The neurofibromatoses: when less is more

The study of cancer predisposition syndromes presents unique opportunities to gain insights into the genetic events associated with tumor pathogenesis. Individuals with two inherited cancer syndromes, neurofibromatosis 1 (NF1) and neurofibromatosis 2 (NF2), develop both benign and malignant tumors. The corresponding genes mutated in these two disorders encode tumor suppressor proteins, termed neurofibromin (NF1) and merlin (NF2), which function in novel ways to regulate cell growth and differentiation. Neurofibromin inhibits cell proliferation, at least in part, by modulating mitogenic pathway signaling through inactivation of p21-ras. In contrast, merlin may act as a membrane-associated molecular switch that regulates cell-cell and cell-matrix signals transduced by cell surface receptors. Significant progress in our understanding of the genetics and biology of NF1 and NF2 has elucidated the roles of these genes in tumor initiation and progression.

Tumorigenesis in neurofibromatosis: new insights and potential therapies

The neurofibromatoses NF1 and NF2 are inherited cancer predisposition syndromes in which affected individuals are prone to development of mostly benign, but occasionally malignant, tumors. The NF1 and NF2 genes function as tumor suppressor genes (negative growth regulators), such that their loss of expression predisposes to tumor formation. Neurofibromin, the protein product of the NF1 gene, acts as a negative regulator of the ras proto-oncogene, to reduce cell growth. Merlin, the NF2 gene product, is involved in regulating cell proliferation and motility, and probably plays a role in integrating multiple cell-signaling pathways. By understanding the function of these tumor suppressors, we have a unique opportunity to develop targeted pharmacotherapeutic interventions for these disorders.

The NF2 interactor, hepatocyte growth factor-regulated tyrosine kinase substrate (HRS), associates with merlin in the "open" conformation and suppresses cell growth and motility

The neurofibromatosis 2 tumor suppressor protein, merlin or schwannomin, functions as a negative growth regulator; however, its mechanism of action is not known. In an effort to determine how merlin regulates cell growth, we analyzed a recently identified novel merlin interactor, hepatocyte growth factor-regulated tyrosine kinase substrate (HRS). We demonstrate that regulated overexpression of HRS in rat schwannoma cells results in similar effects as overexpression of merlin, including growth inhibition, decreased motility and abnormalities in cell spreading. Previously, we showed that merlin forms an intramolecular association between the N- and C-termini and exists in "open" and "closed" conformations. Merlin interacts with HRS in the unfolded, or open, conformation. This HRS binding domain maps to merlin residues 453-557. Overexpression of C-terminal merlin has no effect on HRS function, arguing that merlin binding to HRS does not negatively regulate HRS growth suppressor activity. These results suggest the possibility that merlin and HRS may regulate cell growth in schwannoma cells through interacting pathways.

Neurofibromatosis 1 (NF1) heterozygosity results in a cell-autonomous growth advantage for astrocytes

Individuals with neurofibromatosis 1 (NF1) develop low-grade astrocytomas at an increased frequency. To gain insight into the function of the Nf1 gene product as a growth regulator for astrocytes, we examined mice heterozygous for a targeted Nf1 mutation. In our previous studies, we demonstrated increased numbers of proliferating astrocytes in Nf1 heterozygote (Nf1+/-) mice in vivo. We now show that cultured Nf1+/- astrocytes exhibit a cell-autonomous growth advantage in vitro associated with increased p21-ras pathway activation. Furthermore, we demonstrate that Nf1+/-;wild-type N-ras mice have a similar astrocyte growth advantage in vitro and in vivo as either oncogenic N-ras or Nf1+/-; oncogenic N-ras mice. Lastly, mice heterozygous for targeted defects in both Nf1 and p53 as well as Nf1 and Rb exhibit 3- and 2.5-fold increases in astrocyte proliferation in vivo, respectively, suggesting that abnormalities in Nf1- and p53/Rb-regulated pathways cooperate in the heterozygous state to confer a growth advantage for brain astrocytes. Collectively, these results provide evidence for a cell-autonomous growth advantage in Nf1+/- astrocytes and suggest that some of the brain pathology in individuals with NF1 might result from reduced, but not absent, NF1 gene function.

Merlin, DAL-1, and progesterone receptor expression in clinicopathologic subsets of meningioma: a correlative immunohistochemical study of 175 cases

The molecular pathogenesis of meningiomas is poorly characterized. Loss of NF2 (merlin) expression has been reported in 30%-80% of all sporadic meningiomas. Recently, we found that loss of expression for a second Protein 4.1-family tumor suppressor. DAL-1, is also common. A biologically important role for progesterone receptor (PR) has also been proposed based on its reported inverse relationship with tumor grade. In order to better define the pathogenetic roles of these proteins, we studied the merlin, DAL-1, and PR immunoprofiles in 175 fully characterized meningiomas, including nonrecurring versus recurring benign, proliferative versus brain invasive atypical and anaplastic subtypes. Loss of expression for either Protein 4.1-family tumor suppressor (merlin or DAL-1) was almost universal (92%), with combined losses being common (58%). Individually, absence of merlin or DAL-1 protein was detected in 74% and 76% respectively, with no significant differences among the 5 subsets. PR immunoreactivity was commonly associated with retained DAL-1 expression (p < 0.001) and with tumor grade, with 51% of benign, 21% of atypical, and 11% of anaplastic tumors staining positive (p < 0.001). We conclude that PR immunohistochemistry may have diagnostic utility in meningothelial neoplasms. Protein 4.1-family tumor suppressor losses are likely important early events in meningioma pathogenesis, whereas PR expression is associated with benignity.

