Autoimmune and paraneoplastic channelopathies

Thirty years ago, antibodies against the muscle acetylcholine receptor (AChR) were recognized as the cause of myasthenia gravis. Since then, there has been great interest in identifying other neurological disorders associated with autoantibodies. Several other antibody-mediated neuromuscular disorders have been identified, each associated with an antibody against a ligand- or voltage-gated ion channel. The Lambert-Eaton syndrome is caused by antibodies against voltage-gated calcium channels and often occurs in patients with small cell lung cancer. Acquired neuromyotonia is caused by voltage-gated potassium channel antibodies, and autoimmune autonomic ganglionopathy is caused by antibodies against the neuronal AChR in autonomic ganglia. There is good evidence that antibodies in these disorders cause changes in synaptic function or neuronal excitability by directly inhibiting ion channel function. More recently, studies have identified ion channel antibodies in patients with certain CNS disorders, such as steroid-responsive encephalitis and paraneoplastic cerebellar ataxia. It remains unclear if antibodies can gain access to the CNS and directly cause ion channel dysfunction. Treatment of autoimmune channelopathies includes drugs that help restore normal neuronal function and treatments to remove pathogenic antibodies (plasma exchange) or modulate the immune response (steroids or immunosuppressants). These disabling neurological disorders may be dramatically responsive to immunomodulatory therapy. Future studies will likely lead to identification of other ion channel antibodies and other autoimmune channelopathies.

The nondystrophic myotonias

The nondystrophic myotonias are a heterogeneous set of rare diseases that demonstrate clinical myotonia, electrical myotonia, or both. These disorders are distinguished from myotonic dystrophy type 1 (DM-1), the more recently described proximal myotonic myopathy/myotonic dystrophy type 2 (PROMM/DM-2), and proximal myotonic dystrophy (a variant of DM-2) by characteristic clinical features, lack of abnormal nucleotide repeat expansions in the DM-1 and DM-2 genes, lack of cataracts and endocrine disturbances, and absence of significant histopathology in the muscle biopsy. The present article reviews each of the nondystrophic myotonias by exploring the unique clinical features, electrodiagnostic findings, diagnostic criteria, gene mutations, and response to pharmacologic therapy. These diseases are divided into those with chloride channel dysfunction (the myotonia congenita disorders) and those with sodium channel dysfunction (paramyotonia congenita, potassium-aggravated myotonia, and hyperkalemic periodic paralysis with myotonia). The variants that occur in each of these conditions are commented on. The differentiating features of the nondystrophic myotonias are summarized, and their predominant clinical, electrodiagnostic, and genetic characteristics are tabulated. For a comprehensive review of pertinent research and studies with application to diagnosis and treatment of individuals with nondystrophic myotonic disorders, the present article is best read in the context of other articles in this issue, especially those on ion channel physiology (Cannon) and pharmacology (Conte-Camerino), and on hyperkalemic periodic paralysis (Lehmann-Horn).

Episodic ataxia type 1: a neuronal potassium channelopathy

Episodic ataxia type 1 is a paroxysmal neurological disorder characterized by short-lived attacks of recurrent midline cerebellar dysfunction and continuous motor activity. Mutations in KCN1A, the gene encoding Kv1.1, a voltage-gated neuronal potassium channel, are associated with the disorder. Although rare, the syndrome highlights the fundamental features of genetic ion-channel diseases and serves as a useful model for understanding more common paroxysmal disorders, such as epilepsy and migraine. This review examines our current understanding of episodic ataxia type 1, focusing on its clinical and genetic features, pathophysiology, and treatment.

