Merlin knockdown in human Schwann cells: clues to vestibular schwannoma tumorigenesis

Identification of novel fusion transcripts in meningioma

INTRODUCTION

Meningiomas are the most common primary intracranial tumor. Recent next generation sequencing analyses have elaborated the molecular drivers of this disease. We aimed to identify and characterize novel fusion genes in meningiomas.

METHODS

We performed a secondary analysis of our RNA sequencing data of 145 primary meningioma from 140 patients to detect fusion genes. Semi-quantitative rt-PCR was performed to confirm transcription of the fusion genes in the original tumors. Whole exome sequencing was performed to identify copy number variations within each tumor sample. Comparative RNA seq analysis was performed to assess the clonality of the fusion constructs within the tumor.

RESULTS

We detected six fusion events (NOTCH3-SETBP1, NF2-SPATA13, SLC6A3-AGBL3, PHF19-FOXP2 in two patients, and ITPK1-FBP2) in five out of 145 tumor samples. All but one event (NF2-SPATA13) led to extremely short reading frames, making these events de facto null alleles. Three of the five patients had a history of childhood radiation. Four out of six fusion events were detected in expression type C tumors, which represent the most aggressive meningioma. We validated the presence of the RNA transcripts in the tumor tissue by semi-quantitative RT PCR. All but the two PHF19-FOXP2 fusions demonstrated high degrees of clonality.

CONCLUSIONS

Fusion genes occur infrequently in meningiomas and are more likely to be found in tumors with greater degree of genomic instability (expression type C) or in patients with history of cranial irradiation.

Proteomic screening identifies PML/p53 axis as a potential treatment target of facial nerve schwannomas

Facial nerve schwannomas (FNS) represents one of the more difficult treatment paradigms in neurotology. The aim of this study is to investigate the molecular alterations of FNS, thus providing potential targets treatable in the tumour. We for the first time suggest that the deficiency of merlin (the product of NF2 tumour suppressor) is probably one of the key mechanisms underlying FNS tumourigenesis, although no disease-causing NF2 mutations were demonstrated in tumour samples. TMT-labeled spectrometry analysis was used to identify the proteome of FNS relative to nerve controls. Eighty-four significantly deregulated proteins were identified, among which the PML tumour suppressor showed the most significantly increased expression. The PML protein was distributed in the nucleoplasm of non-tumorous Schwann cells, whereas it was preferentially confined to the cytoplasm of FNS cultures. Overexpression of PML and p53, partner proteins positively regulating each other to trigger apoptosis, was further confirmed in FNS tissues/cultures, and this correlated with a significant decrease in the proliferation of FNS cultures in comparison to Schwann cells. It is therefore probable that PML-p53 overexpression may occur as part of protective cellular mechanisms in response to the proliferation signal mediated by loss of merlin in FNS, in accordance with the fact that the tumour is benign slow-growing. This hypothesis was supported by the finding that the p53 activator nutlin-3 could exert dose-dependent inhibitory effects on FNS cultures via a cooperative induction of PML-p53 levels. Thus, the current study may present a potential treatment target directed on the molecular mechanisms of this disease.

Difference in the hypoxic immunosuppressive microenvironment of patients with neurofibromatosis type 2 schwannomas and sporadic schwannomas

BACKGROUND

Neurofibromatosis type 2 (NF2) patients uniformly develop multiple schwannomas. The tumor-microenvironment (TME) is associated with hypoxia and consists of immunosuppressive cells, including regulatory T cells (Tregs) and tumor-associated macrophages (TAMs). The hypoxic TME of NF2 schwannomas remains unclear. In addition, no comparative study has investigated immunosuppressive cells in NF2 and sporadic schwannomas.

METHODS

In 22 NF2 and 21 sporadic schwannomas, we analyzed the immunohistochemistry for Ki-67, hypoxia-inducible factor-1α (HIF-1α), vascular endothelial growth factor receptor 1 (VEGFR1) and VEGFR2, platelet derived growth factor receptor-beta (PDGFR-β), programmed cell death-1 (PD-1)/ programmed cell death ligand-1 (PD-L1), Foxp3, CD163, CD3, and CD8 to assess the immunosuppressive TME.

RESULTS

Most vessels in sporadic schwannomas exhibited slight or negative VEGFR1 and 2 expressions with pericytes coverage. In contrast, large vessels in NF2 schwannomas exhibited strong VEGFR1 and 2 expressions without pericytes. The number of CD3⁺, CD8⁺, and CD163⁺ cells was significantly higher in NF2 schwannomas than in sporadic ones. The expression of PD-L1 and nestin positive cell ratio was higher in NF2 schwannomas than that in sporadic ones. The number of CD163⁺ cells, nestin positive cell ratio, and HIF-1α expression were significantly associated with shorter progression-free survival in NF2 schwannomas.

