding transcription repressor activity, RNA polymerase II-specific
  downstream:
  - target: Derepression of the Chondrocyte Maturation Program
    causal_link_type: DIRECT
  - target: Loss of Post-Embryonic Restraint on Joint Chondrocyte Proliferation
    causal_link_type: DIRECT
  - target: Loss of Chondrocyte Survival Signalling
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:20004766
    reference_title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Genome wide homozygosity mapping followed by a candidate gene approach resulted in the elucidation of the genetic cause in three new consanguineous families with SMMD."
    explanation: The gene-discovery result establishing NKX3-2 as the SMMD gene.
  - reference: PMID:29704686
    reference_title: "A novel NKX3-2 mutation associated with perinatal lethal phenotype of spondylo-megaepiphyseal-metaphyseal dysplasia in a neonate."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Molecular analysis revealed homozygosity for a novel mutation, c.507-508delCA (p.Gly171Cysfs*55) in exon 2 of NKX3-2."
    explanation: A frameshift allele at the severe end of the reported spectrum.
- name: Derepression of the Chondrocyte Maturation Program
  description: >-
    NKX3-2 normally restrains chondrocyte maturation, acting as a transcriptional repressor of
    Runx2 in the proliferative zone and being switched off as maturation begins. The direction
    of the mechanism is established by a construct experiment: wild-type Nkx3.2 blocks
    maturation, while a reverse-function mutant converted into a transcriptional activator
    accelerates it, and Runx2 mis-expression rescues the Nkx3.2-induced block. Losing NKX3-2
    therefore removes a brake on maturation timing rather than removing a positive signal.
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  cell_types:
  - preferred_term: growth plate chondrocyte
    term:
      id: CL:0000138
      label: chondrocyte
  biological_processes:
  - preferred_term: negative regulation of chondrocyte differentiation
    modifier: DECREASED
    term:
      id: GO:0032331
      label: negative regulation of chondrocyte differentiation
  locations:
  - preferred_term: growth plate
    term:
      id: UBERON:0002516
      label: epiphyseal plate
  downstream:
  - target: Deranged Endochondral Ossification
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:16421188
    reference_title: "Nkx3.2/Bapx1 acts as a negative regulator of chondrocyte maturation."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Although wild-type Nkx3.2 blocks chondrocyte maturation by acting as a transcriptional repressor, a ;reverse function' mutant of Nkx3.2 that has been converted into a transcriptional activator conversely accelerates chondrocyte maturation."
    explanation: >-
      Establishes the direction of the mechanism by reversing it. Quoted verbatim including the
      source's mangled quotation mark in ";reverse function'".
  - reference: PMID:16421188
    reference_title: "Nkx3.2/Bapx1 acts as a negative regulator of chondrocyte maturation."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Nkx3.2 represses expression of the chondrocyte maturation factor Runx2, and Runx2 mis-expression can rescue the Nkx3.2-induced blockade of chondrocyte maturation."
    explanation: >-
      Names the transcriptional target and shows the block is relieved by restoring it, placing
      Runx2 downstream of NKX3-2 in this pathway.
  - reference: PMID:16421188
    reference_title: "Nkx3.2/Bapx1 acts as a negative regulator of chondrocyte maturation."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "In chick and mouse, expression of Nkx3.2/Bapx1 in the growth plate is restricted to the proliferative zone and is down regulated as chondrocyte maturation begins."
    explanation: >-
      Locates the factor spatially and temporally in the growth plate, which is what makes its
      loss a timing defect rather than a global one.
- name: Loss of Chondrocyte Survival Signalling
  description: >-
    A second, parallel arm of NKX3-2 function, distinct from the maturation brake modelled
    above. In proliferating chondrocytes NKX3-2 constitutively activates RelA by a
    ligand-independent route - recruiting the RelA-IkappaB-alpha complex into the nucleus by
    direct protein-protein interaction and degrading IkappaB-alpha there - and that NF-kappaB
    activity is required for chondrocyte viability. Losing it removes a survival signal from
    exactly the cells the maturation brake acts on.

    This makes NKX3-2 a dual-function node rather than a single repressor: a brake on maturation
    and a survival signal for the cells being braked. The two arms converge, which is consistent
    with the mouse null showing increased apoptosis alongside failed chondrogenesis.

    Curated as PROVISIONAL for this disease. The mechanism is established in chick and mouse
    chondrocytes and has not been shown in a patient with SMMD, so its contribution to the human
    phenotype is inferred from the gene rather than demonstrated in the disease.
  biological_scale: CELLULAR
  mechanism_confidence: PROVISIONAL
  cell_types:
  - preferred_term: proliferating chondrocyte
    term:
      id: CL:0000138
      label: chondrocyte
  biological_processes:
  - preferred_term: canonical NF-kappaB signal transduction
    modifier: DECREASED
    term:
      id: GO:0007249
      label: canonical NF-kappaB signal transduction
  - preferred_term: negative regulation of apoptotic process
    modifier: DECREASED
    term:
      id: GO:0043066
      label: negative regulation of apoptotic process
  evidence:
  - reference: PMID:17310243
    reference_title: Constitutive RelA activation mediated by Nkx3.2 controls chondrocyte viability.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Here, we show that the nuclear factor kappa B (NF-kappaB) pathway is required for chondrocyte viability and that Nkx3.2 supports chondrocyte survival by constitutively activating RelA."
    explanation: >-
      The core claim of the survival arm. Graded IN_VITRO: the work is in cultured chondrocytes,
      with no human disease data.
  - reference: PMID:17310243
    reference_title: Constitutive RelA activation mediated by Nkx3.2 controls chondrocyte viability.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Nkx3.2 recruits the RelA-IkappaBalpha heteromeric complex into the nucleus by direct protein-protein interactions and activates RelA through proteasome-dependent IkappaBalpha degradation in the nucleus"
    explanation: >-
      The mechanism, and the reason it is ligand-independent. Quoted with the source's ASCII
      spellings of the Greek letters.
  - reference: PMID:10572046
    reference_title: The murine Bapx1 homeobox gene plays a critical role in embryonic development of the axial skeleton and spleen.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Loss of Bapx1 is accompanied by an increase in apoptotic cell death in affected tissues, although cell cycling rates are unaltered."
    explanation: >-
      The in vivo counterpart: losing the gene raises apoptosis without changing proliferation
      rate, which is what a lost survival signal predicts and a lost proliferation brake does
      not. The dissociation is why this arm is modelled separately.
- name: Deranged Endochondral Ossification
  description: >-
    The consequence is not uniform under- or over-ossification but a disordered one. In the
    axial skeleton, vertebral body ossification is delayed and impaired, in the severe case
    absent altogether. In the appendicular skeleton, epiphyseal ossification centres are large,
    growth plates are wide, and pseudoepiphyses appear in the short tubular bones. NKX3-2 is
    therefore required for endochondral ossification of both the axial and appendicular
    skeleton in humans.
  biological_scale: TISSUE
  mechanism_confidence: ESTABLISHED
  biological_processes:
  - preferred_term: endochondral ossification
    modifier: DYSREGULATED
    term:
      id: GO:0001958
      label: endochondral ossification
  downstream:
  - target: Delayed vertebral ossification
    causal_link_type: DIRECT
  - target: Delayed pubic bone ossification
    causal_link_type: DIRECT
  - target: Coronal cleft vertebrae
    causal_link_type: DIRECT
  - target: Enlarged epiphyses
    causal_link_type: DIRECT
  - target: Thick growth plates
    causal_link_type: DIRECT
  - target: Pseudoepiphyses of hand bones
    causal_link_type: DIRECT
  - target: Disproportionate short-trunk short stature
    causal_link_type: DIRECT
  - target: Flexion contracture
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Cervical Spine Instability and Cord Injury
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:20004766
    reference_title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The most remarkable radiographic features are the delayed and impaired ossification of the vertebral bodies as well as the presence of large epiphyseal ossification centers and wide growth plates in the long tubular bones."
    explanation: >-
      Describes both halves of the ossification derangement, axial deficiency alongside
      appendicular excess, in one sentence.
  - reference: PMID:20004766
    reference_title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This study illustrates that NKX3-2 plays an important role in endochondral ossification of both the axial and appendicular skeleton in humans."
    explanation: The authors' conclusion that the requirement spans both skeletal compartments.
- name: Cervical Spine Instability and Cord Injury
  description: >-
    The clinically decisive consequence of the axial ossification failure, and the reason this
    disorder is dangerous rather than merely deforming. Reduced or absent ossification of the
    cervical vertebrae leaves the cervical spine mechanically unsupported, producing instability
    with anterior or posterior kinking - the swan-neck deformity also called kyknodysostosis -
    and from there cord injury presenting as limb spasticity.

    Two things make this node worth separating from the general ossification derangement. It hit
    five of the six patients in the only series to look at it, so within that series it is the
    rule rather than a complication. And the authors state that the high incidence of cervical
    spine deformation is unique among skeletal dysplasias, which means it is not a generic
    consequence of poor vertebral ossification that could be inherited from a sibling disorder.

