NOG-Related Symphalangism Spectrum Disorder

1. Disease Information

Overview. NOG-SSD is an umbrella term proposed in 2011. It covers five autosomal dominant joint-development syndromes, all caused by heterozygous variants in NOG, the gene for the BMP antagonist noggin.

"The hallmark of NOG-related syndromes is proximal symphalangism, defined by abnormal fusion of the proximal interphalangeal joints of the hands and feet. Many additional features secondary to NOG mutations are commonly but inconsistently observed, including a characteristic facies with a hemicylindrical nose, congenital conductive hearing loss due to stapes fixation, and hyperopia." — PMID:21538686 (Potti et al., Hum Mutat 2011, review)

"Due to the variable phenotypic spectrum within families and among families with the same mutation, we propose a unifying term, NOG-related symphalangism spectrum disorder (NOG-SSD)" — PMID:21538686

The five syndromes it covers, with OMIM numbers (as listed in PMID:34049328):

The gene is NOG, OMIM 602991 (PMID:33308208), HGNC hgnc:7866 ✓ (NOG). Facio-audio-symphalangism syndrome is a historical synonym (PMID:26994744, PMID:29605356).

Identifiers. MONDO:0100521 is the entry's disease term. I could not confirm an Orphanet code for the NOG-SSD umbrella. Orphanet lists the component syndromes separately, and the web search did not find an umbrella ORPHA entry. I did not verify ICD-10/ICD-11 or MeSH codes. The relevant MeSH headings are Synostosis, Stapes/abnormalities and Hand Deformities, Congenital (taken from the MeSH indexing of PMID:21538686).

Data provenance. All of the evidence is aggregated from case reports, family studies and reviews. There are no EHR-derived or registry data. A locus-specific variant database exists (https://NOG.lovd.nl, PMID:21538686).

Suggested subtype modelling: five has_subtypes rows (SYM1, SYNS1, SABTT, TCC, BDB2). BDB2 has a different molecular mechanism (section 6), so it is the only one that is more than a clinical label.

2. Etiology

• Cause. Heterozygous germline NOG variants (missense, nonsense, frameshift, start-loss, in-frame indel), or 17q22 microdeletions that include NOG.
• Genetic heterogeneity of the clinical phenotype. Similar SYNS and SYM phenotypes are also caused by GDF5 (hgnc:4220 ✓) and FGF9 (hgnc:3687 ✓): "Currently, three genes, NOG, GDF5, and FGF9, have been identified as causative of SYNS" (PMID:25241334). Those cases are outside NOG-SSD by definition, but they matter for the differential diagnosis.
• Environmental, protective or gene-environment factors. None reported. Write "not applicable"; do not leave the field empty.
• Modifier effects. Strong intra- and inter-familial variability with the same variant: "the phenotype was expressed differently both within and between the families" (PMID:25241334). In mice, genetic background changes the phenotype: "The Noggin null mouse phenotype is strain dependent" (PMID:16598734), and hearing loss in Nog+/− mice appears "Depending on the genetic background" (PMID:18096605). No human modifier gene has been identified.

3. Phenotypes

The HPO terms below were checked against cache/hp/terms.csv ✓. Frequencies are qualitative because no cohort-level frequency study exists. Onset is congenital unless noted.

Inheritance and expressivity terms: HP:0000006 Autosomal dominant inheritance ✓ and HP:0003828 Variable expressivity ✓.

Quality of life. No validated QoL instrument data exist. The main functional burdens are hearing loss during school age and reduced hand dexterity. The authors of PMID:33588412 give that as the reason for early surgery. This is expert opinion, not measured data.

4. Genetic and Molecular Information

• Gene: NOG at 17q22 (PMID:25815513). It is a single-exon gene: "Since Noggin is a single exon gene, this data strongly suggest that the autosomal dominant disorders associated with NOG mutations are due to haploinsufficiency of NOGGIN" (PMID:18096605; mouse study). Because the gene has one exon, nonsense variants are not expected to trigger nonsense-mediated decay. That is my inference, not a quoted finding.
• Spectrum: "More than 60 mutations of NOG, including both missense and nonsense mutations" (PMID:34049328).

