Insulin gene mutations as a cause of permanent neonatal diabetes

Abstract

We report 10 heterozygous mutations in the human insulin gene in 16 probands with neonatal diabetes. A combination of linkage and a candidate gene approach in a family with four diabetic members led to the identification of the initial INS gene mutation. The mutations are inherited in an autosomal dominant manner in this and two other small families whereas the mutations in the other 13 patients are de novo. Diabetes presented in probands at a median age of 9 weeks, usually with diabetic ketoacidosis or marked hyperglycemia, was not associated with beta cell autoantibodies, and was treated from diagnosis with insulin. The mutations are in critical regions of the preproinsulin molecule, and we predict that they prevent normal folding and progression of proinsulin in the insulin secretory pathway. The abnormally folded proinsulin molecule may induce the unfolded protein response and undergo degradation in the endoplasmic reticulum, leading to severe endoplasmic reticulum stress and potentially beta cell death by apoptosis. This process has been described in both the Akita and Munich mouse models that have dominant-acting missense mutations in the Ins2 gene, leading to loss of beta cell function and mass. One of the human mutations we report here is identical to that in the Akita mouse. The identification of insulin mutations as a cause of neonatal diabetes will facilitate the diagnosis and possibly, in time, treatment of this disorder.

Fig. 1.

Segregation of INS mutations in 16 families. Squares represent male family members, and circles represent female members. Black circles and squares represent persons with diabetes. A slash mark through the square or circle indicates deceased. A slash mark through the branch indicates the couple has divorced. The allele status is indicated under the symbols: N/N, two normal alleles; amino acid change/N, one mutated and one normal allele; N/A, not available for testing. The arrow indicates the proband. Family relationship was confirmed in all families by microsatellite analysis, except in UC2 (parents not available for testing) and UC105.

Fig. 2.

Diagram representing the human preproinsulin molecule showing location of mutations causing ND. The amino acid residues in the signal peptide are indicated in green, the B chain in red, the C-peptide in orange, and the A chain in blue. The dashed circles indicate the basic residues that are the cleavage site for conversion from proinsulin to insulin. The mutations are noted in black circles together with location in the B or A chain.

Fig. 3.

Predicted effect of the substitution of GlyB8 with Ser on folding of the insulin molecule. (Left) Normal conformation of the mature insulin molecule highlighting residues A7, B7, and B8 (shown as space-filling models in both images). (Right) The effect of substitution at B8 with l-serine, which induces a major conformational change resulting in reorientation of residues B1–B8, so as to prevent B7 cysteine from interacting with its intended partner at A7 during folding (see Discussion).