PRA (GPRA) in Shetland Sheepdogs

Background of my PRA investiagations
- Simon Petersen-Jones -

I have studying the molecular genetics of PRA for the last 10 years. My group developed the first DNA test for PRA. This was in the Irish setter breed. We have also identified the cause of PRA in the
Cardigan Welsh corgi and developed a DNA-based test for that form of PRA which has enabled breeders to ensure that no more affected dogs are produced. The test also allows the use of carrier dogs in breeding, thus saving the good features from such dogs while ensuring that PRA does not occur. Eventually this test will enable the complete eradication of PRA from the breed.

We are currently investigating PRA in about 12 different breeds of dog and looking and many potential candidate genes in those breeds. We are able to offer two main different approaches to investigate PRA in any given breed.

1. Firstly we can use a candidate gene approach. This is where we have selected several different genes that could potentially cause PRA and developed markers for those genes. The markers are polymorphic in several different breeds of dog. This enables us to see if that particular gene locus is linked to the PRA causing locus in any given breed of dog. So when we look at PRA in a new breed once we have sufficient DNA samples we can go through our list of candidate gene loci and see if any are linked to the disease in that breed.

2. Secondly we can use conventional gene mapping. In this approach instead of just looking at our candidate gene loci we use DNA markers spread across the entire genome (rather than markers selected only to specific gene locations) to see which are linked to the disease locus. This approach is much more expensive but has the advantage over the first approach that it will eventually find linkage to the disease locus. With the first approach if the disease is not linked to any of your candidate gene loci the second approach must then be used. Once linkage is established it may be possible to use a linked marker for a diagnostic test (although there are several potential problems with this approach). Also it is possible to see which retinal genes are close to the marker and then investigate those genes are candidate for causing PRA in your breed.

PRA in Shetland Sheepdogs in Scandinavia

PRA with an age of onset between 3 and 5 years of age has been recognized as a problem in Shetland sheepdogs in Scandinavia. This would appear to be inherited in an autosomal recessive fashion, this is by far the commonest mode of inheritance of PRA in the dog. This mode of inheritance makes it difficult to eradicate the problem from the breed. Carriers of PRA do not develop the disease themselves but can pass the PRA gene on to their offspring. If a dog receives the PRA gene form both parents it will develop the disease. Carriers are often only identified (if at all) after they have been bred several times and with the later-onset form of PRA affected dogs have often been bred from prior to the disease being identified. These facts make autosomal recessive PRA very difficult to eradicate.

Development and use of a DNA-based test that can identify affected dogs and carriers at an early age makes eradication of the disease easy. Identification of the gene mutation and development of a PRA test is not easy. It requires that DNA samples are collected from several affected dogs and all of the immediate relatives (siblings, parents, grandparents and offspring – where available). If extended pedigrees can be built up where the disease status of the dogs is known gene searching if feasible. These techniques are expensive – we estimate that the second approach costs about $30,000 per year and may take 3 years or longer. The first approach can be performed for much less cost, which is why we most frequently use this approach. Basically the more money and samples available the more likely it is that the search will be successful.

Simon Petersen-Jones
Assistant Professor, Comparative Ophthalmology
Department of
Small Animal Clinical Sciences
Michigan State University
Phone: 517 353 3278

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