Continued lessons from the INS gene: An intronic mutation causing diabetes through a novel mechanism

Background Diabetes in neonates usually has a monogenic aetiology; however, the cause remains unknown in 20-30%. Heterozygous INS mutations represent one of the most common gene causes of neonatal diabetes mellitus. Methods Clinical and functional characterisation of a novel homozygous intronic mutation (c.187+241G>A) in the insulin gene in a child identified through the Monogenic Diabetes Registry (http://monogenicdiabetes. uchicago.edu). Results The proband had insulin-requiring diabetes from birth. Ultrasonography revealed a structurally normal pancreas and C-peptide was undetectable despite readily detectable amylin, suggesting the presence of dysfunctional ß cells. Whole-exome sequencing revealed the novel mutation. In silico analysis predicted a mutant mRNA product resulting from preferential recognition of a newly created splice site. Wild-type and mutant human insulin gene constructs were derived and transiently expressed in INS-1 cells. We confirmed the predicted transcript and found an additional transcript created via an ectopic splice acceptor site. Conclusions Dominant INS mutations cause diabetes via a mutated translational product causing endoplasmic reticulum stress. We describe a novel mechanism of diabetes, without ß cell death, due to creation of two unstable mutant transcripts predicted to undergo nonsense and non-stop-mediated decay, respectively. Our discovery may have broader implications for those with insulin deficiency later in life

Microcephaly, epilepsy, and neonatal diabetes due to compound heterozygous mutations in IER3IP1: Insights into the natural history of a rare disorder

Neonatal diabetes mellitus is known to have over 20 different monogenic causes. A syndrome of permanent neonatal diabetes along with primary microcephaly with simplified gyral pattern associated with severe infantile epileptic encephalopathy was recently described in two independent reports in which disease-causing homozygous mutations were identified in the immediate early response-3 interacting protein-1 (IER3IP1) gene. We report here an affected male born to a non-consanguineous couple who was noted to have insulin-requiring permanent neonatal diabetes, microcephaly, and generalized seizures. He was also found to have cortical blindness, severe developmental delay and numerous dysmorphic features. He experienced a slow improvement but not abrogation of seizure frequency and severity on numerous anti-epileptic agents. His clinical course was further complicated by recurrent respiratory tract infections and he died at 8years of age. Whole exome sequencing was performed on DNA from the proband and parents. He was found to be a compound heterozygote with two different mutations in IER3IP1: p.Val21Gly (V21G) and a novel frameshift mutation p.Phe27fsSer*25. IER3IP1 is a highly conserved protein with marked expression in the cerebral cortex and in beta cells. This is the first reported case of compound heterozygous mutations within IER3IP1 resulting in neonatal diabetes. The triad of microcephaly, generalized seizures, and permanent neonatal diabetes should prompt screening for mutations in IER3IP1. As mutations in genes such as NEUROD1 and PTF1A could cause a similar phenotype, next-generation sequencing approaches-such as exome sequencing reported here-may be an efficient means of uncovering a diagnosis in future cases

Deletion of protein kinase D1 in pancreatic β-cells impairs insulin secretion in high-fat diet-fed mice

Bβ-cell adaptation to insulin resistance is necessary to maintain glucose homeostasis in obesity. Failure of this mechanism is a hallmark of type 2 diabetes (T2D). Hence, factors controlling functional β-cell compensation are potentially important targets for the treatment of T2D. Protein kinase D1 (PKD1) integrates diverse signals in the β-cell and plays a critical role in the control of insulin secretion. However, the role of β-cell PKD1 in glucose homeostasis in vivo is essentially unknown. Using β-cell specific, inducible PKD1 knock-out mice (βPKD1KO), we examined the role of beta-cell PKD1 under basal conditions and during high-fat feeding. βPKD1KO mice under chow diet presented no significant difference in glucose tolerance or insulin secretion compared to mice expressing the Cre transgene alone; however, when compared to wild-type mice, both groups developed glucose intolerance. Under high-fat diet, deletion of PKD1 in β-cells worsened hyperglycemia, hyperinsulinemia and glucose intolerance. This was accompanied by impaired glucose-induced insulin secretion both in vivo in hyperglycemic clamps and ex vivo in isolated islets from high-fat fed βPKD1KO mice, without changes in islet mass. This study demonstrates an essential role for PKD1 in the β-cell adaptive secretory response to high-fat feeding in mice

A Model of Action Potentials and Fast Ca2+ Dynamics in Pancreatic β-Cells

We examined the ionic mechanisms mediating depolarization-induced spike activity in pancreatic β-cells. We formulated a Hodgkin-Huxley-type ionic model for the action potential (AP) in these cells based on voltage- and current-clamp results together with measurements of Ca2+ dynamics in wild-type and Kv2.1 null mouse islets. The model contains an L-type Ca2+ current, a “rapid” delayed-rectifier K+ current, a small slowly-activated K+ current, a Ca2+-activated K+ current, an ATP-sensitive K+ current, a plasma membrane calcium-pump current and a Na+ background current. This model, coupled with an equation describing intracellular Ca2+ homeostasis, replicates β-cell AP and Ca2+ changes during one glucose-induced spontaneous spike, the effects of blocking K+ currents with different inhibitors, and specific complex spike in mouse islets lacking Kv2.1 channels. The currents with voltage-independent gating variables can also be responsible for burst behavior. Original features of this model include new equations for L-type Ca2+ current, assessment of the role of rapid delayed-rectifier K+ current, and Ca2+-activated K+ currents, demonstrating the important roles of the Ca2+-pump and background currents in the APs and bursts. This model provides acceptable fits to voltage-clamp, AP, and Ca2+ concentration data based on in silico analysis