Effect of Diet on Haflinger Horses With GYS1 Mutation (Polysaccharide Storage Myopathy Type 1)

Describing the effects of a carbohydrate-rich diet (CRD) on Haflingers with R309 H glycogensynthase 1 (GYS1) mutation. Haflinger mares heterozygous positive (polysaccharide storage myopathy [PSSM] type 1, n = 7) and negative (control, n = 7) for GYS1 mutation received 6 weeks of CRD and hay only (hay), with a wash out period of two. Each diet was followed by oral glucose tolerance test (oGTF), submaximal treadmill exercise test (STET), and biopsies of semitendinosus (STm) and gluteus medius (GMm) muscles. Insulin sensitivity was evaluated by oGIT. Plasma creatine kinase, aspartate aminotransferase, lactate dehydrogenase, and lactate were measured before and after STET. Semitendinosus and GMm biopsies were evaluated for distribution of fiber types (1, 2A, and 2X), chronic myopathic changes, and complex Periodic Acid Schiff (cPAS) positive amylase resistant inclusions. One Haflinger of control group had cPAS inclusions indicating PSSM type 2; therefore, it was excluded. In PSSM1 horses, cPAS were found in every STm sample, and in 10 of 14 GMm samples, furthermore, STm samples showed a greater total myopathy score and significantly more type 2A and fewer type 2X fibers compared with controls. There were no significant differences between groups regarding enzyme levels and for oGTT. Diet had no effect on muscle enzyme levels after STET and on cPAS inclusions in PSSM1 group. Glycogensynthase 1 mutation in Haflingers was not accompanied by clinical signs, even after STET. Carbohydrate-rich diet for 6 weeks did not induce clinical signs or increase cPAS inclusions. As PSSM type 2 is a possibility in Haflingers, STm muscle biopsy and cPAS evaluation should be performed if GYS1 gene test is negative.