We are investigating the function of tumor suppressor genes and growth factor signaling pathways involved in the molecular pathogenesis of two different human genetic disorders, both of which result in the formation of benign and malignant tumors. We have previously shown that Ras proteins play an important role in the tumors of patients with neurofibromatosis type 1 (NF1), which affects the peripheral nervous system and other organs. We are currently investigating other events that occur in the process of tumor development in NF1, using both human tumors and animal model systems. In addition, we have found that the small GTPase Rap1, which is closely related to Ras, interacts with the protein product (tuberin) encoded by one of the genes (TSC2) mutated in patients with tuberous sclerosis (TSC). TSC patients develop lesions in the brain, kidney, heart, and other organs, and some of these lesions progress to malignancy. The phenotype of TSC suggests a tumor suppressor function for the TSC1 and TSC2 genes, each of which is linked to approximately one-half of TSC cases. Interestingly, it has been recently shown that the products of TSC1 (hamartin) and TSC2 (tuberin) physically associate in cells, suggesting an important role for this complex in the physiological function of these proteins.NF1 We and others have shown that loss of expression of the NF1 product neurofibromin leads to activation of cellular Ras proteins in tumors of NF1 patients. This is because neurofibromin functions as a negative regulatory, GTPase-activating protein (GAP) for Ras in Schwann cells, which are critical to the formation of benign and malignant tumors in this disease. These results defined a novel, direct interaction between a tumor suppressor product and an oncoprotein, with Ras-induced growth activation in the absence of specific mutations in c-ras genes. Our recent NF1 studies have focused on the role of other genetic or epigenetic events that may drive tumorigenesis in NF1 patients. Specifically, we have found that the epidermal growth factor receptor (EGFR) is aberrantly expressed in cells of both benign and malignant NF1 tumors. This result, which we have observed in NF1 patient tumor cell lines as well as primary benign and malignant tumors, is surprising in that Schwann cells do not normally express the EGFR. We found that cell lines derived from NF1 malignant peripheral nerve sheath tumors (MPNST) respond to EGF via activation of tyrosine kinase activity and MAP kinase. Furthermore, similar results were obtained with cells from mice lacking a functional copy of the NF1 gene. In addition, we found that anti-EGFR agents abolished the growth of a cell line derived from an NF1 patient tumor. These findings may represent a new avenue for therapeutic intervention in NF1 patients, and we are currently addressing this possibility through the use of animal model systems. TUBEROUS SCLEROSIS (TSC) We previously found that that tuberin acts as a negative regulator of Rap1 in vitro, and that TSC2 functions as a tumor suppressor gene. We also observed that tuberin is localized to the cis/medial region of the Golgi apparatus, and shows significant overlap with Rap1, consistent with the proteins interacting in vivo. More recently, we have developed antisera that recognize the TSC1 product, hamartin, and found that hamartin is a widely expressed protein. When hamartin is overexpressed, a significant portion of it forms a complex with tuberin. These results suggest that the function of these proteins is intimately linked to their ability to form a complex, and that disruption of the association following mutation of TSC1 or TSC2 abolishes the function(s) of the complex. We are currently investigating the structural requirements for the hamartin-tuberin complex, and analyzing the role of these proteins in the regulation of cell growth. - Biochemistry, Cancer cell growth regulation, cell proliferation, Oncogenes, protein function, proto-oncogene, Transformation, Tumor Suppressor,
