OBJECT:Although the manifestations of neurofibromatosis Type 2 (NF2) vary, the hallmark is bilateral vestibular schwannomas (VSs). The authors studied the clinical course and genetic basis of unilateral VSs associated with other NF2-related tumors.METHODS:Forty-four adults presenting with unilateral VSs and other NF2-related tumors were identified. A comprehensive review of patient records and cranial imaging was conducted. Molecular analysis of the NF2 locus was performed in available tumors and paired blood specimens. Patient age at symptomatic onset ranged from 11 to 63 years (mean 32 years). Twenty-two patients (50%) presented with eighth cranial nerve dysfunction. Twenty-six presented with multiple lesions. Thirty-eight harbored other intracranial tumors and 27 had spinal tumors, with most lesions situated ipsilateral to the VS. No patient had a relative with NF2, although two of 63 offspring had isolated NF2-related findings. A contralateral VS developed in four patients 3 to 46 years after the symptomatic onset of a unilateral VS, and two of these patients experienced rapid progression to total deafness. Results of a Kaplan-Meier analysis identified actuarial chances of developing contralateral VS: 2.9% (3-17 years after the VS symptomatic onset of unilateral VS), 11% (18-24 years), and 28.8% (25-40 years). Mosaicism for the NF2 gene mutation was proven in eight patients.CONCLUSIONS:The authors describe the clinical features of this unique phenotype--unilateral VS with other NF2-related tumors. Persons with this phenotype should undergo evaluation and monitoring similar to that conducted in patients with NF2, and the possibility of aggressive contralateral VS formation should be considered in their treatment. Molecular genetic analysis is best performed using resected tumor specimens and will enable future studies to determine the genetic risks of individuals with mosaicism.

Neurofibromatosis 2 (NF2) is a severe autosomal dominant disorder that predisposes to multiple tumours of the nervous system. About half of all patients are founders with clinically unaffected parents. The purpose of the present study was to examine the extent to which mosaicism is present in NF2 founders. A total of 233 NF2 founders with bilateral vestibular schwannomas (BVS) were screened by exon scanning. NF2 mutations were detected in the blood samples of 122 patients (52%). In 10 of the 122 cases, the ratio of mutant to normal alleles was obviously less than 1, suggesting mosaicism. Tumour specimens were available from 35 of the 111 subjects in whom no mutation could be detected in blood specimens. Mutational analysis by exon scanning detected typical NF2 mutations in 21 of the 35 tumours. In nine subjects, the alterations found in tumours could be confirmed to be the constitutional mutation based on finding of identical mutations in pathologically and/or anatomically distinct second tumours. In six other subjects with only a single tumour available, allelic loss of the NF2 gene was found in addition to the mutation in each tumour, suggesting that either the mutation or the deletion of the NF2 gene is probably the constitutional genetic alteration. Our results suggest that failure to find constitutional mutations in blood specimen from these 15 patients was not because of the limitation of the applied screening technique, but the lack of the mutations in their leucocytes, best explained by mosaicism. Extrapolating the rate (15/35 = 43%) of mosaicism in these 35 cases to the 111 NF2 founders with no constitutional NF2 mutations found in their blood, we inferred 48 mosaic subjects (111 x 0.429). Adding the 10 mosaic cases detected directly in blood specimens, we estimate the rate of mosaicism to be 24.8% (58/233) in our cohort of 233 NF2 founders with bilateral vestibular schwannomas.

Background: Schwannomatosis is a recently recognized disorder, defined as multiple pathologically proven schwannomas without vestibular tumors diagnostic of neurofibromatosis 2 ( NF2). Some investigators have questioned whether schwannomatosis is merely an attenuated form of NF2. Methods: The authors identified eight families in which a proband met their diagnostic criteria for schwannomatosis. Archived and prospectively acquired tumor specimens were studied by mutational analysis at the NF2 locus, loss of heterozygosity analysis along chromosome 22, and fluorescent in situ hybridization analysis of NF2 and the more centromeric probe BCR. Linkage analysis could be performed in six of eight families. Results: Clinical characterization of these kindreds showed that no affected family member harbored a vestibular tumor. Molecular analysis of 28 tumor specimens from 17 affected individuals in these kindreds revealed a pattern of somatic NF2 inactivation incompatible with our current understanding of NF2 as an inherited tumor suppressor gene syndrome. Linkage analysis excluded the NF2 locus in two kindreds, and showed a maximum lod score of 6.60 near the more centromeric marker D22S1174. Conclusions: Schwannomatosis shows clinical and molecular differences from NF2 and should be considered a third major form of neurofibromatosis. Further work is needed to identify the inherited genetic element responsible for familial schwannomatosis.

