Type 2 polysaccharide storage myopathy in Quarter Horses is a novel glycogen storage disease causing exertional rhabdomyolysis.
Authors: Valberg Stephanie J, Williams Zoë J, Finno Carrie J, Schultz Abigail, Velez-Irizarry Deborah, Henry Marisa L, Gardner Keri, Petersen Jessica L
Journal: Equine veterinary journal
Summary
# Editorial Summary: Type 2 Polysaccharide Storage Myopathy in Quarter Horses Polysaccharide storage myopathy manifests in two distinct forms in horses, yet whilst PSSM1's genetic basis (the R309H mutation in GYS1) has long been established, the underlying cause of PSSM2 in Quarter Horses remained undefined and characterisation of glycogen accumulation incomplete. Valberg and colleagues conducted a comprehensive investigation into PSSM2-QH by performing muscle biopsies and biochemical analysis on affected Quarter Horses, comparing muscle glycogen content and polysaccharide accumulation patterns with healthy controls and PSSM1-affected animals. The researchers identified that PSSM2-QH represents a distinct glycogen storage disease with abnormal polysaccharide aggregation in muscle fibres, though glycogen concentrations and the precise genetic mutations differ from the PSSM1 phenotype. This novel classification is clinically significant for farriers, veterinarians and trainers because it explains why some Quarter Horses with exertional rhabdomyolysis do not carry the PSSM1 mutation, enabling more targeted diagnostic approaches and dietary management strategies that may differ from conventional PSSM1 protocols. Recognising PSSM2-QH as a separate metabolic disorder opens avenues for genetic identification and potentially breed-specific nutritional intervention to prevent performance-limiting muscle disease.
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Practical Takeaways
- •PSSM2 in Quarter Horses is a distinct disease entity from PSSM1 requiring different diagnostic and management approaches
- •Genetic testing for R309H GYS1 mutation will not identify PSSM2-QH cases, necessitating muscle biopsy or novel genetic markers for diagnosis
- •Understanding glycogen metabolism abnormalities in PSSM2-QH may help inform exercise management and nutritional strategies to reduce exertional rhabdomyolysis episodes
Key Findings
- •PSSM2 in Quarter Horses is a novel glycogen storage disease distinct from PSSM1, with unknown genetic basis
- •PSSM2-QH is characterized by abnormal polysaccharide aggregates in skeletal muscle with previously undefined glycogen concentrations
- •PSSM2-QH causes exertional rhabdomyolysis in affected horses