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Regulation of protein translation by the NF2 tumor suppressor schwannomin

Of all the genes that are associated with the pathogenesis of brain tumors, the neurofibromatosis 2 (NF2) gene falls into a unique category of tumor suppressors primarily because mutation in the NF2 gene can be the first step in the pathogenesis of multiple brain tumor types originating from different brain tissues. Additionally, NF2 gene mutation is one of the most common causes of benign brain tumors. These facts alone suggest that the NF2 tumor suppressor is a powerful regulator of tumor cell proliferation yet the normal function of the NF2 tumor suppressor protein remains unknown.

There are four main types of tumors that are caused by mutation in the NF2 gene, including schwannomas, meningiomas, ependymomas, and astrocytomas. Any type of NF2 gene mutation (truncating or missense) results in the loss of the NF2 gene encoded tumor suppressor protein, known as either schwannomin or merlin. When tumors occur sporadically (in an individual who does not possess an inherited NF2 gene mutation), schwannomas always lack schwannomin, while various studies have shown that roughly 50% of meningiomas, 10% of ependymomas, and some astrocytomas lack schwannomin. Mutation in the NF2 gene therefore is not always causative for sporadic meningiomas, ependymomas and astrocytomas. However these tumor types are also prominent in the spectrum of tumors found in patients with inherited NF2 mutations demonstrating the global importance of the NF2 gene to tumor suppression. Interestingly, recent investigations have also demonstrated that a subset of malignant mesotheliomas are also caused by NF2 mutation.

Familial NF2 is a debilitating disorder affecting one in 40,000 individuals worldwide. Individuals with NF2 develop multiple tumors of the nervous system requiring surgical intervention, and NF2 nearly always results in death. The hallmark feature of NF2 is the development of bilateral vestibular schwannomas and excision of these tumors is often associated with loss of hearing in both ears. Additionally, meningiomas (tumors of the brain lining) and other schwannoma types affecting the skin and spine are common. Ependymomas and astrocytomas occur less frequently in NF2 patients than do meningiomas. NF2 patients also frequently develop cataracts and other ocular abnormalities.

Since 1995 the research conducted in our laboratory has been steadfastly focused on identifying the function of the NF2 tumor suppressor protein by using a variety of experimental methods designed to determine the identities of other proteins in the cell with which schwannomin interact. While the identity of the NF2 gene has been known since 1993, research on schwannomin function has only partly divulged its normal function. It is clear that schwannomin is a member of a class of proteins that stabilize the cell membrane and the cytoskeleton (a lattice-like structure to which other proteins are bound which when destabilized causes proliferative changes). Our past work has determined that one important cytoskeletal protein known as beta spectrin interacts with schwannomin and that this interaction is important for cytoskeletal organization. We have also demonstrated that schwannomin interacts with HRS, a protein involved in attenuating growth factor signaling, and are actively working on elucidating the functional significance of this interaction. Other laboratories have also made significant progress on the function of schwannomin. Briefly, schwannomin (or merlin) interacts with CD44, NHERF, paxillin, RhoGDI, and with three members of the ERM family of proteins to which schwannomin belongs as well. While each of these interactors suggest cellular pathways regulated by schwannomin, none of the pathways related to these interacting proteins has been verified as being altered in patient tumors. The NF2 pathway of pathogenesis remains unknown.

We discovered a new schwannomin interacting protein called p110 that is a member of a complex of eight to ten proteins known as the eukaryotic initiation factor 3 (eIF3). It is well documented that the initiation of protein synthesis in the cell requires the actions of eIF3. P110 is required for normal eIF3 function, but the role of p110 in this process is not well described. There is strong evidence linking p110 and cellular proliferation that hints at a possible role for p110 in tumorigenesis. P110 is overabundant in some tumor types (seminomas) and other eIF3 proteins are overabundant in tumors as well (eIF3-p48/Int-6 has been described as an oncoprotein associated with the malignant transformation of mouse mammary cells, and the gene encoding eIF3-p40 is often amplified in both breast and prostate tumors). Like p40, the abundance of p110 can also be abnormally elevated by gene amplification. In fact, the chromosomal region where p110 is encoded in the human genome is prone to duplication, and one study showed that the intact p110 gene was amplified in multiple tissue types associated with disease predisposition. It is our hypothesis that schwannomin has a critical function in specific tissues in regulating p110 activity.

The identification of p110 as a schwannomin interacting protein provides a new and exciting avenue for analysis of schwannomin role in tumorigenesis. Our preliminary data demonstrate that schwannomin and p110 interact directly and colocalize in cultured cells similar to human cells that give rise to schwannomas. We have also obtained preliminary data suggesting that schwannomin can inhibit protein translation. We hypothesize that schwannomin is a regulator of protein translation through its interaction with p110, and the proposed work will thoroughly test this hypothesis. Successful demonstration that schwannomin inhibits protein translation by way of its interaction of p110 will provide new targets for therapy for NF2 and for a variety of brain tumors in general.