- Email: [email protected]
Painful Na-channelopathies: an expanding universe
Review
Painful Na-channelopathies: an expanding universe Stephen G. Waxman1,2 1
Department of Neurology and Center for Neuroscience and Regeneration Research, Yale University School of Medicine, New Haven, CT 06510, USA 2 Rehabilitation Research Center, Veterans Affairs Connecticut Healthcare System, West Haven, CT 06516, USA
The universe of painful Na-channelopathies – human disorders caused by mutations in voltage-gated sodium channels – has recently expanded in three dimensions. We now know that mutations of sodium channels cause not only rare genetic ‘model disorders’ such as inherited erythromelalgia and channelopathy-associated insensitivity to pain but also common painful neuropathies. We have learned that mutations of NaV1.8, as well as mutations of NaV1.7, can cause painful Na-channelopathies. Moreover, recent studies combining atomic level structural models and pharmacogenomics suggest that the goal of genomically guided pain therapy may not be unrealistic. ‘Peripheral’ isoforms of the voltage-sensitive sodium channel – NaV1.7, NaV1.8, and NaV1.9 – are expressed selectively or preferentially within peripheral neurons and not within the brain or heart. These Na-channel isoforms have emerged as central foci of pain research because isoform-specific block or knockdown would be expected to ameliorate pain without central or cardiac side effects. Interest in several isoforms has been fueled by the discovery of painful Na-channelopathies that have directly demonstrated roles for these channels in human pain, thus providing a degree of validation that is not possible in animal models. Beginning with the demonstration, less than 10 years ago, that inherited erythromelalgia is caused by missense mutations in Nav1.7, our understanding of painful channelopathies has rapidly progressed. Notably, the universe of painful genetic Na-channelopathies has recently expanded in three ways. First, it has expanded from rare genetic ‘model diseases’ to more common disorders that affect millions. Second, it now includes a larger number of sodium channel isoforms. And third, pharmacogenomic studies on painful channelopathies have begun to yield promising results with potential clinical applications. In this brief review, we describe recent progress in the study of human painful genetic Na-channelopathies, and suggest some questions and promising directions for future research. Because the human painful genetic channelopathies characterized to date involve NaV1.7 and NaV1.8, this Corresponding author: Waxman, S.G. ([email protected]). Keywords: channelopathy; pain; sodium channels. 1471-4914/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.molmed.2013. 04.003
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review focuses on these two isoforms. A synopsis of other Na channel isoforms that appear to be involved in pain can be found in other recent reviews [1,2]. Inherited erythromelalgia and paroxysmal extreme pain disorder Inherited erythromelalgia (IEM), the first painful Nachannelopathy to be identified, is a rare genetic disorder in which affected individuals experience searing, burning pain (usually in the distal extremities) in response to mild warmth. Following linkage studies that implicated a locus (2q 31–32) in chromosome 2 [3], data from two families with IEM identified point mutations in the SCN9A gene that encodes the NaV1.7 sodium channel [4]. Functional profiling of these IEM mutations demonstrated gain-offunction changes that included enhanced channel activation [5]. Subsequent studies on an additional NaV1.7 mutation, in a large a family with IEM, demonstrated that the amino acid substitution segregated with the disease, and showed that the mutant channels produce hyperexcitability in dorsal root ganglion (DRG) neurons, lowering their threshold and increasing their firing frequency in response to stimulation [6]. More than a dozen IEM mutations have now been characterized; all enhance activation and increase DRG neuron excitability [7,8]. A second, distinct group of NaV1.7 mutations has subsequently been reported in another rare genetic disease, paroxysmal extreme pain disorder (PEPD; previously termed familial rectal pain disorder). Beginning in infancy, individuals affected with PEPD experience severe perirectal pain in response to lower body stimulation, which later evolves into periocular and perimandibular pain [9]. PEPD mutations impair channel fast-inactivation and thus differ electrophysiologically from IEM mutations. Although IEM and PEPD typically manifest with different clinical symptoms and are usually regarded by clinicians as distinct disorders, they may represent the endpoints of a continuum. An overlap syndrome with a mixed clinical phenotype including both IEM-like and PEPD-like features has been linked to a NaV1.7 mutation that confers both IEM-like (enhanced activation) and PEPD-like (impaired inactivation) changes to the NaV1.7 channel [10]. Channelopathy-associated insensitivity to pain Shortly after gain-of-function mutations in NaV1.7 were described in the context of IEM, a rare loss-of-function disorder due to the absence of functional NaV1.7 channels
