Gene affected: GYS1
Transmission: Autosomal dominant incomplete
Mutation: EquCab3.0 10 g.19203501C>T c.926G>A
OMIA link: 000785-9796

Breeds : Draft horses, Warmblood, Quarter Horse, American Paint, Appaloosa

Symptoms:
The symptoms of polysaccharide storage myopathy 1 are:
-Muscle atrophy and progressive weakness in draft horses 
-Muscle pain and gait abnormalities in Warmblood breeds, 
-Degradation of skeletal muscle tissue (acute rhabdomyolysis) upon exercise in Quarter Horses.
The severity of the clinical signs of PSSM can be muscle cramps, severe muscle pain and sometimes be a complete inability to stand.

References:
Aleman, M., Scalco, R., Malvick, J., Grahn, R.A., True, A., Bellone, R.R. (2022) Prevalence of genetic mutations in horses with muscle disease from a neuromuscular disease laboratory. J Equine Vet Sci 118:104129, 2022. Pubmed reference: 36150530. DOI: 10.1016/j.jevs.2022.104129.
Naylor, J.M., Nickel, D.D., Trimino, G., Card, C., Lightfoot, K., Adams, G. (1999) Hyperkalaemic periodic paralysis in homozygous and heterozygous horses: a co-dominant genetic condition Equine Veterinary Journal 31:153-159, 1999. Pubmed reference: 10213428.