Mutations in SCN9A, encoding a sodium channel alpha subunit, in patients with primary erythermalgia

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.

Neuronal voltage-gated sodium channel subtypes: Key roles in inflammatory and neuropathic pain

Voltage-gated sodium channels (VGSCs) play an important role in neuronal excitability. Regulation of VGSC activity is a complex phenomenon that occurs at multiple levels in the cell, including transcriptional regulation, post-translational modification and membrane insertion and retrieval. Multiple VGSC subtypes exist that vary in their biophysical and pharmacological properties and tissue distribution. Any alteration of the VGSC subtype profile of a neuron or the mechanisms that regulate VGSC activity can cause significant changes in neuronal excitability. Inflammatory and neuropathic pain states are characterised by alterations in VGSC subtype composition and activity in sensory neurons. This review focuses on the VGSC subtypes involved in such pain states.

Roles of Voltage-Dependent Sodium Channels in Neuronal Development, Pain, and Neurodegeneration

Besides initiating and propagating action potentials in established neuronal circuits, voltage-dependent sodium channels sculpt and bolster the functional neuronal network from early in embryonic development through adulthood (e.g., differentiation of oligodendrocyte precursor cells into oligodendrocytes, myelinating axon; competition between neighboring equipotential neurites for development into a single axon; enhancing and opposing functional interactions with attractive and repulsive molecules for axon pathfinding; extending and retracting terminal arborization of axon for correct synapse formation; experience-driven cognition; neuronal survival; and remyelination of demyelinated axons). Surprisingly, different patterns of action potentials direct homeostasis-based epigenetic selection for neurotransmitter phenotype, thus excitability by sodium channels specifying expression of inhibitory neurotransmitters. Mechanisms for these pleiotropic effects of sodium channels include reciprocal interactions between neurons and glia via neurotransmitters, growth factors, and cytokines at synapses and axons. Sodium channelopathies causing pain (e.g., allodynia) and neurodegeneration (e.g., multiple sclerosis) derive from 1) electrophysiological disturbances by insults (e.g., ischemia/hypoxia, toxins, and antibodies); 2) loss-of-physiological function or gain-of-pathological function of mutant sodium channel proteins; 3) spatiotemporal inappropriate expression of normal sodium channel proteins; or 4) de-repressed expression of otherwise silent sodium channel genes. Na(v)1.7 proved to account for pain in human erythermalgia and inflammation, being the convincing molecular target of pain treatment.

Sodium Channels and Mechanisms of Neuropathic Pain

Na+ channels are large transmembrane proteins with a voltage-gated central pore capable of selectively passing Na+ ions. They are critical determinants of the electrical excitability of sensory neurons and play a key role in pain sensation by controlling afferent impulse discharge. Injury and disease affecting peripheral nerves induces axonopathy and demyelination. These neuropathic changes, in turn, trigger membrane remodeling in injured afferents and perhaps also in uninjured neighbors. A major consequence of the remodeling is increased cellular excitability. This is due in large part to subtype-selective abnormalities in the expression and trafficking of Na+ channels and perhaps also to altered kinetic properties of unitary channels. Hyperexcitable neurons show enhanced membrane resonance, rhythmogenesis, and ectopic spiking. The resulting excess discharge constitutes a primary neuropathic pain signal. In addition, it triggers and maintains central sensitization. This amplifies residual afferent input, yielding tactile allodynia, and it also amplifies ongoing ectopia that exaggerates spontaneous pain. Membrane-stabilizing Na+ channel ligands suppress neuropathic pain by selectively reducing membrane resonance in injured afferents and hence ectopic hyperexcitability. The clinical usefulness of these peripherally acting drugs might be enhanced by reducing their central side effects.

PERSPECTIVE

Neuropathic pain is a complex outcome of multiple pathophysiological changes that develop in the peripheral nervous system (PNS) and the central nervous system (CNS) following nerve injury or disease. All or most of the CNS changes are thought to be due to abnormal signaling from the PNS, notably electrical hyperexcitability of peripheral sensory neurons. Because hyperexcitability is associated with abnormal sodium channel regulation, this process is a prime target for therapeutic intervention.

