Immunohistochemical analysis supports a role for INI1/SMARCB1 in hereditary forms of schwannomas, but not in solitary, sporadic schwannomas

Peripheral nerve tumors

The chapter is focused on the neoplastic peripheral nerve lesions, which primarily involve "cranial and paraspinal nerves," as outlined in the CNS volume (WHO_Classification_of_Tumours_Editorial_Board, 2021). These include classic peripheral nerve sheath tumors such as schwannoma, neurofibroma, intraneural perineurioma, and malignant peripheral nerve sheath tumors, with their variants as well as new and more precisely defined entities, including hybrid nerve sheath tumors and malignant melanotic nerve sheath tumor (previously melanotic schwannoma).

Benign Spinal Tumors

Benign spinal intradural tumors are relatively rare and include intramedullary tumors with a favorable histology such as low-grade astrocytomas and ependymomas, as well as intradural extramedullary tumors such as meningiomas and schwannomas. The effect on the neural tissue is usually a combination of mass effect and neuronal involvement in cases of infiltrative tumors. The new understanding of molecular profiling of different tumors allowed us to better define central nervous system tumors and tailor treatment accordingly. The mainstay of management of many intradural spinal tumors is maximal safe surgical resection. This goal is more achievable with intradural extramedullary tumors; yet, with a meticulous surgical approach, many of the intramedullary tumors are amenable for safe gross-total or near-total resection. The nature of these tumors is benign; hence, a different way to measure outcome success is pursued and usually depends on functional rather than oncological or survival outcomes.

SMARCB1 (INI1) retained but SMARCA4 (BRG1) negative atypical teratoid/rhabdoid tumor arising at the bilateral cerebellopontine angles: a case report

Most atypical teratoid/rhabdoid tumor (AT/RT) of the central nervous system shows an inactivation of SMARCB1 (INI1) and is considered as the hallmark of this neoplasm. However, AT/RT could exceptionally rarely present retained SMARCB1 (INI1) but inactivated SMARCA4 (BRG1). Here, the authors report a rare case of a 2-year-old boy with a SMARCB1 (INI1) retained but SMARCA4 (BRG1) negative AT/RT arising at the bilateral cerebellopontine angles mimicking neurofibromatosis type 2. The tumor was highly aggressive and was refractory to all treatment modalities. This case highlights the challenges during differential diagnosis of atypical cerebellopontine angle tumors of childhood and the importance of thoroughly investigating SMARCB1 (INI1) and SMARCA4 (BRG1) when AT/RT is suspected.

Pathogenic noncoding variants in the neurofibromatosis and schwannomatosis predisposition genes

Neurofibromatosis type 1 (NF1), type 2 (NF2), and schwannomatosis are a group of autosomal dominant disorders that predispose to the development of nerve sheath tumors. Pathogenic variants (PVs) that cause NF1 and NF2 are located in the NF1 and NF2 loci, respectively. To date, most variants associated with schwannomatosis have been identified in the SMARCB1 and LZTR1 genes, and a missense variant in the DGCR8 gene was recently reported to predispose to schwannomas. In spite of the high detection rate for PVs in NF1 and NF2 (over 90% of non-mosaic germline variants can be identified by routine genetic screening) underlying PVs for a proportion of clinical cases remain undetected. A higher proportion of non-NF2 schwannomatosis cases have no detected PV, with PVs currently only identified in around 70%-86% of familial cases and 30%-40% of non-NF2 sporadic schwannomatosis cases. A number of variants of uncertain significance have been observed for each disorder, many of them located in noncoding, regulatory, or intergenic regions. Here we summarize noncoding variants in this group of genes and discuss their established or potential role in the pathogenesis of NF1, NF2, and schwannomatosis.

