Congenital Myotonia in the Miniature Schnauzer

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The Congenital Myotonia (MC) is an inherited disorder characterized by a delay in skeletal muscle relaxation (or simpler - by a disorder of the relaxation of muscle contraction resulting in muscle stiffness). The disease belongs to canalopathies; this means that it is caused by mutation in a gene that encodes the ion channel in muscle fibre. These genes encode proteins that determine the ion conduction by their "opening and closing".

MC has been described in humans as dominant Thomsen´s disease or recessive Becker´s disease. The congenital myotonia has been well described in goats and mice (Rhodes et al. 1999).

The congenital myotonia in Miniature Schnauzer is caused by mutation in chloride channel ClC-1 in the skeletal muscle, and that by substitution of threonine in transmembrane segment D5 for methionine (Vite et al., 1998). Mutation c.803C˃T  (p.Thr268Met) in CLCN1 gene is located near the recently identified pores forming segments in ClC-1 and causes a significant change in dependence of activation on the voltage (Fahlke, 1997). This change of dependence on the voltage reduces significantly the probability of channel opening at physiological voltage and explains the reduction of chloride conduction and the related irritation of membranes on the surface of muscle fibres.

Dogs affected with this disease have a stiff, stilted gait, so-called "bunnyhop" type movement. In some cases an uncontrolled turning and downfalls may occur. In comparison with healthy animals, the affected Schnautzers have a superior prognathism and a deformed lower jaw, caused probably by contraction of jaw muscles.

The congenital myotonia in Miniature Schnauzers connected with the defective chloride ion conductance across the membrane of skeletal muscles has been diagnosed as autosomal recessive inherited disease (Vite et al., 1997). The recessive inheritance means that the offspring must inherit the mutated gene from both parents for the disease to develop. The individuals with one mutated gene (heterozygous) are carriers of the disease.

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References:

Barchi RL, 1994, The pathophysiology of excitation in skeletal Muscle. In: Disorders of volnutary Musile (Walton J., Karpati G., Histon-Jones D, eds.) Edinburgh, Churchill Livingstone, 415-436

Vite, C., Cozzi, F., Rich, M., Klide, A.K., Volk, S. and Lombardo,
R. (1998) J. Vet. Intern. Med. 12, 394^397.

Fahlke, C., Yu, H.T., Beck, C.L., Rhodes, T.H. and George Jr.,
A.L. (1997) Nature 390, 529^532.

Thomas H. Rhodes, Charles H. Vite, Urs Giger, Donald F. Patterson, Christoph Fahlke, Alfred L. George Jr.: A missense mutation in canine ClC-1 causes recessive myotonia congenita
in the dog1(1999)