Channelopathies in idiopathic epilepsy

Approximately 70% of all patients with epilepsy lack an obvious extraneous cause and are presumed to have a predominantly genetic basis. Both familial and de novo mutations in neuronal voltage-gated and ligand-gated ion channel subunit genes have been identified in autosomal dominant epilepsies. However, patients with dominant familial mutations are rare and the majority of idiopathic epilepsy is likely to be the result of polygenic susceptibility alleles (complex epilepsy). Data on the identity of the genes involved in complex epilepsy is currently sparse but again points to neuronal ion channels. The number of genes and gene families associated with epilepsy is rapidly increasing and this increase is likely to escalate over the coming years with advances in mutation detection technologies. The genetic heterogeneity underlying idiopathic epilepsy presents challenges for the rational selection of therapies targeting particular ion channels. Too little is currently known about the genetic architecture of the epilepsies, and genetic testing for the known epilepsy genes remains costly. Pharmacogenetic studies have yet to explain why 30% of patients do not respond to the usual antiepileptic drugs. Despite this, the recognition that the idiopathic epilepsies are a group of channelopathies has, to a limited extent, explained the therapeutic action of the common antiepileptic drugs and has assisted clinical diagnosis of some epilepsy syndromes.

Calcium channelopathies

Intracellular calcium ([Ca2+]i) is highly regulated in eukaryotic cells. The free [Ca2+]i is approximately four orders of magnitude less than that in the extracellular environment. It is, therefore, an electrochemical gradient favoring Ca2+ entry, and transient cellular activation increasing Ca2+ permeability will lead to a transient increase in [Ca2+]i. These transient rises of [Ca2+]i trigger or regulate diverse intracellular events, including metabolic processes, muscle contraction, secretion of hormones and neurotransmitters, cell differentiation, and gene expression. Hence, changes in [Ca2+]i act as a second messenger system coordinating modifications in the external environment with intracellular processes. Notably, information on the molecular genetics of the membrane channels responsible for the influx of Ca2+ ions has led to the discovery that mutations in these proteins are linked to human disease. Ca2+ channel dysfunction is now known to be the basis for several neurological and muscle disorders such as migraine, ataxia, and periodic paralysis. In contrast to other types of genetic diseases, Ca2+ channelopathies can be studied with precision by electrophysiological methods, and in some cases, the results have been highly rewarding with a biophysical phenotype that correlates with the ultimate clinical phenotype. This review outlines recent advances in genetic, molecular, and pathophysiological aspects of human Ca2+ channelopathies.

TRPC6 and FSGS: the latest TRP channelopathy

Focal and segmental glomerulosclerosis (FSGS) is a common cause of nephrotic syndrome in children and adults throughout the world. In the past 50 years, significant advances have been made in the identification and characterization of familial forms of nephrotic syndrome and FSGS. Resultant to these pursuits, several podocyte structural proteins such as nephrin, podocin, alpha-actinin 4 (ACTN4), and CD2-associated protein (CD2AP) have emerged to provide critical insight into the pathogenesis of hereditary nephrotic syndromes. The latest advance in familial FSGS has been the discovery of a mutant form of canonical transient receptor potential cation channel 6 (TRPC6), which causes an increase in calcium transients and essentially a gain of function in this cation channel located on the podocyte cell membrane. The TRP ion channel family is a diverse group of cation channels united by a common primary structure which contains six membrane-spanning domains, with both carboxy and amino termini located intracellularly. TRP channels are unique in their ability to activate independently of membrane depolarization. TRPC6 channels have been shown to be activated via phospholipase C stimulation. The mechanisms by which mutant TRPC6 causes an increase in intracellular calcium and leads to glomerulosclerosis are unknown. Mutant TRPC6 may affect critical interactions with the aforementioned podocyte structural proteins, leading to abnormalities in the slit diaphragm or podocyte foot processes. Mutant TRPC6 may also amplify injurious signals mediated by Ang II, a common final pathway of podocyte apoptosis in various mammalian species. Current evidence also suggests that blocking TRPC6 channels may be of therapeutic benefit in idiopathic FSGS, a disease with a generally poor prognosis. Preliminary experiments reveal the commonly used immunosuppressive agent FK-506 can inhibit TRPC6 activity in vivo. This creates the exciting possibility that blocking TRPC6 channels within the podocyte may translate into long-lasting clinical benefits in patients with FSGS.

