The Expression of Preaxial Polydactyly Is Influenced by Modifying Genetic Elements and Is Not Maintained by Chromosomal Inversion in an Avian Biomedical Model

Abstract

Polydactyly (Po) is a common mutation found in many vertebrates. The UCD-Po.003 congenic chicken line was previously characterized for Po inheritance (autosomal dominant) and the mutation was mapped to chromosome 2p. Here, we describe for the first time the range and variability of the phenotype in this congenic line. Further, we studied the hypothesis that a chromosomal inversion was responsible for the maintenance of a large (6.3 Mb) candidate gene region. Fluorescence in situ hybridization employing BACs encompassing a 10.7-Mb region of GGA2p showed that the Po chromosome was normal, i.e. exhibits the wild-type BAC order. Continued fine-mapping along with a change in breeding strategy reduced the size of the causative region to 1.43 Mb. Recent research indicates that the cause of preaxial Po resides within a 794-bp highly conserved zone of polarizing activity regulatory sequence (ZRS) element located in intron 5 of the LMBR1 gene; however, the ZRS polymorphism of interest is found in some but not all breeds of polydactylous chicken. Therefore, we sequenced the ZRS in 101 heterozygous and 30 unaffected (wild-type) individuals to establish the relevance of this region to the Po condition in the UCD-Po.003 congenic line. A single point mutation (C/A at coordinate GGA2p: 8,414,121) within the ZRS segregated with carrier status. The polydactylous UCD-Silkie line also maintains this SNP in addition to a single base deletion. An inheritance analysis of the phenotypic variation in UCD-Po.003 suggests recessive epistasis as the mode of inheritance for the additional modifying genetic elements, residing outside the ZRS, to impact the preaxial polydactyl phenotype. These results contribute to our understanding of the cause of Po in an important vertebrate model.

Genome-Wide Linkage Analysis and Association Study Identifies Loci for Polydactyly in Chickens

“…Kalsoom et al 2013 have recently identified a novel zinc-finger gene ( ZNF141 ) associated with autosomal-recessive postaxial polydactyly type A through whole-genome sequencing combined with homozygosity mapping and array comparative genomic hybridization analysis. These studies found the gene for one polydactyly allele/locus, but the molecular mechanism and other polydactyly alleles/loci remain incompletely understood (Robb and Delany 2012). …”

Association of SNP rs80659072 in the ZRS with polydactyly in Beijing You chickens

“…In the present study, the ZRS in GGA2 was sequenced in Beijing You chickens, and no other mutations except for SNP rs80659072 were detected, suggesting that the ZRS sequence is highly conserved [30]. The association analysis showed the G/T polymorphism of rs80659072 was significantly associated with polydactyly in the mass Beijing You population ( P <0.0001).…”

“…Zhang et al [4] documented that polydactyly phenotypes in European and Chinese chickens were caused by two closely located but independent mutation events. Robb et al [30] conducted an inheritance analysis of the phenotypic variation in a congenic polydactyly chicken line and suggested that additional modifying genetic elements located outside of the ZRS could impact the preaxial polydactyly phenotype. Besides, in Dorking, Bouldin and Harfe [32] disclosed that preaxial polydactyly was initiated by the up-regulation of fibroblast growth factor 4 ( Fgf4 ) expression instead of Shh .…”

“…The phenotypic variability might be controlled by either genetic or epigenetic mechanisms or both [30]. In addition, incubation temperatures also influenced the expression of a polydactylous phenotype in many chicken breeds [28, 30, 34]. Hence, further studies are recommended to unravel the mechanisms behind the polydactyly phenotype in Beijing You chickens.…”

Defining the Sequence Elements and Candidate Genes for the Coloboma Mutation

“…The classic chick embryo model (see Stern [5]), with its in ovo easy access, provides great value in this regard [6]–[10]. The utility of the chick embryo model was further improved by the availability of the chicken reference genome sequence [11], advanced tools and high-throughput technologies, as well as the existence of well-phenotyped developmental mutations [4], [12]–[14]. Such advantages led to the National Institute of Health recognition of the chicken as a model organism for biomedical research (http://www.nih.gov/science/models/gallus/).…”