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PSSM1 Polysaccharide Storage Myopathy Type 1

The PSSM1 DNA test verifies the presence of the affected allele at the GYS1 locus responsible for Polysaccharide Storage Myopathy...

£38.00 Incl. VAT

    AboutAbout

    The PSSM1 DNA test detects the GYS1 variant c.926G>A (p.Arg309His, commonly written R309H) responsible for Type 1 Polysaccharide Storage Myopathy. The test reports the genotype directly — n/n, n/P1 or P1/P1 — and is valid at any age, from foal to adult, because the genotype does not change over the animal's lifetime.

    The result is issued as a DNA certificate identifying the horse and the genotype found.

    GYS1 R309H is a single, well-characterised variant, reported across multiple breeds and with an established genetic test (McCue et al., 2008, PMID 18691366; Stanley et al., 2009, PMID 19803057).

    Scope: this test detects only the GYS1 PSSM1 variant. It does not detect PSSM2 / MIM / MFM or any other muscle disorder. A horse testing n/n for PSSM1 can still have muscle disease of another cause.

    Why test?Why test?

    Breeding decisions. PSSM1 is dominant, so a single affected parent can transmit it. Knowing both parents' genotypes lets you predict the risk for every mating and avoid producing affected foals.

    Buying and selling. The genotype is objective, permanent information that can be disclosed at the point of sale.

    Management before symptoms appear. PSSM1 cannot be cured, but in horses carrying the variant episodes are largely controlled by a low-starch, high-fat diet and consistent daily exercise. Knowing the genotype early allows that management to begin before muscle damage accumulates.

    Explaining recurrent "tying-up". In a horse with repeated episodes of exertional rhabdomyolysis, a positive result identifies a genetic cause and changes the management plan.

    Clinical signsClinical signs

    Signs are usually triggered by exercise, particularly after a period of rest with continued grain feeding, and vary widely between individuals. Some horses carrying the variant never show obvious signs.

    Commonly reported:

    • Muscle stiffness, reluctance to move forward, short or "shuffling" stride
    • Repeated episodes of exertional rhabdomyolysis ("tying-up"), often shortly after exercise begins
    • Excessive sweating, elevated respiratory rate, apparent pain or colic-like behaviour
    • Firm, painful muscles over the hindquarters and back
    • Dark, coffee-coloured urine (myoglobinuria) during severe episodes
    • Elevated serum creatine kinase (CK) and aspartate aminotransferase (AST)
    • Poor performance, muscle tremors, muscle atrophy in chronic cases
    • In draft and draft-cross breeds, progressive weakness, difficulty rising, and gait abnormalities may predominate over classic tying-up

    Homozygous horses (P1/P1) tend to show signs earlier and more severely than heterozygotes, though this is not absolute.

    Clinical signs are not specific to PSSM1. Diagnosis of disease — as opposed to carrier status — is made by a veterinarian combining history, clinical examination, blood biochemistry and this genetic result.

    TransmissionTransmission

    PSSM1 follows autosomal dominant inheritance with variable expression. The GYS1 variant is on an autosome, so colts and fillies are affected equally, and a single copy is enough to make a horse susceptible.

    Expected offspring genotypes:

    • n/n × n/n → 100% n/n
    • n/n × n/P1 → 50% n/n, 50% n/P1
    • n/n × P1/P1 → 100% n/P1
    • n/P1 × n/P1 → 25% n/n, 50% n/P1, 25% P1/P1
    • n/P1 × P1/P1 → 50% n/P1, 50% P1/P1
    • P1/P1 × P1/P1 → 100% P1/P1

    Because penetrance is variable, a horse can carry one copy, appear clinically normal, and still pass the variant to half of its offspring. Clinical appearance is therefore not a reliable substitute for testing.

    PreventionPrevention

    There is no cure. Prevention has two independent parts.

    Preventing affected foals — breeding. Test breeding stock before mating. Pairing an n/P1 or P1/P1 horse with an n/n partner removes the possibility of a P1/P1 foal but still produces carriers; only n/n × n/n matings produce no carriers at all. Where a positive horse has high value for other traits, breeding it only to n/n partners and testing every foal allows the line to be kept while the variant is progressively bred out.

    Preventing episodes — management of a positive horse. Management is the responsibility of your veterinarian and equine nutritionist; the measures consistently reported as effective are:

    • A diet low in starch and sugar (non-structural carbohydrates), with grain and sweet feed removed
    • Additional calories supplied as fat rather than as starch
    • Daily turnout and consistent, gradually increased exercise; prolonged stall rest should be avoided
    • Avoiding abrupt increases in workload after a rest period
    • Gradual dietary transitions over several weeks, not days

    Ask your veterinarian to define exact energy, starch and fat targets for your horse's bodyweight and workload.

    ResultsResults

    n/n — Normal homozygous. No copy of the PSSM1 variant. Negative for PSSM1. Cannot transmit the variant.

    n/P1 — Heterozygous (positive, carrier). One copy of the PSSM1 variant. Transmits it to approximately 50% of offspring. May show clinical signs; expression varies.

    P1/P1 — Homozygous positive. Two copies of the PSSM1 variant. Transmits it to 100% of offspring. Generally associated with earlier and more severe signs.

