G.P.137: Functional study of five new CLC-1 mutations causing myotonia congenita in Italian families

Abstract

Myotonia congenita is an inherited disease characterized by impaired muscle relaxation after contraction, resulting in muscle stiffness. It is caused by loss-of-function mutations of the muscle ClC-1 chloride channel. We report the functional characterization of five novel mutations found in patients with recessive and dominant myotonia congenita, selected among the largest cohort of Italian families reported to date, on the basis of the associated clinical phenotype and position in the protein 3D-structure. The mutations F484L, L198P and L520P resides within the channel pore and are inherited in a dominant manner. The mutations V640G and L628P occur in the C-terminal domain and show a recessive inheritance. Recombinant hClC-1 channel mutants were expressed in a mammalian cell line for patch-clamp studies of chloride current properties. The mutant channels F484L, L198P and V640G show a large current reduction at every tested potential. In addition, the mutations F484L and L198P induce a dramatic shift of activation voltage-dependence toward more positive potentials, resulting in nearly zero chloride current within physiological voltage range. These effects likely contribute to impaired muscle repolarization and explain the myotonia. Further experiments are required to clarify the dominant inheritance for mutations located outside the common hot-spot for dominant genetic variants, to address the mechanism of ATP modulation for the mutations close to the ATP binding sites, and to clarify genotype-phenotype correlations. Pharmacogenetics studies are also in progress.