Functional analysis of neurofibromatosis 2 (NF2) missense mutations.

Neurofibromatosis 2 (NF2) is a tumor predisposition syndrome in which affected individuals develop nervous system tumors at an increased frequency. The most common tumor in individuals with NF2 is the schwannoma, which is composed of neoplastic Schwann cells lacking NF2 gene expression. Moreover, inactivation of the NF2 gene is observed in nearly all sporadic schwannomas, suggesting that the NF2 gene is a critical growth regulator for Schwann cells. In an effort to gain insights into the function of the NF2 gene product, merlin or schwannomin, we performed a detailed functional analysis of eight naturally occurring non-conservative missense mutations in the NF2 gene. Using a regulatable expression system in rat schwannoma cells, we analyzed proliferation, actin cytoskeleton-mediated events and merlin folding. In this report, we demonstrate that mutations clustered in the predicted alpha-helical region did not impair the function of merlin whereas those in either the N- or C-terminus of the protein rendered merlin inactive as a negative growth regulator. These results suggest that the key functional domains of merlin lie within the highly conserved FERM domain and the unique C-terminus of the protein.     

The location of constitutional neurofibromatosis 2 (NF2) splice site mutations is associated with the severity of NF2

Neurofibromatosis 2 (NF2) patients with constitutional splice site NF2 mutations have greater variability in disease severity than NF2 patients with other types of mutations; the cause of this variability is unknown. We evaluated genotype-phenotype correlations, with particular focus on the location of splice site mutations, using mutation and clinical information on 831 patients from 528 NF2 families with identified constitutional NF2 mutations. The clinical characteristics examined were age at onset of symptoms of NF2 and number of intracranial meningiomas, which are the primary indices of the severity of NF2. Two regression models were used to analyse genotype-phenotype correlations. People with splice site mutations in exons 1–5 had more severe disease than those with splice site mutations in exons 11–15. This result is compatible with studies showing that exons 2 and 3 are required for self-association of the amino terminal of the NF2 protein in vitro, and that deletions of exons 2 and 3 in transgenic and knockout mouse models of NF2 cause a high prevalence of Schwann cell derived tumours.     

Reduced apoptosis rates in human schwannomas

Schwannomas, tumors originating from Schwann cells, represent a frequent neurological tumor and can occur both in a genetic disorder called neurofibromatosis type 2 (NF2) and sporadically. In both cases the genetic background is identical as all schwannomas are caused by biallelic mutations in the tumor suppressor gene NF2 coding for merlin. Mutations in this gene have also been found to be responsible for 50% to 60% of spontaneous and 100% of the NF2 associated meningiomas. The NF2 gene product, merlin, links transmembrane proteins to the cytoskeleton and is involved in intracellular signaling processes. It has previously been shown that reexpression of wild-type merlin in primary human schwannoma cells leads to an increase in the number of apoptotic cells. Here, we report in vivo and in vitro evidence that the basal apoptosis rate of primary human schwannoma cells is reduced in comparison to that of normal Schwann cells, supporting the idea that in this benign tumor type, apoptosis has a role in tumorigenesis.     

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.      

Further genotype – phenotype correlations in neurofibromatosis 2

Selvanathan SK, Shenton A, Ferner R, Wallace AJ, Huson SM, Ramsden RT, Evans DG. Further genotype–phenotype correlations in neurofibromatosis 2. Neurofibromatosis 2 (NF2) is caused by mutations in the NF2 gene predisposing carriers to develop nervous system tumours. Different NF2 mutations result in either loss/reduced protein function or gain of protein function (abnormally behaving mutant allele i.e. truncated protein potentially causing dominant negative effect). We present a comparison between the clinical presentations of patients with mutations that are predicted to produce truncated protein (nonsense/frameshift mutations) to those that results in loss of protein expression (large deletions) to elucidate further genotype–phenotype correlations in NF2. Patients with nonsense/frameshift mutations have a younger age of diagnosis and a higher prevalence/proportion of meningiomas (p = 0.002, p = 0.014), spinal tumours (p = 0.004, p = 0.004) and non‐VIII cranial nerve tumours (p = 0.006, p = 0.003). We also found younger age of diagnosis of vestibular schwannomas (p = 0.007), higher mean numbers of cutaneous lesions (p = 0.003) and spinal tumours (p = 0.006) in these patients. With respect to NF2 symptoms, we found younger age of onset of hearing loss (p = 0.010), tinnitus (p = 0.002), paraesthesiae (p = 0.073), wasting and weakness (p = 0.001) and headaches (p = 0.049) in patients with nonsense/frameshift mutations. Our comparison shows, additional, new correlations between mutations in the NF2 gene and the NF2 disease phenotype, and this further confirms that nonsense/frameshift mutations are associated with more severe NF2 symptoms. Therefore patients with this class of NF2 mutation should be followed up closely.     

