Hyperproinsulinemia

A number sign (#) is used with this entry because hyperproinsulinemia is caused by heterozygous mutation in the INS gene (176730) on chromosome 11p15.

Description

Insulin (INS; 176730) is produced posttranslationally from its precursor molecule, proinsulin, by site-directed proteolysis in beta-cell granules. Conversion involves cleavage at pairs of basic residues that link both the insulin A and B chains to C-peptide. Human proinsulin conversion has a preferred sequential route, such that cleavage at the B-chain/C-peptide junction occurs first, producing des-31,32 split proinsulin as the major conversion intermediate. Under normal circumstances, proinsulin conversion is largely completed before secretion, and low plasma levels of intact proinsulin and conversion intermediates are found. Structural abnormalities in the proinsulin molecule can impair conversion, leading to the accumulation of proinsulin-like material in the circulation. Such defects show an autosomal dominant mode of inheritance and are the main cause of familial hyperproinsulinemia (summary by Warren-Perry et al., 1997).

Clinical Features

Gabbay et al. (1976) described a family in which many persons in an autosomal dominant pattern had proinsulin or a proinsulin-like material as the major fraction of circulating insulin immunoreactivity in both the fasting and the stimulated state. The hyperproinsulinemia was asymptomatic, with no evident relation to hypoglycemia or development of diabetes mellitus. In light of the dominant inheritance, the authors thought a structural abnormality of proinsulin to be more likely than deficiency of the proinsulin cleaving enzyme (or enzymes).

Gabbay et al. (1979) presented data indicating that proinsulin in familial hyperproinsulinemia indeed has a structural abnormality such that cleavage at the B chain/C peptide site is impaired. Kanazawa et al. (1978) described the counterpart of the defect reported by Gabbay et al. (1976, 1979): hyperproinsulinemia due to mutation at the cleavage site connecting the A chain to the C peptide. Inheritance was again autosomal dominant.

Shoelson et al. (1983) studied 3 unrelated patients, 1 of whom was previously reported by Tager et al. (1979) and 1 of whom was previously reported by Shoelson et al. (1983), with diabetes-type hyperglycemia and marked hyperinsulinemia suggestive of insulin resistance but with normal response to exogenously administered insulin. Because the opportunity to study pancreatic tissue was rare, the authors developed a method combining high pressure liquid chromatography and radioimmunoassay. By this method, each of the 3 patients was found to secrete a structurally variant and chemically distinct insulin. All 3 patients were heterozygous. Two patients came from families in which many members of several generations had the variant insulin in an autosomal dominant pedigree pattern and had either normal or very mildly impaired glucose tolerance.

Gruppuso et al. (1984) identified a hyperproinsulinemia kindred in which the proband, a 14-year-old girl with a history of transient hyperglycemia at age 2 years, was studied for symptoms of hypoglycemia. Elevated levels of proinsulin were found in her and 2 sibs, her father, and her paternal grandfather, whereas 4 other close relatives were normal. Gruppuso et al. (1984) suggested that hyperproinsulinemia present in 5 persons in 3 consecutive generations with male-to-male transmission was due not to a structural defect of the proinsulin molecule but to a defect in the conversion mechanism in the pancreas.

In 4 persons in 2 generations, Vinik et al. (1986) described a structural defect of insulin associated with hyperinsulinemia. The insulin was found to elute later than normal human insulin on reverse phase high performance liquid chromatography. It was more hydrophobic than normal human insulin and had only 10% of the activity of normal insulin in terms of ability to bind to isolated rat adipocytes and stimulate glucose metabolism in them. The proband, aged 45 years, had a 9-year history of syncopal attacks. He showed resistance to exogenous insulin. Plasma counterregulatory hormones (glucagon, growth hormone, cortisol, epinephrine, norepinephrine) showed normal concentrations. Binding of exogenous insulin by the patient's cells was normal. The abnormal insulin was demonstrated in 2 of 3 sons and in a sister but not in the mother, brother, or niece. Sensitivity to insulin was normal in the 2 sons who had abnormal insulin. The explanation for the insulin resistance in the proband was unclear. The authors thought that the change in the insulin molecule in this family was different from either the B24 phe-to-ser (176730.0002) or the B25 phe-to-leu (176730.0001) mutations previously described.

Inheritance

The transmission pattern of hyperproinsulinemia in the families reported by Shoelson et al. (1983) and Gruppuso et al. (1984) was consistent with autosomal dominant inheritance.

Molecular Genetics

Tager et al. (1979) studied insulin isolated from the pancreas of a diabetic patient and concluded that one of the allelic genes had undergone a mutation resulting in substitution of leucine for phenylalanine at position 23 or 24 in the insulin B chain. Occurring in the invariant portion of the molecule, the mutation resulted in reduced biologic activity (Given et al., 1980). Kwok et al. (1981) isolated genomic DNA from the leukocytes of a diabetic patient with the mutant insulin identified by Given et al. (1980). After digestion with restriction endonuclease MboII, electrophoresis, and hybridization with cloned human cDNA probes, one MboII cleavage site had been lost, which is consistent with the postulated replacement of phenylalanine by leucine at position 24 of the insulin gene. Shoelson et al. (1983) demonstrated that the substitution in the mutant insulin identified by Tager et al. (1979) and Given et al. (1980) is leucine for phenylalanine at B25 (176730.0001).

In a patient with hyperproinsulinemia, Shoelson et al. (1983) identified a substitution of serine for phenylalanine at position 24 of the insulin B chain (176730.0002).

In a patient with mild diabetes, marked fasting hyperinsulinemia, and a reduced fasting C-peptide:insulin molar ratio, Haneda et al. (1983, 1984) found heterozygosity for the ser24-to-phe mutation in the insulin B chain. The patient, who was found to have abnormal circulating insulin molecules that could be distinguished from each other and from normal insulin, responded normally to exogenous insulin. Five additional family members of both sexes in 3 generations were affected.

In affected members of the family with hyperproinsulinemia reported by Gruppuso et al. (1984), Chan et al. (1987) identified a heterozygous missense mutation in the B chain of the INS gene (H10D; 176730.0003).