New DNA test for genetic mutation which causes a blinding condition in Cocker Spaniels - Veterinary Practice
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New DNA test for genetic mutation which causes a blinding condition in Cocker Spaniels

A DNA test has been developed by vets and scientists, after a genetic mutation that causes retinal dysplasia has been discovered for the first time in this breed

An inherited retinal disease, which presents as retinal detachment and blindness in puppies, has been identified in Cocker Spaniels.

Believed to be the canine equivalent of Norrie’s disease in children, it is a severe form of retinal dysplasia, in which the retina never develops properly.

But now, thanks to the work of vets and scientists at the Animal Health Trust (AHT), a DNA test has been developed to help eradicate this very severe form of the disease within the breed before it takes hold.

Hattie Wright, a Vets4Pets-sponsored research assistant within the AHT’s Canine Genetics research team, led the project.

Hattie said: “Usually, dogs affected by retinal dysplasia appear normal or demonstrate subtle symptoms, such as hesitancy in everyday life or clumsiness. It is very often diagnosed as an incidental finding, but in some cases, it can be progressive and result in vision loss.

“The conventional form of retinal dysplasia, for which there is no treatment, is well documented in nine breeds of dogs but was previously unrecognised within Cocker Spaniels.”

The condition was identified when an affected dog was referred to the Ophthalmology Service in the AHT’s Small Animal Referral Centre. It was revealed that another male puppy within the same litter was also affected, alongside two of the dog’s maternal uncles, which lead researchers to suspect the inherited eye disease was caused by a mutation on the “X” chromosome meaning males are far more likely to be affected than females.

As part of her work on the project, Hattie investigated and identified the causal mutation. Results show the mutation is currently rare within the breed, and thanks to this research, it can now be prevented from becoming more common.

“As this condition in Cocker Spaniels is quite rare, we’re not expecting to find many affected dogs through DNA testing, but it is crucial to identify any carrier females in order to prevent more puppies being born with this blinding condition and to stop it becoming more widespread in future,” Hattie continued.

“With use of this new DNA test, the Cocker Spaniel breeding community can safely eradicate this disease from this lovely breed altogether.”

Dr Huw Stacey, director of clinical services at Vets4Pets, said: “This condition can be so debilitating to puppies born with it, especially if it progresses into full vision loss.

“And although it has sadly now been discovered in a new breed, it’s brilliant that AHT has created a DNA test for breeders to use that will hopefully help to eradicate the disease within Cocker Spaniels. This is an especially important breakthrough as there is no treatment for this condition.

“At Vets4Pets, animal welfare is at the heart of everything we do so we are thrilled to be able to support this ground-breaking work.”

How the mutation was found

In order to find the mutation responsible, the AHT used whole genome sequencing technology to study all 2.4 billion letters of DNA from one of the affected Cocker Spaniel littermates and his unaffected mother.

Traditionally, a genetic investigation into a disease of this nature requires DNA samples from at least twelve affected dogs and the same number of healthy control dogs from the same breed. Now, advanced genome sequencing technology and the use of state-of-the-art computer analysis allows every letter of DNA from a single affected dog to be interrogated and compared to a bank of genome sequences of healthy dogs from different breeds, in order to find the change in DNA responsible for a specific inherited disease. This approach allowed the genetics of the disease in Cocker Spaniels to be investigated as soon as the disease was reported in a small number of dogs, without needing to wait until additional cases had been identified.

The causal mutation was quickly identified as a single letter insertion in the 2.4 billion letter DNA code. Once this mutation had been pinpointed, it was confirmed by comparison against the genotypes of more than 1,200 different dogs from the UK and Europe, including those stored in the AHT’s Canine Genome Bank, which is the largest in the UK. The Cocker Spaniel affected with retinal dysplasia was the only male dog that carried this mutation. Some of the female relatives carried single copies of the mutation, but because the mutation is on the X chromosome carrier, females are unaffected, whereas males with a single copy are clinically affected.

The DNA test for retinal dysplasia in Cocker Spaniels, costing £48, will be available to purchase from the AHT DNA Testing Service in January 2020.

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