    It is also the only point in this disease where an intervention changes the outcome: the
    lesion upstream is embryonic and inaccessible, while cervical instability is detectable and
    stabilisable before the cord is injured.
  biological_scale: ORGANISM
  mechanism_confidence: ESTABLISHED
  downstream:
  - target: Cervical instability
    causal_link_type: DIRECT
  - target: Spasticity
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:22791571
    reference_title: Severe neurologic manifestations from cervical spine instability in spondylo-megaepiphyseal-metaphyseal dysplasia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Reduced or absent ossification of the cervical vertebrae leads to cervical instability with anterior or posterior kinking of the cervical spine (swan neck-like deformity, kyknodysostosis)."
    explanation: >-
      The mechanical step from the ossification defect to the instability, which is why this node
      hangs off Deranged Endochondral Ossification rather than standing alone.
  - reference: PMID:22791571
    reference_title: Severe neurologic manifestations from cervical spine instability in spondylo-megaepiphyseal-metaphyseal dysplasia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "As a result of the cervical spine instability or deformation, five of six patients in our series suffered cervical cord injury that manifested clinically as limb spasticity."
    explanation: >-
      The step from instability to cord injury and its clinical readout, with the authors'
      attribution ("as a result of") rather than a co-occurrence. Six patients, so the
      proportion is fragile even though the direction is not.
- name: Loss of Post-Embryonic Restraint on Joint Chondrocyte Proliferation
  description: >-
    A distinct, later-acting arm identified in an adult viable zebrafish mutant: chondrocytes
    show increased proliferation and upregulation of stress-induced pathways, including
    prostaglandin synthases, and the animals develop cartilage overgrowth and severe spine and
    craniofacial dysmorphology. This arm is what accounts for the skeletal *overgrowth* of SMMD
    rather than its ossification deficit, and it is post-embryonic - the same overgrowth is
    absent in knockdown animals that lack jaw joints, which is the internal control separating
    the embryonic from the later role.
  biological_scale: CELLULAR
  mechanism_confidence: PROVISIONAL
  cell_types:
  - preferred_term: joint-associated chondrocyte
    term:
      id: CL:0000138
      label: chondrocyte
  biological_processes:
  - preferred_term: chondrocyte proliferation
    modifier: INCREASED
    term:
      id: GO:0035988
      label: chondrocyte proliferation
  downstream:
  - target: Scoliosis
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  evidence:
  - reference: PMID:33462117
    reference_title: "Zebrafish model for spondylo-megaepiphyseal-metaphyseal dysplasia reveals post-embryonic roles of Nkx3.2 in the skeleton."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Single-cell RNA-sequencing and in vivo validation reveal increased proliferation and upregulation of stress-induced pathways, including prostaglandin synthases, in mutant chondrocytes."
    explanation: The cellular and transcriptional phenotype defining this arm.
  - reference: PMID:33462117
    reference_title: "Zebrafish model for spondylo-megaepiphyseal-metaphyseal dysplasia reveals post-embryonic roles of Nkx3.2 in the skeleton."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "cartilage overgrowth and scoliosis are absent in rare viable nkx3.2 knockdown animals that lack jaw joints, supporting post-embryonic roles for Nkx3.2"
    explanation: >-
      The internal control that makes this a post-embryonic role: animals with the embryonic
      joint defect but without the later one do not overgrow.
phenotypes:
- category: Skeletal
  name: Disproportionate short-trunk short stature
  description: >-
    Disproportionate short stature with a short and stiff neck and trunk, the limbs appearing
    relatively long by contrast. This is the defining clinical presentation.
  frequency: OBLIGATE
  phenotype_term:
    preferred_term: Disproportionate short stature with short, stiff neck and trunk
    term:
      id: HP:0003521
      label: Disproportionate short-trunk short stature
  evidence:
  - reference: PMID:20004766
    reference_title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Affected individuals have a disproportionate short stature with a short and stiff neck and trunk."
    explanation: The core clinical description of the disorder.
- category: Skeletal
  name: Delayed vertebral ossification
  description: >-
    Delayed and impaired ossification of the vertebral bodies, extending in the most severe
    reported case to total absence of vertebral body, pubic and ischial ossification at birth.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Delayed and impaired vertebral body ossification
    term:
      id: HP:0031096
      label: Delayed vertebral ossification
  evidence:
  - reference: PMID:20004766
    reference_title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the delayed and impaired ossification of the vertebral bodies"
    explanation: The axial ossification defect as a defining radiographic feature.
  - reference: PMID:29704686
    reference_title: "A novel NKX3-2 mutation associated with perinatal lethal phenotype of spondylo-megaepiphyseal-metaphyseal dysplasia in a neonate."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Skeletal survey revealed total absence of the ossification of the vertebral bodies, pubis, and ischia."
    explanation: The severe end of the same defect, showing it is graded rather than all-or-none.
- category: Skeletal
  name: Delayed pubic bone ossification
  description: >-
    Absent or grossly delayed ossification of the pubic bones, reported as a constant
    radiographic feature. Curated separately from the vertebral finding because it is part of
    the diagnostic constellation in its own right - the pubis and ischia were carried inside the
    vertebral snippet and had no phenotype of their own.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Absent pubic and ischial ossification
    term:
      id: HP:0008788
      label: Delayed pubic bone ossification
  evidence:
  - reference: PMID:29704686
    reference_title: "A novel NKX3-2 mutation associated with perinatal lethal phenotype of spondylo-megaepiphyseal-metaphyseal dysplasia in a neonate."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Skeletal survey revealed total absence of the ossification of the vertebral bodies, pubis, and ischia"
    explanation: >-
      The perinatally lethal case, where the deficit is total. preferred_term names the ischia
      as well because the source does; HPO codes the pubis only.
  - reference: PMID:22791571
    reference_title: Severe neurologic manifestations from cervical spine instability in spondylo-megaepiphyseal-metaphyseal dysplasia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "missing ossification of the pubic bones"
    explanation: >-
      Independent confirmation in the six-patient series, where it is listed among the constant
      radiographic features rather than as a finding of the severe end.
- category: Skeletal
  name: Coronal cleft vertebrae
  description: >-
    Sagittal and coronal clefts in the poorly ossified vertebral bodies, part of the
    radiographic pattern the diagnosis rests on.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Sagittal and coronal vertebral clefts
    term:
      id: HP:0003417
      label: Coronal cleft vertebrae
  evidence:
  - reference: PMID:22791571
    reference_title: Severe neurologic manifestations from cervical spine instability in spondylo-megaepiphyseal-metaphyseal dysplasia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Radiographs show a severe ossification delay of the vertebral bodies with sagittal and coronal clefts"
    explanation: >-
      HPO codes the coronal cleft; preferred_term keeps the sagittal component the source
      reports alongside it.
- category: Skeletal
  name: Enlarged epiphyses
  description: >-
    Large, round "balloon-like" epiphyseal ossification centres in the long tubular bones. The
    "megaepiphyseal" element of the disease name.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Enlarged epiphyses
    term:
      id: HP:0010580
      label: Enlarged epiphyses
  evidence:
  - reference: PMID:20004766
    reference_title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the presence of large epiphyseal ossification centers and wide growth plates in the long tubular bones"
    explanation: The epiphyseal half of the appendicular signature. The same sentence supports the growth-plate phenotype below, which HPO codes separately.
- category: Skeletal
  name: Thick growth plates
  description: >-
    Wide growth plates in the long tubular bones. The "metaphyseal" element of the disease name,
    and the only metaphyseal finding this entry carries.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Thick growth plates
    term:
      id: HP:0025369
      label: Thick growth plates
  evidence:
  - reference: PMID:20004766
    reference_title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the presence of large epiphyseal ossification centers and wide growth plates in the long tubular bones"
    explanation: >-
      Curated separately from the epiphyseal finding because HPO codes them distinctly and
      because the growth plate is the metaphyseal compartment - without this the "metaphyseal"
      third of the disease name has no phenotype behind it.
- category: Skeletal
  name: Pseudoepiphyses of hand bones
  description: >-
    Numerous pseudoepiphyses at the short tubular bones of the hands and feet, reported at all
    metacarpals and phalanges. Singled out by both source series as a distinguishing feature
    rather than an incidental radiographic finding.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Pseudoepiphyses of hand and foot bones
    term:
      id: HP:0004288
      label: Pseudoepiphyses of hand bones
  evidence:
  - reference: PMID:20004766
    reference_title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Numerous pseudoepiphyses of the short tubular bones in hands and feet are another remarkable feature of the disorder."
    explanation: >-
      The source calls this "another remarkable feature", which is why it is its own phenotype
      rather than folded into an epiphyseal grouping term. preferred_term extends to the feet
      because the source does; HPO codes the hand bones only.
- category: Skeletal
  name: Scoliosis
  description: >-
    Spinal deformity, listed among the skeletal defects characterising the disorder and
    reproduced as severe spine dysmorphology in the adult zebrafish model. No frequency band is
    asserted: the only source naming it in humans lists it among the disease's defects without
    counting it, and phenotype support is not frequency support.
  phenotype_term:
    preferred_term: Scoliosis
    term:
      id: HP:0002650
      label: Scoliosis
  evidence:
  - reference: PMID:33462117
    reference_title: "Zebrafish model for spondylo-megaepiphyseal-metaphyseal dysplasia reveals post-embryonic roles of Nkx3.2 in the skeleton."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "which is characterized by skeletal defects including scoliosis, large epiphyses, wide growth plates and supernumerary distal limb joints"
    explanation: >-
      Lists scoliosis among the defining human skeletal defects. Graded OTHER rather than
      HUMAN_CLINICAL because this publication presents no human data - the sentence is its
      background characterisation of the disease, not an observation it made.
- category: Skeletal
  name: Supernumerary distal limb joints
  description: >-
    Extra joints in the distal limbs, named among the disease's characteristic skeletal defects.
    A striking finding and the one element of that list this entry had not curated.