Representative variants:

• Functional class. Most variants are hypomorphic or loss of function: "the human disease-causing mutations are hypomorphic alleles that reduce secretion of functional dimeric noggin" (PMID:11562478). That study also found no dominant-negative effect: "Coexpression of mutant noggin with wild-type noggin … did not interfere with wild-type noggin secretion". BDB2 variants are an exception. They "do not indicate a major loss of function … Rather, they presumably alter NOG's ability to bind to BMPs and growth-differentiation factors (GDFs) in a subtle way" (PMID:17668388).
• Structural variants. A 555-kb deletion covering only NOG and ANKFN1 gave a mild, typical NOG-SSD phenotype (PMID:34049328). A 1.6-Mb deletion covering 13 genes gave NOG-SSD without intellectual disability (PMID:25815513). Larger 17q22 deletions cause more severe syndromes, which "are likely due to deletion of genes other than NOG" (PMID:34049328). Recommendation from PMID:25815513: when sequencing finds no variant, screen for 17q22 deletions.
• Mosaicism. Gonadal or low-level somatic mosaicism has been inferred in an unaffected parent (PMID:15736221). That is parental mosaicism, so per the somatic-mosaicism skill it must not be bound to HP:0001442. A de novo variant in an affected mother has also been inferred (PMID:34049328).
• ClinGen, ClinVar, gnomAD. Not checked. Run just check-gene-validity and the ClinGen cache before recording any validity tier; do not assign one.
• Epigenetics: none reported.

5. Environmental Information

No environmental, lifestyle or infectious contributors are known. Treat this as not applicable.

6. Mechanism and Pathophysiology

Causal chain

1. A heterozygous NOG loss-of-function or hypomorphic variant, or a NOG-containing deletion, leads to reduced secretion of functional noggin dimers (PMID:11562478, PMID:34049328, PMID:31628072). For R136C, the defect is reduced HSPG binding rather than reduced secretion (PMID:24735539; in silico only).
2. Reduced extracellular noggin results in unopposed BMP signalling in developing skeletal elements, including BMP4 and GDF5 (PMID:18096605; PMID:24735539, "leads to an excess of BMP signaling" — inferred from docking).
3. Excess BMP activity causes chondrocyte hyperplasia and over-recruitment of cells into cartilage condensations: "Excess BMP activity in the absence of Noggin antagonism may enhance the recruitment of cells into cartilage, resulting in oversized growth plates" (PMID:9603738; mouse null).
4. Leads to failed joint initiation and interzone formation: "initiation of joint development failed as measured by the expression of growth and differentiation factor-5"; "chondrocytes are also refractory to joint-inducing positional cues" (PMID:9603738).
5. Failure of joint cavitation results in bony or cartilaginous fusion:
6. Branch A, digits, carpals, tarsals and elbow: symphalangism and synostoses (PMID:10080184, "NOG is essential for joint formation").
7. Branch B, middle ear: in Nog+/− mice the stapes and styloid process do not separate (PMID:18096605). In humans, "circumferentially calcified cartilage bridging the stapedovestibular joint space" (PMID:26474326). Both lead to stapes fixation and then conductive hearing loss. Human ankylosis may be incomplete at birth and progressive afterwards (PMID:33588412).
8. BDB2 branch (separate mechanism): subtle changes to NOG–BMP/GDF binding alter the balance of BMP signalling and lead to terminal digit deficiency plus symphalangism. The authors propose BMP–ROR2 crosstalk, but that is a hypothesis (PMID:17668388).
9. Not explained: I found no mechanistic study for hyperopia, the facial features or tall stature. Leave these phenotypes without upstream edges, or attach a KNOWLEDGE_GAP discussion, instead of inventing a link.

Dosage and species caveat. Nog−/− mice die at birth. Heterozygous mice were first reported as normal (PMID:11562478), but later work found carpal/tarsal fusions in some strains (PMID:16598734) and conductive hearing loss (PMID:18096605). In other words, the human dominant phenotype only partly reproduces in mice. That fits a HUMAN_MODEL_MISMATCH discussion.

Suggested terms

All checked against the cache ✓.

• GO: GO:0030514 negative regulation of BMP signaling pathway (activity lost, modifier DECREASED); GO:0030509 BMP signaling pathway (INCREASED); GO:0060272 embryonic skeletal joint morphogenesis; GO:0072498 embryonic skeletal joint development; GO:0001502 cartilage condensation; GO:0035988 chondrocyte proliferation; GO:0002062 chondrocyte differentiation; GO:0042474 middle ear morphogenesis; GO:0001958 endochondral ossification; GO:0009306 protein secretion; GO:0042803 protein homodimerization activity; GO:0008201 heparin binding; GO:0005615 extracellular space.
• CL: CL:0000138 chondrocyte; CL:0008019 mesenchymal cell; CL:0000062 osteoblast.
• Genes on nodes: NOG hgnc:7866; BMP4 hgnc:1071 ✓; GDF5 hgnc:4220 ✓.

Omics, single-cell, spatial and CRISPR data: none found for this disease.

