Alsaloum M, Labau JIR, Liu S, Effraim PR, Waxman SG
Brain : a journal of neurology •
Effective treatment of pain remains an unmet healthcare need that requires new and effective therapeutic approaches. NaV1.7 has been genetically and functionally validated as a mediator of pain. Preclinical studies of NaV1.7-selective blockers have shown limited success and translation to clinical studies has been limited. The degree of NaV1.7 channel blockade necessary to attenuate neuronal excitability and ameliorate pain is an unanswered question important for drug discovery. Here, we utilize dynamic clamp electrophysiology and induced pluripotent stem cell-derived sensory neurons (iPSC-SNs) to answer this question for inherited erythromelalgia, a pain disorder caused by gain-of-function mutations in Nav1.7. We show that dynamic clamp can produce hyperexcitability in iPSC-SNs associated with two different inherited erythromelalgia mutations, NaV1.7-S241T and NaV1.7-I848T. We further show that blockade of approximately 50% of NaV1.7 currents can reverse neuronal hyperexcitability to baseline levels.
Eijkenboom I, Sopacua M, Hoeijmakers JGJ, de Greef BTA, Lindsey P , et al.
Journal of neurology, neurosurgery, and psychiatry •
Neuropathic pain is common in peripheral neuropathy. Recent genetic studies have linked pathogenic voltage-gated sodium channel (VGSC) variants to human pain disorders. Our aims are to determine the frequency of , and variants in patients with pure small fibre neuropathy (SFN), analyse their clinical features and provide a rationale for genetic screening. Between September 2009 and January 2017, 1139 patients diagnosed with pure SFN at our reference centre were screened for , and variants. Pathogenicity of variants was classified according to established guidelines of the Association for Clinical Genetic Science and frequencies were determined. Patients with SFN were grouped according to the VGSC variants detected, and clinical features were compared. Among 1139 patients with SFN, 132 (11.6%) patients harboured 73 different (potentially) pathogenic VGSC variants, of which 50 were novel and 22 were found in ≥ 1 patient. The frequency of (potentially) pathogenic variants was 5.1% (n=58/1139) for 3.7% (n=42/1139) for and 2.9% (n=33/1139) for . Only erythromelalgia-like symptoms and warmth-induced pain were significantly more common in patients harbouring VGSC variants. (Potentially) pathogenic VGSC variants are present in 11.6% of patients with pure SFN. Therefore, genetic screening of and should be considered in patients with pure SFN, independently of clinical features or underlying conditions.
Ahn HS, Vasylyev DV, Estacion M, Macala LJ, Shah P , et al.
Brain research •
Sodium channel NaV1.7 is preferentially expressed in dorsal root ganglion (DRG) and sympathetic ganglion neurons. Gain-of-function NaV1.7 mutations/variants have been identified in the painful disorders inherited erythromelalgia and small-fiber neuropathy (SFN). DRG neurons transfected with these channel variants display depolarized resting potential, reduced current-threshold, increased firing-frequency and spontaneous firing. Whether the depolarizing shift in resting potential and enhanced spontaneous firing are due to persistent activity of variant channels, or to compensatory changes in other conductance(s) in response to expression of the variant channel, as shown in model systems, has not been studied. We examined the effect of wild-type NaV1.7 and a NaV1.7 mutant channel, D623N, associated with SFN, on resting potential and membrane potential during interspike intervals in DRG neurons. Resting potential in DRG neurons expressing D623N was depolarized compared to neurons expressing WT-NaV1.7. Exposure to TTX hyperpolarized resting potential by 7mV, increased current-threshold, decreased firing-frequency, and reduced NMDG-induced-hyperpolarization in DRG neurons expressing D623N. To assess the contribution of depolarized resting potential to DRG neuron excitability, we mimicked the mutant channel's depolarizing effect by current injection to produce equivalent depolarization; the depolarization decreased current threshold and increased firing-frequency. Voltage-clamp using ramp or repetitive action potentials as commands showed that D623N channels enhance the TTX-sensitive inward current, persistent at subthreshold membrane voltages, as predicted by a Hodgkin-Huxley model. Our results demonstrate that a variant of NaV1.7 associated with painful neuropathy depolarizes resting membrane potential and produces an enhanced inward current during interspike intervals, thereby contributing to DRG neuron hyperexcitability.
Secondary erythromelalgia is a rare disease characterized by burning pain, marked erythema, edema, and hyperthermia of the affected limbs. Secondary erythromelalgia can be associated with various systemic diseases. Here, we describe a patient who developed secondary erythromelalgia involving the ears and concomitant clinical and laboratory, probably, indicating the initial stage of a developing lupus erythematosus.
