Phenotypic severity of homozygous GCK mutations causing neonatal or childhood-onset diabetes is primarily mediated through effects on protein stability

Abstract

Mutations in glucokinase (GCK) cause a spectrum of glycemic disorders. Heterozygous loss-of-function mutations cause mild fasting hyperglycemia irrespective of mutation severity due to compensation from the unaffected allele. Conversely, homozygous loss-of-function mutations cause permanent neonatal diabetes requiring lifelong insulin treatment. This study aimed to determine the relationship between in vitro mutation severity and clinical phenotype in a large international case series of patients with homozygous GCK mutations. Clinical characteristics for 30 patients with diabetes due to homozygous GCK mutations (19 unique mutations, including 16 missense) were compiled and assigned a clinical severity grade (CSG) based on birth weight and age at diagnosis. The majority (28 of 30) of subjects were diagnosed before 9 months, with the remaining two at 9 and 15 years. These are the first two cases of a homozygous GCK mutation diagnosed outside infancy. Recombinant mutant GCK proteins were analyzed for kinetic and thermostability characteristics and assigned a relative activity index (RAI) or relative stability index (RSI) value. Six of 16 missense mutations exhibited severe kinetic defects (RAI ≤ 0.01). There was no correlation between CSG and RAI (r2 = 0.05, P = 0.39), indicating that kinetics alone did not explain the phenotype. Eighty percent of the remaining mutations showed reduced thermostability, the exceptions being the two later-onset mutations which exhibited increased thermostability. Comparison of CSG with RSI detected a highly significant correlation (r2 = 0.74, P = 0.002). We report the largest case series of homozygous GCK mutations to date and demonstrate that they can cause childhood-onset diabetes, with protein instability being the major determinant of mutation severity.

Association of a homozygous GCK missense mutation with mild diabetes

“…Homozygous inactivating mutations of GCK have been repeatedly reported to contribute to PNDM, with only two exceptions, described in 9 and 15 years old white French Canadians showing mild hyperglycemia totally superimposable to that observed in GCK‐related MODY conditions (Raimondo et al, ). We here describe a novel GCK missense mutation p.E372D that in an Italian family co‐segregates with mild impairment of glucose homeostasis not only in heterozygous patients but also in the only homozygous family member who was diagnosed as diabetic in the adult life.…”

Congenital Diabetes: Comprehensive Genetic Testing Allows for Improved Diagnosis and Treatment of Diabetes and Other Associated Features

“…Most cases will require lifelong insulin therapy, although partial responsiveness to repaglinide and the sulfonylurea glibenclamide have been reported [1]. Phenotypic heterogeneity has been described across recessive mutations, including atypical features such as childhood-onset diabetes, with protein instability playing the largest role in predicted severity [35]. In the heterozygous state, GCK mutations cause stable, mildly elevated fasting blood glucose levels without diabetes-related complications (GCK-MODY, [36, 37]).…”

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

“…Several hundred GCK mutations have been identified to cause diabetes. 44,45 Enzyme kinetics studies of those mutant GCK proteins showed increased S 0.5 for glucose when the mutations were located in glucose-binding sites and increased K m for ATP for ATP-binding site mutations. Some mutations led to unstable GCK proteins, while some mutations only showed decreased K cat .…”