Progressive retinal atrophy (PRA) is a group of inherited diseases that causes progressive degeneration of the light-sensitive cells at the back of the eye. Dogs with PRA have normal vision at birth, but by the age of four or five they will be completely blind. There is no treatment.
Now a team led by the University of Cambridge has identified the genetic mutation that causes PRA in English sheepdogs and developed a DNA test for it. Identifying dogs that are carriers of the disease before their vision deteriorates provides a tool to guide breeding decisions so that the disease is not passed on to puppies.
Owners usually don’t realize their dog has PRA until he is middle-aged, by which time he may have reproduced and passed on the defective gene to his puppies. This makes it a difficult disease to control.
The new discovery means that progressive retinal atrophy can now be completely eliminated from the English Sheepdog population very quickly.
The results are published today in the journal Genes.
“Once the dog’s vision starts to fail, there is no treatment – he will eventually become completely blind,” said Katherine Stanbury, a researcher at the University of Cambridge’s Department of Veterinary Medicine and first author of the report.
She added: “Now that we have a DNA test, there is no reason why another English sheepdog should ever be born with this form of progressive retinal atrophy – it gives breeders a way to eliminate the disease completely.”
The genetic mutation identified by the team is recessive, meaning it will only cause blindness if the English Sheepdog inherits two copies of it. If the dog has only one copy, it is a carrier; he will not develop PRA, but can pass the mutation on to his pups. If two carriers are bred together, approximately one in four puppies will develop PRA.
Dog breeds are highly inbred, with many individuals being related, making them much more likely to be affected by recessive diseases than humans.
The team began the investigation after being contacted by a distraught English Sheepdog owner who had recently been diagnosed with PRA. The dog worked as a search and rescue dog, but had to withdraw due to visual decline that led to total blindness. The researchers put out a call for DNA samples from other owners or breeders of this breed and received samples from six English Shepherds with PRA and twenty without. This was enough for them to pinpoint the genetic mutation responsible for PRA using whole genome sequencing.
The team offers a commercial dog genetic testing service that provides DNA testing to dog breeders to help them avoid breeding dogs that will develop hereditary diseases. As part of this, they will now offer a DNA test for progressive retinal atrophy in English Shepherds. Anyone can buy a test kit, which costs just £48, to take a swab from their dog’s mouth and send it back for testing.
“An owner won’t necessarily notice that their dog has something wrong with his eyes until he bumps into the furniture. Unlike people who say something when their vision is not good, dogs should just get on with it,” said Dr Cathryn Mellersh of the University of Cambridge’s Department of Veterinary Medicine, lead author of the report.
She added: “For the price of a decent bag of dog food, people can now have their English Shepherd tested for progressive retinal atrophy before breeding. It’s about prevention, rather than cure, and it means the world to the people who breed these dogs. They no longer have to worry about whether the puppies will be healthy or will develop this terrible disease in a few years.”
The English Shepherd is a herding dog breed popular in the United States and closely related to the Border Collie.
The new discovery is the thirty-third genetic mutation causing an inherited disease in dogs that the team has found – twenty-three of which cause eye diseases. They say the health and welfare of many dogs have been compromised because of the way they have been bred by humans.
PRA occurs in many dog breeds, including the English Sheepdog. And it is similar to a disease called retinitis pigmentosa in humans, which also causes blindness. The researchers say their work with dogs could shed light on the human version of the disease and potentially identify targets for gene therapy in the future.
