The gene coding for the alpha 1 subunit of the skeletal dihydropyridine receptor (Cchl1a3 = mdg) maps to mouse chromosome 1 and human 1q32

Pharmacogenetics and pathophysiology of CACNA1S mutations in malignant hyperthermia

A review of the pharmacogenetics (PGt) and pathophysiology of calcium voltage-gated channel subunit alpha1 S (CACNA1S) mutations in malignant hyperthermia susceptibility type 5 (MHS5; MIM #60188) is presented. Malignant hyperthermia (MH) is a life-threatening hypermetabolic state of skeletal muscle usually induced by volatile, halogenated anesthetics and/or the depolarizing neuromuscular blocker succinylcholine. In addition to ryanodine receptor 1 (RYR1) mutations, several CACNA1S mutations are known to be risk factors for increased susceptibility to MH (MHS). However, the presence of these pathogenic CACNA1S gene variations cannot be used to positively predict MH since the condition is genetically heterogeneous with variable expression and incomplete penetrance. At present, one or at most six CACNA1S mutations display significant linkage or association either to clinically diagnosed MH or to MHS as determined by contracture testing. Additional pathogenic variants in CACNA1S, either alone or in combination with genes affecting Ca²⁺ homeostasis, are likely to be discovered in association to MH as whole exome sequencing becomes more commonplace.

DHPR alpha1S subunit controls skeletal muscle mass and morphogenesis

The alpha1S subunit has a dual function in skeletal muscle: it forms the L-type Ca(2+) channel in T-tubules and is the voltage sensor of excitation-contraction coupling at the level of triads. It has been proposed that L-type Ca(2+) channels might also be voltage-gated sensors linked to transcriptional activity controlling differentiation. By using the U7-exon skipping strategy, we have achieved long-lasting downregulation of alpha1S in adult skeletal muscle. Treated muscles underwent massive atrophy while still displaying significant amounts of alpha1S in the tubular system and being not paralysed. This atrophy implicated the autophagy pathway, which was triggered by neuronal nitric oxide synthase redistribution, activation of FoxO3A, upregulation of autophagy-related genes and autophagosome formation. Subcellular investigations showed that this atrophy was correlated with the disappearance of a minor fraction of alpha1S located throughout the sarcolemma. Our results reveal for the first time that this sarcolemmal fraction could have a role in a signalling pathway determining muscle anabolic or catabolic state and might act as a molecular sensor of muscle activity.

Cloning, sequencing and identification of single nucleotide polymorphisms of partial sequence on the porcine CACNA1S gene

CACNA1S gene encodes the alpha1 subunit of the calcium channel. The mutation of CACNA1S gene can cause hypokalemic periodic paralysis (HypoKPP) and maliglant hyperthermia synarome (MHS) in human beings. Current research on CACNA1S was mainly in human being and model animal, but rarely in livestock and poultry. In this study, Yorkshire pigs (23), Pietrain pigs (30), Jinhua pigs (115) and the second generation (126) of crossbred of Jinhua and Pietrain were used. Primers were designed according to the sequence of human CACNA1S gene and PCR was carried out using pig genome DNA. PCR products were sequenced and compared with that of human, and then single nucleotide polymorphisms (SNPs) were investigated by PCR-SSCP, while PCR-RFLP tests were performed to validate the mutations. Results indicated: (1) the 5211 bp DNA fragments of porcine CACNA1S gene were acquired (GenBank accession number: DQ767693 ) and the identity of the exon region was 82.6% between human and pig; (2) fifty-seven mutations were found within the cloned sequences, among which 24 were in exon region; (3) the results of PCR-RFLP were in accordance with that of PCR-SSCP. According to the EST of porcine CACNA1S gene published in GenBank (Bx914582, Bx666997), 8 of the 11 SNPs identified in the present study were consistent with the base difference between two EST fragments.

