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.

Turnaround Time
At least 10 business days; may be delayed beyond 10 business days if sample requires additional testing, or a new sample is requested.

Sample Collection

Horse DNA tests are carried out using cells from the roots of a hair sample (roughly 20-30 hairs).

1. Grab about 10 hairs at the base.

2. Wrap the hairs around your finger and give it a quick pull.

3. Check the ends to make sure the pulled hairs have roots.

4. Repeat the process until you have collected about 20-30 hairs with intact roots.

5. You can choose different places on the mane or tail. NOTE: For foals, we recommend pulling all hairs from the tail only. 

6. Tape the hairs to the submission form and fold the form along the dotted line to protect the sample. Do not use ziploc bags as they can cause condensation that allows mold to grow on the hair.

Hairs with roots

7. Place the folded form containing the sample in a paper envelope and mail it to the laboratory.


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.