Clinical manifestations of mutations in the neurofibromatosis type 2 gene in vestibular schwannomas (acoustic neuromas)

Understanding the Molecular Mechanism of Vestibular Schwannoma for Hearing Preservation Surgery: Otologists’ Perspective from Bedside to Bench

Vestibular schwannoma is a clinically benign schwannoma that arises from the vestibulocochlear nerve that causes sensorineural hearing loss. This tumor is clinically and oncologically regarded as a benign tumor as it does not metastasize or invade surrounding tissues. Despite being a benign tumor, its management is difficult and controversial due to the potential serious complications, such as irreversible sensorineural hearing loss, of current interventions. Therefore, preventing hearing loss due to the natural course of the disease and complications of surgery is a challenging issue for an otologist. Improvements have been reported recently in the treatment of vestibular schwannomas. These include advances in intraoperative monitoring systems for vestibular schwannoma surgery where the risk of hearing loss as a complication is decreased. Precise genomic analysis of the tumor would be helpful in determining the characteristics of the tumor for each patient, leading to a better hearing prognosis. These procedures are expected to help improve the treatment of vestibular schwannomas. This review summarizes recent advances in vestibular schwannoma management and treatment, especially in hearing preservation. In addition, recent advances in the understanding of the molecular mechanisms underlying vestibular schwannomas and how these advances can be applied in clinical practice are outlined and discussed, respectively. Moreover, the future directions from the bedside to the bench side are presented from the perspective of otologists.

Contribution of mTOR and PTEN to Radioresistance in Sporadic and NF2-Associated Vestibular Schwannomas: A Microarray and Pathway Analysis

The use of radiation treatment has increased for both sporadic and neurofibromatosis type 2 (NF2)-associated vestibular schwannoma (VS). However, there are a subset of radioresistant tumors and systemic treatments that are seldom used in these patients. We investigated molecular alterations after radiation in three NF2-associated and five sporadically operated recurrent VS after primary irradiation. We compared these findings with 49 non-irradiated (36 sporadic and 13 NF2-associated) VS through gene-expression profiling and pathway analysis. Furthermore, we stained the key molecules of the distinct pathway by immunohistochemistry. A total of 195 differentially expressed genes in sporadic and NF2-related comparisons showed significant differences based on the criteria of p value < 0.05 and a two-fold change. These genes were involved in pathways that are known to be altered upon irradiation (e.g., mammalian target of rapamycin (mTOR), phosphatase and tensin homolog (PTEN) and vascular endothelial growth factor (VEGF) signaling). We observed a combined downregulation of PTEN signaling and an upregulation of mTOR signaling in progressive NF2-associated VS after irradiation. Immunostainings with mTOR and PTEN antibodies confirmed the respective molecular alterations. Taken together, mTOR inhibition might be a promising therapeutic strategy in NF2-associated VS progress after irradiation.

Proposal for Modification of Cahan's Criteria Utilizing Molecular Genetic Analyses for Cases without Baseline Histopathology: A Unique Method Applicable to Primary Radiosurgery

Background  Cahan's criteria have been utilized since 1948 to establish causality between prior radiation treatment and the development of secondary malignancy. One major criterion specifies that histological and radiographic evidence collected before and after radiation treatment must confirm separate tumor types; however, pretreatment biopsy is rarely obtained prior to radiosurgery for vestibular schwannoma and many other skull base and cranial lesions. Therefore, in these cases Cahan's criteria cannot be validly applied. Objective  This article proposes an update to Cahan's criteria using modern molecular genetic analysis for cases lacking baseline histopathology. Methods  Mate-pair sequencing and whole exome sequencing of a cerebellopontine angle undifferentiated high-grade pleomorphic sarcoma (UHGPS) that developed after stereotactic radiosurgery of a presumed benign vestibular schwannoma. Results  Mate-pair sequencing and whole exome sequencing of the sarcoma revealed complex chromosomal aberrations. Notably, the tumor contained a deletion in the NF2 gene at 22q12 and an in-frame deletion on exon 5 of the remaining copy of NF2 . Biallelic events impacting NF2 are atypical for UHGPS but are characteristic for vestibular schwannoma. These findings help support the conclusion that the UHGPS arose from a benign vestibular schwannoma all along. Conclusions  Next-generation sequencing can be successfully applied to a radiation-induced sarcoma when both the original and malignant tumors harbor separate signature genetic markers. As our understanding of the genetic profile of various tumors expand, we believe that next-generation sequencing and other genomic tools will play an increasingly important role in establishing causality between radiation and the development of secondary malignancy.

