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Gene or Region: RYR1
Reference Variant: C
Mutant Variant: G
Affected Breeds: Quarter Horse
Research Confidence: High confidence, findings reproduced multiple studies
Explanation of Results: MH/MH = homozygous for Malignant Hyperthermia, trait expressed MH/n = heterozygous for Malignant Hyperthermia, trait expressed n/n = no variant detected
Malignant Hyperthermia (MH) is a muscle disorder in which anesthesia, stress, or extreme exercise trigger a hyperthermic state. Symptoms include high temperature, increased heart rate, high blood pressure, sweating, acidosis, and muscle rigidity. If symptoms are not immediately resolved, death is likely to occur. As this condition is autosomal dominant, even horses with a single allele (MH/n) exhibit symptoms. One study has shown that horses with both MH and polysaccharide storage myopathy (PSSM) have more severe clinical symptoms.
RYR1 is involved in the control of calcium channels within the muscle. Mutations in both humans and swine have been shown to cause conditions similar to MH seen in the horse. This mutation is a single base substitution that alters an amino acid, likely affected the function of the encoded protein.
Aleman M et al., “Association of a mutation in the ryanodine receptor 1 gene with equine malignant hyperthermia.” (2004) Muscle Nerve. 30: 356-65. PMID: 15318347
McCue ME et al., “Polysaccharide storage myopathy phenotype in quarter horse-related breeds is modified by the presence of an RYR1 mutation.” (2009) Neuromuscul Disord. 19: 37-43. PMID: 19056269
Aleman M et al., “Malignant hyperthermia associated with ryanodine receptor 1 (C7360G) mutation in Quarter Horses.” (2009) J Vet Intern Med. 23: 329-34. PMID: 19220734
Lavender foal syndrome (LFS), also known as coat color dilution lethal, is a neurological dysfunction in newborn foals. Symptoms are apparent at birth and include seizures, severe hyper-extension of limbs, neck and back, stiff paddling leg movements, involuntary eye movement and inability to stand or sit upright. The coat usually has a dilute color that ranges in shade from silver to a pale lavender/pink. As there is no treatment, affected foals are humanely euthanized.
Lordosis (L1, L2, L3, L4), also known as "Swayback", is a curvature or dip in the spine that is often seen in older horses. However, in the American Saddlebred, this condition also affects younger horses. These animals do not appear to experience pain from their condition and are still able to be used under saddle.
Myosin-Heavy Chain Myopathy (MY; previously "IMM") is a genetic muscle disease found most commonly in stock-type horses that can result in two different presentations, Immune-Mediated Myositis (IMM) and Nonexertional Rhabdomyolysis “tying up,” which are both characterized by muscle damage or loss. Both presentations are associated with the same genetic variant. Horses with MYHM may exhibit one or both presentations at different times in their lives, although some horses carrying the variant might not display any symptoms at all.
Myotonia (MYT) is a rare disorder involving a slowed relaxation of muscles after contraction. The most well-known example of myotonia is “fainting goats,” a breed that is characterized by sudden rigidy and/or falling over when startled. In the single documented horse, this resulted in a protruding third eyelid when excited, as well as problems with muscle stiffness.
