Functional Effects of Mutations at F35 in the NH₂-terminus of Kir6.2 (KCNJ11), Causing Neonatal Diabetes, and Response to Sulfonylurea Therapy

Abstract

Heterozygous mutations in the human Kir6.2 gene (KCNJ11), the pore-forming subunit of the ATP-sensitive K⁺ channel (K_ATP channel), cause neonatal diabetes. To date, all mutations increase whole-cell K_ATP channel currents by reducing channel inhibition by MgATP. Here, we provide functional characterization of two mutations (F35L and F35V) at residue F35 of Kir6.2, which lies within the NH₂-terminus. We further show that the F35V patient can be successfully transferred from insulin to sulfonylurea therapy. The patient has been off insulin for 24 months and shows improved metabolic control (mean HbA_1c 7.58 before and 6.18% after sulfonylurea treatment; P < 0.007). Wild-type and mutant Kir6.2 were heterologously coexpressed with SUR1 in Xenopus oocytes. Whole-cell K_ATP channel currents through homomeric and heterozygous F35V and F35L channels were increased due to a reduced sensitivity to inhibition by MgATP. The mutation also increased the open probability (P_O) of homomeric F35 mutant channels in the absence of ATP. These effects on P_O and ATP sensitivity were abolished in the absence of SUR1. Our results suggest that mutations at F35 cause permanent neonatal diabetes by affecting K_ATP channel gating and thereby, indirectly, ATP inhibition. Heterozygous F35V channels were markedly inhibited by the sulfonylurea tolbutamide, accounting for the efficacy of sulfonylurea therapy in the patient.