Evaluation and Management of Facial Nerve Schwannoma

Facial nerve schwannomas are benign peripheral nerve sheath tumors that arise from Schwann cells, and most commonly present with facial paresis and/or hearing loss. Computed tomography and MRI are critical to diagnosis. Management decisions are based on tumor size, facial function, and hearing status. Observation is usually the best option in patients with good facial function. For patients with poor facial function, the authors favor surgical resection with facial reanimation. There is growing evidence to support radiation treatment in patients with progressively worsening moderate facial paresis and growing tumors.

Removal of large or giant sporadic vestibular schwannomas via translabyrinthine approach: a report of 115 cases

BACKGROUND/AIMS

The present study was a retrospective analysis of surgical outcomes of large or giant vestibular schwannomas (VSs) via a translabyrinthine approach (TLA).

METHODS

One hundred and fifteen sporadic large or giant VSs (maximal diameter in the cerebellopontine angle ≥31 mm) were operated via TLA between January 2001 and December 2010. We divided patients into two groups: cystic VS (CVS) and solid VS (SVS) group. The surgical outcomes were recorded and compared.

RESULTS

Total, near total, and subtotal tumor removal rate was 89.6, 7.8, and 2.6%, respectively. The near total removal rate in CVS was higher than that in SVS (16.7 vs. 3.8%). The anatomical facial nerve (FN) integrity was preserved in 101 patients (87.8%). More FNs in CVS were interrupted (22.2%). There were 29.6, 56.5, and 13.9% of patients who had a short-term (1 month) FN function of House-Brackmann grade I or II, III or IV, and V or VI, respectively. The CVS had worse FN function than the SVS group. The mortality rate was 0.9%. Cerebrospinal fluid leakage occurred in 8 patients (7.0%). Four (3.5%) patients had recurrence.

CONCLUSIONS

Cyst formation is a crucial factor for surgical outcome of VS. With near total or subtotal tumor removal, worse functional outcomes in large or giant CVSs should be foreseen.

Neurofibromatosis 2

A comprehensive discussion of neurofibromatosis 2 (NF2) is presented, including clinical characteristics, symptoms, diagnosis, tumor types, prevalence and incidence, genetic testing, imaging, treatment options, and follow-up management of NF2.

Incidence trends of vestibular schwannomas in Denmark, Finland, Norway and Sweden in 1987-2007

Background:

The reported incidence rates of vestibular schwannomas (VS) vary substantially, but it is unclear as to what extent the variation reflects differences in risk or recording practices. Our aim was to describe the incidence rates of VS in Denmark, Finland, Norway and Sweden between 1987 and 2007.

Methods:

Comprehensive data were available from all registries only for the period from 1987 to 2007. An analysis of a longer time period (1965–2007) was conducted with the Norwegian and Swedish data.

Results:

The average age-standardised incidence rates during 1987–2007 varied from 6.1 per 1 000 000 person-years (95% confidence interval (CI), 5.4–6.7) among Finnish men to 11.6 (95% CI, 10.4–12.7) in Danish men, and from 6.4 per 1 000 000 person-years (95% CI, 5.7–7.0) among Swedish women to 11.6 (95% CI, 10.5–12.8) among Danish women. An overall annual increase of 3.0% (95% CI 2.1–3.9) was observed when all countries and both sexes were combined, with considerable differences between countries. However, the practices of both reporting and coding VS cases varied markedly between countries and over time, which poses a challenge for interpretation of the results.

Conclusion:

The overall incidence of VS increased in all the four Nordic countries combined between 1987 and 2007, with marked differences between countries. However, the incidence rates more or less stabilised in the late 1990s, showing relatively constant incidence rates and even some decline after 2000.

