Deletion mutation of sodium channel NaV1.7 in inherited erythromelalgia: enhanced slow inactivation modulates dorsal root ganglion neuron hyperexcitability

Gain-of-function missense mutations of voltage-gated sodium channel NaV1.7 have been linked to the painful disorder inherited erythromelalgia. These mutations hyperpolarize activation, slow deactivation and enhance currents evoked by slow ramp stimuli (ramp currents). A correlation has recently been suggested between the age of onset of inherited erythromelalgia and the extent of hyperpolarizing shifts in mutant NaV1.7 channel activation; mutations causing large activation shifts have been linked to early age of onset inherited erythromelalgia, while mutations causing small activation shifts have been linked to age of onset within the second decade of life. Here, we report a family with inherited erythromelalgia with an in-frame deletion of a single residue-leucine 955 (Del-L955) in DII/S6. The proband did not show symptoms until the age of 15 years, and her affected mother only experienced mild symptoms during adolescence, which disappeared at the age of 38 years. Del-L955 shows no effect on NaV1.7 current density and fast inactivation, but causes an approximately −24 mV shift in activation, together with increases in amplitude of persistent currents and ramp currents. The mutation also produces an approximately −40 mV shift in slow inactivation, which reduces channel availability. Comparison of the effects of the Del-L955 mutation on dorsal root ganglion neuron hyperexcitability with those produced by another inherited erythromelalgia mutation (L858F) that does not enhance slow inactivation suggests that a delayed age of onset and milder symptoms in association with a large shift of channel activation, enhanced persistent and enhanced ramp currents may be related to the approximately −40 mV shift in slow inactivation for Del-L955, the largest shift thus far demonstrated in mutant NaV1.7 channels. Our results suggest that despite the pivotal role of activation shift in inherited erythromelalgia development, slow inactivation may regulate clinical phenotype by altering channel availability.