Exon 17 skipping in CLCN1 leads to recessive myotonia congenita

Association of Three Different Mutations in the CLCN1 Gene Modulating the Phenotype in a Consanguineous Family with Myotonia Congenita

Myotonia congenita is a genetic disease caused by mutations in the CLCN1 gene, which encodes for the major chloride skeletal channel ClC-1, involved in the normal repolarization of muscle action potentials and consequent relaxation of the muscle after contraction. Two allelic forms are recognized, depending on the phenotype and the inheritance pattern: the autosomal dominant Thomsen disease with milder symptoms and the autosomal recessive Becker disorder with a severe phenotype. Before the recent advances of molecular testing, the diagnosis and genetic counseling of families was a challenge due to the large number of mutations in the CLCN1 gene, found both in homozygous or in heterozygous state. Here, we studied a consanguineous family in which three members presented a variable phenotype of myotonia, associated to a combination of three different mutations in the CLCN1 gene. A pathogenic splicing site mutation which causes the skipping of exon 17 was present in homozygosis in one very severely affected son. This mutation was present in compound heterozygosis in the consanguineous parents, but interestingly it was associated to a different second variant in the other allele: c.1453 A > G in the mother and c.1842 G > C in the father. Both displayed variable, but less severe phenotypes than their homozygous son. These results highlight the importance of analyzing the combination of different variants in the same gene in particular in families with patients displaying different phenotypes. This approach may improve the diagnosis, prognosis, and genetic counseling of the involved families.

A large intragenic deletion in the CLCN1 gene causes Hereditary Myotonia in pigs

Mutations in the CLCN1 gene are the primary cause of non-dystrophic Hereditary Myotonia in several animal species. However, there are no reports of Hereditary Myotonia in pigs to date. Therefore, the objective of the present study was to characterize the clinical and molecular findings of Hereditary Myotonia in an inbred pedigree. The clinical, electromyographic, histopathological, and molecular findings were evaluated. Clinically affected pigs presented non-dystrophic recessive Hereditary Myotonia. Nucleotide sequence analysis of the entire coding region of the CLCN1 gene revealed the absence of the exons 15 and 16 in myotonic animals. Analysis of the genomic region flanking the deletion unveiled a large intragenic deletion of 4,165 nucleotides. Interestingly, non-related, non-myotonic pigs expressed transcriptional levels of an alternate transcript (i.e., X2) that was identical to the deleted X1 transcript of myotonic pigs. All myotonic pigs and their progenitors were homozygous recessive and heterozygous, respectively, for the 4,165-nucleotide deletion. This is the first study reporting Hereditary Myotonia in pigs and characterizing its clinical and molecular findings. Moreover, to the best of our knowledge, Hereditary Myotonia has never been associated with a genomic deletion in the CLCN1 gene in any other species.

Sequence CLCN1 and SCN4A in patients with Nondystrophic myotonias in Chinese populations: Genetic and pedigree analysis of 10 families and review of the literature

Myotonia congenita (MC), paramyotonia congenita (PC) and sodium channel myotonias(SCM) were belonged to Non-dystrophic myotonias, in which muscle relaxation is delayed after voluntary or evoked contraction. These diseases can not be simply distinguished only based on symptoms and signs but also on genetics: more than 100 mutations in the CLCN1 gene have been associated with MC, while at least 20 mutations in the SCN4A gene have been associated with PC and SCM. Most of these genetics studies have been conducted outside China, only several MC, PC, and SCM families accepted gene scan were reported in China. Therefore we analyzed genetic mutations in CLCN1 and SCN4A in 10 Chinese families clinically diagnosed with Non-dystrophic myotonias. Our result revealed 12 potential disease-causing mutations(3 mutations were novel) that were present in the probands and affected family members. We also reviewed all available literature on mutations linked to these 3 disease in Chinese populations. Our results may help identify genetic determinants as well as clarify genotype-phenotype relationships.

In vitro analysis of splice site mutations in the CLCN1 gene using the minigene assay

Mutations in the chloride channel gene CLCN1 cause the allelic disorders Thomsen (dominant) and Becker (recessive) myotonia congenita (MC). The encoded protein, ClC-1, is the primary channel that mediates chloride (Cl-) conductance in skeletal muscle. Mutations in CLCN1 lower the channel's threshold voltage, leading to spontaneous action potentials that are not coupled to neuromuscular transmission and resulting in myotonia. Over 120 mutations in CLCN1 have been described, 10% of which are splicing defects. Biological specimens suitable for RNA extraction are not always available, but obtaining genomic DNA for analysis is easy and non-invasive. This is the first study to evaluate the pathogenic potential of novel splicing mutations using the minigene approach, which is based on genomic DNA analysis. Splicing mutations accounted for 23% of all pathogenic variants in our cohort of MC patients. Four were heterozygous mutations in four unrelated individuals, belonging to this cohort: c.563G>T in exon 5; c.1169-5T>G in intron 10; c.1251+1G>A in intron 11, and c.1931-2A>G in intron 16. These variants were expressed in HEK 293 cells, and aberrant splicing was verified by in vitro transcription and sequencing of the cDNA. Our findings confirm the need to further investigate the nature of rearrangements associated with this class of mutations and their effects on mature transcripts. In particular, splicing mutations predicted to generate in-frame transcripts may generate out-of-frame mRNA transcripts that do not produce functional ClC-1. Clinically, incomplete molecular evaluation could lead to delayed or faulty diagnosis.