Anandamide suppression of Na+ currents in rat dorsal root ganglion neurons

Anandamide, the ethanolamide of arachidonic acid, is an endogenous cannabinoid. It is an agonist at CB1 and CB2 cannabinoid receptors as well as the vanilloid receptor, VR1. It is analgesic in inflammatory and neuropathic pain. Both central and peripheral mechanisms are considered to participate in its analgesia. Primary sensory neurons express Na+ currents that are involved in the pathogenesis of pain. We examined the effect of anandamide on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) Na+ currents in rat dorsal root ganglion neurons. Anandamide inhibited both Na+ currents in a concentration-dependent manner. At a membrane potential of -80 mV, the current inhibition was greater in TTX-S than TTX-R currents (K(d); 5.4 microM vs. 38.4 microM). The activation and inactivation became faster in TTX-R current but not in TTX-S current. Anandamide did not alter the activation voltage in either type of current. It, however, produced a hyperpolarizing shift of the steady-state inactivation voltage in both types of currents. The maximum availability at a large negative potential was not reduced by anandamide. Thus, anandamide seems to affect inactivated Na+ channels rather than resting channels. The inhibition of Na+ currents was not reversed by AM 251 (a CB1 antagonist), AM 630 (a CB2 antagonist) or capsazepine (a VR1 antagonist), suggestive of a direct action of anandamide on Na+ channels. The inhibition of Na+ currents in sensory neurons may contribute to the anandamide analgesia.

Sodium channel mutations in epilepsy and other neurological disorders

Since the first mutations of the neuronal sodium channel SCN1A were identified 5 years ago, more than 150 mutations have been described in patients with epilepsy. Many are sporadic mutations and cause loss of function, which demonstrates haploinsufficiency of SCN1A. Mutations resulting in persistent sodium current are also common. Coding variants of SCN2A, SCN8A, and SCN9A have also been identified in patients with seizures, ataxia, and sensitivity to pain, respectively. The rapid pace of discoveries suggests that sodium channel mutations are significant factors in the etiology of neurological disease and may contribute to psychiatric disorders as well.

Inherited disorders of voltage-gated sodium channels

A variety of inherited human disorders affecting skeletal muscle contraction, heart rhythm, and nervous system function have been traced to mutations in genes encoding voltage-gated sodium channels. Clinical severity among these conditions ranges from mild or even latent disease to life-threatening or incapacitating conditions. The sodium channelopathies were among the first recognized ion channel diseases and continue to attract widespread clinical and scientific interest. An expanding knowledge base has substantially advanced our understanding of structure-function and genotype-phenotype relationships for voltage-gated sodium channels and provided new insights into the pathophysiological basis for common diseases such as cardiac arrhythmias and epilepsy.

Challenges in the development of novel treatment strategies for neuropathic pain

Neuropathic pain might best be considered as a collection of various pain states with a common feature, that being symptoms suggestive of dysfunction of peripheral nerves. The development of therapeutic options for the treatment of neuropathic pain is complicated significantly by several factors. Neuropathic pain may arise from widely diverse etiologies such as physical trauma, disease, infection, or chemotherapy. Symptoms indicative of neuropathic pain may also arise in individuals with no evidence of any type of nerve trauma (idiopathic). Although neuropathic pain is a substantial health care issue, it is relatively uncommon and only occurs in a small fraction (<10%) of individuals with these initiating factors. Moreover, the efficacy of treatment protocols, even against the same type of symptoms, differ depending on the underlying initiating cause of the neuropathy. Although these observations strongly suggest that there are predisposing factors that may impart susceptibility to the development of neuropathic pain, no common predisposing factors or genetic markers have been satisfactorily identified. Because of these vagaries, treatment of neuropathic pain has been based on trial and error. However, recent progress in the understanding of neurophysiologic changes that accompany peripheral nerve dysfunction indicate that regulation of ion channels that maintain membrane potentials or generate action potentials may provide an important therapeutic approach. Neuropathic pain is accompanied by increased activity of peripheral nociceptors, which is produced in part by changes in levels of specific calcium and sodium channels. The identification of sodium and/or calcium channels subtypes that are expressed almost exclusively on nociceptors may provide a way of regulating the activity of exaggerated nociceptor function without altering other sensory modalities. Thus, the selective targeting of ion channels may represent a viable therapeutic target for the management of the neuropathic pain state, regardless of etiology.

