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Horse Myotonia (MYO)
| Acronym: | MYO |
| Gene: | CLCN1 |
| Mutation: | c.1775A>C |
| Inheritance: | Autosomal Recessive |
| Sample type: | WBE (Whole Blood EDTA), HA (Hairs - with roots) |
Genetics and characteristics
Myotonia (MYO) is an inherited neuromuscular disease that affects numerous breeds of horses. A characteristic symptom of MYO in horses is delayed relaxation of skeletal muscle after sudden forceful contraction, so it is sometimes associated with clinical signs of weakness. MYO can be recognized early in the development, and the initial signs in foals are well-developed musculature and mild pelvic limb stiffness. MYO is caused by a genetic mutation in the chloride channel 1 (CLCN1) gene responsible for the chloride ion transport in the cell and maintaining the membrane potential. The missense mutation in the CLCN1 gene impacts the formation of the functional ion channel leading to the instability of the muscle cells.
MYO is inherited as an autosomal recessive trait, therefore two copies of the mutation are needed for a horse to be affected. Horses heterozygous for the mutation don’t show any clinical signs of the disease. MYO affects more than ten breeds, such as Quarter Horse, New Forest Pony, Thoroughbred, and Camarillo White Horse. The disease is incurable, but early detection by genetic testing can help identify MYO carriers in the population and prevent further breeding.
Results Reported As
Test Result |
Interpretation of test result |
CLEAR |
Tested mutation was not detected in animal with „clear“ result. Animal tested as clear has wild-type allele in homozygous state (i.e. two pairs of healthy alleles). It will not develop disease caused by tested mutation.* It will pass only wild-type allele to its offspring. |
CARRIER |
Tested mutation was detected in animal with „carrier“ result. Animal tested as carrier has one wild-type and one mutation allele, it is in heterozygous state. It will not develop disease caused by tested mutation.* It can pass wild-type or mutation allele to its offspring. |
AFFECTED |
Tested mutation was detected in animal with „affected“ result. Animal tested as affected has two copies of mutation alleles affecting the gene. It is likely the animal will experience a genetic disorder due to this mutation.** It will pass only mutation allele to its offspring. |
*Test excludes only tested mutation but not possible unknown mutations or factors that can lead to similar condition/symptoms.
** Potential unknown mutations or multiple other factors can possibly affect the likelihood of experiencing a genetic disorder.
References:
Wijnberg, I. D., Owczarek-Lipska, M., Sacchetto, R., Mascarello, F., Pascoli, F., Grünberg, W., … Drögemüller, C. (2012). A missense mutation in the skeletal muscle chloride channel 1 (CLCN1) as candidate causal mutation for congenital myotonia in a New Forest pony. Neuromuscular Disorders, 22(4), 361–367. doi:10.1016/j.nmd.2011.10.001
Reed, S. M., Hegreberg, G. A., Bayly, W. M., Brown, C. M., Paradis, M. R., & Clemmons, R. M. (1988). Progressive myotonia in foals resembling human dystrophia myotonica. Muscle & Nerve, 11(4), 291–296. doi:10.1002/mus.880110403