Researchers suspect tying-up in horses is a heritable condition; however, they have yet to determine the gene–or genes–responsible. But a team of Japanese researchers recently moved the investigation forward with a groundbreaking study of affected Thoroughbred racehorses' DNA.

Muscle disorders such as polysaccharide storage myopathy (PSSM, recognized mainly in Quarter Horses) and recurrent exertional rhabdomyolysis (RER, found primarily in Thoroughbred and Standardbred racehorses) can lead to tying-up. A horse that's tying-up typically displays stiffness, sweating, muscle tremors, and a reluctance to move, among other clinical signs.

"Tying-up in racehorses is important because it affects approximately 5% of Thoroughbred racehorses," explained Teruaki Tozaki, PhD, from the Department of Molecular Genetics, Laboratory of Racing Chemistry, Tochigi, Japan, author of the recent study. "Although the condition is influenced by sex (of the affected horse), temperament, and diet, the current body of evidence suggests that tying-up is a heritable trait that is affected by one or several genetic factors."

Researchers have identified a specific mutation in the GYS1 gene in horses with PSSM as the underlying cause of tying-up in Quarter Horses and Belgians, yet the gene responsible for tying-up in Thoroughbred racehorses is still unknown. However, armed with the completed horse genome, Tozaki and others are attempting to identify the gene(s) responsible for tying-up in Thoroughbreds