Channelopathies: a New Category of Diseases Causing Sudden Death

Identification of familial forms of different diseases has provided a unique opportunity to study how changes in the structure of a gene create a dysfunction in the physiology of the resulting protein. Changes in the genes encoding for ion channels produce modifications in the function of the channel. Changes in the sodium channel are responsible for long QT syndrome, Brugada syndrome and conduction defects. Changes in the potassium channels have been related to long QT syndrome, short QT syndrome and familial atrial fibrillation. Relating genetic modification and dysfunction allows to study the substrate for a disease, understand the physiopathologic mechanism and look for appropriate therapies.

Channel, neuronal and clinical function in sodium channelopathies: from genotype to phenotype

What is the relationship between sodium channel function, neuronal function and clinical status in channelopathies of the nervous system? Given the central role of sodium channels in the generation of neuronal activity, channelopathies involving sodium channels might be expected to cause either enhanced sodium channel function and neuronal hyperexcitability associated with positive clinical manifestations such as seizures, or attenuated channel function and neuronal hypoexcitability associated with negative clinical manifestations such as paralysis. In this article, I review observations showing that changes in neuronal function and clinical status associated with channelopathies are not necessarily predictable solely from the altered physiological properties of the mutated channel itself. I discuss evidence showing that cell background acts as a filter that can strongly influence the effects of ion channel mutations.

TRP channels in disease

"Transient receptor potential" cation channels (TRP channels) play a unique role as cell sensors, are involved in a plethora of Ca(2+)-mediated cell functions, and play a role as "gate-keepers" in many homeostatic processes such as Ca(2+) and Mg(2+) reabsorption. The variety of functions to which TRP channels contribute and the polymodal character of their activation predict that failures in correct channel gating or permeation will likely contribute to complex pathophysiological mechanisms. Dysfunctions of TRPs cause human diseases but are also involved in a complex manner to contribute and determine the progress of several diseases. Contributions to this special issue discuss channelopathias for which mutations in TRP channels that induce "loss-" or "gain-of-function" of the channel and can be considered "disease-causing" have been identified. The role of TRPs will be further elucidated in complex diseases of the intestinal, renal, urogenital, respiratory, and cardiovascular systems. Finally, the role of TRPs will be discussed in neuronal diseases and neurodegenerative disorders.

KATP channel mutation confers risk for vein of Marshall adrenergic atrial fibrillation

BACKGROUND

A 53-year-old female presented with a 10-year history of paroxysmal atrial fibrillation (AF), precipitated by activity and refractory to medical therapy. In the absence of traditional risk factors for disease, a genetic defect in electrical homeostasis underlying stress-induced AF was explored.

INVESTIGATIONS

Echocardiography, cardiac perfusion stress imaging, invasive electrophysiology with isoproterenol provocation, genomic DNA sequencing of K(ATP) channel genes, exclusion of mutation in 2,000 individuals free of AF, reconstitution of channel defect with molecular phenotyping, and verification of pathogenic link in targeted knockout.

DIAGNOSIS

K(ATP) channelopathy caused by missense mutation (Thr1547Ile) of the ABCC9 gene conferring predisposition to adrenergic AF originating from the vein of Marshall.

MANAGEMENT

Disruption of arrhythmogenic gene-environment substrate at the vein of Marshall by radiofrequency ablation.

[Genetics of epilepsy]

In the last few years, the presence of specific genetic abnormalities leading to some of the classical epileptic syndromes has been clearly elucidated. This means that for the first time, it has become possible to create a strong relationship between the presence of specific genomic and/or proteomic abnormalities and epileptic syndromes previously considered to be "idiopathic". The majority of such genetic defects have been found in genes coding for either ion channels and/or membrane receptors, a fact that somehow seems to confirm the previously postulated importance of the latter structures in the electrochemical activity of neurons. This review will focus on the genetic and clinical aspects of such conditions. Some of the most relevant data suggesting the existence of additional genetic defects in many other epileptic syndromes will also be briefly reviewed, even though a definitive relationship to many of them has not yet been established in the form of specific gene defects. In addition, the worrisome fact that despite the importance of such advances, their application in routine clinical practice remains very limited will be emphasized, in particular in the pharmacological management of most patients. Finally, a brief discussion about the intriguing possibilities of such findings, including the development of neuro-pharmacogenomics plus several ethical issues, will also be attempted.

