Mutations in the insulin gene
Insulin plays a key role in the regulation of glucose homeostasis with diabetes resulting from insulin deficiency, whether complete deficiency as in type 1 diabetes or relative deficiency as in type 2 diabetes. Insulin, which is secreted from the pancreatic beta cells in a tightly regulated manner to maintain plasma glucose within a narrow range, is synthesized as the prohormone, preproinsulin. Post-translational proteolytic processing generates the mature two-chain insulin molecule. Mutations in the insulin gene cause disorders of glucose homeostasis through effects of the mutant insulin on beta cell function, insulin receptor affinity, or processing of proinsulin to insulin. The effects of the mutations on glucose homeostasis are variable with associated phenotypes ranging from permanent neonatal diabetes with complete insulin deficiency to near-normal glucose homeostasis. Maturity-onset diabetes of the young (MODY) and type 1b diabetes mellitus are other clinical manifestations of heterozygous insulin gene mutations.
Insulin is the major biosynthetic and secretory product of the beta cell accounting for 50% or more of total protein synthesis when maximally stimulated corresponding to 1.3 x 106 molecules of insulin per minute . The initial product of translation of human insulin mRNA is the single chain prohormone preproinsulin. The signal peptide of preproinsulin interacts with the signal recognition particle in the beta cell cytosol which targets proinsulin to the endoplasmic reticulum (ER). The signal peptide is rapidly cleaved and degraded
Figure 1: (Click to enlarge)Diagrammatic representation of the amino acid sequence of human preproinsulin (signal peptide–green, B-chain–red, C-peptide–orange, A-chain–dark blue) indicating sites of mutations identified in patients with diabetes as well as hyperinsulinemia and hyperproinsulinemia. Mutations shown in black are associated with permanent neonatal diabetes mellitus (PNDM) and type 1b diabetes; mutations in light blue are associated with hyperinsulinaemia; mutations in light green are associated with hyperproinsulinaemia; mutations in purple are associated with maturity-onset diabetes of the young (MODY) and type 1b diabetesand proinsulin then folds in the lumen of the ER forming the characteristic disulfide bonds of the insulin molecule. In proinsulin, the C-peptide bridges the B-chain and the A-chain via two pairs of basic amino acids. In mature insulin, the C-peptide and the four basic residues have been excised and the B- and A-chains are connected covalently only by the two disulfide bonds (figure 1). Upon folding in the ER, proinsulin progresses to the Golgi compartments. Proinsulin is concentrated and sorted into immature secretory granules in the transGolgi network. The processing of proinsulin to insulin and C-peptide takes place mainly in the maturing secretory granules by cleavage sequentially at the B-chain-C-peptide junction and then at the C-peptide-A-chain junction yielding insulin and C-peptide. Insulin and C-peptide are stored in the mature secretory granules and secreted in equimolar amounts .
As described elsewhere (i.e. section on permanent neonatal diabetes) heterozygous insulin gene mutations are associated with permanent neonatal diabetes (PNDM) due to synthesis of structural abnormal preproinsulin or proinsulin. The mutations associated with PNDM are located in the signal peptide, the B-chain and A-chain regions and in the pairs of basic residues that flank the C-peptide. Those located in the signal peptide impair or abolish cleavage of the signal peptide. The mutations in proinsulin affect disulfide bond formation thereby leading to a misfolded proinsulin molecule that is retained in the ER impairing normal beta-cell function and resulting in cell death . A subgroup of mutations is associated with diabetes diagnosed outside the neonatal period/infancy and is associated with varying degrees of insulin deficiency due to beta cell impairment . Finally, a third group of mutations in the insulin gene causes mild diabetes or impaired glucose tolerance by another mechanism. These mutations lead to the synthesis of an insulin with reduced biological potency or impair processing of proinsulin to insulin leading to hyperinsulinemia or hyperproinsulinemia, respectively . In contrast to the insulin gene mutations associated with PNDM, the beta cells are able to synthesize and secrete these molecules.
Insulin gene mutations and diabetes diagnosed outside the neonatal period and infancy
Manifest diabetes diagnosed outside the neonatal period and infancy due to heterozygous mutations in the insulin gene can occasionally be found in patients diagnosed with non-autoimmune type 1 diabetes (type 1b diabetes mellitus) or maturity-onset diabetes of the young (MODY).
Heterozygous mutations in the insulin gene in patients with a diagnosis of type 1b diabetes are rare with less than 1 % of patients with a disease-causing mutation in the insulin gene. These patients are treated with insulin and some of the patients are prone to develop diabetic ketoacidosis. The patients carry mutations that are typically associated with PNDM, but for reasons that are incompletely understood the diabetes presents later in life. Some of the patients in this group carry mutations that are typically associated with the milder phenotype of MODY.