Universal absence of merlin, but not other ERM family members, in schwannomas.

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.     

New variant translocation (8;20)(q22;q13) in bone marrow cells of extramedullary blast crisis in chronic myelogenous leukemia

We used a novel RNase cleavage assay (NIRCA) to screen for neurofibromatosis 2 (NF2) mutations in NF2 schwannomas. Mutations were found in tumors in 16 of 20 patients. Eleven patients (55%) had loss of heterozygosity or loss of one allele, indicating that the mutation was a germ-line mutation. The phenotypes of these patients were consistent with previous NF2 genotype-phenotype correlation studies: patients with nonsense mutations had severe phenotypes, whereas those with splice-site or missense mutations had milder and variable phenotypes. These results confirm the utility of NIRCA as a rapid and convenient method for screening for germ-line NF2 mutations.     

Neuropathology and Molecular Genetics of Neurofibromatosis 2 and Related Tumors

Neurofibromatosis 2 (NF2) is an uncommon, autosomal dominant disorder in which patients are predisposed to neoplastic and dysplastic lesions of Schwann cells (schwannomas and schwannosis), meningeal cells (meningiomas and meningioan-giomatosis) and glial cells (gliomas and glial hamar-tomas). Clinical and genetic criteria that distinguish NF2 from neurofibromatosis 1 have allowed more accurate assignment of specific pathological features to NF2. The NF2 tumor suppressor gene on chromosome 22q12 encodes a widely expressed protein, named merlin, which may link the cytoskeleton and cell membrane. Germline NF2 mutations in NF2 patients and somatic NF2 mutations in sporadic schwannomas and meningiomas have different mutational spectra, but most NF2 alterations result in a truncated, inactivated merlin protein. In NF2 patients, specific mutations do not necessarily correlate with phenotypic severity, although grossly truncating alterations may result in a more severe phenotype. In schwannomas, NF2 mutations are common and may be necessary for tumorigenesis. In meningiomas, NF2 mutations occur more commonly in fibroblastic than meningothelial subtypes, and may cluster in the first half of the gene. In addition, in meningiomas, a second, non- NF2 meningioma locus is probably also involved. Future efforts in NF2 research will be directed toward elucidating the role of merlin in the normal cell and the sequelae of its inactivation in human tumors.     

Characterization of chicken Nf2/merlin indicates regulatory roles in cell proliferation and migration.

The neurofibromatosis 2 (NF2) tumor suppressor protein merlin, or schwannomin, functions as a negative growth regulator such that inactivating mutations in Nf2 predispose humans to tumors. In addition, merlin has a critical role during embryonic development. Nf2-deficient mice die early during embryogenesis, with defects in gastrulation and extraembryonic tissues. To investigate the function of Nf2/merlin during embryonic development, we first identified the homologous Nf2 gene in chicken (cNf2) and examined the distribution of chicken merlin (c-merlin) during myogenesis. cNf2 encoded a full-length mRNA of 1,770 nucleotides and a protein of 589 residues. C-merlin shared high sequence homology and common protein motifs with vertebrate and Drosophila merlins. In addition, cNF2 functions as a negative growth regulator similar to human and Drosophila merlin in vitro. In vivo, c-merlin was expressed diffusely in the forming dermomyotome but down-regulated in migratory muscle precursors in the forelimb. As muscle formed in the limb, c-merlin expression was up-regulated. As an initial examination of c-merlin function during myogenesis, c-merlin was ectopically expressed in muscle precursors and the effects on muscle development were examined. We show that ectopic merlin expression reduces the proliferation of muscle precursors as well as their ability to migrate effectively in limb mesoderm. Collectively, these results demonstrate that c-merlin is developmentally regulated in migrating and differentiating myogenic cells, where it functions as a negative regulator of both muscle growth and motility.     

Genotype-phenotype correlations in von Hippel-Lindau disease

von Hippel-Lindau (VHL) disease is a dominantly inherited familial cancer syndrome resulting from mutations in the VHL tumor suppressor gene. VHL disease displays marked variation in expression and the presence of pheochromocytoma has been linked to missense VHL mutations. We analyzed genotype-phenotype correlations in 573 individuals with VHL disease. Routine clinical and radiological surveillance of VHL patients and at-risk relatives was associated with increased detection of retinal angiomatosis (73 vs. 59% of cases) and a reduction in age at diagnosis of renal cell carcinoma (RCC) (44.0+/-10.9 vs. 39.7+/-10.3 years). We confirmed the association of pheochromocytoma with missense mutations described previously, but stratifying missense mutations into those that resulted in substitution of a surface amino acid and those that disrupted structural integrity demonstrated that surface amino acid substitutions conferred a higher pheochromocytoma risk. Age at first manifestation of VHL disease was significantly earlier (P=0.001), and age-related risks of retinal angiomas and RCC were higher (P=0.022 and P=0.0008, respectively) in individuals with a nonsense or frameshift mutation than in those with deletions or missense mutations that disrupted the structural integrity of the VHL gene product (pVHL). These results extend genotype-phenotype-protein structure correlations in VHL disease and provide a baseline for future chemoprevention studies in VHL disease.     

