Digenic HNF1A and ABCC8 variants provide mechanistic insight into early-onset diabetes

CONTEXT: Oligogenic inheritance in maturity-onset diabetes of the young (MODY) remains poorly characterized, and the contribution of multiple candidate variants to disease pathogenesis is incompletely understood. OBJECTIVE: To investigate the pathogenicity and mechanistic contribution of multiple MODY gene variants identified in a MODY-like family and determine their role in early-onset diabetes. METHODS: Comprehensive genetic analysis of known MODY genes was performed in a MODY-like family. Functional effects of HNF1A and HNF1B variants were assessed using luciferase reporter assays in HEK293T cells. Functional characterization of ABCC8 variants included Kir6.2-dependent thallium (Tl+) flux assays, sulfonylurea responsiveness, and channel stability. RESULTS: Four variants in three MODY genes were identified in the proband: novel p.Ser551Lysfs*2, HNF1B p.Glu102Ala, and ABCC8 p.Arg298Cys and p.Arg521Gln. Functional analysis showed that HNF1A p.Ser551Lysfs*2 retained approximately 5% of wild-type transactivation activity, consistent with loss-of-function, whereas HNF1B p.Glu102Ala and ABCC8 p.Arg521Gln exhibited wild-type-like function. In contrast, ABCC8 p.Arg298Cys reduced channel activity to 77% of wild-type levels while preserving sulfonylurea responsiveness. Segregation analysis identified HNF1A p.Ser551Lysfs*2 and ABCC8 p.Arg298Cys in affected parents. The proband, who inherited both pathogenic variants, developed diabetes earlier than either parent and was exposed to maternal hyperglycemia in utero, which may also have contributed to this early onset. CONCLUSIONS: Functional characterization distinguishes pathogenic from variants of unknown significance and supports digenic inheritance of HNF1A and ABCC8. Their additive effects, together with intrauterine hyperglycemia, likely accelerated disease onset. This study provides mechanistic evidence for oligogenic contributions to MODY and expands the genetic architecture of early-onset diabetes.