The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.
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27 March 2018 THE HORSE TheHorse.com cells. Mutations in the DNA can produce faulty proteins. Sometimes those deviant proteins do not function, whereas other times, proteins like glycogen synthase become more active, going above and beyond the call of duty. The term autosomal means the defec- tive gene occurs on any chromosome except the X and Y "sex" chromosomes. In this case, the GYS1 gene is on ECA10 (equine chromosome 10). Dominant means only one copy of the gene is needed to cause production of the defective enzyme. Recall that all living organisms have two copies of each gene, one coming from the sire and one from the dam. Therefore, the GYS1 mutation doesn't occur on a sex chromosome, requires only one of the two potential copies of the mutation to be effective, and results in excess accumulation of polysaccharides— specifically, glycogen—in muscle cells. "One of our latest research projects found that horses diagnosed with PSSM1 had the same 'amount' of enzyme pro- duced by the GYS1 gene, but that the mutant glycogen synthase enzyme was far more active than the normal enzyme, thereby producing excess glycogen," says McCue. Not all horses with the GYS1 muta- tion for PSSM, however, fully develop PSSM1. Research shows that about 69% of Percherons in America have the GYS1 mutation; however, 69% of those horses do not tie up. "Multiple factors can influence the de- velopment of PSSM1, not just the genetic mutation," McCue says. "Diet, exercise, other genes for tying-up, such as MH, biological variation, the environment, and a multitude of other genes, can all influ- ence whether PSSM1 actually manifests clinically in a horse." This also explains why, as you will find out, rushing straight for a genetic test isn't necessarily the best place to start if you suspect your horse has PSSM. PSSM2 Once researchers identified the genetic mutation for PSSM1, they could test all horses with signs of tying-up and confirmed abnormal glycogen for that mutation. In performing this research, Valberg and colleagues realized some cases with abnormal polysaccharides did not have PSSM1. These cases were termed type 2 PSSM. The cause—or more likely causes—for this type of myopathy remains elusive and is the subject of cur- rent research. "We can't consider PSSM2 one specific disease as yet because we have not identi- fied one or more specific causes," says Valberg. "PSSM2 actually appears to be caused by different things, depending on breed." In other words, just like any horse with PSSM, horses with PSSM2 have abnor- mal muscle biopsies. The distinguishing feature is they lack the GYS1 mutation. Greatness… ADVERTISEMENT We can't consider PSSM2 one specific disease as yet because we have not identified one or more specific causes. PSSM2 actually appears to be caused by different things, depending on breed." DR. STEPHANIE VALBERG