    Deliberately not wired into the pathograph. The joint-chondrocyte proliferation node is the
    tempting target, and both concern joints - but that is a shared anatomical locus, not a
    stated mechanism: no source connects supernumerary joint formation, which is a patterning
    event, to the post-embryonic proliferation arm. An edge drawn on the word "joint" appearing
    in both would be exactly the unsupported inference this entry declines elsewhere.
  phenotype_term:
    preferred_term: Supernumerary distal limb joints
    term:
      id: HP:0011297
      label: Abnormal digit morphology
  evidence:
  - reference: PMID:33462117
    reference_title: "Zebrafish model for spondylo-megaepiphyseal-metaphyseal dysplasia reveals post-embryonic roles of Nkx3.2 in the skeleton."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "which is characterized by skeletal defects including scoliosis, large epiphyses, wide growth plates and supernumerary distal limb joints"
    explanation: >-
      The same background sentence, graded OTHER for the same reason. HPO has no term for a
      supernumerary limb joint, so the binding is the digit-morphology parent and the finding
      itself is carried in preferred_term.
- category: Skeletal
  name: Flexion contracture
  description: Flexion contractures of the distal joints may be present in the relatively long limbs.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Distal joint flexion contractures
    term:
      id: HP:0001371
      label: Flexion contracture
  evidence:
  - reference: PMID:20004766
    reference_title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The limbs appear relatively long and may show flexion contractures of the distal joints."
    explanation: >-
      Reports the contractures with the source's own hedge ("may show"), which is why the
      frequency is recorded as occasional.
- category: Gastrointestinal
  name: Duodenal atresia
  description: >-
    Reported in the perinatally lethal neonate. A single case, so it is recorded as very rare
    and should not be read as an expected feature of SMMD.
  frequency: VERY_RARE
  phenotype_term:
    preferred_term: Duodenal atresia
    term:
      id: HP:0002247
      label: Duodenal atresia
  evidence:
  - reference: PMID:29704686
    reference_title: "A novel NKX3-2 mutation associated with perinatal lethal phenotype of spondylo-megaepiphyseal-metaphyseal dysplasia in a neonate."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The patient required immediate intubation at the delivery room and duodenal atresia was detected during his course in neonatal intensive care unit."
    explanation: >-
      Single-patient observation. Recorded because it drove management and mortality in that
      case, not because it is established as part of the syndrome.
  notes: >-
    Deliberately left unconnected in the pathograph. No mechanism links NKX3-2 loss to foregut
    atresia in any source read here, and NKX3-2 has a known role in gut and spleen patterning
    in mouse, so a plausible-sounding edge would be speculation rather than curation. It stays
    a recorded observation until somebody can source the link.
- category: Skeletal
  name: Cervical instability
  description: >-
    Reduced or absent cervical vertebral ossification leaves the cervical spine unsupported,
    giving anterior or posterior kinking - the swan-neck deformity the original authors also
    call kyknodysostosis. The authors note that the high incidence of cervical spine deformation
    in this disorder is unique among skeletal dysplasias, which is what makes it worth
    surveillance here rather than being folded into the general vertebral abnormality.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Cervical instability
    term:
      id: HP:0008462
      label: Cervical instability
  evidence:
  - reference: PMID:22791571
    reference_title: Severe neurologic manifestations from cervical spine instability in spondylo-megaepiphyseal-metaphyseal dysplasia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Although the number of individuals observed is small, the high incidence of cervical spine deformation in SMMD is unique among skeletal dysplasias."
    explanation: >-
      The frequency claim and its own caveat in one sentence. Graded VERY_FREQUENT on "high
      incidence" together with the 5-of-6 cord injury rate, but note the authors themselves flag
      the small denominator, and this remains the only series to have looked.
- category: Neurologic
  name: Spasticity
  description: >-
    Limb spasticity is the clinical presentation of cervical cord injury in this disorder, not
    a primary neurological feature: the lesion is mechanical compression from an unstable
    cervical spine. Five of six patients in the only series to examine this developed it.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Limb spasticity
    term:
      id: HP:0001257
      label: Spasticity
  evidence:
  - reference: PMID:22791571
    reference_title: Severe neurologic manifestations from cervical spine instability in spondylo-megaepiphyseal-metaphyseal dysplasia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "five of six patients in our series suffered cervical cord injury that manifested clinically as limb spasticity"
    explanation: >-
      The frequency and the causal attribution. 5/6 is 83%, in the VERY_FREQUENT band, but on a
      denominator of six from a single series - the band is what the number supports and the
      confidence in it is lower than the band alone conveys.
prevalence:
- population: Worldwide, reported cases
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    Ultra-rare, with only a few cases in the literature at the time the gene was identified.
    No population rate has been estimated and none is asserted here.
  evidence:
  - reference: PMID:20004766
    reference_title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "is a rare skeletal dysplasia with only a few cases reported in the literature."
    explanation: The stated rarity of the disorder in its founding genetic description.
diagnosis:
- name: Clinical and radiological diagnosis, confirmed molecularly
  description: >-
    Diagnosis rests on the radiographic pattern: delayed and impaired vertebral body
    ossification together with large epiphyseal ossification centres, wide growth plates and
    pseudoepiphyses of the short tubular bones. That combination of axial under-ossification
    with appendicular excess is what distinguishes SMMD from other spondyloepiphyseal
    dysplasias. Molecular confirmation is by NKX3-2 sequencing. Because the disorder is so
    rarely seen, the review describing the perinatally lethal case notes explicitly that the
    diagnostic features are not yet fully established.
  evidence:
  - reference: PMID:29704686
    reference_title: "A novel NKX3-2 mutation associated with perinatal lethal phenotype of spondylo-megaepiphyseal-metaphyseal dysplasia in a neonate."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A diagnosis of SMMD was made on clinical and radiological grounds."
    explanation: States that the diagnosis is made clinically and radiographically.
  - reference: PMID:29704686
    reference_title: "A novel NKX3-2 mutation associated with perinatal lethal phenotype of spondylo-megaepiphyseal-metaphyseal dysplasia in a neonate."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Because of the rarity of the disorder, the diagnostic feature has not been fully established yet."
    explanation: >-
      Records the authors' own caveat that the diagnostic criteria are not settled, which is the
      honest limit on the section above.
- name: Cervical spine surveillance
  description: >-
    Distinct from the diagnostic workup above, and with a different purpose: not to establish
    the diagnosis but to detect instability before the cord is injured. It is the step that
    makes the one effective intervention in this disorder possible, and it is prompted by
    recognising the radiographic pattern rather than by neurological symptoms - by the time
    spasticity appears, the injury has happened.
  evidence:
  - reference: PMID:22791571
    reference_title: Severe neurologic manifestations from cervical spine instability in spondylo-megaepiphyseal-metaphyseal dysplasia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Early diagnosis of SMMD by recognition of the radiographic pattern might prevent of the neurologic complications via prophylactic cervical spine stabilization."
    explanation: >-
      Ties recognition of the radiographic pattern to the preventive action. The same sentence
      supports the treatment entry; it is cited in both places because it makes both claims and
      the surveillance is useless without the intervention it enables.
  - reference: PMID:22791571
    reference_title: Severe neurologic manifestations from cervical spine instability in spondylo-megaepiphyseal-metaphyseal dysplasia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Because of the rarity of the condition, its diagnostic features and natural course are not well known."
    explanation: >-
      The caveat that governs this whole section. Recorded because a surveillance recommendation
      reads as settled practice unless the state of knowledge behind it is stated.
treatments:
- name: Prophylactic Cervical Spine Stabilization
  description: >-
    The one intervention in this disorder that changes the outcome rather than managing it. The
    upstream lesion is an embryonic transcription-factor failure and is not therapeutically
    accessible after birth; cervical instability is. Stabilization is prophylactic in the strict
    sense - performed on the basis of the radiographic pattern before cord injury has occurred,
    not as treatment for an established myelopathy.

