A rare mutation in ABCC8/SUR1 leading to altered K_ATP channel activity and β-cell glucose sensing is associated with type 2 diabetes mellitus in adults

Abstract

Objective: ATP-sensitive K⁺ (K_ATP) channels link glucose metabolism to the electrical activity of the pancreatic β-cell to regulate insulin secretion. Mutations in either the Kir6.2 or SUR1 subunit of the channel have previously been shown to cause neonatal diabetes. We describe here an activating mutation in the ABCC8 gene, encoding SUR1, that is associated with the development of type 2 diabetes mellitus only in adults.

Research Design and Methods: Recombinant K_ATP channel subunits were expressed using pIRES2-based vectors in HEK293 or INS1(832/13) cells and the subcellular distribution of c-myc-tagged SUR1 channels analysed by confocal microscopy. K_ATP channel activity was measured in inside-out patches, and plasma membrane potential in perforated whole-cell patches. Cytoplasmic [Ca²⁺] was imaged using Fura-red.

Results: A mutation in ABCC8/SUR1, leading to a Y356C substitution in the seventh membrane-spanning α-helix, was observed in a patient diagnosed with hyperglycemia at age 39, and in two adult offspring with impaired insulin secretion. Single K_ATP channels incorporating Y356C-SUR1 displayed lower sensitivity to MgATP (IC₅₀=24μ mol/l and 95μ mol/l for wild-type and mutant channels, respectively). Similar effects were observed in the absence of Mg²⁺, suggesting an allosteric effect via associated Kir6.2 channels. Over-expression of Y356C-SUR1 in INS1(832/13) cells impaired glucose-induced cell depolarisation and increases in intracellular free Ca²⁺ concentration, albeit more weakly than neonatal diabetes-associated SUR1 mutants.

Conclusions: An ABCC8/SUR1 mutation with relatively minor effects on K_ATP channel activity and β-cell glucose sensing causes diabetes in adulthood. These data suggest a close correlation between altered SUR1 properties and clinical phenotype. [249 words]