A novel alteration of muscle chloride channel gating in myotonia levior

Abstract

Mutations in the voltage-dependent skeletal muscle chloride channel, ClC-1, result in dominant or recessive myotonia congenita. The Q552R mutation causes a variant of dominant myotonia with a milder phenotype, myotonia levior. To characterise the functional properties of this mutation, homodimeric mutant and heterodimeric wild-type (WT) mutant channels were expressed in tsA201 cells and studied using the whole-cell recording technique. Q552R ClC-1 mutants formed functional channels with normal ion conduction but altered gating properties. Mutant channels were activated by membrane depolarisation, with a voltage dependence of activation that was shifted by more than +90 mV compared to WT channels. Q552R channels were also activated by hyperpolarisation, and this process was dependent upon the intracellular chloride concentration ([Cl _ ] i ). Together, these alterations resulted in a substantial reduction in the open probability at _85 mV at a physiological [Cl _ ] i . Heterodimeric WT-Q552R channels did not exhibit hyperpolarisation-activated gating transitions. As was the case for WT channels, activation occurred upon depolarisation, but the activation curve was shifted by 28 mV to more positive potentials. The functional properties of heterodimeric channels suggest a weakly dominant effect, a finding that correlates with the inheritance pattern and symptom profile of myotonia levior.

Phenotypic variability in myotonia congenita

“…1). 114 Likewise, Ryan et al87 found a +28‐mV shift for heterodimeric WT:Q552R constructs. Thus, in agreement with the mild phenotype, the G200R and Q552R mutations produce functional heterodimers with only slightly altered gating characteristics.…”

Multidisciplinary study of a new CIC‐1 mutation causing myotonia congenita: a paradigm to understand and treat ion channelopathies

“…One possible interpretative hypothesis could be that, as a consequence of this impaired interaction between the 2 T335N subunits, chloride currents lose fast deactivation at negative potentials that is typical of ClC-1 WT channels observed in patch-clamp recordings of T335N channels. Of interest, a hyperpolarization-activated gating, which occurs via a cooperative conformational change of 2 mutant subunits, has been previously shown for other heterozygous MC mutations, including Q552R of loop P-Q ( 44 – 48 ). Thus, similar to T335N, activation at negative voltages of Q552R homomers could likely be ascribed to disruption of the intermonomeric H-bond between Q552 and I553.…”

Impaired surface membrane insertion of homo- and heterodimeric human muscle chloride channels carrying amino-terminal myotonia-causing mutations

“…Our results on function, assembly and trafficking of hetero- and homodimeric Q43R hClC-1 channels predict - assuming equal expression of mutant and WT hClC-1 - a reduction of the sarcolemmal chloride conductance to about 25% in such heterozygous patients. This drastic reduction is expected to result in sarcolemmal hyperexcitability 10 12 49 50 and is therefore in disagreement with the apparently recessive inheritance of this particular mutation. A possible explanation for recessive inheritance might be that the number of Q43R hClC-1 subunits is much lower than of WT subunits, either by decreased Q43R or increased WT expression 51 .…”