Current models suggest that the FERM domain protein Merlin, encoded by the tumor suppressor NF2, inhibits mitogenic signaling at or near the plasma membrane. We have discovered that the closed, growth inhibitory form of Merlin accumulates in the nucleus and binds to DCAF1, the receptor component of the E3 ubiquitin ligase CRL4DCAF1. Genetic and biochemical evidence indicates that Merlin functions as a negative regulator of CRL4DCAF1. Depletion of DCAF1 blocks exit from contact inhibition and progression through the cell cycle in Merlin-deficient cells. Expression of Merlin and silencing of DCAF1 induce a largely overlapping program of gene expression, which includes the upregulation of growth arrest and proapototic genes and the downregulation of mitogenic and survival genes. Tumor-derived mutants of Merlin fail to accumulate into the nucleus, to bind to DCAF1, or to inhibit CRL4DCAF1. Finally, depletion of DCAF1 suppresses the ability of Merlin-deficient tumor cells to grow in soft agar and to form tumors in nude mice. These findings strongly suggest that Merlin suppresses tumorigenesis by translocating to the nucleus to inhibit CRL4DCAF1-dependent gene expression. We propose to pursue four Specific Aims. I) To determine if Merlin mediates contact inhibition through inhibition of CRL4DCAF1, we will examine if expression of a Merlin-insensitive form of DCAF1 causes loss of contact inhibition. In addition, we will examine if the tumor-derived missense mutants of Merlin, which have lost the ability to suppress CRL4DCAF1, are unable to mediate contact inhibition. II) To examine if Merlin suppresses tumorigenesis in vivo through inhibition of CRL4DCAF1, we will generate mice lacking DCAF1 in their Schwann cells and breed them to mouse models of NF2. We will examine tumor onset and progression in control and DCAF1 mutant mice, as we have done previously in our studies on integrin signaling. III) To identify the physiological substrates of CRL4DCAF1 and to examine their involvement in Merlin-mediated tumor suppression, we will conduct Tandem Affinity Purification followed by mass spectrometry using wild type or mutationally inactivated DCAF1. Alternatively, we will subject control and DCAF1-silenced cells to Global Stability Profiling, as recently described by the Elledge laboratory. IV) To examine if CRL4DCAF1 displays pro- oncogenic activity, we will examine if overexpression of DCAF1 enhances the ability of Merlin-deficient, Merlin- re-expressing, or both types of schwannoma cells to proliferate in vitro as well as their ability to form tumors in vivo. In addition, we will examine if the gene encoding DCAF1 is amplified or constitutively activated by C- terminal deletion in NF2-related or sporadic meningiomas and in Schwannomas. Taken together, these studies should help to elucidate the mechanism through which Merlin mediates contact inhibition and suppresses tumorigenesis.
Contact inhibition of growth is critical for proper tissue morphogenesis and repair and it contributes to suppress tumorigenesis. The Ezrin Radixin Moesin (ERM) protein Merlin, encoded by the tumor suppressor gene NF2, localizes at the plasma membrane and mediates contact inhibition of growth. We have found that Merlin accumulates also in the nucleus, where it binds to and inhibits the novel E3 ubiquitin ligase CRL4DCAF1. Our Preliminary Studies indicate that Merlin needs to translocate to the nucleus and to inhibit CRL4DCAF1 in order to suppress tumorigenesis. We intend to test the hypothesis that Merlin mediates contact inhibition of growth by inhibiting CRL4DCAF1 and to place this biochemical reaction within a cellular pathway of growth inhibition. We also wish to test if inhibition of CRL4DCAF1 blocks tumor growth in vivo and if DCAF1 is a novel oncogene. These studies will contribute to elucidate the mechanisms through which Merlin mediates contact inhibition and tumor suppression. It is expected that a mechanistic understanding of Merlin's function will shed light on contact inhibition and on the pathogenesis of NF2 mutant tumors.
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