Genetic Testing Abbreviations With Explainations

PRA = Progressive Retinal Atrophy

There are several genetically distinct forms of Progressive Retinal Atrophy in dogs. PRA is a gradual degeneration of the retinal tissue. The form found in Australian Shepherds is Progressive Rod Cone Degeneration (PRCD). Age of onset is usually in the prime of life. The first things an owner might notice is night-blindness. The disease progresses, over months or years, until the dog is blind. The disease is caused by a recessive gene mutation.

Progressive Retinal Atrophy (PRA) is an inherited disease of the retina (the “film in the camera”) in dogs, in which the rod cells in the retina are programmed to die. PRA occurs in both eyes simultaneously and is nonpainful. There are many different types of inherited retinal degenerative diseases in purebred dogs, and discussing these are beyond the scope of this article. PRA occurs in most breeds of dogs and also occurs in mixed breeds. It is recessively inherited in all breeds studied, with the following exceptions: PRA is dominantly inherited in Old English Mastiffs and Bullmastiffs, and is sex-linked and found primarily in male dogs in Siberian Husky and Samoyed breeds.

A reprint from Animal Eye Care

CEA = Collie Eye Anomaly

Collie Eye Anomaly (CEA) is a sometimes blinding congenital inherited eye disease. All dogs with CEA have bilateral choroidal hypoplasia (CH), also called chorioretinal dysplasia, a thinning of the vascular tissue in the back of the eye which does not significantly impair vision. Some dogs will also have optic nerve coloboma, in which the nerve tissue is not fully developed where the optic nerve enters the back of the eye. A few will have retinal detachment. Both of these more serious defects can be blinding. CEA is present at birth and does not progress, so there is no worry that an affected puppy will get worse as it gets older. It also does not cause the affected puppy any pain.

CEA affected puppies generally appear normal because very few have defects that seriously impair vision. Those defects are within the eye and cannot be detected without special instruments so diagnosis can only be made by a veterinary ophthalmologist or with a DNA test. While coloboma and retinal detachment will be identified on exam, CH may not be apparent in puppies over 6-8 weeks of age because it is can be obscured as the pigment develops in the back of the eye, a condition called masked affected.

CH is caused by a recessive gene mutation. Even though this gene does not by itself cause the more serious defects, dogs that do not have two copies of the CH mutation will not develop CEA defects. Other as yet unidentified genes, environmental effects, or regulatory DNA play a role in determining exactly what defects a dog will have. Carrier dogs, having only one copy of the mutation, are normal. There is no known relationship between CEA and coat or eye color. CEA has been identified in other dog breeds, including Collies, Shetland Sheepdogs and Border Collies. The CEA mutation is the same in all affected breeds.

A reprint from Australian Shepherd Health and Genetics Institute

HC = Hereditary Cataracts (HSF4)

Hereditary Cataracts (HC) are a clouding of the lens of the eye caused by a breakdown of tissue in the eye. This condition generally results in aninability to see clearly and can cause total blindness. In canines, cataracts are often familial; this type is known as Hereditary Cataracts. A mutation in the HSF4 gene causes this type of cataracts in several breeds of dogs. In this case, the dog is typically affected bilaterally, in that both eyes are affected by the cataracts. The cataracts associated with HSF4 also occur in the posterior region of the lens. They usually start by being small and grow progressively, though the speed of growth is highly variable. Some cataracts will grow so slowly that the dog's vision remains relatively clear, while others will grow such a way that the dog will quickly go blind. Corrective surgery is possible, though it is costly and is not always effective.

A mutation of the HSF4 gene is linked to a form of Hereditary Cataracts in Australian Shepherds. This mutation affects Aussies differently then Boston Terriers, French Bulldogs and Staffordshire Bull Terriers in that the disease is dominant but not completely penetrant. This means that only one copy of the mutation is necessary to predispose a dog to the disease. However, incomplete penetrance means that a dog that has this mutation will not always develop HC. Research suggests that the mutation makes a dog 12 times more likely to develop posterior bilateral cataracts at some point in their lifetime. It is likely that a secondary gene interaction occurs in the small percentage of dogs possessing the HC mutation but does not develop cataracts. This interaction is not yet understood.