CONCLUSIONS

This study presents the clinicopathological features of the differences in immunosuppressive cells and the expression of immune checkpoint molecules between NF2 and sporadic schwannomas. Hypoxic TME was first detected in NF2-schwannomas, which was associated with the tumor progression.

The Role of Merlin/NF2 Loss in Meningioma Biology

Mutations in the neurofibromin 2 (NF2) gene were among the first genetic alterations implicated in meningioma tumorigenesis, based on analysis of neurofibromatosis type 2 (NF2) patients who not only develop vestibular schwannomas but later have a high incidence of meningiomas. The NF2 gene product, merlin, is a tumor suppressor that is thought to link the actin cytoskeleton with plasma membrane proteins and mediate contact-dependent inhibition of proliferation. However, the early recognition of the crucial role of NF2 mutations in the pathogenesis of the majority of meningiomas has not yet translated into useful clinical insights, due to the complexity of merlin's many interacting partners and signaling pathways. Next-generation sequencing studies and increasingly sophisticated NF2-deletion-based in vitro and in vivo models have helped elucidate the consequences of merlin loss in meningioma pathogenesis. In this review, we seek to summarize recent findings and provide future directions toward potential therapeutics for this tumor.

Nf2 Mutation in Schwann Cells Delays Functional Neural Recovery Following Injury

Merlin is the protein product of the NF2 tumor suppressor gene. Germline NF2 mutation leads to neurofibromatosis type 2 (NF2), characterized by multiple intracranial and spinal schwannomas. Patients with NF2 also frequently develop peripheral neuropathies. While the role of merlin in SC neoplasia is well established, its role in SC homeostasis is less defined. Here we explore the role of merlin in SC responses to nerve injury and their ability to support axon regeneration. We performed sciatic nerve crush in wild-type (WT) and in P0SchΔ39-121 transgenic mice that express a dominant negative Nf2 isoform in SCs. Recovery of nerve function was assessed by measuring mean contact paw area on a pressure pad 7, 21, 60, and 90 days following nerve injury and by nerve conduction assays at 90 days following injury. After 90 days, the nerves were harvested and axon regeneration was quantified stereologically. Myelin ultrastructure was analyzed by electron microscopy. Functional studies showed delayed nerve regeneration in Nf2 mutant mice compared to the WT mice. Delayed neural recovery correlated with a reduced density of regenerated axons and increased endoneurial space in mutants compared to WT mice. Nevertheless, functional and nerve conduction measures ultimately recovered to similar levels in WT and Nf2 mutant mice, while there was a small (∼17%) reduction in the percent of regenerated axons in the Nf2 mutant mice. The data suggest that merlin function in SCs regulates neural ultrastructure and facilitates neural regeneration, in addition to its role in SC neoplasia.

Neurovascular patterning cues and implications for central and peripheral neurological disease

The highly branched nervous and vascular systems run along parallel trajectories throughout the human body. This stereotyped pattern of branching shared by the nervous and vascular systems stems from a common reliance on specific cues critical to both neurogenesis and angiogenesis. Continually emerging evidence supports the notion of later-evolving vascular networks co-opting neural molecular mechanisms to ensure close proximity and adequate delivery of oxygen and nutrients to nervous tissue. As our understanding of these biologic pathways and their phenotypic manifestations continues to advance, identification of where pathways go awry will provide critical insight into central and peripheral nervous system pathology.

"Wait and scan" management of patients with vestibular schwannoma and the relevance of non-contrast MRI in the follow-up

Vestibular schwannoma (VS) is a slow-growing benign neoplasm. There has been an evolution in the management of VS from active treatments (microsurgery and stereotactic radiotherapy) to conservative management (wait and scan). Regular MRI scanning is necessary to monitor tumor progression. Conservative management causes significantly less complications and offers a higher quality of life compared with active treatments. The mean growth rate of VS varies from 0.4 to 2.9 mm/year, and spontaneous shrinkage is observed in 3.8 percent of tumors during observation. If significant growth occurs, active treatment is considered. Significant growth is defined as an increase of at least 3 mm in the largest extrameatal diameter in any plane between the first and last available scans. The vestibulocochlear nerve is surrounded by cerebrospinal fluid, which provides natural contrast for MRI; thus, gadolinium may not be needed to detect VS. Specific sequences have high sensitivity, specificity, and accuracy for detection of progression. Hypointense signal in the ipsilateral inner ear fluid might be a useful sign to distinguish VS from meningioma. In this paper, we summarize the current status of research on conservative management and non-contrast MRI for the detection of VS.

