ACTIVATION OF GLUCOSE-TRANSPORT BY A NATURAL MUTATION IN THE HUMAN INSULIN-RECEPTOR

Naturally occurring mutations in the insulin receptor gene cause heritable severe insulin resistance. These mutations usually impair insulin receptor signaling in cells cultured from affected individuals. However, fibroblasts cultured from a patient with intrauterine growth restriction and severe insulin resistance (leprechaun Atl-1) had normal amounts of insulin receptor protein and defective insulin binding but constitutive activation of insulin-receptor autophosphorylation and kinase activity and of glucose transport. In the same fibroblasts, growth was impaired. Homozygosity for a mutation in the insulin receptor gene was suspected, since he inherited identical DNA haplotypes for this gene from both related parents. Here we report that the proband was homozygous and both parents were heterozygous for a point mutation in the insulin receptor gene converting the Arg86 codon (CGA) to Pro (C-CA) (R86P). The R86P substitution is contiguous to the hydrophobic beta-sheet of the receptor alpha subunit implicated by DeMeyts et al. [DeMeyts, P., Gu, J.-L., Shymko, R. M., Kaplan, B. E., Bell, G. 1. & Whittaker, J. (1990) Mol. Endocrinol. 4, 409-416] in the binding of aromatic residues of the insulin molecule. The R86P mutant insulin receptor cDNA was inserted into a plasmid under control of a simian virus 40 promoter and transfected into Chinese hamster ovary (CHO) cells. In contrast with fibroblasts from patient Atl-1, which had normal insulin receptor processing, CHO cells stably transfected with the R86P mutant cDNA (CHO-R86P) had altered posttranslational processing. The R86P mutant receptor failed to bind insulin but caused a significant increase in basal glucose transport in CHO cells. As in fibroblasts cultured from the patient, the R86P mutant insulin receptor did not stimulate growth in transfected CHO cells. These results suggest that the R86P mutation in the insulin receptor activates glucose transport without promoting cell growth and that distinct cell types process this mutant insulin receptor differently.