A reprint from Animal Genetics

DM - Degenerative Myelopathy

Degenerative myelopathy is a progressive disease of the spinal cord in older dogs. The disease has an insidious onset typically between 8 and 14 years of age. It begins with a loss of coordination (ataxia) in the hind limbs. The affected dog will wobble when walking, knuckle over or drag the feet. This can first occur in one hind limb and then affect the other. As the disease progresses, the limbs become weak and the dog begins to buckle and has difficulty standing. The weakness gets progressively worse until the dog is unable to walk. The clinical course can range from 6 months to 1 year before dogs become paraplegic. If signs progress for a longer period of time, loss of urinary and fecal continence may occur and eventually weakness will develop in the front limbs. Another key feature of DM is that it is not a painful disease.

The “A” allele is very common in some breeds. In these breeds, an overly aggressive breeding program to eliminate the dogs testing A/A or A/G might be destructive to the breed as a whole because it would eliminate a large fraction of the high quality dogs that would otherwise contribute desirable qualities to the breed. Nonetheless, DM should be taken seriously. It is a fatal disease with devastating consequences for the dogs and a very unpleasant experience for the owners who care for them. Thus, a realistic approach when considering which dogs to select for breeding would be to consider dogs with the A/A or A/G test result to have a fault, just as a poor top-line or imperfect gait would be considered faults. Dogs that test A/A should be considered to have a worse fault than those that test A/G. Dog breeders could then continue to do what conscientious breeders have always done: make their selections for breeding stock in light of all of the dogs’ good points and all of the dogs’ faults. Using this approach over many generations should substantially reduce the prevalence of DM while continuing to maintain or improve those qualities that have contributed to the various dog breeds.

Summary: We recommend that dog breeders take into consideration the DM test results as they plan their breeding programs; however, they should not over-emphasize this test result. Instead, the test result is one factor among many in a balanced breeding program.

A reprint from Canine Genetic Diseases

MDR1 = Multi-Drug Resistance 1

MDR1 is the abbreviated name of a gene called Multi-Drug Resistance 1. A mutation of this gene causes sensitivity to Ivermectin and a number of other drugs. Dogs with the mutation will react to those drugs. Whether a dog reacts depends on the dosage of the drug. A dog may not react to very low doses, as with the amount of Ivermectin found in heart worm products. Typical doses of a variety of medications will cause reactions in dogs with two copies of the mutation, but some drugs most notably several chemotherapy agents can cause reactions in dogs with only one. Dogs with this mutation have a transport defect the drug goes in to their brains, fails to be transported out, and builds up to toxic levels. This causes serious neurological problems including seizures and sometimes death.

A reprint from Australian Shepherd Health and Genetics Institute

Problem Drugs

Many different drugs and drug classes have been reported to cause problems in Collies and other herding breed dogs that carry the MDR1 mutation. We and other researchers have documented the toxicity that occurs with several of these drugs.

Drugs that have been documented to cause problems in dogs with the MDR1 mutation include:
  • Acepromazine
  • Butorphanol
  • Emodepside
  • Erythromycin
  • Ivermectin
  • Loperamide
  • Selamectin, milbemycin, and moxidectin
  • Vincristine, Vinblastine, Doxorubicin

Drugs that are known to be pumped out of the brain by the protein that the MDR1 gene is responsible for producing but appear to be safely tolerated by dogs with the MDR1 mutation:
  • Cyclosporin
  • Digoxin
  • Doxycycline

Drugs that may be pumped out by the protein that the MDR1 is responsible for producing, but appear to be safely tolerated by dogs with the MDR1 mutation:
  • Morphine
  • Buprenorphine
  • Fentanyl

A reprint from Washington State University

This article was reserached by Breeze Burress

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