TECTA mutations in Japanese with mid-frequency hearing loss affected by zona pellucida domain protein secretion

TECTA gene encodes α-tectorin, the major component of noncollagenous glycoprotein of the tectorial membrane, and has a role in intracochlear sound transmission. The TECTA mutations are one of the most frequent causes of autosomal dominant (AD) hearing loss and genotype-phenotype correlations are associated with mutations of TECTA in exons according to α-tectorin domains. In this study, we investigated the prevalence of hearing loss caused by TECTA mutations in Japanese AD hearing loss families, and confirmed genotype-phenotype correlation, as well as the intracellular localization of missense mutations in the α-tectorin domain. TECTA mutations were detected in 2.9% (4/139) of our Japanese AD hearing loss families, with the prevalence in moderate hearing loss being 7.7% (4/52), and all patients showed typical genotype-phenotype correlations as previously described. The present in vitro study showed differences of localization patterns between wild type and mutants, and suggested that each missense mutation may lead to a lack of assembly of secretion, and may reduce the incorporation of α-tectorin into the tectorial membrane.     

Missense mutations in the forkhead domain of FOXL2 lead to subcellular mislocalization, protein aggregation and impaired transactivation

Mutations of the FOXL2 gene have been shown to cause blepharophimosis syndrome (BPES), characterized by an eyelid malformation associated with premature ovarian failure or not. Recently, polyalanine expansions and truncating FOXL2 mutations have been shown to lead to protein mislocalization, aggregation and altered transactivation. Here, we study the molecular consequences of 17 naturally occurring FOXL2 missense mutations. Most of them map to the conserved DNA-binding forkhead domain (FHD). The subcellular localization and aggregation pattern of the mutant FOXL2 proteins in COS-7 cells was variable and ranged from a diffuse nuclear distribution like the wild-type to extensive nuclear aggregation often in combination with cytoplasmic mislocalization and aggregation. We also studied the transactivation capacity of the mutants in FOXL2 expressing granulosa-like cells (KGN). Several mutants led to a loss-of-function, while others are suspected to induce a dominant negative effect. Interestingly, one mutant that is located outside the FHD (S217F), appeared to be hypermorphic and had no effect on intracellular protein distribution. This mutation gives rise to a mild BPES phenotype. In general, missense mutations located in the FHD lead to classical BPES and cannot be correlated with expression of the ovarian phenotype. However, a potential predictive value of localization and transactivation assays in the making of genotype-phenotype correlations is proposed. This is the first study to demonstrate that a significant number of missense mutations in the FHD of FOXL2 lead to mislocalization, protein aggregation and altered transactivation, and to provide insights into the pathogenesis associated with missense mutations of FOXL2 in human disease.     

A homology model for human alpha-l-iduronidase: insights into human disease

Genotype-phenotype correlations in genetic diseases for which missense mutations lead to disease remain a challenge. This is particularly true for diseases caused by alterations of proteins for which no three-dimensional structure is available. One such disease is Mucopolysaccharidosis type I, a disorder arising from a lack of activity of the lysosomal enzyme alpha-l-iduronidase (IDUA, EC 3.2.1.76). This deficiency compromises the degradation pathway of glycosaminoglycans such as heparan sulfate and dermatan sulfate, leading to substrate accumulation, which ultimately results in a multisystem disorder. Patients with IDUA deficiency have a wide spectrum of disease ranging from an early onset, rapidly progressive form leading to death in the first decade of life, to an attenuated disease which manifests in adolescence and leads to progressive joint and cardiac disease but is associated with a normal life span. Many patients fit into a disease phenotype intermediate to these extremes. While a number of point mutations have been described as leading to varying degrees of disease severity, a structural basis for these genotype-phenotype correlations has not been available owing to the lack of a three-dimensional structure for IDUA. A homology model for the IDUA enzyme was constructed based on the recently solved crystal structure of the beta-xylosidase from Thermoanaerobacterium saccharolyticum (XyTS, EC 3.2.1.37), both of which belong to the same sequence-related family (CAZY family 39). This model provides insights into why certain point mutations produce severely misfolded proteins and thus lead to severe disease, and why other mutations produce proteins with only minor structural perturbations and therefore the attenuated form of the disease.     