Long and short QT syndrome

In the past decade molecular genetic analysis has greatly expanded our knowledge about inherited arrhythmogenic syndromes. The congenital long QT syndrome (LQTS) and the recently described short QT syndrome (SQTS), with the defining characteristic of abnormal prolongation or shortening of the QTc interval on the surface electrocardiogram, are caused by cardiac ion channel dysfunctions. These "channelopathies" show a high degree of genetic heterogeneity of the molecular pathways in terms of the relationships between genetic defects and phenotypic expression. In this brief review we summarize the current understanding of the molecular basis of long and short QT syndrome with focus on the impact of molecular genetics on the clinical management of these diseases.

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.

TRPpathies

Many human diseases are caused by mutations in ion channels. Dissecting the pathogenesis of these 'channelopathies' has yielded important insights into the regulation of vital biological processes by ions and has become a productive tool of modern ion channel biology. One of the best examples of a synergism between the clinical and basic science aspects of a modern biological topic is cystic fibrosis. Not only did the identification of the ion channel mutated in cystic fibrosis pinpoint the root cause of this disease, but it also has significantly advanced our understanding of basic biological processes as diverse as protein folding and epithelial fluid and electrolyte secretion. The list of confirmed 'channelopathies' is growing and several members of the TRP family of ion channels have been implicated in human diseases such as mucolipidosis type IV (MLIV), autosomal dominant polycystic kidney disease (ADPKD), familial focal segmental glomerulosclerosis (FSG), hypomagnesemia with secondary hypocalcaemia (HSH), and several forms of cancer. Analysing pathogenesis of the diseases linked to TRP dysregulation provides an exciting means of identifying novel functions of TRP channels.

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.

Voltage-gated calcium channels in genetic diseases

Voltage-gated calcium channels (VGCCs) mediate calcium entry into excitable cells in response to membrane depolarization. During the past decade, our understanding of the gating and functions of VGCCs has been illuminated by the analysis of mutations linked to a heterogeneous group of genetic diseases called "calcium channelopathies". Calcium channelopathies include muscular, neurological, cardiac and vision syndromes. Recent data suggest that calcium channelopathies result not only from electrophysiological defects but also from altered alpha(1)/Ca(V) subunit protein processing, including folding, posttranslational modifications, quality control and trafficking abnormalities. Overall, functional analyses of VGCC mutations provide a more comprehensive view of the corresponding human disorders and offer important new insights into VGCC function. Ultimately, the understanding of these pathogenic channel mutations should lead to improved treatments of such hereditary diseases in humans.

Molecular basis of inherited calcium channelopathies: role of mutations in pore-forming subunits

The pore-forming alpha subunits of voltage-gated calcium channels contain the essential biophysical machinery that underlies calcium influx in response to cell depolarization. In combination with requisite auxiliary subunits, these pore subunits form calcium channel complexes that are pivotal to the physiology and pharmacology of diverse cells ranging from sperm to neurons. Not surprisingly, mutations in the pore subunits generate diverse pathologies, termed channelopathies, that range from failures in excitation-contraction coupling to night blindness. Over the last decade, major insights into the mechanisms of pathogenesis have been derived from animals showing spontaneous or induced mutations. In parallel, there has been considerable growth in our understanding of the workings of voltage-gated ion channels from a structure-function, regulation and cell biology perspective. Here we document our current understanding of the mutations underlying channelopathies involving the voltage-gated calcium channel alpha subunits in humans and other species.

Voltage-gated sodium channels: action players with many faces

Voltage-gated sodium channels are responsible for the upstroke of the action potential and thereby play an important role in propagation of the electrical impulse in excitable tissues like muscle, nerve and the heart. Duplication of the sodium channels encoding genes during evolution generated the sodium channel gene family with the different isoforms differing in biophysical properties and tissue distribution. In this review article, mutations in these genes leading to various inherited disorders are discussed.