PSSM2/MFM FAQ


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Frequently Asked Questions


Q. How do I pull hair?

A. Make sure that your horse is dry. Wrap 30-50 strands of hair from the mane or tail around a comb, a pen, or your finger and pull sharply. You should be able to see the hair bulbs (roots) at the base of the hair. Hair that is cut or broken, and that does not contain the hair bulb, will not provide a DNA sample.

Q. Why can't horse owners outside the United States submit hair samples?

A. The USDA has a set of requirements to insure that diseases affecting horses are not imported from outside the United States. It is difficult for a small operation like EquiSeq to comply with these requirements. We have developed an alternative to hair samples: dried blood on a card. Horse owners outside the United States need to use this method.

Q. How old does my horse need to be for genetic testing?

A. A horse's genetic material does not change over its lifetime. A newborn foal can be tested. For young horses, it is best to take hair samples from the tail rather than the mane.

Q. I read that they aren't sure that Polysaccharide Storage Myopathy type 2 (PSSM2) is inherited. Why do you say that it is?

A. Researchers at EquiSeq have identified three genetic variants associated with PSSM2/MFM. The genetic variants alter genes of known function that are required in skeletal muscle.

Q. What is Myofibrillar Myopathy (MFM)?

A. Myofibrillar Myopathy (MFM) is a subtype of PSSM2 that has a specific kind of defect in muscle revealed by a recently developed staining technique. Prior to the development of this stain, these horses were classified as having PSSM2.

Q. How can three different genetic variants cause the same disease?

A. PSSM2/MFM are umbrella terms that describe a set of symptoms. Variant forms of different genes cause symptoms that are similar.

Q. I heard that PSSM2/MFM can only be diagnosed by muscle biopsy. Do I still need a muscle biopsy for diagnosis?

A. Until now, PSSM2 has been defined as having symptoms of exercise intolerance, testing negative for a genetic variant of glycogen synthase associated with PSSM1 (GYS1-R309H or P1), and having abnormal findings on muscle biopsy. If your horse tests positive for the P2, P3, or P4 variants, researchers at EquiSeq don't think that a muscle biopsy is necessary to confirm the diagnosis.

Q. Can my horse get PSSM2/MFM from another horse?

A. PSSM2/MFM is an inherited (genetic) condition. It can be passed down to offspring, but it is not contagious.

Q. Can a horse have both PSSM1 and PSSM2?

A. PSSM1 and PSSM2/MFM are the names of diseases. A horse can have all four variants (for example, n/P1 n/P2 n/P3 n/P4), but by definition, if this horse has symptoms of exercise intolerance and tests positive for GYS1-R309H (n/P1 or P1/P1), it has PSSM1.

Q. How accurate is your test for PSSM2/MFM?

A. Genetic tests score the presence or absence of a genetic variant. Apart from rare cases of human error, the tests are completely accurate. Because the P2, P3, and P4 variants are not fully penetrant, some horses that are n/P2, n/P3, or n/P4 will not develop exercise intolerance. This means that the tests are not completely predictive. P2/P2, P3/P3, and P4/P4 horses are much more likely to be affected.

Q. What breeds have these variants?

A. The P2, P3, and P4 variants have been seen in stock breeds (Quarter Horses, Paints, Appaloosas) and Thoroughbreds. They are likely to be present in other related breeds as well.

Q. My horse tests positive for the P2 variant (n/P2 or P2/P2). What should I do?

A. Researchers at EquiSeq regard PSSM2 as untreatable. Symptoms can be managed though a diet and exercise program. Consult your veterinarian for recommendations.

Q. My horse tests positive for the P3 variant (n/P3 or P3/P3). What should I do?

A. Researchers at EquiSeq regard PSSM2/MFM as untreatable. Symptoms can be managed though a diet and exercise program. Consult your veterinarian for recommendations.

Q. My horse tests positive for the P4 variant (n/P4 or P4/P4). What should I do?

A. Researchers at EquiSeq regard PSSM2/MFM as untreatable. Symptoms can be managed though a diet and exercise program. Consult your veterinarian for recommendations.

Q. My horse has symptoms of exercise intolerance but tests negative for the P2, P3, and P4 variants (n/n). What does this mean?

A. There are other genetic causes of PSSM2/MFM besides the P2, P3, and P4 variants. Researchers at EquiSeq are working to identify these variants. There are also other conditions that produce symptoms similar to PSSM2/MFM (for example, Lyme disease or EPM). Consult with your veterinarian to obtain the best possible diagnosis for your horse.

Q. I don't understand some of the information on this site. Is there a Genetics For Dummies page?

A. Please see our DNA Learning Center and the Glossary. You may find additional useful information on Wikipedia (try searching Wikipedia with terms that you don't understand).

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