Biological aspects of the neurofibromatosis type 2 gene product

Neurofibromatosis 2 (NF2) is an inherited disorder characterized by a predisposition to multiple intracranial tumors. The protein encoded by the NF2 gene has striking similarities to ezrin, radixin and, moesin (ERM) proteins which link membrane proteins to the cytoskeleton. To elucidate the function of NF2 protein (merlin), we attempted to identify the merlin- binding proteins. The protein-binding assay detected five merlin-binding cellular proteins (MBPs). The entire ERM-homology domain of merlin was found to be essential for binding to all five proteins. Since most reported NF2 mutations were nonsense or frameshift mutations, we next performed a protein truncation test and found that 11 of 14 NF2 patients had truncational mutations (79%). Seven of the 11 patients (64%) had a splicing abnormality which leads to absence of exons in the ERM-homology domain. To examine the biological significance of the exon-missing mutations in the ERM-homology domain, we expressed the wild-type (wt-) and the various mutant (mu-) NF2s in a fibroblast cell line. The wt-NF2 showed intense punctate staining in the perinuclear cytoplasm in addition to overall staining of the submembranous area, whereas the mu-NF2s lacking exons in the ERM-homology domain showed granular staining at the perinuclear region without any accumulation at the submembrane region. Microinjection of wt-NF2 cDNA into the nucleus of fibroblasts revealed that wt-NF2 protein induced a progressive elongation of cell processes. Furthermore, cells that expressed mu-NF2 had decreased adhesion, which resulted in their detachment from the substratum. These findings suggest that the exon-missing mutations in the ERM-homology domain, through the loss of binding to MBPs, affect cell membrane-cytoskeleton signaling and consequently disrupt cell-to-cell or cell-to-matrix interaction.