Congenital hyperinsulinism due to mutations in HNF1A.

Congenital hyperinsulinism is a rare but significant cause of severe and persistent hypoglycaemia in infancy. Although a biphasic phenotype of congenital hyperinsulinism in infancy followed by Maturity-Onset Diabetes of the Young (MODY) in later life has been established for HNF4A, the existence of a similar phenotype for a related MODY gene, HNF1A, is less clear. We describe two cases of congenital hyperinsulinism in association with dominantly inherited variants in HNF1A. They presented in the early neonatal period with unequivocal biochemical evidence of congenital hyperinsulinism and persistence into childhood with ongoing need for medical therapy. Both cases inherited HNF1A variants from a parent with a diabetes phenotype consistent with MODY, without obesity, insulin resistance or other metabolic syndrome features. In the first case, a paternally inherited novel c.-230_-101del variant was found that deletes the minimal promoter region presumably required for HNF1A expression. In the second case, a maternally inherited missense variant (c.713G>T, p.(Arg238Met)) was identified. This variant is predicted to cause haploinsufficiency via aberrant splicing and has previously been associated with MODY but not congenital hyperinsulinism. Our cases further strengthen the evidence for HNF1A as a CHI-causing gene requiring long-term follow-up.