Neurofibromatosis type 1: from genotype to phenotype

Although neurofibromatosis 1 (NF1) is a common Mendelian disorder with autosomal-dominant inheritance, its expression is highly variable and unpredictable. Many NF1 patients have been genotyped but few allele-phenotype correlations have been identified. NF1 genotype-phenotype correlations are difficult to identify because of the complexity of the NF1 phenotype, its strong age dependency, the relatedness of many clinical features and the huge heterogeneity of pathogenic NF1 mutations. Some NF1 patients with a given NF1 mutation may develop very severe disease while others with the same mutation have only mild symptoms. This phenotypic variability may be due to both modifier genes and environmental factors. Recent targeted strategies have identified several interesting candidate modifier genes.     

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.     

Bioinformatic analysis of protein structure-function relationships: case study of leukocyte elastase (ELA2) missense mutations

Cyclic and congenital neutropenia are caused by mutations in the human neutrophil elastase (HNE) gene (ELA2), leading to an immunodeficiency characterized by decreased or oscillating levels of neutrophils in the blood. The HNE mutations presumably cause loss of enzyme activity, consequently leading to compromised immune system function. To understand the structural basis for the disease, we implemented methods from bioinformatics to analyze all the known HNE missense mutations at both the sequence and structural level. Our results demonstrate that the 32 different mutations have diverse effects on HNE structure and function, affecting structural disorder and aggregation tendencies, stability maintaining contacts, and electrostatic properties. A large proportion of the mutations are located at conserved amino acids, which are usually essential in determining protein structure and function. The majority of the disease-causing HNE missense mutations lead to major structural changes and loss of stability in the protein. A few mutations also affect functional residues, leading into decreased catalytic activity or altered ligand binding. Our analysis reveals the putative effects of all known missense mutations in HNE, thus allowing the structural basis of cyclic and congenital neutropenia to be elucidated. We have employed and analyzed a set of some 30 different methods for predicting the effects of amino acid substitutions. We present results and experience from the analysis of the applicability of these methods in the analysis of numerous genes, proteins, and diseases to reveal protein structure-function relationships and disease genotype-phenotype correlations.     

Differential involvement of protein 4.1 family members DAL-1 and NF2 in intracranial and intraspinal ependymomas.

Ependymomas are malignant CNS neoplasms with highly variable biologic behavior, including a generally better prognosis for intraspinal tumors. Inactivation of the NF2 gene on 22q12 and loss of its protein product, merlin, have been well documented in subsets of meningiomas and ependymomas. DAL-1, a related tumor suppressor and protein 4.1 family member on 18p11.3, has also been recently implicated in meningioma pathogenesis, though its role in ependymoma remains unknown. Therefore, we evaluated 27 ependymomas (12 intracranial and 15 spinal) using fluorescence in situ hybridization (FISH) and immunohistochemistry (IHC) to determine NF2/merlin and DAL-1/DAL-1 status at the DNA and protein levels. Demonstrable NF2 and DAL-1 gene deletions were each detected in 6 (22%) ependymomas. All 5 merlin losses by IHC occurred in spinal ependymomas (P =.047), whereas 5 (71%) DAL-1-negative cases were intracranial (P =.185). The former result is consistent with prior observations that NF2 mutations are generally limited to spinal ependymomas. In contrast to meningiomas, simultaneous merlin and DAL-1 losses were not encountered. Our findings suggest that (1) NF2 and DAL-1 losses are involved in the pathogenesis of spinal and intracranial ependymoma subsets, respectively and (2) given the number of cases with no demonstrable losses, other cellular perturbations must also be critical for tumori-genesis.     

Functional analysis of the relationship between the neurofibromatosis 2 tumor suppressor and its binding partner,hepatocyte growth factor-regulated tyrosine kinase substrate

Individuals with the neurofibromatosis 2 (NF2) inherited tumor predisposition syndrome are prone to the development of nervous system tumors, including schwannomas and meningiomas. The NF2 tumor suppressor protein, merlin or schwannomin, inhibits cell growth and motility as well as affects actin cytoskeleton-mediated processes. Merlin interacts with several proteins that might mediate merlin growth suppression, including hepatocyte growth factor-regulated tyrosine kinase substrate (HRS or HGS). Previously, we demonstrated that regulated overexpression of HRS in RT4 rat schwannoma cells had the same functional consequences as regulated overexpression of merlin. To determine the functional significance of this interaction, we generated a series of HRS truncation mutants and defined the regions of HRS required for merlin binding and HRS growth suppression. The HRS domain required for merlin binding was narrowed to a region (residues 470-497) containing the predicted coiled-coil domain whereas the major domain responsible for HRS growth suppression was distinct (residues 498-550). To determine whether merlin growth suppression required HRS, we demonstrated that merlin inhibited growth in HRS (+/+), but not HRS( -/-) mouse embryonic fibroblast cells. In contrast, HRS could suppress cell growth in the absence of Nf2 expression. These results suggest that merlin growth suppression requires HRS expression and that the binding of merlin to HRS may facilitate its ability to function as a tumor suppressor.