Diagnostic Pathology of Tumors of Peripheral Nerve

Neoplasms of the peripheral nervous system represent a heterogenous group with a wide spectrum of morphological features and biological potential. They range from benign and curable by complete excision (schwannoma and soft tissue perineurioma) to benign but potentially aggressive at the local level (plexiform neurofibroma) to the highly malignant (malignant peripheral nerve sheath tumors [MPNST]). In this review, we discuss the diagnostic and pathologic features of common peripheral nerve sheath tumors, particularly those that may be encountered in the intracranial compartment or in the spine and paraspinal region. The discussion will cover schwannoma, neurofibroma, atypical neurofibromatous neoplasms of uncertain biological potential, intraneural and soft tissue perineurioma, hybrid nerve sheath tumors, MPNST, and the recently renamed enigmatic tumor, malignant melanotic nerve sheath tumor, formerly referred to as melanotic schwannoma. We also discuss the diagnostic relevance of these neoplasms to specific genetic and familial syndromes of nerve, including neurofibromatosis 1, neurofibromatosis 2, and schwannomatosis. In addition, we discuss updates in our understanding of the molecular alterations that represent key drivers of these neoplasms, including neurofibromatosis type 1 and type 2, SMARCB1, LZTR1, and PRKAR1A loss, as well as the acquisition of CDKN2A/B mutations and alterations in the polycomb repressor complex members (SUZ12 and EED) in the malignant progression to MPNST. In summary, this review covers practical aspects of pathologic diagnosis with updates relevant to neurosurgical practice.

Rhabdoid tumor predisposition syndrome with renal tumor 10 years after brain tumor

We report a case of rhabdoid tumor predisposition syndrome with a renal tumor developing 10 years after a brain tumor, which demonstrated an unexpectedly favorable outcome. A 2-year-old boy underwent gross total resection of a brain tumor located in the fourth ventricle, and received adjuvant chemotherapy and radiotherapy. At the age of 11 years, a renal tumor was found and nephrectomy was performed. He is currently alive without evidence of disease over 2 years without postoperative therapy. Histologically, rhabdoid cells were observed in both brain and renal tumors. Loss of SMARCB1 (also known as INI1) expression was found in the nucleus of both tumor cells. Genetic testing revealed pathogenic variants of SMARCB1 exon 5 in the renal tumor and SMARCB1 exon 9 in the brain tumor. In addition, heterozygous deletion of 22q11.21-q11.23 containing the SMARCB1 locus was shared by both tumors and this deletion was identified in normal peripheral blood. Considering the histopathological and genetic findings, our case was considered to be rhabdoid tumor predisposition syndrome with atypical teratoid/rhabdoid tumor and late-onset rhabdoid tumor of the kidney.

Update on SWI/SNF-related gynecologic mesenchymal neoplasms: SMARCA4-deficient uterine sarcoma and SMARCB1-deficient vulvar neoplasms

Our knowledge regarding the role of genes encoding the chromatin remodeling switch/sucrose non-fermenting (SWI/SNF) complex in the initiation and progression of gynecologic malignancies continues to evolve. This review focuses on gynecologic tumors in which the sole or primary genetic alteration is in SMARCA4 or SMARCB1, two members of the SWI/SNF chromatin remodeling complex. In this review, we present a brief overview of the classical example of such tumors, ovarian small cell carcinoma of hypercalcemic type, and then a detailed review and update of SMARCB1-deficient and SMARCA4-deficient tumors of the uterus and vulva.

What's new in nerve sheath tumors

Peripheral nerve sheath tumors are commonly encountered and frequently pose challenges to the pathologist and the clinician. This review discusses the wide range of entities with an emphasis on new discoveries in the past decade. Clinical, histologic, immunohistochemical, and pathogenetic findings are discussed with an emphasis on clinical implications and differential diagnosis.

Update on selected advances in the immunohistochemical and molecular genetic analysis of soft tissue tumors

Although traditional morphological evaluation remains the cornerstone for the diagnosis of soft tissue tumors, ancillary diagnostic modalities such as immunohistochemistry and molecular genetic analysis are of ever-increasing importance in this field. New insights into the molecular pathogenesis of soft tissue tumors, often obtained from high-throughput sequencing technologies, has enabled significant progress in the characterization and biologic stratification of mesenchymal neoplasms, expanding the spectrum of immunohistochemical tests (often aimed towards recently discovered genetic events) and molecular genetic assays (most often fluorescence in situ hybridization and reverse transcription-polymerase chain reaction). This review discusses selected novel molecular and immunohistochemical assays with diagnostic applicability in mesenchymal neoplasms, with emphasis on diagnosis, refinement of tumor classification, and treatment stratification.

