Breeds Affected: New Forest Pony
Samples Accepted: Blood, Hair
Disease Information: Congenital Myotonia is a disorder in which it takes an unusually long period of time for skeletal muscles to relax after contracting. Affected animals show stiffness when moving, particularly when starting to move, and overgrowth (hypertrophy) of the muscles. Predominant signs include a stiff gait, eye retraction and third eyelid protrusion.
Inheritance Information: Congenital Myotonia is autosomal recessive, meaning that animals with two copies of this allele will be affected. Animals with one copy of the gene will be clinically-normal carriers.
The possible genotypes are:
N/N The horse is normal, and cannot produce affected offspring.
N/cm The horse is a carrier of congenital Myotonia, and can pass the allele on to approximately 50% of any offspring. If bred to another N/cm carrier, approximately 25% of the offspring will be normal, 50% will be carriers, and 25% will be affected.
cm/cm The horse is affected with congenital Myotonia. If bred to a normal animal, 100% of the offspring will be carriers. If bred to a N/cm carrier, 50% of the offspring will be carriers and 50% will be affected.
– Carriers may be bred to normal animals (N/cm x N/N) without any risk of producing affected offspring. The offspring should also be tested before breeding, to determine if they are carriers or normal.
– Breeding two carriers (N/cm x N/cm) is not recommended due to the possibility of 25% of the offspring being affected.
– Affected animals (cm/cm) should not be used for breeding.
Test Information: This mutation test is based on a single base pair change in exon 15 of the CLCN1 gene.
Wijnberg, I.D., Owczarek-Lipska, M., Sacchetto, R., Mascarello, F., Pascoli, F., Grünberg, W., van der Kolk, J.H., Drögemüller, C.: A missense mutation in the skeletal muscle chloride channel 1 (CLCN1) as candidate causal mutation for congenital myotonia in a New Forest pony. Neuromuscul Disord 22:361-7, 2012. Pubmed reference: 22197188. Doi: 10.1016/j.nmd.2011.10.001.
Further information is available at the Online Mendelian Inheritance in Animals website.