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 are members of the phakomatoses characterized by 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 been studying the molecular function of the genes 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. One interesting aspect of TSC is the fact that this disease can develop as a result of the inactivation of either of two tumor suppressor genes, designated TSC1 and TSC2. Consistent with these proteins acting in the same pathway, it has been shown that the products of TSC1 (hamartin) and TSC2 (tuberin) physically associate in cells. Thus we have been exploring the role of this complex in the molecular pathology of TSC. NF1 Loss of expression of the NF1 product neurofibromin leads to activation of cellular Ras proteins, as neurofibromin functions as a negative regulator of Ras. This is particularly true in Schwann cells, which are critical to the formation of benign and malignant tumors in NF1 patients. Our recent NF1 studies have focused on the role of other genetic or epigenetic events that may drive tumorigenesis in NF1 patients. We have found that the epidermal growth factor receptor (EGFR) is aberrantly expressed in cells of both benign and malignant NF1 tumors, and in animal models of NF1. Cell lines derived from NF1 malignant peripheral nerve sheath tumors (MPNST), as well as Schwann cells from mice lacking a functional copy of the NF1 gene respond to EGF by activation of mitogenic signaling pathways and increased growth. EGFR inhibitors blocked the growth of these cells, suggesting that these findings may represent a new avenue for therapeutic intervention in NF1 patients. Recently, we have found similar results in tumors from compound heterozygous mice bearing lesions in p53 and NF1. TUBEROUS SCLEROSIS (TSC) We have used overexpression of TSC1 and TSC2 to investigate the molecular function of the hamartin and tuberin. Overexpression of these genes directly inhibits cell growth, and growth inhibition by hamartin is associated with stabilization of tuberin, by preventing the ubiquitin-mediated degradation of tuberin. These results suggest that the stabilization of tuberin by hamartin is important for its tumor suppressor function. We also have investigated the structural requirements for the hamartin-tuberin complex, and found that specific TSC patient mutations can disrupt the association of these proteins, suggesting that their binding is critical for tumor suppression. Finally, we have recently shown that the hamartin/tuberin complex increases adhesion through increased E-cadherin function in human kidney epithelial cells.
