Background and aims: Neonatal diabetes diagnosed before 6 months is caused by mutations that reduce. β cell number (reduced formation or increased destruction) or impair β cell function. We investigated the genetic cause of a syndrome characterised by neonatal diabetes, microcephaly and epilepsy.
Materials and methods: We performed whole genome sequencing for two unrelated patients with neonatal diabetes, epilepsy and microcephaly. Replication studies were performed in 394 patients with neonatal diabetes, using a targeted next generation sequencing assay. YIPF5 was silenced in the human β cell line EndoC-βH1 using RNA interference. Cells were exposed to the endoplasmic reticulum (ER) stressors thapsigargin and brefeldin A. Apoptosis was evaluated by staining with DNA-binding dyes or real-time annexin V binding assay. mRNA expression was assessed by qPCR and in situ hybridization.
Results: The two patients were of Turkish and Indian origin, both born to consanguineous parents. They were diagnosed with diabetes at the age of 9 and 15 weeks and were treated with insulin. Genetic testing revealed homozygous likely deleterious mutations (missense and in-frame deletion) in YIPF5. Replication studies identified 2 homozygous YIPF5 mutations in 3 patients (2 siblings) with insulin-treated diabetes diagnosed below age 12 months. All patients had epilepsy and microcephaly. Based on the patients consistent phenotype, we examined YIPF5 mRNA expression. YIPF5 was abundantly expressed in human pancreatic islets and brain. In situ hybridization in foetal brain showed ubiquitous expression of YIPF5 throughout the developing brain, in neurons, progenitor cells and the choroid plexus. We next examined the impact of YIPF5 depletion in β cells. As YIPF5 is thought to play a role in trafficking between ER and Golgi compartments, we examined β cell survival during ER stress, a stress response activated by the accumulation of proteins in the ER. YIPF5 silencing did not affect basal β cell survival, but it increased apoptosis in ER stress conditions. When exposed to chemical ER stressors, YIPF5-depleted cells showed increased expression of the ER stress markers CHOP, sXBP1 and BiP, indicating an exaggerated activation of the ER stress response. YIPF5 silencing enhanced the expression of the pro-apoptotic proteins PUMA and DP5.Silencing the pro-apoptotic transcription factor CHOP protected YIPF5-depleted cells from apoptosis.
Conclusions: We identified homozygous YIPF5 mutations as a novel cause of neonatal diabetes associated with microcephaly and epilepsy. YIPF5 was abundantly expressed in islets and brain. Functional studies show that YIPF5 deficiency reduces human β cell survival. This is the first form of neonatal diabetes caused by dysregulated ER-to-Golgi trafficking, resulting in increased β cell ER stress and apoptosis.
19 - 20 Oct 2018
Belgian Endocrine Society