    The evidence is a proposal rather than a demonstration, and the entry records it that way.
    The authors write that early diagnosis "might prevent" the neurologic complications; no
    series has reported outcomes in patients stabilized prophylactically, and with six patients
    described in total there is no prospect of one soon. This is curated because the reasoning
    is sound and the stakes are high, not because it has been shown to work.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: cervical spine stabilization
    term:
      id: NCIT:C157986
      label: Spinal Fusion
  target_mechanisms:
  - target: Cervical Spine Instability and Cord Injury
    description: >-
      Mechanically stabilizes the unossified cervical spine, interrupting the step from
      instability to cord compression. It does not address the ossification defect itself.
  evidence:
  - reference: PMID:22791571
    reference_title: Severe neurologic manifestations from cervical spine instability in spondylo-megaepiphyseal-metaphyseal dysplasia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Early diagnosis of SMMD by recognition of the radiographic pattern might prevent of the neurologic complications via prophylactic cervical spine stabilization."
    explanation: >-
      The recommendation, quoted with the authors' hedge ("might prevent") and with their own
      grammatical slip left in place, since a snippet does not correct its source. This is a
      proposal from a case series, not an outcome.
- name: Respiratory and Surgical Supportive Care
  description: >-
    There is no disease-modifying treatment. Management is supportive and, at the severe end,
    intensive: the perinatally lethal case required immediate intubation in the delivery room
    and surgery for duodenal atresia on day 7, and died of sepsis and respiratory failure on
    day 14. The clinical picture in that infant suggested very early cervical cord compression,
    which is the specific risk the axial ossification defect creates and the reason airway and
    cervical spine assessment matter in this disorder.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:29704686
    reference_title: "A novel NKX3-2 mutation associated with perinatal lethal phenotype of spondylo-megaepiphyseal-metaphyseal dysplasia in a neonate."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The patient required immediate intubation at the delivery room and duodenal atresia was detected during his course in neonatal intensive care unit."
    explanation: Documents the supportive interventions required at the severe end of the spectrum.
  - reference: PMID:29704686
    reference_title: "A novel NKX3-2 mutation associated with perinatal lethal phenotype of spondylo-megaepiphyseal-metaphyseal dysplasia in a neonate."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the findings of clubfoot, neuromuscular respiratory insufficiency requiring invasive mechanical ventilation and downward sloping or tented appearance of the ribs were suggestive of very early cervical cord compression leading to perinatal mortality"
    explanation: >-
      Identifies cervical cord compression as the proposed mechanism of perinatal death, which
      is what makes airway and cervical spine assessment the priority. Stated by the authors as
      suggestive rather than demonstrated; no imaging was possible.
classifications:
  harrisons_chapter:
  - classification_value: GENETICS_ENVIRONMENT_DISEASE
    evidence:
    - reference: PMID:20004766
      reference_title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "it defines SMMD as yet another skeletal dysplasia with autosomal-recessive inheritance and a distinct phenotype"
      explanation: Places the disorder among the Mendelian skeletal dysplasias.
genetic:
- name: NKX3-2
  gene_term:
    preferred_term: NKX3-2
    term:
      id: hgnc:951
      label: NKX3-2
  relationship_type: CAUSATIVE
  notes: >-
    Homeobox-containing transcription factor at 4p15.33, also known as BAPX1. Acts as a
    transcriptional repressor in the proliferative zone of the growth plate, downstream of PTHrP
    signalling, holding chondrocytes immature by repressing Runx2. All reported disease alleles
    are inactivating.

    The deep-research report supplies gnomAD constraint metrics for this gene (pLI 0.0009,
    LOEUF 1.07, o/e 0.67) as the quantitative basis for the haplosufficiency that explains the
    recessive inheritance. They are not curated as evidence: they are a database lookup with no
    citable publication behind them in the report, and they could not be independently verified
    from this environment. Recorded here as a lead rather than asserted.
  evidence:
  - reference: PMID:20004766
    reference_title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Each proband was homozygous for a different inactivating mutation in NKX3-2, a homeobox-containing gene located on chromosome 4p15.33."
    explanation: >-
      Identifies the gene, its class and locus, and establishes that the disease alleles are
      inactivating rather than of mixed consequence.
  - reference: PMID:16421188
    reference_title: "Nkx3.2/Bapx1 acts as a negative regulator of chondrocyte maturation."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Nkx3.2/Bapx1 expression is lost in the growth plates of mice engineered to lack PTHrP signaling and, conversely, is maintained by ectopic expression of PTHrP in developing bones."
    explanation: >-
      Places NKX3-2 downstream of PTHrP signalling in the growth plate, in both directions of
      the manipulation.
- name: NKX3-1
  gene_term:
    preferred_term: NKX3-1
    term:
      id: hgnc:7838
      label: NKX3-1
  relationship_type: MODIFIER
  notes: >-
    A functionally overlapping paralog, coexpressed with NKX3-2 in somites during early
    development. Whether it modifies severity in human SMMD is not established - the evidence is
    entirely murine, and Nkx3.1 single nulls have no skeletal phenotype at all. Recorded because
    it is the only candidate modifier this literature offers and because the double-null result
    bears on why the human phenotype is survivable: partial redundancy would predict that a
    residual NKX3-1 contribution buffers the loss.
  evidence:
  - reference: PMID:12204261
    reference_title: Transcription factors Nkx3.1 and Nkx3.2 (Bapx1) play an overlapping role in sclerotomal development of the mouse.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Double mutants exhibited enhanced defects of vertebrae compared with Bapx1-deficient animals."
    explanation: >-
      The redundancy result. Enhancement over the single null is what makes this a modifier
      rather than an independent cause.
  - reference: PMID:12204261
    reference_title: Transcription factors Nkx3.1 and Nkx3.2 (Bapx1) play an overlapping role in sclerotomal development of the mouse.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "In contrast, inactivation of the Nkx3.1 gene causes no apparent skeletal phenotype despite its early expression in sclerotomal cells."
    explanation: >-
      The negative half, and the reason this is a modifier and not a second disease gene: losing
      NKX3-1 alone does nothing to the skeleton.
animal_models:
- name: Nkx3-2 null mouse
  species: Mouse
  genotype: Nkx3-2 null
  publication: PMID:10572046
  description: >-
    Reproduces the human vertebral ossification defect with striking similarity, but diverges in
    two directions: the mice are asplenic, which SMMD patients are not, and they lack the limb
    radiographic abnormalities seen in patients. The authors attribute the missing limb findings
    to perinatal death of the mutant animals rather than to a species difference in gene
    function.
  modeled_mechanisms:
  - target: Deranged Endochondral Ossification
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    model_scale: TISSUE
    description: >-
      Vertebral ossification defects closely resemble those of SMMD patients; appendicular
      findings are absent.
    limitations: >-
      Asplenia occurs in the mutant mice but not in SMMD patients, and the limb radiographic
      abnormalities that are a defining human feature are not seen. The perinatal lethality of
      the model plausibly explains the missing limb phenotype but has not been shown to, so the
      appendicular arm of the human disease is effectively unmodelled here.
    evidence:
    - reference: PMID:20004766
      reference_title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Striking similarities were found when comparing the vertebral ossification defects in SMMD patients with those observed in the Nkx3-2 null mice."
      explanation: Establishes the axial correspondence between model and human disease.
    - reference: PMID:20004766
      reference_title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
      supports: REFUTE
      evidence_source: MODEL_ORGANISM
      snippet: "Distinguishing features were the asplenia found in the mutant mice and the radiographic abnormalities in the limbs only observed in SMMD patients."
      explanation: >-
        Refutes full recapitulation: names the two features on which model and human disease
        diverge, in both directions.
  evidence:
  - reference: PMID:10572046
    reference_title: The murine Bapx1 homeobox gene plays a critical role in embryonic development of the axial skeleton and spleen.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Bapx1 null mice are affected by a perinatal lethal skeletal dysplasia and asplenia, with severe malformation or absence of specific bones of the vertebral column and cranial bones of mesodermal origin"
    explanation: >-
      The primary description of this model, cited directly rather than through the human
      genetics paper that summarised it.
  - reference: PMID:10572046
    reference_title: The murine Bapx1 homeobox gene plays a critical role in embryonic development of the axial skeleton and spleen.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "downregulation of several molecular markers required for normal chondroblast differentiation (&agr; 1(II) collagen, Fgfr3, Osf2, Indian hedgehog, Sox9)"
    explanation: >-
      The molecular readout: the chondrogenic network collapses downstream of the loss, which is
      what makes this a failure of cartilage development rather than of ossification alone.
      Quoted with the source's own "&agr;" HTML entity where the character should be alpha - the
      report's validator flagged this quote as unsupported purely on that encoding difference,
      and a snippet does not correct its source.
- name: Adult viable nkx3.2 mutant zebrafish
  species: Zebrafish
  genotype: nkx3.2 mutant (adult viable)
  publication: PMID:33462117
  description: >-
    Survives to adulthood, unlike the embryonic-lethal mouse and knockdown models, and is the
    only model to reproduce the skeletal overgrowth arm of SMMD. Shows cartilage overgrowth in
    place of a missing jaw joint, severe facial, skullcap and spine dysmorphology.
  modeled_mechanisms:
  - target: Loss of Post-Embryonic Restraint on Joint Chondrocyte Proliferation
    relationship: RECAPITULATES
    fidelity: MODERATE
    model_scale: CELLULAR
    description: >-
      Generated specifically to model the skeletal overgrowth defects of SMMD, which prior
      models could not show, and identifies increased chondrocyte proliferation and a stress
      response as the cellular basis.
    limitations: >-
      Zebrafish skeletal anatomy differs substantially from human: the overgrowth phenotype is
      read out at a jaw joint that has no direct human counterpart, and the fish spine is not
      loaded as a human spine is. The prostaglandin-synthase upregulation has not been shown in
      human SMMD tissue.
    evidence:
    - reference: PMID:33462117
      reference_title: "Zebrafish model for spondylo-megaepiphyseal-metaphyseal dysplasia reveals post-embryonic roles of Nkx3.2 in the skeleton."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "By generating a zebrafish model for the skeletal overgrowth defects of SMMD, we reveal post-embryonic roles for Nkx3.2 in dampening proliferation and buffering the stress response in joint-associated chondrocytes."
      explanation: The authors' statement of what the model was built for and what it showed.
discussions:
- discussion_id: smmd_overgrowth_unmodelled_in_mammals
  kind: HUMAN_MODEL_MISMATCH
  prompt: >-
    Does the skeletal overgrowth of SMMD have a mammalian model, and can the zebrafish
    chondrocyte proliferation and stress-response mechanism be assumed to hold in human growth
    plates?
  rationale: >-
    Evidence for the overgrowth arm exists, but only in fish. Mouse Nkx3-2 mutants show skeletal
    *reductions* and nkx3.2 knockdown zebrafish show embryonic lethal jaw joint fusions; both
    lack the skeletal overgrowth seen in SMMD patients. Only the adult viable zebrafish mutant
    reproduces it, and the proposed mechanism - increased chondrocyte proliferation with
    upregulated stress-induced pathways including prostaglandin synthases - rests on single-cell
    RNA-sequencing in that fish. This is a translational-validity question rather than an
    absence of evidence, which is why it is recorded as HUMAN_MODEL_MISMATCH: the mammalian
    models diverge from the human phenotype in the specific direction that matters, and the
    mouse additionally shows asplenia that SMMD patients do not. The relevant node is marked
    PROVISIONAL for this reason. Resolving it would need chondrocyte proliferation and
    prostaglandin-pathway activity measured in human SMMD growth-plate or joint cartilage, or a
    conditional mouse allele that survives past the perinatal period.
  attaches_to:
  - pathophysiology#Loss of Post-Embryonic Restraint on Joint Chondrocyte Proliferation
  - animal_models#Mouse
  evidence:
  - reference: PMID:33462117
    reference_title: "Zebrafish model for spondylo-megaepiphyseal-metaphyseal dysplasia reveals post-embryonic roles of Nkx3.2 in the skeleton."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Whereas nkx3.2 knockdown zebrafish and mouse Nkx3.2 mutants display embryonic lethal jaw joint fusions and skeletal reductions, respectively, they lack the skeletal overgrowth seen in SMMD patients."
    explanation: >-
      States the mismatch explicitly: two model systems fail to reproduce the human overgrowth
      phenotype, which is the gap this discussion records.
  - reference: PMID:20004766
    reference_title: "Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "The absence of the latter anomalies in the murine model may be due to the perinatal death of the affected animals."
    explanation: >-
      The proposed explanation for the mouse gap, offered by the authors as a hypothesis ("may
      be due to") rather than a demonstrated cause.
📚

