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PubMed2005

Two patterns of ion channelopathy in the myocardium: perspectives for development of anti-arrhythmic agents.

Cardiac arrhythmia remains a significant problem, due to the high morbidity and mortality associated with cardiovascular diseases with prominent cardiac remodeling. There is still a lack of effective drugs with which to combat this life-threatening disorder. The abnormal electrophysiological properties of the heart can be explained in terms of ion channels and channelopathy and, in recent years, advances have been made in understanding these properties. There are two patterns of ion channelopathies in the diseased heart: Single insufficiency disorder, which is attributed to mutations in genes, and a multiple derangement of channels. Malignant arrhythmias in a diseased heart usually occur when ventricular hypertrophy is evident, and when they are associated with abnormal repolarization. Abnormalities in the ryanodine receptor-calcium release channel complex (RyR)2, FK-506 binding protein (FKBP 12.6), cardiac sarcoplasmic reticulum calcium ATPase (SERCA2a) and phospholamban (PLB) are involved in the initiation of cardiac arrhythmias, and can be identified as targets for therapeutic interventions.

Animals↗

PubMed2006

Molecular genetics of infantile nervous system channelopathies.

Inherited or de novo mutations in at least a dozen genes encoding ion channels may present as paroxysmal disorders during the neonatal period or first year of life. These channelopathies include genes encoding voltage-gated channels specific for sodium (SCN1A, SCN2A, SCN1B, SCN9A) and potassium (KCNQ2, KCNQ3) which account for a variety of epilepsy phenotypes ranging from mild, such as Benign familial neonatal seizures (BFNS) to severe, such as Dravet syndrome (severe myoclonic epilepsy of infancy, SMEI) and the rare and unusual syndrome paroxysmal extreme pain disorder (PEPD). Ligand-gated channels involved include the GABA(A) receptor in a variety of epilepsy phenotypes and the human glycine receptor. Mutations in five genes encoding subunits of this receptor and accessory molecules underlie hyperekplexia or stiff-baby syndrome. All these conditions are rare but correct diagnosis is of value not only for genetic counselling but to allow the specific treatment which is available.

Channelopathies↗

PubMed2006

Chloride channelopathy in myotonic dystrophy resulting from loss of posttranscriptional regulation for CLCN1.

Transmembrane chloride ion conductance in skeletal muscle increases during early postnatal development. A transgenic mouse model of myotonic dystrophy type 1 (DM1) displays decreased sarcolemmal chloride conductance. Both effects result from modulation of chloride channel 1 (CLCN1) expression, but the respective contributions of transcriptional vs. posttranscriptional regulation are unknown. Here we show that alternative splicing of CLCN1 undergoes a physiological splicing transition during the first 3 wk of postnatal life in mice. During this interval, there is a switch to production of CLCN1 splice products having an intact reading frame, an upregulation of CLCN1 mRNA encoding full-length channel protein, and an increase of CLCN1 function, as determined by patch-clamp analysis of single muscle fibers. In a transgenic mouse model of DM1, however, the splicing transition does not occur, CLCN1 channel function remains low throughout the postnatal interval, and muscle fibers display myotonic discharges. Thus alternative splicing is a posttranscriptional mechanism regulating chloride conductance during muscle development, and the chloride channelopathy in a transgenic mouse model of DM1 results from a failure to execute a splicing transition for CLCN1.

Alternative Splicing↗

PubMed2002

Skeletal muscle channelopathies.

Ion channelopathies have common clinical features, recurrent patterns of mutations, and almost predictable mechanisms of pathogenesis. In skeletal muscle, disorders are associated with mutations in voltage-gated Na(+), K(+), Ca(2+), and Cl(-) channels leading to hypoexcitability, causing periodic paralysis and to hyperexcitabilty, resulting in myotonia or susceptibility to malignant hyperthermia.

Humans↗

PubMed2005

Human skeletal muscle sodium channelopathies.