The molecular biology and novel treatments of vestibular schwannomas

Vestibular schwannomas are histopathologically benign tumors arising from the Schwann cell sheath surrounding the vestibular branch of cranial nerve VIII and are related to the NF2 gene and its product merlin. Merlin acts as a tumor suppressor and as a mediator of contact inhibition. Thus, deficiencies in both NF2 genes lead to vestibular schwannoma development. Recently, there have been major advances in our knowledge of the molecular biology of vestibular schwannomas as well as the development of novel therapies for its treatment. In this article the authors comprehensively review the recent advances in the molecular biology and characterization of vestibular schwannomas as well as the development of modern treatments for vestibular schwannoma. For instance, merlin is involved with a number of receptors including the CD44 receptor, EGFR, and signaling pathways, such as the Ras/raf pathway and the canonical Wnt pathway. Recently, merlin was also shown to interact in the nucleus with E3 ubiquitin ligase CRL4(DCAF1). A greater understanding of the molecular mechanisms behind vestibular schwannoma tumorigenesis has begun to yield novel therapies. Some authors have shown that Avastin induces regression of progressive schwannomas by over 40% and improves hearing. An inhibitor of VEGF synthesis, PTC299, is currently in Phase II trials as a potential agent to treat vestibular schwannoma. Furthermore, in vitro studies have shown that trastuzumab (an ERBB2 inhibitor) reduces vestibular schwannoma cell proliferation. With further research it may be possible to significantly reduce morbidity and mortality rates by decreasing tumor burden, tumor volume, hearing loss, and cranial nerve deficits seen in vestibular schwannomas.

Evaluation of the vestibulo-ocular reflex using sinusoidal off-vertical axis rotation in patients with acoustic neurinoma

The vestibulo-ocular reflex (VOR) was studied to examine the utility of off-vertical axis rotation (OVAR) in the diagnosis of acoustic neurinoma. Subjects were sinusoidally rotated with eyes open in complete darkness at frequencies of 0.4 and 0.8 Hz with a maximum angular velocity of 60 degrees /s at either earth-vertical axis rotation (EVAR) or OVAR. Thirteen patients with acoustic neurinomas were investigated. Results showed that VOR gain during OVAR at 0.8 Hz and in a 30 degrees nose-up position in patients with internal auditory canal tumors was significantly less than the gain measured during EVAR. The VOR gain measured from all patients (including those with tumors extending to the cerebellopontine angle) was not significantly different when the patients were subjected to EVAR and OVAR. These observations were possibly due to superior vestibular nerve dysfunction. We concluded that certain stimulating parameters--patient's nose tilted up 30 degrees; sinusoidal OVAR at 0.8 Hz and 60 degrees /s maximum angular head velocity--were useful for evaluating vestibular function in patients suffering from an acoustic neurinoma located within the internal auditory canal.

Hearing preservation surgery for neurofibromatosis Type 2–related vestibular schwannoma in pediatric patients

OBJECT

The authors reviewed the proportion of pediatric patients with neurofibromatosis Type 2 (NF2) in whom hearing was preserved after middle fossa resection of vestibular schwannoma (VS).

METHODS

In this retrospective chart review the authors examined the cases of 35 children with NF2 who had undergone middle fossa resection (47 surgeries) between 1992 and 2004 in a neurotological tertiary care center. Surgical outcome was assessed using pure-tone average (PTA) thresholds obtained before and immediately after resection. Speech discrimination scores (SDSs) and pre- and postfacial nerve grades were also recorded. In 55% of surgeries, hearing of less than or equal to 70 dB PTA was maintained postoperatively. The American Academy of Otolaryngology-Head and Neck Surgery Class A hearing (PTA < or = 30 dB and SDS > or = 70%) was preserved in 47.7%. Facial nerve function was good (House-Brackmann Grades I or II) in 81% of the patients. Twelve patients had bilateral middle fossa resections; in nine (75%) of these patients hearing was maintained postoperatively in both ears.

CONCLUSIONS

More than half of the children with NF2 in the authors' cohort experienced hearing preservation after middle fossa resection was performed for VS. The authors recommend this approach for preserving hearing in children with NF2.

The molecular biology of vestibular schwannomas: dissecting the pathogenic process at the molecular level

OBJECTIVE

The goal of this article was to review concisely what is currently known about the tumorigenesis of vestibular schwannomas.

BACKGROUND

Recent advances in molecular biology have led to a better understanding of the cause of vestibular schwannomas. Mutations in the neurofibromatosis type 2 tumor suppressor gene (NF2) have been identified in these tumors. In addition, the interactions of merlin, the protein product of the NF2 gene, and other cellular proteins are beginning to give us a better idea of NF2 function and the pathogenesis of vestibular schwannomas.

METHODS

Review of the relevant basic science studies at our institution as well as the basic science and clinical literature.

RESULTS

The clinical characteristics of vestibular schwannomas and neurofibromatosis type 2 syndromes are reviewed and related to alterations in the NF2 gene. Studies demonstrating our current understanding of tumor developmental pathways are highlighted. In addition, methods of clinical and genetic screening for neurofibromatosis type 2 disease are outlined. Avenues for the development of potential future research and therapies are discussed.

