Glycogen Branching Enzyme Deficiency (GBED)

Quick Summary

Glycogen branching enzyme deficiency (GBED) is a fatal genetic disorder that results from the inability to correctly store glycogen in several organs of the body.

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Phenotype: Glycogen branching enzyme deficiency (GBED) is an inherited disease found primarily in Quarter Horses and related breeds in which the inability to correctly store glycogen is fatal for foals.

Mode of Inheritance: Autosomal recessive

Alleles: N = Normal/Unaffected, G = Glycogen branching enzyme deficiency

Breeds appropriate for testing: Quarter Horse and related breeds

Explanation of Results:

  • Horses with N/N genotype will not have glycogen branching enzyme deficiency and cannot transmit this glycogen branching enzyme deficiency variant to their offspring.
  • Horses with N/G genotype will not be affected by glycogen branching enzyme deficiency, but are carriers. They may transmit this GBED variant to 50% of their offspring. Matings between two carriers result in a 25% chance of producing a GBED-affected foal.
  • Horses with G/G genotype will have glycogen branching enzyme deficiency, a fatal condition.

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Turnaround Time
At least 15 business days; may be delayed beyond 15 business days if sample requires additional testing, or a new sample is requested.
Additional Details

Glycogen branching enzyme deficiency (GBED) is a fatal disease that is seen in Quarter Horses and related breeds. Affected animals may be aborted or stillborn, and foals that survive to term typically die or are euthanized by 18 weeks due to severe muscle weakness. GBED affected horses lack glycogen branching enzyme, the enzyme necessary to properly make the branched sugar storage molecules known as glycogen. Therefore affected horses cannot properly store sugar and tissues that rely on glycogen for energy are impaired. This disease is fatal as the brain, heart muscle, and skeletal muscles become weak and are unable to function.

GBED is inherited as an autosomal recessive trait, meaning that two copies of the affected allele are required to cause the disease. The mutation responsible for GBED is in the gene glycogen branching enzyme or GBE1 for short. The mutation, a single base change in the DNA (c.102C>A), results in the production of a nonfunctional form of the glycogen branching enzyme. Specifically, this change is a nonsense mutation that results in a shortened protein product (denoted as p.Y34*, because the protein is truncated after the 34th amino acid).

Prevalence of the GBED carriers in the United States Quarter Horse population is estimated to be between 8-11%, but can vary by discipline of Quarter Horse, and is reported to be highest in the Western Pleasure and Cutting disciplines.

Testing for GBED is important to assist veterinarians to make the correct diagnosis and to assist owners in breeding decisions. Matings between two GBED carriers have a 25% chance of producing GBED-affected foals and thus should be avoided.

Type of Sample


Type of Test
Results Reported As
Test Result Glycogen Branching Enzyme Deficiency (GBED)
N/N Normal. No copies of the GBED allele detected.
N/G Carrier. One copy of the GBED allele detected.
G/G Affected. Two copies of the GBED allele detected. This is a fatal disease.

Ward, T., Valberg, S., Adelson, D., Abbey, C., Binns, M., & Mickelson, J. (2004). Glycogen branching enzyme (GBE1) mutation causing equine glycogen storage disease IV. Mammalian Genome, 15(7). doi: 10.1007/s00335-004-2369-1

Wagner, M. L., Valberg, S. J., Ames, E. G., Bauer, M. M., Wiseman, J. A., Penedo, M. C. T., Kinde, H., Abbitt, B., & Mickelson, J. R. (2006). Allele Frequency and Likely Impact of the Glycogen Branching Enzyme Deficiency Gene in Quarter Horse and Paint Horse Populations. Journal of Veterinary Internal Medicine, 20(5), 1207-1211. doi: 10.1111/j.1939-1676.2006.tb00724.x

Araujo, C. E., Delfiol, D. J., Badial, P. R., Oliveira-Filho, J. P., Araujo-Junior, J. P., & Borges, A. S. (2018). Prevalence of the Glycogen Branching Enzyme Deficiency Mutation in Quarter Horses in Brazil. Journal of Equine Veterinary Science, 62, 81-84. doi: 10.1016/j.jevs.2017.10.010

License Info

This test is performed under license from the University of Minnesota.