Molecular genetic testing of patients with monogenic diabetes and hyperinsulinism

Abstract

Genetic sequencing has become a critical part of the diagnosis of certain forms of pancreatic beta cell dysfunction. Despite great advances in the speed and cost of DNA sequencing, determining the pathogenicity of variants remains a challenge, and requires sharing of sequence and phenotypic data between laboratories. We reviewed all diabetes and hyperinsulinism-associated molecular testing done at the Seattle Children's Molecular Genetics Laboratory from 2009 to 2013. 331 probands were referred to us for molecular genetic sequencing for Neonatal Diabetes (NDM), Maturity-Onset Diabetes of the Young (MODY), or Congenital Hyperinsulinism (CHI) during this period. Reportable variants were identified in 115 (35%) patients with 91 variants in one of 6 genes: HNF1A, GCK, HNF4A, ABCC8, KCNJ11, or INS. In addition to identifying 23 novel variants, we identified unusual mechanisms of inheritance, including mosaic and digenic MODY presentations. Re-analysis of all reported variants using more recently available databases led to a change in variant interpretation from the original report in 30% of cases. These results represent a resource for molecular testing of monogenic forms of diabetes and hyperinsulinism, providing a mutation spectrum for these disorders in a large North American cohort. In addition, they highlight the importance of periodic review of molecular testing results.

Keywords: Hyperinsulinism; MODY; Mosaicism; Mutation spectrum; Neonatal diabetes; Variant classification.

Fig. 1.

Recurrence of GCK-related MODY due to mosaicism. The pedigree of the family is shown. Genotypes based on Sanger sequencing shown below each individual, with N representing reference. Filled-in symbols represent individuals with MODY. Restriction-fragment based genotypes are shown in the gel image below the pedigree. The mutant allele is represented by the lower, 494 bp band (arrowhead), while the reference allele is represented by the upper, 539 bp band. The faint presence of the mutant band from blood-derived DNA from the father indicates evidence of mosaicism for this mutation.

Fig. 2.

Coinheritance of HNF1A and GCK pathogenic variants. The pedigree of the family is shown. Filled-in symbols represent individuals with MODY. Results of Sanger sequencing are shown below each individual, with N representing reference.

Fig. 3.

Variant re-classification. A total of 99 variants were originally reported out as either VUS or pathogenic. 30% (30/99 variants) changed classification during the 4-year review period. The types of classification changes are indicated in the legend, with number of variants within each class indicated in parentheses. “LB” is likely benign and “LP” is likely pathogenic.