Proceedings of the National Academy of Sciences of the United States of America •
Voltage-gated sodium channel Nav1.8 plays a crucial role in regulating excitability of small dorsal root ganglion (DRG) neurons and is an emerging target for pain therapeutics. Using dynamic clamp, we systematically manipulated Nav1.8 conductance to assess its impact on action potential (AP) electrogenesis, rheobase, and repetitive firing in native rat DRG neurons and those expressing the gain-of-function Nav1.7L858H mutation which underlies inherited erythromelalgia, a human genetic pain disorder. Our findings reveal that the Nav1.8 contribution to net sodium current is highly correlated with AP voltage threshold. Nav1.8 conductance regulated AP overshoot and voltage threshold without significantly affecting undershoot or resting membrane potential. We identified two populations of wild-type DRG neurons: strong responders (50% of cells), which exhibited substantial rheobase modulation with alterations in Nav1.8 conductance, and weak responders (50% of cells), which remained largely unaffected. In hyperexcitable Nav1.7L858H-expressing neurons, partial Nav1.8 subtraction (50%) restored rheobase above control levels in 63% of cells. However, weak responders (37%) remained hyperexcitable. The effect of Nav1.8 subtraction in responsive neurons supports the conclusion that Nav1.8 inhibition can reduce neuropathic pain. However, the presence of weakly responsive DRG neurons suggests that other channels might need to be targeted for full pain relief.
Shim J, Tanaka B, Taub DG, Mis MA, Schulman BR , et al.
Brain : a journal of neurology •
Inherited erythromelalgia, small fibre neuropathy and paroxysmal extreme pain disorder are caused by gain-of-function mutations in the voltage-gated sodium channel Nav1.7. It remains unknown how different mutations in the same channel enhancing electrogenesis in sensory neurons results in such distinct disease presentations. Most of the work analysing the impact of these mutations on electrophysiological properties has used overexpression systems in cell lines and rodent sensory neurons, which might differ from the natural context. We have differentiated sensory neurons from induced pluripotent stem cells derived from patient samples that have the Nav1.7 A1632G mutation. This strategy reveals changes in electrophysiological properties, not previously observed in cell lines, that might be important for disease presentation. Furthermore, using CRISPR/Cas9, we corrected this mutation, which reduced the underlying hyperexcitability, providing a path for personalized medicine to treat these disorders, and we introduced the mutation into control induced pluripotent stem cells, which generated hyperexcitability, providing causality. Induced pluripotent stem cell sensory neurons are a robust, scalable and relevant model to study the effects of gain-of-function mutations in ion channels in pain-related disorders.
Inherited erythromelalgia (IEM) is a chronic pain disorder caused by gain-of-function mutations of peripheral sodium channel Nav1.7, in which warmth triggers severe pain. Little is known about the brain representation of pain in IEM. Here we study two subjects with the IEM Nav1.7-S241T mutation using functional brain imaging (fMRI). Subjects were scanned during each of five visits. During each scan, pain was first triggered using a warming boot and subjects rated their thermal-heat pain. Next, the thermal stimulus was terminated and subjects rated stimulus-free pain. Last, subjects performed a control visual rating task. Thermal-heat induced pain mapped to the frontal gyrus, ventro-medial prefrontal cortex, superior parietal lobule, supplementary motor area, insula, primary and secondary somato-sensory motor cortices, dorsal and ventral striatum, amygdala, and hippocampus. Stimulus-free pain, by contrast, mapped mainly to the frontal cortex, including dorsal, ventral and medial prefrontal cortex, and supplementary motor area. Examination of time periods when stimulus-free pain was changing showed further activations in the valuation network including the rostral anterior cingulate cortex, striatum and amygdala, in addition to brainstem, thalamus, and insula. We conclude that, similar to other chronic pain conditions, the brain representation of stimulus-free pain during an attack in subjects with IEM engages brain areas involved in acute pain as well as valuation and learning.
Cao L, McDonnell A, Nitzsche A, Alexandrou A, Saintot PP , et al.