Loss of neurofibromin is associated with activation of RAS/MAPK and PI3-K/AKT signaling in a neurofibromatosis 1 astrocytoma

Neurofibromatosis 1 (NF1) is a common autosomal dominant cancer predisposition syndrome, in which 15% to 20% of affected individuals develop astrocytomas. Neurofibromin, the protein product of the NF1 gene, functions as a tumor suppressor, largely by inhibiting Ras activity. While loss of neurofibromin has been implicated in the molecular pathogenesis of other NF1-associated tumors, there is no formal evidence demonstrating loss of neurofibromin function in NF1-associated astrocytomas. In this report, we describe an NF1 patient from whom both astrocytoma tumor tissue as well as corresponding non-neoplastic white matter were available for analysis. Loss of neurofibromin expression was observed in the tumor and was associated with elevated levels of Ras-GTP. However, elevated Ras-GTP levels were not the result of oncogenic Ras mutations, altered p120-GAP function, growth factor receptor activation, or abnormal p53, Rb, or p16 expression. Furthermore, increased Raf-MAPK and PI3-K/Akt activity was detected in the NF1 astrocytoma compared with the corresponding normal white matter. These results support a role for neurofibromin as the critical GAP in the molecular pathogenesis of NF1 astrocytomas.

Malignant peripheral nerve sheath tumors in neurofibromatosis 1

One of the most clinically aggressive cancers associated with neurofibromatosis 1 (NF1) is the malignant peripheral nerve sheath tumor (MPNST). To determine the incidence and relative risk (RR) of MPNSTs in individuals with NF1, 1,475 individuals with NF1 were included from a cohort of patients examined by a single experienced geneticist from 1977 to 1996. The end points were incidence of MPNST, relative risk of MPNST, and relative risk associated with specific NF1 physical findings. Thirty-four individuals were identified with MPNST (2%). The relative risk of MPNST was higher than expected with an RR value of 113 (95% confidence interval [CI] = 78-158). The average 10-year annual incidence of MPNST between the second and fifth decade of life was roughly the same with a range of 0.0013 and 0.0068 MPNST per patient year. Most lesions occurred in the limbs (n = 18; 53%), and those with limb lesions survived longer than those with nonlimb MPNSTs. Pain associated with a mass was the greatest risk factor associated with MPNST development (RR = 31.4; 95% CI = 13.2-75.1). Further biological and epidemiological studies are needed to determine other factors that influence the risk of MPNST development in individuals affected with NF1. Am. J. Med. Genet. 93:388-392, 2000. Published 2000 Wiley-Liss, Inc.

Loss of neurofibromatosis 1 (NF1) gene expression in NF1-associated pilocytic astrocytomas

The critical role of the neurofibromatosis 1 (NF1) gene as a tumour suppressor has been clearly demonstrated for malignancies arising in NF1 patients. However, little is known about the more common benign tumours, such as the pilocytic astrocytoma. Most NF1-associated astrocytomas are benign and clinically non-progressive, though aggressive tumours are occasionally encountered. In this study, eight pilocytic astrocytomas from six individuals affected with NF1 were analysed for NF1 expression. All eight tumours demonstrated loss of neurofibromin expression by immunohistochemistry, which was confirmed in one case using Western blot analysis. Microsatellite analysis showed loss of a single NF1 allele (LOH) in two of four NF1-associated tumours. These results demonstrate that, in contrast to sporadic astrocytomas, loss of NF1 expression is an important primary genetic event in the pathogenesis of NF1-associated pilocytic astrocytomas.

The neurofibromatosis 2 tumor suppressor protein interacts with hepatocyte growth factor-regulated tyrosine kinase substrate

The neurofibromatosis 2 tumor suppressor protein schwannomin/merlin is commonly mutated in schwannomas and meningiomas. Schwannomin, a member of the 4.1 family of proteins, which are known to link the cytoskeleton to the plasma membrane, has little known function other than its ability to suppress tumor growth. Using yeast two-hybrid interaction cloning, we identified the HGF-regulated tyrosine kinase substrate (HRS) as a schwannomin interactor. We verified the interaction by both immunoprecipitation of endogenous HRS with endogenous schwannomin in vivo as well as by using bacterially purified HRS and schwannomin in vitro. We narrowed the regions of interaction to include schwannomin residues 256-579 and HRS residues from 480 to the end of either of two HRS isoforms. Schwannomin molecules with a L46R, L360P, L535P or Q538P missense mutation demonstrated reduced affinity for HRS binding. As HRS is associated with early endosomes and may mediate receptor translocation to the lysosome, we demonstrated that schwannomin and HRS co-localize at endosomes using the early endosome antigen 1 in STS26T Schwann cells by indirect immunofluorescence. The identification of schwannomin as a HRS interactor implicates schwannomin in HRS-mediated cell signaling.

