Stockholm, 2007-09-14

Dear Ms Duncan,

Thank you for your long letter. It poses a lot of interesting questions in regards to PRA in the Abyssinian and Somali cat breeds and in regards to the testing for the mutation that we have found to be prevalent in this group of cats.

Some background first of all: I discovered the disease in a Swedish Abyssinian cat in 1977 and was able to set up a breeding colony for affected cats only a few years later at the Swedish University of Agricultural Sciences, in Uppsala. A research project was initiated to allow for a detailed characterization of the disease in order to proceed thereafter with molecular genetic studies. I published my PhD thesis at the Medical School in Linköping, Sweden, in 1985, which concerned the clinical, hereditary, electrophysiological and morphological description of the disease in large groups of Swedish Abyssinian cats (1). Almost a decade later, collaboration with the Laboratory for Genomic Diversity at the National Cancer Institute (NCI), in Frederick, USA, was initiated. My then 7 generation purebred Abyssinian pedigree was found to be the perfect model to use for the gene search. In order to increase heterozygocity in the cat colony, they were thereafter outcrossed to normal cats and then backcrossed to affected individuals in order to, through linkage analysis in a group of 55 affected and unaffected cats, find the affected gene, and later the specific mutation.

At NCI my foremost collaborators are Dr. Marilyn Menotti-Raymond and Victor David at the Laboratory for Genomic Diversity. This laboratory is led by Dr. Stephen O'Brien, a world leader in cat genetics. Actually, one of my purebred Abyssinians, Cinnamon, was chosen by him and others, to be utilized in order to sequence the feline genome (2).

When the retinal degeneration of Abyssinian cats (rdAc) was first described in Sweden the prevalence was extremely high in the breed. It was found that 45% of purebred Abyssinians that were 2 years old or older were affected by hereditary retinal disease. The estimated proportion of genetically free animals was then only 11%, which would give 44% nonaffected carriers (3). According to Swedish breeders the frequency of the disease is now very low (below 1%) in the Swedish population. So it is clear that the breeders have done a tremendous job reducing the disease frequency through a strict breeding scheme, primarily eliminating affected cats from breeding.

The past year we have performed studies in order to elucidate the rdAc allele frequency in Scandinavian and American populations of cats. The Scandinavian studies have shown an allele frequency of 0.133 for rdAc (4), which would mean that the incidence of affected cats would be 0.133 x 0.133 = 0.017, or 17 affected cats in one thousand cats. The results we have obtained so far through the screening of approximately 300 samples from Abyssinian and Somali cats in Scandinavia are compatible with these numbers. The cats used for these studies have not been randomly chosen, however, in most part picked by the breeders or breed-clubs. This could cause the results to be somewhat incorrect for the overall population.

Most affected cats can be diagnosed with a funduscopic exam (also called ophthalmoscopic test) at the age of 1.5 to 2 years (5). However, through examination of my own cats in the pedigree, bred to develop the disease, I have found early retinal changes indicative of disease in two 4-6 month old cats, but also in a 4-year old cat. This means that there is a great diversity of when typical funduscopic changes appear. The changes are then slowly progressive and lead to a complete photoreceptor cell death within another 3-4 years. At this time the inner retina is still functional and the cats still have more or less normal pupillary light reflexes. They also have a normal lens, which means that no secondary cataract is found in affected cats, as is seen for example in most dogs affected with a hereditary retinal degeneration.

All of this shows that there is a great variation as to clinical signs of the disease, especially when using ophthalmoscopy. A more reliable clinical test than ophthalmoscopy is actually electroretinography (ERG). There are several methods for performing ERGs, however, and a special diagnostic protocol is needed in order to obtain an early diagnosis, which can be obtained at approximately age 8 months (6,7), using procedures that have been developed in my laboratory, but so far not performed by most "eye" veterinarians.