Case-based review: atypical teratoid/rhabdoid tumor

Atypical teratoid/rhabdoid tumor (AT/RT) is a rare CNS cancer that typically occurs in children younger than 3 years of age. Histologically, AT/RTs are embryonal tumors that contain a rhabdoid component as well as areas with primitive neuroectodermal, mesenchymal, and epithelial features. Compared to other CNS tumors of childhood, AT/RTs are characterized by their rapid growth, short symptomatic prodrome, and large size upon presentation, often leading to brain compression and intracranial hypertension requiring urgent intervention. For decades, the mainstay of care has been a combination of maximal safe surgical resection followed by adjuvant chemotherapy and radiotherapy. Despite advances in each of these modalities, the relative paucity of data on these tumors, their inherently aggressive course, and a lack of molecular data have limited advances in treatment over the past 3 decades. Recent large-scale, multicenter interdisciplinary studies, however, have significantly advanced our understanding of the molecular pathogenesis of these tumors. Multiple clinical trials testing molecularly targeted therapies are underway, offering hope for patients with AT/RT and their families.

Germline variants in SMARCB1 and other members of the BAF chromatin-remodeling complex across human disease entities: a meta-analysis

Germline variants that affect function are found in seven genes of the BAF chromatin-remodeling complex. They are linked to a broad range of diseases that, according to the gene affected, range from non-syndromic or syndromic neurodevelopmental disorders to low-grade tumors and malignancies. In the current meta-analysis, we evaluate genetic and clinical data from more than 400 families and 577 patients affected by BAF germline alterations. We focus on SMARCB1, including 43 unpublished patients from the EU-RHAB registry and our institution. For this gene, we further demonstrate whole gene as well as exon deletions and truncating variants to be associated with malignancy and early-onset disease. In contrast, non-truncating variants are associated with non-malignant disorders, such as Coffin-Siris syndrome or late-onset tumors like schwannoma or meningioma (p < 0.0001). SMARCB1 germline variants are distributed across the gene with variants in exons 1, 2, 8, and 9 being associated with low-grade entities, and single-nucleotide variants or indels outside of exon 9 that appear in patients with malignancies (p < 0.001). We attribute variants in specific BAF genes to certain disease entities. Finally, single-nucleotide variants and indels are sometimes detected in the healthy relatives of tumor patients, while Coffin-Siris syndrome and Nicolaides-Baraitser syndrome generally seem to appear de novo. Our findings add further information on the genotype-phenotype association of germline variants detected in genes of the BAF complex. Functional studies are urgently needed for a deeper understanding of BAF-related disorders and may take advantage from the comprehensive information gathered in this article.

[Pathogenesis and molecular pathology of vestibular schwannoma]

Schwannomas are benign Schwann cell-derived tumors of the peripheral nerve sheath often involving the vestibular cranial nerve (vestibular schwannoma). Histologically, they consist of bipolar spindle cells and show a moderate cellularity. Typically, Antoni A regions with a storiform pattern and loose Antoni B regions are intermingled. Verocay bodies are the pathognomonic palisading structures. Malignant transformation is rare. Merlin (schwannomin), the protein product of NF2, is inactivated by mutations, loss of heterozygosity or methylation. Within neurofibromatosis type 2, a germline mutation is present in about half of cases, whereas tumors demonstrate an additional second hit of the NF2 gene. A loss of chromosome 22 or 22q is common. Merlin links the cell membrane with the cytoskeleton and regulates intracellular signaling pathways leading to dysorganization when merlin is inactivated. Loss of merlin activates Rac1 and Ras, and the PAK1, mTORC1, EGFR-Ras-ERK, PI3K-Akt, WNT and Hippo pathways as well as receptor tyrosine kinases. Furthermore, merlin locates to the nucleus and inhibits E3 ubiquitin ligase CRL4^DCAF1. Besides biallelic inactivation of NF2 in schwannomas, other genes are involved in the pathogenesis of schwannomatosis-associated schwannomas such as LZTR1, SMARCB1, COQ6 indicating an important role of SWI/SNF chromatin-remodeling complex for schwannoma development. Our own investigations point to deregulation of BAF170, another essential SWI/SNF complex component. Knowledge of mechanisms allows targeted molecular therapy, especially in vestibular schwannomas, using antagonists against mTOR (rapamycin/sirolmus/everolimus), EGFR (lapatinib) or VEGF (bevacizumab), although clinical studies have been in part disappointing so far.

