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A.V. Kukekova, M.A. Richardson, J.L. Johnson, J.A. Jordan, G.D. Aguirre, G.M. Acland; Fine Mapping of the Canine Rod Cone Dysplasia Type 2 Locus (rcd2) . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3276.
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© ARVO (1962-2015); The Authors (2016-present)
Canine rod cone dysplasia type 2 (rcd2) is an early onset autosomal recessive form of PRA that segregates in the collie breed. The rcd2 locus has been mapped to canine chromosome 7, the ortholog of human 1q32. CRB1, the only known retinal degeneration gene near this region, has been excluded as a positional candidate for rcd2. This indicates that rcd2 represents a novel retinal degeneration gene.
To fine map the rcd2 interval.
Linkage and linkage disequilibrium (LD) mapping approaches were applied.
25 additional rcd2 informative pedigrees (138 individuals in informative generations) have been genotyped with gene specific microsatellite markers from the rcd2 interval. Linkage mapping of these new pedigrees allowed reduction of the rcd2 interval to approximately 2 Mb. To further reduce the rcd2 interval, an LD mapping approach was applied. 43 informative SNPs were retrieved from the canine SNP database (http://www.broad.mit.edu/mammals/dog/snp) amplified and sequenced for a subset of unrelated or distantly related rcd2 affected and carrier dogs to identify ancestral haplotype(s) in phase with the rcd2 mutation. Allelism of rcd2 disease among these unrelated affected individuals was confirmed by experimental breeding. The average interSNP distance for the rcd2 interval is now 45 Kb with the biggest gap 84 Kb. The history and the structure of canine breeds favor the existence within breeds of extended LD regions which often exceed 400 Kb. Within small families with recent common ancestors, LD surrounding the rcd2 region was extensive (> 2Mb), but comparison of haplotypes of rcd2–affected dogs with no known recent common ancestors, did not identify a common haplotype within the rcd2 interval.
The rcd2 candidate region was reduced to a 2 Mb interval by meiotic mapping but, unexpectedly, LD mapping failed to reveal a consistent SNP haplotype common to all rcd2–bearing chromosomes. Absence of an extended common SNP haplotype might indicate that the rcd2 trait is caused by multiple independently arising mutations in the same gene within this breed. Alternatively, the history of multiple strenuous test breeding programs to eliminate the rcd2 mutation within the breed might have dramatically reduced the extent of LD across the rcd2 interval. Retina expressed genes from the region of the interest are being evaluated as positional candidates for rcd2.
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