7. Anatomical Structures Affected

• Primary sites: proximal interphalangeal joints of the hands and feet; carpal and tarsal bones; elbow; middle ear (stapes and stapedovestibular joint); cervical spine.
• UBERON ✓: UBERON:0002496 stapes base; UBERON:0001756 middle ear; UBERON:0001490 elbow joint; UBERON:0002413 cervical vertebra. Proximal interphalangeal joint, carpal bone and tarsal bone were not in the cache and need a lookup.
• Tissue: cartilage and bone (connective tissue).
• Subcellular: secretory pathway and extracellular space. The signal-peptide variant L14P causes degradation of the protein inside the cell (PMID:34049328).
• Laterality: usually bilateral and roughly symmetric. Unilateral CHL has been reported (PMID:34049328, II-4).
• Otic capsule: "fixation of the stapes footplate to the otic capsule because of a circumferential bridge of calcified cartilage" (PMID:26474326). There is also an unusually long incus–footplate distance: 4.62 mm, against 3.72 ± 0.38 mm in 40 otosclerosis temporal bones (PMID:34049328).

8. Temporal Development

• Onset: congenital for the skeletal features.
• Hearing: traditionally described as congenital. However, "the stapes ankylosis in our SABTT patient seemed to be incomplete at birth and progressive in early childhood" (PMID:33588412).
• Joints: SYNS is described as "progressive fusion of multiple joints" (PMID:25241334).
• Growth: children are tall and appendicular bone mineral content is low; both normalise by adulthood (PMID:18204269).
• Course: lifelong, with no remission. The critical window for intervention is early childhood, to limit hearing loss during school age (PMID:33588412).
• Suggested progression: phases: prenatal joint-formation failure; congenital or early-childhood stapes fixation; childhood progression of CHL and joint fusion; stable adult phenotype.

9. Inheritance and Population

• Mode: autosomal dominant, HP:0000006 (PMID:21538686).
• Penetrance: appears high, but expressivity varies widely. Non-penetrant carriers have not been documented in the cached sources. Do not claim incomplete penetrance without a source.
• De novo variants and mosaicism: both occur (PMID:10080184; PMID:15736221; PMID:34472207).
• Prevalence: no numeric estimate. Orphanet-derived data for SYNS1 say "No data available" (Phenodis). Use prevalence_class: NOT_YET_DOCUMENTED with measure_type: UNKNOWN.
• Populations: reported in European, North American, Israeli, Japanese, Korean, Chinese and Indian families. No founder variant is known.
• Sex ratio: no bias reported.
• Anticipation and consanguinity: not relevant.

10. Diagnostics

• Clinical diagnosis: proximal symphalangism or other synostoses, plus CHL with normal tympanic membranes and absent stapedial reflexes, hyperopia and the characteristic nose (PMID:26474326, PMID:21538686).
• Imaging: hand and foot radiographs (symphalangism, carpal/tarsal coalition) and temporal bone CT. CT can look normal: "No obvious abnormalities were found … on ear computed tomography" (PMID:33308208).
• Audiology: pure-tone audiometry and tympanometry (type A). Newborn ABR may be normal: "newborn hearing screening may not detect conductive hearing loss in patients with NOG-SSD" (PMID:33588412).
• Genetic testing:
• Single-gene NOG sequencing, which is straightforward because the gene has one exon.
• A hearing-loss or skeletal dysplasia panel, or exome sequencing. CNV calling from exome data detected the 555-kb deletion (PMID:34049328).
• Chromosomal microarray or aCGH when sequencing is negative (PMID:25815513).
• GDF5 and FGF9 should be on the panel for SYNS-like phenotypes.
• Differential diagnosis:
• Otosclerosis. The key distinction: "Mutations in the NOG gene are commonly found in congenital stapes ankylosis with symphalangism, but not in otosclerosis" (PMID:22288654 [uncached]).
• GDF5- and FGF9-related synostoses.
• ROR2-related brachydactyly type B1 (PMID:17668388).
• Larger 17q22 deletion syndromes, which include intellectual disability.
• Osteogenesis imperfecta with stapes fixation (PMID:33308208).
• GeneReviews: I am not aware of a NOG-SSD chapter. Confirm with just check-genereviews before writing a "no chapter exists" note.
• Screening: periodic hearing tests for at-risk children (PMID:33588412). Prenatal and preimplantation testing are possible once the familial variant is known; this is standard practice and no source is cited.

