Identification of two novel mutations in Shh long-range regulator associated with familial pre-axial polydactyly

Abstract

Pre-axial polydactyly type II (PPDII, 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 date, 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. We present two PPDII families with fully penetrant point mutations in ultra-conserved predicted binding sites for transcription factors SOX9 and PAX3, two possible candidates for regulating SHH expression. Screening for point mutations or copy-number variation of the ZRS, high-resolution array-CGH, and screening of other conserved non-coding sequences (CNS) surrounding SHH in a third family are negative. This is the sixth PPDII pedigree with possible linkage to 7q36 that presents with no detectable ZRS mutation. We hypothesize that another nearby regulatory sequence, or an undetected position effect between ZRS and SHH, could be responsible for negative familial cases linked to 7q36.