Ion channels are transmembrane proteins that allow ions to flow in or out of the cell. Sodium and potassium channel activation and inactivation are the basis of action potential's production and conduction. During the past 15 years, ion channels have been implicated in diseases that have come to be known as the channelopathies. Over 30 mutations of the muscle channel gene SCN4A, which encodes the muscle voltage-gated sodium channel, have been described and associated with neuromuscular disorders like hypo- and hyper-kalaemic periodic paralyses (hypoPP and hyperPP), paramyotonia congenita, sodium channel myotonias and congenital myasthenic syndrome. Different mutations within the same gene (SCN4A) cause distinct clinical disorders, while mutations in different channel genes may result in similar phenotypes. In addition, identical sodium channel mutations can result in different clinical phenotypes (hyperPP or paramyotonia) in different members of the same family, suggesting that the genetic background and perhaps other epigenetic factors may influence the clinical expression of a particular mutation. This article reviews the clinical features of the skeletal muscle sodium channel diseases and highlights the phenotypic or genetic overlap in these disorders.

Humans↗

PubMed2000

Therapy in myotonic disorders and in muscle channelopathies.

Myotonia and muscle weakness are cardinal features of myotonic disorders including the myotonic dystrophies and the non-dystrophic myotonias. Despite the recent progress in molecular genetics of these myotonic disorders, the precise mechanisms responsible for myotonia and for permanent or episodic muscle weakness are still unclear. Treatment has been mostly symptomatic, independent of the disease process involved. Moreover, there have been few randomized controlled trials of treatment for myotonic disorders and consequently no standardized treatment regimens are available. We present a review of selected treatment trials in the myotonic disorders and in muscle channelopathies, and discuss, on the basis of our experience in the myotonic disorders, the limits and advantages of treatment trials in this field. Future genotype-phenotype correlations using the patch-clamp technique are also illustrated.

Calcium Channels↗

PubMed2002

Periodic paralysis: understanding channelopathies.

Familial periodic paralyses are typical channelopathies (i.e., caused by functional disturbances of ion channel proteins). The episodes of flaccid muscle weakness observed in these disorders are due to underexcitability of sarcolemma leading to a silent electromyogram and the lack of action potentials even upon electrical stimulation. Interictally, ion channel malfunction is well compensated, so that special exogenous or endogenous triggers are required to produce symptoms in the patients. An especially obvious trigger is the level of serum potassium (K+), the ion responsible for resting membrane potential and degree of excitability. The clinical symptoms can be caused by mutations in genes coding for ion channels that mediate different functions for maintaining the resting potential or propagating the action potential, the basis of excitability. The phenotype is determined by the type of functional defect brought about by the mutations, rather than the channel effected, because the contrary phenotypes hyperkalemic periodic paralysis (HyperPP) and hypokalemic periodic paralysis (HypoPP) may be caused by point mutations in the same gene. Still, the common mechanism for inexcitability in all known episodic-weakness phenotypes is a long-lasting depolarization that inactivates sodium ion (Na+) channels, initiating the action potential.

Electrophysiology↗

PubMed2000

Channelopathies.

In patients with mutations in the genes that encode the chloride, sodium, and calcium channels in skeletal muscle, there is abnormal function of the muscle membrane, which can cause myotonia or attacks of weakness. Mutations in the chloride and sodium channels can lead to myotonia, which typically begins in early childhood. Mexiletine is usually effective in controlling myotonia in these patients. Mexiletine is also effective in preventing attacks of cold-provoked muscle paralysis in patients with paramyotonia congenita, a sodium channel disorder. Certain mutations in the sodium channel cause attacks of hyperkalemic periodic paralysis; these attacks are often controlled with thiazide diuretics. Mutations in the skeletal muscle calcium channel cause periodic attacks of weakness, but hypokalemia (not hyperkalemia) occurs during these episodes. The carbonic anhydrase inhibitors acetazolamide and dichlorphenamide prevent attacks of hypokalemic periodic paralysis, although the mechanism by which they produce this protective effect remains a mystery. Interestingly, the hypokalemic attacks with periodic weakness that occur in some thyrotoxic patients are made worse by acetazolamide. This undesirable response to treatment emphasizes that not all disorders associated with hypokalemic periodic paralysis will benefit from carbonic anhydrase inhibitor therapy. DNA analysis to search for a mutation in the genes that encode for chloride, sodium, or calcium channels in skeletal muscle is helpful to establish the diagnosis. Some patients may eventually require provocative testing, however, to evaluate the attack of weakness and to reach a final diagnosis. Fortunately, there are effective treatments for the channelopathies that affect the skeletal muscle membrane.

