Fatal Genetic Quarter Horse Disease Identified

An inherited disorder called glycogen branching enzyme deficiency (GBED) in American Quarter Horses and related breeds that appears to be the cause for many unexplained stillbirths and deaths of young foals was identified by the University of Minnesota's Stephanie Valberg, DVM, PhD, Department of Clinical and Population Sciences, and Jim Mickelson, PhD, associate professor veterinary pathobiology, both in the College of Veterinary Medicine. The disease was found to be an autosomal recessive trait, meaning each normal parent carries one allele that is defective and passes that "mutant" along to the foal, which ends up with a pair of defective alleles and expression of the disease.

This discovery means GBED could be prevented prior to breeding if parents are tested and carriers are not mated. (This type of recessive gene passage is similar to HC/HERDA found in Quarter Horses, but there isn't a genetic test available to identify carriers of HC/HERDA.)

GBED-affected foals are either born dead or are weak at birth with a low body temperature, according to Valberg. With assistance to nurse and intense nurturing, such foals often become stronger and nurse on their own. However, their activity level is typically less than that of healthy foals; they might have flexural deformities of the lower limbs, show a more rapid rate of breathing, and have intermittent seizures or signs easily interpreted as sepsis or infection. Most foals with GBED die suddenly by eight weeks of age due to a fatal drop in blood glucose levels or sudden heart failure. Some foals are euthanized because they become progressively weaker and are unable to rise without assistance.

A new type of equine disease was first suspected when muscle biopsies from a foal at Kansas State University were submitted to Valberg's Neuromuscular Diagnostic Laboratory. She found that this foal--as well as muscles from other affected foals--had a lower-than-expected amount of glycogen (the intracellular storage form of glucose) and an abundance of abnormal polysaccharide that formed both crystalline rods and large globules within the cells. This histopathological picture was very similar to human glycogen storage disease type IV that is due to a deficiency in the glycogen branching enzyme (GBE).

Further biochemical analysis in Mickelson's laboratory revealed that muscle and liver tissues from affected foals contained an unbranched form of polysaccharide, and that little or no GBE activity was present. Glycogen within cells normally exists as a 12-tiered tree-like structure with thousands of branch points that allow for the rapid storage and mobilization of glucose for energy metabolism. The lack of branch points on the glycogen molecule--due to the missing GBE enzyme activity--meant that none of the GBED foal's tissues could efficiently store and metabolize glucose. The lack of available energy from stored glucose in GBED likely affects tissues differently at different times. In the brain it could produce seizures, in the muscles it could produce weakness and contracted tendons, and in the heart it could cause sudden cardiac arrest.

Valberg said DVM/PhD student Tara Ward, working in Mickelson's laboratory, was able to identify the location of the GBE1 gene on horse chromosome 26, and the complete DNA sequence for the region of the GBE1 gene that codes for amino acids in the GBE protein. When the control horse and GBED foal DNA sequences were compared, a single DNA base pair alteration was evident. This GBE1 mutation caused a stop codon for protein synthesis to be inserted very early in the GBE1 mRNA, and meant that GBE protein could not be produced in GBED foals. GBED foals carry two copies (alleles) of the mutant GBE1 gene, and their parents each carried one normal and one mutant GBE1 allele. This pattern of distribution of GBE1 alleles is consistent with an autosomal recessive disease. To date, 12 foals have been identified with GBED, but analysis of their extended pedigrees indicates this is likely only the tip of the iceberg.

The discovery of the equine GBE1 mutation will be published in the July issue of the journal Mammalian Genome, noted Valberg. Work is now ongoing to determine the prevalence of the mutant GBE1 allele in the U.S. Quarter Horse population and to estimate its overall role in foal death and late-term abortion. A DNA-based test for horse breeders to use to determine if mares and stallions are carriers of the GBE1 mutation, and enable them to reduce or eliminate production of GBED foals, will be offered.

Financial support for this research comes from the American Quarter Horse Association, the Minnesota Racing Commission, and the University of Minnesota Equine Center.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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