Loss of DAL-1, a protein 4.1-related tumor suppressor, is an important early event in the pathogenesis of meningiomas

Meningiomas are common nervous system tumors, whose molecular pathogenesis is poorly understood. To date, the most frequent genetic alteration detected in these tumors is loss of heterozygosity (LOH) on chromosome 22q. This finding led to the identification of the neurofibromatosis 2 (NF2) tumor suppressor gene on 22q12, which is inactivated in 40% of sporadic meningiomas. The NF2 gene product, merlin (or schwannomin), is a member of the protein 4.1 family of membrane-associated proteins, which also includes ezrin, radixin and moesin. Recently, we identified another protein 4.1 gene, DAL-1 (differentially expressed in adenocarcinoma of the lung) located on chromosome 18p11.3, which is lost in approximately 60% of non-small cell lung carcinomas, and exhibits growth-suppressing properties in lung cancer cell lines. Given the homology between DAL-1 and NF2 and the identification of significant LOH in the region of DAL-1 in lung, breast and brain tumors, we investigated the possibility that loss of expression of DAL-1 was important for meningioma development. In this report, we demonstrate DAL-1 loss in 60% of sporadic meningiomas using LOH, RT-PCR, western blot and immunohistochemistry analyses. Analogous to merlin, we show that DAL-1 loss is an early event in meningioma tumorigenesis, suggesting that these two protein 4.1 family members are critical growth regulators in the pathogenesis of meningiomas. Furthermore, our work supports the emerging notion that membrane-associated alterations are important in the early stages of neoplastic transformation and the study of such alterations may elucidate the mechanism of tumorigenesis shared by other tumor types.

Clinic-based study of plexiform neurofibromas in neurofibromatosis 1

Individuals with neurofibromatosis 1 (NF1) develop both benign and malignant tumors at an increased frequency. One of the most common benign tumors in NF1 is the plexiform neurofibroma. These tumors cause significant morbidity and mortality on account of their propensity to grow and affect adjacent normal tissues. To determine the clinical profile of plexiform neurofibromas in NF1, we conducted a retrospective review of 68 NF1 patients with plexiform neurofibroma. In our series, 44% of tumors were detected by 5 years of age and most were located in the trunk and extremities. Only two patients developed malignant peripheral nerve sheath tumors in their preexisting plexiform neurofibromas. Lastly, we demonstrate that there were no specific clinical features of NF1 associated with the presence of plexiform neurofibroma. These results underscore the importance of careful serial examinations in the evaluation of patients with NF1.

Genetic and cellular defects contributing to benign tumor formation in neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is a common inherited cancer predisposition syndrome. The NF1 gene product, neurofibromin, is hypothesized to function as a tumor suppressor and nearly all NF1 patients develop benign peripheral nerve tumors. These neurofibromas presumably arise from NF1 inactivation in S100(+)Schwann cells, but there is no formal proof for this mechanism. We demonstrate that fibro-blasts isolated from neurofibromas carried at least one normal NF1 allele and expressed both NF1 mRNA and protein, whereas the S100(+)cells typically lacked the NF1 transcript. Our findings further indicate that additional molecular events aside from NF1 inactivation in Schwann cells and/or other neural crest derivatives contribute to neurofibroma formation.

Expression of the tuberous sclerosis complex gene products, hamartin and tuberin, in central nervous system tissues

Tuberous sclerosis complex (TSC) is a common genetic disorder in which affected individuals can develop mental retardation, developmental brain defects, and seizures. Two genetic loci are responsible for TSC: TSC1 on chromosome 9q and TSC2 on chromosome 16p. Here, we report our analysis of TSC1 (hamartin) and TSC2 (tuberin) protein expression in the central nervous system (CNS). Both tuberin and hamartin are expressed in neurons and astrocytes where they physically interact. In the mouse cerebellum in vivo, tuberin predominantly localizes to the perinuclear region of the Purkinje cell, whereas hamartin is distributed along neuronal or astrocytic processes. In contrast, both hamartin and tuberin demonstrate similar neuronal expression patterns in pure neuronal cultures in vitro. Additionally, hamartin is highly expressed in astrocytes in mixed neuron-glia cultures in vitro, suggesting that hamartin may be important for astrocyte growth control. Unlike tuberin, loss of hamartin expression was not observed in sporadic astrocytomas. These results suggest that tuberin and hamartin may differentially contribute to the CNS pathology in TSC.

Neurofibromatosis 2 tumor suppressor protein, merlin, forms two functionally important intramolecular associations

The neurofibromatosis 2 (NF2) tumor suppressor gene product, merlin (schwannomin) forms an intramolecular association that is required for negative growth regulation in vitro and in vivo. In an effort to develop a molecular model for merlin relevant to its tumor suppressor function, we further characterized merlin intramolecular folding. We now demonstrate that merlin forms two intramolecular associations, one between the amino terminal (N-term) domain and the carboxyl terminal (C-term) domain and another within the amino terminal domain (N-term/N-term) itself. The N-term/C-term domain interaction requires contact between residues 302-308 in the N-term and an intact exon 17 (residues 580-595) in the C-term domain. In addition, we demonstrate that the N-term/N-term domain self-interaction is required for N-term/C-term domain association. Lastly, we identify NF2 patient mutations that dramatically reduce each of these interactions in vitro. Based on these findings, we propose a model for merlin folding critical to its ability to function as a tumor suppressor protein.

Developmental regulation of a neuron-specific neurofibromatosis 1 isoform

We have identified a protein isoform of the neurofibromatosis 1 (NF1) gene (neurofibromin) containing the alternatively spliced exon 9a that is expressed in forebrain neurons. Exon 9a neurofibromin is localized in the cytoplasm, sediments in a P100 fraction, and is expressed throughout the soma and processes in cortical neurons in vitro. Expression of exon 9a neurofibromin is developmentally regulated, with expression first detected after postnatal day 2. The identification of this novel protein isoform with restricted neuronal expression suggests novel functions for neurofibromin in the postmitotic brain that are perhaps relevant to the learning disabilities observed in children with NF1.