Journal Article↗

PubMed2005

Sudden infant death syndrome: how significant are the cardiac channelopathies?

Having an apparently healthy, thriving infant fail to reach his/her first birthday is profoundly tragic. This tragedy is compounded when the infant's death is unexpected and unexplained, signed out as sudden infant death syndrome (SIDS). Despite impressive success and welcome reductions in these tragic deaths due in large measure to "Back-to-Sleep" campaigns, the fundamental pathogenic mechanisms precipitating such deaths remain dimly exposed. Here, we review the causal link between SIDS and mutations involving the SCN5A-encoded cardiac sodium channel, provide new findings following extensive postmortem genetic testing of long QT syndrome (LQTS)-associated potassium channel genes in a population-based cohort of SIDS, and summarize the current understanding regarding the spectrum and prevalence of cardiac channelopathies in the pathogenesis of SIDS.

Autopsy↗

PubMed2005

Noradrenergic blockade prevents attacks in a model of episodic dysfunction caused by a channelopathy.

Episodic neurological dysfunction often results from ion channel gene mutations. Despite knowledge of the mutations, the factors that precipitate attacks in channelopathies are not clear. In humans, mutations of the calcium channel gene CACNA1A are associated with attacks of neurological dysfunction in familial hemiplegic migraine and episodic ataxia type-2. In tottering mice, a mutation in the same gene causes attacks resembling paroxysmal dyskinesia. Stress, a trigger associated with human episodic disorders, reliably elicits attacks in tottering mice. Because noradrenergic neurotransmission is critical to the stress response and because noradrenergic hyperinnervation is observed in tottering mice, the role of norepinephrine in stress-induced attacks was investigated. Drugs that act at alpha-adrenergic receptors to block noradrenergic transmission prevented attacks. However, agents that facilitate noradrenergic neurotransmission failed to induce attacks. These results suggest that, while noradrenergic neurotransmission may be necessary for attacks, an increase in norepinephrine is not sufficient to induce attacks.

Adrenergic Agonists↗

PubMed2004

Presynaptic Ca2+ channels compete for channel type-preferring slots in altered neurotransmission arising from Ca2+ channelopathy.

Several human channelopathies result from mutations in alpha1A, the pore-forming subunit of P/Q-type Ca2+ channels, conduits of presynaptic Ca2+ entry for evoked neurotransmission. We found that wild-type human alpha1A subunits supported transmission between cultured mouse hippocampal neurons equally well as endogenous mouse alpha1A, whereas introduction of impermeant human alpha1A hampered the effect of endogenous subunits. Thus, presynaptic P/Q-type channels may compete for channel type-preferring "slots" that limit their synaptic effectiveness. The existence of slots generates predictions for how neurotransmission might be affected by changes in Ca2+ channel properties, which we tested by studying alpha1A mutations that are associated with familial hemiplegic migraine type 1 (FHM1). Mutant human P/Q-type channels were impaired in contributing to neurotransmission in precise accord with their deficiency in supporting whole-cell Ca2+ channel activity. Expression of mutant channels in wild-type neurons reduced the synaptic contribution of P/Q-type channels, suggesting that competition for type-preferring slots might support the dominant inheritance of FHM1.

Animals↗

PubMed1998

Episodic ataxia and channelopathies.

