Short description

Progressive retinal atrophy (PRA, formerly called generalized PRA) is a group of hereditary conditions that result in a progressive degeneration of the retina and loss of vision in cats. PRA is the veterinary equivalent of the retinitis pigmentosa in man.

When I searched the literature to find a more in depth description for PRA, I came across to the following:
A single-nucleotide polymorphism designated "rdAc" in the CEP290 gene produces a defective protein which is associated with progressive retinal atrophy (PRA) in these breeds.
The late onset photoreceptor degeneration rdAC-PRA is affecting Abyssinian and Somali cats. This genetic disorder causes the degeneration of retinal cells in the eye. A mutation in the CEP290 gene which has been suggested to cause rdAc-PRA has recently been published by the group of Kristina Narfström at the University of Missouri-Columbia, Columbia.
In the study, researchers found that cats with the late-onset form of the disease have a mutation in the CEP290 gene, which is the same mutation found in humans with Joubert syndrome and Leber's congenital amaurosis. In both of these diseases, the genetic mutations result in changes in the function and structure of the photoreceptors.

There were many new words, which I did not understand, and several questions arose:

• What is a nucleotide?
• What is polymorphism?
• What does rdAC stand for?
• What is the CEP290 gene?
  Which function does it have?
  For which traits and functions of the organism is it responsible?
  On which chromosome is this CEP290-gene located?
• What is retinitis pigmentosa?
• What is the Joubert syndrome?
• What is the Leber's congenital amaurosis?
• Do I need to know all these genotechnical terms?
• Which breeds are actually affected?

I ended up with reading many of these scientific articles about rdAC published in scientific journals, but understanding very little of the content of these articles. But I'm not a veterinary, I'm only the owner of a patient. Thus, eventually it is not necessary that I do understand the genetic reasons of PRA (?).
I ended up with reading many lessons and articles about the basics of genetics, as described by scientists - not that kind of genetics, which we have learned in our cat world.
BTW this disaster that I did understand so little in these scientific articles and lessons was one of the reasons, why I started the chapter about anatomy.

So, I've started to set up pages about the
Cat Genome, DNA, genes, etc.

Symptoms and signs

The initial signs in typical PRA are a loss of rod photoreceptor responses, which leads first to night blindness. This is then followed by a slower loss of cone responses and a progressive deterioration in daytime vision. As the condition progresses the owners may notice a more dilated pupil and increased reflection from the eye. Secondary cataracts accompany the loss of vision in the later-onset and more slowly progressive forms of PRA.

There are several forms of PRA in cats:

• Rod cone dysplasia
• Progressive rod cone degeneration
• X-linked PRA
• Cinnamon linked PRA

Inheritance in cats

Most forms of PRA are inherited autosomal (i.e. sex independent) recessive. This goes for rdAc.
But there are also dominantly inherited forms of PRA, called Rdy.
This type of PRA is caused by a defect in the gene CRX.
Unfortunately Abyssinians and Somali may have both forms, recessively and dominantly inherited form.

Detailed research had been made in breeding lines in the UK, US, Sweden and the Netherlands.

Cat breeds, likely to inherit PRA (rdAc)

• Abyssinian
• Exotic
• Persian
• Somali
• Ocicat
• Siamese

Cat breeds, likely to inherit PRA (Rdy)

• Abyssinian
• Somali

Test methods in cats

PRA is not a painful disease, and thus the early stages might not be recognized by the pet owners.
The veterinary ophthalmologist examines the retina with an instrument called an indirect ophthalmoscope. Changes in the retinal blood vessel pattern, the optic nerve head, and the reflective substance within the cat's eye called the tapetum can be seen.
Diagnosis is made and confirmed by the ERG (electroretinogram). This test involves sophisticated instrumentation used to measure the response of the retina to flashes of light. Your pet would be anesthetized for this test. A healthy retina will produce a characteristic wave form that builds from the time the lights are turned out.

a typical curve of  the ERG for a healthy cat

DNA-test for rdAC

Since begin of 8/2007 there is a DNA-test available for the recessive progressive rode cone degeneration in Abyssinian and Somali - rdAC. See:

• Laboklin (Genetic - Genetic diseases)
• Mutation in CEP290 discovered for cat model of human retinal degeneration.
  Menotti-Raymond M, David VA, Schäffer AA, Stephens R, Wells D, Kumar-Singh R, O'Brien SJ, Narfström K.
  Oxford Journal of Hereditary, 2007 May-Jun; 98(3):211-20. Epub 2007 May 16

It is mainly Kristina Narfström from the Mason Eye Institute at Columbia University of Missouri, who studies the PRA in Abyssinian and Somali already for many years.

Marilyn A. Menotti-Raymond works at the National Cancer Institute of the Frederick University in Maryland.

The DNA-test is a blood test, there is needed 1cc (about 1ml) of so called EDTA-blood (EDTA = ethylene-diamine-tetra-acetic acid).

Description about the DNA-test for Rdy follows.

Consequences for breeding cats

Cats tested positive for PRA, even they themselves are healthy and they are only carriers (when they have the recessive type of PRA), shall not be permitted for breeding and have to be neutered.