Differential effects of cAMP in neurons and astrocytes. Role of B-raf

Mitogen-activated protein kinase (MAPK) activation provides cell type-specific signals important for cellular differentiation, proliferation, and survival. Cyclic AMP (cAMP) has divergent effects on MAPK activity depending on whether signaling is through Ras/Raf-1 or Rap1/B-raf. We found that central nervous system-derived neurons, but not astrocytes, express B-raf. In neurons, cAMP activated MAPK in a Rap1/B-raf-dependent manner, while in astrocytes, cAMP decreased MAPK activity. Inhibition of MAPK in neurons decreased neuronal growth factor-mediated survival, and activation of MAPK by cAMP analogues rescued neurons from death. Furthermore, constitutive expression of B-raf in astrocytoma cells increased MAPK activation, as seen in neurons, and enhanced proliferation. These data provide the first experimental evidence that B-raf is the molecular switch which dominantly permits differential cAMP-dependent regulation of MAPK in neurons versus astrocytes, with important implications for both survival and proliferation.

Overexpression of bax in human glioma cell lines

OBJECT

Cells that lose their ability to undergo apoptosis may promote the development of neoplasms and result in resistance to clinical treatment with DNA-damaging modalities such as radio- and chemotherapy. Four established human glioma cell lines that are resistant to apoptosis were transfected with the proapoptotic gene bax and assessed for their sensitivity to a proapoptotic stimulus.

METHODS

Two cell lines had a wild-type p53 genotype (U87 and D247MG) and two had mutant p53 genotypes (U138 and U373). Constitutive overexpression of murine bax was achieved in U138 and U373 only, which resulted in an increased sensitivity of these lines to the apoptosis-inducing effect of cytosine arabinoside (ara-C). Multiple attempts to produce constitutive overexpression of bax in U87 and D247MG cells resulted in spontaneous, near-complete cell loss. Vector-only control transfections were successful in all four cell lines. Inducible overexpression of bax was achieved in the U87 cells and elevated levels of BAX were observed as early as 6 hours after gene induction. This overexpression of BAX resulted in the spontaneous induction of apoptosis in these cells.

CONCLUSIONS

Overexpression of BAX in four human glioma cell lines resulted in increased sensitivity to apoptosis. In the two lines that had a wild-type p53 genotype, overexpression of BAX produced spontaneous apoptosis. In contrast, the lines that had mutant, nonfunctional P53 did not undergo spontaneous apoptosis, but they were rendered more sensitive to the apoptosis-inducing effect of ara-C. Modulation of BAX expression may be a useful therapeutic modality for gliomas, regardless of p53 genotype.

Haploinsufficiency for the neurofibromatosis 1 (NF1) tumor suppressor results in increased astrocyte proliferation

Individuals affected with neurofibromatosis 1 (NF1) harbor increased numbers of GFAP-immunoreactive cerebral astrocytes and develop astrocytomas that can lead to blindness and death. Mice heterozygous for a targeted Nf1 mutation (Nf1+/-) were employed as a model for the human disease to evaluate the hypothesis that reduced NF1 protein (neurofibromin) expression may confer a growth advantage for astrocytes, such that inactivation of only one NF1 allele is sufficient for abnormal astrocyte proliferation. Here, we report that Nf17+/- mice have increased numbers of cerebral astrocytes and increased astrocyte proliferation compared to wild-type littermates. Intriguingly, primary Nf1+/- astrocyte cultures failed to demonstrate a cell-autonomous growth advantage unless they were cocultured with C17 neuronal cells. This C17 neuronal cell-induced Nf1+/- increase in proliferation was blocked by MEK inhibition (PD98059), suggesting a p21-ras-dependent effect. Furthermore, mice heterozygous for a targeted mutation in another GAP molecule, p120-GAP, demonstrated no increases in cerebral astrocyte number. These findings suggest that reduced NF1 expression results in a cell context-dependent increase in astrocyte proliferation that may be sufficient for the development of astrocytic growth abnormalities in patients with NF1.

Molecular analysis of malignant triton tumors

Triton tumors are rare variants of malignant peripheral nerve sheath tumor (MPNST) with muscle differentiation, often seen in patients with neurofibromatosis 1 (NF1). Individuals affected with NF1 harbor mutations in the NF1 tumor suppressor gene and develop neurofibromas and MPNSTs. The NF1 gene is expressed in Schwann cells and its expression is lost in schwannian neoplasms, suggesting a role in malignant development. Separately, there is evidence that p53 suppressor gene mutations are involved in MPNSTs. To determine the role of the NF1 and p53 genes in the development of the malignant Triton tumor we examined 2 such tumors, 1 from a 3-year-old boy without clinical manifestations of NF1 and another from a 24-year-old man with NF1. Histological analysis of these tumors showed both neural and muscle differentiation with S-100 and desmin immunoreactivity, respectively. Reverse transcribed RNA polymerase chain reaction (RT-PCR) of NF1 mRNA showed NF1 expression in the sporadic tumor. Strong nuclear immunoreactivity for p53 was observed throughout the malignant population in both tumors. This was confirmed by loss of heterozygosity for p53 in the non-NF1 patient, suggesting that p53 is involved in both hereditary and sporadic Triton tumors. The finding of preserved NF1 gene expression in the non-NF1-related Triton tumor suggests that different genetic events predispose to the development of this rare neoplasm in sporadic cases.

