Disease relevance of CLCN1

• Autosomal dominant myotonia congenita is an inherited disorder of skeletal muscle caused by mutations in a voltage-gated Cl- channel gene (CLCN1, 7q35) [1].
• In another family previously diagnosed as having Thomsen's disease, we unexpectedly found a CLCN1 14 bp deletion known to cause recessive myotonia, and a rare Trp-118-Gly polymorphism [2].
• Linkage to two loci implicated in other myotonic disorders, the muscle chloride channel (CLCN1) gene, and the muscle sodium channel (SCN4A) gene, was assessed and excluded [3].
• Loss of the muscle-specific chloride channel in type 1 myotonic dystrophy due to misregulated alternative splicing [4].
• The cause of several familial muscular diseases have recently been linked to mutations within skeletal muscle sodium and chloride channel genes [5].

High impact information on CLCN1

• The glycine receptor chloride channel (GlyR) is a member of the nicotinic acetylcholine receptor family of ligand-gated ion channels [6].
• Molecular structure and function of the glycine receptor chloride channel [6].
• CFTR is a conductance regulator as well as a chloride channel [7].
• We now demonstrate linkage of this phenotype to a segment of chromosome 1 containing the gene encoding a renal chloride channel, CLCNKB [8].
• The chloride channel gene, CLCN4, has been previously mapped to the X chromosome in humans [9].

Chemical compound and disease context of CLCN1

• Multimeric structure of ClC-1 chloride channel revealed by mutations in dominant myotonia congenita (Thomsen) [10].
• The cystic fibrosis gene encodes a cyclic AMP-gated chloride channel (CFTR) that mediates electrolyte transport across the luminal surfaces of a variety of epithelial cells [11].
• A point mutation (D136G) predicting the substitution of glycine for aspartate in position 136 of the human muscle Cl- channel (hClC-1) causes recessive generalized myotonia [12].
• BACKGROUND & AIMS: Cystic fibrosis transmembrane conductance regulator (CFTR) is an adenosine 3',5'-cyclic monophosphate-dependent chloride channel that is defective in cystic fibrosis [13].
• Extracellular adenine nucleotides also serve as precursors for adenosine, which promotes cyclic AMP-mediated activation of the cystic fibrosis transmembrane regulator chloride channel via A(2b) adenosine receptors [14].

Biological context of CLCN1

• Recently, these two phenotypes have been associated with mutations in the major muscle chloride channel gene CLCN1 on human chromosome 7q35 [15].
• Mutations within CLCN1, the gene encoding the major skeletal muscle chloride channel, cause either dominant Thomsen disease or recessive Becker-type myotonia, which are sometimes difficult to discriminate, because of reduced penetrance or lower clinical expressivity in females [16].
• We have systematically screened the open reading frame of the CLCN1 gene for mutations by SSC analysis (SSCA) in a panel of 24 families and 17 single unrelated patients with human myotonia [15].
• Dominant myotonia congenita (Thomsen's disease) is linked to CLCN1, the gene encoding the major muscle chloride channel, localized on chromosome 7q35 [2].
• We now show that the protein-coding sequence of the CLCN1 gene is organized into 23 exons [17].

Anatomical context of CLCN1

• Current understanding does not include a molecular identification of most of the observed channel functions, but details of the molecular properties of these channel molecules should allow future identification and further understanding of chloride channel function in hepatocytes [18].
• Expression of CLCN voltage-gated chloride channel genes in human blood vessels [19].
• Differential expression of the human chloride channel genes in the trabecular meshwork under stress conditions [20].
• Expression of the chloride channel CLC-K in human airway epithelial cells [21].
• Identification and functional characterization of a voltage-gated chloride channel and its novel splice variant in taste bud cells [22].

Associations of CLCN1 with chemical compounds

• The functional consequences of the novel CLCN1 sequence variants were explored by recording chloride currents from human embryonic kidney cells transiently expressing homo- or heterodimeric mutant channels [23].
• This glycine residue is conserved in all known members of this class of chloride channel proteins [24].
• An unusual restriction site in the CLC-1 locus in two GM families identified a mutation associated with that disease, a phenylalanine-to-cysteine substitution in putative transmembrane domain D8 [25].
• Voltage-dependent blocks by intracellular and extracellular iodide help to distinguish two distinct ion binding sites within the hClC-1 conduction pathway [1].
• These findings suggest that Gly 230 is critical for normal ion conductance in hClC-1 and that this residue resides within the channel pore [1].

Physical interactions of CLCN1

• Conservation of chloride channel structure revealed by an inhibitor binding site in ClC-1 [26].
• The chloride channel and drug transport activities of P-glycoprotein appear to reflect two distinct functional states of the protein that can be interconverted by changes in tonicity [27].
• Production of a CFTR with reduced Cl(-) transport on the basis of abnormal regulation of the chloride channel is the basis of class III [28].
• The alpha(1)-inhibitory glycine receptor is a ligand-gated chloride channel composed of three ligand-binding alpha1-subunits and two structural beta-subunits that are clustered on the postsynaptic membrane of inhibitory glycinergic neurons [29].
• Channel-active mammalian porin binds ATP and the stilbene disulphonate grouping of the chloride channel inhibitor DIDS [30].

Regulatory relationships of CLCN1

• A common sequence variation of the CLCNKB gene strongly activates ClC-Kb chloride channel activity [31].
• Human ClC-3 is not the swelling-activated chloride channel involved in cell volume regulation [32].
• CIC-2 is a voltage- and volume-regulated chloride channel expressed in many tissues [33].
• We speculate that the NEG2 region interacts with other cytosolic domains of CFTR to control opening and closing transitions of the chloride channel [34].
• Recently, the calcium-activated chloride channel hCLCA1 has been described to be upregulated by IL-9 and has been thought to regulate the expression of soluble gel-forming mucins [35].

Other interactions of CLCN1

• An expansion in the ZNF9 gene causes PROMM in a previously described family with an incidental CLCN1 mutation [36].
• The family was not sufficiently informative to exclude linkage to the sodium channel gene SCN4A or the chloride channel gene CLC1 [37].
• The GM locus was again completely linked to both the CLCN1 and the TCRB gene in all families with a combined lod score of Z = 9.26 at a recombination fraction of theta = 0.00 [38].
• These findings suggest that RPE 28 SV4 cells possess regulated chloride channels including CFTR and members of the ClC chloride channel family [39].
• Modulation of ClC-Kb chloride channel activity by polymorphic variations of the CLCNKB gene, thus, could form a molecular basis for salt sensitivity of blood pressure regulation [31].

Analytical, diagnostic and therapeutic context of CLCN1

• METHODS: The authors used a mammalian cell (human embryonic kidney 293) expression system and the whole-cell voltage-clamp technique to functionally express and physiologically characterize five CLCN1 mutations [40].
• All 23 exons of the CLCN1 gene were analysed by direct sequencing of PCR products to detect the nucleotide changes [41].
• By means of two microelectrode voltage clamp recordings, we found that S(-)-CPP shifted the activation curve of the ClC-1 currents toward more positive potentials and decreased the residual conductance at negative membrane potential; both effects probably account for the decrease of gCl at resting potential of native muscle fibers [42].
• Skeletal muscle CLC-1 mRNA levels were decreased by denervation [43].
• Real-time quantitative RT-PCR did not reveal any obvious association between the total CLCN1 mRNA level in muscle and the mode of inheritance, but the dominant family with the most severe phenotype expressed twice the expected amount of the R894X mRNA allele [44].

References