References & Deep Research

References

5
PMID:20004766
Homozygous inactivating mutations in the NKX3-2 gene result in spondylo-megaepiphyseal-metaphyseal dysplasia.
No top-level findings curated for this source.
PMID:29704686
A novel NKX3-2 mutation associated with perinatal lethal phenotype of spondylo-megaepiphyseal-metaphyseal dysplasia in a neonate.
No top-level findings curated for this source.
PMID:33462117
Zebrafish model for spondylo-megaepiphyseal-metaphyseal dysplasia reveals post-embryonic roles of Nkx3.2 in the skeleton.
No top-level findings curated for this source.
PMID:16421188
Nkx3.2/Bapx1 acts as a negative regulator of chondrocyte maturation.
No top-level findings curated for this source.
PMID:22791571
Severe neurologic manifestations from cervical spine instability in spondylo-megaepiphyseal-metaphyseal dysplasia.
No top-level findings curated for this source.

Deep Research

1
OpenScientist ▸
Spondylo-Megaepiphyseal-Metaphyseal Dysplasia (SMMD): A Comprehensive Disease Characteristics Report
openscientist-autonomous 13 citations 2026-09-04T19:34:27.391074

Spondylo-Megaepiphyseal-Metaphyseal Dysplasia (SMMD): A Comprehensive Disease Characteristics Report

Disease: Spondylo-megaepiphyseal-metaphyseal dysplasia (SMMD) MONDO: MONDO:0013228 · OMIM: 613330 · Orphanet: ORPHA:228387 · MeSH: C567639 · GARD: 0017154 · UMLS: C2750066 · MedGen: 412869 Causal gene: NKX3-2 (formerly BAPX1), HGNC:951, 4p15.33 Category: Mendelian (autosomal recessive)

Autonomous literature-based discovery report. Evidence note: SMMD is an ultra-rare disorder with only a small number of molecularly/clinically documented patients worldwide. Nearly all clinical evidence derives from small case series and single-case reports (human clinical, descriptive), while mechanistic understanding comes from mouse, chick, and zebrafish developmental biology (model organism / in vitro).

Summary

Spondylo-megaepiphyseal-metaphyseal dysplasia (SMMD) is an ultra-rare, autosomal recessive skeletal dysplasia caused by biallelic loss-of-function (LoF) mutations in NKX3-2 (BAPX1), a homeobox transcription factor located on chromosome 4p15.33. The disease is defined by a characteristic disproportion — a short trunk and short neck combined with relatively long limbs — together with a distinctive radiographic picture of delayed/absent vertebral ossification with sagittal and coronal clefts, large "balloon-like" (mega-) epiphyses, wide/abnormal metaphyses, absent pubic ossification, and multiple pseudoepiphyses of the hand and foot tubular bones. The single most clinically important complication is cervical spine instability with spinal cord compression, which drives major morbidity and can be life-threatening.

Mechanistically, NKX3-2 sits at the center of the sclerotome-to-cartilage developmental program. It is induced during sclerotome specification by Sonic hedgehog (SHH) signaling combined with BMP antagonism (Noggin), acting upstream of the master chondrogenic transcription factor SOX9. Within the growth plate, NKX3-2 functions downstream of PTHrP to repress the pro-hypertrophy factor RUNX2, thereby restraining premature chondrocyte maturation; independently, it sustains the survival of proliferating chondrocytes through ligand-independent activation of RelA/NF-κB. Biallelic loss of NKX3-2 therefore removes a maturation brake and a survival signal simultaneously, disrupting endochondral ossification and producing the skeletal phenotype. gnomAD constraint metrics (pLI ≈ 0, LOEUF ≈ 1.07) confirm that a single functional allele is sufficient (haplosufficiency), which explains the recessive inheritance pattern — disease requires loss of both alleles.

There is no disease-modifying therapy. Management is supportive and centers on early recognition and surveillance of cervical spine instability, surgical stabilization when indicated, respiratory and orthopedic care, and genetic counseling for affected families. Animal models — the Bapx1-null mouse and a zebrafish nkx3.2 mutant — recapitulate the core axial skeletal defects and have illuminated both embryonic and post-embryonic roles of the gene, providing platforms for future mechanistic and therapeutic study.

Section-by-Section Report

1. Disease Information

SMMD is a Mendelian skeletal dysplasia affecting the spine (spondylo-), the epiphyses (which become abnormally large — megaepiphyseal), and the metaphyses (metaphyseal). Affected individuals present in infancy/early childhood with disproportionate short stature (short trunk and neck, comparatively long limbs), joint limitation, and progressive skeletal deformity. The disease is characterized clinically and radiographically rather than biochemically.

Key identifiers (verified programmatically via EBI OLS4 and HGNC REST — Finding F007):

Resource Identifier
MONDO MONDO:0013228
OMIM (phenotype) 613330
Orphanet ORPHA:228387
MeSH C567639
GARD 0017154
UMLS C2750066
MedGen 412869
Gene (HGNC) HGNC:951 (NKX3-2)
Gene (NCBI) 579
Gene (Ensembl) ENSG00000109705
Gene (UniProt) P78367
Gene OMIM *602183
Cytoband 4p15.33

Synonyms / alternative names: SMMD; spondylomegaepiphyseal-metaphyseal dysplasia. The causal gene was historically named BAPX1 (bagpipe homeobox homolog 1), with aliases NKX3B and NKX3.2.

Data source type: Information is derived from aggregated disease-level resources (OMIM, Orphanet, Mondo) and small published patient case series/reports rather than large EHR cohorts, reflecting the disease's rarity.

2. Etiology

Disease causal factors — genetic. SMMD is a monogenic disorder caused by biallelic inactivating (loss-of-function) mutations in NKX3-2 (Finding F001). Genome-wide homozygosity mapping combined with candidate-gene sequencing in three consanguineous families identified three distinct homozygous inactivating mutations in NKX3-2 on chromosome 4p15.33 PMID: 20004766. A later perinatal-lethal neonatal case carried the homozygous frameshift variant c.507-508delCA (p.Gly171Cysfs*55) in exon 2 PMID: 29704686.

"Each proband was homozygous for a different inactivating mutation in NKX3-2, a homeobox-containing gene located on chromosome 4p15.33." — PMID: 20004766

Genetic risk factors. The only established risk factor is inheritance of two loss-of-function NKX3-2 alleles. Consanguinity is a major contributor: the founding cases were identified in consanguineous families through homozygosity mapping, and consanguineous unions increase the probability of homozygosity for a rare recessive allele. Heterozygous carriers are unaffected (see gnomAD constraint, Finding F008).

Environmental risk factors. None identified or expected — this is a fully penetrant Mendelian developmental disorder. Age, sex, and lifestyle exposures are not causal contributors.

Protective factors. No genetic or environmental protective factors are described. Because a single intact allele is sufficient for normal development (haplosufficiency), the presence of one functional NKX3-2 allele is fully "protective" in carriers.

Gene–environment interactions. None documented; the phenotype is driven by the developmental genetic lesion.

3. Phenotypes

SMMD phenotypes are physical/skeletal manifestations and clinical/neurological signs. The characteristic radiographic and clinical features derive largely from the six-patient series of Simon et al. and related reports (Finding F003).

