| Contents |
|---|
| Glycogen Branching Enzyme Deficiency (GBED) |
| Summary |
| Understanding the Results |
| Disease Name and Genes |
| Inheritance |
| Resources |
Glycogen Branching Enzyme Deficiency (GBED)
Summary
Glycogen branching enzyme deficiency (GBED) is an inherited disease that leads to stillbirths and spontaneous abortions, and cardiac or respiratory failure, seizures, muscle weakness, and sudden death or euthanasia in foals under 18 weeks old. GBED results from a variant of the gene encoding the glycogen branching enzyme, GBE1. The variant form of this enzyme is unable to form α1,6 glycogen branches, leading to an inability to store functional glycogen in muscle and liver. Glycogen is used as energy storage, and is converted to glucose to provide energy to the body between meals, particularly the heart, skeletal muscles, and brain. Lack of energy to these tissues leads to heart, lung, and muscle weakness and failure typical of the disease. Many embryos affected with the disease will die in utero. Most foals affected with the disease will die within eight weeks. There is no treatment for the disease.
About 8.3% of Quarter Horses and 7.1% of Paint Horses have been found to be heterozygotes (carriers) of GBED, and all are related. At least 3% of spontaneous abortions in Quarter Horses are caused by GBED. No homozygous horses have lived more than 18 weeks. The American Quarter Horse Association requires genetic testing for GBED and four other genetic conditions (MH, PSSM1, HYPP, and HERDA).
Date of Last Update: 08/02/2016
Results
Understanding the Results
Results of the genetic test for GBED are presented as shown below.
| Glycogen branching enzyme deficiency (GBED) | ||
|---|---|---|
| N/N | Clear | This horse tested negative for GBE1-T34X and does not carry the GBED mutation identified in Quarter Horses. |
| N/Gb | Carrier | Both the normal and mutant alleles are present. This horse is positive for the GBE1-T34X mutation but will not develop symptoms. |
| Gb/Gb | Affected | This horse carries two copies of the GBE1-T34X mutation and will die of the disease. |
Disease Name and Genes
Glycogen branching enzyme deficiency, a glycogen storage disease also termed GSD IV, is caused by a C to A substitution at base 102 of the gene for glycogen branching enzyme GBE1. The change creates a termination codon in the codon 34 of the 699 amino acid GBE1 protein (GBE1-T34X). The variant is fully recessive.
Inheritance
GBED is caused by a recessive variant of the GBE1 gene (GBE1-T34X). The recessive allele is commonly abbreviated as Gb, with the dominant wild-type allele abbreviated as N.
Carriers of the recessive allele (N/Gb) have no symptoms of the disease. If two carriers are bred, each foal has a 25% chance of having two copies of the normal allele (N/N), a 50% chance of being a carrier (N/Gb), and a 25% chance of being affected (Gb/Gb).
If a carrier of the recessive allele (N/Gb) is bred to a normal horse (N/N), each foal has a 50% chance of having two copies of the normal allele (N/N) and a 50% chance of being a carrier (N/Gb).
