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PSSM1 is a muscle disease caused by a disorder of glycogen metabolism. Excessive activity of the enzyme GYS1 leads to abnormal accumulation of glycogen granules in muscle cells.


PSSM2 is a collective term for a number of related muscle disorders with similar symptoms. Six genetic variants (called P2, P3, P4, P8, Px, K1) associated with PSSM2 are currently testable. These variants lead to defects in the structure, stability, or normal function of muscle fibrils and thus a risk of developing symptoms.


It was previously thought that PSSM2 was also a glycogen storage disease like PSSM1 because horses showed similar symptoms and tissue biopsies sometimes looked similar. This is how the name "PSSM2" came about. However, it is now known that PSSM2 is NOT a glycogen storage disease. For this reason, it can be said that the name PSSM2 is technically incorrect, but it has been retained because it has already become established among horse owners.


Both PSSM1 and PSSM2 are multifactorial disorders. The risk of developing symptoms is greatly influenced by environment conditions such as diet and exercise.