A leucine to proline mutation at position 233 in the insulin receptor inhibits cleavage of the proreceptor and transport to the cell surface.

We have previously shown that a homozygous mutation encoding a substitution of proline for leucine at position 233 in the insulin receptor is linked with the syndrome of leprechaunism, being a lethal form of insulin resistance in newborn children. Specific binding of insulin and insulin-stimulated autophosphorylation of the insulin receptor are nearly absent in fibroblasts from the leprechaun patient. To examine the molecular basis of the observed insulin receptor abnormalities, CHO cell lines overexpressing mutant insulin receptors were made by transfection. The results show that the mutation inhibits cleavage and transport of the proreceptor from intracellular sites to the cell surface. As the mutant receptor is poorly precipitated by two different monoclonal antibodies recognizing epitopes on undenatured wild-type alpha-subunits, the mutation probably affects overall folding of the alpha-subunit. The mutant proreceptor is unable to bind insulin and exhibits no insulin-stimulated autophosphorylation. These data explain the abnormalities seen in the patient's fibroblasts. Pulse-chase labeling experiments on transfected cells show that the mutant precursor has an extended half-life (approximately 5 h) compared to the precursor of wild-type insulin receptors (approximately 2 h). This mutation is the first example of a naturally occurring mutation in the insulin receptor which completely blocks cleavage of the proreceptor and transport to the cell surface.

Individuals with only one allele for a functional insulin receptor have a tendency to hyperinsulinaemia but not to hyperglycaemia.

Recently, we described a leprechaun patient with a genetically transmitted severe insulin resistance due to the absence of functional insulin receptors as inferred from the loss of insulin binding to the patients' fibroblasts and the impaired autophosphorylation of the beta-chain of the receptor. This patient was homozygous for the genetic defect which was recently found to be a leucine to proline mutation at position 233 in the alpha-chain of the insulin receptor. In the present study we have examined insulin receptor functions in relatives of this patient. Some of these individuals are heterozygous for the genetic defect and have only one allele coding for a functional insulin receptor. Insulin binding to cultured fibroblasts from the heterozygous individuals is only 20-40% of control values indicating a Mendelian mode of inheritance of the binding defect. In contrast, insulin stimulated autophosphorylation of the beta-chain of the insulin receptor shows normal values, indicating compensation mechanisms operating on this process. The stimulation of the basal level of 2-deoxyglucose uptake by insulin in fibroblasts from the homozygous patient is 1.2 fold whereas the heterozygous and control individuals show stimulation values of approximately 1.65 fold. Basal levels of 2-deoxyglucose uptake are similar in these fibroblasts. Oral glucose tolerance tests on the heterozygous individuals indicate an increased requirement for insulin of the target tissues as concluded from the tendency towards hyperinsulinaemia with no observed hyperglycaemia.(ABSTRACT TRUNCATED AT 250 WORDS)

A leucine-to-proline mutation in the insulin receptor in a family with insulin resistance.

We have determined the primary structure of a mutant insulin receptor of a leprechaun patient born from a consanguineous marriage. A characteristic feature of leprechaunism is an extreme resistance to insulin. In this patient the insulin resistance seems to result from an observed lack of insulin binding to intact cells. Solubilization of cells in non-ionic detergents leads to the appearance of insulin receptors which can bind insulin. However, the insulin-stimulated autophosphorylation of the receptor's beta subunit is markedly reduced. Cloning and sequencing of cDNA derived from insulin receptor mRNA of this patient revealed a leucine-to-proline mutation at position 233 in the alpha subunit. By means of DNA amplification we found that the patient is homozygous for this mutation and that the parents and two grandparents from the consanguineous line are heterozygous. The heterozygous individuals all show decreased insulin binding to cultured fibroblasts. In addition, they are mildly insulin resistant in vivo. These observations show a linkage between the leucine-to-proline mutation and the observed insulin resistance in this family. We therefore conclude that the mutation in the homozygous form is responsible for the extreme insulin resistance in the leprechaun patient. The mutation for the first time characterizes a region in the insulin receptor which seems to be involved in transmitting the insulin binding signal to the tyrosine kinase domain.

Fibroblasts from a leprechaun patient have defects in insulin binding and insulin receptor autophosphorylation.

Leprechaunism is an inherited human disorder associated with an extreme resistance of the target cells towards the action of insulin. We have examined the properties of the insulin receptor in fibroblasts from a leprechaun patient (Geldermalsen, the Netherlands). In vitro, severe insulin resistance is reflected by a low level of insulin stimulated uptake of 2-deoxyglucose by these fibroblasts. This defect seems to be caused by a combination of two factors: a low level of insulin binding to intact cells and a strong decrease of insulin stimulated autophosphorylation of the receptor. The stimulation of autophosphorylation by insulin was approximately six-fold in control subjects and less than two-fold in the patient. No abnormalities were observed in the total number of insulin receptors in these cells and the molecular weights of the receptor subunits. In addition, the insulin concentration required for half maximal autophosphorylation is similar for the solubilised receptor from control and patient fibroblasts.

Improper expression of insulin receptors on fibroblasts from a leprechaun patient.

Leprechaunism is an inherited human disorder characterized by severe insulin resistance. We have examined the properties of the insulin receptor in fibroblasts from a leprechaun patient. In vitro, severe insulin resistance is reflected by a low level of insulin binding to the patients fibroblasts and impaired insulin-mediated uptake of 2-deoxyglucose. Quantification of the receptor in detergent-solubilized total glycoprotein indicates a normal receptor number, in agreement with the observed normal level of insulin receptor mRNA on northern blots. The insulin-stimulated autophosphorylation of the patient's receptor shows a normal profile. The insulin receptor is present on the plasma membrane as indicated by cell-surface iodination experiments. No abnormalities in the molecular masses of the receptor's alpha and beta chains were observed. The results indicate that an apparently normal receptor is synthesized in sufficient amounts but functional expression of the receptor on the plasma membrane is impaired.

Plant expression signals of the Agrobacterium T-cyt gene.

Within the 5' and 3' non-coding regions of the T-cyt gene from the octopine T-DNA of Agrobacterium tumefaciens sequences required for expression of this gene in plant cells were identified by deletion mutagenesis. The results show that 184 bp of the 5' non-coding region and 270 bp of the 3' non-coding region are sufficient for wild-type expression. Within the 5' non-coding region two essential expression signals were identified: (1.) an activator element located between -185 and -129 with respect to the ATG start codon and (2.) one out of two TATA boxes. Deletions of the activator element or the two TATA boxes resulted in nonfunctional genes. Deletion of the upstream TATA box and both putative CAAT boxes did not significantly affect expression. Within the 3' non-coding region, the polyadenylation box most distal to the stop codon was not essential for expression, but sequences more upstream, including a second polyadenylation box were found to be required for wild-type expression.