Glycogen storage diseases in horses are characterized by aggregates of abnormal polysaccharide in skeletal muscle. Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR, professor emeritus at Michigan State University’s McPhail Equine Performance Center, and colleagues investigated exertional rhabdomyolysis in Quarter-Horse-related breeds (Quarter Horses, Paints, Appaloosas) with type 2 polysaccharide storage myopathy (PSSM2-QH).
The researchers evaluated a retrospective selection of PSSM2-QH cases from the Michigan State University Neuromuscular Diagnostic Laboratory database for horse submissions from December 1993 to November 2021. These horses were Quarter-Horse-related breeds with histories of rhabdomyolysis that were negative for the GYS1 mutation found in PSSM1 horses and had specific muscle staining characteristics on muscle biopsies.
Glycogen Concentrations in Quarter Horses With PSSM2
Quarter-Horse-related breeds with PSSM2-QH have a 1.6-fold higher muscle glycogen concentration than normal control horses, which is lower than horses affected with PSSM1. The research group found that PSSM2-QH “is a novel glycogenosis potentially caused by variants in genes not currently associated with skeletal muscle glycogen storage disease or in genes that either alter the regulation of glycogen-related genes.”
PSSM2 in Quarter Horse Pedigrees
The researchers compared the pedigrees of Quarter-Horse-related breeds with PSSM2 and identified three families descended from three popular sires in barrel racing and working cow/roping horse pursuits. Seventeen of the 30 PSSM2-QH in the study descended from one of three stallions over four generations. This high prevalence of PSSM2-QH in horses in these disciplines differs from PSSM1 found mostly in halter and pleasure horse pedigrees. PSSM2-QH occurs fairly evenly in male and female horses, so it doesn’t correspond to inheritance through an X chromosome. Mutations that disrupt glycogen synthesis or ubiquitin assembly are prevalent in PSSM2-QH. (Ubiquitin is a protein that helps regulate other protein processes, including immune responses, cell proliferation, angiogenesis, DNA repair, and apoptosis.)
Diagnosing PSSM2
The study authors reported that approximately 60% of Quarter Horses might possess variant genes. However, they do not have clinical or histopathological evidence of PSSM2, making diagnostic genetic testing uncertain. Muscle biopsy samples are currently the best means of diagnosis since PSSM2 is a histopathological description of PAS-stained (periodic acid-Schiff) polysaccharide rather than a specific etiology. Muscle biopsies do pose diagnostic challenges because 72% of affected horses lack amylase-resistant polysaccharide (Ar-PS), which are abnormal polysaccharide aggregates typical of PSSM2. In addition, samples must be well-chilled and shipped immediately to the lab to preserve glycogen concentrations without degradation.
Management Strategies
In humans with this condition, dietary intervention can be helpful. Controlled exercise is important to reduce dependence on glycogen while increasing fatty acid oxidation. Regular exercise is found to be helpful for Quarter Horses with PSSM2, and anecdotal reports suggest a low-nonstructural carbohydrate diet with fat supplementation might be beneficial.
Reference
Valberg SJ, Williams ZJ, Finno CF, et al. Type 2 polysaccharide storage myopathy in Quarter Horses is a novel glycogen storage disease causing exertional rhabdomyolysis. Equine Veterinary Journal Aug 2022; DOI: 10.1111/evj.13876
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