cord1 PRA Update November 2009
Summary of Presentations made by Dr. Cathryn Mellersh and Keiko Miyadera at the Dachshund Breed Council’s Conference: 29th November 2009.
The number of MLHDs being tested has levelled out to around 200 per year (2007-2009) and MSHDs has remained at a level of around 270 per year over the same period. In total, 1366 MLHDs and 815 MSHDs have been tested since 2005, although MSHD testing only started in 2007.
In MLHDs the percentage of cord1 “affected” dogs has dropped from 18% to 5% between 2005 and 2009. Approx. 40% of MLHDs are now “Clear” of the mutation.
In MSHDs, the percentage of cord1 “affected” dogs has dropped from 26% in 2007 to 14% in 2009. The number of “Clear” dogs has increased from 28% to 38% in that same period.
In both MLHDs and MSHDs not all dogs that have two copies of the cord1 mutation (“Affected” dogs) develop sight problems at the same age and some dogs don’t develop clinical symptoms until middle to late age and in some others, visual problem could be unnoticed by the owner within the dog’s lifetime. This suggests some additional factors are impacting on what would be expected for early onset PRA.
In collaboration with David Sargan at the University of Cambridge Vet School the causes of this variation have been investigated by Keiko Miyadera and Claudia Busse.
Claudia Busse’s clinical examinations using Electroretinogarpahy (ERG) have shown the earliest signs of retinal degeneration in apparently normal dogs. She showed that there was a significant decline in ERG response in dogs that had been DNA tested as “Affected”. It is likely that dogs with no apparent visual dysfunction on routine clinical assessment could develop visual deficits in later life. Even if a dog looks normal to its owner, ERG does demonstrate reduced retinal response in cord1 “Affected” dogs.
Keiko Miyadera’s search for a genetic modifier has identified an extra single locus that determines early onset PRA in cord1 “Affected” dogs. It is now believed that cord1“Affected” dogs undergo retinal degeneration that could result in late-onset PRA in which blindness becomes apparent at any mid to late age, up to 15 years old. Dogs that have the second mutation as well as being cord1 “Affected” will develop early-onset PRA. A DNA test for this second mutation is not yet available. Read the Research Paper.
Breeding strategies to reduce the prevalence of cord1 mutations in the breed will reduce the occurrence of PRA (in either late or early-onset forms). The recommendation is still to use at least one “Clear” parent and to have regular clinical eye checks done on dogs’ eyes.