Review was described in human subjects [11–13]. Channelopathyassociated insensitivity to pain (CIP), caused by null mutations in SCN9A, is clinically characterized by an inability to sense noxious stimuli or events as painful; affected individuals experience painless injuries, such as fractures or burns, and undergo dental extractions and childbirth without experiencing pain. Although NaV1.7 is expressed in sympathetic ganglion neurons as well as DRG neurons [14,15], individuals with this disorder do not appear to manifest autonomic insufficiency. They do, however, display anosmia [16], consistent with the presence of NaV1.7 in olfactory sensory neurons [17]. NaV1.7 mutations and painful peripheral neuropathy IEM, PEPD, and CIP are an ensemble of rare genetic ‘model diseases’ that establish a strong link between NaV1.7 and human pain at both the gain-of-function and loss-of-function levels. More recent studies have established a link between NaV1.7, NaV1.8, and pain in a common painful disorder, painful peripheral neuropathy. Small fiber neuropathy is a form of painful neuropathy that is characterized by autonomic dysfunction and severe pain, usually in a distal ‘stocking-and-glove’ pattern [18]. Gainof-function NaV1.7 mutations have recently been identified in 28% of patients with biopsy confirmed idiopathic small fiber neuropathy [19]. These mutations produce relatively subtle enhancements of channel function such as impaired slow-inactivation or modestly impaired fast- and slowinactivation. These mutations share a common effect on DRG neuron function, reducing current thresholds and producing higher-than-normal firing frequencies in response to stimulation, two changes that appear to explain stimulus-evoked pain in these patients. The mutant channels also cause DRG neurons to fire spontaneously in the absence of stimulation, a finding that may explain spontaneous pain in these patients [19]. One mutation in NaV1.7 causes acromesomelia, reduced size of the distal limbs, in addition to painful neuropathy, suggesting that it may have produced an intrauterine neuropathy, impairing interactions between nerves and target tissue during development [20]. NaV1.8 mutations and painful peripheral neuropathy Recently, analysis of the SCN10A gene, which encodes the NaV1.8 sodium channel, revealed seven NaV1.8 mutations in nine subjects from a series of 104 patients with painful, predominantly small fiber neuropathy who did not carry mutations in SCN9A [21]. Three mutations met the criteria for potential pathogenicity based on predictive algorithms; two of these three mutations enhanced the response of the channels to depolarization and produced hyperexcitability of DRG neurons. This observation extends the list of channels likely to be involved in the channelopathy-associated pain syndromes to include NaV1.8 as well as NaV1.7. Genotype–phenotype correlations Even within a relatively discrete diagnostic category, such as IEM or small fiber neuropathy, there can be differences in clinical presentations for patients harboring different mutations. Most patients with IEM, for example, manifest
Trends in Molecular Medicine July 2013, Vol. 19, No. 7
pain beginning very early in life, during infancy or early childhood, but occasional patients display late onset of pain. The age of pain onset appears, in at least some cases, to be related to the degree of activation enhancement [22,23], but there is also evidence indicating that, in addition to age of onset, the nature of the symptoms depends on the electrophysiological changes conferred on cells by the mutant channel. For example, Han et al. [24] showed that the degree of autonomic involvement in small fiber neuropathy associated with NaV1.7 mutations depends on whether the mutant channel depolarizes the resting potential; depolarization would be expected to have profound depressive effects on sympathetic ganglion neurons, due to their lack of NaV1.8 sodium channels, which are present within DRG neurons and do not inactivate with depolarization within the physiological range in these cells [14]. There is also evidence suggesting that modifier genes, epigenetic factors, or other factors may contribute to phenotypic variation between, or even within, families [25]. Acquired changes in channel expression and pain Evidence also exists for a broader link between these sodium channels and human pain. Estacion et al. [26] characterized a single nucleotide polymorphism in SCN9A, present in approximately 30% of the control population, that modestly increases DRG neuron excitability and suggested that this polymorphism might bias sensitivity to pain. Subsequently reported genetic association studies found a correlation between expression of the minor (hyperexcitability associated) allele and increased pain sensitivity, together with increased pain scores, in patients with osteoarthritis, compressive radiculopathy, and