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New Research Questions Validity of Some Genetic Testing

With the rise of commercial genetics testing, we now have access to a load of new information that may indicate disease, nutritional preferences, and the behaviour of our horses. However, are these genetic tests reliable when it comes to diagnosing disease, particularly muscle disorders? Well, that is exactly what researchers out of Michigan State University set out to determine.

The lab of Dr. Stephanie Valberg, DVM, PHD, DACVIM, ACVSMR, has demonstrated some intriguing conclusions about the validity of commercial genetic testing for horses with suspected PSSM Type 2 and Myofibrillar Myopathy (MFM).

What is PSSM?

While often referred to as a form of ‘tying-up’, Polysaccharide Storage Myopathy (PSSM) is a disorder characterized by abnormal build-up of glycogen in muscle cells.

Sugar is stored in the body in the form of glycogen, which can be broken down to be used as energy. Muscle is a common storage location for glycogen as it means the muscle has easy access to the fuel it needs to function.

Essentially, PSSM horses have a condition where the storage or the use of glycogen from the muscle is altered, causing a build-up of glycogen in the muscle cells. This can have varying impacts on the horse, with muscle soreness or tremors and exercise intolerance being the most common symptoms, often accompanied by sweating, lameness, and poor performance under saddle (reluctance to go forward or collect, etc.). In more severe cases colic, or the inability to move at all, can occur in these horses.

What is the difference between PSSM 1 and PSSM 2?

PSSM is separated into two types: PSSM Type 1 (PSSM 1) and PSSM Type 2 (PSSM 2).

PSSM 1 describes horses who have a genetic mutation in the glycogen synthase 1 (GYS1) gene that causes the excess storage of glycogen in the muscle. PSSM 1 is most often seen in draft breeds and stock-type breeds such as Quarter Horses (particularly halter-bred Quarter Horses), Paints and Appaloosas. It is very low in light breeds like Arabians and Thoroughbreds.

PSSM Type 2 (PSSM 2), on the other hand, is the diagnosis for horses who do not have the GYS1 gene mutation but do have abnormal storage of glycogen. PSSM 2 can also be seen in stock breeds but can also occur in Warmbloods as well.

What is MFM?

Myofibrillar myopathy, or MFM, is a muscle disorder most often found in Arabian and Warmblood horses. It occurs when there is the abnormal build-up of glycogen in normal microtears that form in muscle fibres following exercise. Horses with MFM had historically been diagnosed with PSSM2; however, research from the Valberg lab has brought light to the fact that MFM is a unique issue that is separate from PSSM 2.

There is some belief that PSSM 2 is a precursor to MFM, but there is a lack of research available on this topic to give scientists a clear conclusion.

How do you test for PSSM?

PSSM 1 is diagnosed by genetic testing of hair or blood for the GYS1 gene mutation. Muscle biopsy, which is where a small core of muscle is extracted from the haunches, can be used to support the diagnosis.

When it comes to testing for PSSM 2, the GYS1 gene mutation testing would be negative, but a muscle biopsy would identify abnormal storage of glycogen.

Since muscle biopsies are relatively invasive, a genetic test to look for PSSM2 is ideal. This is where independent companies come in. They claim that their own researchers have found genetic mutations, which they call ‘P variants’ that indicated whether your horse is predisposed to PSSM 2, and through that, MFM.

Researchers have yet to discover the exact cause of PSSM 2. Despite this, commercial companies have developed genetic testing for horse owners as a diagnostic tool for identifying horses with PSSM 2 which they allege will evolve into MFM. Given these companies charge several hundred dollars for testing, the test can put significant financial burden on horse owners.

So, are these tests a good tool for veterinarians and horse owners to diagnose PSSM 2 or MFM? That is exactly with Dr. Valberg’s team set to find out.

The Research Findings

This research assessed muscle biopsies and genetic profiles of 163 horses with confirmed PSSM 2 and 229 horses without PSSM or other muscle disorders. The horses were all Quarter Horse or Quarter-Horse related breeds (such as Paint and Appaloosa horses).

Researchers looked at the occurrence of P variants in the genetic profiles and the presence of MFM in muscle biopsies. This was done for both groups of horses.

The results of this research are surprising. Interestingly, they found that there was no significant difference in the prevalence of any of the individual P variants between either groups of horses (those with confirmed PSSM 2 and those without).

So, what does this mean? This suggests that the use of P variants may not be an accurate indicator of PSSM 2 in Quarter Horse-type horses.

As for MFM, there also was no difference in prevalence between the groups of horses. In fact, of the 392 horses in this study, none had evidence for MFM, suggesting that this muscle disorder may be rare or non-existent in Quarter Horse types.


While a simple a genetic test for PSSM 2 using a hair or blood sample would be a easy way to diagnose horses with this muscle disorder, research by the Valberg lab suggests that it may not be as accurate as we would hope.

If you suspect your horse has PSSM 2, the best way to diagnose would be to reach out to your veterinarian to perform a muscle biopsy.

You can read up more about this paper by following this link.

Original article: New Research Questions Validity of Some Genetic Testing (horsesport.com)

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