Congenital Myotonia

Congenital myotonia is an inherited muscle disease. Affected cats usually show muscle hypertrophy, are stiff and walk with a jerky gait with an abnormally short stride. They have a limited ability to open their jaws, swallow and make sounds.

The disease belongs to the so-called channelopathies, i.e. it is caused by a mutation in the gene encoding an ion channel in a muscle fibre. These genes encode proteins that determine the ionic conductivity by “opening and closing”. Three different mutations (c.1930+1G>T, c.428_433+1del, c.991G>C) in the CLCN1 gene have been identified in cats that cause congenital myotonia. The c.428_433+1del mutation occurs in the long-haired domestic cat, while the other two are not breed-specific. CLCN1 encodes a voltage-dependent chloride channel of the skeletal muscle. These channels cannot open fully and there is a delay in muscle fibre relaxation after the action potential is terminated.

It is reported that the inheritance of the mutation is probably autosomal recessive. This means that only individuals who inherit the mutated gene from both parents will develop the disease. Carriers of the mutated gene are clinically healthy but pass the mutation on to their offspring. In the case of a mating between two heterozygous individuals, theoretically 25% of the offspring will be completely healthy, 50% of the offspring will be carriers and 25% of the offspring will inherit the mutated gene from both parents and will therefore be affected by the disease.

The genetic test can clearly reveal the genotype of the animal and is a useful tool for breeders to prevent unintentional breeding of affected kittens.

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References:

Gandolfi, B., Daniel, R.J., O'Brien, D.P., Guo, L.T., Youngs, M.D., Leach, S.B., Jones, B.R., Shelton, G.D., Lyons, L.A. : A novel mutation in CLCN1 associated with feline myotonia congenita. PLoS One 9:e109926, 2014. Pubmed reference: 25356766

Woelfel, C., Meurs, K., Friedenberg, S., DeBruyne, N., Olby, N.J. : A novel mutation of the CLCN1 gene in a cat with myotonia congenita: Diagnosis and treatment. J Vet Intern Med 36:1454-1459, 2022. Pubmed reference: 35815860

Corrêa, S., Basso, R.M., Cerri, F.M., de Oliveira-Filho, J.P., Araújo, J.P., Torelli, S.R., Salán, L.P.C.D.C., Salán, M.O., Macedo, I.Z., Borges, A.S. : Hereditary myotonia in cats associated with a new homozygous missense variant p.Ala331Pro in the muscle chloride channel ClC-1. J Vet Intern Med 37:2498-2503, 2023. Pubmed reference: 37668104