Identification of three novel Ca(2+) channel gamma subunit genes reveals molecular diversification by tandem and chromosome duplication

Gene duplication is believed to be an important evolutionary mechanism for generating functional diversity within genomes. The accumulated products of ancient duplication events can be readily observed among the genes encoding voltage-dependent Ca(2+) ion channels. Ten paralogous genes have been identified that encode isoforms of the alpha(1) subunit, four that encode beta subunits, and three that encode alpha(2)delta subunits. Until recently, only a single gene encoding a muscle-specific isoform of the Ca(2+) channel gamma subunit (CACNG1) was known. Expression of a distantly related gene in the brain was subsequently demonstrated upon isolation of the Cacng2 gene, which is mutated in the mouse neurological mutant stargazer (stg). In this study, we sought to identify additional genes that encoded gamma subunits. Because gene duplication often generates paralogs that remain in close syntenic proximity (tandem duplication) or are copied onto related daughter chromosomes (chromosome or whole-genome duplication), we hypothesized that the known positions of CACNG1 and CACNG2 could be used to predict the likely locations of additional gamma subunit genes. Low-stringency genomic sequence analysis of targeted regions led to the identification of three novel Ca(2+) channel gamma subunit genes, CACNG3, CACNG4, and CACNG5, on chromosomes 16 and 17. These results demonstrate the value of genome evolution models for the identification of distantly related members of gene families.

Voltage-gated ion channels and hereditary disease

By the introduction of technological advancement in methods of structural analysis, electronics, and recombinant DNA techniques, research in physiology has become molecular. Additionally, focus of interest has been moving away from classical physiology to become increasingly centered on mechanisms of disease. A wonderful example for this development, as evident by this review, is the field of ion channel research which would not be nearly as advanced had it not been for human diseases to clarify. It is for this reason that structure-function relationships and ion channel electrophysiology cannot be separated from the genetic and clinical description of ion channelopathies. Unique among reviews of this topic is that all known human hereditary diseases of voltage-gated ion channels are described covering various fields of medicine such as neurology (nocturnal frontal lobe epilepsy, benign neonatal convulsions, episodic ataxia, hemiplegic migraine, deafness, stationary night blindness), nephrology (X-linked recessive nephrolithiasis, Bartter), myology (hypokalemic and hyperkalemic periodic paralysis, myotonia congenita, paramyotonia, malignant hyperthermia), cardiology (LQT syndrome), and interesting parallels in mechanisms of disease emphasized. Likewise, all types of voltage-gated ion channels for cations (sodium, calcium, and potassium channels) and anions (chloride channels) are described together with all knowledge about pharmacology, structure, expression, isoforms, and encoding genes.

A genome wide search for susceptibility loci in three European malignant hyperthermia pedigrees

Malignant hyperthermia (MH) is an autosomal dominant disorder which is potentially lethal in susceptible individuals on exposure to commonly used inhalational anaesthetics and depolarising muscle relaxants. Crises reflect the consequences of disturbed skeletal muscle calcium homeostasis. Susceptibility was first localised to chromosome 19q13.1 and the skeletal muscle ryanodine receptor, RYR1 (the calcium release channel of the sarcoplasmic reticulum). Defects in this gene have been identified which cosegregate with the MHS phenotype and evidence as to their potential causal roles has accumulated. MH has, however, been shown to be genetically heterogeneous, additional loci on chromosomes 3q, 17q and 7q being proposed. Pedigrees remain in Europe where linkage status is still unclear. In a collaborative search of the human genome conducted with three pedigrees whose disease status was classified according to the European IVCT protocol we have evidence to suggest that at least two further loci exist for MH susceptibility. One of these locates to chromosome 1q, the site of a candidate gene, CACNL1A3, encoding the alpha-subunit of the dihydropyridine receptor. The second region resides on chromosome 5p to where no known candidate has been mapped to date. The third family exhibited inconclusive results which suggests the existence of at least one other locus. This study adds to the evidence for considerable genetic heterogeneity in MH and will provide a route to further our understanding of the molecular pathology of the condition.

Channelopathies: the nondystrophic myotonias and periodic paralyses

The term channelopathy does not indicate a new group of neuromuscular conditions, but a re-orientation of well- and long-known muscular conditions, the congenital myotonias, and the periodic paralyses. Although, in the past, they have overlapped clinically here and there, both groups were classified differently, as myotonias and as metabolic myopathies, respectively. The discovery of mutations in several ion channels has rewritten nosography of these disorders and procured a new term, the channelopathy-clinical, electrophysiological, and molecular genetic details of which are discussed in this chapter.