Phenotype Type Onset Severity/Progression Suggested HPO term
Disproportionate short stature (short trunk/neck, long limbs) Physical manifestation Congenital/infancy Moderate–severe, progressive HP:0004322 (Short stature); HP:0003521 (Disproportionate short-trunk short stature)
Delayed/absent vertebral body ossification with sagittal & coronal clefts Radiographic sign Congenital Severe HP:0008428 (Abnormal vertebral ossification); HP:0003312 (Abnormal form of the vertebral bodies)
Cervical spine instability ("swan-neck" deformity, kyknodysostosis) Clinical/radiographic sign Early childhood Severe, progressive HP:0003316 (Abnormality of the cervical spine); HP:0008443 (Cervical instability)
Cervical cord injury → limb spasticity Neurological sign Childhood Severe; life-threatening HP:0001257 (Spasticity); HP:0002385 (Paraparesis)
Large "balloon-like" (mega-) epiphyses of long bones Radiographic sign Childhood — HP:0003065 (Epiphyseal dysplasia); HP:0010577 (Enlarged epiphyses)
Metaphyseal abnormalities Radiographic sign Childhood — HP:0000944 (Abnormal metaphysis)
Multiple pseudoepiphyses of metacarpals/phalanges Radiographic sign Childhood — HP:0006262 (Pseudoepiphyses of the hand bones)
Absent/delayed pubic bone ossification Radiographic sign Congenital — HP:0008788 (Delayed pubic bone ossification)
Perinatal lethality (severe end of spectrum) Outcome Neonatal Fatal HP:0001522 (Death in infancy)

"Radiographs show a severe ossification delay of the vertebral bodies with sagittal and coronal clefts, missing ossification of the pubic bones, large round 'balloon-like' epiphyses of the long bones, and presence of multiple pseudoepiphyses at all metacarpals and phalanges." — PMID: 22791571

Quality-of-life impact. Cervical cord compression causing spasticity has profound effects on mobility and daily function; short stature and skeletal deformity affect ambulation and independence. No formal EQ-5D/SF-36 data exist for this ultra-rare disease.

4. Genetic / Molecular Information

Causal gene. NKX3-2 (BAPX1), HGNC:951, NCBI Gene 579, Ensembl ENSG00000109705, UniProt P78367, gene OMIM *602183, cytoband 4p15.33. NKX3-2 is a NK-family homeobox transcription factor.

Pathogenic variants (Findings F001, F008).

Variant Type Consequence Classification Reference
Three distinct homozygous inactivating mutations (3 families) Inactivating/LoF Loss of function Pathogenic PMID: 20004766
c.507-508delCA (p.Gly171Cysfs*55), exon 2 Frameshift deletion LoF / truncation Pathogenic (perinatal-lethal) PMID: 29704686
  • Variant classification: Pathogenic (biallelic LoF) per ACMG framework (null variants in a gene where LoF is the established mechanism).
  • Variant type/class: Inactivating LoF, including frameshift/truncating variants.
  • Origin: Germline.
  • Functional consequence: Loss of function of the NKX3-2 transcription factor.
  • Allele frequency: Extremely rare / private variants; not present at appreciable frequency in population databases.

gnomAD constraint (Finding F008). For NKX3-2 (ENSG00000109705, GRCh38): pLI = 0.0009 (LoF-tolerant, not haploinsufficient); LOEUF = 1.07; observed/expected LoF point estimate = 0.67 (13 observed vs 19.4 expected); missense Z = −1.42 (no missense constraint). These metrics confirm that heterozygous LoF is tolerated in the general population, consistent with haplosufficiency and the recessive inheritance of SMMD.

Modifier genes. NKX3-1 is a functionally overlapping paralog: in mouse, Nkx3.1/Nkx3.2 double-null embryos show enhanced vertebral defects and embryonic lethality (E12.5–E17.5) beyond the Bapx1 single-null phenotype PMID: 12204261, indicating partial redundancy. Whether NKX3-1 modifies human SMMD severity is not established. Upstream regulators Meox1/Meox2 directly activate Bapx1 transcription and are required for sclerotomal Bapx1 expression PMID: 15024065; MEOX1 loss remodels cranio-cervical joints and alters Bapx1 expression PMID: 19520072.

Epigenetic information / chromosomal abnormalities. No disease-specific epigenetic signatures or large-scale chromosomal abnormalities are reported; SMMD is caused by point/small LoF mutations rather than structural variants.

5. Environmental Information

No environmental factors, lifestyle factors, or infectious agents are implicated in SMMD. It is a purely genetic developmental disorder. This section is not applicable beyond noting that consanguinity (a demographic/social factor, not an environmental exposure) increases recessive-disease risk.

6. Mechanism / Pathophysiology

Ordered causal chain (initiating lesion → clinical manifestation)

  1. Biallelic loss-of-function mutation in NKX3-2 (germline) → complete loss of functional NKX3-2 transcription factor in developing skeleton (demonstrated; PMID: 20004766).
  2. Loss of NKX3-2 → failure of the normal sclerotome→chondrogenesis program. Normally SHH + BMP-antagonism induce Pax1/Bapx1(NKX3-2), which acts upstream of Sox9 to launch chondrogenesis; without NKX3-2 this program is impaired (demonstrated in ESC model; PMID: 25294938).
  3. Branch A — loss of maturation brake: NKX3-2 normally represses RUNX2 downstream of PTHrP to keep chondrocytes proliferating. Its loss → derepression of RUNX2 → dysregulated/premature chondrocyte maturation (demonstrated in growth-plate models; PMID: 16421188).
  4. Branch B — loss of survival signal: NKX3-2 normally sustains proliferating-chondrocyte viability via ligand-independent RelA/NF-κB activation. Its loss → reduced chondrocyte survival (demonstrated in vitro; PMID: 17310243).
  5. Branches A + B converge → downregulation of the chondrogenic gene network (Sox9, Col2a1, Fgfr3, Ihh, Runx2) and failure of normal cartilage differentiation (demonstrated in Bapx1-null mouse; PMID: 10572046).
  6. Disrupted cartilage template → defective endochondral ossification of vertebrae, epiphyses, and metaphyses → delayed/cleft vertebral ossification, mega-epiphyses, abnormal metaphyses, pseudoepiphyses, absent pubic ossification (inferred from radiographic phenotype; PMID: 22791571).
  7. Poor cervical vertebral ossification → cervical spine instability ("swan-neck"/kyknodysostosis) → spinal cord compression → limb spasticity and neurological compromise (demonstrated clinically in 5/6 patients; PMID: 22791571).
  8. (Severe genotypes) → perinatal lethality (demonstrated; PMID: 29704686).

Detail by category

Upstream induction (Finding F006). In an embryonic-stem-cell–directed somitic chondrogenesis model, isolated paraxial mesoderm treated with SAG1 (Hedgehog agonist) plus LDN193189 (BMP type-I receptor inhibitor) induced Pax1 and Bapx1(NKX3-2), then Sox9, producing cartilaginous nodules; canonical Wnt (Wnt3a/CHIR99021) + Noggin generated the upstream paraxial mesoderm. TGFβ supported Sox9/chondrogenesis but did not induce Pax1/Bapx1, showing the sclerotome route is specifically SHH- and BMP-antagonism-dependent.

"Pax1 and Bapx1 expression was induced when the isolated paraxial mesodermal progeny were treated with SAG1 (a hedgehog receptor agonist) and LDN193189, then Sox9 expression was induced, leading to cartilaginous nodules." — PMID: 25294938

Molecular pathways. SHH signaling; BMP antagonism (Noggin); canonical Wnt/β-catenin (upstream mesoderm); PTHrP–NKX3-2–RUNX2 growth-plate axis; NF-κB (RelA) survival signaling.

Cellular processes. Chondrocyte fate specification, proliferation, maturation/hypertrophy control, and chondrocyte survival (anti-apoptotic).

Repression of RUNX2 downstream of PTHrP (Finding F002). Nkx3.2/Bapx1 expression in the growth plate is restricted to the proliferative zone, is lost when PTHrP signaling is absent, and is maintained by ectopic PTHrP. NKX3-2 represses RUNX2, and RUNX2 mis-expression rescues the NKX3-2-induced blockade of maturation — placing NKX3-2 as a PTHrP-controlled brake on chondrocyte hypertrophy.

"Nkx3.2 represses expression of the chondrocyte maturation factor Runx2, and Runx2 mis-expression can rescue the Nkx3.2-induced blockade of chondrocyte maturation." — PMID: 16421188

"Nkx3.2/Bapx1 expression is lost in the growth plates of mice engineered to lack PTHrP signaling and, conversely, is maintained by ectopic expression of PTHrP." — PMID: 16421188

Chondrocyte survival via RelA/NF-κB (Finding F005). NKX3-2 sustains proliferating-chondrocyte viability by constitutively activating RelA. It recruits the RelA–IκBα complex into the nucleus by direct protein–protein interaction and activates RelA via proteasome-dependent nuclear IκBα degradation — a stage-specific, ligand-independent mode of NF-κB activation.

"Nkx3.2 supports chondrocyte survival by constitutively activating RelA." — PMID: 17310243

"Nkx3.2 recruits the RelA-IkappaBalpha heteromeric complex into the nucleus by direct protein-protein interactions and activates RelA through proteasome-dependent IkappaBalpha degradation in the nucleus." — PMID: 17310243

Protein dysfunction. NKX3-2 is a homeodomain transcription factor; LoF mutations abolish its DNA-binding/transcriptional-regulatory activity (loss of function; not gain of function or dominant negative — consistent with recessive inheritance).

Downstream target network. Bapx1-null mice show downregulation of Sox9, Col2a1 (α1(II) collagen), Fgfr3, Indian hedgehog (Ihh), and Runx2/Osf2 (Finding F004).

Cell types & GO terms. Cell types: chondrocyte (CL:0000138), proliferating chondrocyte, sclerotome-derived chondroprogenitor. Suggested GO biological processes: chondrocyte differentiation (GO:0002062), endochondral ossification (GO:0001958), cartilage development (GO:0051216), negative regulation of chondrocyte differentiation (GO:0032331), positive regulation of NF-κB transcription factor activity (GO:0051092), and somite/sclerotome patterning.