Intracranial gliomas in neurofibromatosis type 1

Optic pathway gliomas and brainstem gliomas are the predominant intracranial neoplasms associated with neurofibromatosis type 1 (NF1). Before the past 15 years, studies of optic pathway gliomas in NF1 were hampered by the inaccurate diagnosis of NF1, the unavailability of noninvasive neuroimaging techniques, and the frequent rendering of what would now be considered unnecessary, overly aggressive therapy. When studied systematically, these tumors behave in a much more benign fashion than their counterparts in children who do not have NF1. While they may cause symptoms in as many of 50% of cases, progression to the point where specific intervention is deemed necessary is unusual. Consequently, screening neuroimaging of asymptomatic patients is unwarranted. Because optic pathway tumors universally arise in children younger than 7 years of age, all such children should undergo yearly ophthalmologic evaluations and annual assessments of growth to monitor for signs of precocious puberty. Am. J. Med. Genet. (Semin. Med. Genet.) 89:38-44, 1999.

Establishment of primary vestibular schwannoma cultures from neurofibromatosis type-2 patients

Primary cultures were established from vestibular schwannomas of NF2 patients. The cultured tumor cells were selectively amplified by growth factor supplemented medium and characterized by immunocytochemistry. NF2 cDNA was amplified by RT-PCR and mutations were detected by both the non-isotopic RNase cleavage assay and direct DNA sequencing, no detectable wild-type NF2 transcript was found in cDNA from the cultured cells. Distinguishable morphology and growth rate differences have been observed in different passages of the primary cells. The data suggest that a pure schwannoma primary culture can be established and could be very useful in vitro model for further understanding the NF2 gene function in Schwann cells.

Increased expression of the NF2 tumor suppressor gene product, merlin, impairs cell motility, adhesionand spreading

The neurofibromatosis 2 ( NF2 ) gene product, merlin, is a tumor suppressor protein mutated in schwanno-mas and several other tumors. Merlin, which shares significant homology with the actin-associated proteins ezrin, radixin and moesin (ERM proteins), inhibits cell growth when overexpressed in cell lines. The similarities between merlin and ERM proteins suggest that merlin's growth-regulatory capabilities may be due to alterations in cytoskeletal function. We examined this possibility in rat schwannoma cell lines overexpressing wild-type merlin isoforms and mutant merlin proteins. We found that overexpression of wild-type merlin resulted in transient alterations in F-actin organization, cell spreading and cell attachment. Merlin overexpression also impaired cell motility as measured in an in vitro motility assay. These effects were only observed in cells overexpressing a merlin isoform capable of inhibiting cell growth and not with mutant merlin molecules (NF2 patient mutations) or a merlin splice variant (isoform II) lacking growth-inhibitory activity. These data indicate that merlin may function to maintain normal cytoskeletal organization, and suggest that merlin's influence on cell growth depends on specific cytoskeletal rearrangements.

Parallels between tuberous sclerosis complex and neurofibromatosis 1: common threads in the same tapestry

Neurofibromatosis type 1 (NF1) and tuberous sclerosis complex (TSC) represent two neurocutaneous disorders in which affected individuals develop tumors at an increased frequency. Although the clinical manifestations of these disorders are distinctive, the identification of the genes responsible for these disorders has demonstrated remarkable similarities on a molecular level between the NF1 and TSC tumor suppressor gene products. The NF1 and TSC2 gene products are hypothesized to function as growth regulators by modulating the activities of small GTPase molecules. The overlap between the functions of these tumor suppressor genes has yielded important insights into the molecular pathogenesis underlying each of these disorders and suggested possible pharmacological therapies specifically targeted for affected individuals.

Neurofibromatosis type 1: piecing the puzzle together

Neurofibromatosis type 1 (NF1) was first described in 1882 and is characterized by a diverse spectrum of clinical manifestations, including neurofibromas, café au lait spots, and Lisch nodules. NF1 is also noted for the higher risk of associated malignancies, making it the most common tumour-predisposing disease in humans. Transmitted in an autosomal dominant manner, the NF1 gene was cloned in 1990, and belongs to the family of tumour suppressor genes. Since then, there has been an explosion in our understanding of how the gene product, neurofibromin, functions in normal cellular physiology, and how its loss in NF1 relates to the wide spectrum of clinical findings, including NF1-associated tumours. Neurofibromin is a major negative regulator of a key signal transduction pathway in cells, the Ras pathway, which transmits mitogenic signals to the nucleus. Loss of neurofibromin leads to increased levels of activated Ras (bound to GTP), and thus increased downstream mitogenic signaling. Our understanding of neurofibromin's role within cells has allowed for the development of pharmacological therapies which target the specific molecular abnormalities in NF1 tumours. These include the farnesyl transferase inhibitors, which inhibit the post-translational modification of Ras, and other agents which modulate Ras-mediated signaling pathways.

