Neurofibromatosis type 1 (NF1) is a common autosomal dominant disease characterized by abnormalities of tissues predominantly derived from the neural crest. One of these neural crest-derived cells, the Schwann cell, is thought to be intrinsically abnormal in patients with NF1 and may contribute to the development of peripheral neurofibromas. The NF1 gene product, neurofibromin, which is expressed at relatively high levels in Schwann cells, contains a functional GAP-related domain that is able to inactivate p21-ras by stimulating its intrinsic GTPase activity. It has been suggested that neurofibromin functions as either an upstream regulator of ras or a downstream effector but both roles are not incompatible. In cell types where ras-GTP is growth promoting, the diminished capacity of mutant neurofibromin to stimulate the hydrolysis of ras-GTP may allow enhanced ras signalling, causing continuous cell proliferation. In cell types like Schwann cells where ras-GTP appears to be growth inhibiting, mutant neurofibromin may disrupt a ras-GTP/neurofibromin complex required for transduction of the growth inhibiting signal. We recently found that Schwann cell differentiation is associated with the specific induction of an alternate form of neurofibromin with reduced GTPase activity, which could serve to facilitate a ras signal transduction pathway. Our long term goal is to investigate direct cause-and-effect relationships between NF1 gene defects, abnormal ras regulation, and loss of Schwann cell function. Since ras-GTP is a major regulator of cell growth and differentiation, mutant neurofibromin might interfere with ras signalling pathways and contribute to tumor formation. In this proposal, we intend to isolate and culture Schwann cells from neurofibromas so that defects in neurofibromin expression can be directly correlated with abnormalities in cell function or alterations in p21-ras regulation. By expressing an oncogenic form of ras in normal human Schwann cells, we will examine how unregulated p21-ras interferes with growth or differentiation. Lastly, we will determine if expressing the GAP-related domain of NF1 in Schwann cells from neurofibromas restores ras regulation or normal cellular function. Identifying differences in biological properties and neurofibromin expression between normal and neurofibroma Schwann cells may provide impact on the development of future improved treatment strategies for neurofibromas in patients with NF1, and potentially have relevance to other malignancies given the more general role of neurofibromin as a tumor suppressor gene product.
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