Voltage-gated Na+ channels are heteromeric protein complexes, consisting of a pore-forming α subunit and auxiliary β subunits . Nine α subunits (NaV1.1–NaV1.9) have been reported to date, and have been shown to be expressed in a tissue-specific manner . Each α subunit is composed of four homologous domains (DI-DIV), with each domain consisting of six transmembrane segments (S1–S6), with S4 acting as a voltage-sensor and S5 and S6 lining the pore . Voltage-gated Na+ channels are essential for the production of action potentials and are thus pivotal for excitability of neurons, myocytes, and neuroendocrine cells . Mutations of voltage-gated Na+ channels have been linked to a number of human diseases including epilepsy, periodic paralysis, cardiac disorders, and pain disorders [3–7].
Primary erythromelalgia is a rare, inherited, autosomal dominant disorder, characterized by intermittent burning pain in the extremities and skin redness of the affected areas. Episodes can be triggered by warm stimuli or mild exercise . The onset of symptoms varies from childhood, adolescence, to adulthood , with onset of pain most commonly occurring during the first decade of life. Primary erythromelalgia has been linked to mutations in SCN9A, the gene that encodes voltage-gated sodium channel NaV1.7 . NaV1.7 is preferentially expressed in nociceptive dorsal root ganglia (DRG) neurons and sympathetic ganglia neurons [11–14], and it produces a fast-activating and -inactivating TTX-sensitive (TTX-S) current with a slow recovery from inactivation [12, 15]. NaV1.7 also exhibits slow kinetics of closed-state inactivation, which enables it to respond to small, slow depolarizing inputs and boost subthreshold depolarizations . Thus, gain-of-function mutations of the NaV1.7 channel might be expected to contribute to the hyperexcitability of nociceptive neurons which underlies chronic pain in patients with erythromelagia.
Nine mutations of NaV1.7 have been identified in patients with primary erythromelalgia, with eight of them, all from families with onset in childhood (≤ 10 years of age), having been characterized by electrophysiological studies [16–22]. All eight mutations cause a hyperpolarizing shift in voltage dependence of activation in NaV1.7 channels. Slow deactivation kinetics and larger ramp currents were observed in all but the F1449V mutant channels [16–22]. The changes in the biophysical properties of mutant NaV1.7 are predicted to increase excitability of DRG neurons and thereby contribute to the pathophysiology of erythromelalgia. In fact, three of these mutations (F1449V, A863P, and L858H) have been studied using current clamp and shown to increase excitability of DRG neurons in culture [18, 20, 23].
Recently, a new erythromelalgia mutation of NaV1.7 was reported by Lee et al in a Taiwanese family, affecting family members of three generations , with a later age of onset. In this family, pain appeared first in the feet at ages varying from 9 to 22 years, with onset at ages ≥ 17 years in five of seven family members; pain was reported in the hands in one-half of the affected family members, 8–10 years later (C-C Yang and M-J Lee, personal communication). The clinical phenotype was unusual in that the proband reported onset of pain in feet at 11 years of age, later than in most of familial cases [16, 18, 21, 22, 25] and in the patients with de novo founder mutations that have been studied to date [19, 20]. Genetic analysis identified a substitution of isoleucine 136 with valine (I136V) in NaV1.7 in all affected family members, but not in unaffected family members or control DNA . This amino acid substitution is located in DIS1 of the NaV1.7 channel, in contrast to all NaV1.7 mutations reported to date, which have been localized in the voltage sensor S4, the linker joining segments S4 and S5 or pore-lining segments S5 and S6 in DI, II and III. Here, we investigate the effect of this mutation on biophysical properties of NaV1.7 channels.