Glycogen Branching Enzyme Deficiency in Quarter Horse Foals

Seven related Quarter Horse foals that died by 7 weeks of age were examined for glycogen branching enzyme (GBE) deficiency. Clinical signs varied from stillbirth, transient flexural limb deformities, seizures, and respiratory or cardiac failure to persistent recumbency. Leukopenia (5 of 5 foals) as well as high serum creatine kinase (CK; 5 of 5), aspartate transaminase (AST; 4 of 4), and gamma glutamyl transferase (GGT; 5 of 5) activities were present in most foals, and intermittent hypoglycemia was present in 2 foals. Gross postmortem lesions were minor, except for pulmonary edema in 2 foals. Muscle, heart, or liver samples from the foals contained abnormal periodic acid Schiff's (PAS)‐positive globular or crystalline intracellular inclusions in amounts proportional to the foal's age at death. Accumulation of an unbranched polysaccharide in tissues was suggested by a shift in the iodine absorption spectra of polysaccharide isolated from the liver and muscle of affected foals. Skeletal muscle total polysaccharide concentrations were reduced by 30%, but liver and cardiac muscle glycogen concentrations were normal. Several glycolytic enzyme activities were normal, whereas GBE activity was virtually absent in cardiac and skeletal muscle, as well as in liver and peripheral blood cells of affected foals. GBE activities in peripheral blood cells of dams of affected foals and several of their half‐siblings or full siblings were ∼50% of controls. GBE protein in liver determined by Western blot was markedly reduced to absent in affected foals, and in a half‐sibling of an affected foal, it was approximately one‐half the amount of normal controls. Pedigree analysis also supported an autosomal recessive mode of inheritance. The affected foals have at least 2,600 half‐siblings. Consequently, GBE deficiency may be a common cause of neonatal mortality in Quarter Horses that is obscured by the variety of clinical signs that resemble other equine neonatal diseases.