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Myosin-Heavy Chain Myopathy (MYHM)
| Acronym: | MYHM |
| Gene: | MYH1 |
| Mutation: | g.53345548T>C |
| Inheritance: | Autosomal codominant |
| Sample type: | WBE (Whole Blood EDTA), HA (Hairs - with roots) |
Genetics and characteristics
Myosin-Heavy Chain Myopathy (MYHM) is an inherited muscle disease that affects horses. Horses with MYHM have dramatic muscle atrophy, often accompanied by stiffness and nonspecific malaise. Severely affected horses can develop weakness and frequent episodes of recumbency (laying down). MYHM is caused by a genetic mutation in the myosin heavy chain 1 (MYH1) gene responsible for building muscles and converting chemical energy into mechanical energy for muscle movement. The missense mutation in the MYHM gene alters myosin composition by the destruction of myofibers and impacts protein stability, leading to weaker muscles and characteristic symptoms of the disease.
MYHM is inherited as an autosomal codominant trait with variable penetrance, meaning that horses with one or two copies of the mutation are susceptible to disease, but not all horses with the mutation will develop signs of the disease. However, horses with two copies of the mutation that show clinical signs are more severely affected. MYHM affects primarily Quarter Horses and related breeds. The disease is incurable, but early detection by genetic testing can help with the managing of the symptoms, as well as with a careful selection of mating pairs to prevent further breeding of affected horses.
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. |
AFFECTED HETEROZYGOTE |
Tested mutation was detected in animal with „affected heterozygote“ result. Animal tested as affected heterozygote has one wild-type and one mutation allele, it is in heterozygous state. It is likely to 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.
**Penetrance of tested mutation, and potential unknown mutations or multiple other factors can possibly affect the likelihood of experiencing a genetic disorder.
References:
Durward-Akhurst, S.A. and Valberg, S.J. (2018). Immune-mediated muscle diseases of the horse. Veterinary pathology, 55(1), doi: 10.1177/0300985816688755
Gianino, G. M., Valberg, S. J., Perumbakkam, S., Henry, M. L., Gardner, K., Penedo, C., & Finno, C. J. (2019). Prevalence of the E321G MYH1 variant for immune-mediated myositis and nonexertional rhabdomyolysis in performance subgroups of American Quarter Horses. Journal of Veterinary Internal Medicine. doi:10.1111/jvim.15393