    An inconclusive result is reported when sample quality does not allow a confident genotype call; in that case a new sample is requested at no additional cost.

    SampleSample

    Hair roots: 20–40 hairs with the roots (bulbs) attached, pulled — not cut — from the mane or tail. Tape the hairs onto the printable sample submission form, inside the marked area. Hairs without visible roots cannot be analysed.

    Blood: 5 mL of whole blood in a K3-EDTA tube, collected by a veterinarian. Label the tube with the horse's name and send it together with the printable sample submission form.

    Keep samples dry and at room temperature; do not freeze. Do not send hair that is wet, mouldy, or contaminated with bedding or faeces.

    Send your sample by regular mail or express delivery to:

    Equigerminal Lab HIESE
    Rua da Quinta do Sobreiro Nº25
    3230-343 Penela, Portugal

    TurnaroundTurnaround

    Standard processing: results within 10 working days of the sample arriving at the laboratory. Shipping is arranged and paid for by the client, and transit time is not included in the 10 working days.

    Samples that fail DNA extraction or amplification are repeated at no extra cost, which may add a few working days. We contact you if a new sample is required.

    How it worksHow it works

    🛒 Purchase the test: select and buy the test online.

    📧 Receive instructions: after payment confirmation you receive sample collection instructions by e-mail.

    ✨ Collect the sample yourself: pull 20 to 40 hair roots with the bulb attached, or ask your veterinarian to collect blood in a K3-EDTA tube.

    📄 Complete the form: print and complete the submission form with the animal identification.

    📮 Send it to the laboratory: Equigerminal, S.A., HIESE, Rua da Quinta do Sobreiro, 25, Quinta Vale do Espinhal, 3230-343 Penela, PORTUGAL.

    📄 Receive your report: your certified report is issued as soon as the analysis is validated.

    FAQsFAQs

    Which breeds are affected?
    The variant has been reported across a wide range of breeds, including Quarter Horses and related stock breeds, Belgian and Percheron draft horses and their crosses, and several Warmblood populations. It has not been reported in all breeds, and frequency differs substantially between populations.

    How is it inherited?
    Autosomal dominant. One copy (n/P1) is enough for a horse to be susceptible, and enough to transmit the variant to half of its offspring.

    My horse is n/P1 but has never had an episode. Is the test wrong?
    No. PSSM1 shows variable penetrance: some horses carrying the variant never develop clinical signs, particularly when their diet is low in starch and their exercise is regular. The genotype is still transmitted to offspring.

    At what age can a horse be tested?
    Any age. The genotype is fixed at conception and does not change.

    Does a negative result mean my horse cannot tie up?
    No. This test covers only the GYS1 PSSM1 variant. Exertional rhabdomyolysis has several other causes, genetic and non-genetic, including PSSM2/MFM, RER, and electrolyte or management problems.

    Can this test also tell me if my horse is PSSM2?
    No. PSSM2 — also marketed as MIM (Muscle Integrity Myopathy) — is a distinct condition and is not detected by this test. It is also important to know that the commercial variant tests offered for PSSM2/MIM (MYOT P2, FLNC P3a/P3b, MYOZ3 P4) have not been confirmed by independent validation: genotype frequencies for those variants did not differ between horses with a histopathological diagnosis and healthy controls (Valberg et al., 2021, PMID 32896939; Valberg et al., 2023, PMID 35288976). For suspected PSSM2/MFM the reference diagnosis remains muscle biopsy with desmin staining, not a commercial DNA test.

    Can I test a newborn foal?
    Yes. Hair roots or EDTA blood can be collected at any age; for very young foals, hair roots from the tail are usually easiest.

    Do I need my veterinarian?
    Hair roots can be collected by the owner. Blood must be collected by a veterinarian.

    MoreMore

    PSSM1 is a hereditary muscle disorder caused by a point mutation in the GYS1 gene, which encodes the enzyme glycogen synthase 1 in skeletal muscle. The p.R309H substitution makes the enzyme abnormally active, so muscle accumulates excessive glycogen together with an amylase-resistant polysaccharide. This stored material is not readily usable during exercise, and the resulting energy deficit in the muscle fibre underlies the stiffness, pain and muscle-fibre breakdown seen in affected horses.

    • Gene: GYS1 (glycogen synthase 1)
    • Variant: c.926G>A, p.Arg309His (R309H)
    • Inheritance: autosomal dominant, variable penetrance
    • Species: Equus caballus
    • Sample types accepted: hair roots, EDTA whole blood
    • Report: DNA certificate delivered by e-mail

    Histological confirmation by muscle biopsy is a separate diagnostic procedure and is not part of this test.

    Key references

    • McCue, M., Valberg, S., Lucio, M., & Mickelson, J. (2008). Glycogen synthase 1 (GYS1) mutation in diverse breeds with polysaccharide storage myopathy. Journal of Veterinary Internal Medicine, 22, 1228–1233. PMID 18691366
    • Stanley, R., McCue, M., Valberg, S., Mickelson, J., Mayhew, I., McGowan, C., et al. (2009). A glycogen synthase 1 mutation associated with equine polysaccharide storage myopathy and exertional rhabdomyolysis occurs in a variety of UK breeds. Equine Veterinary Journal, 41, 597–601. PMID 19803057

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