Myotonia congenita

Myotonia is a symptom of many different acquired and genetic muscular conditions that impair the relaxation phase of muscular contraction. Myotonia congenita is a specific inherited disorder of muscle membrane hyperexcitability caused by reduced sarcolemmal chloride conductance due to mutations in CLCN1, the gene coding for the main skeletal muscle chloride channel ClC-1. The disorder may be transmitted as either an autosomal-dominant or recessive trait with close to 130 currently known mutations. Although this is a rare disorder, elucidation of the pathophysiology underlying myotonia congenita established the importance of sarcolemmal chloride conductance in the control of muscle excitability and demonstrated the first example of human disease associated with the ClC family of chloride transporting proteins.

Functional characterization of CLCN1 mutations in Taiwanese patients with myotonia congenita via heterologous expression

Mutations in the CLCN1 gene frequently associate with myotonia congenita (MC). We have recently reported several CLCN1 mutants in Taiwanese patients. To further elucidate the correlation between the genotypes and phenotypes, in this study, we used Xenopus oocyte as a system to investigate the functional effects of these mutants. The fs793X and G482R mutants, which were suggested to have a dual inheritance pattern, were found to cause a functional loss of CLCN1 channels. While co-expression of fs793X and wild-type (WT) showed a reduction of chloride conductance by about half of WT channels, the activation curve of voltage-dependence was not shifted. A compound heterozygous mutant, P575S/D644G, was found in a patient. When both mutants were co-expressed in oocytes, they caused a shift of the voltage-dependence of activation curve to more positive values than individual mutant. This indicates that both P575S and D644G mutants may contribute cooperatively to change the gating property of CLCN1 channel. Interestingly, the S471F mutant did not cause significant alternation of functional properties. Consistent with the fact that T631I mutant was found in three asymptomatic individuals, the electrophysiological parameters of T631I were similar to those of WT CLCN1 channels, suggesting that T631I is a neutral mutation. These results further clarify the correlation between the mutations and their functional implications of CLCN1 channels.

Functional complementation of truncated human skeletal-muscle chloride channel (hClC-1) using carboxyl tail fragments

Crystal structures of bacterial CLC (voltage-gated chloride channel family) proteins suggest the arrangement of permeation pores and possible gates in the transmembrane region of eukaryotic CLC channels. For the extensive cytoplasmic tails of eukaryotic CLC family members, however, there are no equivalent structural predictions. Truncations of cytoplasmic tails in different places or point mutations result in loss of function or altered gating of several members of the CLC family, suggesting functional importance. In the present study, we show that deletion of the terminal 100 amino acids (N889X) in human ClC-1 (skeletal-muscle chloride channel) has minor consequences, whereas truncation by 110 or more amino acids (from Q879X) destroys channel function. Use of the split channel strategy, co-injecting mRNAs and expressing various complementary constructs in Xenopus oocytes, confirms the importance of the Gln879-Arg888 sequence. A split between the two CBS (cystathionine b-synthase) domains (CBS1 and CBS2) gives normal function (e.g. G721X plus its complement), whereas a partial complementation, eliminating the CBS1 domain, eliminates function. Surprisingly, function is retained even when the region Gly721-Ala862 (between CBS1 and CBS2, and including most of the CBS2 domain) is omitted from the complementation. Furthermore, even shorter peptides from the CBS2-immediate post-CBS2 region are sufficient for functional complementation. We have found that just 26 amino acids from Leu863 to Arg888 are necessary since channel function is restored by co-expressing this peptide with the otherwise inactive truncation, G721X.

Identification and functional characterization of a voltage-gated chloride channel and its novel splice variant in taste bud cells

Taste bud cells are epithelial cells with neuronal properties. Voltage-dependent ion channels have been physiologically described in these cells. Here, we report the molecular identification and functional characterization of a voltage-gated chloride channel (ClC-4) and its novel splice variant (ClC-4A) from taste bud cells. ClC-4A skipped an exon near its 5'-end, incurring the loss of 60 amino acids at the N terminus. In situ hybridization and immunohistochemistry localized these two channels' transcripts and proteins to a subset of taste bud cells. Electrophysiological recordings of the heterologously expressed channels in Xenopus oocytes showed that ClC-4 and ClC-4A have opposite sensitivity to pH and unique ion selectivity. The chloride channel blockers niflumic acid and 5-nitro-2-(3-phenylpropylamino)benzoic acid had a slight or no inhibitory effect on the conductance of ClC-4, but both blockers inhibited ClC-4A, suggesting that ClC-4A is a candidate channel for an acid-induced 5-nitro-2-(3-phenylpropylamino)benzoic acid-sensitive current. Furthermore, these two channels may play a role in bitter-, sweet-, and umami-mediated taste transmission by regulating transmitter uptake into synaptic vesicles.

Phenotypic variability in myotonia congenita

Myotonia congenita is a hereditary chloride channel disorder characterized by delayed relaxation of skeletal muscle (myotonia). It is caused by mutations in the skeletal muscle chloride channel gene CLCN1 on chromosome 7. The phenotypic spectrum of myotonia congenita ranges from mild myotonia disclosed only by clinical examination to severe and disabling myotonia with transient weakness and myopathy. The most severe phenotypes are seen in patients with two mutated alleles. Heterozygotes are often asymptomatic but for some mutations heterozygosity is sufficient to cause pronounced myotonia, although without weakness and myopathy. Thus, the phenotype depends on the mutation type to some extent, but this does not explain the fact that severity varies greatly between heterozygous family members and may even vary with time in the individual patient. In this review, existing knowledge about phenotypic variability is summarized, and the possible contributing factors are discussed.

Atomic Absorption Spectroscopy in Ion Channel Screening

This article examines the utility of atomic absorption spectroscopy, in conjunction with cold flux assays, to ion channel screening. The multiplicity of ion channels that can be interrogated using cold flux assays and atomic absorption spectroscopy is summarized. The importance of atomic absorption spectroscopy as a screening tool is further elaborated upon by providing examples of the relevance of ion channels to various physiological processes and targeted diseases.