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preaxial polydactyly

Preaxial polydactyly is less frequent than postaxial one (but could be more frequent in Caucasians).

When present, one should search for an associated syndrome (particularly if a triphalangeal thumb is present), such as Holt-Oram syndrome (atrial and ventricular septal defects, abnormal thumb - from absent to triphalangeal), short ribs polydactyly syndromes (SRPS), Carpenter syndrome, trisomy 21, VACTERL association, Fanconi disease, etc.

Classification

- PPD1: preaxial polydactyly type 1 (thumb polydactyly) (MIM.174400)

  • thumb polydactyly involves duplication of one or more of the skeletal components of a biphalangeal thumb. Severity varies from mere broadening of the distal phalanx with slight bifurcation at the tip to full duplication of the thumb including the metacarpals.
    - PPD2 at 7q36: preaxial polydactyly type 2 (polydactyly of a triphalangeal thumb) (MIM.174500)
    - PPD3: preaxial polydactyly type 3: polydactyly of an index finger (MIM.174600)
    - PPD4 at 7p13: preaxial polydactyly type 4: polysyndactyly (MIM.174700) at 7p13: mutation in the GLI3 gene (MIM.165240)

Etiology

- Holt-Oram syndrome (atrial and ventricular septal defects, abnormal thumb - from absent to triphalangeal)
- short ribs polydactyly syndromes
- Carpenter syndrome
- trisomy 21
- VACTERL association
- Fanconi syndrome (Fanconi anemia)
- familial preaxial polydactyly

Pre-axial polydactyly type 2

Pre-axial polydactyly type 2 (PPD2, MIM.174500), Werner mesomelic syndrome (MIM.188770) and Haas polysyndactyly (MIM #186200) are a group of closely related conditions caused by mutations in a long-range Sonic hedgehog (SHH, MIM.600725) regulator called ZRS.

To 2010, 19 point mutations, 10 duplications and 1 triplication of the ZRS associated with those pre-axial polydactylies have been reported in humans, mice, cats and chickens.

Some of these have been shown to cause ectopic expression of SHH in the limb bud in mice, leading to a polydactylous phenotype, but the precise mechanism by which ZRS mutations generate this phenotype remains unknown.

See also

- polydactylies

  • postaxial polydactyly

- - familial preaxial polydactyly

References

- Identification of two novel mutations in Shh long-range regulator associated with familial pre-axial polydactyly. Albuisson J, Isidor B, Giraud M, Pichon O, Marsaud T, David A, Le Caignec C, Bezieau S. Clin Genet. 2010 May 13. PMID: #20569257#

- Lettice LA, Hill RE. Preaxial polydactyly: a model for defective long-range regulation in congenital abnormalities. Curr Opin Genet Dev. 2005 Jun;15(3):294-300. PMID: #15917205#