Allele frequency and likely impact of the glycogen branching enzyme deficiency gene in Quarter Horse and Paint Horse populations.
Authors: 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
Journal: Journal of veterinary internal medicine
Summary
# Glycogen Branching Enzyme Deficiency in Quarter Horses and Paint Horses Glycogen branching enzyme deficiency (GBED) causes fatal glycogen accumulation in foetal and neonatal foals of Quarter Horse and Paint Horse descent, resulting from a nonsense mutation in the GBE1 gene that prevents synthesis of functional branching enzyme protein. Wagner and colleagues screened random samples from major breeds to establish carrier frequencies and retrospectively identified GBED cases amongst fetal and neonatal deaths archived at veterinary diagnostic laboratories, finding mutant allele frequencies of 4.1% in Quarter Horses, 3.6% in Paint Horses, and notably absent in Thoroughbreds. Approximately one in forty fetal and early neonatal deaths in Quarter Horse-related breeds were homozygous for the mutation, predominantly representing abortions, with characteristic periodic acid Schiff-positive glycogen inclusions visible in cardiac or skeletal muscle of eight of nine cases examined histologically. Pedigree analysis confirmed simple autosomal recessive inheritance from a single founder, meaning carriers (heterozygotes) remain clinically normal but risk producing affected foals when mated together. Given that GBED accounts for a meaningful proportion of reproductive loss in these populations, DNA-based genetic testing now permits breeders and veterinarians to identify carriers and strategically plan matings to prevent affected conceptions—making this screening particularly valuable where carrier frequencies are highest.
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Practical Takeaways
- •DNA testing can identify carrier and affected horses to enable selective breeding that prevents GBED conception in Quarter Horse and Paint Horse lines
- •If unexplained fetal loss or neonatal death occurs in these breeds, GBED should be included in diagnostic considerations and histopathology may reveal characteristic glycogen accumulation
- •Quarter Horse and Paint Horse breeders should consider genotyping breeding stock given the significant association of GBED homozygosity with abortion and early neonatal mortality
Key Findings
- •Mutant GBE1 allele frequency was 4.1% in Quarter Horses, 3.6% in Paint Horses, and 0% in Thoroughbreds
- •Approximately 2.5% of fetal and early neonatal deaths in Quarter Horse-related breeds were homozygous for the mutant GBE1 allele
- •GBED is inherited as a simple recessive trait from a single founder
- •PAS-positive inclusions in cardiac or skeletal muscle characteristic of GBED were detected in 8 of 9 homozygous cases