Background: Schwannomatosis is a recently recognized disorder, defined as multiple pathologically proven schwannomas without vestibular tumors diagnostic of neurofibromatosis 2 (NF2). Some investigators have questioned whether schwannomatosis is merely an attenuated form of NF2.

Onion bulbs are concentric lamellar structures formed by Schwann or perineurial cells, which may be seen in several generalized or localized diseases of the peripheral nerve. There is debate regarding the pathogenesis of localized tumefactions displaying these microscopic structures. We report the fifth case, to our knowledge, of a Schwann cell-onion bulb tumor, which arose in the trigeminal nerve of a child with an unclassifiable, probably distinct, neurocutaneous syndrome; we also provide evidence for a neoplastic or hamartomatous origin. Molecular studies failed to establish an abnormality in the NF1, NF2, PMP22, or Connexin 32 genes. Similar and previously reported cases are discussed, as well as other onion bulb-forming entities.

BSTRACT Neurofibromatosis 2 (NF2) is an autosomal dominant disorder characterized by schwannomas and meningiomas that develop after inactivation of both copies of the NF2 gene. Approximately half of all patients with NF2 have unaffected parents and the disease results from new mutations at the NF2 locus. Loss of heterozygosity (LOH) in tumor specimens due to deletions covering the normal NF2 allele can be used to infer the haplotypes surrounding underlying mutations and determine the allelic origin of new mutations. We studied 71 sporadic NF2 patients using both LOH and pedigree analysis and compared the parental origin of the new mutation with the underlying molecular change. In the 45 informative individuals, 31 mutations (69%) were of paternal and 14 (31%) were of maternal origin ( P =0.016). Comparison with corresponding constitutional mutations revealed no correlation between parental origin and the type or location of the mutations. However, in 4 of 6 patients with somatic mosaicism the NF2 mutation was of maternal origin. A slight parent of origin effect on severity of disease was found. Further clinical and molecular studies are needed to determine the basis of these unexpected observations.

Neurofibromatosis type 2 (NF2) is a genetic disorder characterized by formation of multiple schwannomas and meningiomas due to inactivating mutations in the NF2 tumor suppressor gene on chromosome 22. We describe a polymorphism in the 3' untranslated region of the NF2 gene that is informative in about one-third of individuals. This polymorphism permitted an assessment of the relative expression of NF2 transcripts in lymphoblastoid cell RNA from 22 unrelated NF2 patients heterozygous for a germline NF2 mutation, along with 6 schwannomatosis patients, and 14 unaffected controls. Unequal allelic expression (1.8- to 20-fold) was detected in 15 of the NF2 cases, but in none of the schwannomatosis or control individuals. Underexpression of the NF2 mutant allele was documented for all 6 nonsense or frameshift mutations, 3 of 6 splice mutations, and 1 of 4 missense mutations, which, unexpectedly, was shown to alter the NF2 transcript and create a premature stop codon. In contrast, equal expression or slight overexpression of NF2 mutant alleles was observed for 2 in-frame deletions, 2 splice alterations, and 3 missense mutations. In the remaining 5 cases, the allele representing the mutant transcript was not known. Thus, truncating NF2 mutations, which are the most frequent alterations in NF2 patients and NF2-associated tumors, were associated with underexpression of the mutant allele, whereas the less common in-frame alterations usually showed normal or slight overexpression of the mutant transcript.

In recent years, it has become clear that the ERMs occupy a crucial position as protein linkers that both respond to and participate in reorganization of membrane-cytoskeletal interactions. With the identification of new binding partners, the ERMs are also implicated in linked regulation of the activities of particular membrane proteins. Thus, they reside at a junction in a complex web of interactions that must respond to stimuli from both outside and inside the cell. As expected from its structural motifs, merlin behaves in a manner similar to the ERM proteins, but with some notable differences. Chief among these is the absence of intramolecular interaction to mask intermolecular interaction domains in isoform 2. The full range of merlin's intermolecular interactions remains to be delineated, but it can be expected from the comparison to ERMs that merlin also sits within a web of interactions that may involve multiple partners and signaling pathways, some of which it shares with the ERMs. Defining merlin's tumor suppressor function will likely require identifying those differences that are peculiarly important in the target cell types of NF2. However, the fact that inactivation of merlin in the mouse by targeted mutagenesis produces a variety of malignant tumors with a high rate of metastasis [33] suggests that merlin's suppression of tumor formation may involve different partners and pathways in different cell types and genetic backgrounds. Consequently, the disruptions due to merlin inactivation in the progression of malignant mesothelioma may represent a tumor suppressor role operating by a different pathway than that in schwannoma or meningioma.