Endogenous cardiac Ca2+ channels do not overcome the E-C coupling defect in immortalized dysgenic muscle cells: evidence for a missing link

The expression of subunit genes of the Ca2+ channel complex was studied in differentiating, immortalized mouse mdg cells. These cells expressed alpha 1 and alpha 2/delta transcripts of the skeletal muscle Ca2+ channel genes, a cardiac Ca2+ channel alpha 1 subunit gene and several known transcript variants of skeletal, cardiac and brain beta genes. The mdg mutation is retained in the 129DA3 cell line and occurs exclusively at nucleotide position 4010 in the skeletal alpha 1 transcript in which a cytosine residue is deleted. In early stages of differentiation and fusion, Ba2+ currents were detected in dysgenic myotubes the same as the cardiac L-type Ca2+ channel. These data provide specific structural evidence [Chaudhari, N. (1992) J. Biol. Chem. 267, 25636-25639] for the major genetic defect in mouse muscular dysgenesis and show a change in the expression levels of alpha 1S and alpha 1C. The upregulation of the expression of alpha 1C results in functional Ca2+ channel activity, however, presumably not sufficient for excitation-contraction coupling.

Myotube driven myogenic recruitment of cells during in vitro myogenesis

Muscular dysgenesis (mdg) is a recessive lethal mutation in the mouse which drastically affects skeletal muscle development during embryonic life. Physiologically, the disease is characterized by a complete paralysis resulting from a lack of excitation-contraction coupling. Existing electrophysiological, biochemical, and genetic evidence shows that mdg/mdg mice express a basic alteration of L-type voltage-sensitive Ca2+ channels in skeletal muscle. Studies on mdg/mdg myotubes in primary culture have shown that +/+ fibroblasts or +/+ Schwann cells may fuse with them and correct their functional deficiency by genetic complementation. As the spontaneous formation of heterocaryons is thought to be an exclusive property of myoblasts, we asked whether fibroblasts may have changed their properties before fusion occurred. We used primary cells issued from sciatic nerves dissected from newborn transgenic mice carrying the pHuDes1-nls-LacZ transgene (Des-LacZ cells) as non-muscle cells. These cells were mainly fibroblasts (80%) positive for Thy1.1 and Schwann cells positive for S100. The cultures were negative for myogenic markers (desmin, troponin T), did not form myotubes long-term, and did not display significant activation of the muscle reporter gene (pHuDes1-nls-LacZ). After a few days in coculture with dysgenic or normal myotubes, the muscle reporter gene (beta-galactosidase) was detected both within dysgenic myotubes, correlating with the restoration of normal contractile activity, and normal myotubes. As well as confirming that fusion takes place, this shows that Des-LacZ cells nuclei incorporated into recipient myotubes express their own myogenic genes. Moreover, individual mononucleated Des-LacZ cells expressing beta-galactosidase were observed, indicating that myogenic genes were being expressed before fusion. This suggests a mechanism of myotube driven myogenic recruitment of cells during the in vitro myogenesis. Analysis of the distribution of the induced Des-LacZ cells (positive for beta-galactosidase) in compartmentalized muscle cocultures showed that in the presence of dysgenic myotubes, these cells were equally distributed in both myotube free and enriched areas, whereas in the presence of normal myotubes, the positive cells remained in close vicinity of the myotubes. This difference could be explained by the fact that the dysgenic phenotype might include release of the induction process from its normal controls. Our results are consistent with the idea of a transcellular mechanism triggering myogenic differentiation in non-muscle cells, and that myotubes themselves are able to drive myogenic recruitment of cells during the in vitro myogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Restoration of normal ultrastructure after expression of the alpha 1 subunit of the L-type Ca2+ channel in dysgenic myotubes

Muscular dysgenesis (mdg) is a spontaneous mutation affecting the alpha 1 subunit of the skeletal L-type Ca2+ channel. mdg/mdg mice suffer from a skeletal muscle disease characterised by low levels of the slow Ca2+ current, lack of contractile activity, and immature organisation of skeletal muscle. Microinjections of a cDNA encoding alpha 1 into mutant myotubes restore excitation-contraction coupling. We checked here that dysgenic myotubes transfected with expression vectors, including a full-length alpha 1 cDNA, also recover normal ultrastructural features. Transfection of alpha 1 cDNA partially deleted on the 5' end leads to the recovery of a good structural organisation without any improvement in the mutant physiological phenotype. These results suggest that: (i) the proper expression of alpha 1 is required for the full muscle differentiation of muscular dysgenesis myotubes, and (ii) portions of the alpha 1 molecule may be involved in the structural organisation of a muscle fiber, independent of its known functional properties.