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Polysaccharide Storage Myopathy

Polysaccharide Storage Myopathy (PSSM1)


Gene or region: GYS1

Reference allele: C

Mutant allele: T

Affected Breeds: many


Research Confidence:                            High confidence, findings reproduced multiple studies


What it does:


Polysaccharide Storage Myopathy (PSSM) is a glycogen storage disorder in which affected horses have chronic episodes of exertional rhabdomyolysis (“tying-up”). Acute symptoms are an unwillingness to move and muscle damage following exercise. Heterozygous horses (PSSM1/N) experience more moderate symptoms that can usually be managed through a low-sugar/starch diet and exercise. However, homozygous horses (PSSM1/PSSM1) are generally more severely affected and are thus harder to manage. Additionally, one study has found that horses with PSSM1 and a malignant hyperthermia (MH) exhibit more severe clinical symptoms.


GYS1 is an enzyme involved in the production of glycogen that is found specifically within the muscle. Mutations in this gene have been associated with glycogen storage diseases in several species. This particular mutation is a single base substitution that alters an amino acid, likely affected the function of the encoded protein.




McCue ME et al., “Glycogen synthase (GYS1) mutation causes a novel skeletal muscle glycogenosis.” (2008) Genomics. 91: 458-66. PMID: 18358695


McCue ME et al., “Glycogen synthase 1 (GYS1) mutation in diverse breeds with polysaccharide storage myopathy.” (2008) J Vet Intern Med. 22: 1228-33. PMID: 18691366


Herszberg B et al., “A GYS1 gene mutation is highly associated with polysaccharide storage myopathy in Cob Normand draught horses.” (2009) Anim Genet. 40: 94-6. PMID: 18822097


McCue ME et al., “Polysaccharide storage myopathy phenotype in quarter horse-related breeds is modified by the presence of an RYR1 mutation.” (2009) Neuromuscul Disord. 19: 37-43. PMID: 19056269


Tryon RC et al., “Evaluation of allele frequencies of inherited disease genes in subgroups of American Quarter Horses.” (2009) J Am Vet Med Assoc. 234: 120-5. PMID: 19119976


Stanley RL et al., “A glycogen synthase 1 mutation associated with equine polysaccharide storage myopathy and exertional rhabdomyolysis occurs in a variety of UK breeds.” (2009) Equine Vet J. 41: 597-601. PMID: 19803057


Baird JD et al., “Presence of the glycogen synthase 1 (GYS1) mutation causing type 1 polysaccharide storage myopathy in continental European draught horse breeds.” (2010) Vet Rec. 167: 781-4. PMID: 21262610


McCoy AM et al., “Evidence of positive selection for a glycogen synthase (GYS1) mutation in domestic horse populations.” (2014) J Hered. 105: 163-72. PMID: 24215078

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