A mosaic pattern of INI1/SMARCB1 protein expression distinguishes Schwannomatosis and NF2-associated peripheral schwannomas from solitary peripheral schwannomas and NF2-associated vestibular schwannomas

BACKGROUND

The INI1/SMARCB1 gene protein product has been implicated in the direct pathogenesis of schwannomas from patients with one form of schwannomatosis [SWNTS1; MIM # 162091] showing a mosaic pattern of loss of protein expression by immunohistochemistry [93% in familial vs. 55% in sporadic cases].

AIM OF STUDY

To verify whether such INI1/SMARCB1 mosaic pattern could be extended to all schwannomas arising in the sporadic and familial schwannomatoses [i.e. to SMARCB1-related (SWNTS1) or LZTR1-related (SWNTS2) schwannomatosis or to SMARCB1/LZTR1-negative schwannomatosis] and whether it could be involved in classical NF2 or solitary peripheral schwannomas METHODS: We blindly analysed schwannoma samples obtained from a total of 22 patients including (a) 2 patients (2 males; aged 38 and 55 years) affected by non-familial SMARCB1-associated schwannomatosis (SWTNS1); (b) 1 patient (1 female; aged 33 years) affected by familial schwannomatosis (SWTNS1/ SMARCB1 germ line mutations); (c) 5 patients (3 males, 2 females; aged 33 to 35 years) affected by non-familial (sporadic) LZTR1-associated schwannomatosis (SWNTS2); (d) 3 patients (3 males; aged 35 to 47 years) affected by familial schwannomatosis (SWTNS2/ LZTR1 germ line mutations); (e) 2 patients (1 male, 1 female; aged 63 and 49 years, respectively) affected by non-familial schwannomatosis (SWTNS, negative for SMARCB1, LZTR1 and NF2 gene mutations); (f) 4 patients (3 males, 1 females; aged 15 to 24 years) affected by classical NF2 (NF2: harbouring NF2 germ line mutations; and (g) 5 patients (3 males, 2 females; aged 33 to 68 years) who had solitary schwannomas. [follow-up = 15-30 years; negative for constitutional/somatic mutation analysis for the SMARCB1, LZTR1 and NF2 genes] were (blindly) analyzed. The INI1/SMARCB1 immunostaining pattern was regarded as (1) diffuse positive nuclear staining [= retained expression] or (2) mosaic pattern [mixed positive/negative nuclei = loss of expression in a subset of tumour cells].

RESULTS

All solitary peripheral schwannomas and NF2-associated vestibular schwannomas showed diffuse nuclear INI1/SMARCB1 staining in 97-100% of neoplastic cells; schwannomas obtained from all cases of non-familial and familial schwannomatosis and NF2-associated non-vestibular schwannomas showed a mosaic pattern ranging from 10 to 70% of INI1/SMARCB1-positive expression. We did not record a complete lack of nuclear staining.

CONCLUSIONS

The present data suggests that (a) mosaic loss of immunohistochemical INI1/SMARCB1 expression, despite the interlesional variability, is a reliable marker of schwannomatosis regardless of the involved gene and it might help in the differential diagnosis of schwannomatosis vs. solitary schwannomas and (b) INI1/SMARCB1 expression is not useful in the differential with mosaic NF2, since NF2-associated peripheral schwannomas show the same immunohistochemical pattern.

