An actively mobile accessory digit arising from the dorsum of the foot: an unusual example of polydactyly

Clinical Genetics of Polydactyly: An Updated Review

Polydactyly, also known as hyperdactyly or hexadactyly is the most common hereditary limb anomaly characterized by extra fingers or toes, with various associated morphologic phenotypes as part of a syndrome (syndromic polydactyly) or may occur as a separate event (non-syndromic polydactyly). Broadly, the non-syndromic polydactyly has been classified into three types, i.e.; preaxial polydactyly (radial), central polydactyly (axial), and postaxial polydactyly (ulnar). Mostly inherited as an autosomal dominant entity with variable penetrance and caused by defects that occur in the anterior-posterior patterning of limb development. In humans, to-date at least 10 loci and six genes causing non-syndromic polydactyly have been identified, including the ZNF141, GLI3, MIPOL1, IQCE, PITX1, and the GLI1. In the present review, clinical, genetic and molecular characterization of the polydactyly types has been presented including the recent genes and loci identified for non-syndromic polydactyly. This review provides an overview of the complex genetic mechanism underlie polydactyly and might help in genetic counseling and quick molecular diagnosis.

Polydactyly: phenotypes, genetics and classification

Polydactyly is one of the most common hereditary limb malformations featuring additional digits in hands and/or feet. It constituted the highest proportion among the congenital limb defects in various epidemiological surveys. Polydactyly, primarily presenting as an additional pre-axial or post-axial digit of autopod, is a highly heterogeneous condition and depicts broad inter- and intra-familial clinical variability. There is a plethora of polydactyly classification methods reported in the medical literature which approach the heterogeneity in polydactyly in various ways. In this communication, well-characterized, non-syndromic polydactylies in humans are reviewed. The cardinal features, phenotypic variability and molecular advances of each type have been presented. Polydactyly at cellular and developmental levels is mainly a failure in the control of digit number. Interestingly, GLI3 and SHH (ZRS/SHH enhancer), two antagonistic factors known to modulate digit number and identity during development, have also been implicated in polydactyly. Mutations in GLI3 and ZRS/SHH cause overlapping polydactyly phenotypes highlighting shared molecular cascades in the etiology of additional digits, and thus suggesting the lumping of at least six distinct polydactyly entities. However, owing to the extreme phenotypic and clinical heterogeneity witnessed in polydactyly a substantial genetic heterogeneity is expected across different populations and ethnic groups.