Identification of a Heterozygous Sulfonylurea Receptor 1 Mutation that Exerts a Strong Dominant-Negative Effect on KATP Channel Response to MgADP

Abstract

ATP-sensitive potassium (KATP) channels couple cell metabolism to cell excitability thus mediating a range of physiological responses to metabolic stress. In pancreatic β-cells, KATP channels regulate insulin secretion according to plasma glucose concentrations. Mutations in the channel genes ABCC8 encoding the regulatory sulfonylurea receptor 1 or KCNJ11 encoding the pore-forming inwardly rectifying potassium channel Kir6.2 that lead to loss of channel function are causes of congenital hyperinsulinism, characterized by inappropriate insulin secretion despite severe hypoglycemia. The disease-causing mutations can be recessively inherited, which are usually associated with severe disease phenotype, or dominantly inherited, which are commonly associated with less severe disease phenotype and are clinically responsive to the KATP channel opener diazoxide. The most prominent channel gating defects caused by mutations identified in congenital hyperinsulinism is loss of channel response to the stimulatory effect of MgADP and diazoxide. Here, we have identified a heterozygous in-frame insertion mutation in exon 37 of the ABCC8 gene that results in duplication of two amino acids ala-ser at position 1508 in the second nucleotide binding fold 2 (NBF2) from a patient with severe congenital hyperinsulinism unresponsive to diazoxide. Functional characterization of mutant channels reconstituted in COS cells show that the mutation does not disrupt surface expression of the channel but abolishes channel response to MgADP and diazoxide. Strikingly, in simulated heterozygous expression condition, the mutant SUR1 subunit exhibited a strong dominant negative effect on WT SUR1 subunit such that the MgADP and diazoxide response are nearly identical to homomeric mutant channels. This clinical and in-vitro strong dominant negative effect is distinct from other heterozygous mutations reported previously present an interesting case for understanding the structural mechanisms underlying channel response to MgADP and diazoxide.