Molecular Genetic Testing for the Irish Setter Progressive Retinal Atrophy (PRA) Mutation

Patrick J. Venta, Ph.D.
Departments of Microbiology and Small Animal Clinical Sciences, and the Program in Genetics, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824-1314

Irish setter progressive retinal atrophy is an inherited disease that leads to blindness in affected dogs. It is inherited in a simple autosomal recessive fashion.1 That is, the disease is not related to the sex of the animal and it requires that both copies of the culprit gene be mutant. The disease in Irish setters was first described in the scientific literature in 1949, although its existence was known much earlier than this.1,2 The discovery of a biological phenomenon called "second messenger signal transduction" in the early 1960s by Sutherland and colleagues gave Farber and Lolley a clue as to what might lie behind certain inherited forms of blindness in various mammalian species.3,4 After considerable effort, they discovered that at least one form of blindness in a certain mouse strain was caused by a defect in a "second messenger" system.5 Aguirre and colleagues, in collaboration with these investigators, found that Irish setter PRA was caused by a defect in this same system.6 They eventually showed that a particular protein involved in a second messenger pathway, encoded by a gene called PDEB, was missing from the retinas of affected animals.7 Using this information and expertise in molecular biology, Hurwitz and colleagues discovered the mutation in the canine gene responsible for this disease.8 The mutation, designated W807X, causes the protein produced by the gene to be shorter than normal, and consequently, nonfunctional. The biochemical imbalance caused by the lack of a functional protein leads to the atrophy of the retina, and eventually, to a state of blindness.

It is believed that the mutation discovered by Hurwitz and colleagues accounts for all of the PRA present in the Irish setter breed. There is no evidence that any other gene or any other mutation is involved. In addition, prevalent genetic diseases in purebred animal populations and in human population isolates are almost always caused by a single mutation in a single gene.9 In these cases, the mutation originates in one individual that contributes substantially to the gene pool of the breed or isolate. This is called the Founder Effect, because the individual is quite often one of a few founders of the population. There are numerous examples of single mutations causing a prevalent disease in animal populations. These include Porcine Stress Syndrome, Bovine Leukocyte Adhesion Deficiency, Hyperkalemic Periodic Paralysis ("Impressive Syndrome") in Quarterhorses, von Willebrand's Disease in Scottish terriers and Doberman pinschers, and many other examples.10,11,12,13,14,15 Statistical arguments from the fields of quantitative and population genetics show that a prevalent disease in an animal population will almost always be caused by a single mutation in one gene.16

The identification of a mutation that causes a recessive disease is extremely useful. It allows molecular tests to be developed that can accurately determine if an animal is homozygous normal, a carrier, or affected by the disease. Using this information, breeders can make knowledgeable breeding decisions that will lead to the removal of the disease gene from their lines, while at the same time maintaining the desirable characteristics. Molecular geneticists have developed a number of ways of detecting mutations.17 Sargan and colleagues and Aguirre and colleagues as well as scientists at VetGen LLC have all developed different tests that are capable of accurately detecting the Irish setter PRA W807X mutation.18,19,20 There is no question that many groups with expertise in molecular biology can detect an identical mutation with extremely high accuracy. For example, there are well over fifty different groups that have all developed tests that can detect the most common mutation that causes cystic fibrosis in humans.21,22,23,24

The scientists at VetGen LLC, together with their collaborators at Michigan State University and the University of Michigan, have over 60 years of experience in molecular genetics. The test that they have developed for the Irish setter PRA mutation is completely accurate and includes external and internal controls to ensure reliability. It has been tested on both normal and mutant alleles from animals whose genetic state is known and the results are exactly as predicted. Furthermore, a convenient, simple, non-invasive cheek swab DNA collection method has been developed that can be used for a dog at any age, including puppyhood. This method of collection has been very popular among breeders. In addition, the staff at VetGen has extensive experience in blood-based DNA collection and testing methods as well. The results from this test are not only completely accurate, but are probably better controlled than other methods currently in use for detection of the Irish setter PRA W807X mutation.

References:

