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Precision medicine starts with a precision diagnosis. Yet up to 80% of cases of monogenic diabetes, a form of diabetes characterized by mutations in a single gene, are either overlooked or misdiagnosed. A genetic test for monogenic diabetes does not always lead to a precise diagnosis, as novel variants are often classified as variants of unknown significance. Variant interpretation requires collation of a framework of evidence, including population, computational, and segregation data, and can be assisted by functional analysis. The inclusion of functional data can be challenging, depending on the number of benign and pathogenic variants available for benchmarking assays. Glucokinase is the rate-limiting step for glucose metabolism in the pancreatic beta-cell and governs the threshold for glucose-stimulated insulin release. Loss-of-function alleles in the glucokinase (GCK) gene are a cause of stable fasting hyperglycemia from birth and/or diabetes. In this study, we functionally characterized 25 variants identified during diagnostic testing or in exome sequencing studies. We assessed their kinetic characteristics, stability, and interaction with pharmacological and physiological regulators. We integrated our functional data with existing data from the ClinGen Monogenic Diabetes Variant Curation Expert Review panel using a gene-specific framework to assist variant classification. We show how functional evidence can aid variant classification, thus enabling diagnostic certainty.

We report a 9-year-old female who presented with mild, persistent hyperglycemia, with hemoglobin A1c and oral glucose tolerance test results in the prediabetes range, relatively preserved C-peptide, and mildly positive glutamate decarboxylase 65 autoantibody levels. A strong family history of lean, noninsulin-dependent diabetes suggested a possible monogenic cause. Genetic testing with a 28-gene maturity-onset diabetes of the young (MODY) panel identified a heterozygous variant of uncertain significance in GCK (NM_000162.5:c.179C>T [p.Thr60Ile]). Following review by the ClinGen Monogenic Diabetes Variant Curation Expert Panel, the GCK variant was reclassified as pathogenic, confirming a diagnosis of glucokinase (GCK)-MODY/GCK-hyperglycemia, a stable form of mild hyperglycemia that does not require pharmacologic treatment. Subsequent genetic testing of the father confirmed paternal inheritance. Due to the presence of a positive islet autoantibody, she was counseled on the potential for autoimmune diabetes and advised to continue periodic monitoring. This case illustrates the diagnostic complexity of differentiating monogenic hyperglycemia from autoimmune diabetes in children.