Genotype-phenotype discrepancy among family members carrying a novel glucokinase mutation: insights into the interplay of GCK-MODY and insulin resistance

Aims/Hypothesis: Heterozygous inactivating mutations in the glucokinase (GCK) gene are known to cause maturity-onset diabetes of the young (GCK-MODY). We identified a novel variant of uncertain significance (VUS) GCK mutation (c.77A>T, p.Q26L) in two family members presenting markedly different severities of diabetic phenotypes. This study aimed to elucidate the potential diabetogenic effect of GCK-Q26L and to explore the mono- and poly-genetic background attributing to different diabetes phenotypes. Methods: Whole- exome sequencing (WES) and genetic analyses, including polygenic risk score (PRS) assessments, were performed in three members of a family with early-onset diabetes. To elucidate the impact of the GCK-Q26L mutation on glucose homeostasis, a global knock-in mouse model harboring this mutation in both heterozygous and homozygous states was generated. Insulin content and insulin secretion response to glucose and potassium were evaluated in isolated islets. Furthermore, the effects of dorzagliatin (a glucokinase activator, GKA) and liraglutide (a glucagon like peptide 1 receptor agonist, GLP-1RA) on glucose tolerance and insulin secretion were assessed in GCK-Q26L mutant mice. Results: The proband, who inherited the GCK-Q26L mutation from her father (presenting with non-progressive, mildly elevated blood glucose), exhibited severe diabetic phenotypes including polydipsia, polyuria, polyphagia, weight loss, and ketosis, accompanied by significant dyslipidemia. Genetic analyses revealed that the proband's severe phenotypes and metabolic profiles were associated with a high polygenic risk score (PRS) for insulin resistance that was inherited from her mother. Global heterozygous GCK-Q26L knock-in mice showed a mild increased fasting blood glucose, impaired glucose tolerance (IGT), and decreased serum insulin. Homozygous GCK-Q26L mice presented more severe phenotypes compared to their heterozygous counterparts, confirming the diabetogenic nature of the GCK-Q26Lmutation. Further analyses indicated that GCK-Q26L did not affect insulin sensitivity and islet insulin content. However, GCK-Q26L blunted islet responsiveness to different glucose concentrations and markedly impaired glucose-stimulated insulin secretion (GSIS) without affecting potassium chloride-stimulated insulin secretion (KSIS) and glucose inhibitory effects on glucagon secretion. Both GKA and GLP-1RA enhanced insulin secretion and improved glucose tolerance in mutant mice. Conclusions/Interpretation: This study demonstrates that GCK-Q26L is a GCK-MODY causing mutation. The interplay of GCK-Q26L with a high PRS for insulin resistance contributes to severe diabetic phenotypes. The findings not only expends the list of GCK-MODY causing mutations originally classified as VUS mutations, but also provides insights into interactions of GCK-MODY with polygenic risks of type 2 diabetes, highlighting the importance of considering polygenic backgrounds in the assessment and management of monogenic diabetes. Keywords: monogenic diabetes, glucokinase, GCK-MODY, insulin resistance, polygenic risk score (PRS)