1. Hodgman, S.F.J., H.B. Parry, W.J. Rasbridge, and J.D. Steel (1949) Progressive retinal atrophy in dogs. 1. The disease in Irish Setters (Red). Vet. Rec. 61: 185-90.
2. Rasbridge, W.J. (1944) Our Dogs, 6th May.
3. Nobel prize for the second messenger (Earl Sutherland). (1971) Lancet. 2: 911.
4. Farber, D.B. and R.N. Lolley (1974) Cyclic guanosine monophosphate: elevation in degenerating photoreceptor cells of the C3H mouse retina. Science 186: 449-451.
5. Lolley, R.N. and D.B. Farber (1976) Abnormal guanosine 3', 5'-monophosphate during photoreceptor degeneration in the inherited retinal degeneration in C3H/HeJ mice. Ann. Ophthalmol. 8: 469-473.
6. Aguirre, G., D. Farber, R. Lolley, P. O'Brien, J. Alligood, R.T. Fletcher, and G. Chadler (1982) Retinal degeneration in the dog. III. Abnormal cyclic nucleotide metabolism in rod-cone dysplasia. Exp. Eye Res. 35: 625-642.
7. Farber, D.B., J.S. Dancier, and G. Aguirre (1992) The subunit of cyclic GMP phosphdiesterase mRNA is deficient in canine rod-cone dysplasia 1. Neuron 9: 349-356.
8. Suber, M.L., S.J. Pittler, N. Qin, G.C. Wright, V. Holcombe, R.H. Lee, C.M. Craft, R.N. Lolley, W. Baehr, and R.L. Hurwitz (1993) Irish setter dogs affected with rod/cone dysplasia contain a nonsense mutation in the rod cGMP -subunit gene. Proc. Natl. Acad. Sci. USA 90: 3968-3972.
9. Venta, P.J., unpublished survey. See also reference 15 and Georges, M. and L. Andersson (1996) Livestock genomics comes of age. Genome Res. 6: 907-921, and Livingstone, F.B. (1987) Simulation of the Founder Effect and its role in the determination of the polymorphic frequencies of deleterious genes in human populations. Hum. Biol. 59: 59-75.
10. O'Brien, P.J. et al. (1993) Use of a DNA-based test for the mutation associated with porcine stress syndrome (malignant hyperthermia) in 10,000 breeding swine. J. Am. Vet. Med. Assoc. 203: 842-851.
11. Shuster, D.E. et al. (1992) Identification and prevalence of a genetic disease that causes leukocyte adhesion deficiency in Holstein cattle. Proc. Natl. Acad. Sci. USA 89: 9225-9229.
12. Rudolph, J.A. et al. (1992) Periodic paralysis in Quarter Horses: a sodium channel mutation disseminated by selective breeding. Nat. Genet. 2: 144-147.
13. Venta, P.J., J. Li, V. Yuzbasiyan-Gurkan, W.D. Schall, and G.J. Brewer (1997) Von Willebrand's disease in the Scottish terrier is caused by a single base deletion in exon four of the von Willebrand factor gene. In review, Am. J. Vet. Res.
14. Venta, P.J., J. Li, V. Yuzbasiyan-Gurkan, W.D. Shall, and G.J. Brewer (1997) Von Willebrand's disease in the Doberman pinscher is due to a splice site defect. In preparation.
15. Healy, P.J. and J.A. Dennis (1993) Inherited enzyme deficiencies in livestock. Vet. Clin. North Am. Food Anim. 9: 55-63.
16. Falconer, D. and T.B. Mackay (1996) Introduction to Population Genetics, Fourth Ed., Longman House, Burnt Hill, England.
17. Dianzani, I., C. Camashcella, A. Ponzone, and R.G.H. Cotton (1993) Dilemmas and progress in mutation detection. Trends Genet. 9: 403-405.
18. Clements, P.J.M., C.Y. Gregory, S.M. Peterson-Jones, D.R. Sargan, and S.S. Bhattacharya (1993) Confirmation of the rod cGMP phosphodiesterase subunit (PDE) nonsense mutation in affected rcd-1 Irish setters in the UK and development of a diagnostic test. Curr. Eye Res. 12: 861-866.
19. Ray, K., V.J. Baldwin, G.M. Acland, S.H. Blanton, and G.D. Aguirre (1994) Cosegregation of the codon 807 mutation of the canine rod cGMP phosphodiesterase gene and rcd1. Invest. Ophthal. Vis. Sci. 35: 4291-4299.
20. Unpublished results. For a similar test using the same methodology, see: Venta, P.J., Hewett-Emmett, D., and Tashian, R.E. Simple method to convert DNA sequence variation into sites cut by restriction endonucleases: utility shown by typing the human CA3 and mouse strain Car-2 polymorphisms. Am. J. Hum. Genet. 1991:49;a445.
21. Population variation of common cystic fibrosis mutations. (1994) The Cystic Fibrosis Genetic Analysis Consortium. Hum. Mutat. 4: 167-77.
22. Correlation between genotype and phenotype in patients with cystic fibrosis. (1993) The Cystic Fibrosis Genotype-Phenotype Consortium. N. Engl. J. Med. 329: 1308-1313.
23. Schwartz, M.J., G.M. Malone, A. Haworth, J.P. Cheadle, A.L. Meredith, A. Gardner, I.H. Sawyer, M. Connarty, N. Dennis, A. Seller, et al. (1995) Cystic fibrosis mutation analysis: report form 22 U.K. regional genetic laboratories. Hum. Mutat. 6: 326-333.
24. Cuppens, H. and J.J. Cassiman (1995) A quality control study of CFTR mutation screening in 40 different European laboratories. The European Concerted Action on Cystic Fibrosis. Eur. J. Hum. Genet. 3: 235-245.

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