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Gene or Region: SCN4A
Reference Variant: C
Mutant Variant: G
Affected Breeds: Stock Horses
Research Confidence: High - Findings reproduced in multiple studies
Explanation of Results: HYPP/HYPP = homozygous for Hyperkalemic Periodic Paralysis, extreme trait expressed HYPP/n = heterozygous for Hyperkalemic Periodic Paralysis, trait expressed n/n = no variant detected
Hyperkalemic Periodic Paralysis (HYPP) is a dominant genetic disorder resulting from defects in sodium channels in the muscle. Affected animals experience episodes of muscular weakness or tremors, and can lead to paralysis, collapse, and sudden death. Heterozygous horses (HYPP/n) experience more moderate symptoms that can usually be managed through diet and exercise. However, homozygous horses (HYPP/HYPP) are much more severely affected, and some breed registries will not accept these animals.
SCN4A is a sodium channel involved in normal muscular control. Mutations in humans are well documented to cause HYPP. This mutation is a single base substitution that alters an amino acid, likely changing the function of the encoded protein. This was the first successfully mapped genetic disorder of the horse.
Rudolph JA et al., “Periodic paralysis in quarter horses: a sodium channel mutation disseminated by selective breeding.” (1992) Nat Genet. 2: 144-7. PMID:1338908
Bowling AT et al., “Evidence for a single pedigree source of the hyperkalemic periodic paralysis susceptibility gene in quarter horses.” (1996) Anim Genet. 27: 279-81. PMID: 8856926
Naylor JM et al., “Hyperkalaemic periodic paralysis in homozygous and heterozygous horses: a co-dominant genetic condition.” (1999) Equine Vet J. 31: 153-9. PMID: 10213428
Tryon RC et al., “Evaluation of allele frequencies of inherited disease genes in subgroups of American Quarter Horses.” (2009) J Am Vet Med Assoc. 234: 120-5. PMID: 19119976
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.
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.
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.