Loss of the neurofibromatosis 2 (NF2) gene-encoded protein merlin is a universal finding in sporadic and NF2-associated schwannomas. Certain NF2 patients may develop numerous minute Schwann cell tumorlets of the spinal nerve roots in addition to larger, frank schwannomas and thereby provide an opportunity to investigate the timing of NF2 gene/merlin loss in Schwann cell tumorigenesis. We studied an NF2 patient with a germline NF2 gene frameshift mutation who had many Schwann cell tumorlets and schwannomas. Loss of heterozygosity studies of DNA from microdissected specimens showed allelic loss of the NF2 region of chromosome 22q in tumorlets as well as schwannomas. Immunohistochemistry further demonstrated loss of merlin expression in tumorlets as well as schwannomas, with intact expression in adjacent nerve. Thus, loss of both NF2 alleles and merlin occur early in Schwann cell tumorigenesis, before the tumorlet stage. The study of tumorlets and schwannomas in such patients may also provide an opportunity to elucidate mechanisms responsible for the subsequent growth of Schwann cell lesions into symptomatic tumors.

NF2 (neurofibromatosis 2, encoding the merlin protein) gene mutations and chromosome 22q loss have been demonstrated in the majority of sporadic and NF2-associated schwannomas, but many schwannomas fail to demonstrate genetic evidence of biallelic NF2 gene inactivation. In addition, the role of the merlin-related ERM family members (ezrin, radixin, and moesin) remains unclear in these tumors. We therefore studied expression of NF2-encoded merlin as well as ezrin, radixin, and moesin in 22 vestibular and peripheral schwannomas that had been evaluated for NF2 mutations and chromosome 22q loss. Western blotting and immunohistochemistry with antibodies directed against the amino and carboxy termini of merlin demonstrated loss of merlin expression in all studied schwannomas, including 12 tumors lacking genetic evidence of biallelic NF2 gene inactivation. Western blotting with antibodies directed against ezrin, radixin, and moesin, however, showed expression of these proteins in all schwannomas. In addition, immunohistochemistry with an antibody to moesin revealed widespread expression in tumor and endothelial cells. These data indicate that the specific loss of merlin is universal to schwannomas and is not linked to loss of ezrin, radixin, or moesin expression.

Patients with multiple schwannomas without vestibular schwannomas have been postulated to compose a distinct subclass of neurofibromatosis (NF), termed "schwannomatosis." To compare the molecular-genetic basis of schwannomatosis with NF2, we examined the NF2 locus in 20 unrelated schwannomatosis patients and their affected relatives. Tumors from these patients frequently harbored typical truncating mutations of the NF2 gene and loss of heterozygosity of the surrounding region of chromosome 22. Surprisingly, unlike patients with NF2, no heterozygous NF2-gene changes were seen in normal tissues. Examination of multiple tumors from the same patient revealed that some schwannomatosis patients are somatic mosaics for NF2-gene changes. By contrast, other individuals, particularly those with a positive family history, appear to have an inherited predisposition to formation of tumors that carry somatic alterations of the NF2 gene. Further work is needed to define the pathogenetics of this unusual disease mechanism.

Sporadic and inherited schwannomas were scanned for the nature, frequency, and distribution of mutations in the NF2 locus encoding the merlin tumor suppressor protein on 22q. Of 58 tumors, 47% displayed loss of heterozygosity for NF2, leaving a total of 89 NF2 alleles to be examined. Pathogenic alterations were identified in 62 of these alleles, including 36 frameshifts with premature termination, 14 nonsense mutations, and 12 changes presumed to affect splicing. Effects of ten of the latter were confirmed in the NF2 transcript and indicated that activation of cryptic splice sites in coding sequence is another frequent mechanism leading to truncation of merlin. The mutations were relatively evenly distributed across both the protein 4.1 superfamily (exons 1-9) and the alpha-helical (exons 10-15) domains of merlin, but they did not occur at all in exons 16 and 17, which encode the protein's alternative COOH-termini. The data support the "two-hit" tumor suppressor model for formation of schwannomas and indicate that loss of merlin function can be achieved by truncation at various locations in the protein. However, the absence of mutations in exons 16 and 17 suggests that an inactivating mutation affecting only one of the merlin's alternative termini may not be sufficient to eliminate tumor suppressor function.

Schwannomas and meningiomas occur as multiple tumors in sufferers of neurofibromatosis 2 (NF2) and as solitary tumors in the general population due to the inactivation of a gene at chromosome 22q12. In 1993, a location cloning approach revealed this tumor suppressor, dubbed merlin, as a novel member of a family of proteins that link elements of the cytoskeleton and the cell membrane. Subsequent investigations have confirmed merlin's role in tumor formation, but have yet to reveal its mechanism of action.