traumatic limb amputation [27]. In another link to human disease, immunocytochemical studies have demonstrated abnormal accumulations of NaV1.7 and NaV1.8 within neuromas from patients with intractable pain following peripheral nerve injury or traumatic limb amputation, suggesting that these channels contribute to ectopic impulse generation underlying pain after nerve injury in humans [28]. Pharmacogenomics Finally, advances in structural modeling have facilitated progress in pharmacogenomics. Capitalizing on the recently solved crystal structure of bacterial sodium channels [29], Yang et al. [30] constructed an atomic level structural model of the human NaV1.7 channel and used this to predict channel pharmacoresponsiveness in the context of various mutations. Beginning with a previously described IEM mutation [31] that, in addition to producing disease, endows the mutant channel with enhanced responsiveness to the clinically used sodium channel blocker carbamazepine (CBZ), the model was interrogated to identify other variants that might increase responsiveness to this drug. Increased pharmacoresponsiveness was predicted by this model for a second variant, and was confirmed when CBZ was shown to decrease the excitability of DRG neurons expressing the mutant channel. Although additional work remains to be done, this initial result reinforces the expectation that pharmacogenomically guided, individualized pain therapy may not be unrealistic. In 407
Review
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Table 1. Human painful Na-channelopathies Disorder Inherited erythromelalgia
Channel NaV1.7
PEPD
NaV1.7
Overlap syndrome
NaV1.7
Channelopathy associated insensitivity to pain Painful peripheral neuropathies
NaV1.7 NaV1.7
Painful peripheral neuropathies
NaV1.8
Mutation Gain-of-function, primarily enhanced activation Gain-of-function, primarily impaired fast-inactivation Enhanced activation + impaired fast-inactivation Loss-of-function Gain-of-function, multiple effects on channel Gain-of-function, multiple effects on channel
fact, methods similar to this may make it possible to identify polymorphisms in the general population that increase, or decrease, responsiveness of human subjects to various sodium channel blockers, so that pain pharmacotherapy can be transformed from a process that is largely ‘trial-and-error’, to a more effective, pharmacogenomics guided ‘first-time-around’ approach. Open questions and future directions The universe of painful Na-channelopathies has expanded from rare genetic model disorders to more commonly observed diseases, and the underlying causes now include both NaV1.7 and NaV1.8 (Table 1). Additional painful channelopathies may exist, but how should their causes be identified? Large kindred analysis may provide compelling information, and genome-wide association studies may also yield important insights, although these studies require large numbers of patients and may not, in themselves, establish a mechanistic link to disease. Wholeexome or whole-genome sequencing may also identify candidate variants, although, again, the identification of disease-causing mutations will require mechanistic studies. Functional profiling – via voltage clamp to assess channel function, and current clamp to assess neuronal function – can, as illustrated by the papers summarized in this review, provide a mechanistic link to pathogenicity. Several important open questions remain to be answered. For example, what is the basis for the temporal or spatial pattern in which pain manifests in the channelopathies? Most patients with IEM and PEPD experience pain beginning in early childhood. We do not fully understand why occasional patients with IEM develop pain later in life, or why patients with painful neuropathy associated with Na-channelopathies develop pain in adulthood. A growing number of partner molecules and modulatory factors are being shown to interact with Na channels [32] and one or more of these may be involved. Modifier genes, epigenetic factors, other environmental factors, or a ‘multi-hit’ mechanism must be considered, especially in view of the length-dependent manifestation of pain, which often begins in the feet and the hands in patients with peripheral neuropathies. Why patients with IEM experience pain in distal extremities, whereas patients with PEPD experience pain more proximally, is also not understood. Cell background, that is, the ensemble of molecules that modulate or interact biochemically or physiologically with Na channels in a 408
Pattern Distal limbs
Refs [7,8]
Perirectal, periorbital, perimandibular Mixed
[9] [10]
Early pain usually distal
[11–13] [19]
Early pain usually distal
[21]
