Objective: A 5 month old, premature male infant with history of hydrops fetalis and hernia repair presented unresponsive in respiratory distress requiring intubation and was found to have pH 6.93, bicarbonate 6 mmol/L, anion gap 23 mmol/L, glucose 669 mg/dL and HbA1c 14.2%. His condition improved with insulin drip, fluid and electrolyte management. Work-up revealed negative autoantibodies and the University of Chicago Neonatal Diabetes Sequencing Panel was performed. Testing revealed a heterozygous sequence variant of unknown significance in the INS gene (c.289A>C), resulting in the amino acid change p.T97P. In silico pathogenicity prediction tools provided contradictory results. Neither parent carried the variant. INS gene mutations have been associated with Mutant INS-gene-induced Diabetes of Youth (MIDY) that includes permanent neonatal diabetes and MODY 10. Expression of mutant proinsulin is thought to lead to misfolded complexes in the endoplasmic reticulum (ER), leading to ER stress and β-cell death.
Methods: To determine the effects of this variant on insulin processing and release, we utilized a validated luminescent insulin secretion assay where Gaussia luciferase is inserted into the C-peptide portion of proinsulin. We compared luciferase secretion for wild type and variant INS sequences.
Results: The mutation decreased luciferase release ∼10-fold versus wild type. Proteasome inhibition led to partial rescue. However, overall expression of proinsulin-luciferase peptide was unchanged between the wild type and mutant. Therefore, it is likely that the mutation causes disruptions in processes downstream of transcription and translation, such as folding, proteolytic processing and transport.
Conclusions: The novel INS p.T97P is the first identified A-chain mutation resulting in MIDY not involving the creation or deletion of cysteine residues. The substitution of the polar threonine with nonpolar proline may be sufficient to disrupt nearby disulfide bonding in the proinsulin folding pathway.
R. Lal: Consultant; Self; Abbott, Biolinq. H.P. Moeller: None. R. Freeman: None. P. Prahalad: None. J.P. Annes: None.
National Institutes of Health (1K12DK122550, T32DK007217, R01DK101530, T32GM113854, P30DK116074)