AN IN-FRAME INSERTION IN EXON-3 AND A NONSENSE MUTATION IN EXON-2 OF THE INSULIN-RECEPTOR GENE ASSOCIATED WITH SEVERE INSULIN-RESISTANCE INA PATIENT WITH RABSON-MENDENHALL SYNDROME

We have studied the structure and function of the insulin receptor in a patient (PK) with severe insulin resistance and Rabson-Mendenhall sy ndrome. Insulin binding to cultured fibroblasts from PK was almost not detectable and insulin-induced insulin receptor autophosphorylation a nd glucose uptake was abolished. The structure of the receptor gene wa s analysed by sequencing amplified products of the 22 exons with the f lanking intron regions directly as well as after subcloning in pUCBM20 plasmids. Two mutant alleles of the insulin receptor gene were detect ed. One allele contains in-frame 12 additional base pairs in exon 3 co ding for the amino acids Leu-His-Leu-Val located between Asp-261 and L eu-262 in the receptor's extracellular domain, being the first report of an insertion mutation of the insulin receptor gene. In the other al lele Arg-86 in exon 2 is changed into a stop codon. Therefore, PK is c ompound heterozygous at the insulin receptor locus. Direct cDNA sequen cing indicates that both mutant alleles are expressed in the patient's fibroblasts. Studies of the parents' fibroblasts revealed that PK inh erited the insertion mutation from the father and the nonsense mutatio n from the mother. Insulin binding to fibroblasts of the mother was re duced (63 % of control cells) and hormone binding to the father's cell s shows a larger reduction (37 % of control cells), but less severe th an the patient's cells (11 % of control). This investigation provides further evidence that the Rabson-Mendenhall syndrome is causally relat ed to mutations in the insulin receptor gene.