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Description
Polysaccharide Storage Myopathy Type 1 (PSSM1) is an inherited muscle disorder and a genetic cause of “tying-up” (exertional rhabdomyolysis). It is caused by a mutation in the GYS1 gene, which encodes glycogen synthase, the enzyme that builds glycogen, the main energy store in muscle. The mutation makes the enzyme overactive, so affected horses build up excess glycogen and an abnormal form of stored sugar (polysaccharide) in their muscles, which disrupts normal muscle function.
PSSM1 is inherited as an autosomal dominant trait, so a horse needs only one copy of the mutation to be at risk. It has been identified in more than 20 breeds. It is especially common in draft breeds such as Belgians, and is also found in about 8% of Quarter Horses and related breeds.
Signs may begin by 2 to 3 years of age, while some horses never show noticeable signs. Clinical signs can include muscle stiffness, firm and painful muscles, sweating, skin twitching, weakness and reluctance to move, even during light exercise. Gait abnormalities, mild colic and muscle wasting occasionally occur. Many horses that test positive have no history of tying-up.
A second mutation, in the RYR1 gene that causes Malignant Hyperthermia (MH), can increase the severity of PSSM1 signs in Quarter Horses and related breeds. Animal Genetics offers both the PSSM1 and MH tests.
Not all cases of tying-up are caused by PSSM1. A horse that tests N/N for PSSM1 but shows signs of tying-up or muscle pain may have a different muscle disorder, such as PSSM Type 2, which is not detected by this test. Please contact us about such horses so they can be included in our ongoing research.
Managing horses that test positive
Horses with one or two copies of the PSSM1 mutation should be managed carefully through diet and exercise. A low-starch, low-sugar diet with added fat, combined with regular, consistent exercise, can reduce or even prevent signs in many horses. These steps may not work in every case, so always let your veterinarian know if a horse has tested positive for PSSM1.
Possible results
| Genotype | Description |
|---|---|
| P1/P1 | At risk: Horse has two copies of the PSSM1 mutation and may show signs of the disorder. It will pass one copy of the mutation to every offspring. |
| N/P1 | At risk: Horse has one copy of the PSSM1 mutation and may show signs of the disorder. Each offspring has a 50% chance of inheriting the mutation. |
| N/N | Clear: Horse does not carry the PSSM1 mutation and cannot pass it on to offspring. |
Breeds Affected
American Quarter Horse, American Paint Horse, Appaloosa, Belgian, Percheron and other draft breeds; identified in more than 20 breeds
Reference
Glycogen synthase (GYS1) mutation causes a novel skeletal muscle glycogenosis.
[PMID: 18358695]