Myotonia congenita and periodic hypokalemia paralysis in a consanguineous marriage pedigree: Coexistence of a novel CLCN1 mutation and an SCN4A mutation

Abstract

Myotonia congenita and hypokalemic periodic paralysis type 2 are both rare genetic channelopathies caused by mutations in the CLCN1 gene encoding voltage-gated chloride channel CLC-1 and the SCN4A gene encoding voltage-gated sodium channel Nav1.4. The patients with concomitant mutations in both genes manifested different unique symptoms from mutations in these genes separately. Here, we describe a patient with myotonia and periodic paralysis in a consanguineous marriage pedigree. By using whole-exome sequencing, a novel F306S variant in the CLCN1 gene and a known R222W mutation in the SCN4A gene were identified in the pedigree. Patch clamp analysis revealed that the F306S mutant reduced the opening probability of CLC-1 and chloride conductance. Our study expanded the CLCN1 mutation database. We emphasized the value of whole-exome sequencing for differential diagnosis in atypical myotonic patients.

Fig 1. Genetic analysis.

(A) The pedigree with myotonia congenital and hypokalemic periodic paralysis type 2. (B) Sanger sequencing revealed a homozygous p.F306S CLCN1 mutation in V: 5 and V: 6, while IV: 2 and VI: 3 carried a heterozygous sequence. There was a heterozygous p.R222W SCN4A mutation in V.7 and VI.3.

Fig 2. Electrophysiological properties of the CLC-1_F306S mutant channel in HEK293T cells.

(A-C) Representative current traces from HEK293T cells expressing WT, CLC-1_F306L and CLC-1_F306S. (D and E) Instantaneous (denoted by an arrow) and steady-state (denoted by a triangle) current-voltage association of CLC-1_F306S and CLC-1_F306L mutant channels compared with the WT channel. (F) Voltage-dependent activation of CLC-1_F306S and CLC-1_F306L mutant channels compared with the WT channel.

Fig 3. Schematic of CLC-1 channel and multiple sequence alignment.

(A) The localization of p.F306S mutation on the CLC-1 channel. (B) The Phe306 amino acid residue is conserved in every member of the CLC family in humans except for CLC-6. (C) The Phe306 amino acid residue is conserved from C. elegans to H. sapiens.