Neuropathic pain develops normally in mice lacking both Na(v)1.7 and Na(v)1.8

Two voltage gated sodium channel α-subunits, Na_v1.7 and Na_v1.8, are expressed at high levels in nociceptor terminals and have been implicated in the development of inflammatory pain. Mis-expression of voltage-gated sodium channels by damaged sensory neurons has also been implicated in the development of neuropathic pain, but the role of Na_v1.7 and Na_v1.8 is uncertain. Here we show that deleting Na_v1.7 has no effect on the development of neuropathic pain. Double knockouts of both Na_v1.7 and Na_v1.8 also develop normal levels of neuropathic pain, despite a lack of inflammatory pain symptoms and altered mechanical and thermal acute pain thresholds. These studies demonstrate that, in contrast to the highly significant role for Na_v1.7 in determining inflammatory pain thresholds, the development of neuropathic pain does not require the presence of either Na_v1.7 or Na_v1.8 alone or in combination.

SCN9A mutations define primary erythermalgia as a neuropathic disorder of voltage gated sodium channels

Primary erythermalgia is a rare disorder characterized by recurrent attacks of red, warm and painful hands, and/or feet. We previously localized the gene for primary erythermalgia to a 7.94 cM region on chromosome 2q. Recently, Yang et al identified two missense mutations of the sodium channel alpha subunit SCN9A in patients with erythermalgia. The presence of voltage-gated sodium channels in sensory neurons is thought to play a crucial role in several chronic painful neuropathies. We examined four different families and two sporadic cases and detected missense sequence variants in SCN9A to be present in primary erythermalgia patients. A total of five of six mutations were located in highly conserved regions. One family with autosomal dominantly inherited erythermalgia was double heterozygous for two separate SCN9A mutations. These data establish primary erythermalgia as a neuropathic disorder and offers hope for treatment of this incapacitating painful disorder.

Electrophysiological properties of mutant Nav1.7 sodium channels in a painful inherited neuropathy

Although the physiological basis of erythermalgia, an autosomal dominant painful neuropathy characterized by redness of the skin and intermittent burning sensation of extremities, is not known, two mutations of Na(v)1.7, a sodium channel that produces a tetrodotoxin-sensitive, fast-inactivating current that is preferentially expressed in dorsal root ganglia (DRG) and sympathetic ganglia neurons, have recently been identified in patients with primary erythermalgia. Na(v)1.7 is preferentially expressed in small-diameter DRG neurons, most of which are nociceptors, and is characterized by slow recovery from inactivation and by slow closed-state inactivation that results in relatively large responses to small, subthreshold depolarizations. Here we show that these mutations in Na(v)1.7 produce a hyperpolarizing shift in activation and slow deactivation. We also show that these mutations cause an increase in amplitude of the current produced by Na(v)1.7 in response to slow, small depolarizations. These observations provide the first demonstration of altered sodium channel function associated with an inherited painful neuropathy and suggest that these physiological changes, which confer hyperexcitability on peripheral sensory and sympathetic neurons, contribute to symptom production in hereditary erythermalgia.

Genetic heterogeneity and exclusion of a modifying locus at 2q in a family with autosomal dominant primary erythermalgia

BACKGROUND

Primary erythermalgia is a rare disorder characterized by recurrent attacks of red, warm and painful hands and/or feet. In a previous study we reported localization of a gene for primary erythermalgia to a 7.94-cM region on chromosome 2q. A recent study reported voltage-gated sodium channel gene SCN9a sequence variants in a family and a single individual with primary erythermalgia.

OBJECTIVES

To describe the clinical characteristics of a large three-generation family with primary erythermalgia and to test for genetic linkage to chromosome 2q.

METHODS

We collected clinical data of a 10-member three-generation family with autosomal dominant primary erythermalgia. In addition, we performed linkage analysis and searched for SCN9a variants using a restriction fragment length polymorphism assay.

RESULTS

We established the diagnosis of autosomal dominant primary erythermalgia in six of 10 family members. We excluded linkage to chromosome 2q and could not detect SCN9A variants in this family.

