Polysaccharide Storage Myopathy / PSSM1 (GYS1-related) - Horse

What does this test examine?

This genetic test examines the GYS1 c.926G>A variant in horses. The condition is known as Polysaccharide Storage Myopathy type 1, PSSM1, PSSM type 1, equine rhabdomyolysis syndrome, exertional rhabdomyolysis and tying-up.

What does this mean for the horse?

GYS1 encodes glycogen synthase in muscle tissue. The PSSM1 variant increases enzyme activity and causes abnormal glycogen accumulation in skeletal muscle. Horses with the variant can develop muscle pain, stiffness, sweating, reluctance to move, muscle twitching, weakness or exercise-related muscle breakdown.

One copy can already cause signs; horses with two copies can be more severely affected. The variant explains PSSM type 1, but not all forms of abnormal glycogen storage or all muscle problems in horses.

Practical value of this test

• The test shows whether the horse carries zero, one or two copies of the PSSM1 variant.
• The result supports breeding selection, risk management and targeted interpretation of muscle signs.
• In positive horses, the result helps guide training, diet and management more precisely.

Inheritance

PSSM1 is inherited as an autosomal incomplete dominant trait. A horse with one copy can develop PSSM1 and passes the variant to about half of its offspring; a horse with two copies passes the variant to all offspring.