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Purpose: Bestrophin-1 gene (BEST1) mutations are responsible for a broad spectrum of human retinal phenotypes, jointly called bestrophinopathies. Canine multifocal retinopathy (cmr), caused by mutations in the dog gene ortholog, shares numerous phenotypic features with human BEST1-associated disorders. The purpose of this study was the assessment of molecular consequences and pathogenic outcomes of the cmr1 (C(73)T/R(25)X) premature termination and the cmr2 (G(482)A/G(161)D) missense mutation of the canine model compared with the C(87)G/Y(29)X mutation observed in human patients., Methods: Dogs carrying the BEST1 mutation were introduced into a breeding colony and used to produce either carrier or affected offspring. Eyes were collected immediately after euthanatization at the disease-relevant ages and were harvested for expression studies. In parallel, an in vitro cell culture model system was developed and compared with in vivo, Results: The results demonstrate that cmr1 and human C(87)G-mutated transcripts bypass the nonsense-mediated mRNA decay machinery, suggesting the AUG proximity effect as an underlying transcriptional mechanism. The truncated protein, however, is not detectable in either species. The in vitro model accurately recapitulates transcriptional and translational expression events observed in vivo and, thus, implies loss of bestrophin-1 function in cmr1-dogs and Y(29)X-affected patients. Immunofluorescence microscopy of cmr2 mutant showed mislocalization of the protein., Conclusions: Molecular evaluation of cmr mutations in vivo and in vitro constitutes the next step toward elucidating genotype-phenotype interactions concerning human bestrophinopathies and emphasizes the importance of the canine models for studying the complexity of the BEST1 disease mechanism.

Purpose: Mutations in bestrophin 1 (BEST1) are associated with a group of retinal disorders known as bestrophinopathies in man and canine multifocal retinopathies (cmr) in the dog. To date, the dog is the only large animal model suitable for the complex characterization and in-depth studies of Best-related disorders. In the first report of cmr, the disease was described in a group of mastiff-related breeds (cmr1) and the Coton de Tulear (cmr2). Additional breeds, e.g., the Lapponian herder (LH) and others, subsequently were recognized with similar phenotypes, but linked loci are unknown. Analysis of the BEST1 gene aimed to identify mutations in these additional populations and extend our understanding of genotype-phenotype associations., Methods: Animals were subjected to routine eye exams, phenotypically characterized, and samples were collected for molecular studies. Known BEST1 mutations were assessed, and the canine BEST1 coding exons were amplified and sequenced in selected individuals that exhibited a cmr compatible phenotype but that did not carry known mutations. Resulting sequence changes were genotyped in several different breeds and evaluated in the context of the phenotype., Results: Seven novel coding variants were identified in exon 10 of cBEST1. Two linked mutations were associated with cmr exclusive to the LH breed (cmr3). Two individuals of Jämthund and Norfolk terrier breeds were heterozygous for two conservative changes, but these were unlikely to have disease-causing potential. Another three substitutions were found in the Bernese mountain dog that were predicted to have a deleterious effect on protein function. Previously reported mutations were excluded from segregation in these populations, but cmr1 was confirmed in another mastiff-related breed, the Italian cane corso., Conclusions: A third independent canine model for human bestrophinopathies has been established in the LH breed. While exhibiting a phenotype comparable to cmr1 and cmr2, the novel cmr3 mutation is predicted to be based on a distinctly different molecular mechanism. So far cmr2 and cmr3 are exclusive to a single dog breed each. In contrast, cmr1 is found in multiple related breeds. Additional sequence alterations identified in exon 10 of cBEST1 in other breeds exhibit potential disease-causing features. The inherent genetic and phenotypic variation observed with retinal disorders in canines is complicated further by cmr3 being one of four distinct genetic retinal traits found to segregate in LH. Thus, a combination of phenotypic, molecular, and population analysis is required to establish a strong phenotype-genotype association. These results indicate that cmr has a larger impact on the general dog population than was initially suspected. The complexity of these models further confirms the similarity to human bestrophinopathies. Moreover, analyses of multiple canine models will provide additional insight into the molecular basis underlying diseases caused by mutations in BEST1.