A gain-of-function sodium channel β2-subunit mutation in painful diabetic neuropathy

Diabetes mellitus is a global challenge with many diverse health sequelae, of which diabetic peripheral neuropathy is one of the most common. A substantial number of patients with diabetic peripheral neuropathy develop chronic pain, but the genetic and epigenetic factors that predispose diabetic peripheral neuropathy patients to develop neuropathic pain are poorly understood. Recent targeted genetic studies have identified mutations in alpha-subunits of voltage-gated sodium channels (Na(v)s) in patients with painful diabetic peripheral neuropathy. Mutations in proteins that regulate trafficking or functional properties of Na(v)s could expand the spectrum of patients with Na-v-related peripheral neuropathies. The auxiliary sodium channel beta-subunits (beta 1-4) have been reported to increase current density, alter inactivation kinetics, and modulate subcellular localization of Na-v. Mutations in beta-subunits have been associated with several diseases, including epilepsy, cancer, and diseases of the cardiac conducting system. However, mutations in beta-subunits have never been shown previously to contribute to neuropathic pain. We report here a patient with painful diabetic peripheral neuropathy and negative genetic screening for mutations in SCN9A, SCN10A, and SCN11A-genes encoding sodium channel alpha-subunit that have been previously linked to the development of neuropathic pain. Genetic analysis revealed an aspartic acid to asparagine mutation, D109N, in the beta 2-subunit. Functional analysis using current-clamp revealed that the beta 2-D109N rendered dorsal root ganglion neurons hyperexcitable, especially in response to repetitive stimulation. Underlying the hyperexcitability induced by the beta 2-subunit mutation, as evidenced by voltage-clamp analysis, we found a depolarizing shift in the voltage dependence of Na(v)1.7 fast inactivation and reduced use-dependent inhibition of the Na(v)1.7 channel.