given cell type, may be different in DRG neurons innervating different parts of the body. Whether this can be studied in animal models, where nerve lengths are much shorter than in humans, is not known. These and other questions about painful Nachannelopathies will hopefully be answered in the near future. The universe of painful channelopathies may continue to expand. These experiments of nature will continue to teach us important lessons about sodium channels, about the molecular basis for pain, and, hopefully, about new therapeutic approaches. References 1 Dib-Hajj, S.D. et al. (2010) Sodium channels in normal and pathological pain. Annu. Rev. Neurosci. 33, 325–347 2 Eijkelkamp, N. et al. (2012) Neurological perspectives on voltage-gated sodium channels. Brain 135, 2585–2612 3 Drenth, J.P. et al. (2001) The primary erythermalgia-susceptibility gene is located on chromosome 2q31-32. Am. J. Hum. Genet. 68, 1277–1282 4 Yang, Y. et al. (2004) Mutations in SCN9A, encoding a sodium channel alpha subunit, in patients with primary erythermalgia. J. Med. Genet. 41, 171–174 5 Cummins, T.R. et al. (2004) Electrophysiological properties of mutant Nav1.7 sodium channels in a painful inherited neuropathy. J. Neurosci. 24, 8232–8236 6 Dib-Hajj, S.D. et al. (2005) Gain-of-function mutation in Nav1.7 in familial erythromelalgia induces bursting of sensory neurons. Brain 128, 1847–1854 7 Dib-Hajj, S.D. et al. (2012) The Na(V)1.7 sodium channel: from molecule to man. Nat. Rev. Neurosci. 14, 49–62 8 Drenth, J.P. and Waxman, S.G. (2007) Mutations in sodium-channel gene SCN9A cause a spectrum of human genetic pain disorders. J. Clin. Invest. 117, 3603–3609 9 Fertleman, C.R. et al. (2006) SCN9A mutations in paroxysmal extreme pain disorder: allelic variants underlie distinct channel defects and phenotypes. Neuron 52, 767–774 10 Estacion, M. et al. (2008) NaV1.7 gain-of-function mutations as a continuum: A1632E displays physiological changes associated with erythromelalgia and paroxysmal extreme pain disorder mutations and produces symptoms of both disorders. J. Neurosci. 28, 11079–11088 11 Ahmad, S. et al. (2007) A stop codon mutation in SCN9A causes lack of pain sensation. Hum. Mol. Genet. 16, 2114–2121 12 Cox, J.J. et al. (2006) An SCN9A channelopathy causes congenital inability to experience pain. Nature 444, 894–898 13 Goldberg, Y.P. et al. (2007) Loss-of-function mutations in the Nav1.7 gene underlie congenital indifference to pain in multiple human populations. Clin. Genet. 71, 311–319 14 Rush, A.M. et al. (2006) A single sodium channel mutation produces hyper- or hypoexcitability in different types of neurons. Proc. Natl. Acad. Sci. U.S.A. 103, 8245–8250 15 Toledo-Aral, J.J. et al. (1997) Identification of PN1, a predominant voltage-dependent sodium channel expressed principally in peripheral neurons. Proc. Natl. Acad. Sci. U.S.A. 94, 1527–1532
Review 16 Weiss, J. et al. (2011) Loss-of-function mutations in sodium channel Nav1.7 cause anosmia. Nature 472, 186–190 17 Ahn, H.S. et al. (2011) Nav1.7 is the predominant sodium channel in rodent olfactory sensory neurons. Mol. Pain 7, 32 18 Hoeijmakers, J.G. et al. (2012) Small-fibre neuropathies – advances in diagnosis, pathophysiology and management. Nat. Rev. Neurol. 8, 369–379 19 Faber, C.G. et al. (2012) Gain of function NaV1.7 mutations in idiopathic small fiber neuropathy. Ann. Neurol. 71, 26–39 20 Hoeijmakers, J.G. et al. (2012) Small nerve fibres, small hands and small feet: a new syndrome of pain, dysautonomia and acromesomelia in a kindred with a novel NaV1.7 mutation. Brain 135, 345–358 21 Faber, C.G. et al. (2012) Gain-of-function Nav1.8 mutations in painful neuropathy. Proc. Natl. Acad. Sci. U.S.A. 109, 19444–19449 22 Cheng, X. et al. (2008) Mutation I136V alters electrophysiological properties of the Na(v)1.7 channel in a family with onset of erythromelalgia in the second decade. Mol. Pain 4, 1 23 Han, C. et al. (2009) Early- and late-onset inherited erythromelalgia: genotype–phenotype correlation. Brain 132, 1711–1722 24 Han, C. et al. (2012) Functional profiles of SCN9A variants in dorsal root ganglion neurons and superior cervical ganglion neurons correlate
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25
26 27 28
29 30
31 32
with autonomic symptoms in small fibre neuropathy. Brain 135, 2613– 2628 Estacion, M. et al. (2011) Intra- and interfamily phenotypic diversity in pain syndromes associated with a gain-of-function variant of NaV1.7. Mol. Pain 7, 92 Estacion, M. et al. (2009) A sodium channel gene SCN9A polymorphism that increases nociceptor excitability. Ann. Neurol. 66, 862–866 Reimann, F. et al. (2010) Pain perception is altered by a nucleotide polymorphism in SCN9A. Proc. Natl. Acad. Sci. U.S.A. 107, 5148–5153 Black, J.A. et al. (2008) Multiple sodium channel isoforms and mitogenactivated protein kinases are present in painful human neuromas. Ann. Neurol. 64, 644–653 Payandeh, J. et al. (2011) The crystal structure of a voltage-gated sodium channel. Nature 475, 353–358 Yang, Y. et al. (2012) Structural modelling and mutant cycle analysis predict pharmacoresponsiveness of a Na(v)1.7 mutant channel. Nat. Commun. 3, 1186 Fischer, T.Z. et al. (2009) A novel Nav1.7 mutation producing carbamazepine-responsive erythromelalgia. Ann. Neurol. 65, 733–741 Dib-Hajj, S.D. and Waxman, S.G. (2010) Isoform-specific and panchannel partners regulate trafficking and plasma membrane stability; and alter sodium channel gating properties. Neurosci. Lett. 486, 84–91