Defects in neurofibromatosis 2 protein function can arise at multiple levels

Neurofibromatosis 2 (NF2) is an inherited cancer syndrome resulting from mutations in the NF2 tumor suppressor gene. Analysis of NF2 mutations has revealed some general genotype-phenotype correlations. Severe disease has been associated with mutations that produce a premature termination while more mild disease has been associated with missense mutations. Here, we provide experimental proof for these genotype-phenotype correlations by demonstrating that nonsense mutations fail to produce stable merlin protein while missense mutations result in the generation of merlin proteins defective in negative growth regulation. This inability to suppress cell growth may result from defects in the function of merlin at several levels, including failure to form an intramolecular complex. Based on these findings, we propose a model for merlin growth suppression that provides a framework for analyzing NF2 patient mutations and merlin function.

Merlin differentially associates with the microtubule and actin cytoskeleton

The neurofibromatosis 2 (NF2) suppressor gene encodes a protein termed merlin (or schwannomin) with sequence similarity to a family of proteins that link the actin cytoskeleton to cell surface glycoproteins. Members of this ERM family of proteins include ezrin, radixin, and moesin. These proteins contain a carboxyl (C-) terminus actin binding site. In contrast to the ERM proteins, merlin lacks the conventional C-terminal actin binding site, but still localizes to the ruffling edge of plasma membranes. In this study, we investigate the ability of merlin to interact with actin through a nonconventional actin binding domain. We demonstrate for the first time that merlin can associate with polymerized actin in vitro by virtue of an amino (N-) terminal actin binding domain including residues 178-367. Merlin actin binding is not affected by several naturally-occurring NF2 patient mutations or alternatively spliced isoforms. These results suggest that merlin, like other ERM proteins, can directly interact with the actin cytoskeleton. In addition, merlin associates with polymerized microtubules in vitro using a novel microtubule binding region in the N-terminal region of merlin that is masked in the full-length merlin molecule, such that wild-type functional merlin in the "closed" conformation fails to bind polymerized microtubules. These microtubule association results confirm the notion that merlin exists in "open" and "closed" conformations relevant to its function as a negative growth regulator.

Transfection of C6 glioma cells with the bax gene and increased sensitivity to treatment with cytosine arabinoside

OBJECT

Genes known to be involved in the regulation of apoptosis include members of the bcl-2 gene family, such as inhibitors of apoptosis (bcl-2 and bcl-xl) and promoters of apoptosis (bax). The authors investigated a potential approach for the treatment of malignant gliomas by using a gene transfection technique to manipulate the level of an intracellular protein involved in the control of apoptosis.

METHODS

The authors transfected the murine bax gene, which had been cloned into a mammalian expression vector, into the C6 rat glioma cell line. Overexpression of the bax gene resulted in a decreased growth rate (average doubling time of 32.96 hours compared with 22.49 hours for untransfected C6, and 23.11 hours for clones transfected with pcDNA3 only), which may be caused, in part, by an increased rate of spontaneous apoptosis (0.77 +/- 0.15% compared with 0.42 +/- 0.08% for the vector-only transfected C6 cell line; p = 0.038, two-tailed Student's t-test). Treatment with 1 microM cytosine arabinoside (ara-C) resulted in significantly more cells undergoing apoptosis in the cell line overexpressing bax than in the vector-only control cell line (23.57 +/- 2.6% compared with 5.3 +/- 0.7% terminal deoxynucleotidyl transferase-mediated biotinylated-deoxyuridine triphosphate nick-end labeling technique-positive cells; p = 0.007). Furthermore, measurements of growth curves obtained immediately after treatment with 0.5 microM ara-C demonstrated a prolonged growth arrest of at least 6 days in the cell line overexpressing bax.

CONCLUSIONS

These results can be used collectively to argue that overexpression of bax results in increased sensitivity of C6 cells to ara-C and that increasing bax expression may be a useful strategy, in general, for increasing the sensitivity of gliomas to antineoplastic treatments.

Interdomain binding mediates tumor growth suppression by the NF2 gene product

The neurofibromatosis 2 (NF2) tumor suppressor gene encodes an intracellular membrane-associated protein, called merlin (or schwannomin), that belongs to the band 4.1 family of cytoskeleton-associated proteins. Inactivating NF2 mutations occur in several sporadic tumor types and have been linked to the NF2 disease, whose hallmark is the development of bilateral Schwann cell tumors (schwannomas) of the eighth cranial nerve. Two major alternatively spliced NF2 variants are expressed in normal tissues: 'NF2-17' lacking exon 16 and 'NF2-16' that contains exon 16 and encodes a merlin protein truncated at the C-terminus. We report that overexpression of NF2-17 in rat schwannoma cells inhibits their growth in vitro and in vivo, while NF2-16 fails to influence schwannoma growth. Tumor growth inhibition by merlin depends on an interdomain association occurring either in cis or in trans between the N- and C-termini. This association does not occur in the truncated NF2-16 protein nor in a mutant NF2-17 protein lacking C-terminal sequences. These data indicate that merlin has a unique mechanism of tumor suppression, inhibiting cell proliferation via self-association.

