Extremes of Clinical and Enzymatic Phenotypes in Children With Hyperinsulinism Caused by Glucokinase Activating Mutations

Abstract

OBJECTIVE-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. RESEARCH DESIGN AND METHODS-Mutationswere 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.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; wild type ϭ 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 ϭ 5.0 mmol/l).CONCLUSIONS-These results confirm the potency of glucokinase as the pancreatic ␤-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.

Large Islets, Beta-Cell Proliferation, and a Glucokinase Mutation

“…Normal histologic findings in four previous cases of glucokinase-related hypoglycemia have been reported, but none of these patients underwent detailed quantitative morphometric analysis. [2][3][4] In a previously reported case that included quantitative histologic analysis, a similar increase in the mean islet profile was confirmed (it was 2.5 times larger than that of control subjects and 8.0 to 10.0 times larger than that of patients with a K ATP -channel deficiency). 5 In both that patient and our patient, the routine pathology report did not indicate any abnormality in islet size; this emphasizes the importance of quantitative morphometric analysis to determine islet size.…”

Nutrient sensing in pancreatic islets: lessons from congenital hyperinsulinism and monogenic diabetes

“…ATP K m was similar to that of wild-type proteins and the calculated activity index was increased. 36,38 Clinical investigation has confirmed the role of GCK activating mutations in the regulation of insulin secretion. GCK-CHI patients show typical features of hyperinsulinism (HI), including abnormal insulin secretion, suppressed ketone production during fasting, and an inappropriate positive response to glucagonstimulated blood glucose elevation after prolonged fasting.…”

“…[36][37][38] The enzyme kinetics of the activating GCK mutant protein showed a leftward shift of the glucose dosedependency curve with reduced glucose S 0.5 . ATP K m was similar to that of wild-type proteins and the calculated activity index was increased.…”

“…GCK-CHI patients show typical features of hyperinsulinism (HI), including abnormal insulin secretion, suppressed ketone production during fasting, and an inappropriate positive response to glucagonstimulated blood glucose elevation after prolonged fasting. 17,36,[38][39][40] A mouse disease model has been generated to express one of the GCK activating mutants (A456V). 41 As we expected, the threshold for glucose ramp-stimulated insulin secretion in isolated islets was left-shifted in heterozygous mouse islets, and even more left-shifted for islets isolated from the homozygous A456V GCK mutation.…”

“…However, diazoxide has not been very effective in many GCK-CHI patients. 36,38 Some patients with severe hypoglycemia and no response to diazoxide therapy required near-total pancreatectomy. 42 The plausible explanation for the ineffective response to diazoxide is that activating GCK mutations will override any effort to raise blood glucose above the threshold or the set-point for GSIS.…”