CONCLUSION

Great strides have been made to identify why vestibular schwannomas develop at the molecular level. Continued research is needed to find targeted therapies with which to treat these tumors.

A Novel Method in Predicting Immediate Postoperative Facial Nerve Function Post Acoustic Neuroma Excision

To determine whether the percentage calculated by dividing the amplitude of postexcision direct facial nerve stimulus responses (at pontomedullary junction) by the amplitude of distal ipsilateral transcutaneous (stylomastoid region) maximal stimulus responses and response amplitude progression by increasing stimulus intensities have predictive value for determining normal or near-normal (House-Brackmann Grade 1 or 2) immediate postoperative facial nerve function.

STUDY DESIGN

Intraoperative recordings of three muscle groups: 1) frontalis, 2) orbicularis oculi, and 3) orbicularis oris. Postexcision direct facial nerve stimulation at the pontomedullary junction and transcutaneous maximal facial nerve stimulation at the ipsilateral stylomastoid region and their associated response amplitudes were recorded.

SETTING

Tertiary referral center.

PATIENTS AND METHODS

Patients who underwent acoustic neuroma surgery from January 2004 to March 2006 with intraoperative facial nerve monitoring and an intact facial nerve after tumor excision were included. Recordings were available for 38 patients.

RESULTS

With a stimulus intensity of 0.3 mA at the root exit zone, there was an 81% positive predictive value in patients that exhibited a compound action potential of greater than 20% of maximum (sensitivity, 81%). This increased to 93% when the compound action potential was greater than 50% of maximum. When the amplitude increase was greater than 5 microV, there was a 77% positive predictive value (sensitivity, 87%).

CONCLUSION

The percentage of the response amplitude of direct facial nerve stimulation at the pontomedullary junction when compared with the maximum response amplitude of ipsilateral transcutaneous stimulation at the stylomastoid foramen is a good predictor of normal to near-normal immediate postoperative facial nerve function. Progression of amplitude response also seems to be a good predictor of normal to near-normal immediate postoperative facial nerve function.

Cyclin D1 and D3 Expression in Vestibular Schwannomas

OBJECTIVES

The G1 regulators of the cell cycle, cyclin D(1) and D(3), have been implicated in the regulation of Schwann cell proliferation and differentiation. The purpose of this study is to evaluate cyclin D(1) and D(3) protein expression and the corresponding clinical characteristics of vestibular schwannomas.

STUDY DESIGN AND METHODS

Tissue sections of 15 sporadic vestibular schwannomas were prepared. Immunohistochemical analysis of the vestibular schwannomas was performed with anticyclin D(1) and anticyclin D(3) antibodies. The immunoreactivity was evaluated in comparison with adjacent vestibular nerves. Tissue sections of breast carcinoma and prostate carcinoma were used as positive controls for cyclin D(1) and D(3) staining, respectively. Patient demographics, tumor characteristics, and cyclin D expression were reviewed, and statistical analysis was performed.

RESULTS

While the breast carcinoma control expressed abundant cyclin D(1) protein, none of the 15 vestibular schwannomas showed detectable cyclin D(1) staining. In contrast, seven of 15 vestibular schwannomas stained positive for the cyclin D(3) protein. Cyclin D(3) staining was taken up in the nucleus of schwannoma tumor cells in greater proportion than Schwann cells of adjacent vestibular nerve. Although sample size was small, no significant difference in the average age of presentation, tumor size, and male to female ratios for the cyclin D(3)(+) or cyclin D(3)(-) groups was found.

CONCLUSION

The Cyclin D(1) protein does not appear to play a prominent role in promoting cell cycle progression in vestibular schwannomas. In contrast, cyclin D(3) expression was seen in nearly half of the tumors examined, suggesting that it may have a growth-promoting role in some schwannomas. Further studies are needed to define its cellular mechanism.

Current Concepts in the Evaluation and Treatment of Neurofibromatosis Type II

This article presents the current diagnostic and treatment options for the hereditary disease neurofibromatosis type II, reviews clinical presentation and diagnosis, highlights indications for and methods of clinical and genetic screening, discusses treatment approaches for surgery and stereotactic radiation, and summarizes potential future therapeutic avenues.

Facial Nerve Monitoring Parameters As a Predictor of Postoperative Facial Nerve Outcomes after Vestibular Schwannoma Resection

OBJECTIVE

To evaluate whether the intraoperative stimulus threshold and response amplitude measurements from facial electromyography can predict facial nerve function at 1 year after vestibular schwannoma resection.

STUDY DESIGN

Prospective study.

