Preaxial polydactyly (PPD; OMIM 603596), which is characterized as having supernumerary fingers, is an unusual congenital hand abnormality. Triphalangeal thumb (TPT; OMIM 190600) is identified by an extra phalangeal bone and is often found in association with PPD. When in combination, the disease is referred to as PPD type II (PPD II; OMIM 174500). Previous studies have demonstrated that variations in the zone of polarizing activity regulatory sequence (ZRS; chr7:156,583,796-156,584,569; hg19) region are associated with PPD II. In this study, our patient was diagnosed with PPD II, having bilateral thumb duplication and unilateral TPT (on the right hand). Further investigation of possible causative genes identified a de novo heterozygous ZRS mutation (ZRS 428T>A). This novel mutation was neither found in 200 normal controls nor reported in online databases. Moreover, the bioinformatics program Genomic Evolutionary Rate Profiling (GERP) revealed this site (ZRS428) to be evolutionarily highly conserved, and the 428T>A point mutation was predicted to be deleterious by MutationTaster. In conclusion, the affected individual shows bilateral thumb duplication, but unilateral TPT making this case special. Thus, our findings not only further support the important role of ZRS in limb morphogenesis and expand the spectrum of ZRS mutations, but also emphasize the significance of genetic diagnosis and counseling of families with digit number and identity alterations as well.
A novel 13 base pair insertion in the sonic hedgehog ZRS limb enhancer (ZRS/LMBR1) causes preaxial polydactyly with triphalangeal thumb