Differential NF2 Gene Status in Sporadic Vestibular Schwannomas and its Prognostic Impact on Tumour Growth Patterns

The great majority of sporadic vestibular schwannomas (VSs) are due to the inactivation of the NF2 gene. In this study, we found age-dependent differences in the clinical parameters of sporadic VSs. Young patients were characterized by progressive tumour behaviours, including earlier onset of initial symptoms, shorter symptom duration and larger tumour size. An increased rate of "two-hits" of both NF2 alleles, usually by mutation and allelic loss, was observed in young cases compared to older, and this correlated with the loss of protein and mRNA expression. In contrast, the tumours with a single mutation (referred to as 'one-hit') exhibited obvious expression levels. Moreover, a mixture of merlin-expressing tumour cells and non-expressing tumour cells was observed in 'one-hit' schwannomas, suggesting that a subset of 'one-hit' tumour cells was present in these tumours. To mimic the growth promoting effects by the second hit, we performed lentivirus-mediated NF2 knockdown in the 'one-hit' schwannoma cultures. Following the loss of NF2 expression, schwannoma cultures demonstrated increased proliferation rates. Above all, we have identified a correlation between the NF2 status and the growth patterns of sporadic VSs. The treatment decision-making, microsurgery or "wait and scan" strategy, should be carried out according to the tumour's genetic background.

Differential NF2 Gene Status in Sporadic Vestibular Schwannomas and its Prognostic Impact on Tumour Growth Patterns

The great majority of sporadic vestibular schwannomas (VSs) are due to the inactivation of the NF2 gene. In this study, we found age-dependent differences in the clinical parameters of sporadic VSs. Young patients were characterized by progressive tumour behaviours, including earlier onset of initial symptoms, shorter symptom duration and larger tumour size. An increased rate of “two-hits” of both NF2 alleles, usually by mutation and allelic loss, was observed in young cases compared to older, and this correlated with the loss of protein and mRNA expression. In contrast, the tumours with a single mutation (referred to as ‘one-hit’) exhibited obvious expression levels. Moreover, a mixture of merlin-expressing tumour cells and non-expressing tumour cells was observed in ‘one-hit’ schwannomas, suggesting that a subset of ‘one-hit’ tumour cells was present in these tumours. To mimic the growth promoting effects by the second hit, we performed lentivirus-mediated NF2 knockdown in the ‘one-hit’ schwannoma cultures. Following the loss of NF2 expression, schwannoma cultures demonstrated increased proliferation rates. Above all, we have identified a correlation between the NF2 status and the growth patterns of sporadic VSs. The treatment decision-making, microsurgery or “wait and scan” strategy, should be carried out according to the tumour’s genetic background.

p53 performs an essential role in mediating the oncogenic stimulus triggered by loss of expression of neurofibromatosis type 2 during in vitro tumor progression

The loss of the tumor suppressor neurofibromatosis type 2 gene, encoding merlin, has been considered to be a fundamental event during the malignant progression of various cell types. However, a consensus for the mainstream mechanism, by which merlin deficiency contributes to uncontrolled cellular proliferation, has not been reached. The present study aimed to determine whether silencing of merlin using lentivirus-based short hairpin RNA potentiates cellular proliferation and cell cycle progression in human colon carcinoma HCT116 cell lines, expressing p53. The present results demonstrated that merlin knockdown contributed to cellular proliferation and G1/S cell cycle progression to a greater extent in HCT116 cells wide-type for p53 (p53^wt) compared with p53-null (p53^−/−) cells. This was supported by overexpression experiments which demonstrated a significant inhibitory effect of excess merlin on cellular proliferation only in HCT116 p53^wt cells. In order to investigate the underlying mechanisms of action, the expression of p53-involved G1/S transition genes was evaluated by western blot analysis. For HCT116 p53^wt cells, merlin loss suppressed p53 expression, and therefore the dysregulation of cell cycle regulatory proteins, including p21, cyclin D1/cyclin-dependent kinase (CDK)4 and cyclin E1/CDK2 complexes. However, merlin knockdowns had no impact on the expression of any of the aforementioned molecules in p53^−/− cells, indicating that lack of merlin resulted in G1/S cell cycle progression, and thereby uncontrolled cellular proliferation mainly via the regulation of p53-mediated pathways. Taken together, it was proposed that p53 performs an essential role in mediating the oncogenic stimulus triggered by merlin loss, and p53 is a molecule that should be investigated for its potential in targeted drug therapy for merlin-deficient malignancies.