Oncogenic roles of SMARCB1/INI1 and its deficient tumors

SMARCB1/INI1 is one of the core subunit proteins of the ATP-dependent SWI/SNF chromatin remodeling complex, and is identified as a potent and bona fide tumor suppressor. Interactions have been demonstrated between SMARCB1/INI1 and key proteins in various pathways related to tumor proliferation and progression: the p16-RB pathway, WNT signaling pathway, sonic hedgehog signaling pathway and Polycomb pathway. Initially, no detectable SMARCB1/INI1 protein expression was found in malignant rhabdoid tumor cells, whereas all other kinds of tumor cells and non-tumorous tissue showed SMARCB1/INI1 protein expression. Therefore, immunohistochemical testing for the SMARCB1/INI1 antibody has been considered useful in confirming the histologic diagnosis of malignant rhabdoid tumors. However, recently, aberrant expression of SMARCB1/INI1 has been found in various tumors such as epithelioid sarcomas, schwannomatosis, synovial sarcomas, and so on. In addition, it has been reported that aberrant expression can be classified into three patterns: complete loss, mosaic expression and reduced expression. Although the various pathways related to mechanisms of tumorigenesis and tumor proliferation are complexly intertwined, the clarification of these mechanisms may contribute to therapeutic strategies in SMARCB1/INI1-deficient tumors. In terms of pathological classifications, SMARCB1/INI1-deficient tumors may be re-classified by genetic backgrounds.

The molecular pathogenesis of schwannomatosis, a paradigm for the co-involvement of multiple tumour suppressor genes in tumorigenesis

Schwannomatosis is characterized by the predisposition to develop multiple schwannomas and, less commonly, meningiomas. Despite the clinical overlap with neurofibromatosis type 2 (NF2), schwannomatosis is not caused by germline NF2 gene mutations. Instead, germline mutations of either the SMARCB1 or LZTR1 tumour suppressor genes have been identified in 86% of familial and 40% of sporadic schwannomatosis patients. In contrast to patients with rhabdoid tumours, which are due to complete loss-of-function SMARCB1 mutations, individuals with schwannomatosis harbour predominantly hypomorphic SMARCB1 mutations which give rise to the synthesis of mutant proteins with residual function that do not cause rhabdoid tumours. Although biallelic mutations of SMARCB1 or LZTR1 have been detected in the tumours of patients with schwannomatosis, the classical two-hit model of tumorigenesis is insufficient to account for schwannoma growth, since NF2 is also frequently inactivated in these tumours. Consequently, tumorigenesis in schwannomatosis must involve the mutation of at least two different tumour suppressor genes, an occurrence frequently mediated by loss of heterozygosity of large parts of chromosome 22q harbouring not only SMARCB1 and LZTR1 but also NF2. Thus, schwannomatosis is paradigmatic for a tumour predisposition syndrome caused by the concomitant mutational inactivation of two or more tumour suppressor genes. This review provides an overview of current models of tumorigenesis and mutational patterns underlying schwannomatosis that will ultimately help to explain the complex clinical presentation of this rare disease.

Cribriform neuroepithelial tumor: molecular characterization of a SMARCB1-deficient non-rhabdoid tumor with favorable long-term outcome