Clinical details are given of different types of episodic ataxia: type 1, with myokymia, and attacks which usually last a few minutes, and may occur several times a day, and treatment with acetazolamide can reduce the number of attacks; type 2, with interictal nystagmus, and attacks which last for several hours to a day or more, and treatment with acetazolamide is very effective; paroxysmal choreoathetosis with episodic ataxia, with attacks lasting for about 20 min and occurring at varying intervals; and familial hemiplegic migraine, with transient hemiplegia presenting during the aura of a migraine headache, the symptoms improving on treatment with acetazolamide. Their inheritance is of dominant type; and the gene for type 1 is mapped to chromosome 12p near to a cluster of potassium channel genes, and that for type 2 and for familial hemiplegic migraine to chromosome 19p near to calcium channel genes. The differential diagnosis from other conditions with a periodic symptomatology is discussed, especially from a number of metabolic disorders. Treatment is effective for many of these, and the treatment of the episodic ataxias with acetazolamide can sometimes have a dramatic effect. The possible role of the channelopathies in the causation of some periodic neurological disorders is considered; with the expectation that further research will improve the identification of specific diseases, and lead to more effective treatment.

Acetazolamide↗

PubMed1997

Channelopathies: ion channel disorders of muscle as a paradigm for paroxysmal disorders of the nervous system.

Some of the most common diseases in humans occur intermittently in people who are otherwise healthy and active. Such disorders include migraine headache, epilepsy, and cardiac arrhythmias. Because electrical signals are critical to the function of neurons, muscle cells, and heart cells, proteins that regulate electrical signaling in these cells are logical sites where abnormalities might lead to disease. All of these diseases have prominent genetic components. Difficulty in understanding these diseases arises from the complexity of the clinical phenotypes as well as from the genetic heterogeneity that is almost certain to exist. Therefore, early work in may laboratory was aimed at understanding the pathogenesis of rare disorders that are similar in their episodic nature. These disorders of muscle (the periodic paralyses), lead to attacks of weakness that occur intermittently in otherwise normal people. We, and others, have shown that hyperkalemic periodic paralysis (hyperKPP) and paramyotonia congenita (PC) result from mutations in a gene encoding a skeletal muscle sodium channel. We have also shown that hypokalemic periodic paralysis (hypoKPP) is caused by mutations in a gene encoding a voltage-gated calcium channel. The characterization of these diseases as channelopathies has served as a paradigm for other episodic disorders. One example is periodic ataxia, which results from mutations in voltage-gated potassium calcium channels. Long QT syndrome, an episodic cardiac dysrhythmia syndrome, is known to result from mutations in either voltage-gated sodium or potassium channels. We have recently mapped genes that cause a familial paroxysmal dyskinesia (non-kinesiogenic paroxysmal dystonia/choreoathetosis) in humans and a reflex epilepsy in mice. The similarities among all these disorders, including their episodic nature, precipitating factors, and therapeutic responses, are striking. Understanding gained from work in these rare monogenic episodic disorders is not only allowing characterization of the molecular and physiologic basis of these diseases, but may ultimately shed light on our understanding of the pathophysiology of more common and genetically complex disorders of the central nervous system.

Arrhythmias, Cardiac↗

PubMed2001

Inherited sodium channelopathies: novel therapeutic and proarrhythmic molecular mechanisms.

Voltage-gated sodium (Na) channels, transmembrane proteins that produce the ionic current responsible for the rapid upstroke of the cardiac action potential, are key elements required for rapid conduction through the myocardium and maintenance of the cardiac rhythm. The exquisite sensitivity of the cardiac rhythm to Na channel function is manifest in the proarrhythmic complications of "antiarrhythmic" Na channel blockade in patients with myocardial ischemia. More recently, studies of inherited single amino acid substitutions in Na channels have unveiled a remarkable array of cardiac rhythm disturbances, as well as surprising pharmacologic sensitivities. Hence, the sodium channelopathies are providing new molecular insights into mechanisms whereby altered ion channel behavior precipitates cardiac arrhythmias.

Arrhythmias, Cardiac↗

PubMed2000

Calcium channelopathies.

Calcium is an important intracellular signaling molecule, and altered calcium channel function can cause widespread cellular changes. Genetic mutations in calcium channels that cause what appear to be trivial alterations of calcium currents in vitro can result in serious diseases in muscles and the nervous system. This article reviews calcium channelopathies in humans and mice.

Animals↗

PubMed2004

Cardiac channelopathies: from men to mice.

