Mucopolysaccharidosis Type VI (MPS-VI) in Chesapeakes
Mucopolysaccharidoses comprise a class of metabolic diseases that are known in both people and dogs. The cause of the disorder is that an enzyme that breaks down large sugars (polysaccharides) is unable to do its job because of a mutation. Although we tend to think of cells as static and unchanging, normal cells are constantly breaking down their components and building new versions of the same. When the balance between synthesis of new molecules and degradation of old molecules is not maintained, the consequences can be severe (for example, in diabetes). In each type of mucopolysaccharidosis (MPS), a different enzyme is nonfunctional. The symptoms and severity of the diseases vary, but many are fatal. In animals (and people) with MPS, these polysaccharides build up and are stored in parts of the cell known as lysosomes. These diseases are therefore also called lysosomal storage disorders.
A few years ago, the first known case of an MPS disease in Chesapeake Bay Retrievers came to the attention of the scientific establishment, when an approximately 8-month-old puppy was diagnosed with MPS VI (each type of MPS is assigned a Roman numeral). The male puppy was diagnosed by Dr. Urs Giger of the vet school at the University of Pennsylvania in Philadelphia. The puppy was first seen by a vet who remembered something vague from a class taken with Dr. Giger. Samples were sent to Penn after the puppy had been euthanized. A particular area of interest and expertise at Penn has been the lysosomal storage diseases, including mucopolysaccharidoses, so the samples happened to go to one of the few places where the correct diagnosis could be made.
In MPS VI, there is a deficiency in the enzyme known as arylsulfatase B. MPS VI had been reported in humans, Siamese and domestic shorthair cats, and several breeds of dogs, including Miniature Pinschers, Miniature Schnauzers, and Welsh Corgis. The mutations that cause MPS VI occur on non-sex (i.e., autosomal) chromosomes; males and females are equally affected. Individuals that carry one copy of the mutated gene (carriers) are entirely normal in their appearance and they live normal asymptomatic lives.
Puppies with MPS VI have serious skeletal abnormalities, including stunted growth, defects of the breastbone (sternum), hip dysplasia, and vertebral abnormalities. The head is unusually large relative to the size of the body, and the long bones of the legs are twisted outward. The cornea of the eye may be clouded, and there may be internal organ failures, too. These changes are progressive, so that puppies look essentially normal at 8 weeks-of age. The disease is diagnosed by a total evaluation of the animalís clinical signs, a positive urinary MPS spot test, and abnormalities in the white blood cells. Compared to normal animals, animals with MPS VI have much lower arylsulfatase B enzyme activity in their white blood cells, and this is used as a confirmatory test of the disease. Unfortunately, testing of enzyme levels has not been as effective as had been hoped in differentiating between carriers and animals affected by the disease. Recent work has characterized the disease-causing mutations in Miniature Pinschers and Miniature Schnauzers. Dr. Gigerís research group has found that the mutation (or mutations) in Chesapeakes differs from the mutations found in other breeds, but additional samples are needed.
It is likely that MPS VI was not diagnosed in Chesapeakes until recently because 1) it is a rare disease in the breed and 2) relatively few veterinarians are aware of the symptoms of lysosomal storage disorders. It is hoped that, given the information provided here, Chesapeake fanciers and breeders will be on the look-out for puppies that may have this condition and will provide assistance to Dr.Gigerís research group. Once the mutation is found in Chesapeake Bay Retrievers, Dr. Gigerís group plans to develop a DNA test (as has been done for other breeds), which would provide an easier and more accurate diagnosis of both carriers and animals with the disease.
Urine for the MPS spot test to detect affected animals can be shipped by overnight mail Monday to Thursday. Testing for possible carriers must be pre-arranged by the Metabolic Genetic Laboratory. A submission form with instructions is available at their website www.vet.upenn.edu/penngen. If you don't have Internet access, you can also request a submission form by sending your request with a self-addressed, stamped envelope to: PennGen, VHUP 4006, 3900 Delancey Street, Philadelphia, PA 19104; by fax at 215-573-2162; or by phone at 215-898-8894. Checks should be made out to: University of Pennsylvania/Dr. Giger. Visa and MasterCard are also accepted. For additional information, please visit the website or contact the Josephine Deubler Genetic Disease Testing Laboratory at 215-898-3375. The identity of dogs and/or their breeders will not be made public.
©2004 Claire McCall