A new explanation for recessive myotonia congenita: exon deletions and duplications in CLCN1

Abstract

Objective: To assess whether exon deletions or duplications in CLCN1 are associated with recessive myotonia congenita (MC).

Methods: We performed detailed clinical and electrophysiologic characterization in 60 patients with phenotypes consistent with MC. DNA sequencing of CLCN1 followed by multiplex ligation-dependent probe amplification to screen for exon copy number variation was undertaken in all patients.

Results: Exon deletions or duplications in CLCN1 were identified in 6% of patients with MC. Half had heterozygous exonic rearrangements. The other 2 patients (50%), with severe disabling infantile onset myotonia, were identified with both a homozygous mutation, Pro744Thr, which functional electrophysiology studies suggested was nonpathogenic, and a triplication/homozygous duplication involving exons 8-14, suggesting an explanation for the severe phenotype.

Conclusions: These data indicate that copy number variation in CLCN1 may be an important cause of recessive MC. Our observations suggest that it is important to check for exon deletions and duplications as part of the genetic analysis of patients with recessive MC, especially in patients in whom sequencing identifies no mutations or only a single recessive mutation. These results also indicate that additional, as yet unidentified, genetic mechanisms account for cases not currently explained by either CLCN1 point mutations or exonic deletions or duplications.

Figure 1. Breakdown of results of gene sequencing and multiplex ligation-dependent probe amplification (MLPA) in our cohorts of patients with myotonia congenita

Flow chart shows all patients with myotonia congenita investigated. Patients were split according to dominant or recessive pedigrees. Patients from recessive pedigrees with a single recessive mutation, no mutation, or homozygous mutations went on to have MLPA. Those with known dominant mutations or compound heterozygotes did not have MLPA analysis because the cause of their disease was already fully genetically established.

Figure 2. Functional expression of wild-type ClC-1 and Pro744Thr mutation by whole-cell patch clamp of transfected human embryonic kidney 293Tcells

(A) Representative wild-type recording (2 MΩ series resistance, 70% series resistance compensation). (B) Representative mutant recording (3 MΩ series resistance, 70% series compensation). (C) Boltzmann fits of the normalized tail currents from 6 wild-type and 4 mutant recordings to show the similar voltage dependence of activation.