SETTING

Tertiary academic center.

PATIENTS

Seventy-four consecutive vestibular schwannoma patients.

INTERVENTION

The minimal stimulus intensity (in milliamperes) and electromyographic response amplitude (in microvolts) were recorded during stimulation applied to the proximal facial nerve after vestibular schwannoma removal.

MAIN OUTCOME MEASURE

Facial nerve outcomes at 1 year were evaluated using the House-Brackmann scale. Analysis was then performed to evaluate whether these electrophysiologic recordings and tumor size could predict facial nerve functional outcomes.

RESULTS

Of the 74 patients, 66 of 74 (89%) had House-Brackmann Grade I or II facial nerve function and 8 of 74 (11%) had House-Brackmann Grade III-VI function at 1 year after surgery. If standards were set for intraoperative minimal stimulus intensity of 0.05 mA or less and response amplitude of 240 microV or greater, the authors were able to predict a House-Brackmann Grade I or II outcome in 56 of 66 (85%) patients at 1 year after surgery. With these same electrophysiologic parameters, only 1 of 8 (12%) House-Brackmann Grade III-VI patients also met this standard and thus gave a false-positive result. Logistic regression analysis of the data showed that both a stimulus threshold of 0.05 mA or less and a response amplitude of 240 microV or greater predicted a House-Brackmann Grade I or II outcome with a 98% probability. However, stimulus threshold or response amplitude alone had a much lower probability of the same result. In addition, although tumor size was found to independently predict facial nerve outcomes at 1 year, it did not improve the ability to predict facial nerve function over a model using stimulus intensity and amplitude alone.

CONCLUSION

Individually, minimal stimulus intensity or response amplitude was less successful in predicting long-term postoperative facial nerve function. However, if both parameters are considered together, the study demonstrates that they are good prognostic indicators for facial nerve function at 1 year after surgery.

Spinal tumors in neurofibromatosis type 2. Is emerging knowledge of genotype predictive of natural history?

OBJECT

The authors conducted a study to examine the incidence, classification, and progression of spinal tumors in patients with neurofibromatosis Type 2 (NF2) treated at a single center, and to examine relationships with the known mutational subtypes of NF2.

METHODS

They performed a retrospective review of clinical records, neuroimaging studies, and genetic data obtained in 61 patients with NF2. Forty-one (67%) of 61 patients harbored one or more spinal tumors. Thirty-four patients had undergone serial spinal magnetic resonance imaging during a mean follow-up period of 52 months (range 10-103 months; median 53 months). In 16 patients there were multiple extramedullary tumors smaller than 5 mm, which did not progress. Fourteen patients harbored at least one extramedullary tumor that was greater than 5 mm; of these, radiological progression was demonstrated or spinal tumor excision was performed during the follow-up period in eight cases (57%). Eleven patients harbored intramedullary cord tumors in addition to small and large extramedullary tumors, three (27%) of which exhibited radiological progression. In cases in which genotypes were known, protein-truncating mutations were significantly more likely to be associated with the presence of spinal tumors than in other types (p = 0.03, Fisher exact test). No associations between clinical behavior of spinal tumors and genotype, however, could be demonstrated.

CONCLUSIONS

Spinal tumors in cases involving NF2 are heterogeneous in type, distribution, and behavior but larger-size tumors are more likely to progress significantly. Intramedullary tumors usually accompany multiple extramedullary tumors. In the authors' experience subtyping of the NF2 mutation has not yet influenced management. Protein-truncating mutations are associated with an increased prevalence of spinal tumors.

Vestibular schwannoma growth rates in neurofibromatosis type 2 natural history consortium subjects

OBJECTIVE

To determine the amount of growth in vestibular schwannomas in Neurofibromatosis type 2 (NF2) patients from diagnosis through short-term (up to 2 yr) and long-term (up to 4 yr) follow-up.

STUDY DESIGN

Retrospective magnetic resonance imaging (MRI) films were obtained on subjects enrolled in the NF2 Natural History study and examined for changes in vestibular schwannoma size over time.

SETTING

Data were collected from nine foreign and domestic NF2 centers, including hospital-based, academic, and tertiary care centers.

SUBJECTS

NF2 patients with MRI data and at least one follow-up examination within 9 months to 2 years of diagnosis were included; n=56 patients with 84 lesions for evaluation of growth.

INTERVENTION

Routine, clinically obtained, magnetic resonance images were digitized and measured using image management software. Short-term follow-up was defined as up to 2 years (n=84 lesions), and long-term follow-up was defined as 3 to 4 years (n=29 lesions).

