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Tying-up gene found

S. Valberg, DVM, University of Minnesota

For hundreds of years, horses have suffered from painful muscle cramping following light exercise. In draft horses, this condition was called Monday morning disease because if tended to occur after a day of rest. In light breed horses, it has been called tying-up.

Veterinarians at the University of Minnesota recognized that there was a specific form of tying up called polysaccharide storage myopathy (PSSM), characterized by excessive and abnormal storage of sugar (polysaccharide) in muscle cells. Fifteen years after this discovery, researchers found the gene mutation that is responsible for PSSM. An article about the research will be published in an upcoming issue of Genomics.

The researchers found that the mutated gene is inherited in a dominant fashion, meaning that one copy of the mutation can cause PSSM. Each horse has two copies of every gene, one inherited from the dam and one from the sire. To develop PSSM, a horse only needs to inherit the mutant gene from one parent.

Up to 50 percent of some draft horse breeds and 10 percent of quarter horses are affected by PSSM. The research also showed that the PSSM gene likely arose 1,600 years ago at the time that the "Great Horse" was being developed to take knights into battle. The genetic mutation has since been disseminated to more than 20 horse breeds.

Horse owners and veterinarians can now test for PSSM using a DNA blood or hair test commercially available through the University's Veterinary Diagnostic Laboratory.

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