Syndrom der multiplen Synostosen

Multiple synostoses syndrome (SYNS), proximal symphalangism (SYM), tarsal-carpal coalition (TCC) syndrome, stapes ankylosis with broad thumbs and toes (SABTT), and brachydactyly B2 (BDB2) are overlapping autosomal dominant conditions united by typically displaying ankylosis of the proximal interphalangeal (PIP) joints, and carpal and tarsal bones.

Pathology

They all typically display ankylosis of the PIP joints, carpal and tarsal bones. More specifically:

• SYM displays variable fusion of the PIP joints and occasionally metacarpophalangeal joints
• SYNS is distinguished from SYM by more severe joint involvement which may include the hips and vertebrae; affected individuals may have characteristic facial features and at times conductive hearing loss
• TCC has a focus on carpal and tarsal fusion in addition to brachydactyly, varus/valgus foot anomalies
• SABTT is known for its broad thumbs and toes in addition to conductive hearing loss
• brachydactyly is as the name describes

Genetics

Mutations in specific genes have been found to be responsible for more than one syndrome type. For example, different mutations within the NOG locus (the gene encoding for the noggin protein( can result in all five overlapping syndromes  and the same specific mutations in NOG, in different families, has resulted in the separate diagnoses of SYM1, SABTT and BDB2 , and TCC and SYM . Similarly, an allele of GDF5 results in both SYNS and BDA1 in different families . This has led to the suggestion, that these syndromes are part of a spectrum with variable penetrance .

Whilst these syndromes overlap with respect to causative genes, four SYNS subtypes have been described. Each SYNS subtype is associated with a specific genetic mutation. This may suggest that the above-described syndromes (SYNS, SYM, TCC, SABTT and BDB2) could be better classified as variable penetrance of the four subtypes of SYNS.

All four subtypes of SYNS have variable carpal and tarsal fusion . In addition, all multiple synostoses syndromes with the exception of SYN4 display symphalangism in the hands and humeroradial synostoses. Other differentiating features include :

• SYNS1
  • hands and feet: brachydactyly
  • elbow: elbow joint dysplasia
  • spine: cervical vertebral fusions
  • hearing: early conductive hearing loss
  • facial: long face, broad tubular nose, short philtrum, thin upper vermilion, strabismus
  • other: pectus carinatum
• SYNS2
  • hands and feet: brachydactyly
  • spine: vertebral fusions
  • facial: broad hemicylindrical nose
• SYNS3
  • elbow: semi-dislocation or cubital valgus
  • spine: lumbar vertebral fusions
• SYNS4
  •  feet: symphalangism
  •  hearing: late onset (after 40 years) conductive hearing loss

The four genes and the various mutations relating to the SYNS subtypes are listed below.

• SYNS1
  • NOGGIN
  • NOGGIN W205C
  • NOGGIN W217G
  • NOGGIN g58delC (Frameshift) 
  • NOGGIN C232W
• SYNS2
  • GDF5
  • GDF5 (R438L) 
    • initially described as representing SYNS1  but the phenotype is more suggestive of SYNS2 and has thus been categorized as an SYNS2 phenotype mutation
  • GDF5 (N445K, N445T) 
    • initially described as representing SYNS1 but the phenotype is more suggestive of SYNS2 and has thus been categorized as an SYNS2 phenotype mutation
  • GDF5 (W414R) 
• SYNS3
  • FGF9
  • FGF9 (S99N) 
• SYNS4
  • GDF6
  • GDF6 (Y444N) 

Additionally, some patients with features of SYNS additionally have craniosynostosis . Unfortunately, the genetic mutation(s) in these patients were not characterized.

Siehe auch:

• Karpale Koalition
• Tarsale Koalition
• Synostose
• Symphalangismus
• Koalition (tarsal / carpal)