Background: Primary erythromelalgia is an autosomal dominant pain disorder characterized by burning pain and skin redness in the extremities, with onset of symptoms during the first decade in the families whose mutations have been physiologically studied to date. Several mutations of voltage-gated Na+ channel NaV1.7 have been linked with primary erythromelalgia. Recently, a new substitution Na(v)1.7/I136V has been reported in a Taiwanese family, in which pain appeared at later ages (9-22 years, with onset at 17 years of age or later in 5 of 7 family members), with relatively slow progression (8-10 years) to involvement of the hands. The proband reported onset of symptoms first in his feet at the age of 11, which then progressed to his hands at the age of 19. The new mutation is located in transmembrane segment 1 (S1) of domain I (DI) in contrast to all Na(v)1.7 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.
Results: In this study, we characterized the gating and kinetic properties of I136V mutant channels in HEK293 cells using whole-cell patch clamp. I136V shifts the voltage-dependence of activation by -5.7 mV, a smaller shift in activation than the other erythromelalgia mutations that have been characterized. I136V also decreases the deactivation rate, and generates larger ramp currents.
Conclusion: The I136V substitution in Na(v)1.7 alters channel gating and kinetic properties. Each of these changes may contribute to increased excitability of nociceptive dorsal root ganglion neurons, which underlies pain in erythromelalgia. The smaller shift in voltage-dependence of activation of Na(v)1.7, compared to the other reported cases of inherited erythromelalgia, may contribute to the later age of onset and slower progression of the symptoms reported in association with this mutation.