Alterations in the rap1 signaling pathway are common in human gliomas

Several inherited predisposition to cancer syndromes are associated with the development of nervous system tumors. Tuberous sclerosis complex (TSC) is an autosomal dominant disorder in which affected individuals are at risk for developing astrocytomas. One of the genes responsible for this disorder is TSC2, located on chromosome 16p, and encoding a 180 kDa protein (tuberin) that functions in part as a negative regulator of rap1. Previous studies from our laboratory demonstrated that 30% of sporadic astrocytomas have reduced or absent tuberin expression. In addition to loss of tuberin in sporadic astrocytomas, aberrant rap1 mediated signaling may also result from overexpression of rap1. In this study, we test the hypothesis that alterations in the rap1 signaling pathway are frequently observed in certain subsets of gliomas compared to other tumors of the nervous system. Analysis of sporadic astrocytomas and ependymomas demonstrated either increased rap1 or reduced/absent tuberin protein expression in 50-60% of different cohorts of these gliomas, compared to 30-33% of sporadic schwannomas and meningiomas and none of eight oligodendrocyte tumors. These results suggest that alterations in the rap1 signaling pathway are important in the development of certain sporadic human gliomas.

Reduced TSC2 RNA and protein in sporadic astrocytomas and ependymomas

Individuals affected with tuberous sclerosis complex (TSC) develop several benign and malignant tumors at increased frequency, including astrocytomas. Tuberin, the protein product of the tuberous sclerosis complex-2 (TSC2) tumor suppressor gene, has been shown to directly inhibit cell growth and is expressed at high levels in normal central nervous system neurons and astrocytes. To determine whether TSC2 RNA and protein are reduced in astrocytomas from individuals without tuberous sclerosis, reverse transcriptase-polymerase chain reaction and immunoblotting analyses were performed on 49 adult astrocytomas, 10 pediatric astrocytomas, and 13 ependymomas. Eighteen of 40 (45%) high-grade (World Health Organization [WHO] grade III/IV) astrocytomas and 4 of 8 (50%) adult low-grade (WHO grade II) astrocytomas demonstrated reduced or absent TSC2 expression, including 1 giant cell astrocytoma, whereas none of the 10 pediatric low-grade astrocytomas analyzed showed a reduction in TSC2 expression. Reduced or absent tuberin was observed in 2 of 6 (33%) ependymomas analyzed. These data demonstrate, for the first time, that reduced or absent TSC2 expression may represent one of the critical genetic events associated with the development of sporadic adult, but not pediatric, astrocytomas.

The diagnostic evaluation and multidisciplinary management of neurofibromatosis 1 and neurofibromatosis 2

OBJECTIVE

Neurofibromatosis 1 and neurofibromatosis 2 are autosomal dominant genetic disorders in which affected individuals develop both benign and malignant tumors at an increased frequency. Since the original National Institutes of Health Consensus Development Conference in 1987, there has been significant progress toward a more complete understanding of the molecular bases for neurofibromatosis 1 and neurofibromatosis 2. Our objective was to determine the diagnostic criteria for neurofibromatosis 1 and neurofibromatosis 2, recommendations for the care of patients and their families at diagnosis and during routine follow-up, and the role of DNA diagnostic testing in the evaluation of these disorders.

DATA SOURCES

Published reports from 1966 through 1996 obtained by MEDLINE search and studies presented at national and international meetings.

STUDY SELECTION

All studies were reviewed and analyzed by consensus from multiple authors.

DATA EXTRACTION

Peer-reviewed published data were critically evaluated by independent extraction by multiple authors.

DATA SYNTHESIS

The main results of the review were qualitative and were reviewed by neurofibromatosis clinical directors worldwide through an Internet Web site.

CONCLUSIONS

On the basis of the information presented in this review, we propose a comprehensive approach to the diagnosis and treatment of individuals with neurofibromatosis 1 and neurofibromatosis 2.

The diagnostic evaluation and multidisciplinary management of neurofibromatosis 1 and neurofibromatosis 2

OBJECTIVE

Neurofibromatosis 1 and neurofibromatosis 2 are autosomal dominant genetic disorders in which affected individuals develop both benign and malignant tumors at an increased frequency. Since the original National Institutes of Health Consensus Development Conference in 1987, there has been significant progress toward a more complete understanding of the molecular bases for neurofibromatosis 1 and neurofibromatosis 2. Our objective was to determine the diagnostic criteria for neurofibromatosis 1 and neurofibromatosis 2, recommendations for the care of patients and their families at diagnosis and during routine follow-up, and the role of DNA diagnostic testing in the evaluation of these disorders.

DATA SOURCES

Published reports from 1966 through 1996 obtained by MEDLINE search and studies presented at national and international meetings.

STUDY SELECTION

All studies were reviewed and analyzed by consensus from multiple authors.

DATA EXTRACTION

Peer-reviewed published data were critically evaluated by independent extraction by multiple authors.

DATA SYNTHESIS

The main results of the review were qualitative and were reviewed by neurofibromatosis clinical directors worldwide through an Internet Web site.

CONCLUSIONS

On the basis of the information presented in this review, we propose a comprehensive approach to the diagnosis and treatment of individuals with neurofibromatosis 1 and neurofibromatosis 2.

Loss of merlin expression in sporadic meningiomas, ependymomas and schwannomas

Neurofibromatosis 2 (NF2) is an inherited disorder in which affected individuals develop schwannomas and meningiomas. NF2 is mapped to chromosome 22q in a region where frequent loss of heterozygosity also occurs in sporadic meningiomas, ependymomas, and schwannomas. Using NF2 protein (merlin or schwannomin)-specific antibodies, 11 of 14 sporadic schwannomas, three of eight sporadic ependymomas, and 16 of 19 sporadic meningiomas demonstrated significantly reduced or absent merlin expression, suggesting that NF2 may be involved in the pathogenesis of these sporadic tumors.

