IER3IP1 Mutations Cause Neonatal Diabetes Due to Impaired Proinsulin Trafficking

Abstract

Immediate early response 3 interacting protein 1 (IER3IP1) is an endoplasmic reticulum (ER) resident protein, highly expressed in pancreatic cells and the developing brain cortex. Homozygous mutations in IER3IP1 have been found in individuals with microcephaly and neonatal diabetes, yet the underlying mechanism causing b-cell failure remains unclear. Here, we used differentiation of genomeedited stem cells into pancreatic islet cells to elucidate the molecular basis of IER3IP1 neonatal diabetes. Using CRISPR/Cas9 we generated two distinct IER3IP1 mutant human embryonic stem cell lines: a homozygous knock-in model of a patient mutation (IER3IP1V21G), and a knockout (KO) model (IER3IP12/2). While these mutant stem cell lines differentiated normally into definitive endoderm and pancreatic progenitors, we observed that IER3IP1-KO stem cell-derived islets (SC-islets) presented a significant decrease in b-cell numbers and elevated ER stress. Retention using selective hooks assay revealed a threefold reduction in ER-to-Golgi trafficking of proinsulin in IER3IP1 mutant b-cells. Additionally, IER3IP1 mutant SCislets implanted into immunocompromised mice displayed defective human insulin secretion, indicating the deleterious impact of IER3IP1 mutations on b-cell function. Our study provides valuable insights into the role of IER3IP1 in human b-cell biology and establishes a useful model to investigate ER-to-Golgi trafficking defects within b-cells.