I wanted to share this report since it has a lot of valuable information. I hope it copy and pastes ok. I have to post this report in parts, since it is too long to post as a whole:
University of Minnesota
Veterinary Diagnostic Laboratory
Veterinary Diagnostic Laboratory
Age: 1995 Sex: Female
Specimen: Hair from Belle.
Diagnostic report: Your horse was tested for type 1 PSSM characterized by a mutation in the GYS1 gene.
The genetic test result for the submitted sample is as follows:Type 1 PSSM (GYS1 ): P/N
Interpretation of genetic test
Your horse has tested positive for Type 1 PSSM. Each horse has two copies of every gene (known as its genotype), one inherited from the dam and one from the sire. Every horse inherits either a normal form of the GYS1 gene or a mutant gene of the GYS1 gene form each parent. We have designated the letter P to indicate the mutant PSSM GYS1 gene and N to indicate the normal GYS1 gene. A normal horse is designated as N/N. A horse with Type 1 PSSM may be heterozygous P/N or rarely homozygous P/P for the mutation. Those horses that are P/P are often more severely affected and harder to manage.
Other acronyms have been used to describe polysaccharide storage myopathy including EPSM and EPSSM. Our laboratory has consistently used the term PSSM. Our research (January 2008) also indicates that there are two forms of PSSM (Type 1 and Type 2) in horses. Type 1 PSSM is due to a specific genetic mutation in the
GYS1 gene affecting glycogen synthesis. It occurs in over 20 different breeds to date including Quarter Horses, American Paint Horses and Appaloosas as well as Draft, Draft crossbreeds, Hafflingers, some Warmblood breeds, Morgans, Tennessee Walkers and other breeds. At present no Standardbreds, Arabs or Thoroughbred have been diagnosed with Type 1 PSSM, although sample numbers are limited. Horses with Type 2 PSSM do not possess the GYS1 genetic mutation present in Type 1 PSSM. Type 2 PSSM can only be diagnosed in horses by muscle biopsy where glycogen and abnormal polysaccharide has a characteristic appearance. We are currently working to
identify the basis for Type 2 PSSM and to determine which breeds are affected. Thus, although the type 1 PSSM (GYS1 mutation) is the most common genetic cause of PSSM and tying up in Draft and Quarter Horse related breeds there are other causes of tying-up. If horses affected with chronic muscle diseases are negative for Type 1 PSSM we recommend follow up with a muscle biopsy to investigate other possible causes. See http://www.cvm.umn.edu/umec/lab/home.html, for information on obtaining and submitting a muscle biopsy to the Neuromuscular Diagnostic Laboratory.
The type 1 PSSM mutation we have discovered is inherited in a dominant fashion, meaning that one copy of the mutation is sufficient to cause PSSM. This is different from diseases such as HERDA and GBED, which are inherited in a recessive fashion, and 2 copies of the mutant gene are required for disease. Because type 1 PSSM is inherited in a dominant fashion the chances of an affected foal being born are dependant on the genotype of the parents as follows;
(Go to University of Minnesota's website to view chart)
For example, breeding a P/N horse to an N/N horse gives a 50% chance that the offspring will have type 1 PSSM. Thus any time a horse with type 1 PSSM is bred there is a minimum chance of 50% of an affected foal being born even if the selected mate is completely normal. The risk of producing an affected offspringwhen breeding a horse with PSSM is much higher because it is a dominant disease. Unlike the recessive diseases, where a horse with one copy of the gene is a ˇ§carrierˇ¨ a horse with one copy of the type 1 PSSM mutation has PSSM.
Genetic modifiers of PSSM
Some horses with particularly severe or recurrent forms of PSSM possess a second mutation in another gene, known as the RYR1 gene that is responsible for MH (also known as malignant hyperthermia). We recommend testing for this mutation if this describes your horse (see website for testing http://www.vdl.umn.edu/vdl/ourservic...omuscular.html) The MH mutation is rare in the Quarter Horse population but when it occurs together with the type 1 PSSM mutation it makes signs of tying up more severe. In addition, in some horses this mutation results in a higher risk of developing severe anesthetic reactions such as very high body temperature, lactic acidosis, muscle damage and death under general anesthesia.
What Causes PSSM?
Horses with polysaccharide storage myopathy (PSSM) accumulate excessive amounts of the normal form of sugar (glycogen) in their muscles as well as an abnormal form of sugar (polysaccharide). Clinical signs of PSSM are usually those of tying-up, where horses develop muscle stiffness, soreness and reluctance to work with exercise. However, signs found in Draft, Draft crossbreeds, include muscle atrophy, weakness and gait abnormalities. Some horses with PSSM that are managed well show no clinical signs. Type 1 PSSM is caused by an inherited defect in the GYS1 gene (called glycogen synthase) that results in abnormal regulation of glycogen metabolism in skeletal muscle. This appears to disrupt energy metabolism. We do not know yet what the defect is that causes Type 2 PSSM.