Title: Regulation and function of the NF1 tumor supressor
Description: The Ras GAP activity of the NF1 tumor suppressor is extensively studied. However, the precise regulatory mechanisms governing the catalytic activity of neurofibromin are largely undefined. Previous studies have demonstrated that neurofibromin is a target of the ubiquitin proteasome system, but the signaling pathway mediating this degradation was unknown. This dissertation describes the investigation of the signaling pathway promoting neurofibromin proteasomal degradation, and the possibility that deregulation of this pathway contributes to tumorigenesis. During the course of these studies we find that in a subset of gliomas, neurofibromin is aberrantly degraded and in another subset of gliomas, the NF1 gene is mutated. Our data further suggest that complete genetic inactivation of NF1 can only be tolerated in the presence of p53 mutations. These studies reveal that neurofibromin is inactivated in gliomagenesis via proteasomal and genetic mechanisms and illustrate how cooperating mutations may impact the mechanism by which neurofibromin is suppressed.

As mentioned above, the Ras GAP domain of neurofibromin is well characterized, however relatively little is known about the remainder of the protein. Importantly, clusters of NF1 patient missense mutations lie outside the catalytic GAP domain suggesting that other regions of the protein are essential for neurofibromin function. Furthermore, a variety of studies suggest that neurofibromin participates in cellular functions distinct from its role as a Ras GAP. As a first step in exploring these hypotheses, we conduct a mass spectrometry screen to identify novel neurofibromin protein binding partners. In this screen we repeatedly detect a set of candidate neurofibromin protein binding partners with a high degree of confidence based on the mass spectrometry results indicating the number of peptides detected and overall protein coverage. We focus our initial validation efforts on the candidate neurofibromin interactor, PDZ-GEF1, a GEF for the Rap family of GTP-binding proteins. Our preliminary experiments confirm the existence of a neurofibromin/PDZ-GEF1 complex, and suggest that neurofibromin negatively regulates PDZ-GEF1 function and consequently, downstream Rap activity. Because Rap activity is associated with tumorigenesis, it is possible that this complex plays an important role in mediating the tumor suppressor activity of neurofibromin.