OUTCOME MEASURES

Vestibular schwannoma size was assessed using anterior-posterior, medial-lateral, and greatest diameter linear measurements.

RESULTS

Vestibular schwannomas increased in size (at least 5 mm) in 8% of the vestibular schwannomas across short-term follow-up. At long-term follow-up, 13% of the tumors had increased in size. On average, schwannomas increased in greatest diameter 1.3 mm per year across short-term follow-up.

CONCLUSION

Slightly greater than 1 in 10 diagnosed NF2-related vestibular schwannomas increased in size by at least 5 mm by 4 years of follow-up, if still untreated at that time.

Serial Analysis of Gene Expression in Neurofibromatosis Type 2–Associated Vestibular Schwannoma

HYPOTHESIS

The genesis, morphology, and growth characteristics of vestibular schwannomas are determined by genetic alterations which vary gene transcript expression and this transcript expression can be qualitatively and quantitatively evaluated using the SAGE technique. By use of such technique, gene products with tumorigenic potential may be identified, providing insight and targets for future study.

BACKGROUND

Serial analysis of gene expression (SAGE) is a powerful new technique that allows detailed qualitative and quantitative evaluation of cellular gene transcript expression. Tissue in limited quantity (5 x 10 to 2 x 10 cells) may be analyzed by a modified version of SAGE called microSAGE. Application of SAGE or microSAGE to study vestibular schwannoma gene expression has not been previously reported.

METHODS

Fresh, vestibular schwannoma specimen from an individual with the diagnosis of neurofibromatosis type 2 was attained intraoperatively and maintained in a sealed container at -80degreesC until the time of analysis. The tissue was processed according to the microSAGE protocol, using 180 mg of vestibular schwannoma as starting material.

RESULTS

The protocol resulted in the generation and sequencing of a tag library involving 458 tags representing 277 different gene products, including many transcripts known to be expressed in vestibular schwannomas. Several gene products with tumorigenic potential were identified.

CONCLUSIONS

These data demonstrate that microSAGE is a useful technique to study vestibular schwannoma gene expression. Future studies will include building more comprehensive libraries and comparing libraries from various vestibular schwannoma phenotypes to identify useful diagnostic or prognostic markers, and targets for therapeutic intervention.

Identification of recurrent regions of chromosome loss and gain in vestibular schwannomas using comparative genomic hybridisation

BACKGROUND

Schwannomas are benign tumours of the nervous system that are usually sporadic but also occur in the inherited disorder neurofibromatosis type 2 (NF2). The NF2 gene is a tumour suppressor on chromosome 22. Loss of expression of the NF2 protein product, merlin, is universal in both sporadic and NF2 related schwannomas. The GTPase signalling molecules RhoA and Rac1 regulate merlin function, but to date only mutation in the NF2 gene has been identified as a causal event in schwannoma formation.

METHODS

Comparative genomic hybridisation (CGH) was used to screen 76 vestibular schwannomas from 76 patients (66 sporadic and 10 NF2 related) to identify other chromosome regions that may harbour genes involved in the tumorigenesis.

RESULTS

The most common change was loss on chromosome 22, which was more frequent in sporadic than in NF2 related tumours. Importantly, eight tumours (10%) showed gain of copy number on chromosome 9q34. Each of the two NF2 patients who had received stereotactic radiotherapy had non-chromosome 22 changes, whereas only one of eight non-irradiated NF2 patients had any chromosome changes. Three tumours had gain on 17q, which has also been reported in malignant peripheral nerve sheath tumours that are associated with neurofibromatosis type 1. Other sites that were identified in three or fewer tumours were regions on chromosomes 10, 11, 13, 16, 19, 20, X, and Y.

CONCLUSIONS

These findings should be verified using techniques that can detect smaller genetic changes, such as microarray-CGH.

Gene discovery using a human vestibular schwannoma cDNA library constructed from a patient with neurofibromatosis type 2 (NF2)

BACKGROUND

Despite a strong association of schwannomin/merlin gene mutations with vestibular schwannoma formation, the regulatory mechanisms and biologic pathways involved are still largely unknown. The hypothesis of this study is that the genesis and growth characteristics of neurofibromatosis type 2 (NF2)-associated vestibular schwannomas are determined by genetic alterations that vary in gene transcript expression; this transcript expression includes oncogenic gene products that may be identified by construction and sequencing of a cDNA library from NF2-associated vestibular schwannoma.

