Clinical characteristics and biochemical mechanisms of congenital hyperinsulinism associated with dominant KATP channel mutations

Abstract

Congenital hyperinsulinism is a condition of dysregulated insulin secretion often caused by inactivating mutations of the ATP-sensitive K+ (KATP) channel in the pancreatic beta cell. Though most disease-causing mutations of the 2 genes encoding KATP subunits, ABCC8 (SUR1) and KCNJ11 (Kir6.2), are recessively inherited, some cases of dominantly inherited inactivating mutations have been reported. To better understand the differences between dominantly and recessively inherited inactivating KATP mutations, we have identified and characterized 16 families with 14 different dominantly inherited KATP mutations, including a total of 33 affected individuals. The 16 probands presented with hypoglycemia at ages from birth to 3.3 years, and 15 of 16 were well controlled on diazoxide, a KATP channel agonist. Of 29 adults with mutations, 14 were asymptomatic. In contrast to a previous report of increased diabetes risk in dominant KATP hyperinsulinism, only 4 of 29 adults had diabetes. Unlike recessive mutations, dominantly inherited KATP mutant subunits trafficked normally to the plasma membrane when expressed in COSm6 cells. Dominant mutations also resulted in different channel-gating defects, as dominant ABCC8 mutations diminished channel responses to magnesium adenosine diphosphate or diazoxide, while dominant KCNJ11 mutations impaired channel opening, even in the absence of nucleotides. These data highlight distinctive features of dominant KATP hyperinsulinism relative to the more common and more severe recessive form, including retention of normal subunit trafficking, impaired channel activity, and a milder hypoglycemia phenotype that may escape detection in infancy and is often responsive to diazoxide medical therapy, without the need for surgical pancreatectomy.

Figure 1. Pedigrees of 16 children with dominant hyperinsulinism associated with K_ATP channel mutations.

The pedigrees are labeled in ascending order of SUR1 and Kir6.2 codons. Males are depicted as squares, females as circles, and a stillborn fetus as a diamond. Identical twins are indicated by squares connected by a triangle. Arrows indicate probands. Deceased individuals are depicted with a slash. n/M, mutation positive; n/n, mutation negative; filled squares, hypoglycemia diagnosed; gray squares, hypoglycemia suspected; open squares, asymptomatic.

Figure 2. Locations of dominant K_ATP channel mutations.

Secondary structures of SUR1 and Kir6.2 are shown, including the amino (N) and carboxy (C) termini of the proteins, SUR1 glycosylation sites, and SUR1 nuclear binding fold 1 (NBF1) and NBF2, which contain Walker A and B motifs (A and B) associated with regulatory nucleotide binding. SUR1 mutations are clustered predominantly in NBF2.

Figure 3. Analysis of mutant channels expressed in COSm6 cells.

(A) Western blot analysis of fSUR1. In cells coexpressing Kir6.2 and S1386P-fSUR1, 2 bands were observed, the lower core-glycosylated band (black arrow) and the upper complex-glycosylated band (white arrow), as was seen with WT-fSUR1. (B) Confocal images of cells coexpressing Kir6.2 and either WT-fSUR1 or S1386P-fSUR1 and immunostained for surface SUR1. A and B illustrate that dominant K_ATP mutant proteins are processed and trafficked to the membrane like the WT protein. (C and D) Representative inside-out patch-clamp recordings of WT, S1386P mutant, or WT + S1386P channels showing channel response to MgADP (C) or diazoxide (D). Currents were measured at –50 mV in symmetrical K-INT solution, and inward currents are shown as upward deflections. Patches were exposed to ATP, ADP, or diazoxide, as indicated by the bars above the records (the patch was also exposed to 1 mM ATP at the very beginning and end of each recording). Free Mg²⁺ concentration was maintained at 1 mM in all ATP-containing solutions. Compared with WT channels, S1386P mutant channels showed a lack of response to MgADP and diazoxide, while channels from simulated heterozygous expression (WT + S1386P) showed a partial response. (E) Quantification of MgADP and diazoxide responses. Currents in 0.1 mM ATP, 0.1 mM ATP + 0.5 mM ADP, or 0.1 mM ATP + 0.2 mM diazoxide were normalized to that seen in K-INT and expressed as percentage of currents. Each bar represents mean ± SEM of 11 (WT) or 7 (S1386P and WT + S1386P) patches.