| Summary: | Non-dystrophic myotonias have been linked to loss-of-function mutations in the ClC-1 chloride channel or gain-of-function mutations in the Na<sub>v</sub>1.4 sodium channel. Here, we describe a family with members diagnosed with Thomsen’s disease. One novel mutation (p.W322*) in <i>CLCN1</i> and one undescribed mutation (p.R1463H) in <i>SCN4A</i> are segregating in this family. The <i>CLCN1</i>-p.W322* was also found in an unrelated family, in compound heterozygosity with the known <i>CLCN1</i>-p.G355R mutation. One reported mutation, <i>SCN4A</i>-p.T1313M, was found in a third family. Both <i>CLCN1</i> mutations exhibited loss-of-function: <i>CLCN1</i>-p.W322* probably leads to a non-viable truncated protein; for <i>CLCN1</i>-p.G355R, we predict structural damage, triggering important steric clashes. The <i>SCN4A</i>-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 <i>CLCN1</i>-p.W322* and <i>SCN4A</i>-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.
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