409
Painful Na-channelopathies: an expanding universe Stephen G. Waxman1,2 1
Department of Neurology and Center for Neuroscience and Regeneration Research, Yale University School of Medicine, New Haven, CT 06510, USA 2 Rehabilitation Research Center, Veterans Affairs Connecticut Healthcare System, West Haven, CT 06516, USA
The universe of painful Na-channelopathies – human disorders caused by mutations in voltage-gated sodium channels – has recently expanded in three dimensions. We now know that mutations of sodium channels cause not only rare genetic ‘model disorders’ such as inherited erythromelalgia and channelopathy-associated insensitivity to pain but also common painful neuropathies. We have learned that mutations of NaV1.8, as well as mutations of NaV1.7, can cause painful Na-channelopathies. Moreover, recent studies combining atomic level structural models and pharmacogenomics suggest that the goal of genomically guided pain therapy may not be unrealistic. ‘Peripheral’ isoforms of the voltage-sensitive sodium channel – NaV1.7, NaV1.8, and NaV1.9 – are expressed selectively or preferentially within peripheral neurons and not within the brain or heart. These Na-channel isoforms have emerged as central foci of pain research because isoform-specific block or knockdown would be expected to ameliorate pain without central or cardiac side effects. Interest in several isoforms has been fueled by the discovery of painful Na-channelopathies that have directly demonstrated roles for these channels in human pain, thus providing a degree of validation that is not possible in animal models. Beginning with the demonstration, less than 10 years ago, that inherited erythromelalgia is caused by missense mutations in Nav1.7, our understanding of painful channelopathies has rapidly progressed. Notably, the universe of painful genetic Na-channelopathies has recently expanded in three ways. First, it has expanded from rare genetic ‘model diseases’ to more common disorders that affect millions. Second, it now includes a larger number of sodium channel isoforms. And third, pharmacogenomic studies on painful channelopathies have begun to yield promising results with potential clinical applications. In this brief review, we describe recent progress in the study of human painful genetic Na-channelopathies, and suggest some questions and promising directions for future research. Because the human painful genetic channelopathies characterized to date involve NaV1.7 and NaV1.8, this Corresponding author: Waxman, S.G. ([email protected]). Keywords: channelopathy; pain; sodium channels. 1471-4914/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.molmed.2013. 04.003
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review focuses on these two isoforms. A synopsis of other Na channel isoforms that appear to be involved in pain can be found in other recent reviews [1,2]. Inherited erythromelalgia and paroxysmal extreme pain disorder Inherited erythromelalgia (IEM), the first painful Nachannelopathy to be identified, is a rare genetic disorder in which affected individuals experience searing, burning pain (usually in the distal extremities) in response to mild warmth. Following linkage studies that implicated a locus (2q 31–32) in chromosome 2 [3], data from two families with IEM identified point mutations in the SCN9A gene that encodes the NaV1.7 sodium channel [4]. Functional profiling of these IEM mutations demonstrated gain-offunction changes that included enhanced channel activation [5]. Subsequent studies on an additional NaV1.7 mutation, in a large a family with IEM, demonstrated that the amino acid substitution segregated with the disease, and showed that the mutant channels produce hyperexcitability in dorsal root ganglion (DRG) neurons, lowering their threshold and increasing their firing frequency in response to stimulation [6]. More than a dozen IEM mutations have now been characterized; all enhance activation and increase DRG neuron excitability [7,8]. A second, distinct group of NaV1.7 mutations has subsequently been reported in another rare genetic disease, paroxysmal extreme pain disorder (PEPD; previously termed familial rectal pain disorder). Beginning in infancy, individuals affected with PEPD experience severe perirectal pain in response to lower body stimulation, which later evolves into periocular and perimandibular pain [9]. PEPD mutations impair channel fast-inactivation and thus differ electrophysiologically from IEM mutations. Although IEM and PEPD typically manifest with different clinical symptoms and are usually regarded by clinicians as distinct disorders, they may represent the endpoints of a continuum. An overlap syndrome with a mixed clinical phenotype including both IEM-like and PEPD-like features has been linked to a NaV1.7 mutation that confers both IEM-like (enhanced activation) and PEPD-like (impaired inactivation) changes to the NaV1.7 channel [10]. Channelopathy-associated insensitivity to pain Shortly after gain-of-function mutations in NaV1.7 were described in the context of IEM, a rare loss-of-function disorder due to the absence of functional NaV1.7 channels
