Progressive Retinal Atrophy in the Sloughi

What is Progressive Retinal Atrophy? 
      Progressive Retinal Atrophy, or PRA, is a hereditary blinding disorder found in most purebred dogs. PRA is a degenerative disease of the retina. The retina, a tissue located inside the back of the eye, contains specialized cells or photoreceptors which absorb the light focused on them by the eye's lens. These photoreceptors convert that light into electrical nerve signals. The nerve signals from the retina are passed through the optic nerve to the brain where they are perceived as vision. The retinal photoreceptors are of two kinds: rods for night vision, and cones for day and color vision. PRA usually affects the rods first, leading to poor vision in darkness and twilight, and then cones in later stages of the disease when day vision becomes impaired too. As their vision deteriorates, affected dogs adjust to their handicap by relying on other sensory modalities (touch, hearing, smell) as long as their environment remains constant. As the disease progresses, the pupils of their eyes become noticeably very "shiny" and the lens of their eyes may become opaque sometimes resulting in a cataract. In humans a similar disorder is called retinitis pigmentosa. 

When was Progressive Retinal Atrophy identified in the Sloughi?
      Isolated cases of Sloughis becoming "early blind" have occurred occasionally during the past 25 years of breeding in Europe. However, it was not before summer of 2000 that PRA was properly diagnosed and the genetic defect responsible for it (an 8-bp insertion in exon 21 of the PDE6B gene) identified in the breed by a German team of scientists: Gabriele Dekomien, Maren Munte, Rene Gödde and Jörg Thomas Epplen of the department of Molecular Human Genetics, Ruhr University, in Bochum, Germany. 

How is Progressive Retinal Atrophy in the Sloughi characterized?
      The mode of inheritance and the age of onset of PRA vary tremendously from breed to breed. In the Sloughi, PRA has a late onset, and affected dogs appear normal when young but develop PRA as adults. From the few cases that are known, it seems that PRA starts around the age of 2 to 3 years and develops slowly over the following years. There are individual differences and some Sloughis affected with PRA might develop the disease faster than others. 

How is Progressive Retinal Atrophy in the Sloughi inherited?
      In the Sloughi, both dogs and bitches can inherit and pass on PRA to their offspring. It is inherited recessively in the Sloughi. Sloughis can be genotyped as being homozygous for the defect gene (PRA affected), heterozygous for the defect gene (carrier for PRA) or homozygous for the healthy gene (PRA clear). PRA carriers and PRA clear Sloughis are healthy and will never develop the disease. However PRA carriers can pass on the disease to their offspring if they are bred to another carrier or a dog affected with PRA. More specifically, the laws of inheritance for autosomal recessive inheritance tell us the following about breeding: 
1) 2 Sloughis affected with PRA bred with each other will result in 100% affected offspring. 
2) One affected Sloughi bred with a carrier will result in 50% affected offspring and 50% carriers. 
3) One affected Sloughi bred with a PRA clear Sloughi will result in 100% carriers. 
4) One carrier bred with another carrier will result in 50% carriers, 25% affected with PRA, and 25% PRA clear Sloughis. 
5) One carrier bred with a PRA clear Sloughi will result in 50% carriers and 50% PRA clear Sloughis. 
6) One PRA clear Sloughi bred with another PRA clear Sloughi will result in 100% PRA clear offspring. 
      Of course these percentages are calculated on a potential of 100 puppies resulting from each breeding. In reality, the percentages may vary somewhat from litter to litter in scenarios 2, 4 and 5. 

What to do when breeding Sloughis?
     Genotyping Sloughis for PRA has been possible since summer of 2000 by sending blood samples to the Department of   Molecular Human Genetics of the Ruhr University   Germany. In the USA the Sloughi Fanciers Association of America, together with OptiGen and the German team, have worked to develop the same test for Sloughis in the USA. Genotyping Sloughis for PRA is now possible in the Western Hemisphere, as of January 2001, by simply sending blood samples to OptiGen

     It is important to keep in mind that, at the beginning of year 2001, although less than 10 Sloughis have been found to be affected with PRA, 30% of the Sloughis genotyped so far have been shown to be carriers for PRA. It is therefore extremely important at this point in time not to go the path of other extremes, and to deplete the gene pool of the Sloughi by excluding 1/3 of the current population from breeding. In doing so, one could run an even greater risk of developing other problems in the breed, far more difficult to genotype and control. At the same time, because of the high incidence of PRA carriers in the breed, serious breeders will screen all their breeding stock for PRA. It is, however, suggested that only outstanding Sloughis found to be PRA carriers be bred with, and that from generation to generation fewer and fewer PRA carriers be used for breeding, until ideally and ultimately PRA is eradicated in the breed while at the same time maintaining the overall genetic health of the breed. 

© with the Library of Congress, Dominique de Caprona 2001

Note: a sentence published in the article  on line "SLOUGHIS AND PROGRESSIVE RETINAL ATROPHY
 By Timothy Anderson PhD, Ermine Moreau-Sipiere, and Karin Schirmer
states the following

"Dogs, just like humans, have many Genetic disorders. Common in breeds other than the Sloughi are genetic disorders such as cardiovascular problems, displasia, and epilepsy. Until recently, the Sloughi has had no known genetic disorders. Unfortunately, the genetic disease called progressive retinal atrophy (PRA) has now appeared in Sloughis in Europe and the United States, probably through spontaneous mutation or introduced by North African Sloughis."

The reference to North African Sloughis having brought PRA to the European dogs is totally uncalled for as all the North African imports since the test exists have been shown to be PRA clear. Anybody who is knowledgeable about the Sloughi knows that PRA did not just "appear", it had been in the European Sloughi population for many years before the test was developed.

References and addresses:

Dekomien G, Maren Munte, Rene Gödde, Jörg Thomas Epplen (2000): Generalized Progressive Retinal Atrophy of Sloughi dogs is due to an 8-bp insertion in exon 21 of the PDE6B gene. Cytogenet. Cell Genet. 93: 261-267 Department of Molecular Human Genetics, Ruhr University, 44780 Bochum, Germany Web:  http://mhg.uni-bochum.de

Acland Gregory, Gustavo Aguirre (1995): PRA today - PRA background and diagnosis. From the Web:  http://www.sheepdog.com

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