b'cells. Mutations in the DNA can produceWe cant considera multitude of other genes, can all influ-faulty proteins. Sometimes those deviantence whether PSSM1 actually manifests proteins do not function, whereas otherPSSM2 one specificclinically in a horse.times, proteins like glycogen synthasedisease as yet becauseThis also explains why, as you will find become more active, going above andout, rushing straight for a genetic test beyond the call of duty. we have not identifiedisnt necessarily the best place to start if The term autosomal means the defec- one or more specificyou suspect your horse has PSSM. tive gene occurs on any chromosomePSSM2Once researchers identified the except the X and Y sex chromosomes.causes. PSSM2 actuallygenetic mutation for PSSM1, they could In this case, the GYS1 gene is on ECA10appears to be caused bytest all horses with signs of tying-up and (equine chromosome 10).confirmed abnormal glycogen for that Dominant means only one copy of thedifferent things,mutation. In performing this research, gene is needed to cause production of thedepending on breed. Valberg and colleagues realized some defective enzyme. Recall that all livingcases with abnormal polysaccharides organisms have two copies of each gene,DR. STEPHANIE VALBERG did not have PSSM1. These cases were one coming from the sire and one fromtermed type 2 PSSM. The causeor more the dam.thereby producing excess glycogen, sayslikely causesfor this type of myopathy Therefore, the GYS1 mutation doesntMcCue.remains elusive and is the subject of cur-occur on a sex chromosome, requiresNot all horses with the GYS1 muta- rent research. only one of the two potential copies of thetion for PSSM, however, fully developWe cant consider PSSM2 one specific mutation to be effective, and results inPSSM1. Research shows that about 69%disease as yet because we have not identi-excess accumulation of polysaccharides of Percherons in America have the GYS1fied one or more specific causes, says specifically, glycogenin muscle cells. mutation; however, 69% of those horsesValberg. PSSM2 actually appears to be One of our latest research projectsdo not tie up.caused by different things, depending on found that horses diagnosed with PSSM1Multiple factors can influence the de- breed.had the same amount of enzyme pro- velopment of PSSM1, not just the geneticIn other words, just like any horse with duced by the GYS1 gene, but that themutation, McCue says. Diet, exercise,PSSM, horses with PSSM2 have abnor-mutant glycogen synthase enzyme wasother genes for tying-up, such as MH,mal muscle biopsies. The distinguishing far more active than the normal enzyme,biological variation, the environment, andfeature is they lack the GYS1 mutation. ADVERTISEMENTGreatnessMarch 2018 THE HORSE TheHorse.com 27'