Review was described in human subjects [11–13]. Channelopathyassociated insensitivity to pain (CIP), caused by null mutations in SCN9A, is clinically characterized by an inability to sense noxious stimuli or events as painful; affected individuals experience painless injuries, such as fractures or burns, and undergo dental extractions and childbirth without experiencing pain. Although NaV1.7 is expressed in sympathetic ganglion neurons as well as DRG neurons [14,15], individuals with this disorder do not appear to manifest autonomic insufficiency. They do, however, display anosmia [16], consistent with the presence of NaV1.7 in olfactory sensory neurons [17]. NaV1.7 mutations and painful peripheral neuropathy IEM, PEPD, and CIP are an ensemble of rare genetic ‘model diseases’ that establish a strong link between NaV1.7 and human pain at both the gain-of-function and loss-of-function levels. More recent studies have established a link between NaV1.7, NaV1.8, and pain in a common painful disorder, painful peripheral neuropathy. Small fiber neuropathy is a form of painful neuropathy that is characterized by autonomic dysfunction and severe pain, usually in a distal ‘stocking-and-glove’ pattern [18]. Gainof-function NaV1.7 mutations have recently been identified in 28% of patients with biopsy confirmed idiopathic small fiber neuropathy [19]. These mutations produce relatively subtle enhancements of channel function such as impaired slow-inactivation or modestly impaired fast- and slowinactivation. These mutations share a common effect on DRG neuron function, reducing current thresholds and producing higher-than-normal firing frequencies in response to stimulation, two changes that appear to explain stimulus-evoked pain in these patients. The mutant channels also cause DRG neurons to fire spontaneously in the absence of stimulation, a finding that may explain spontaneous pain in these patients [19]. One mutation in NaV1.7 causes acromesomelia, reduced size of the distal limbs, in addition to painful neuropathy, suggesting that it may have produced an intrauterine neuropathy, impairing interactions between nerves and target tissue during development [20]. NaV1.8 mutations and painful peripheral neuropathy Recently, analysis of the SCN10A gene, which encodes the NaV1.8 sodium channel, revealed seven NaV1.8 mutations in nine subjects from a series of 104 patients with painful, predominantly small fiber neuropathy who did not carry mutations in SCN9A [21]. Three mutations met the criteria for potential pathogenicity based on predictive algorithms; two of these three mutations enhanced the response of the channels to depolarization and produced hyperexcitability of DRG neurons. This observation extends the list of channels likely to be involved in the channelopathy-associated pain syndromes to include NaV1.8 as well as NaV1.7. Genotype–phenotype correlations Even within a relatively discrete diagnostic category, such as IEM or small fiber neuropathy, there can be differences in clinical presentations for patients harboring different mutations. Most patients with IEM, for example, manifest
Trends in Molecular Medicine July 2013, Vol. 19, No. 7
pain beginning very early in life, during infancy or early childhood, but occasional patients display late onset of pain. The age of pain onset appears, in at least some cases, to be related to the degree of activation enhancement [22,23], but there is also evidence indicating that, in addition to age of onset, the nature of the symptoms depends on the electrophysiological changes conferred on cells by the mutant channel. For example, Han et al. [24] showed that the degree of autonomic involvement in small fiber neuropathy associated with NaV1.7 mutations depends on whether the mutant channel depolarizes the resting potential; depolarization would be expected to have profound depressive effects on sympathetic ganglion neurons, due to their lack of NaV1.8 sodium channels, which are present within DRG neurons and do not inactivate with depolarization within the physiological range in these cells [14]. There is also evidence suggesting that modifier genes, epigenetic factors, or other factors may contribute to phenotypic variation between, or even within, families [25]. Acquired changes in channel expression and pain Evidence also exists for a broader link between these sodium channels and human pain. Estacion et al. [26] characterized a single nucleotide polymorphism in SCN9A, present in approximately 30% of the control population, that modestly increases DRG neuron excitability and suggested that this polymorphism might bias sensitivity to pain. Subsequently reported genetic association studies found a correlation between expression of the minor (hyperexcitability associated) allele and increased pain sensitivity, together with increased pain scores, in patients with osteoarthritis, compressive radiculopathy, and traumatic limb amputation [27]. In another link to