Muscle Integrity Myopathy (formerly "PSSM2") in horses
The smooth and highly complex interaction required for proper muscular function depends on many genes and requires an environment that meets the horse’s physiological needs. With regard to the role of genes, some act as critical control points, while others function more like gears within a large mechanical system. Defects may have dramatic consequences or may be compensated, at least partially or temporarily, through various biological mechanisms. The resulting muscle disorders often present with a similar and diffuse spectrum of clinical signs, collectively referred to as “exertional myopathy.”
We use the term MIM (Muscle Integrity Myopathy) to describe those forms of exertional myopathy in which the muscle cells are primarily impaired in their structure (due to defective proteins) and/or in their function (such as impaired detoxification processes or disturbed signal conduction).
As with all forms of exertional myopathy, environmental conditions also play a major role in the development and severity of MIM.
What are the possible symptoms of Muscle Integrity Myopathy?
Common symptoms include:
• Pain-related changes in temperament and behaviour
• Difficulty with gait changes, coordination
• Lack of power from the hindquarters
• Difficulty building muscle (especially hindquarters and topline/shoulders)
• Shifting, unexplained lameness
• Muscle stiffness in the hindquarters or topline, often requiring a long warm-up time
• High muscle tension or muscle tremors/cramping, often aggravated by cold weather or stress
• Muscle loss in the hindquarters and topline, and/or focal muscle atrophy (appearing like kick marks)
• Ataxic gait/coordination problems
• Blood levels of Creatinine Kinase (CK) and Aspartate Aminotransferase (AST) are usually within normal limits, even with obvious symptoms of muscle disease
Breeds affected of Muscle Integrity Myopathy (formerly known as 'PSSM2')
Which breeds can be affected from Muscle Integrity Myopathy (formerly known as ('PSSM2')?
Most breeds can be affected with the symptoms of MIM.
The P2, P3, P4, Px, P8, and K1 variants have been found in nearly all breeds of horses; in Icelandic horses only the P8, K1 and (rarely) P3 variants have been found.
Which Genes are involved with Muscle Integrity Myopathy (formerly known as 'PSSM2') /Test Information
Six semi-dominant gene variants (P2, P3, P4, P8, Px and K1) are currently being investigated as risk factors for the occurrence of Muscle Integrity Myopathy symptoms.
This Panel test identifies the changes in the MYOT (P2), FLNC (P3), MYOZ3 (P4), PYROXD1 (P8), COL6A3 (K1) and CACNA2D3 (Px) genes.
The Equine Myopathy panel is offered under license from EquiSeq Inc. and is based upon their research.
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Literature
McCue ME et al. (2008). „Glycogen synthase (GYS1) mutation causes a novel skeletal muscle glycogenosis.“ Genomics. 91(5):458-66. PMID: 18358695.
McCue ME et al. (2008). „Glycogen synthase 1 (GYS1) mutation in diverse breeds with polysaccharide storage myopathy.“ Journal of Veterinary Internal Medicine. 22(0):1228–1233. PMID: 18691366.
McCue ME et al. (2009). „Polysaccharide storage myopathy phenotype in quarter horse-related breeds is modified by the presence of an RYR1 mutation.“ Neuromuscular Disorders. 19(0):37–43. PMID: 19056269.
McCue ME et al. (2009). „Comparative skeletal muscle histopathologic and ultrastructural features in two forms of polysaccharide storage myopathy in horses.“ Vet Pathol. 46(6):1281-1291. PMID: 19605906.
Maile CA et al. (2017). „A highly prevalent equine glycogen storage disease is explained by constitutive activation of a mutant glycogen synthase.“ Biochim Biophys Acta.. 1861(1):3388-3398. PMID: 27592162.
Valberg SJ et al. (2016). „Suspected myofibrillar myopathy in Arabian horses with a history of exertional rhabdomyolysis.“ Equine Vet J.. 48(5):548-556. PMID: 26234161.
Lewis SS et al. (2017). „Clinical characteristics and muscle glycogen concentrations in warmblood horses with polysaccharide storage myopathy“ Am J Vet Res. 78(11):1305-1312. PMID: 29076373.
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