page_title: Polydactyly
section_title: Causes
hierarchical_section_title: Polydactyly / Causes
caption_attribution_description: English: Polydactyly ectopic Shh expression at Hemingway Mutant
context_page_description: Polydactyly or polydactylism, also known as hyperdactyly, is an anomaly in humans and animals resulting in supernumerary fingers and/or toes. Polydactyly is the opposite of oligodactyly.
context_section_description: Polydactyly is associated with different mutations, either mutations in a gene itself or in a cis-regulatory element responsible for the expression of a specific gene. Mutations in Hoxa- or Hoxd clusters are reported leading to polydactyly. Interactions of Hoxd13 and GLI3 induce synpolydactyly, a combination of extra and consolidated digits. Other signal transduction pathways in this context are the Wnt signaling pathway or Notch.
  In the specific case of preaxial polydactyly (Hemingway mutant), a cis-acting mutation approximately 1Mb upstream of Shh gene has been implicated. Normally Shh is expressed in an organiser region, called the zone of polarizing activity (ZPA) on the posterior limb side. From there it diffuses anteriorly, laterally to the growth direction of the limb. In the mutant, smaller ectopic expression in a new organiser region is seen on the anterior side of the limb. This ectopic expression causes cell proliferation delivering the raw material for one or more new digits.
  In addition to the study of genetic causes of polydactyly limb patterning models are used to simulate the congenital disorder at the limb, being able to explain the development paths of polydactyly.
  Polydactyly can occur by itself, or more commonly, as one feature of a syndrome of congenital anomalies. When it occurs by itself, it is associated with autosomal dominant mutations in single genes, i.e. it is not a multifactorial trait. But mutation in a variety of genes can give rise to polydactyly. Typically the mutated gene is involved in developmental patterning, and a syndrome of congenital anomalies results, of which polydactyly is one feature or two.
  Types include:
  97 genetic syndromes have been associated with different kinds of polydactyly.
caption: Preaxial polydactyly: Ectopic Shh-expression, Hemingway mutant, mouse, right forelimb