Canine Multifocal Retinopathy (CMR) is an autosomal recessive disease that affects Pyrenees, English Mastiffs, Bullmastiffs, Coton de Tulear and other related breeds. Autosomal recessive disorders are disorders that can be passed from either parent and require two copies of the gene to show symptoms.
CMR can cause of range of symptoms from minor retinal folding to detached retinas and is very similar to Best Macular Dystrophy disease (BMD) in humans. Both BMD and CMR are retinal disorders caused by a mutation in the VMD2 gene (Vitelliform Macular Dystrophy 2 Gene). The VMD2 gene is responsible for coding a protein called bestrophin. Bestrophin is responsible for the correct formation of pigment epithelium (a thin tissue similar to skin) in the retina. Mutations in VMAD2 gene cause pigment epithelium atrophy and a potential reduction of sight.
In affected puppies, symptoms of CMR disease usually begin before 4 months of age. Rose-grey colored lesions of different sizes and shapes occur in both eyes. Total blindness usually develops as the dog matures.
Because CMR2 is a recessive disorder, a dog must have two copies of the mutation in order for the disease to manifest. This means that a dog can have one copy of the mutation and not experience any signs or symptoms of CMR2. This dog would be known as a carrier. The carrier can then pass on either the normal gene or the mutated gene to any offspring. If two carriers are bred, there is a 25% per puppy that they will develop symptoms of CMR2. Additionally, since retinal defects can be caused by other conditions, testing can verify that a dog actually has CMR rather than some other eye condition.
|Affected: Dog has two copies of the CMR2 mutation and may develop symptoms of the disorder. The dog will always pass on the mutation to all offspring.
|Carrier: Dog carries one copy of the Canine Multifocal Retinopathy Type 2 mutation and may pass it on to any offspring.
|Clear: Dog is negative for the mutation associated with CMR2.
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Zangerl B, Wickström K, Slavik J, Lindauer SJ, Ahonen S, Schelling C, Lohi H, Guziewicz KE, Aguirre GD. Assessment of canine BEST1 variations identifies new mutations and establishes an independent bestrophinopathy model (cmr3). Mol Vis. 2010 Dec 16; 16:2791-804. [PubMed: 21197113]