PSSM1
Polysaccharide Storage Myopathy Type 1
dominant traitWhat is PSSM1?
PSSM1 (Polysaccharide Storage Myopathy Type 1) is a muscle glycogen storage disorder caused by a mutation in the GYS1 gene. Affected horses accumulate abnormal glycogen in muscle cells, which impairs normal muscle function and causes episodes of stiffness, reluctance to move, muscle pain, and "tying-up" (exertional rhabdomyolysis). Because it is dominant, a single copy of the mutation is enough to cause the condition — but severity varies considerably between horses.
How is it inherited?
PSSM1 is autosomal dominant. One copy (N/P1) causes the condition and can be passed to ~50% of offspring. Two copies (P1/P1) typically produces more severe signs. Affected horses are manageable with appropriate diet and exercise — it is not a death sentence.
What do the results mean?
| Result | Status | What it means |
|---|---|---|
| N/N | Normal | No PSSM1 mutation. Horse has normal muscle glycogen storage and will not develop PSSM1-related tying-up. |
| N/P1 | Heterozygous (affected) | One PSSM1 allele. Horse is affected and may experience stiffness or tying-up, especially with high-starch diets or inconsistent work. Well-managed horses often perform comfortably. |
| P1/P1 | Homozygous (more severely affected) | Two PSSM1 alleles. Clinical signs are typically more pronounced. Strict dietary and exercise management is required. |
No PSSM1 mutation. Horse has normal muscle glycogen storage and will not develop PSSM1-related tying-up.
One PSSM1 allele. Horse is affected and may experience stiffness or tying-up, especially with high-starch diets or inconsistent work. Well-managed horses often perform comfortably.
Two PSSM1 alleles. Clinical signs are typically more pronounced. Strict dietary and exercise management is required.
What should buyers know?
N/P1 horses can lead full athletic lives with a low-starch, high-fat diet and daily exercise. If you are prepared to manage the condition, a positive PSSM1 result should not automatically rule out a horse — but go in with clear expectations and a vet-reviewed management plan.
Which breeds are affected?
Very common in Quarter Horses, Warmbloods, and draft breeds. Estimated prevalence in Quarter Horses is 6–10%. Also seen in Paints, Appaloosas, and many European sport horse breeds.
This page is for general education only. Always consult a veterinarian or equine geneticist before making purchasing or breeding decisions based on genetic test results.
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