Mutations of the Same Conserved Glutamate Residue in NBD2 of the Sulfonylurea Receptor 1 Subunit of the KATP Channel Can Result in Either Hyperinsulinism or Neonatal Diabetes

Abstract

OBJECTIVETwo novel mutations (E1506D, E1506G) in the nucleotide-binding domain 2 (NBD2) of the ATP-sensitive K+ channel (KATP channel) sulfonylurea receptor 1 (SUR1) subunit were detected heterozygously in patients with neonatal diabetes. A mutation at the same residue (E1506K) was previously shown to cause congenital hyperinsulinemia. We sought to understand why mutations at the same residue can cause either neonatal diabetes or hyperinsulinemia.RESEARCH DESIGN AND METHODSNeonatal diabetic patients were sequenced for mutations in ABCC8 (SUR1) and KCNJ11 (Kir6.2). Wild-type and mutant KATP channels were expressed in Xenopus laevis oocytes and studied with electrophysiological methods.RESULTSOocytes expressing neonatal diabetes mutant channels had larger resting whole-cell KATP currents than wild-type, consistent with the patients’ diabetes. Conversely, no E1506K currents were recorded at rest or after metabolic inhibition, as expected for a mutation causing hyperinsulinemia. KATP channels are activated by Mg-nucleotides (via SUR1) and blocked by ATP (via Kir6.2). All mutations decreased channel activation by MgADP but had little effect on MgATP activation, as assessed using an ATP-insensitive Kir6.2 subunit. Importantly, using wild-type Kir6.2, a 30-s preconditioning exposure to physiological MgATP concentrations (>300 µmol/L) caused a marked reduction in the ATP sensitivity of neonatal diabetic channels, a small decrease in that of wild-type channels, and no change for E1506K channels. This difference in MgATP inhibition may explain the difference in resting whole-cell currents found for the neonatal diabetes and hyperinsulinemia mutations.CONCLUSIONSMutations in the same residue can cause either hyperinsulinemia or neonatal diabetes. Differentially altered nucleotide regulation by NBD2 of SUR1 can explain the respective clinical phenotypes.

Reinterpreting the Action of ATP Analogs on KATP Channels

“…SUR1 E1507Q has a somewhat reduced affinity for MgADP versus WT, but a more precise estimate is difficult because its affinity for ATP is high and it is hard to suppress the endogenous adenylate kinase in our membrane preparations. The results are consistent with electrophysiologic data on the E1507D substitution that, when assembled with Kir6.2, produces hyperactive K ATP channels that are less sensitive to stimulation by MgADP (52).…”

“…The SUR1 E1507D /Kir6.2 channels exhibit a comparable reduced sensitivity to stimulation by MgADP (52). Interestingly, although the SUR1 E1507Q substitution has a reduced affinity for MgADP, SUR1 Q1178R , which also hyperactivates Kir6.2 pores to produce ND, has a significantly higher apparent affinity for MgADP (31) (Fig.…”

Expression and function of ATP-dependent potassium channels in zebrafish islet β-cells

“…Again, this property is indistinguishable that of from mammalian K ATP channels formed form Kir6.2 + SUR1 subunits [24,25]. These channels are inhibited by increasing concentrations of ATP at the intracellular surface (figure 2 b,c ), with IC 50 of 22.6 µM ( n H   =  1.01), very similar to reported values for mammalian β-cell K ATP channels in the same conditions (IC 50  ∼ 10–20 µM [21,24,26,27]). Furthermore, these channels are activated by addition of Mg-ADP to the cytoplasmic face (figure 2 d,e ), again similar to properties of mammalian K ATP channels [28].…”

Neonatal Diabetes Caused by Activating Mutations in the Sulphonylurea Receptor

“…For example, mutations of the conserved glutamate residue E1506 in NBD2 can result in either hyperinsulinism or ND [30]. …”