Science translational medicine •
In common with other chronic pain conditions, there is an unmet clinical need in the treatment of inherited erythromelalgia (IEM). TheSCN9Agene encoding the sodium channel Nav1.7 expressed in the peripheral nervous system plays a critical role in IEM. A gain-of-function mutation in this sodium channel leads to aberrant sensory neuronal activity and extreme pain, particularly in response to heat. Five patients with IEM were treated with a new potent and selective compound that blocked the Nav1.7 sodium channel resulting in a decrease in heat-induced pain in most of the patients. We derived induced pluripotent stem cell (iPSC) lines from four of five subjects and produced sensory neurons that emulated the clinical phenotype of hyperexcitability and aberrant responses to heat stimuli. When we compared the severity of the clinical phenotype with the hyperexcitability of the iPSC-derived sensory neurons, we saw a trend toward a correlation for individual mutations. The in vitro IEM phenotype was sensitive to Nav1.7 blockers, including the clinical test agent. Given the importance of peripherally expressed sodium channels in many pain conditions, our approach may have broader utility for a wide range of pain and sensory conditions.
Estacion M, Han C, Choi JS, Hoeijmakers JG, Lauria G , et al.
Molecular pain •
Sodium channel NaV1.7 is preferentially expressed within dorsal root ganglia (DRG), trigeminal ganglia and sympathetic ganglion neurons and their fine-diamter axons, where it acts as a threshold channel, amplifying stimuli such as generator potentials in nociceptors. Gain-of-function mutations and variants (single amino acid substitutions) of NaV1.7 have been linked to three pain syndromes: Inherited Erythromelalgia (IEM), Paroxysmal Extreme Pain Disorder (PEPD), and Small Fiber Neuropathy (SFN). IEM is characterized clinically by burning pain and redness that is usually focused on the distal extremities, precipitated by mild warmth and relieved by cooling, and is caused by mutations that hyperpolarize activation, slow deactivation, and enhance the channel ramp response. PEPD is characterized by perirectal, periocular or perimandibular pain, often triggered by defecation or lower body stimulation, and is caused by mutations that severely impair fast-inactivation. SFN presents a clinical picture dominated by neuropathic pain and autonomic symptoms; gain-of-function variants have been reported to be present in approximately 30% of patients with biopsy-confirmed idiopathic SFN, and functional testing has shown altered fast-inactivation, slow-inactivation or resurgent current. In this paper we describe three patients who house the NaV1.7/I228M variant. We have used clinical assessment of patients, quantitative sensory testing and skin biopsy to study these patients, including two siblings in one family, in whom genomic screening demonstrated the I228M NaV1.7 variant. Electrophysiology (voltage-clamp and current-clamp) was used to test functional effects of the variant channel. We report three different clinical presentations of the I228M NaV1.7 variant: presentation with severe facial pain, presentation with distal (feet, hands) pain, and presentation with scalp discomfort in three patients housing this NaV1.7 variant, two of which are from a single family. We also demonstrate that the NaV1.7/I228M variant impairs slow-inactivation, and produces hyperexcitability in both trigeminal ganglion and DRG neurons. Our results demonstrate intra- and interfamily phenotypic diversity in pain syndromes produced by a gain-of-function variant of NaV1.7.
The voltage-gated sodium-channel type IX alpha subunit, known as Na(v)1.7 and encoded by the gene SCN9A, is located in peripheral neurons and plays an important role in action potential production in these cells. Recent genetic studies have identified Na(v)1.7 dysfunction in three different human pain disorders. Gain-of-function missense mutations in Na(v)1.7 have been shown to cause primary erythermalgia and paroxysmal extreme pain disorder, while nonsense mutations in Na(v)1.7 result in loss of Na(v)1.7 function and a condition known as channelopathy-associated insensitivity to pain, a rare disorder in which affected individuals are unable to feel physical pain. This review highlights these recent developments and discusses the critical role of Na(v)1.7 in pain sensation in humans.