human disease, immunocytochemical studies have demonstrated abnormal accumulations of NaV1.7 and NaV1.8 within neuromas from patients with intractable pain following peripheral nerve injury or traumatic limb amputation, suggesting that these channels contribute to ectopic impulse generation underlying pain after nerve injury in humans [28]. Pharmacogenomics Finally, advances in structural modeling have facilitated progress in pharmacogenomics. Capitalizing on the recently solved crystal structure of bacterial sodium channels [29], Yang et al. [30] constructed an atomic level structural model of the human NaV1.7 channel and used this to predict channel pharmacoresponsiveness in the context of various mutations. Beginning with a previously described IEM mutation [31] that, in addition to producing disease, endows the mutant channel with enhanced responsiveness to the clinically used sodium channel blocker carbamazepine (CBZ), the model was interrogated to identify other variants that might increase responsiveness to this drug. Increased pharmacoresponsiveness was predicted by this model for a second variant, and was confirmed when CBZ was shown to decrease the excitability of DRG neurons expressing the mutant channel. Although additional work remains to be done, this initial result reinforces the expectation that pharmacogenomically guided, individualized pain therapy may not be unrealistic. In 407
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Table 1. Human painful Na-channelopathies Disorder Inherited erythromelalgia
Channel NaV1.7
PEPD
NaV1.7
Overlap syndrome
NaV1.7
Channelopathy associated insensitivity to pain Painful peripheral neuropathies
NaV1.7 NaV1.7
Painful peripheral neuropathies
NaV1.8
Mutation Gain-of-function, primarily enhanced activation Gain-of-function, primarily impaired fast-inactivation Enhanced activation + impaired fast-inactivation Loss-of-function Gain-of-function, multiple effects on channel Gain-of-function, multiple effects on channel
fact, methods similar to this may make it possible to identify polymorphisms in the general population that increase, or decrease, responsiveness of human subjects to various sodium channel blockers, so that pain pharmacotherapy can be transformed from a process that is largely ‘trial-and-error’, to a more effective, pharmacogenomics guided ‘first-time-around’ approach. Open questions and future directions The universe of painful Na-channelopathies has expanded from rare genetic model disorders to more commonly observed diseases, and the underlying causes now include both NaV1.7 and NaV1.8 (Table 1). Additional painful channelopathies may exist, but how should their causes be identified? Large kindred analysis may provide compelling information, and genome-wide association studies may also yield important insights, although these studies require large numbers of patients and may not, in themselves, establish a mechanistic link to disease. Wholeexome or whole-genome sequencing may also identify candidate variants, although, again, the identification of disease-causing mutations will require mechanistic studies. Functional profiling – via voltage clamp to assess channel function, and current clamp to assess neuronal function – can, as illustrated by the papers summarized in this review, provide a mechanistic link to pathogenicity. Several important open questions remain to be answered. For example, what is the basis for the temporal or spatial pattern in which pain manifests in the channelopathies? Most patients with IEM and PEPD experience pain beginning in early childhood. We do not fully understand why occasional patients with IEM develop pain later in life, or why patients with painful neuropathy associated with Na-channelopathies develop pain in adulthood. A growing number of partner molecules and modulatory factors are being shown to interact with Na channels [32] and one or more of these may be involved. Modifier genes, epigenetic factors, other environmental factors, or a ‘multi-hit’ mechanism must be considered, especially in view of the length-dependent manifestation of pain, which often begins in the feet and the hands in patients with peripheral neuropathies. Why patients with IEM experience pain in distal extremities, whereas patients with PEPD experience pain more proximally, is also not understood. Cell background, that is, the ensemble of molecules that modulate or interact biochemically or physiologically with Na channels in a 408
Pattern Distal limbs
Refs [7,8]
Perirectal, periorbital, perimandibular Mixed
[9] [10]
Early pain usually distal
[11–13] [19]
Early pain usually distal
[21]