When to suspect a mutation in the insulin gene in a patient with type 1b diabetes:
- Positive family history of diabetes with similar phenotype although de novo mutations are common
- Undetectable pancreatic auto-antibodies at diagnosis of diabetes
- No extra-pancreatic features (late-diabetic complications may develop)
- Patients typically require insulin in replacement doses to achieve satisfactory glycemic control
Mutations in the insulin gene can be found in about 1 % of patients with MODY. These patients are picked up on routine MODY-screening that is performed due to a classical MODY-phenotype and a positive family history of diabetes. These patients harbor a distinct group of mutations that are not associated with PNDM. The patients achieve a satisfactory glycemic control on a variety of treatment regimens ranging from lifestyle interventions to insulin-based regimens.
When to suspect a mutation in the insulin gene in a patient with MODY:
- Clinical features as described in the MODY section
- Positive family history of diabetes with similar phenotype
- Undetectable pancreatic auto-antibodies at diagnosis of diabetes
- Detectable or low-levels of C-peptide
- No extra-pancreatic features ( late-diabetic complications may develop)
- Variable treatment requirements (from lifestyle interventions to low-dose insulin treatment)
- Mutations in common MODY genes have been ruled out by sequencing
The mutations associated with type 1b diabetes and MODY are located in the signal peptide, B-chain and at the pairs of basic residues that flank the C-peptide of preproinsulin. These mutations are also predicted to affect signal peptide function or folding of proinsulin leading to impaired beta-cell function.
Currently there are no clinical studies to suggest how to treat these patients best. The aims of treating the patients are to maintain near-normal glycemia to avoid the development of late-diabetic complications, but another aim could be to preserve beta cell function to sustain the endogenous insulin synthesis. This could, hypothetically, be achieved through administration of exogenous insulin to suppress the endogenous insulin synthesis thereby limiting the toxic effects on the beta cells of the mutant insulin protein.
Four rare heterozygous missense mutations in the insulin gene have been identified in patients with a disorder characterized by hyperproinsulinemia and variable degrees of glucose intolerance. Overt diabetes can develop in insulin resistant individuals, but it is typically mild and is manageable with lifestyle interventions alone. The mutations affect the processing of proinsulin to insulin and C-peptide due to their location at the residues involved in cleavage of the C-peptide from the A-chain. The patients have high levels in the circulation of proinsulin-like material that has been only partially cleaved. One of the mutations (H34D) causes hyperproinsulinemia due to secretion of unprocessed proinsulin in a glucose-independent manner via an unregulated pathway and enters the circulation as proinsulin. The reason for the missorting of this proinsulin is unclear but it may relate to higher biological potency of the variant proinsulin.
Three rare missense mutations have been identified in individuals with variable glucose tolerance and concomitant hyperinsulinemia. These mutations affect amino acids residues involved in binding of insulin to its receptor and as a consequence the mutant insulin has reduced insulin receptor affinity and biological activity. Mutation carriers have hyperinsulinemia, but impaired glucose tolerance or overt diabetes only develops in adults with insulin resistance. When diabetes develops, it is mild and satisfactory glycemic control can be achieved with lifestyle interventions, oral hypoglycemic agents or small doses of insulin.
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^ Steiner DF et al. 2009 A brief perspective on insulin production. Diabetes Obes Metab 11 Suppl 4:189-196
^ Støy J et al. 2007 Insulin gene mutations as a cause of permanent neonatal diabetes. Proc Natl Acad Sci U S A 104:15040-15044
^ Colombo C et al. Diabetes 2008 Seven mutations in the human insulin gene linked to permanent neonatal/infancy-onset diabetes mellitus. J Clin Invest 118:2148-2156
^ Edghill EL et al. 2008 Insulin mutation screening in 1,044 patients with diabetes: mutations in the INS gene are a common cause of neonatal diabetes but a rare cause of diabetes diagnosed in childhood or adulthood. Diabetes 57:1034-1042
^ Molven A et al. 2008 Mutations in the insulin gene can cause MODY and autoantibody-negative type 1 diabetes. Diabetes 57:1131-1135
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^ Steiner DF et al. 1995 Familial syndromes of hyperproinsulinemia and hyperinsulinemia with mild diabetes. In: Scriver CR, Beaudet, A.L., Sly, A.S., Valle, D. ed. The metabolic and molecular bases of inherited disease: McGraw-Hill; 897-904