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Breeds: American Indian, Appaloosa, Appaloosa Cross, Appaloosa/Friesian, Appaloosa/Quarter Horse, Appaloosa/Thoroughbred, Appendix, AraAppaloosa, Arabian/Paint, Arabian/Quarter Horse, Australian Stock Horse, Azteca, Baroque Pinto, Canadian, Cleveland Bay, Clydesdale/Paint Cross, Colonial Spanish Horse, Crossbred, Curly Horse, Grade Horse, Grade Pony, Miniature Appaloosa, Mixed Breed, Mixed Breed (Horse), Morgan Horse, Origine Constatée, Paint Cross, Paint Horse, Paint Horse/Thoroughbred, Paint/Warmblood, Percheron Cross, Ponies of America, Pura Raza Espanola, Quarter Cross, Quarter Horse, Quarter Pony, Quarter/Paint Horse, Quarter/Stock Horse, Quarter/Thoroughbred, Quarter/Warmblood, Spanish Barb, Spanish Mustang, Stock Horse, Stonewall Sporthorse, Sugarbush Harlequin Draft, Thoroughbred, Unspecified, Welsh Mountain/Quarter Horse
Description
Glycogen Branching Enzyme Deficiency (GBED), also known as Glycogen Storage Disease Type IV (GSD IV), is a fatal inherited disorder in which the body cannot properly store sugar. Normally, glucose is stored as glycogen, a highly branched molecule that serves as an energy reserve for the muscles, heart, and brain. Horses with GBED lack the glycogen branching enzyme needed to build this structure, so they cannot store or use energy effectively.
GBED is caused by a nonsense mutation in the GBE1 gene (c.102C>A, p.Tyr34Ter) that creates a premature stop codon and prevents production of a functional enzyme. The mutation was identified by researchers at the University of Minnesota. Many affected fetuses are aborted or stillborn. Foals that are born alive show weakness, difficulty rising, low body temperature, contracted muscles, seizures, and sudden death. There is no treatment, and affected foals typically die or are euthanized before 8 weeks of age. Research suggests that approximately 3% of aborted Quarter Horse fetuses are homozygous for the mutation.
GBED is inherited as an autosomal recessive trait. Studies estimate that roughly 4 to 11% of Quarter Horses and Paint Horses are carriers. Carriers show no signs of the disorder, so DNA testing is the only way to identify them. Screening breeding stock allows breeders to avoid pairings (N/Gb x N/Gb) that carry a 25% risk of producing an affected foal.
Possible results
| Genotype | Description |
|---|---|
| Gb/Gb | Affected: Horse has two copies of the GBED variant and is expected to develop signs of the disorder. |
| N/Gb | Carrier: Horse has one copy of the GBED variant and is not expected to show signs of the disorder. Each offspring has a 50% chance of inheriting the variant. |
| N/N | Clear: Horse does not carry the GBED variant and cannot pass it on to offspring. |
Breeds Affected
American Indian, Appaloosa, Appaloosa Cross, Appaloosa/Friesian, Appaloosa/Quarter Horse, Appaloosa/Thoroughbred, Appendix, AraAppaloosa, Arabian/Paint, Arabian/Quarter Horse, Australian Stock Horse, Azteca, Baroque Pinto, Canadian, Cleveland Bay, Clydesdale/Paint Cross, Colonial Spanish Horse, Crossbred, Curly Horse, Grade Horse, Grade Pony, Miniature Appaloosa, Mixed Breed (Horse), Morgan Horse, Origine Constatée, Paint Cross, Paint Horse, Paint Horse/Thoroughbred, Paint/Warmblood, Ponies of America, Pura Raza Espanola, Quarter Cross, Quarter Horse, Quarter Pony, Quarter/Paint Horse, Quarter/Stock Horse, Quarter/Thoroughbred, Quarter/Warmblood, Spanish Barb, Spanish Mustang, Stock Horse, Stonewall Sporthorse, Sugarbush Harlequin Draft, Thoroughbred
Reference
Glycogen branching enzyme (GBE1) mutation causing equine glycogen storage disease IV.
[PMID: 15366377]
[PMID: 17063718]