Achromatopsia Caused by Genomic CNGB3 Deletion Identical by Descent in Multiple Canine Breeds

Abstract

Purpose: : To report clinical phenotype and molecular genetic analysis of CNGB3 achromatopsia in a Miniature Australian Shepherd (MAS), and to screen for the affected allele in other breeds.

Methods: : One affected, two-years-old male-neutered MAS from a family of dayblind dogs underwent complete ophthalmic examination. Both scotopic and photopic ERGs were recorded. DNA was evaluated for mutations in the CNGB3 gene that are known to cause achromatopsia in dogs (genomic deletion and D262N missense mutation). PCR sequencing for known single nucleotide polymorphisms (SNPs) was performed on genomic DNA to establish the haplotype surrounding the CNGB3 locus in the MAS and to compare it to a purebred Alaskan Malamute (AM) affected with achromatopsia cause by the genomic CNGB3 deletion, as well as a colony-AM derived dog affected with the same disease. The DNA was also tested through the Mars Wisdom Panel to determine breed association. Screening for the affected allele in the Australian Shepherd, MAS, Siberian Huskies, Samoyeds and other Alaskan-sled dogs (sprints runners and distance runners) was done by pooling DNA samples for PCR reaction.

Results: : The affected MAS showed the typical achromatopsia phenotype: The dog performed normally under dim light conditions, but showed clear signs of visual impairment in daylight. Ophthalmic examination was normal. ERGs revealed normal rod function but the absence of cone function. The Mars Wisdom Panel analysis places the dog as a purebred MAS . Genotyping revealed that the dog was homozygous for a complete genomic deletion of the CNGB3 gene. Sequencing the affected PCR product showed that the genomic deletion was identical to the one found in achromatopsia-affected AM dogs and 404,820bp in size. Pooling screening identified one Siberian Husky and three sled- dogs carrying the same affected allele. To establish if the 3 breeds (AM, MAS, and Siberian Husky) carrying the mutated allele were identical by descent (IBD), 147 SNPs were identified in a 3.9 Mb interval and confirmed an identical haplotype to the purebred affected AM in a 1.9 Mb interval.

Conclusions: : We report the occurrence of achromatopsia in an additional canine breed, the MAS. The phenotype is based on the identical genomic CNGB3 deletion that was previously reported in the AM dog. Since this mutation affects multiple breeds of dogs, it may represent a founder effect.

Keywords: genetics • gene screening • retina: distal (photoreceptors, horizontal cells, bipolar cells)