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What Is GBED?

Glycogen-branching enzyme deficiency is an autosomal recessive genetic trait. Foals must inherit two copies of the faulty gene, one from each parent, to suffer from GBED symptoms. The disease is passed on from carrier horses that have one normal gene and one recessive GBED gene. Carrier horses are healthy because the GBED gene is recessive to its normal counterpart gene, but a mating of two carrier horses has a 1-in-4 chance of producing an affected foal. The disorder follows the same pattern as HERDA, hereditary equine regional dermal asthenia, another autosomal recessive trait, which causes fragile, slow-healing skin.

“It’s exactly the same as HERDA, in that it’s recessive,” said Stephanie Valberg, D.V.M., Ph.D., professor of large animal medicine and director of the University of Minnesota’s Equine Center. She spearheaded the research that identified GBED and developed the DNA test to pinpoint carriers. “The genetic mutation of GBED was identified before HERDA, but it just didn’t get the same press. I’m hoping that information [about GBED] will get out and become more of a discussion point among owners.”

Dr. Valberg’s GBED research began in 1996, as an offshoot of a study already underway on another genetic glucose-storage disorder, polysaccharide storage myopathy (PSSM). Both projects received funding from the American Quarter Horse Association (AQHA) Foundation.

Foals that inherit two copies of the GBED gene lack the enzyme responsible for storing glucose in the form of big-branched glycogen molecules. Glycogen must be organized this way for the body to break it down and utilize it for energy. All life processes are fueled by glycogen, so a GBED-affected foal experiences total system failure. Muscles, heart, liver and brain all malfunction.

Sick foals display a multitude of symptoms including weakness, contracted tendons and low body temperature. Some are able to stand and nurse, but then collapse with seizures when their blood sugar plummets. Heroic measures can sometimes prolong a GBED foal’s life, but the ending is always the same.

“You cannot save them. You can keep them alive by providing them with adequate nutrition – the maximum amount of time would be 18 weeks that we’ve seen foals live – but they just don’t have enough energy to keep their heart beating and their muscles working, and they die,” Valberg said.

Read the June 1 issue of Quarter Horse News to read the full article.