7. Anatomical Structures Affected

Organ/system level. Primary: axial and appendicular skeleton (skeletal system, UBERON:0001434). The vertebral column (UBERON:0001130) — especially the cervical spine (UBERON:0002413) — is most severely affected. Secondary: the nervous system via spinal cord (UBERON:0002240) compression from cervical instability. In the mouse model, the spleen is also affected (asplenia), though splenic involvement is not a prominent feature of human SMMD.

Anatomical sites (UBERON). Vertebral body (UBERON:0002347), epiphysis (UBERON:0006589), metaphysis (UBERON:0003914), growth plate (UBERON:0003078), pubis (UBERON:0002367), metacarpal/phalangeal bones (UBERON:0002374 / UBERON:0003221).

Tissue and cell level. Cartilage tissue (UBERON:0002418) and the chondrocyte (CL:0000138) — specifically proliferating growth-plate chondrocytes — are the central affected cell population. Connective tissue of the developing skeleton is broadly involved.

Subcellular level (GO Cellular Component). Nucleus (GO:0005634) — the site of NKX3-2 transcription-factor and RelA/NF-κB activity; proteasome-mediated IκBα degradation (cytoplasm/nucleus) participates in the survival pathway.

Lateralization. Bilateral and symmetric (axial midline and paired long bones).

8. Temporal Development

Onset. Congenital; radiographic abnormalities (delayed vertebral/pubic ossification) are present at birth. Clinical presentation is typically in infancy/early childhood. A severe end of the spectrum presents as perinatal-lethal disease PMID: 29704686.

Onset pattern. Chronic/insidious for the surviving milder phenotype; the perinatal-lethal form is evident at/before birth.

Progression. Skeletal deformity and, critically, cervical spine instability are progressive. Cervical instability can worsen and lead to cord injury and spasticity during childhood (5/6 patients in the reported series; PMID: 22791571). Disease duration is chronic/lifelong for survivors.

Critical periods. Two windows are important: (1) embryonic sclerotome/chondrogenesis (the mechanistic origin, not therapeutically accessible postnatally), and (2) infancy–childhood, when cervical spine surveillance and timely stabilization can prevent catastrophic cord injury (the key intervention window).

9. Inheritance and Population

Inheritance pattern. Autosomal recessive (biallelic LoF NKX3-2), OMIM 613330 PMID: 20004766.

Penetrance / expressivity. Penetrance appears complete for biallelic LoF. Expressivity is variable, ranging from perinatal-lethal to survival into childhood/adulthood with progressive skeletal and neurological disease.

Consanguinity / founder effects. Consanguinity is a prominent feature of reported families; index cases were identified via homozygosity mapping in consanguineous pedigrees. No specific founder allele is established — the reported mutations are distinct/private.

Carrier frequency. Heterozygous carriers are asymptomatic (haplosufficiency confirmed by gnomAD, Finding F008). Given the disease rarity, carrier frequency is very low in the general population.

Epidemiology. SMMD is ultra-rare, with only a small number of families/cases reported worldwide; precise prevalence and incidence are not established (below reliable estimation). No strong sex bias is expected for an autosomal recessive disorder (theoretical male:female ≈ 1:1). Age distribution: presents congenitally/in childhood.

10. Diagnostics

Imaging (primary diagnostic modality). Skeletal radiography is central. Characteristic findings (Finding F003): severe ossification delay of vertebral bodies with sagittal and coronal clefts, absent pubic bone ossification, large round "balloon-like" epiphyses of long bones, and multiple pseudoepiphyses at all metacarpals and phalanges. Cervical spine imaging (dynamic flexion/extension radiographs, CT, MRI) is essential to detect instability and cord compression.

"five of six patients in our series suffered cervical cord injury that manifested clinically as limb spasticity." — PMID: 22791571

Genetic testing (confirmatory). Molecular confirmation is by sequencing NKX3-2 — via single-gene testing, a skeletal-dysplasia gene panel, or whole-exome/whole-genome sequencing. Homozygosity mapping was historically used in consanguineous families. Detection of biallelic inactivating NKX3-2 variants confirms the diagnosis.

Laboratory tests / biomarkers. No specific biochemical biomarker exists; diagnosis rests on radiographic pattern + molecular confirmation. Routine biochemistry (calcium, phosphate, ALP) is generally unremarkable, helping distinguish SMMD from metabolic bone disease.

Clinical criteria / differential diagnosis. Diagnosis integrates the disproportionate short-trunk phenotype, the characteristic radiographic constellation, and NKX3-2 genotyping. Differential diagnoses include other spondylometaphyseal/spondyloepimetaphyseal dysplasias, spondyloepiphyseal dysplasia congenita (COL2A1), and other short-trunk dysplasias — distinguished by the unique mega-epiphyses + vertebral clefts + pubic non-ossification pattern and by molecular testing.

Screening. Cascade genetic testing of at-risk relatives and prenatal/preimplantation genetic testing are available for families with known NKX3-2 variants. No population newborn screening exists.

11. Outcome / Prognosis

Survival/mortality. Prognosis spans a wide spectrum. The severe end is perinatal-lethal PMID: 29704686. For survivors, the principal life-threatening risk is cervical cord injury from cervical spine instability, which can cause severe neurological disability or death if unrecognized.

Morbidity/function. Major morbidity arises from (1) neurological compromise (spasticity, myelopathy) due to cord compression, and (2) skeletal deformity and short stature affecting mobility and daily function. In the reported series, 5 of 6 patients developed cervical cord injury with limb spasticity (PMID: 22791571).

Complications. Cervical instability/cord compression is the dominant complication; respiratory compromise and orthopedic complications (deformity, contractures) also occur.

Prognostic factors. Severity and timing of cervical instability, and whether it is detected and stabilized before cord injury, are the key modifiable prognostic determinants. The specific genotype (e.g., truncating variants associated with perinatal lethality) also influences outcome.

12. Treatment

No disease-modifying therapy exists. Management is entirely supportive and preventive.

  • Surgical/interventional (most important). Cervical spine stabilization/fusion and decompression for instability and cord compression; timely neurosurgical/orthopedic intervention is the key to preventing or limiting neurological injury. (Suggested NCIT: cervical spinal fusion / spinal stabilization procedures.)
  • Supportive care. Respiratory support, orthopedic management of deformity and contractures, pain management, and mobility aids.
  • Rehabilitation. Physical and occupational therapy to preserve function and manage spasticity.
  • Genetic counseling. For affected families given autosomal recessive recurrence risk (25% per pregnancy for carrier couples).
  • Pharmacotherapy / advanced therapeutics. No pharmacologic, gene, cell, or RNA-based therapies are established or in trials specifically for SMMD. There are no relevant pharmacogenomic considerations.

13. Prevention

  • Primary prevention. Not possible for an inherited developmental disorder; the only means of avoiding recurrence is reproductive planning in carrier couples (prenatal diagnosis, preimplantation genetic testing).
  • Secondary prevention. Early detection and surveillance of the cervical spine in diagnosed patients to catch instability before cord injury — the single most impactful preventive measure.
  • Tertiary prevention. Cervical stabilization, spasticity management, and orthopedic/respiratory care to prevent complications and disability progression.
  • Genetic screening/counseling. Carrier testing and cascade screening in affected families; genetic counseling regarding 25% recurrence risk and reproductive options.
  • Immunization, behavioral, and public-health interventions are not applicable.

14. Other Species / Natural Disease

Comparative biology (Finding F004). NKX3-2/Bapx1 is deeply conserved. It was first identified in Drosophila as bagpipe (bap), essential for midgut musculature; the vertebrate ortholog acquired axial/limb skeletogenesis functions after the jawless-fish/gnathostome split PMID: 11523821. Orthologs include mouse Bapx1/Nkx3.2 (chromosome 5) and zebrafish nkx3.2. Human BAPX1 has 87% amino-acid identity to the Drosophila homeodomain and 100% homeodomain identity to mouse PMID: 9426254.

Model organisms as "natural disease" analogs. No naturally occurring SMMD-equivalent disease is documented in companion animals or wildlife; disease knowledge comes from engineered models (below).

Suggested NCBI Taxa: Homo sapiens (9606), Mus musculus (10090), Danio rerio (7955), Drosophila melanogaster (7227).

15. Model Organisms

Mouse — Bapx1(Nkx3.2)-null (Finding F004). Bapx1-null mice display a perinatal-lethal skeletal dysplasia with asplenia, featuring severe malformation or absence of vertebral column elements and cranial bones of mesodermal origin (most severe in ventral, notochord-associated structures). Failure of cartilage development is accompanied by downregulation of Sox9, Col2a1, Fgfr3, Ihh, and Runx2/Osf2.

"Bapx1 null mice are affected by a perinatal lethal skeletal dysplasia and asplenia, with severe malformation or absence of specific bones of the vertebral column and cranial bones of mesodermal origin." — PMID: 10572046

"downregulation of several molecular markers required for normal chondroblast differentiation (α1(II) collagen, Fgfr3, Osf2, Indian hedgehog, Sox9)." — PMID: 10572046

Mouse — Nkx3.1/Nkx3.2 double-null. Simultaneous loss of both paralogs causes embryonic lethality (E12.5–E17.5) and enhanced vertebral defects versus Bapx1 single-null, demonstrating partial functional redundancy PMID: 12204261.

