Polysaccharide storage myopathy (PSSM1)

Affected

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Your horse has ONE polysaccharide storage myopathy gene mutation.

This means that your horse has polysaccharide storage myopathy.

As your horse has only one polysaccharide storage myopathy genetic mutation it may have less severe symptoms.

Breeding implications

If you breed your affected horse with another horse clear of the gene mutation for polysaccharide storage myopathy, there is:

  • A 50% chance the foal will not have polysaccharide storage myopathy.
  • A 50% chance the foal could develop polysaccharide storage myopathy, just like your horse.

If you breed your affected horse with another horse that has one gene mutation for polysaccharide storage myopathy, there is:

  • A 25% chance the foal will not have polysaccharide storage myopathy.
  • A 50% chance the foal could develop polysaccharide storage myopathy, just like your horse.
  • A 25% chance the foal could develop the more severe form of polysaccharide storage myopathy.

If you breed your affected horse with another horse that has two gene mutations for polysaccharide storage myopathy, there is:

  • A 50% chance the foal could develop polysaccharide storage myopathy, just like your horse.
  • A 50% chance the foal could develop the more severe form of polysaccharide storage myopathy.
    What is polysaccharide storage myopathy?

    Polysaccharide storage myopathy causes a dysfunction of muscle metabolism that takes sugar out of the bloodstream and transfers it into the muscle at a faster rate than normal. Affected horses make and store around four times as much glycogen as other horses. Some affected horses are asymptomatic, but most horses with polysaccharide storage myopathy will show signs of by four years of age.

    Though there is no cure for polysaccharide storage myopathy, it can be managed with careful nutrition and exercise practices. Scientific studies have found that 75 percent of horses stopped tying-up when feeding and training routines were changed.

    What are the symptoms?
    • Muscle stiffness (especially in the hindquarters).
    • Tremors in their flank area.
    • Sweating.
    • Reluctance to move.
    • Some horses will try to roll immediately after exercise or stretch out as if to urinate to relieve the pain.
    • In severe episodes, muscle proteins are released into the bloodstream, leading to the passing of dark urine.
    How is it inherited?

    Type 1 polysaccharide storage myopathy is a dominant genetic condition. A foal born with only one or two copies of the gene mutation will be affected. With polysaccharide storage myopathy there are no ‘carriers;’ a foal either has the condition or it doesn’t.

    Genomic Details

    The GSY1 gene gives instructions for making an enzyme called glycogen synthase. Horses with the GSY1 mutation have too much glycogen enzyme activity. This leads to a high amount of sugar being stored in the muscle. When this happens, there is shortage of muscle energy, which leads to low energy, muscle pain, stiffness, and tiredness.

    Polysaccharide storage myopathy is a dominant genetic disease. This means only one gene mutation from one parent is needed for the disease to occur.

    The mutation that causes polysaccharide storage myopathy is a switch in one of the four base pairs that make up a DNA molecule. Guanine (G) is replaced by Adenine (A) changing the PPIB gene. Therefore, a horse’s genetic result for polysaccharide storage myopathy asthenia is one of three possibilities:

    1. G/G: Normal. Does not have the disease. Does not have the mutation.
    2. A/A: Affected. Has one disease mutation from one parent. This is enough to cause the disease. It might be passed to their foal.
    3. A/A: Affected. Has two disease gene mutations, one from each parent. One mutation will be passed to their foal. This is the result for your horse.