Genetic basis of CMR3 - is an autosomal recessive inherited retinal disease caused by mutations in the BEST1 gene (Bestrophin 1). Specific mutations linked to CMR3 include a deletion (c.1388delC) and a substitution (c.1466G>T) in exon 10 of BEST1. Dogs must inherit two copies of the mutated gene (homozygous) to be affected. Carriers with one copy do not develop the disease but can pass the mutation to offspring. CMR3 has been documented in Nordic breeds such as the Finnish Lapphund, Lapponian Herder, and Swedish Lapphund.
Pathophysiology - The mutation causes multiple areas of retinal detachment and retinal pigment epithelial abnormalities. Lesions appear as multiple tan, pink, gray, orange, or raised “blisters” in both the tapetal and non-tapetal areas of the retina. The condition is non-progressive or slowly progressive, with some lesions shrinking or healing as the dog matures. Vision is usually not significantly impaired, though mild vision loss may occur in severe cases.
Complications - Typical onset of retinal lesions is between 9 weeks and 2 years of age. Most affected dogs are asymptomatic; signs of visual impairment are rare but may include bumping into objects or reduced ability to track moving objects. Retinal lesions remain stable or improve over time in many affected dogs. Blindness is uncommon but has been reported in rare severe cases.
Why This Matters to Breeders and Vets - Genetic testing is essential for identifying carriers and affected dogs to prevent producing affected puppies. Understanding CMR3 helps avoid misdiagnosis of retinal disease and supports monitoring of affected dogs. Breeders can use testing to inform mating decisions and reduce disease prevalence while maintaining genetic diversity.