Abnormal Regulation of Hepatic Glucose Output in Maturity-Onset Diabetes of the Young Caused by a Specific Mutation of the Glucokinase Gene

A subtype of maturity-onset diabetes of the young (MODY) is caused by mutations of the glucokinase gene, an enzyme expressed in pancreatic β-cells and the liver. To assess the consequences of a functional alteration of glucokinase at the level of the liver, endogenous (hepatic) glucose production and glucose cycling (an indirect assessment of hepatic glucokinase activity) were measured with 2-²H glucose and 6,6-²H glucose in patients who developed MODY because of the V203A mutation of glucokinase, and in control subjects at similar levels of glycemia. Measurements were performed in the postabsorptive state and after ingestion of ¹³C-labeled glucose. In the postabsorptive state, MODY patients had normal glucose production (10.9 ± 1.3 vs. 11.3 ± 0.6 μmol · kg⁻¹ · min⁻¹) but decreased glucose cycling (0.6 ± 0.3 vs. 1.5 ± 0.3 μmol · kg⁻¹ · min⁻¹; P < 0.05) when compared with control subjects. However, at plasma glucose and insulin levels similar to those observed in MODY patients, control subjects' glucose production was markedly lower (3.2 ± 1.5 μmol · kg⁻¹ · min⁻¹). After glucose ingestion, endogenous glucose production was reduced by only 29% in MODY patients compared with 80% in control subjects at a similar level of hyperglycemia (P < 0.05). This suggests that the V203A mutation of glucokinase results in decreased activity of glucokinase in liver cells. Thus endogenous glucose production is inadequately inhibited by hyperglycemia in MODY patients, possibly as a result of impaired hepatic glucokinase activity. These alterations contribute to the pathogenesis of hyperglycemia.