Abstract
Congenital hyperinsulinism is the most common cause of persistent hypoglycemia in early infancy. Mutations in the gene for heterozygous hepatocyte nuclear transcription factor 4-alpha (HNF4A) account for approximately 5% of cases and are inherited in an autosomal dominant fashion or arise as de novo mutations. This case describes a unique presentation of parental gonadal, or germline, mosaicism as the suspected inheritance pattern for siblings with congenital hyperinsulinism caused by HNF4A mutations. Two siblings presented with hypoglycemia in the first hours of life and were subsequently confirmed to have hyperinsulinism. In each patient, glycemic control was achieved at relatively low doses of diazoxide. Both siblings tested positive for the same HNF4A mutation, whereas the parents tested negative for HNF4A mutations. Gonadal, or germline, mosaicism became the presumed leading diagnosis, given 2 unaffected parents with 2 children with congenital hyperinsulinism. The older sibling demonstrated additional clinical features of liver disease and renal Fanconi syndrome, both of which are associated with HNF4A mutations. Genetic testing plays an important role in the diagnosis and management of congenital hyperinsulinism. HNF4A mutations may arise by a range of mechanisms, including gonadal, or germline, mosaicism. HNF4A mutations have phenotypic variance that may affect multiple organ systems at any age.