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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
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”.
