1750-P: Differential Impacts of Two Novel HNF1A Variants Associated with Familial Young-Onset Diabetes on Protein Function and Insulin Secretion In Vitro

Introduction: Mutations in the HNF1A are a cause of monogenic diabetes and are usually dominantly inherited. We report in vitro and clinical studies on two novel HNF1A variants to determine pathogenicity: a recessive (homozygous) HNF1A variant (p.A251T) and a dominant HNF1A variant (p.S19L). We also investigated the mechanisms by which the former variant affects insulin secretion using beta cells derived from induced pluripotent stem cells (iPSCs).

Methods: Nuclear localization, DNA binding, and transactivation activity were examined using standard techniques. iPSC lines from proband carrying the p.A251T variant were differentiated into beta-like cells according to Sui et al, CPHG, 2018. Ca²⁺ dynamics were monitored with Cal-520, and insulin secretion was measured by homogeneous time-resolved fluorescence assay compared to wild-type HNF1A (WT).

Results: HNF1A p.A251T showed a mild (10.4% ± 7.3, p= 0.16) decrease in transactivation activity compared with WT, and decreased (48% of WT, p=0.12) DNA binding. These reductions were significant in p.S19L (DNA binding 57% of WT, p=0.049). Significant impaired nuclear uptake (63% vs 90% cells for WT, p<0.0001) was only apparent for p.S19L. Glucose-stimulated insulin secretion was decreased in A251T iPSCs compared with WT (0.29% vs. 0.46% of total insulin). In contrast, glucose-induced Ca²⁺ increases were increased in mutant cells (arbitrary units: 7.39 ± 1.23, mean +/− SD, n=725 cells vs. 4.58 ± 0.77, n=724, p= < 0.001)

Conclusion: We demonstrate the value of combining multiple in vitro  functional studies to decipher novel HNF1A variant pathogenicity. The recessive p.A251T variant has relatively mild effects on protein function compared to a dominant p.S19L variant. However, when studying proband iPSC derived beta-cell A251T impairs glucose-stimulated insulin secretion despite preserved Ca²⁺ entry, suggesting a novel mechanism of action not captured by standard tests of HNF1A function.