Rhabdoid phenotype and loss of SMARCB1 expression in a brain tumor are characteristic features of atypical teratoid/rhabdoid tumors (ATRT). Rare non-rhabdoid brain tumors showing cribriform growth pattern and SMARCB1 loss have been designated cribriform neuroepithelial tumor (CRINET). Small case series suggest that CRINETs may have a relatively favorable prognosis. However, the long-term outcome is unclear and it remains uncertain whether CRINET represents a distinct entity or a variant of ATRT. Therefore, 10 CRINETs were clinically and molecularly characterized and compared with 10 ATRTs of each of three recently described molecular subgroups (i.e. ATRT-TYR, ATRT-SHH and ATRT-MYC) using Illumina Infinium HumanMethylation450 arrays, FISH, MLPA, and sequencing. Furthermore, outcome was compared to a larger cohort of 27 children with ATRT-TYR. Median age of the 6 boys and 4 girls harboring a CRINET was 20 months. On histopathological examination, all CRINETs demonstrated a cribriform growth pattern and distinct tyrosinase staining. On unsupervised cluster analysis of methylation data, all CRINETs examined exclusively clustered within the ATRT-TYR molecular subgroup. As ATRT-TYR, CRINETs mainly showed large heterozygous 22q deletions (9/10) and SMARCB1 mutations of the other allele. In two patients, SMARCB1 mutations were also present in the germline. Estimated mean overall survival in patients with CRINETs was 125 months (95% confidence interval 100-151 months) as compared to only 53 (33-74) months in patients with ATRTs of the ATRT-TYR subgroup (Log-Rank P < 0.05). In conclusion, CRINET represents a SMARCB1-deficient non-rhabdoid tumor, which shares molecular similarities with the ATRT-TYR subgroup but has distinct histopathological features and favorable long-term outcome.

Multiple Unilateral Vestibular Schwannomas: Segmental NF2 or Sporadic Occurrence?

Objective To report a case of a patient presenting with two separate unilateral vestibular schwannomas (VSs) without other stigmata of neurofibromatosis type 2 (NF2).

Study Design This article discusses a case report and review of the literature.

Setting Tertiary academic referral center.

Participants A 41-year-old female was referred for evaluation of a left-sided 1.8-cm cerebellopontine angle tumor centered on the porus acusticus and a separate ipsilateral 3-mm intracanalicular tumor appearing to arise from the superior vestibular nerve. The patient denied a family history of NF2. Neurotologic examination was unremarkable and close review of magnetic resonance imaging did not find any other stigmata of NF2.

Results The patient underwent left-sided retrosigmoid craniotomy with gross total resection of both tumors. Final pathology confirmed benign schwannoma. The INI1/SMARCB1 staining pattern did not suggest NF2 or schwannomatosis.

Conclusions This is only the third report of a case with multiple unilateral VSs occurring in a patient without other features of NF2. Herein, the authors review the two other reports and discuss potential mechanisms for this rare phenomenon.

Immortalized Human Schwann Cell Lines Derived From Tumors of Schwannomatosis Patients

Schwannomatosis, a rare form of neurofibromatosis, is characterized predominantly by multiple, often painful, schwannomas throughout the peripheral nervous system. The current standard of care for schwannomatosis is surgical resection. A major obstacle to schwannomatosis research is the lack of robust tumor cell lines. There is a great need for mechanistic and drug discovery studies of schwannomatosis, yet appropriate tools are not currently available. Schwannomatosis tumors are difficult to grow in culture as they survive only a few passages before senescence. Our lab has extensive experience in establishing primary and immortalized human Schwann cell cultures from normal tissue that retain their phenotypes after immortalization. Therefore we took on the challenge of creating immortalized human Schwann cell lines derived from tumors from schwannomatosis patients. We have established and fully characterized 2 schwannomatosis cell lines from 2 separate patients using SV40 virus large T antigen. One patient reported pain and the other did not. The schwannomatosis cell lines were stained with S100B antibodies to confirm Schwann cell identity. The schwannomatosis cells also expressed the Schwann cell markers, p75NTR, S100B, and NGF after multiple passages. Cell morphology was retained following multiple passaging and freeze/ thaw cycles. Gene expression microarray analysis was used to compare the cell lines with their respective parent tumors. No differences in key genes were detected, with the exception that several cell cycle regulators were upregulated in the schwannomatosis cell lines when compared to their parent tumors. This upregulation was apparently a product of cell culturing, as the schwannomatosis cells exhibited the same expression pattern of cell cycle regulatory genes as normal primary human Schwann cells. Cell growth was also similar between normal primary and immortalized tumor cells in culture. Accurate cell lines derived directly from human tumors will serve as invaluable tools for advancing schwannomatosis research, including drug screening.