given cell type, may be different in DRG neurons innervating different parts of the body. Whether this can be studied in animal models, where nerve lengths are much shorter than in humans, is not known. These and other questions about painful Nachannelopathies will hopefully be answered in the near future. The universe of painful channelopathies may continue to expand. These experiments of nature will continue to teach us important lessons about sodium channels, about the molecular basis for pain, and, hopefully, about new therapeutic approaches. References 1 Dib-Hajj, S.D. et al. (2010) Sodium channels in normal and pathological pain. Annu. Rev. Neurosci. 33, 325–347 2 Eijkelkamp, N. et al. (2012) Neurological perspectives on voltage-gated sodium channels. Brain 135, 2585–2612 3 Drenth, J.P. et al. (2001) The primary erythermalgia-susceptibility gene is located on chromosome 2q31-32. Am. J. Hum. Genet. 68, 1277–1282 4 Yang, Y. et al. (2004) Mutations in SCN9A, encoding a sodium channel alpha subunit, in patients with primary erythermalgia. J. Med. Genet. 41, 171–174 5 Cummins, T.R. et al. (2004) Electrophysiological properties of mutant Nav1.7 sodium channels in a painful inherited neuropathy. J. Neurosci. 24, 8232–8236 6 Dib-Hajj, S.D. et al. (2005) Gain-of-function mutation in Nav1.7 in familial erythromelalgia induces bursting of sensory neurons. Brain 128, 1847–1854 7 Dib-Hajj, S.D. et al. (2012) The Na(V)1.7 sodium channel: from molecule to man. Nat. Rev. Neurosci. 14, 49–62 8 Drenth, J.P. and Waxman, S.G. (2007) Mutations in sodium-channel gene SCN9A cause a spectrum of human genetic pain disorders. J. Clin. Invest. 117, 3603–3609 9 Fertleman, C.R. et al. (2006) SCN9A mutations in paroxysmal extreme pain disorder: allelic variants underlie distinct channel defects and phenotypes. Neuron 52, 767–774 10 Estacion, M. et al. (2008) NaV1.7 gain-of-function mutations as a continuum: A1632E displays physiological changes associated with erythromelalgia and paroxysmal extreme pain disorder mutations and produces symptoms of both disorders. J. Neurosci. 28, 11079–11088 11 Ahmad, S. et al. (2007) A stop codon mutation in SCN9A causes lack of pain sensation. Hum. Mol. Genet. 16, 2114–2121 12 Cox, J.J. et al. (2006) An SCN9A channelopathy causes congenital inability to experience pain. Nature 444, 894–898 13 Goldberg, Y.P. et al. (2007) Loss-of-function mutations in the Nav1.7 gene underlie congenital indifference to pain in multiple human populations. Clin. Genet. 71, 311–319 14 Rush, A.M. et al. (2006) A single sodium channel mutation produces hyper- or hypoexcitability in different types of neurons. Proc. Natl. Acad. Sci. U.S.A. 103, 8245–8250 15 Toledo-Aral, J.J. et al. (1997) Identification of PN1, a predominant voltage-dependent sodium channel expressed principally in peripheral neurons. Proc. Natl. Acad. Sci. U.S.A. 94, 1527–1532
Review 16 Weiss, J. et al. (2011) Loss-of-function mutations in sodium channel Nav1.7 cause anosmia. Nature 472, 186–190 17 Ahn, H.S. et al. (2011) Nav1.7 is the predominant sodium channel in rodent olfactory sensory neurons. Mol. Pain 7, 32 18 Hoeijmakers, J.G. et al. (2012) Small-fibre neuropathies – advances in diagnosis, pathophysiology and management. Nat. Rev. Neurol. 8, 369–379 19 Faber, C.G. et al. (2012) Gain of function NaV1.7 mutations in idiopathic small fiber neuropathy. Ann. Neurol. 71, 26–39 20 Hoeijmakers, J.G. et al. (2012) Small nerve fibres, small hands and small feet: a new syndrome of pain, dysautonomia and acromesomelia in a kindred with a novel NaV1.7 mutation. Brain 135, 345–358 21 Faber, C.G. et al. (2012) Gain-of-function Nav1.8 mutations in painful neuropathy. Proc. Natl. Acad. Sci. U.S.A. 109, 19444–19449 22 Cheng, X. et al. (2008) Mutation I136V alters electrophysiological properties of the Na(v)1.7 channel in a family with onset of erythromelalgia in the second decade. Mol. Pain 4, 1 23 Han, C. et al. (2009) Early- and late-onset inherited erythromelalgia: genotype–phenotype correlation. Brain 132, 1711–1722 24 Han, C. et al. (2012) Functional profiles of SCN9A variants in dorsal root ganglion neurons and superior cervical ganglion neurons correlate
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with autonomic symptoms in small fibre neuropathy. Brain 135, 2613– 2628 Estacion, M. et al. (2011) Intra- and interfamily phenotypic diversity in pain syndromes associated with a gain-of-function variant of NaV1.7. Mol. Pain 7, 92 Estacion, M. et al. (2009) A sodium channel gene SCN9A polymorphism that increases nociceptor excitability. Ann. Neurol. 66, 862–866 Reimann, F. et al. (2010) Pain perception is altered by a nucleotide polymorphism in SCN9A. Proc. Natl. Acad. Sci. U.S.A. 107, 5148–5153 Black, J.A. et al. (2008) Multiple sodium channel isoforms and mitogenactivated protein kinases are present in painful human neuromas. Ann. Neurol. 64, 644–653 Payandeh, J. et al. (2011) The crystal structure of a voltage-gated sodium channel. Nature 475, 353–358 Yang, Y. et al. (2012) Structural modelling and mutant cycle analysis predict pharmacoresponsiveness of a Na(v)1.7 mutant channel. Nat. Commun. 3, 1186 Fischer, T.Z. et al. (2009) A novel Nav1.7 mutation producing carbamazepine-responsive erythromelalgia. Ann. Neurol. 65, 733–741 Dib-Hajj, S.D. and Waxman, S.G. (2010) Isoform-specific and panchannel partners regulate trafficking and plasma membrane stability; and alter sodium channel gating properties. Neurosci. Lett. 486, 84–91
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