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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
Occipitoatlantoaxial Malformation 1 (OAAM1) is an inherited developmental condition predominantly found in Arabian horses, characterized by abnormal development of the cervical vertebrae leading to compression of the upper cervical cord and subsequent neurological damage.
Polysaccharide Storage Myopathy (PSSM) is a glycogen storage disorder in which affected horses have chronic episodes of exertional rhabdomyolysis (“tying-up”). Acute symptoms are an unwillingness to move and muscle damage following exercise. Moderate symptoms can usually be managed through a low-sugar/starch diet and exercise.
Recurrent Laryngeal Neuropathy (RLN) is a disease that results in loss of the neurons that open the larynx, significantly affecting performance in Thoroughbreds and other sport horses. It is an important issue for horses in competitive events because the resultant paralysis of the larynx leads to obstruction of air flow during intense exercise, creating the abnormal inspiratory noise that gives RLN its common name: “roaring”.
