Progressive Retinal Atrophy
in the Sloughi
By Dominique Crapon de Caprona, Ph.D.
Traduction en Français
What is progressive Retinal Atrophy?
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?
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
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, 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.
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?
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
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:
OptiGen, LLC Cornell Business and Technology
Park 767 Warren Road, Suite 300, Ithaca, NY 14850
For information on shipping samples and forms
please visit OptiGen's Web site: http://www.optigen.com