Seven mutations in the human insulin gene linked to permanent neonatal/infancy-onset diabetes mellitus
Carlo Colombo, … , Fabrizio Barbetti, the Early Onset Diabetes Study Group of the Italian Society of Pediatric Endocrinology and Diabetes (SIEDP)
Carlo Colombo, … , Fabrizio Barbetti, the Early Onset Diabetes Study Group of the Italian Society of Pediatric Endocrinology and Diabetes (SIEDP)
Published May 1, 2008
Citation Information: J Clin Invest. 2008;118(6):2148-2156. https://doi.org/10.1172/JCI33777.
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Seven mutations in the human insulin gene linked to permanent neonatal/infancy-onset diabetes mellitus

Abstract

Permanent neonatal diabetes mellitus (PNDM) is a rare disorder usually presenting within 6 months of birth. Although several genes have been linked to this disorder, in almost half the cases documented in Italy, the genetic cause remains unknown. Because the Akita mouse bearing a mutation in the Ins2 gene exhibits PNDM associated with pancreatic β cell apoptosis, we sequenced the human insulin gene in PNDM subjects with unidentified mutations. We discovered 7 heterozygous mutations in 10 unrelated probands. In 8 of these patients, insulin secretion was detectable at diabetes onset, but rapidly declined over time. When these mutant proinsulins were expressed in HEK293 cells, we observed defects in insulin protein folding and secretion. In these experiments, expression of the mutant proinsulins was also associated with increased Grp78 protein expression and XBP1 mRNA splicing, 2 markers of endoplasmic reticulum stress, and with increased apoptosis. Similarly transfected INS-1E insulinoma cells had diminished viability compared with those expressing WT proinsulin. In conclusion, we find that mutations in the insulin gene that promote proinsulin misfolding may cause PNDM.

Authors

Carlo Colombo, Ottavia Porzio, Ming Liu, Ornella Massa, Mario Vasta, Silvana Salardi, Luciano Beccaria, Carla Monciotti, Sonia Toni, Oluf Pedersen, Torben Hansen, Luca Federici, Roberta Pesavento, Francesco Cadario, Giorgio Federici, Paolo Ghirri, Peter Arvan, Dario Iafusco, Fabrizio Barbetti

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Figure 3

Secondary structure computer modeling of WT and mutant proinsulins.

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Secondary structure computer modeling of WT and mutant proinsulins. Show...
Shown are superimposed Cα traces of WT (blue) and all mutated insulins (purple), with the exception of LB6V. The positions of disulfide bridges are also marked. None of the mutations caused substantial distortion of the secondary structure, with the exception of LB15YB16delinsH (yellow trace at left), where an α-helical disruption is apparent (arrow). This was also evident in the superimposition of WT insulin and LB15YB16delinsH. The LB6P mutation alters the hydrophobic core of the protein. The LB11P mutation affects the hydrophobic core of the protein. The CA6Y mutation disrupts the A6–A11 disulfide bridge; the tyrosine is oriented inside the hydrophobic core of the protein, where it engages LB6 in a stacking interaction. The hAkita mutation — used in this study as positive control — disrupts the B7–A7 disulfide bridge, and the tyrosine is solvent exposed.