CONCLUSIONS

In this family with autosomal dominant primary erythermalgia, exclusion of linkage to chromosome 2q is strongly suggestive for genetic heterogeneity.

Gain-of-function mutation in Nav1.7 in familial erythromelalgia induces bursting of sensory neurons

Erythromelalgia is an autosomal dominant disorder characterized by burning pain in response to warm stimuli or moderate exercise. We describe a novel mutation in a family with erythromelalgia in SCN9A, the gene that encodes the Na(v)1.7 sodium channel. Na(v)1.7 produces threshold currents and is selectively expressed within sensory neurons including nociceptors. We demonstrate that this mutation, which produces a hyperpolarizing shift in activation and a depolarizing shift in steady-state inactivation, lowers thresholds for single action potentials and high frequency firing in dorsal root ganglion neurons. Erythromelalgia is the first inherited pain disorder in which it is possible to link a mutation with an abnormality in ion channel function and with altered firing of pain signalling neurons.

Resveratrol inhibits Na+ currents in rat dorsal root ganglion neurons

Resveratrol, a phytoalexin found in grapevines, exerts neuroprotective, cancer chemopreventive, antiinflammatory and cardioprotective activities. Studies have also shown that resveratrol exhibits analgesic effects. Cyclooxygenase inhibition and K+ channel opening have been suggested as underlying mechanisms for the resveratrol-induced analgesia. Here, we investigated the effects of resveratrol on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) Na+ currents in rat dorsal root ganglion (DRG) neurons. Resveratrol suppressed both Na+ currents evoked at 0 mV from -80 mV. TTX-S Na+ current (K(d), 72 microM) was more susceptible to resveratrol than TTX-R Na+ current (K(d), 211 microM). Although the activation voltage of TTX-S Na+ current was shifted in the depolarizing direction by resveratrol, that of TTX-R Na+ current was not. Resveratrol caused a hyperpolarizing shift of the steady-state inactivation voltage and slowed the recovery from inactivation of both Na+ currents. However, no frequency-dependent inhibition of resveratrol on either type of Na+ current was observed. The suppression and the unfavorable effects on the kinetics of Na+ currents in terms of the excitability of DRG neurons may make a great contribution to the analgesia by resveratrol.

Primary erythromelalgia in a child responding to intravenous lidocaine and oral mexiletine treatment

Erythromelalgia is a rare, chronic, debilitating condition characterized by redness, warmth, and severe burning pain of the distal extremities. The feet are more commonly affected than the hands. Pain is precipitated by increases in temperature and by exercise. Patients often obtain relief by immersing the affected extremity in cold water. The pain is often refractory to treatment. For many patients, multiple pain medications have been useless in achieving complete relief of pain symptoms. Previous reports of erythromelalgia among adolescents indicated prolonged relief of pain with sodium nitroprusside infusions, epidural infusions of local anesthetics, or gabapentin treatment. We present a case of an 11-year-old, white, male child with primary erythromelalgia, whose initial symptoms started in his preschool years and whose childhood was marked by escalating episodes of pain with warmth and redness of his feet, precipitated especially by increases in temperature and by activity. All conventional pain management techniques had failed to relieve our patient of his symptoms, and he obtained some relief only by soaking his affected extremities in ice water. He had experienced minimal benefit from seeing a pain psychologist, who helped him develop techniques to cope with the pain. At the time of presentation, the patient's episodes of pain had increased to 15 to 20 per day, and there was evidence of chronic immersion injury to the skin of his feet. Before his most recent hospitalization, the pain had spread to involve his hands as well. The patient was overwhelmed with anxiety and could not participate in school or social activities at the time of admission. During his current hospitalization, he did show some therapeutic response to sodium nitroprusside infusion, which unfortunately had to be discontinued because of side effects and because his family desired to leave the ICU environment, which was stressful to the patient. He also had some response to lumbar epidural infusion of local anesthetics, which could not be continued because he found the motor blockade that accompanied his analgesia intolerable. However, intravenous lidocaine infusion, with subsequent transition to oral mexiletine therapy, proved very effective in reducing the frequency and severity of the pain episodes. The patient was discharged from the hospital with oral mexiletine therapy and has been monitored at the pain management clinic. He returned to and completed school, attended summer camp, and enjoys an active happy life. He walks without precipitating pain in his feet and sleeps 9 to 10 hours every night. He has needed to soak his feet on only 4 occasions in the 6 months since his discharge from the hospital. His quality of life has improved significantly. He has shown no evidence of liver toxicity, and his mexiletine levels have been stable.

