Functional and Structural Characterization of ClC-1 and Nav1.4 Channels Resulting from CLCN1 and SCN4A Mutations Identified Alone and Coexisting in Myotonic Patients.

Non-dystrophic myotonias have been linked to loss-of-function mutations in the ClC-1 chloride channel or gain-of-function mutations in the Nav1.4 sodium channel. Here, we describe a family with members diagnosed with Thomsen's disease. One novel mutation (p.W322*) in CLCN1 and one undescribed mutation (p.R1463H) in SCN4A are segregating in this family. The CLCN1-p.W322* was also found in an unrelated family, in compound heterozygosity with the known CLCN1-p.G355R mutation. One reported mutation, SCN4A-p.T1313M, was found in a third family. Both CLCN1 mutations exhibited loss-of-function: CLCN1-p.W322* probably leads to a non-viable truncated protein; for CLCN1-p.G355R, we predict structural damage, triggering important steric clashes. The SCN4A-p.R1463H produced a positive shift in the steady-state inactivation increasing window currents and a faster recovery from inactivation. These gain-of-function effects are probably due to a disruption of interaction R1463-D1356, which destabilizes the voltage sensor domain (VSD) IV and increases the flexibility of the S4-S5 linker. Finally, modelling suggested that the p.T1313M induces a strong decrease in protein flexibility on the III-IV linker. This study demonstrates that CLCN1-p.W322* and SCN4A-p.R1463H mutations can act alone or in combination as inducers of myotonia. Their co-segregation highlights the necessity for carrying out deep genetic analysis to provide accurate genetic counseling and management of patients.

Duchenne-Becker muscular dystrophy and the nondystrophic myotonias. Paradigms for loss of function and change of function of gene products.

Recessively inherited disorders can most often be considered loss of function: the patient has only defective copies of the defective gene (homozygous or hemizygous), with little or no functional protein products produced. Dominantly inherited disorders can most often be considered change of function: the patient has both mutant and normal copies of the gene (heterozygous); however, the mutant gene produces an abnormal protein product that causes dysfunction of the cell. Categorization of inherited disorders simply by their inheritance pattern thus affords some predictions concerning the underlying biochemical defect. To illustrate these generalizations, the molecular data on two important human inherited neurologic disorders will be described. X-linked recessive Duchenne/Becker muscular dystrophy has been shown to caused by loss of function of the dystrophin product. Dominantly inherited hyperkalemic periodic paralysis and paramyotonia congenita have been shown to be the result of single amino acid changes of the skeletal muscle voltage-sensitive sodium channel that alter the channel's function in response to environmental or physiologic stimuli (change of function).