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  • Extremes of clinical and enzymatic phenotypes in children with hyperinsulinism caused by glucokinase activating mutations.

Extremes of clinical and enzymatic phenotypes in children with hyperinsulinism caused by glucokinase activating mutations.

Diabetes (2009-04-02)
Samir Sayed, David R Langdon, Stella Odili, Pan Chen, Carol Buettger, Alisa B Schiffman, Mariko Suchi, Rebecca Taub, Joseph Grimsby, Franz M Matschinsky, Charles A Stanley
ABSTRACT

Heterozygous activating mutations of glucokinase have been reported to cause hypoglycemia attributable to hyperinsulinism in a limited number of families. We report three children with de novo glucokinase hyperinsulinism mutations who displayed a spectrum of clinical phenotypes corresponding to marked differences in enzyme kinetics. Mutations were directly sequenced, and mutants were expressed as glutathionyl S-transferase-glucokinase fusion proteins. Kinetic analysis of the enzymes included determinations of stability, activity index, the response to glucokinase activator drug, and the effect of glucokinase regulatory protein. 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; wild type = 1.0). Allosteric responses to inhibition by glucokinase regulatory protein 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; wild type = 5.0 mmol/l). These results confirm the potency of glucokinase as the pancreatic beta-cell glucose sensor, and they demonstrate that responsiveness to diazoxide varies with genotype in glucokinase hyperinsulinism resulting in hypoglycemia, which can be more difficult to control than previously believed.

MATERIALS
Product Number
Brand
Product Description

Millipore
Glutathione−Agarose, lyophilized powder
Millipore
Glutathione−Agarose, set of 3 pre-packed columns (2.5 ml each), (1:1 suspension in a 0.5 M NaCl + 20% ethanol solution)