Substitution of glutamic acid for alanine 1135 in the putative "catalytic loop" of the tyrosine kinase domain of the human insulin receptor. A mutation that impairs proteolytic processing into subunits and inhibits receptor tyrosine kinase activity

The intracellular domain of the insulin receptor possesses activity as a tyrosine-specific protein kinase which is stimulated by insulin binding to the extracellular domain of the receptor. We have identified a patient with a genetic form of insulin resistance who is heterozygous for a mutation substituting Glu for Ala1135 in the putative "catalytic loop" of the tyrosine kinase domain of the receptor. In this investigation, the Glu1135 mutant receptor was expressed by transfection of mutant cDNA into NIH-3T3 cells. Like previously described mutations in the tyrosine kinase domain, the Glu1135 mutation impairs receptor tyrosine kinase activity and inhibits the ability of insulin to stimulate thymidine incorporation and receptor endocytosis. These data support the hypothesis that the receptor tyrosine kinase activity plays a necessary role in the ability of the receptor to mediate insulin action in vitro and in vivo. However, unlike previously described mutations in the intracellular domain of the receptor, the Glu1135 mutation impairs proteolytic cleavage of the proreceptor into separate subunits and impairs the transport of the receptor to the cell surface. These latter defects provide an explanation for the decrease in the number of receptors on the cell surface observed in the patient's circulating monocytes despite the fact that the mutant receptor is resistant to endocytosis and insulin-induced down-regulation.

Postbinding characterization of five naturally occurring mutations in the human insulin receptor gene: impaired insulin-stimulated c-jun expression and thymidine incorporation despite normal receptor autophosphorylation

Some patients with extreme insulin resistance have mutations in their insulin receptor gene. We previously identified five such mutations located in the extracellular domain of the insulin receptor (Asn-->Lys15, His-->Arg209, Phe-->Val382, Lys-->Glu460, and Asn-->Ser462) and studied the effects of these mutations upon posttranslational processing, insulin binding, and tyrosine autophosphorylation. We now characterize the ability of these mutant receptors to mediate biological actions of insulin in transfected NIH-3T3 fibroblasts. All cell lines expressing mutant receptors showed marked impairment in insulin-stimulated c-jun expression and thymidine incorporation when compared with cells expressing wild-type human insulin receptors. The most severe impairment was seen in cells expressing the Val382 mutant (a mutation which causes an intrinsic defect in receptor autophosphorylation). These cells had insulin responses similar to the untransfected cells (used as a negative control). In contrast, cells expressing the Lys15 mutant have the ability to achieve a normal level of maximal autophosphorylation but require an abnormally high concentration of insulin to do so (as the result of decreased insulin binding affinity). These cells show a higher basal rate and much lower insulin stimulation of both c-jun expression and thymidine incorporation when compared with the cells expressing the wild-type human insulin receptors. This pattern is also seen in the cells expressing the other mutants with normal autophosphorylation (Arg209, Glu460, and Ser462). Although the most severe defects in insulin action are seen with the mutation which has an intrinsic defect in receptor autophosphorylation, the ability to undergo normal autophosphorylation does not seem to preclude mutations from impairing the ability of receptors to mediate some of the actions of insulin.

Mutations in the insulin receptor gene in patients with genetic syndromes of insulin resistance and acanthosis nigricans

Mutations of the insulin receptor gene have been identified in patients with genetic syndromes of insulin resistance associated with acanthosis nigricans. These mutations impair insulin responses by reducing the number of insulin receptors on the surface of target cells, or by reducing the receptor's ability to bind insulin or to undergo insulin-stimulated autophosphorylation, an important step in insulin action. Studies of mutant receptors expressed in transfection systems have contributed to our understanding of the structure-function relationships of the insulin receptor.

Substitution of isoleucine for methionine at position 1153 in the beta-subunit of the human insulin receptor. A mutation that impairs receptor tyrosine kinase activity, receptor endocytosis, and insulin action

The intracellular domain of the insulin receptor possesses activity as a tyrosine-specific protein kinase. The receptor tyrosine kinase is stimulated by insulin binding to the extracellular domain of the receptor. Previously, we have identified a patient with a genetic form of insulin resistance who is heterozygous for a mutation substituting Ile for Met1153 in the tyrosine kinase domain of the receptor near the cluster of the three major autophosphorylation sites (Tyr1158, Tyr1162, and Tyr1163). In this investigation, the Ile1153 mutant receptor was expressed by transfection of mutant cDNA into NIH-3T3 cells. The mutation impairs receptor tyrosine kinase activity and also inhibits the ability of insulin to stimulate 2-deoxyglucose uptake and thymidine incorporation. These data support the hypothesis that the receptor tyrosine activity plays a necessary role in the ability of the receptor to mediate insulin action in vivo. Furthermore, expression of the Ile1153 mutant receptor exerted a dominant negative effect to inhibit the ability of endogenous murine receptors for insulin and insulin-like growth factor I to mediate their actions upon the cell. This observation is consistent with previous suggestions that mutant receptors dimerize with wild type receptors, thereby creating hybrid molecules which lack biological activity. The dominant negative effect of the mutant receptor may explain the dominant mode of inheritance of insulin resistance caused by the Ile1153 mutation. Finally, the mutation inhibits the ability of insulin to stimulate receptor endocytosis. This may explain the normal number of insulin receptors on the surface of the patient's cells in vivo. Despite the presence of markedly elevated levels of insulin in the patient's plasma, the receptors were resistant to down-regulation.