METHODS

Approximately 3 mL of fresh tumor was obtained during resection of a 4-cm vestibular schwannoma from a patient with NF2. Poly(A)(+) mRNA was isolated, synthesized into double-stranded cDNA, and unidirectionally inserted into Uni-Zap XR (Stratagene, La Jolla, CA) bacteriophage vectors. Bacteriophage vectors containing cDNA inserts were processed into phagemids according to Uni-Zap XR protocol, and inserted vectors were sequenced and analyzed using BLAST software (National Institutes of Health, Bethesda, MD) with GenBank, EMBL, DDBJ, and PBD databases.

RESULTS

The cDNA library contained 2.4 million primary plaques. Inserts averaged 1.8 kilobases (kb) in length, with a range of 0.8 to 3.0 kb. BLAST multidatabase comparison of the sequence data obtained from 50 randomly selected clones yielded identification of 13 sequences representing known human genes and 17 sequences representing cloned sequences with unknown function. Three clones represented sequences not previously described in vestibular schwannomas but strongly implicated in oncogenesis within other tissues.

CONCLUSIONS

These data have implications for understanding the molecular mechanisms of vestibular schwannoma tumor biology. Identified genes may provide future diagnostic/prognostic markers and therapeutic targets.

cDNA microarray analysis of vestibular schwannomas

BACKGROUND

Vestibular schwannomas are known to harbor mutations in the neurofibromatosis type 2 tumor suppressor gene, but the mechanism of the neurofibromatosis type 2 tumor suppressor gene action is not well understood. Identification of genes differentially expressed in normal and diseased tissues through the use of a large-scale, cDNA microarray approach may lead to increased understanding of pathways that lead to tumor formation.

OBJECTIVE

The objectives of this study were to evaluate the gene expression profiles in vestibular schwannomas in comparison with normal vestibular nerve tissues and to identify pathways that may be altered in schwannomas.

METHODS

Total RNA was extracted from one normal vestibular nerve and seven vestibular schwannomas. The normal vestibular nerve was from one of the seven patients with small vestibular schwannomas. Radiolabeled cDNA was synthesized and hybridized to cDNA microarray filters that contained 25,920 known genes or expressed sequence tags. Expression profiles were imaged and analyzed. Selected genes that showed three-fold or greater difference in the intensity between the normal nerve and the schwannomas were further examined by real-time polymerase chain reaction and by immunohistochemical staining.

RESULTS

Forty-two genes (0.2%) were upregulated 3-fold or more in at least 5 of the 7 tumors when the filter images were compared with a normal adjacent vestibular nerve. Among them, osteonectin, an angiogenesis mediator, and RhoB GTPase, which is important in cell signaling, were significantly upregulated in 5 of 7 tumors. Among genes that were downregulated, an apoptosis-related LUCA-15 gene was highly underexpressed in 6 of 7 schwannomas when compared with the normal nerve. Also, ezrin, a relative of the NF2 protein, was significantly downregulated in 5 of 7 tumors. Real-time PCR and immunohistochemistry data support the cDNA microarray findings.

CONCLUSION

Our cDNA microarray analysis of schwannomas suggested several interesting and potentially important tumorigenesis pathways associated with vestibular schwannoma formation. Further in vivo study is necessary to define the roles of these identified genes and their potential relationships with the neurofibromatosis type 2 tumor suppressor gene.

Retinoblastoma-cyclin-dependent kinase pathway deregulation in vestibular schwannomas

OBJECTIVES

The purpose of the study was to identify genes of the retinoblastoma protein (pRb)-cyclin-dependent kinase (CDK) pathway that are deregulated in vestibular schwannomas when compared with normal vestibular nerve tissues.

STUDY DESIGN

Expression profiles in eight vestibular schwannomas (four sporadic tumors, one neurofibromatosis type 2 tumor, and three cystic tumors) and a paired normal vestibular nerve from one of the eight patients were chosen. Genes examined included the retinoblastoma susceptibility gene (Rb-1); cyclins D1, D2, A, and E; the CDK inhibitors p18, p19, and p27; CDK2 and CDK6; transcription factors E2F-4, E2F-5, and DP-1; and the neurofibromatosis type 2 gene.

METHODS

Total RNA samples were extracted from normal vestibular nerve and vestibular schwannoma tissues and used to generate radiolabeled complementary DNA (cDNA) samples. Labeled cDNA probes were then hybridized to cDNA microarray filters. The hybridization signal was captured and quantified. Differential gene expression profiles between the normal vestibular nerve and vestibular schwannoma were compared. Real-time polymerase chain reaction and immunohistochemistry were used to further confirm the cDNA microarray data.

