Genetic Mutation for Polysaccharide Storage Myopathy Widespread

Veterinary researchers from the University of Minnesota have identified a gene potentially responsible for polysaccharide storage myopathy (PSSM) in Quarter Horses and, in doing so, they uncovered a number of research leads to pursue. In a recent study investigators found the gene in nearly half of 36 different breeds of horses tested. They consider this surprising, as PSSM is most often thought of as a disease of Quarter Horses and Drafts.

PSSM is a disease characterized by the abnormal accumulation of the polysaccharide glycogen--long chains of sugar molecules used for storing energy--in the skeletal muscles. The disease can be debilitating and life-threatening.

"Our previous research efforts revealed that if a gene called glycogen synthase 1 (GYS1) has a specific, single mutation then the activity of the enzyme is increased resulting in the overproduction of glycogen in muscle tissues," explained Molly McCue, DVM, MS, PhD, Dipl. ACVIM, a member of the research team.

Muscle biopsy

Performing a muscle biopsy.

The next step in the group's PSSM research schedule was to determine how many horses, regardless of breed, diagnosed with PSSM have the GYS1 mutation and if horses diagnosed with either Grade 1 or Grade 2 PSSM had the mutation.

From the Neuromuscular Disease Laboratory database, the researchers obtained genetic material (DNA) from 831 horses diagnosed with PSSM by muscle biopsy. They analyzed the DNA to determine which horses had the mutation.

Of the 36 different breeds of horses included in the study, 17 breeds had the GYS1 mutation.

"This included 87% of the Draft horses diagnosed with PSSM by muscle biopsy, 72% of all Quarter Horse-related breeds, but only 18% of Warmbloods and 24% of other light horse breeds," explained McCue.

This study also evaluated the prevalence of the genetic mutation based on severity grade of the biopsy and found that 70% of horses diagnosed with a Grade 2 biopsy (moderate to severe disease) had the GYS1 mutation, while only 16% of horses with Grade 1 biopsy (mild disease) had the mutation.

This data confirms the subjective nature of a Grade 1 biopsy diagnosis, and it also indicates that there might be other causes of PSSM besides the GYS1 mutation.

McCue said that based on their research findings they are now using different terms to indicate whether the horse has PSSM caused by the GYS1 mutation versus a horse lacking the mutation but showing abnormal accumulation of glycogen on muscle biopsy.

"At present, the best way to diagnose PSSM appears to be the genetic test for PSSM on hair or blood samples in those breeds affected by this disease, followed up with a muscle biopsy to look for other causes of muscle disease if the DNA test is negative," relayed McCue.

For more information on PSSM testing, please visit:

The study, "Glycogen synthase 1 (GYS1) mutation in diverse breeds with polysaccharide storage myopathy," was published in the September/October edition of the Journal of Veterinary Internal Medicine.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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