Expression of histone H3 cell cycle-related gene, vimentin and MYC genes in pediatric brain tumors. A preliminary analysis showing the different malignant cell growth potential

Eleven pediatric brain tumors were studied for the histone H3, Vimentin and MYC gene expression. H3, an S phase cell cycle-related gene (ccr), was found prevalently expressed in tumors with a high mitotic index (MI). Vimentin gene, which contributes to maintaining the cell structure but is also demonstrated to be an early responder gene to growth stimulation was found variously expressed. The different expression of Vimentin gene in the examined samples suggests the active proliferation of the tumor cells. Analysis of MYC gene expression was found increased only in a mesenchymal chondrosarcoma while in other samples MYC mRNA was undetectable. Medulloblastoma, chondrosarcoma, and choroid plexus carcinoma have high S phase H3 gene expression associated with a high MI. Differently an astrocytoma shows a low MI associated with high H3 gene expression. This first preliminary report of H3, Vimentin and MYC gene expression in brain tumors demonstrates that malignant cells are characterized by a different gene expression and different growth potentials.

Detection of mutations in insulin receptor gene by denaturing gradient gel electrophoresis

Denaturing gradient gel electrophoresis (DGGE) has been used to screen for mutations in the insulin receptor gene. Each of the 22 exons was amplified by the polymerase chain reaction (PCR). For each exon, one of the two PCR primers contained a guanine-cytosine (GC) clamp at its 5' end. The DNA was analyzed by electrophoresis through a polyacrylamide gel containing a gradient of denaturants. Two geometries for the gels were compared; the gradient of denaturants was oriented either parallel or perpendicular to the electric field. The sensitivity of the technique was evaluated by determining whether DGGE succeeded in detecting known mutations and polymorphisms in the insulin receptor gene. With parallel gels, 12 of 16 sequence variants were detected. The use of perpendicular gels increased the sensitivity of detection so that all 16 sequence variants were successfully detected when DNA was analyzed by a combination of perpendicular and parallel gels. Furthermore, DGGE was used to investigate a patient with leprechaunism whose insulin receptor genes had not previously been studied. Two mutant alleles were identified in this patient. The allele inherited from the father had a mutation substituting alanine for Val-28; in the allele inherited from the mother, arginine was substituted for Gly-366.

N-myc oncogene amplification in a pediatric case of glioblastoma multiforme

A sample of a primary brain tumor of glial cell origin was surgically removed from a 7-year-old girl. The histopathological analysis showed a heterogeneous tumor containing highly cellular areas composed of small, poorly differentiated cells with frequent mitoses suggestive of a glioblastoma multiforme. There were also areas presenting as features of lower-grade astrocytoma. Strong immunohistochemical staining for glial fibrillar acidic protein was demonstrated, while vimentin, neurofilament, and S-100 protein were all positive in just the astrocytic part of the tumor. The DNA extracted from a fresh tumor sample at diagnosis was processed by Southern blot analysis and hybridized with a 2.0 kb N-myc oncogene probe recognizing the first intron and the second exon of the human gene. A 20-fold amplification of the oncogene was found. The possible role of such a molecular alteration is discussed in light of the clinical presentation and histopathological features.

A mutation in the tyrosine kinase domain of the insulin receptor associated with insulin resistance in an obese woman

Insulin resistance is frequently associated with acanthosis nigricans and hyperandrogenism. In patients with type A insulin resistance, this has been shown to be due to genetic defects in insulin receptor function. However, other patients with a similar clinical syndrome have been reported to have a variant of this syndrome, in which assays of insulin receptor function were normal. We have sequenced a portion of the insulin receptor gene in one such patient, a 29-yr-old woman with obesity and insulin resistance. The patient is heterozygous for a mutation substituting isoleucine for methionine at position 1153. Met1153 is located in the intracellular domain of the receptor near the cluster of tyrosine phosphorylation sites at positions 1158, 1162, and 1163. Studies of the mutant receptor expressed in NIH-3T3 cells demonstrated that the Ile1153-mutation impairs the ability of insulin to stimulate autophosphorylation of solubilized insulin receptors. In addition, the mutation impairs the ability of insulin to stimulate receptor tyrosine kinase activity to phosphorylate an artificial substrate [poly(Glu-Tyr)]. It seems likely that this defect in receptor tyrosine kinase activity explains the defect in the ability of the patient's insulin receptors to mediate insulin action in vivo. Furthermore, this patient provides a paradigm in which genetic factors act in concert with other risk factors, such as obesity, to cause clinically important insulin resistance.