Functional characterization of activating mutations in the sulfonylurea receptor 1 (ABCC8) causing neonatal diabetes mellitus in Asian Indian children

Background: Gain-of-function of ATP-sensitive K ⁺ (K _ATP ) channels because of mutations in the genes encoding SUR1 (ABCC8) or Kir6.2 (KCNJ11) is a major cause of neonatal diabetes mellitus (NDM). Our aim is to determine molecular defects in K _ATP channels caused by ABCC8 mutations in Asian Indian children with NDM by in vitro functional studies. Methods: Wild-type (WT; NM_000352.4) or mutant sulfonylurea receptor 1 (SUR1) and Kir6.2 were co-expressed in COSm6 cells. Biogenesis efficiency and surface expression of mutant channels were assessed by immunoblotting and immunostaining. The response of mutant channels to cytoplasmic ATP and ADP was assessed by inside-out patch-clamp recordings. The response of mutant channels to known K _ATP inhibitors in intact cells were determined by ⁸⁶ Rb efflux assays. Results: Five SUR1 missense mutations, D212Y, P254S, R653Q, R992C, and Q1224H, were studied and showed increased activity in MgATP/MgADP. Two of the mutants, D212Y and P254S, also showed reduced response to ATP ⁴⁻ inhibition, as well as markedly reduced surface expression. Moreover, all five mutants were inhibited by the K _ATP channel inhibitors glibenclamide and carbamazepine. Conclusions: The study shows the mechanisms by which five SUR1 mutations identified in Asian Indian NDM patients affect K _ATP channel function to cause the disease. The reduced ATP ⁴⁻ sensitivity caused by the D212Y and P254S mutations in the L0 of SUR1 provides novel insight into the role of L0 in channel inhibition by ATP. The results also explain why sulfonylurea therapy is effective in two patients and inform how it should be effective for the other three patients.