Role of Merlin/NF2 inactivation in tumor biology

Merlin (Moesin-ezrin-radixin-like protein, also known as schwannomin) is a tumor suppressor protein encoded by the neurofibromatosis type 2 gene NF2. Loss of function mutations or deletions in NF2 cause neurofibromatosis type 2 (NF2), a multiple tumor forming disease of the nervous system. NF2 is characterized by the development of bilateral vestibular schwannomas. Patients with NF2 can also develop schwannomas on other cranial and peripheral nerves, as well as meningiomas and ependymomas. The only potential treatment is surgery/radiosurgery, which often results in loss of function of the involved nerve. There is an urgent need for chemotherapies that slow or eliminate tumors and prevent their formation in NF2 patients. Interestingly NF2 mutations and merlin inactivation also occur in spontaneous schwannomas and meningiomas, as well as other types of cancer including mesothelioma, glioma multiforme, breast, colorectal, skin, clear cell renal cell carcinoma, hepatic and prostate cancer. Except for malignant mesotheliomas, the role of NF2 mutation or inactivation has not received much attention in cancer, and NF2 might be relevant for prognosis and future chemotherapeutic approaches. This review discusses the influence of merlin loss of function in NF2-related tumors and common human cancers. We also discuss the NF2 gene status and merlin signaling pathways affected in the different tumor types and the molecular mechanisms that lead to tumorigenesis, progression and pharmacological resistance.

p75NTR is highly expressed in vestibular schwannomas and promotes cell survival by activating nuclear transcription factor κB

Vestibular schwannomas (VSs) arise from Schwann cells (SCs) and result from the loss of function of merlin, the protein product of the NF2 tumor suppressor gene. In contrast to non-neoplastic SCs, VS cells survive long-term in the absence of axons. We find that p75(NTR) is overexpressed in VSs compared with normal nerves, both at the transcript and protein level, similar to the response of non-neoplastic SCs following axotomy. Despite elevated p75(NTR) expression, VS cells are resistant to apoptosis due to treatment with proNGF, a high affinity ligand for p75(NTR) . Furthermore, treatment with proNGF protects VS cells from apoptosis due to c-Jun N-terminal kinase (JNK) inhibition indicating that p75(NTR) promotes VS cell survival. Treatment of VS cells with proNGF activated NF-κB while inhibition of JNK with SP600125 or siRNA-mediated knockdown reduced NF-κB activity. Significantly, proNGF also activated NF-κB in cultures treated with JNK inhibitors. Thus, JNK activity appears to be required for basal levels of NF-κB activity but not for proNGF-induced NF-κB activity. To confirm that the increase in NF-κB activity contributes to the prosurvival effect of proNGF, we infected VS cultures with Ad.IκB.SerS32/36A virus, which inhibits NF-κB activation. Compared with control virus, Ad.IκB.SerS32/36A significantly increased apoptosis including in VS cells treated with proNGF. Thus, in contrast to non-neoplastic SCs, p75(NTR) signaling provides a prosurvival response in VS cells by activating NF-κB independent of JNK. Such differences may contribute to the ability of VS cells to survive long-term in the absence of axons.

The role of NF2 gene mutations and pathogenesis-related proteins in sporadic vestibular schwannomas in young individuals

Vestibular schwannomas (VSs) are benign tumors arising from eighth cranial nerve and most often occur sporadically in individuals of middle age group. Sporadic VSs are rarely reported in the young population. In this study, we evaluated clinical behaviors of 12 young sporadic VSs by the statistical comparison with a matched series of 145 adult cases. We found that young tumors were characterized by an earlier onset of initial symptom, shorter duration from the first symptom to diagnosis, and larger tumor size than adult ones. Standard sequencing demonstrated the presence of NF2 mutations in eight tumors. All NF2 mutations identified were truncating mutations (nonsense, frameshift, and splicing-site mutations). Earlier formation of VSs in young patients was evidenced by the high incidence of NF2 mutations (66.7%) far beyond our previous study in the adult case series (34.5%). Furthermore, young tumors exhibited deficient merlin or heightened phosphorylated merlin that was subsequently demonstrated to be well correlated with increased tumor size. Finally, we compared protein levels of four pathogenesis-related molecules between young and adult group but there was no significant difference. These results led us to suggest that high frequency of NF2 mutations may play a critical role in early tumorigenesis of young VSs. Moreover, merlin deficiency or phosphorylation status of merlin was involved in their earlier development. Further study remains to fully understand the mechanism for the rapid growth of young VSs.

