VARIANT SEQUENCES OF INSULIN-RECEPTOR SUBSTRATE-1 IN PATIENTS WITH NONINSULIN-DEPENDENT DIABETES-MELLITUS

The pathophysiology of noninsulin-dependent diabetes mellitus (NIDDM) is characterized by insulin resistance and insulin deficiency. To search for genetic defects causing NIDDM, we have screened for mutations in the gene encoding insulin receptor substrate-1 (IRS-1), an intracellular protein that is phosphorylated by the insulin receptor and is thought to play an important role in mediating insulin action. The coding sequence of the IRS-1 gene (divided into 12 overlapping fragments) was amplified by polymerase chain reaction and screened for the presence of single stranded conformational polymorphisms. This led to the identification of 6 variants in the nucleotide sequence. There were 3 nonconservative amino acids substitutions: Gly(819)-->Arg, Gly(972)-->Arg, and Arg(1221)-->Cys. In addition, there were three silent polymorphisms: GAC vs. GAT encoding Asp(90), GGG vs. GGA encoding Gly(235), and GCA us. GCG encoding Ala(805). The previously reported Arg(972) substitution was identified in 7 of 31 patients with NIDDM, 4 of 32 normal. subjects, and 4 of 16 nondiabetic obese individuals. The 2 novel amino acid substitutions (Arg(819) and Cys(1221)) were both detected in 1 patient with NIDDM, but not in either of the other 2 groups of nondiabetic individuals. All 3 amino acid residues are identically conserved in the amino acid sequences of human, mouse, and rat IRS-1, suggesting that Gly(819), Gly(972), and Arg(1221),, important for the normal function of IRS-1. Furthermore, the prevalence of amino acid substitutions in IRS-1 is increased in patients with NIDDM. These observations suggest that mutations in the IRS-1 gene may play a causal role in the pathogenesis of NIDDM.