Myotonia congenita: mutation spectrum of CLCN1 in Spanish patients.

Myotonia congenita (MC) is a Mendelian inherited genetic disease caused by the mutations in the CLCN1 gene, encoding the main skeletal muscle ion chloride channel (ClC-1). The clinical diagnosis of MC should be suspected in patients presenting myotonia, warm-up phenomenon, a characteristic electromyographic pattern, and/or family history. Here, we describe the largest cohort of MC Spanish patients including their relatives (up to 102 individuals). Genetic testing was performed by CLCN1 sequencing and multiplex ligation-dependent probe amplification (MLPA). Analysis of selected exons of the SCN4A gene, causing paramyotonia congenita, was also performed. Mutation spectrum and analysis of a likely founder effect of c.180+3A>T was achieved by haplotype analysis and association tests. Twenty-eight different pathogenic variants were found in the CLCN1 gene, of which 21 were known mutations and seven not described. Gross deletions/duplications were not detected. Four probands had a pathogenic variant in SCN4A. Two main haplotypes were detected in c.180+3A>T carriers and no statistically significant differences were detected between case and control groups regarding the type of haplotype and its frequencies. A diagnostic yield of 51% was achieved; of which 88% had pathogenic variants in CLCN1 and 12% in SCN4A. The existence of a c.180+3A>T founder effect remains unsolved.

Directional coupling of oligodendrocyte connexin-47 and astrocyte connexin-43 gap junctions.

Intercellular communication via gap junction channels between oligodendrocytes and between astrocytes as well as between these cell types is essential to maintain the integrity of myelin in the central nervous system. Oligodendrocyte gap junction connexin-47 (Cx47) is a key element in this crosstalk and indeed, mutations in human Cx47 cause severe myelin disorders. However, the permeation properties of channels of Cx47 alone and in heterotypic combination with astrocyte Cx43 remain unclear. We show here that Cx47 contains three extra residues at 5' amino-terminus that play a critical role in the channel pore structure and account for relative low ionic conductivity, cationic permselectivity and voltage-gating properties of oligodendrocyte-oligodendrocyte Cx47 channels. Regarding oligodendrocyte-astrocyte coupling, heterotypic channels formed by Cx47 with Cx43 exhibit ionic and chemical rectification, which creates a directional diffusion barrier for the movement of ions and larger negatively charged molecules from cells expressing Cx47 to those with Cx43. The restrictive permeability of Cx47 channels and the diffusion barrier of Cx47-Cx43 channels was abolished by a mutation associated with leukodystrophy, the Cx47P90S, suggesting a novel pathogenic mechanism underlying myelin disorders that involves alterations in the panglial permeation.

A SCN4A mutation causing paramyotonia congenita.

Paramyotonia congenita (OMIM 168300) is a non-dystrophic myopathy caused by mutations in the SCN4A gene that sometimes can be confused with myotonia congenita. Another disease also caused by mutations in the gene SCN4A is called myotonia aggravated by potassium (OMIM 170500, 613345). It is estimated that more than 20% of patients with suspected myotonia congenita suffer paramyotonia congenita. The two related SCN4A phenotypes exhibit an autosomal dominant inheritance and are the result of mutations that cause an increase in the function of the protein coded by this gene. In this study we present a case of paramyotonia congenita in a family with several affected members and in which a mutation in the SCN4A gene was identified. Evolutionary conservation data and predictive algorithms of pathogenicity allow us to conclude that this DNA variant is the cause of the disease in this family.

Fast skeletal myofibers of mdx mouse, model of Duchenne muscular dystrophy, express connexin hemichannels that lead to apoptosis.

Skeletal muscles of patients with Duchenne muscular dystrophy (DMD) show numerous alterations including inflammation, apoptosis, and necrosis of myofibers. However, the molecular mechanism that explains these changes remains largely unknown. Here, the involvement of hemichannels formed by connexins (Cx HCs) was evaluated in skeletal muscle of mdx mouse model of DMD. Fast myofibers of mdx mice were found to express three connexins (39, 43 and 45) and high sarcolemma permeability, which was absent in myofibers of mdx Cx43(fl/fl)Cx45(fl/fl):Myo-Cre mice (deficient in skeletal muscle Cx43/Cx45 expression). These myofibers did not show elevated basal intracellular free Ca(2+) levels, immunoreactivity to phosphorylated p65 (active NF-κB), eNOS and annexin V/active Caspase 3 (marker of apoptosis) but presented dystrophin immunoreactivity. Moreover, muscles of mdx Cx43(fl/fl)Cx45(fl/fl):Myo-Cre mice exhibited partial decrease of necrotic features (big cells and high creatine kinase levels). Accordingly, these muscles showed similar macrophage infiltration as control mdx muscles. Nonetheless, the hanging test performance of mdx Cx43(fl/fl)Cx45(fl/fl):Myo-Cre mice was significantly better than that of control mdx Cx43(fl/fl)Cx45(fl/fl) mice. All three Cxs found in skeletal muscles of mdx mice were also detected in fast myofibers of biopsy specimens from patients with muscular dystrophy. Thus, reduction of Cx expression and/or function of Cx HCs may be potential therapeutic approaches to abrogate myofiber apoptosis in DMD.

Regulation of connexin hemichannel activity by membrane potential and the extracellular calcium in health and disease.

Connexins are thought to solely mediate cell-to-cell communication by forming gap junction channels composed of two membrane-spanning hemichannels positioned end-to-end. However, many if not all connexin isoforms also form functional hemichannels (i.e., the precursors of complete channels) that mediate the rapid exchange of ions, second messengers and metabolites between the cell interior and the interstitial space. Electrical and molecular signaling via connexin hemichannels is now widely recognized to be important in many physiological scenarios and pathological conditions. Indeed, mutations in connexins that alter hemichannel function have been implicated in several diseases. Here, we present a comprehensive overview of how hemichannel activity is tightly regulated by membrane potential and the external calcium concentration. In addition, we discuss the genetic mutations known to alter hemichannel function and their deleterious effects, of which a better understanding is necessary to develop novel therapeutic approaches for diseases caused by hemichannel dysfunction. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.