Erythromelalgia: A hereditary pain syndrome enters the molecular era

In contrast with acquired pain syndromes, molecular substrates for hereditary pain disorders have been poorly understood. Familial erythromelalgia (Weir Mitchell's disease), also known as primary erythermalgia, is an autosomal dominant disorder characterized by burning pain in the extremities in response to warm stimuli or moderate exercise. The cause of this disorder has been enigmatic, and treatment has been empirical and not very effective. Recent studies, however, have shown that familial erythromelalgia is a channelopathy caused by mutations in the gene encoding the Na(v)1.7 sodium channel which lead to altered channel function. Selective expression of Na(v)1.7 within dorsal root ganglion neurons including nociceptors (in which this channel is targeted to sensory terminals, close to impulse trigger zones) and within sympathetic ganglion neurons explains why patients experience pain but do not suffer from seizures or other manifestations of altered excitability within central nervous system neurons. Erythromelalgia is the first human disorder in which it has been possible to associate an ion channel mutation with chronic neuropathic pain. Identification of mutations within a peripheral neuron-specific sodium channel suggests the possibility of rational therapies that target the affected channel. Moreover, because some other pain syndromes, including acquired disorders, involve altered sodium channel function, erythromelalgia may emerge as a model disease that holds more general lessons about the molecular neurobiology of chronic pain.

Nociceptor-specific gene deletion reveals a major role for Nav1.7 (PN1) in acute and inflammatory pain

Nine voltage-gated sodium channels are expressed in complex patterns in mammalian nerve and muscle. Three channels, Na(v)1.7, Na(v)1.8, and Na(v)1.9, are expressed selectively in peripheral damage-sensing neurons. Because there are no selective blockers of these channels, we used gene ablation in mice to examine the function of Na(v)1.7 (PN1) in pain pathways. A global Na(v)1.7-null mutant was found to die shortly after birth. We therefore used the Cre-loxP system to generate nociceptor-specific knockouts. Na(v)1.8 is only expressed in peripheral, mainly nociceptive, sensory neurons. We knocked Cre recombinase into the Na(v)1.8 locus to generate heterozygous mice expressing Cre recombinase in Na(v)1.8-positive sensory neurons. Crossing these animals with mice where Na(v)1.7 exons 14 and 15 were flanked by loxP sites produced nociceptor-specific knockout mice that were viable and apparently normal. These animals showed increased mechanical and thermal pain thresholds. Remarkably, all inflammatory pain responses evoked by a range of stimuli, such as formalin, carrageenan, complete Freund's adjuvant, or nerve growth factor, were reduced or abolished. A congenital pain syndrome in humans recently has been mapped to the Na(v)1.7 gene, SCN9A. Dominant Na(v)1.7 mutations lead to edema, redness, warmth, and bilateral pain in human erythermalgia patients, confirming an important role for Na(v)1.7 in inflammatory pain. Nociceptor-specific gene ablation should prove useful in understanding the role of other broadly expressed genes in pain pathways.

Voltage-gated sodium channels and pain pathways

Acute, inflammatory, and neuropathic pain can all be attenuated or abolished by local treatment with sodium channel blockers such as lidocaine. The peripheral input that drives pain perception thus depends on the presence of functional voltage-gated sodium channels. Remarkably, two voltage-gated sodium channel genes (Nav1.8 and Nav1.9) are expressed selectively in damage-sensing peripheral neurons, while a third channel (Nav1.7) is found predominantly in sensory and sympathetic neurons. An embryonic channel (Nav1.3) is also upregulated in damaged peripheral nerves and associated with increased electrical excitability in neuropathic pain states. A combination of antisense and knock-out studies support a specialized role for these sodium channels in pain pathways, and pharmacological studies with conotoxins suggest that isotype-specific antagonists should be feasible. Taken together, these data suggest that isotype-specific sodium channel blockers could be useful analgesics.