Atypical ossifying fibromyxoid tumor unusually located in the mediastinum: report of a case showing mosaic loss of INI-1 expression

Ossifying fibromyxoid tumor (OFMT) is a rare soft tissue tumor. OFMT mostly arises in subcutaneous tissue or skeletal muscle of the extremities and is extremely unusual in the mediastinum. OFMT is classified as typical, atypical, or malignant as tumor aggressiveness increases. Herein, we presented a case of atypical OFMT that developed in the mediastinum of a 43-year-old woman. Because of its predominant hypercellular area and some tumor cells with high nuclear grade, it was not a typical OFMT. However, it did not have a sufficient number of mitotic figures to be classified as malignant. Hence, we classified it as atypical OFMT with some apparent characteristic features of OFMT, such as the presence of spicules of bone at the periphery of the tumor. Upon immunohistochemistry, it was positive for vimentin, S-100 protein, and CD10, which was consistent with a diagnosis of OFMT. Particularly noteworthy was the mosaic loss of INI-1 expression. Some OFMT and other exceptionally rare tumors have been reported to exhibit mosaic INI-1 loss. Inactivation of INI-1 gene and deregulation of PHF1 gene are thought to be involved in tumorigenesis of OFMT. Therefore, we speculated that the mosaic loss of INI-1 observed in the present case might also be related to a kind of abnormality of INI-1 as was reported previously.

Segmental neurofibromatosis type 2: discriminating two hit from four hit in a patient presenting multiple schwannomas confined to one limb

Background

A clinical overlap exists between mosaic Neurofibromatosis Type 2 and sporadic Schwannomatosis conditions. In these cases a molecular analysis of tumors is recommended for a proper genetic diagnostics. This analysis is challenged by the fact that schwannomas in both conditions bear a somatic double inactivation of the NF2 gene. However, SMARCB1-associated schwannomas follow a four-hit, three-step model, in which both alleles of SMARCB1 and NF2 genes are inactivated in the tumor, with one of the steps being always the loss of a big part of chromosome 22 involving both loci.

Case presentation

Here we report a 36-year-old woman who only presented multiple subcutaneous schwannomas on her right leg. To help discriminate between both possible diagnoses, an exhaustive molecular genetic and genomic analysis was performed on two schwannomas of the patient, consisting in cDNA and DNA sequencing, MLPA, microsatellite multiplex PCR and SNP-array analyses. The loss of a big part of chromosome 22 (22q12.1q13.33) was identified in both tumors. However, this loss involved the NF2 but not the SMARCB1 locus. SNP-array analysis revealed the presence of the same deletion breakpoint in both schwannomas, indicating that this alteration was actually the first NF2 inactivating hit. In addition, a distinct NF2 point mutation in each tumor was identified, representing independent second hits. In accordance with these results, no deletions or point mutations in the SMARCB1 gene were identified. None of the mutations were present in the blood. Two of the patient’s children inherited chromosome 22 deleted in schwannomas of the mother, but in its wild type form.

Conclusions

These results conclusively confirm the segmental mosaic NF2 nature of the clinical phenotype presented.

Primary atypical teratoid/rhabdoid tumor of central nervous system in children: a clinicopathological analysis and review of literature in China

Atypical teratoid/rhabdoid tumor (AT/RT) is a very rare and highly malignant embryonal tumor in the central nervous system (CNS). Five patients (4 girls and 1 boy) with AT/RT were treated in our hospital. The clinical histories, symptoms, neuroimaging aspects, therapies, histological and immunohistochemical findings and follow-up information were reviewed. The patients ranged from 8 to 40 months with a mean age of 20.6 months. One tumor was located in the spinal cord, two in cerebellum and two in the pineal region. The imagings of the tumors resemble medulloblastomas. Pathological examinations showed that one patient had medulloblastoma differentiation, one had choroid plexus carcinoma differentiation, and one had mesenchymal components. Immunohistochemical staining showed that all of the tumors lost the nuclear expression of integrase interactor 1 (INI1), and were positive for Vimentin, S-100 protein and epithelial membrane antigen. One case with no recurrence after 24 months may have benefited from radical excision and postoperative radiotherapy. The other 4 patients died 8, 4, 1 and 1-month respectively after operation without radiotherapy. The diagnosis of AT/RT depends on full sampling, careful observation the morphological characteristics and INI1 examination, even when the tumor are presented in uncommon sites, such as the spinal cord and the pineal region.

