title: Myotonia levior is a chloride channel disorder.
abstract: The group of dominant non-dystrophic myotonias, comprising disorders characterized by clinically similar forms of myogenic muscle stiffness, is genetically inhomogeneous. Dominant myotonia congenita (Thomsen's disease) is linked to CLCN1, the gene encoding the major muscle chloride channel, localized on chromosome 7q35. In contrast, dominant myotonias sensitive to potassium are caused by point mutations in SCN4A on chromosome 17q, the gene for the alpha subunit of the adult skeletal muscle sodium channel. No linkage or molecular genetic data are as yet available on 'myotonia levior' characterized by milder symptoms and later onset of myotonia than in Thomsen's disease, and absence of muscle hypertrophy. We report a CLCN1 Gln-552-Arg substitution for a family with dominant inheritance previously diagnosed to have myotonia levior. Thus, this disorder appears as a variant of Thomsen's disease due to mutations leading to low clinical expressivity. In addition, we report a novel Ile-290-Met CLCN1 mutation for a typical Thomsen pedigree. In another family previously diagnosed as having Thomsen's disease, we unexpectedly found a CLCN1 14 bp deletion known to cause recessive myotonia, and a rare Trp-118-Gly polymorphism.
authors.affiliation: Department of Applied Physiology, University of Ulm, Germany.
authors.forename: F
authors.initials: F
authors.lastname: Lehmann-Horn
authors.forename: V
authors.initials: V
authors.lastname: Mailänder
authors.forename: R
authors.initials: R
authors.lastname: Heine
authors.forename: A L
authors.initials: AL
authors.lastname: George
mesh_terms: D000328:Adult; D000595:Amino Acid Sequence; D001483:Base Sequence; D018118:Chloride Channels / Q000235:genetics* / Q000378:metabolism; D017931:DNA Primers / Q000235:genetics; D005260:Female; D005799:Genes, Dominant; D006801:Humans; D008297:Male; D008969:Molecular Sequence Data; D018482:Muscle, Skeletal / Q000378:metabolism; D009222:Myotonia / Q000235:genetics* / Q000378:metabolism*; D009224:Myotonia Congenita / Q000235:genetics / Q000378:metabolism; D010375:Pedigree; D017354:Point Mutation; D016133:Polymerase Chain Reaction
chemical_list: D018118:Chloride Channels; D017931:DNA Primers