Schwannomas and their pathogenesis

Schwannomas may occur spontaneously, or in the context of a familial tumor syndrome such as neurofibromatosis type 2 (NF2), schwannomatosis and Carney's complex. Schwannomas have a variety of morphological appearances, but they behave as World Health Organization (WHO) grade I tumors, and only very rarely undergo malignant transformation. Central to the pathogenesis of these tumors is loss of function of merlin, either by direct genetic change involving the NF2 gene on chromosome 22 or secondarily to merlin inactivation. The genetic pathways and morphological features of schwannomas associated with different genetic syndromes will be discussed. Merlin has multiple functions, including within the nucleus and at the cell membrane, and this review summarizes our current understanding of the mechanisms by which merlin loss is involved in schwannoma pathogenesis, highlighting potential areas for therapeutic intervention.

Primary intraocular ancient schwannoma: a case report and review of the literature

BACKGROUND

Schwannomas comprise 8% to 10% of all primary intracranial tumors. Primary intraorbital schwannomas are rare, accounting for 1% to 2% of all intracranial tumors. Schwannomas arise from Schwann cells in the myelin sheaths of sensory and cranial nerves.

METHODS

We present the case of a 56-year-old white man blind since 1970 who presented to an outside hospital for a painful right eye. Physicians identified no clinical or radiologic features of neurofibromatosis.

RESULTS

The patient underwent enucleation; the final pathologic diagnosis was ancient schwannoma.

CONCLUSION

We describe herein a case of intrascleral schwannoma with ancient changes arising within the globe of the right eye occupying the subretinal space and completely replacing the vitreous body.

Clinicopathologic and molecular analysis of a choroidal pigmented schwannoma in the context of a PTEN hamartoma tumor syndrome

PURPOSE

To report the first case of choroidal schwannoma in a patient affected by PTEN hamartoma tumor syndrome (PHTS) and investigate the molecular involvement of the phosphatase and tensin homolog (PTEN) and neurofibromin 2 (NF2) genes in this rare intraocular tumor.

DESIGN

Observational case report.

PARTICIPANT

A 10-year-old girl diagnosed with PHTS.

METHODS

The enucleated specimen underwent histologic, immunohistochemical, and transmission electronic microscopy. The expression of PTEN and NF2 and their protein products were evaluated by reverse transcription-polymerase chain reaction and immunohistochemistry. Somatic mutations of PTEN and NF2, as well as allelic loss, were investigated by direct sequencing of DNA extracted from the tumor. PTEN epigenetic silencing was investigated by pyrosequencing.

MAIN OUTCOME MEASURES

Histopathologic and molecular characterization of a choroidal pigmented schwannoma.

RESULTS

Histopathologic, immunohistochemical, and electron microscopic analysis demonstrated features consistent with a pigmented cellular schwannoma of the choroid. We found no loss of heterozygosity at the genomic level for the PTEN germline mutation and no promoter hypermethylation or other somatic intragenic mutations. However, we observed an approximate 40% reduction of PTEN expression at both the mRNA and the protein level, indicating that the tumor was nonetheless functionally deficient for PTEN. Although DNA sequencing of NF2 failed to identify any pathologic variants, its expression was abolished within the tumor.

CONCLUSIONS

We report the first description of a pigmented choroidal schwannoma in the context of a PHTS. This rare tumor showed a unique combination of reduction of PTEN and absence of NF2 expression.

Fourth ventricular schwannoma: identical clinicopathologic features as schwann cell-derived schwannoma with unique etiopathologic origins

Background. To our knowledge, this is the sixth reported case in the literature of fourth ventricular schwannoma. The etiology and natural history of intraventricular schwannomas is not well understood. A thorough review of potential etiopathogenic mechanisms is provided in this case report. Case Description. A 69-year-old man presented with an incidentally found fourth ventricular tumor during an evaluation for generalized weakness, gait instability, and memory disturbance. Magnetic resonance imaging (MRI) revealed a heterogeneously enhancing lesion in the fourth ventricle. A suboccipital craniotomy was performed to resect the lesion. Histopathological examination confirmed the diagnosis of schwannoma (WHO grade I). Conclusions. Schwannomas should be considered in the differential diagnosis of intraventricular tumors. Although the embryologic origins may be different from nerve sheath-derived schwannomas, the histologic, clinical, and natural history appear identical and thus should be managed similarly.

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.