Myotonia Congenita (CLCN1 gene)

Myotonia congenita is a disease that affects skeletal muscles and is characterized by the inability of the muscles to relax properly after contraction. This condition is the result of mutations that affect the functioning of the chloride channel in muscle cells.

Symptoms

Clinical signs described include generalized stiffness in movement, severe skeletal muscle hypertrophy and increased breath sounds. Other symptoms include difficulty in swallowing and excessive salivation (appears from 2-3 months of age). In some cases, a close relationship between myotonia congenita and upper prognathism has been observed in this breed.

Disease Management

No cure for myotonia congenita has been reported, however, affected dogs are usually kept stable with drug treatment, such as procainamide.

Genetic basis

This disease follows an autosomal recessive mode of inheritance. Autosomal recessive inheritance means that the dog, regardless of sex, must receive two copies of the mutation or pathogenic variant to be at risk of developing the disease. Both parents of an affected dog must carry at least one copy of the mutation. Animals with only one copy of the mutation are not at increased risk of developing the disease, but may pass the mutation on to future generations. Breeding between dogs carrying genetic variants that can cause disease, even if they do not show symptoms, is not recommended.

Technical report

Myotonia congenita is a disorder caused by mutations in the CLCN1 gene encoding a voltage-dependent chloride channel in skeletal muscle. Mutations in the gene cause the chloride channel to be unable to open fully and therefore, depolarization lasts longer than usual. Consequently, skeletal muscle relaxation is delayed after muscle contraction. A base pair insertion (c.2647_2648insA) in the CLCN1 gene that causes a reading frame shift has been identified in the Australian Cattle breed. The result is impaired chloride channel conduction in the skeletal muscles of the dog.

Most affected breeds

Australian Cattle Dog
Border Collie

Bibliography

Finnigan DF, Hanna WJ, Poma R,et al. A novel mutation of the CLCN1 gene associated with myotonia hereditaria in an Australian cattle dog. J Vet Intern Med. 2007 May-Jun;21(3):458-63.