11. Outcome and Prognosis

• Life expectancy is normal; no disease-specific mortality has been reported.
• Morbidity comes from hearing loss, restricted joint motion and joint pain.
• Surgical outcomes:
• "stapedectomy had lasting effect without renewed fixation of the stapes in a follow up period of 18 months-38 years" (PMID:29605356).
• Hearing improved by about 20 dB at 6 months, though "Some reports describe progressive hearing loss due to bone regrowth 2–5 years after surgery" (PMID:33308208).
• Postoperative thresholds returned to normal in a SABTT child (PMID:33588412).
• Histology suggests the CHL "should be improved after stapedectomy" (PMID:26474326).
• Prognostic factors: none validated. The literature disagrees on whether genotype predicts phenotype: "disease expression can be independent of the location and type of NOG mutation" (PMID:33308208), but C-terminal truncations are associated with SABTT (PMID:12089654).

12. Treatment

There is no disease-modifying therapy. NCIT IDs below are not verified in this session; check them with runoak or cache/ncit/terms.csv before binding.

Pharmacotherapy, gene therapy, RNA therapies and clinical trials: none. I found no NCT or ICTRP registrations; this was a web search, and ClinicalTrials.gov itself was not queried.

13. Prevention

• Primary prevention: not applicable beyond reproductive genetic counselling (50% recurrence risk for an affected parent). Prenatal or preimplantation diagnosis is possible.
• Recurrence after unaffected parents: consider parental germline mosaicism when counselling (PMID:15736221).
• Secondary prevention: serial audiometry in at-risk children, because newborn screening can miss later-onset CHL (PMID:33588412). Ophthalmology screening for hyperopia and strabismus.
• Tertiary prevention: early stapes surgery; long-term audiological follow-up for refixation (PMID:33308208).

14. Other Species and Natural Disease

No naturally occurring animal disease was found; OMIA was not searched in this session. Noggin is highly conserved across vertebrates. The causal variants "alter evolutionarily conserved amino acid residues" (PMID:10080184), and L14 is conserved in more than 100 species (PMID:34049328). Joint requirements differ between species: "NOG requirements during skeletogenesis differ between species and between specific skeletal elements within species" (PMID:10080184).

15. Model Organisms

Gaps to Record as Knowledge Gaps

1. The mechanism of hyperopia, the facial features and childhood tall stature.
2. A genotype–phenotype rule, especially why some families have no hearing loss (C230Y).
3. Prevalence.
4. Whether stapes fixation is congenital or progressive across the spectrum.
5. How often ossicles other than the stapes are fixed.
6. Whether EA/TEF is truly part of the spectrum (a single case so far).

References Needing just fetch-reference Before Use

• PMID:22288654 (Ishino et al., Clin Genet 2012)
• Sonoyama 2023 (PMC10097708)
• Start-codon variant 2019 (PMC6679842)
• Ma 2019 (BMC Med Genet)
• Takano 2016 (Hum Genome Var)

Sources

• Cached references in references_cache/: PMID:21538686, 10080184, 11562478, 12089654, 15736221, 16598734, 17668388, 18096605, 18204269, 24735539, 25241334, 25815513, 26474326, 26994744, 29159868, 29605356, 31628072, 33308208, 33588412, 34049328, 34472207, 9603738 (36004948 and 9634411 are cached with no abstract content)
• PubMed 21538686
• Phenodis: Multiple synostoses syndrome 1
• MedlinePlus: Tarsal-carpal coalition syndrome
• DisGeNET: TCC
• NORD overview
• SAIL concept library: MONDO:0008521
• Sonoyama 2023, PMC10097708
• Start-codon variant, PMC6679842
• Ma 2019, BMC Med Genet
• Takano 2016, PMC4972895
• PubMed 22288654
• Stapes ankylosis family
• Springer s41439-023-00236-x
• DOAJ, Case Reports in Genetics 2019
• DisGeNET C0039075
• encyclopedia.pub entry
• SAIL concept library: MONDO:0000151

Reference Validation

Checked with linkml-reference-validator 0.3.0.

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:

1 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:17668388 (abstract only): "do not indicate a major loss of function … Rather, they presumably alter NOG's ability to bind to BMPs and growth-differentiation factors (GDFs) in a subtle way"
• closest text in source: "Rather, they presumably alter NOG's ability to bind to BMPs and growth-differentiation factors (GDFs) in a subtle way, thus disturbing the intricate balance of BMP signaling"
• PMID:33308208: "No obvious abnormalities were found … on ear computed tomography"
• closest text in source: "No obvious abnormalities were found in acoustic conductivity tests or on ear computed tomography (Fig"

Term Validation

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

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:0005615 (obsolete extracellular space) (1 mention) - replaced by GO:0005576

54 of 55 terms resolved to a current term; the rest could not be looked up either way.