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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
Impaired Acrosomal Reaction Subfertility (IAR) causes sub- or infertility in males. In normal fertilization, the the head of a sperm binds to the egg and releases the contents of a structure known as the acrosome. However, some males with IAR are unable to properly carry out this process.
Junctional Epidermolysis Bullosa (JEB), also known as red foot disease or hairless foal syndrome, results in the inability to produce the proteins needed to keep the skin on the body. Affected foals exhibit symptoms within days of birth, including blisters at the pressure points, detatchment of the hooves, and oral ulcers. As there is no treatment, affected foals are humanely euthanized.
Kissing Spines Susceptibility (KSS) evaluates a horses genetic risk for developing Kissing Spines. Horses diagnosed with Kissing Spines will have two or more dorsal spinous processes that are too close together, touch, or even overlap. This bone-on-bone grinding causes varying degrees of pain, and thus decreased mobility.
