Insights into the pathogenicity of rare missense GCK variants from the identification and functional characterization of compound heterozygous and double mutations inherited in cis

Abstract

Objective: To demonstrate the importance of using a combined genetic and functional approach to correctly interpret a genetic test for monogenic diabetes.

Research design and methods: We identified three probands with a phenotype consistent with maturity-onset diabetes of the young (MODY) subtype GCK-MODY, in whom two potential pathogenic mutations were identified: [R43H/G68D], [E248 K/I225M], or [G261R/D217N]. Allele-specific PCR and cosegregation were used to determine phase. Single and double mutations were kinetically characterized.

Results: The mutations occurred in cis (double mutants) in two probands and in trans in one proband. Functional studies of all double mutants revealed inactivating kinetics. The previously reported GCK-MODY mutations R43H and G68D were inherited from an affected father and unaffected mother, respectively. Both our functional and genetic studies support R43H as the cause of GCK-MODY and G68D as a neutral rare variant.

Conclusions: These data highlight the need for family/functional studies, even for previously reported pathogenic mutations.

Figure 1

Pedigrees for the three probands. Squares and circles denote males and females, respectively. Filled symbols represent hyperglycemic individuals, and open symbols correspond to those with normoglycemia. Fasting plasma glucose values and mutation status for each family member are given underneath the appropriate symbol. An arrow indicates the proband in each family. N, no mutation; N/A, data not available.