Due to the difficulties in obtaining a reliable diagnosis early, before cats are used for breeding purposes, a mutation detection test is of utmost importance to have. That is why we decided to provide such a test as soon as we had discovered the mutation causing the disease in the Scandinavian pedigree of cats. Precisely controlled studies have been performed in preserved blood from my colony of cats, consisting of hundreds of animals over the years. All affected cats have been homozygous for the mutation; we have found no discrepancies.

The mutation detection test has been extensively validated (4). Never-the-less there have been some surprising findings that need to be discussed further. In two cats examined in Sweden this summer, retinal changes compatible with those caused by the rdAc mutation were found. Surprising was, however, that the cats were not homozygous for the disease allele. In this regard it should be noted that there are several other causes of retinal degenerative disease. One such cause is taurine deficiency, which may cause a generalized retinal degeneration in cats, a problem previously seen quite often when cats were fed deficient diets, not containing enough taurine. Another is retinal toxicity, as that caused by enrofloxacin, a bacteriocidal drug found to cause blindness and retinal degeneration in cats, if higher than recommended doses are prescribed. Autoimmune disorders can cause retinal degeneration, as well as malabsorbtion syndromes. So it should be noted that there are other causes of bilateral generalized retinal degeneration causing blindness in cats, than that caused by the mutation recently found in the CEP290 gene of Abyssinian and Somali cats.

My collaborators and I are very interested to continue this research. We want to continue our research into the disease process in the affected cats, into studies of disease mechanisms (on the cellular level) caused by the rdAc mutation, and into treatment modalities using gene transfer for affected cats in order to be able to correct the defect in the future. By this I want to say that we are also interested to know about all discrepancies as to diagnostics that occur. There could definitely still be other mutations in the CEP290 gene that are causative to retinal degeneration in cats. For instance in humans, there have been approximately 10 mutations described that cause disease in this gene. In the animal population, however, most commonly only one mutation is causative to a specific disease entity, due to founder effects and extensive inbreeding and thereby a propagation of the allele frequency in a population. We cannot, however, exclude the possibility that there is more than one mutation responsible for retinal degeneration of Abyssinian and Somali cats, something that we need to look into further.

References:

1. Narfström, K.: Retinal degeneration in a strain of Abyssinian cats: A hereditary, clinical, electrophysiological and morphological study. Linköping University Medical Dissertations No. 208, Linköping,1985.

2. Pontius JU, Mullikin JC, Smith D, Lindblad-Toh K, Gnerre S, Clamp M, Chang J, Stephens R, Neelam B, Volfovsky N, Schaffer AA, Agarwala R, Narfström, K, Murphy WJ, Giger U, Roca AL, Antunes A, Menotti-Raymond M, Yuhki N, Pecon-Slattery J, Johnson WE, Bourque G, Tesler G, O'Brien J: The Domestic Cat Genome Sequence: Annotation and Comparative Inferences. Genome Research, Accepted, 2007.

3. Narfström K: Hereditary progressive retinal atrophy in the Abyssinian cat. Journal of Heredity, 74:273-276, 1983.

4. Menotti-Raymond, M., David, V.A., Schäffer, A.A., Stephens, R., Wells D, Kumar-Singh, R., O'Brien, S.J., Narfström, K.: Mutation in CEP290 discovered for cat model of human retinal degeneration. Journal of Heredity, In press, 2007.

5. Narfström, K.: Progressive retinal atrophy in the Abyssinian cat: Clinical characteristics. Invest Ophthalmol Vis Sci 26:193-200, 1985.

6. Hyman, J., Vaegan, Lei, B., Narfström, K.: Electrophysiologic differentiation of homozygous and heterozygous Abyssinian-crossbred cats with late-onset hereditary retinal degeneration. American Journal of Veterinary Research 66(11):1914-21, 2005.

7. Vaegan, Narfström, K.: A(max) is the best a-wave measure for classifying Abyssinian cat rod/cone dystrophy. Documenta Ophthalmologica 111(1):33-38 2005.

Best wishes,

KRISTINA NARFSTRÖM