What is glycogen branching enzyme deficiency?
Glycogen branching enzyme deficiency (GBED) is a fatal disease of developing fetuses or newborn foals. It is inherited as an autosomal recessive disease in Quarter Horses and related breeds. Affected horses lack the enzyme necessary to store glycogen (storage sugar molecule) in its branched form and therefore cannot store sugar molecules needed by the heart and skeletal muscle, liver and brain. As a result, these organs are not able to function properly. A causative genetic mutation has been identified and a genetic test is available.
What are the clinical signs of glycogen branching enzyme deficiency?
Glycogen branching enzyme deficiency (GBED) may result in late-term abortions or stillbirths. Affected foals that are born are weak with decreased muscle tone (hypotonia). They may be hypothermic (i.e. have low body temperatures), have limb deformities and exhibit seizures, cardiac arrest, and respiratory failure. Veterinary care may initially be sought for other illnesses, including septicemia or pneumonia. All known cases of GBED affected foals have died or were euthanized within five months after birth.
How is glycogen branching enzyme deficiency diagnosed?
Genetic testing is required for a definitive diagnosis of GBED. Muscle biopsies may show the absence of normal glycogen staining, but this alone is not conclusive for a diagnosis. Blood work may show a low white blood cell count, low blood glucose, high muscle enzymes (CK and AST) and high GGT (a specific liver enzyme).
How is glycogen branching enzyme deficiency treated?
There is no treatment for GBED.
What is the prognosis for glycogen branching enzyme deficiency?
The prognosis is grave. Most GBED-affected foals die while they are hospitalized, shortly after discharge, or are euthanized.
How can glycogen branching enzyme deficiency be prevented?
The GBED carrier frequency is estimated to be 8.3% in Quarter Horses and 7.1% in Paint Horses. Quarter Horse and Paint Horse sires and dams should be genetically tested prior to breeding to avoid carrier x carrier matings. Hair samples (with the roots attached) can be submitted to the UC Davis Veterinary Genetics Laboratory. Offspring of two GBED carriers have a 25% chance of being affected. Breeding a horse with no copies of the GBED mutation to a carrier (those with one copy) will not result in an affected foal but will produce a GBED carrier 50% of the time. The American Quarter Horse Association (AQHA) requires GBED genetic testing for all breeding stallions and it is one of the tests included in the AQHA 5 panel test.
For more information:
Valberg, S.J., and J. Mickelson. 2006. Glycogen-Branching Enzyme Deficiency. AAEP Proceedings 52: 351-353.
Ward T.L., Valberg S.J., Adelson D.L., et al. 2004. Glycogen branching enzyme (GBE1) mutation causing equine glycogen storage disease IV. Mammalian Genome 15: 570 –577.