Tying-Up in Thoroughbreds: Narrowing the Genetic Search

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.

Tozaki and colleagues isolated DNA from 328 related Thoroughbred racehorses in Japan, 55 of which were affected by tying-up. The researchers then scanned the entire genome of each horse using microsatellite markers (segments of DNA containing simple nucleotides).

"We found two regions of DNA that could contain the gene responsible for tying-up," Tozaki relayed. "However, the most promising is a region of DNA ... on equine chromosome 12."

Tozaki and colleagues are aiming to identify the gene(s) responsible for tying-up in this chromosome region by looking at candidate genes--those that are expressed in the horse's skeletal muscles and are known to play a role in human muscular dystrophy).

Tozaki said that he believes that this research will lead to a better understanding of the heritability and genetics of tying-up in the future

The study, "A genome-wide scan for tying-up syndrome in Japanese Thoroughbreds," was published in a special edition of Animal Genetics. The entire journal supplement, funded by the Dorothy Russell Havemeyer Foundation is available for free online.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.