Dominantly inherited hyperinsulinism caused by a mutation in the sulfonylurea receptor type 1

Abstract

ATP-sensitive potassium channels play a major role in linking metabolic signals to the exocytosis of insulin in the pancreatic beta cell. These channels consist of two types of protein subunit: the sulfonylurea receptor SUR1 and the inward rectifying potassium channel Kir6.2. Mutations in the genes encoding these proteins are the most common cause of congenital hyperinsulinism (CHI). Since 1973, we have followed up 38 pediatric CHI patients in Finland. We reported previously that a loss-of-function mutation in SUR1 (V187D) is responsible for CHI of the most severe cases. We have now identified a missense mutation, E1506K, within the second nucleotide binding fold of SUR1, found heterozygous in seven related patients with CHI and in their mothers. All patients have a mild form of CHI that usually can be managed by long-term diazoxide treatment. This clinical finding is in agreement with the results of heterologous coexpression studies of recombinant Kir6.2 and SUR1 carrying the E1506K mutation. Mutant K(ATP) channels were insensitive to metabolic inhibition, but a partial response to diazoxide was retained. Five of the six mothers, two of whom suffered from hypoglycemia in infancy, have developed gestational or permanent diabetes. Linkage and haplotype analysis supported a dominant pattern of inheritance in a large pedigree. In conclusion, we describe the first dominantly inherited SUR1 mutation that causes CHI in early life and predisposes to later insulin deficiency.

Figure 1

Demonstration of the mutation E1506K. (a) SSCP-analysis of the SUR1 exon 37. Results are shown for two individuals carrying the E1506K mutation (Mt) and two controls (Wt). The abnormally migrating DNA fragment is marked with an arrow. The gel was run for 4 hours at 38°C. (b) Sequence analysis reveals a G→A mutation in the genomic sequence resulting in the substitution of glutamic acid for lysine. (c) Detection of the mutation by restriction analysis of PCR-amplified genomic DNA. Mutation E1506K causes the disappearance of a MnlI restriction site, leading to the formation of a new 89-bp digestion product for the mutated allele (arrow). MW, molecular weight marker; Mt, mutant; Wt, wild type.

Figure 2

Pedigree and haplotype analysis of a large pedigree carrying the SUR1 mutation E1506K. The order of markers of the haplotype is as follows: D11S1890, D11S921, D11S1888. The haplotypes are presented below each individual. The haplotype 3-4-4 associates with E1506K in all cases.

Figure 3

Pancreatic histology of case 1. (a) Hematoxylin-eosin–stained section showing abnormal β cell nuclei (arrows). Objective magnification ×40. (b) Immunoperoxidase staining of insulin reveals irregular islets and small clusters of β cells. Objective magnification ×10.

Figure 4

Effects of metabolic inhibition on wild-type and mutant Kir6.2/SUR1 currents. Mean whole-cell current amplitudes recorded at –50 mV first in control solution (black bars), 15 minutes after exposure to 3 mM azide (hatched bars), then in the continued presence of azide plus 340 μM diazoxide (white bars), and, finally, after the further addition of 500 μM tolbutamide (shaded bars). Oocytes were co-injected with mRNA encoding Kir6.2 and either SUR1, SUR1-E1506K, or a 1:1 mixture of SUR1 plus SUR1-E1506K, as indicated. The number of oocytes is given above the error bars.

Figure 5

(a) Current amplitudes recorded in excised patches. Mean current amplitudes recorded at –100 mV in the cell-attached or inside-out patch configuration from oocytes co-injected with Kir6.2 and either SUR1 or SUR-E1506K as indicated. The number of oocytes is given above the bars. (b) ATP sensitivity of Kir6.2/SUR1-E1506K currents; mean ATP concentration-response relationships for Kir6.2/SUR1-E1506K currents (n = 4). The slope conductance (G) is expressed as a fraction of the mean (G_c) of that obtained in control solution before and after the exposure to ATP: The line is the best fit of the data to Hill equation (IC₅₀ = 9.3 μM and h = 0.96).

Figure 6

Effects of MgADP, tolbutamide, and diazoxide on Kir6.2/SUR1-E1506K currents. (a) Macroscopic Kir6.2/SUR1-E1506K currents recorded from inside-out patches in response to series of voltage ramps from –110 mV to +100 mV. Tolbutamide (100 μM), ADP (100 μM), diazoxide (340 μM), and ATP (100 μM) were added to the internal solution as indicated by the bars. (b) Mean macroscopic slope conductance recorded in the presence of ATP, ATP plus diazoxide, ADP, or tolbutamide, expressed as percentage of the slope conductance in control solution. The number of oocytes is given above the bars. Inset: Mean macroscopic slope conductance recorded in the presence of 100 μM ATP plus 340 μM diazoxide, expressed as a percentage of the current in the presence of 100 μM ATP for Kir6.2/SUR and Kir6.2/SUR1-E1506K currents. The dashed line indicates the conductance in the presence of 100 μM ATP. The number of oocytes is given above the bars.