Abstract
Myotonia congenita-inducing mutations in the muscle chloride channel CLC-1 normally result in reduced open probability (P o) of this channel. One well-accepted mechanism of the dominant inheritance of this disease involves a dominant-negative effect of the mutation on the function of the common-gate of this homodimeric, double-barreled molecule. We report here a family with myotonia congenita characterized by muscle stiffness and clinical and electrophysiologic myotonic phenomena transmitted in an autosomal dominant pattern. DNA sequencing of DMPK and ZNF9 genes for myotonic muscular dystrophy types I and II was normal, whereas sequencing of CLC-1 encoding gene, CLCN1, identified a single heterozygous missense mutation, G233S. Patch-clamp analyses of this mutant CLC-1 channel in Xenopus oocytes revealed an increased P o of the channel’s fast-gate, from ~0.4 in the wild type to >0.9 in the mutant at −90 mV. In contrast, the mutant exhibits a minimal effect on the P o of the common-gate. These results are consistent with the structural prediction that the mutation site is adjacent to the fast-gate of the channel. Overall, the mutant could lead to a significantly reduced dynamic response of CLC-1 to membrane depolarization, from a fivefold increase in chloride conductance in the wild type to a twofold increase in the mutant—this might result in slower membrane repolarization during an action potential. Since expression levels of the mutant and wild-type subunits in artificial model cell systems were unable to explain the disease symptoms, the mechanism leading to dominant inheritance in this family remains to be determined.
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Acknowledgments
This study was supported by research grants to Dr. Richman from National Institutes of Health (R21NS071325-01), Muscular Dystrophy Association (MDA114815), Myasthenia Gravis Foundation of California; grants to Dr. Maselli from National Institutes of Health (R01NS049117-01), Muscular Dystrophy Association, Myasthenia Gravis Foundation of America and Myasthenia Gravis Foundation of California; grants to Dr. Tang from National Science Council, Taiwan (NSC 96-2320-B-002-069-MY3); and grants to Dr. Chen from National Institutes of Health (R01GM065447).
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Richman, D.P., Yu, Y., Lee, TT. et al. Dominantly Inherited Myotonia Congenita Resulting from a Mutation That Increases Open Probability of the Muscle Chloride Channel CLC-1. Neuromol Med 14, 328–337 (2012). https://doi.org/10.1007/s12017-012-8190-1
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DOI: https://doi.org/10.1007/s12017-012-8190-1