Extremes of clinical and enzymatic phenotypes in children with hyperinsulinism due to glucokinase activating mutations.

Abstract

Objective. Heterozygous activating mutations of glucokinase have been reported to cause hypoglycemia due to hyperinsulinism (GK-HI) in a limited number of families. We report three children with de novo GK-HI mutations who displayed a spectrum of clinical phenotypes corresponding with marked differences in enzyme kinetics.

Research Design and Methods. Mutations were directly sequenced and mutants were expressed as GST-GK fusion proteins. Kinetic analysis of the enzymes included determinations of stability, activity index, the response to glucokinase activator drug and the affect of glucokinase regulatory protein (GKRP).

Results. Child 1 had an ins454A mutation, child 2 a W99L mutation and child 3 an M197I mutation. Diazoxide treatment was effective in child 3 but ineffective in child 1 and only partially effective in child 2. Expression of the mutant glucokinase ins454A, W99L and M197I enzymes revealed a continuum of high relative activity indexes in the three children (26, 8.9 and 3.1 respectively; WT = 1.0). Allosteric responses to inhibition by GKRP and activation by the drug RO0281675 were impaired by the ins454A but unaffected by the M197I mutation. Estimated thresholds for glucose stimulated insulin release were more severely reduced by the ins454A than the M197I mutation and intermediate in the W99L mutation (1.1, 3.5 and 2.2 mmol/l respectively; WT = 5.0 mmol/l).

Conclusions. These results confirm the potency of glucokinase as the pancreatic ß-cell glucose sensor and demonstrate that responsiveness to diazoxide varies with genotype in GK-HI resulting in hypoglycemia that can be more difficult to control than previously believed.