EMBRYOLOGY, GENETICS, ORTHOPAEDICS
Definition Polydactyly—preaxial 2 is a form (OMIM:174500) of polydactyly characterised by duplication of the distal phalanx. The thumb is usually opposable and has a normal metacarpal.
Molecular pathology Defects in LMBR1, a gene with an SHH regulatory element—ZRS that resides in LMBR1’s intron 5, cause both polydactyly—preaxial 2 and triphalangeal thumb-polysyndactyly syndrome.
*SHH is the gene that encodes Sonic Hedgehog, a protein central to early embryonic patterning for the anterior-posterior limb axis, the ventral somites and ventral neural tube.