Neither inspection of individual pedigree log of odds scores nor formal tests of heterogeneity suggested a subgroup in which linkage of NIDDM and insulin-receptor gene was likely.
Differences in the ratio of RNA encoding two isoforms of the insulin receptor between control and NIDDM patients. The RNA variant without Exon 11 predominates in both groups.
An asymmetrical reduction in the levels of the insulin receptor mRNA transcribed from one allele was reported in some patients with severe insulin resistance and non-insulin-dependent diabetes mellitus (NIDDM).
To study genetic susceptibility to non-insulin dependent diabetes mellitus (NIDDM), association of insulin receptor gene, a candidate gene for NIDDM, with NIDDM was studied.
Needle biopsies obtained from vastus lateralis muscle from 20 patients with noninsulin-dependent diabetes mellitus (NIDDM) and 20 normal control subjects were analyzed for the relative expression of insulin receptor mRNA variants in a novel assay using fluorescence-labeled primers and subsequent analysis on an automated DNA sequencer.
Our findings provide the first direct evidence linking hyperinsulinemia to alterations in insulin receptor mRNA splicing, and suggest that alterations of insulin receptor mRNA splicing in muscle is an early molecular marker that may play an important role in NIDDM.
We investigated two potential mechanisms that might be relevant for the abnormal function of the insulin receptor in NIDDM, i.e. changes in the expression of the receptor isoforms and the effect of hyperglycaemia on insulin receptor tyrosine kinase activity.
These data indicate that the region of the insulin receptor gene coding for the intracellular portion of the beta-subunit is highly polymorphic and that polymorphisms surrounding specific exons can be identified by denaturing gradient gel blotting, but there is no evidence that variation at this locus contributes to NIDDM susceptibility in most individuals.
Insulin receptor (IR) and insulin-responsive glucose transporter (Glut4) represent two candidate genes involved in the development of non-insulin dependent diabetes mellitus (NIDDM); detection of molecular alterations in these genes might explain their possible contribution to NIDDM.
Since the insulin receptor substrate-1 (IRS-1) is the major substrate of the insulin receptor tyrosine kinase and has been shown to activate phosphatidylinositol (PI) 3-kinase and promote GLUT4 translocation, the IRS-1 gene is a potential candidate for development of non-insulin-dependent diabetes mellitus (NIDDM).
In conclusion, the increased expression of the insulin receptor isoform with lower insulin binding affinity in patients with primary non-genetically determined hyperinsulinaemia supports a role for insulin in the regulation of alternative splicing of insulin receptor pre-mRNA and suggests that in NIDDM an altered receptor isoform distribution might be secondary to the ambient hyperinsulinaemia rather than representing a primary defect.
These studies provide the first direct evidence in vivo that the relative expression of the two IR mRNA-splicing variants is altered in liver and suggest that increased expression of the exon 11- IR isoform may contribute to hepatic insulin resistance and NIDDM or may compensate for some yet unidentified defect.
The t1/2 of intracellular dissociation of insulin-receptor complexes measured by a polyethylene glycol assay was lower in normal (6 +/- 1 min) than in obese (12 +/- 2 min, p < 0.03) and NIDDM subjects (14 +/- 3 min, p < 0.02).
Some evidence of involvement has been produced for insulin-receptor substrate-1, glycogen synthase, the glucagon receptor, a ras-related protein (Rad), histocompatibility antigens, PC-1, and fatty acid binding protein, but the contributions of these genes to NIDDM is probably small.Other candidate genes (e.g. insulin, insulin receptor, glucose transporters) have been excluded as major diabetogenes.
In conclusion, diabetes mellitus per se does not alter the expression of IR isoforms in the liver and skeletal muscle, and therefore, at least in this animal model of NIDDM, impaired insulin action develops independently from a relative increase in IR36+ mRNA expression in skeletal muscle.
Mutations in or near the insulin receptor gene are unlikely to be the major cause of inherited predisposition to NIDDM in Chinese pedigrees that are related in this study, not excluding the principal role in the other Chinese or populations, while the mutations/variations near or at the glucose transporter 2 gene locus might be something of the cause, which needs more data to be ascertained, of familial NIDDM in Chinese pedigrees.
A reduction in tyrosine phosphorylation of both the insulin receptor (IR) and the insulin receptor substrate-1 (IRS-1) has been noted in both animal and human type 2 diabetes.