Canine Multi-focal Retinopathy

Canine Multi-focal Retinopathy

Background:

The OptiGen® CMR test is a DNA-based test that accurately diagnoses multi-focal retinopathy occurring in Mastiffs, Great Pyrenees and Coton de Tulear. The test also detects CARRIERS of this condition and clears dogs that are genetically NORMAL.

Canine Multi-focal Retinopathy (CMR) is a recently identified recessively inherited eye disease known so far to affect the Mastiffs (English, Bullmastiff, French mastiff or Dogue de Bordeaux), Great Pyrenees and Coton de Tulear. Early clinical studies in 1998 by Dr. Bruce Grahn at the University of Saskatchewan, Canada, first described CMR in the Great Pyrenees. The condition observed in each of the named breeds at an ophthalmologist’s exam includes numerous distinct (i.e. multi-focal), roughly circular patches of elevated retina with accumulation of material that produces gray-tan-pink colored lesions. These lesions, looking somewhat like blisters, vary in location and size, although typically they are present in both eyes of the affected dog. Discrete areas of tapetal hyper-reflectivity might also be seen.

The disease generally develops in young dogs before 4 months and might progress slowly, might appear to heal, or might even appear and then go away again. Some lesions disappear with no remaining sign, while some lesions leave a wrinkled area – a fold. Some leave the lasting lesion of a blister formation. Most dogs exhibit no noticeable problem with vision despite their abnormal appearing retinas. And in almost all cases, CMR does not progress significantly over time. The disease seems to have a consistent pattern among the breeds identified so far, although lesions in the Coton de Tulear are often more serious and seem to remain longer than in some of the other CMR-affected breeds. In rare severe cases, the clinical diagnosis could be confused with progressive retinal atrophy (PRA). The full range of clinical symptoms will learned as more dogs are tested for their genetic status.

The clinical presentation and pathology of CMR closely resembles lesions of “Best vitelliform dystrophy”, a human disease with variable clinical expression but usually with serious affects on central vision. Identification of the gene mutation responsible for CMR was based on these similarities. A mutation in the human VMD2 gene – Vitelliform Macular Dystrophy 2 Gene – causes dominantly inherited human Best Disease. Analysis of the canine version of the VMD2 gene indicates that mutations in it cause CMR as a recessively inherited canine condition. The normal form of the VMD2 gene produces a protein named “bestrophin”. The bestrophin protein assembles, in the cells of the retinal pigment epithelium, in a group of four or five units that form a pore through which chloride ions pass.

Our current understanding is that CMR is inherited in an autosomal recessive pattern. This means the gene mutation responsible for CMR is located on an autosome (that is, a chromosome that is not a sex chromosome) and CMR disease results when the gene mutation is passed to the offspring by both the mother and the father. It should be noted that the human disease that mirrors CMR in dogs is an autosomal dominant disease with incomplete penetrance. This means that sometimes, but not always, only one copy of the disease gene needs to be present in order for the disease to be observed clinically. At this point CMR in dogs is NOT considered to be an autosomal dominant disease however as more animals are characterized genetically with the CMR test, it is possible that we will find a similar form of inheritance as is seen in humans.

There is complete concordance of the mutation with the disease among affected dogs in the Mastiffs, Great Pyrenees and Coton de Tulear. However, retinal dysplasia described in other breeds, for example in Labradors, Samoyeds or English Springer Spaniels, is very distinct in comparison to CMR and these conditions are not caused by the CMR mutation.

Due to the abnormal appearance of the CMR-affected retina, CERF, ACVO, ECVO and other ophthalmologist’s eye exam reports typically record these multi-focal lesions as “retinal dysplasia” or “retinal folds”, to denote a defect in formation of the retina. Such findings might disqualify the dog from breeding. Presently CERF doesn’t list CMR as a specific condition, but does fail a dog for “retinal dysplasia/retinopathy – folds, detached.”

The genetic test for CMR is valuable for identifying the cause of a retinal deformation. Given the exact genetic diagnosis, the owner can be reassured that there probably will be little or no vision loss due to this condition. All the same, future cases of the condition can be prevented using the CMR test as an information tool for breeding.

Read more about CMR & how we can stamp it out with proper breeding practices here


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