January 1, 2016;
Neonatal diabetes caused by a homozygous KCNJ11 mutation demonstrates that tiny changes in ATP sensitivity markedly affect diabetes risk.
AIMS/HYPOTHESIS: The pancreatic ATP-sensitive potassium (KATP) channel plays a pivotal role in linking beta cell metabolism to insulin
secretion. Mutations in KATP channel genes can result in hypo- or hypersecretion of insulin
, as in neonatal diabetes mellitus and congenital hyperinsulinism, respectively. To date, all patients affected by neonatal diabetes due to a mutation in the pore-forming subunit of the channel (Kir6.2, KCNJ11
) are heterozygous for the mutation. Here, we report the first clinical case of neonatal diabetes caused by a homozygous KCNJ11
METHODS: A male patient was diagnosed with diabetes shortly after birth. At 5 months of age, genetic testing revealed he carried a homozygous KCNJ11
mutation, G324R, (Kir6.2-G324R) and he was successfully transferred to sulfonylurea therapy (0.2 mg kg(-1) day(-1)). Neither heterozygous parent was affected. Functional properties of wild-type, heterozygous and homozygous mutant KATP channels were examined after heterologous expression in Xenopus oocytes.
RESULTS: Functional studies indicated that the Kir6.2-G324R mutation reduces the channel ATP sensitivity but that the difference in ATP inhibition between homozygous and heterozygous channels is remarkably small. Nevertheless, the homozygous patient developed neonatal diabetes, whereas the heterozygous parents were, and remain, unaffected. Kir6.2-G324R channels were fully shut by the sulfonylurea tolbutamide, which explains why the patient''s diabetes was well controlled by sulfonylurea therapy.
CONCLUSIONS/INTERPRETATION: The data demonstrate that tiny changes in KATP channel activity can alter beta cell electrical activity and insulin
secretion sufficiently to cause diabetes. They also aid our understanding of how the Kir6.2-E23K variant predisposes to type 2 diabetes.
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Fig. 1. Metabolic activation of WT and mutant KATP channels. (a) Representative whole-cell currents recorded from oocytes expressing WT or homG324R channels in response to 500 ms voltage steps of ±20 mV from a holding potential of −10 mV. Sodium azide (azide, 3 mmol/l) and tolbutamide (tolb., 0.5 mmol/l) were added as indicated. (b) Current in control solution for WT (n = 13), hetG324R (het; n = 9) and homG324R (hom; n = 10) expressed as a percentage of that in the presence of 3 mmol/l sodium azide (per cent maximal current). (c) Mean tolbutamide block for WT (98.2 ± 0.2%, n = 7), hetG324R (het; 97.9 ± 0.3%, n = 6) and homG324R (hom; 98.1 ± 0.4%, n = 7) channels (measured in the presence of sodium azide)
Fig. 2. MgATP inhibition of KATP channels is reduced by the Kir6.2-G324R mutation. (a) Representative currents recorded at −60 mV from inside-out patches excised from oocytes expressing WT or mutant KATP channels. The dashed line indicates the zero current level. MgATP (10 μmol/l) was applied as indicated. (b, c) Relationship between KATP current and MgATP concentration for WT (black squares), homG324R (grey filled squares; b) and hetG324R (grey empty squares; c) channels. Current in the presence of nucleotide (I) is expressed as a fraction of that in its absence (IC). The curves are the best fit to the Hill equation. ***p < 0.001 compared with WT (t test)
Fig. 3. Comparison of ATP sensitivities. (a) IC50 for MgATP inhibition and (b) fraction of unblocked current at 3 mmol/l MgATP for the indicated channels: WT (n = 13), homE23K (n = 5), hetG324R (n = 6), homG324R (n = 8), hetE227L (n = 7). Mean values for TNDM (white hatched bars), PNDM (grey hatched bars) and DEND/iDEND (grey bars) channels are given for comparison (for references, see ESM Fig. 1). Data are mean ± SEM. **p < 0.01 compared with WT (t test)
Fig. 4. Molecular model of the Kir6.2 tetramer . (a) Top view from the extracellular side, showing the position of G324 (red), D323 (yellow) and R325 (cyan). ATP (orange sticks) is shown docked into its putative binding site. (b) Detail of two adjacent Kir6.2 subunits (one green, one grey). The first 32 amino acids are not included in the model so N-term denotes residue 32