Mutations in the GAP-related domain impair the ability of neurofibromin to associate with microtubules

The neurofibromatosis 1 (NF1) gene encodes a cytoplasmic protein with structural and functional homology to GTPase activating proteins (GAPs) for p21-ras. Double-labeling immunofluorescence experiments using neurofibromin antibodies and in vitro microtubule assembly have demonstrated that the NF1 gene product, neurofibromin, interacts with cytoplasmic microtubules. The region critical for this interaction was shown to reside within the NF1-GAP-related domain (NF1GRD). Examination of the NFIGRD reveals a number of residues that are highly conserved amongst all GAP molecules. Mutational analysis of a representative number of these conserved residues demonstrated differential effects on NF1GRD p21-ras GAP activity. In this study, we examined the effect of these selected NF1GRD mutations on the ability of neurofibromin to associate with microtubules. Mutations at residues R1391, P1400 and K1423 disrupted microtubule association. In contrast, mutations at residues E1264, Q1426 and the insertion of exon 23a, critical for p21-ras regulation, did not impair microtubule association. These results demonstrate that some residues important for p21-ras regulation are also required for microtubule binding while other residues within the NF1GRD differentially affect p21-ras regulation and microtubule association.

Expression of the neurofibromatosis 2 tumor suppressor gene product, merlin, in Schwann cells

The neurofibromatosis 2 (NF2) tumor suppressor gene encodes a protein with sequence similarity to a family of molecules linking cell membrane proteins to the actin cytoskeleton. Individuals affected with NF2 develop schwannomas at an increased frequency. In this paper, we report that merlin is expressed in Schwann cells, where it is localized in vitro to the cell membrane by immunohistochemistry and subcellular fractionation. Exogenous expression of merlin fragments confirms this subcellular distribution and suggests that both the N-terminal and C-terminal portions of the molecule are required for this localization. In addition, merlin is expressed in rat sciatic nerve Schwann cells at paranodal membranes, where it colocalizes with RhoA. Lastly, expression of the NF2 gene increases during postnatal rat sciatic nerve development, consistent with its role as a negative growth regulator for Schwann cells. These results collectively suggest that merlin may function at the cell surface to modulate cell growth in Schwann cells and to link cell membrane proteins to the cytoskeleton.

Up-regulation of specific NF 1 gene transcripts in sporadic pilocytic astrocytomas

Pilocytic astrocytomas of the optic nerve (optic nerve gliomas) are closely associated with neurofibromatosis 1 (NF1), and allelic losses of the NF1 gene region on chromosome 17q occur in sporadic pilocytic astrocytomas. We therefore hypothesized that the NF1 gene acts as a tumor suppressor gene in pilocytic astrocytomas, and that NF1 gene expression would be reduced or absent in these tumors. To evaluate this possibility, we examined quantitative and qualitative aspects of NF1 gene expression in six sporadic pilocytic astrocytomas. Surprisingly, the NF1 gene was overexpressed up to fourfold in these tumors when compared with normal brain. This up-regulation was accompanied by immunohistochemical positivity using a carboxyl-terminal antibody and by the absence of mutations in one kilobase of the NF1 coding sequence, consistent with the expressed transcript and protein being full length and probably wild type. Pilocytic astrocytomas showed a marked predominance of transcripts containing exon 23a and lacking exon 9br, the same isoforms that are expressed by normal and reactive astrocytes and malignant astrocytomas. These data illustrate that pilocytic astrocytomas overexpress specific NF1 gene transcripts, perhaps as a regulatory response to growth stimuli. The role of the NF1 gene as a tumor suppressor in pilocytic astrocytomas, however, remains to be proven.

Expression of a developmentally-regulated neuron-specific isoform of the neurofibromatosis 1 (NF1) gene

Neurofibromatosis 1 (NF1) is a common autosomal dominant disorder in which affected individuals develop benign and malignant tumors as well as non-tumor-related abnormalities, such as seizures and learning disabilities. Here, we report an NF1 isoform arising from the alternative splicing of exon 9a with predominant central nervous system (CNS) expression. Exon 9a expression is enriched in neurons of the forebrain, specifically septum, striatum, cortex, hippocampus and olfactory bulb with significantly less expression in brainstem, cerebellum and spinal cord. This pattern of NF1 exon 9a expression correlates with the postnatal maturation of these neurons and suggests a role for NF1 in neuronal differentiation.

Increased neurofibromatosis 1 gene expression in astrocytic tumors: positive regulation by p21-ras

The neurofibromatosis 1 (NF1) gene has been implicated in astrocyte growth regulation in several studies. To determine whether loss of NF1 expression is associated with progression towards malignancy in sporadic astrocytomas from individuals without NF1, twenty-eight fresh astrocytoma operative specimens (low and high grade tumors) and seven primary human astrocytoma cell lines were examined for NF1 mRNA and protein expression. In all astrocytomas examined, increased NF1 expression was observed in the tumors relative to normal resting astrocytes. This increased neurofibromin expression correlated with elevated levels of activated p21-ras measured in both the fresh tumor specimens and the primary cell lines. Furthermore, when levels of activated p21 ras were decreased in astrocytoma cells expressing the ras inhibitory Asn-17 dominant-negative mutant, levels of neurofibromin expression decreased. In addition, fibroblasts induced to express oncogenic activated p21-ras(val12) had increased expression of NF1. These results suggested that neurofibromin expression is increased in human astrocytic tumors as a result of positive feedback regulation by increased levels of activated p21-ras.