guanosine-triphosphate and Neurilemmoma

Neurofibromin, the gene product of NF1, is a Ras GTPase Activating Protein. The absence of neurofibromin leads to increased levels of Ras-GTP, which contributes to the proliferation of NF1 neurogenic sarcoma cell lines. Whether this pathogenic mechanism is applicable to benign and malignant peripheral nerve tumours from NF1 and non NF1 patients is not known, due to lack of a tissue based assay. We have adapted a colorimetric enzymatic assay for determining levels of Ras bound guanine nucleotides in tissues. Ras-GTP levels were increased in NF1 neurogenic sarcomas (15 times) and benign NF1 neurofibromas (four times), compared to non NF1 schwannomas. Neurofibromin was not expressed in NF1 sarcomas, in support of its important negative Ras regulatory role in the pathogenesis of NF1 peripheral nerve tumors.

Topics: Base Sequence; Cell Line, Transformed; DNA Primers; Genes, Neurofibromatosis 1; Genes, ras; GTPase-Activating Proteins; Guanosine Diphosphate; Guanosine Triphosphate; Humans; Immunohistochemistry; Molecular Sequence Data; Neurilemmoma; Neurofibroma; Neurofibromin 1; Peripheral Nervous System Neoplasms; Polymerase Chain Reaction; Protein Biosynthesis; Proteins; ras GTPase-Activating Proteins; Sarcoma

Tumor cell lines derived from malignant schwannomas removed from patients with neurofibromatosis type 1 (NF1) have been examined for the level of expression of NF1 protein. All three NF1 lines examined expressed lower levels of NF1 protein than control cells, and the level in one line was barely detectable. The tumor lines expressed normal levels of p120GAP and p21ras. Although the p21ras proteins isolated from the tumor cells had normal (nonmutant) biochemical properties in vitro, they displayed elevated levels of bound GTP in vivo. The level of total cellular GAP-like activity was reduced in extracts from the tumor line that expresses very little NF1 protein. Introduction of the catalytic region of GAP into this line resulted in morphological reversion and lower in vivo GTP binding by endogenous p21ras. These data implicate NF1 protein as a tumor suppressor gene product that negatively regulates p21ras and define a "positive" growth role for ras activity in NF1 malignancies.

Topics: Animals; Gene Expression Regulation, Neoplastic; Genes, Tumor Suppressor; Guanosine Triphosphate; Humans; Mice; Neurilemmoma; Neurofibromatosis 1; Neurofibromin 1; Protein Binding; Proteins; Proto-Oncogene Proteins p21(ras); Tumor Cells, Cultured