Warmblood horses, which have long been bred as riding horses in Europe, are becoming increasingly popular in North America as performance horses. Exertional myopathies cause performance limitations in warmblood horses, and researchers have found that PSSM is the most frequently diagnosed myopathy in North American and European warmblood horses.1–3 A diagnosis of PSSM is based on identification of aggregates of amylase-resistant or amylase-sensitive PAS-positive polysaccharide inclusions in skeletal muscle fibers.4–6 Since the identification of a dominant nonsynonymous gain-of-function R309H mutation in GYS1, PSSM has been subdivided into PSSM1, which represents horses with the GYS1 mutation, and PSSM2, which represents horses with abnormal PAS staining but that lack the GYS1 mutation.4,7 It is known that PSSM1 is a glycogen storage disorder with muscle glycogen concentrations that are elevated (typically 1.8-fold as high) as a result of increased and poorly regulated glycogen synthase activity.5,7 Muscle glycogen concentrations have not been reported for PSSM2, and the cause or causes of PSSM2 are not known.
Warmblood horses reportedly have both PSSM1 and PSSM2.8,9 The R309H GYS1 mutation was found in a family of Selle Français with PSSM, and many of these horses developed clinical signs of exertional rhabdomyolysis.8 In warmblood horses, PSSM2 is more common than PSSM1, with only 8% of warmblood horses with PSSM having the GYS1 mutation.9 In contrast, the GYS1 mutation is found in 76% of Quarter Horses with PSSM.9 Investigators of 1 report1 described the clinical signs of PSSM in warmblood horses; however, that report preceded the discovery of the GYS1 mutation. Thus, although PSSM2 appears to be common in warmblood horses, no studies have determined whether the clinical features of PSSM2 in warmblood horses are distinct from those of warmblood horses with PSSM1 or from those of nonwarmblood breeds with PSSM2.
The purpose of the study reported here was to use information in a database to characterize the signalment and primary clinical signs of warmblood horses with PSSM1 and PSSM2 and to determine whether muscle glycogen concentrations are elevated in warmblood horses with PSSM1 and PSSM2.
Supported by the Endowment of the Mary Anne McPhail Equine Dressage Chair.
Dr. Valberg is a member of the group that licenses the genetic test for PSSM1 mentioned in this report and receives royalties from sale of the test.
The authors thank Keri Gardner for technical assistance.
Glycogen synthase 1 gene
Polysaccharide storage myopathy
Type 1 polysaccharide storage myopathy
Type 2 polysaccharide storage myopathy
Prism, version 5.0, GraphPad Software Inc, La Jolla, Calif.
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