Glycogen Branching Enzyme Deficiency in a foal
History: A 1 week-old male Quaterhorse was submitted for necropsy. At 12 hours old he nursed 3 times but became progressively recumbent and failed to nurse. Upon arrival to the referral hospital he was laterally recumbent with poor mentation. Initial blood chemistry revealed blood glucose of 93 mg/dL, BUN 27 mg/dL, Creatinine of 4.3 mg/dL, lactate of 2.2 mmol/L, CK > 30,000 u/L, and hypoproteinemia. Muscle biopsies were sent to the University of Minnesota. CBC indicated left shift. Later CBC/Chem indicated leukopenia, anemia, hypoproteinemia, hyperfibrinogenemia, and hyperglycemia. He died acutely.
The foal was thin with minimal body fat stores. The cranial lung lobes were bilaterally dark red, and firm. There was mild interlobular edema and collapse of the cranial lung lobes. The trachea, primary bronchi, and itra-lobular bronchi contained serous and fibrinous exudate.
The skeletal muscles were diffusely soft pale and pink.
Skeletal muscle: There is mild multifocal myocyte degeneration and necrosis. Diffusely large numbers of myocyes contain intracytoplasmic pale basophilic round inclusions which replace the sarcoplasm. Myocytes also have pale vacuoles in the cytoplasm.
Heart: Cardiac myocytes are similar to skeletal muscle. The most striking feature is the large basophilic inclusions in Purkinje fibers.
Heart, Skeletal muscle PAS w/ glycogen digestion: Both the pale basophilic inclusions and the pale vacuolar areas in the cytoplasm are positive with PAS/with glycogen digestion (using diastase or amylase).
Skeletal muscle: Generalized multifocal myocyte necrosis, with diffuse pale basophilic amylase resistant sarcoplasmic inclusions, and amylase resistant polysaccharide.
Heart: Diffuse cardiomyocyte and Purkinje fiber inclusions and amylase resistant polysaccharide.
Glycogen Branching Enzyme Deficiency is an inherited defect in an enzyme that creates branches in glycogen for storage in tissues. This disease is similar to the Type IV glycogenosis in humans (glycogen storage disease caused by a deficiency in alpha-1,4-glucan 6-glycosyl transferase). It is also similar to adult polyglucosan body disease in humans, also caused by a defect in a glycogen branching enzyme. Organs affected with these disease include liver, CNS, PNS, and muscles. The defect is an inherited autosomal recessive mutation in the GBE1 gene, causing a premature stop codon. This mutation results in poorly functional enzyme and therefore poorly branched glycogen, which is not capable of being transformed into glucose by debranching enzyme.
The University of California Davis and Vetgen are licensed to conduct testing to test a foal, mare or stallion for carrier status. You can find more information at www.vgl.ucdavis.edu and www.vetgen.com.
Ward, T.L. et al. 2004. Glycogen branching enzyme (GBE1) mutation causing equine glycogen storage disease IV. Mammalian Genome vol 15: 570-577.
Valberg, S.J. et al. 2001. Glycogen Branching Enzyme Deficiency in Quarter Horse Foals. J. Vet. Intern. Med. Vol 15:572-580.