Zebrafish — nkx3.2 mutant (Finding F004). A zebrafish nkx3.2 mutant models SMMD and, importantly, reveals post-embryonic roles of Nkx3.2 in growth plates and joints — extending mechanistic understanding beyond embryonic patterning PMID: 33462117.

Model characteristics. Recapitulation: the mouse null captures axial skeletal malformation and the chondrogenic gene-network collapse; the zebrafish captures post-embryonic joint/growth-plate roles. Limitations: mouse asplenia is not a prominent human feature; the mouse null's perinatal lethality limits study of postnatal cervical instability, which the zebrafish partly addresses. Genetic model types available: knockout (mouse, zebrafish), double-knockout (Nkx3.1/Nkx3.2), and in vitro ESC/iPSC-directed chondrogenesis systems.

Resources: MGI (mouse), ZFIN (zebrafish), Alliance of Genome Resources.

Mechanistic Model / Interpretation

   SHH agonist + BMP antagonism (Noggin)                [PMID 25294938]
 │
 ▼
   Sclerotome specification:  PAX1 ──▶ NKX3-2 (BAPX1)
 │                        │
 │                        ▼
 │                    SOX9  ──▶ chondrogenesis (Col2a1, cartilage template)
 │
   ┌─────────────┴───────────── NKX3-2 functions in growth plate ─────────────┐
   │                                                                            │
   ▼ Branch A (maturation brake)                        ▼ Branch B (survival)
 PTHrP ──▶ NKX3-2 ──┤ represses RUNX2   [PMID 16421188]  NKX3-2 ──▶ RelA/NF-κB  [PMID 17310243]
   → keeps chondrocytes proliferating                    → proliferating-chondrocyte survival
   │                                                                            │
   └──────────────────────────┬─────────────────────────────────────────────┘
              ▼
      BIALLELIC LoF NKX3-2  → both brake AND survival signal LOST
              ▼
   Chondrogenic network collapse (↓Sox9, Col2a1, Fgfr3, Ihh, Runx2)  [PMID 10572046]
              ▼
   Defective endochondral ossification
     → vertebral clefts, mega-epiphyses, metaphyseal defects,
       pseudoepiphyses, absent pubic ossification            [PMID 22791571]
              ▼
   Poor cervical vertebral ossification → CERVICAL INSTABILITY
              ▼
   Spinal cord compression → limb spasticity / neurological injury  [PMID 22791571]
              ▼
   (severe genotypes) perinatal lethality                  [PMID 29704686]

The unifying insight is that NKX3-2 is a dual-function node: it both times chondrocyte maturation (by repressing RUNX2 downstream of PTHrP) and protects proliferating chondrocytes from death (via ligand-independent NF-κB/RelA activation). Its complete loss therefore does not merely slow one process — it simultaneously removes a maturation brake and a survival signal, causing a broad collapse of the chondrogenic program and thus the multi-site skeletal dysplasia. Because a single allele suffices for normal development (gnomAD LOEUF ≈ 1.07, pLI ≈ 0), only individuals with biallelic loss are affected, explaining the recessive inheritance and the association with consanguinity.

Evidence Base

PMID Title (abbrev.) Supports
20004766 Homozygous inactivating NKX3-2 mutations cause SMMD Causal gene, LoF mechanism, 4p15.33 locus, AR inheritance (F001)
29704686 Novel NKX3-2 mutation, perinatal-lethal SMMD Specific frameshift LoF variant; severe end of spectrum (F001)
22791571 Cervical spine instability in SMMD Cervical cord injury frequency (5/6); radiographic features (F003)
16421188 Nkx3.2/Bapx1 negatively regulates chondrocyte maturation RUNX2 repression downstream of PTHrP (F002)
17310243 Constitutive RelA activation by Nkx3.2 Chondrocyte survival via NF-κB (F005)
25294938 Small-molecule sclerotome/somitic chondrogenesis SHH + BMP-antagonism induces Bapx1 upstream of Sox9 (F006)
10572046 Murine Bapx1 in axial skeleton & spleen Mouse KO phenotype; downstream targets (F004)
12204261 Nkx3.1 & Nkx3.2 overlap in sclerotome Paralog redundancy; double-null enhanced defects
33462117 Zebrafish nkx3.2 SMMD model Post-embryonic skeletal roles (F004)
11523821 Bapx1 in axial skeleton development/evolution Evolutionary conservation; vertebral phenotype
15024065 Meox proteins activate Bapx1 Upstream Meox→Bapx1 regulation in sclerotome
19520072 MEOX1 and cranio-cervical joints Upstream sclerotome polarity affecting Bapx1
9426254 Cloning of human BAPX1 Gene identification, expression, chromosomal mapping
27158253 Role of Nkx3.2 in chondrogenesis (review) Synthesis of NKX3-2 role in chondrocyte fate/survival

Limitations and Knowledge Gaps

  • Ultra-rarity: Only a handful of families/cases are reported; there are no reliable prevalence/incidence estimates, no formal natural-history cohorts, and no quality-of-life data.
  • Genotype–phenotype correlation is incompletely defined — why some biallelic LoF variants are perinatal-lethal while others permit survival into childhood remains unclear.
  • Human vs model discrepancies: Mouse Bapx1-null asplenia is not a prominent human feature; the mouse KO's perinatal lethality limits study of the clinically dominant cervical instability, only partly addressed by the zebrafish model.
  • Mechanistic detail of how the two NKX3-2 functions (RUNX2 repression vs RelA/NF-κB survival) are individually weighted in human disease is inferred from model systems, not directly demonstrated in patients.
  • No therapeutic pipeline: There are no disease-modifying agents, gene-therapy programs, or clinical trials specific to SMMD.
  • Modifier gene contribution (e.g., NKX3-1) to human phenotypic variability is untested.

Proposed Follow-up Experiments / Actions

  1. Establish an international patient registry to define prevalence, natural history, genotype–phenotype correlations, and the timeline of cervical instability — directly informing surveillance guidelines.
  2. Standardize cervical-spine surveillance protocols (dynamic imaging schedule from diagnosis) and evaluate outcomes of prophylactic vs reactive stabilization, given that 5/6 reported patients developed cord injury.
  3. Dissect the two NKX3-2 functions in vivo using separation-of-function alleles (RUNX2-repression-deficient vs RelA-activation-deficient) in mouse/zebrafish to quantify each branch's contribution to the skeletal phenotype.
  4. Exploit the zebrafish post-embryonic model (PMID: 33462117) to test whether modulating downstream nodes (e.g., RUNX2 dosage, NF-κB activity) can partially rescue growth-plate/joint defects — a route toward candidate therapeutics.
  5. iPSC-derived chondrocyte models from patient cells (using the ESC/small-molecule sclerotome-chondrogenesis protocol, PMID: 25294938) to model human chondrogenesis and screen for corrective compounds.
  6. Test modifier hypotheses (e.g., NKX3-1) via targeted sequencing across the patient cohort to explain variable expressivity.

Report generated from a 5-iteration autonomous discovery investigation: 8 confirmed findings, 14 papers reviewed. Ontology IDs (HP, GO, CL, UBERON) are best-available suggestions and should be verified against current ontology releases before database ingestion.

Artifacts

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Outcome Count
References checked 14
Resolved 14
Unresolved (possible confabulation) 0
Unverifiable 0
Quoted claims checked 10
Quoted claims found in source 9
Quoted claims not found in source 1
References weighed for topical relevance 14
On topic 7
Off topic 1

Quotes not found in the cited source

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

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

  • PMID:10572046 (abstract only): "downregulation of several molecular markers required for normal chondroblast differentiation (α1(II) collagen, Fgfr3, Osf2, Indian hedgehog, Sox9)."
  • closest text in source: "We provide evidence that the failure of the formation of skeletal elements in Bapx1 null embryos is a consequence of a failure of cartilage development, as demonstrated by downregulation of several molecular markers required for normal chondroblast differentiation (&agr; 1(II) collagen, Fgfr3, Osf2, Indian hedgehog, Sox9), as well as a chondrocyte-specific alpha1 (II) collagen-lacZ transgene"

References that may not be about this subject

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

  • PMID:17310243 (9 mentions) - Constitutive RelA activation mediated by Nkx3.2 controls chondrocyte viability.
  • shared terms: survival, genetic

Weighed against this report's own most characteristic terms: nkx3-2, smmd, cervical, instability, skeletal, model, bapx1, cord, phenotype, spine, gene, disease, mouse, vertebral, survival, function, patient, via, runx2, genetic.

Term Validation

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

Outcome Count
Terms checked 36
Resolved 33
Unresolved (possible confabulation) 0
Obsolete 1
Unverifiable 2
Terms whose name was checked 5
Terms named correctly 2
Terms named as a different term 2
Terms whose name is worth a second look 1

Terms the report names something else

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

  • MONDO:0013228 (2 mentions) - the report calls it "MONDO"; MONDO calls it spondylo-megaepiphyseal-metaphyseal dysplasia
  • HP:0006262 (1 mention) - the report calls it "Pseudoepiphyses of the hand bones"; HP calls it Aplasia/Hypoplasia of the 5th finger

Obsolete terms

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

  • GO:0051092 (obsolete positive regulation of NF-kappaB transcription factor activity) (1 mention)

Terms whose name is worth a second look

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

  • HP:0000944 (1 mention) - the report calls it "Abnormal metaphysis"; HP calls it Abnormal metaphysis morphology

Prefixes with no resolver

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