Composite Digenic Diabetes Linked to Heterozygous Variants of GCK and NEUROD1: A Case Report

We wanted to establish the etiologic cause of diabetes in a female subject with mild hyperglycemia since childhood, that suddenly worsened in her late 40s. We retrieved the proband's laboratory data from the age of 5 years. We assessed type 1 diabetes autoantibodies and performed genetic screening by clinical exome.

The proband showed stable hyperglycemia not requiring pharmacological therapy for 42 years. The proband's fasting plasma glucose increased from 120 to 130 mg/dL (6.1-7.2 mmol/L) to 150-159 mg/dL (8.3-8.8 mmol/L) at the age of 47 years. Four type 1 diabetes autoantibodies resulted repeatedly negative. A spontaneous glucokinase pathogenic variant (c.645C>A, p. Tyr215Ter) and a NEUROD1 variant (c.616dupC, p.His206ProfsTer38) were identified in the proband. Her mother, who carries the NEUROD1 variant, was diagnosed with diabetes by OGTT (120' = 211 mg/dL) when 77 years old. NEUROD1, a low penetrance maturity onset diabetes of the young (MODY) gene, is known to regulate gene transcription of GCK and SLC2A2, encoding for GLUT2, a functional partner of GCK in glucose sensing of the β cell.

We conclude that the low penetrance NEUROD1 variant is likely responsible of the peculiar trajectory of fasting glucose in a subject who presented with classical metabolic phenotype associated with glucokinase haploinsufficiency from childhood to adulthood.