During recent years, genetic manipulations in the mouse aimed to settle animal models of cardiac channelopathies. Among this group of diseases, the genetically heterogeneous long-QT (LQT) syndrome has instigated several models. Models of the LQT1 syndrome have been obtained by invalidation of Kcnq1 encoding a voltage-dependent K+ channel alpha-subunit, LQT2 syndrome by invalidation of Merg also encoding a voltage-dependent K+ channel alpha-subunit, LQT3 by knocking-in a gain-of-function deletion (delta KPQ) of the Scn5A cardiac Na+ channel gene, LQT4 by invalidating the ankyrin B gene. LQT5 by invalidating the Kcne1 K+ channel beta-subunit and the Andersen syndrome by invalidation of the KCNJ2 gene encoding for a cardiac inward rectifier K+ channel. Among these LQT models, the LQT3 and LQT4 mice exhibit spontaneous or exercise-induced life-threatening arrhythmias characteristics of long-QT patients. In addition, a model for the SCN5A-linked Brugada syndrome and for the inherited Lenègre disease has been established by heterozygous knock-out of Scn5A. These mice demonstrate progressive cardiac conduction defect similar to that observed in Lenègre patients and an increased susceptibility to arrhythmias as found in Brugada patients. In the future, the mouse model should prove instrumental to investigate the myocardial remodeling that is likely to result from gene invalidation either in man or in mice.

Animals↗

PubMed2006

Kv1.5 channelopathy due to KCNA5 loss-of-function mutation causes human atrial fibrillation.

Atrial fibrillation is a rhythm disorder characterized by chaotic electrical activity of cardiac atria. Predisposing to stroke and heart failure, this common condition is increasingly recognized as a heritable disorder. To identify genetic defects conferring disease susceptibility, patients with idiopathic atrial fibrillation, lacking traditional risk factors, were evaluated. Genomic DNA scanning revealed a nonsense mutation in KCNA5 that encodes Kv1.5, a voltage-gated potassium channel expressed in human atria. The heterozygous E375X mutation, present in a familial case of atrial fibrillation and absent in 540 unrelated control individuals, introduced a premature stop codon disrupting the Kv1.5 channel protein. The truncation eliminated the S4-S6 voltage sensor, pore region and C-terminus, preserving the N-terminus and S1-S3 transmembrane domains that secure tetrameric subunit assembly. Heterologously expressed recombinant E375X mutant failed to generate the ultrarapid delayed rectifier current I(Kur) vital for atrial repolarization and exerted a dominant-negative effect on wild-type current. Loss of channel function translated into action potential prolongation and early after-depolarization in human atrial myocytes, increasing vulnerability to stress-provoked triggered activity. The pathogenic link between compromised Kv1.5 function and susceptibility to atrial fibrillation was verified, at the organism level, in a murine model. Rescue of the genetic defect was achieved by aminoglycoside-induced translational read-through of the E375X premature stop codon, restoring channel function. This first report of Kv1.5 loss-of-function channelopathy establishes KCNA5 mutation as a novel risk factor for repolarization deficiency and atrial fibrillation.

Animals↗

PubMed1999

Channelopathies of inwardly rectifying potassium channels.

Mutations in genes encoding ion channels have increasingly been identified to cause disease conditions collectively termed channelopathies. Recognizing the molecular basis of an ion channel disease has provided new opportunities for screening, early diagnosis, and therapy of such conditions. This synopsis provides an overview of progress in the identification of molecular defects in inwardly rectifying potassium (Kir) channels. Structurally and functionally distinct from other channel families, Kir channels are ubiquitously expressed and serve functions as diverse as regulation of resting membrane potential, maintenance of K(+) homeostasis, control of heart rate, and hormone secretion. In humans, persistent hyperinsulinemic hypoglycemia of infancy, a disorder affecting the function of pancreatic beta cells, and Bartter's syndrome, characterized by hypokalemic alkalosis, hypercalciuria, increased serum aldosterone, and plasma renin activity, are the two major diseases linked so far to mutations in a Kir channel or associated protein. In addition, the weaver phenotype, a neurological disorder in mice, has also been associated with mutations in a Kir channel subtype. Further genetic linkage analysis and full understanding of the consequence that a defect in a Kir channel would have on disease pathogenesis are among the priorities in this emerging field of molecular medicine.

Animals↗