Nav1.7 mutations associated with paroxysmal extreme pain disorder, but not erythromelalgia, enhance Navβ4 peptide‐mediated resurgent sodium currents

Abstract

Non-technical summary Abnormal pain sensitivity associated with inherited and acquired pain disorders occurs through increased excitability of peripheral sensory neurons in part due to changes in the properties of voltage-gated sodium channels (Navs). Resurgent sodium currents (I NaR ) are atypical currents believed to be associated with increased excitability of neurons and may have implications in pain. Mutations in Nav1.7 (peripheral Nav isoform) associated with two genetic pain disorders, inherited erythromelalgia (IEM) and paroxysmal extreme pain disorder (PEPD), enhance Nav1.7 function via distinct mechanisms. We show that changes in Nav1.7 function due to mutations associated with PEPD, but not IEM, are important in I NaR generation, suggesting that I NaR may play a role in pain associated with PEPD. This knowledge provides us with a better understanding of the mechanism of I NaR generation and may lead to the development of specialized treatment for pain disorders associated with I NaR .Abstract Inherited erythromelalgia (IEM) and paroxysmal extreme pain disorder (PEPD) are inherited pain syndromes predominantly caused by missense mutations in the peripheral neuronal voltage-gated sodium channel (Nav) isoform Nav1.7. While both IEM and PEPD mutations increase neuronal excitability, IEM mutations primarily enhance activation and PEPD mutations impair inactivation. In addition, one PEPD mutation, Nav1.7-I1461T, has been shown to increase resurgent sodium currents in dorsal root ganglion (DRG) neurons. Because resurgent currents have been implicated in increased neuronal excitability, we asked whether (1) additional PEPD mutations located within the putative inactivation gate and docking sites and (2) IEM mutations might also increase these unusual currents. Resurgent currents are generated following open-channel block at positive potentials by an endogenous blocking particle and subsequent expulsion of this blocker upon repolarization to moderately negative potentials. Here we used a mimetic of the putative blocking particle, the Navβ4 peptide, to determine if enhanced resurgent currents are induced by three distinct PEPD mutations and two IEM mutations in stably transfected HEK293 cells. We demonstrate that (1) Nav1.7-mediated resurgent currents are observed in HEK293 cells with the Navβ4 peptide in the recording pipette, (2) while the PEPD mutants M1627K, T1464I and V1299F exhibit enhanced resurgent current amplitudes compared to wild-type, the IEM mutants I848T and L858H do not, and (3) there is a strong correlation between the decay time constant of open-channel fast inactivation and resurgent current amplitude. These data suggest that resurgent currents may play a role in the neuronal hyperexcitability associated with PEPD, but not IEM, mutations.

Inherited Pain

“…To measure this current in a heterologous expression system, a peptide with the sequence of the ␤4 C-terminal section was added to the pipette solution (13). In contrast to A1632T (Fig.…”

“…Slowed transition from the open to the inactivated state, as observed for A1632E, promotes the occurrence of resurgent currents (13). To measure this current in a heterologous expression system, a peptide with the sequence of the ␤4 C-terminal section was added to the pipette solution (13).…”

“…These depolarizing currents arise in the decaying phase of an action potential and have been suggested to contribute to high-frequency firing in physiological and pathological conditions (10,11). PEPD mutations have been shown to enhance resurgent currents in Nav1.7, most likely because of the slowing of fast inactivation kinetics, providing more time for the blocking particle to interact with the open channel (11)(12)(13) and, thereby, priming the channel for resurgent currents (11).…”

A Novel SCN9A Gene Mutation in a Patient with Carbamazepine-ResistantParoxysmal Extreme Pain Disorder

“…The (adult) patients with the M1627K mutation were carbamazepine-responsive with 600 mg/d, whereas a dose of 400 mg/d in a two-year-old boy weighing 8 kg merely halved the frequency of the pain attacks to one every two days. A number of voltage-clamp studies have shown that the M1627K mutation has a marked effect on fast inactivation of NaV1.7 [1][2][3] and resurgent sodium currents [3]. However none of these electrophysiological studies assessed the effect of carbamazepine in the M1627K mutant.…”

“…The SIFT, PolyPhen-2, MutationTaster and Align GVGD prediction tools all suggested that Met1627Arg was likely to affect protein function. Lastly, another mutation of the same amino acid (Met1627Lys) has been reported in carbamazepine-responsive paroxysmal extreme pain disorder [2,3].…”