RESULTS

Among genes in the pRb-CDK pathway, CDK2 was substantially underexpressed in seven of the eight vestibular schwannoma tumors examined. Quantitative RNA expression analysis using real-time polymerase chain reaction also showed consistent downregulation of CDK2 in the tumors. Anti-CDK2 antibody stained predominantly in the vestibular nerve and ganglion cells but only weakly in the vestibular schwannoma tissues.

CONCLUSIONS

The pRb-CDK pathway was altered in all vestibular schwannoma tumors examined, with CDK2 significantly downregulated in seven of the eight tumors. Further investigation into the regulatory mechanisms governing CDK2 expression may lead to a better understanding of vestibular schwannoma tumorigenesis.

Multiple transcription initiation sites, alternative splicing, and differential polyadenylation contribute to the complexity of human neurofibromatosis 2 transcripts

Northern blot analysis has shown that the human neurofibromatosis type 2 (NF2) cDNA hybridizes to multiple RNA species. To examine whether these hybridizing RNA species represent NF2 transcripts, we cloned the complete NF2 cDNA by a combination of techniques: 5' and 3' rapid amplification of cDNA ends, RT-PCR, and searching and sequencing the NF2-related cDNA clones from the IMAGE consortium. We showed that human NF2 transcripts initiate at multiple positions. Analogous to those reported previously, NF2 transcripts undergo alternative splicing in the coding exons. We isolated eight alternatively spliced NF2 cDNA isoforms, including one that contains a new exon termed exon 2', which potentially could encode proteins of different sizes. We assembled the overlapping cDNA fragments, and the longest NF2 cDNA, containing all 17 exons, consists of 6067 nucleotides, which is consistent with the size of the major RNA species hybridized to the NF2 probe. The cDNA has a 425-nucleotide 5' untranslated region upstream from the ATG start codon, and a long 3' untranslated region of 3869 nucleotides. We also isolated two shorter NF2 cDNAs that were terminated by different polyadenylation signal sequences, which indicates that differential usage of multiple polyadenylation sites also contributes to the complexity of human NF2 transcripts. By reference to the transcription initiation site mapped, we analyzed the 5' flanking sequence of the human NF2 gene. Transient transfection analysis in human 293 kidney, SK-N-AS neuroblastoma, and NT2/D1 teratocarcinoma cells with NF2 promoter-luciferase chimeric constructs revealed a core promoter region extending 400 base pairs from the major transcription initiation site. Although multiple regions are required for full promoter activity, a site-directed mutagenesis experiment identified a GC-rich sequence (position -58 to -46), which could be bound by transcription factor Sp1, as a positive cis-acting regulatory element. Cotransfection studies in Drosophila melanogaster SL2 cells showed that Sp1 could activate the NF2 promoter through the GC-rich sequence.

Evaluation of surgical approaches to endoscopic auditory brainstem implantation

OBJECTIVES

This study investigates the use of endoscopy for the placement of an auditory brainstem implant by translabyrinthine, retrosigmoid (suboccipital), and middle cranial fossa approaches.

STUDY DESIGN

Cadaver dissection and endoscope-assisted placement of the auditory brainstem implant.

METHODS

Translabyrinthine, retrosigmoid, and middle cranial fossa dissections were performed bilaterally in five cadaveric heads. An auditory brainstem implant was placed within the lateral recess of the fourth ventricle under endoscopic visualization. The implantation was performed with all approaches and documented by digital image capture followed by production of dye-sublimation photographic prints.

RESULTS

The lateral recess was visualized with the endoscope in all three approaches to the brainstem. The 30 degrees endoscope provided the best visualization by translabyrinthine and retrosigmoid dissection and was essential for the middle cranial fossa approach. Refinement of implant position was readily achieved, as even the deepest portion of the recess could be seen with all three approaches.

CONCLUSIONS

This study finds that endoscopy provides superior visualization of the lateral recess of the fourth ventricle than the operating microscope with all approaches. The retrosigmoid approach is recommended, as it provides the best view of the implantation site and the easiest angle for placement of the prosthesis. The use of the endoscope may allow for a smaller craniotomy than with conventional microscopic techniques, depending on tumor size. The translabyrinthine approach provides a good view of the lateral recess but had no advantage over other approaches. The middle cranial fossa approach is only possible with angled endoscopes; however, it is technically the most difficult and places the facial nerve at greatest risk.