Clinical Neuropathology practice guide 6-2013: morphology and an appropriate immunohistochemical screening panel aid in the identification of synovial sarcoma by neuropathologists

Aims: Pathologists are under increasing pressure to accurately subclassify sarcomas, yet neuropathologists have limited collective experience with rare sarcoma types such as synovial sarcoma. We reviewed 9 synovial sarcomas affecting peripheral nerve diagnosed by neuropathologists and explored the morphologic and immunohistochemical differences between these and MPNST. Our goal was to make practical recommendations for neuropathologists regarding which spindle cell tumors affecting nerve should be sent for SYT-SSX testing. Methods: Clinical records and genetics were reviewed retrospectively and central pathology review of 9 synovial sarcomas and 6 MPNST included immunohistochemistry for SOX10, S100, BAF47, CK (lmw, pan, CK7, CK19), EMA, CD34, bcl2, CD99, and neurofilament. Results: Common synovial sarcoma sites were brachial plexus, spinal and femoral nerve, none were “intra-neural”, all had the SYT-SSX1 translocation, and 6/9 were monophasic with myxoid stroma and distinct collagen. Half of the monophasic synovial sarcomas expressed CK7, CK19 or panCK in a “rare positive cells pattern”, 8/9 (89%) expressed EMA, and all were SOX10 immunonegative with reduced but variable BAF47 expression. Conclusions: We recommend that upon encountering a cellular spindle cell tumor affecting nerve neuropathologists consider the following: 1) SYT-SSX testing should be performed on any case with morphology suspicious for monophasic synovial sarcoma including wiry or thick bands of collagen and relatively monomorphous nuclei; 2) neuropathologists should employ a screening immunohistochemical panel including one of CK7, panCK or CK19, plus EMA, S100 and SOX10, and 3) SYT-SSX testing should be performed on any spindle cell tumor with CK and/or EMA immunopositivity if SOX10 immunostaining is negative or only labels entrapped nerve elements.

CTF meeting 2012: Translation of the basic understanding of the biology and genetics of NF1, NF2, and schwannomatosis toward the development of effective therapies

The neurofibromatoses (NF) are autosomal dominant genetic disorders that encompass the rare diseases NF1, NF2, and schwannomatosis. The NFs affect more people worldwide than Duchenne muscular dystrophy and Huntington's disease combined. NF1 and NF2 are caused by mutations of known tumor suppressor genes (NF1 and NF2, respectively). For schwannomatosis, although mutations in SMARCB1 were identified in a subpopulation of schwannomatosis patients, additional causative gene mutations are still to be discovered. Individuals with NF1 may demonstrate manifestations in multiple organ systems, including tumors of the nervous system, learning disabilities, and physical disfigurement. NF2 ultimately can cause deafness, cranial nerve deficits, and additional severe morbidities caused by tumors of the nervous system. Unmanageable pain is a key finding in patients with schwannomatosis. Although today there is no marketed treatment for NF-related tumors, a significant number of clinical trials have become available. In addition, significant preclinical efforts have led to a more rational selection of potential drug candidates for NF trials. An important element in fueling this progress is the sharing of knowledge. For over 20 years the Children's Tumor Foundation has convened an annual NF Conference, bringing together NF professionals to share novel findings, ideas, and build collaborations. The 2012 NF Conference held in New Orleans hosted over 350 NF researchers and clinicians. This article provides a synthesis of the highlights presented at the conference and as such, is a "state-of-the-field" for NF research in 2012.

Update from the 2011 International Schwannomatosis Workshop: From genetics to diagnostic criteria

Schwannomatosis is the third major form of neurofibromatosis and is characterized by the development of multiple schwannomas in the absence of bilateral vestibular schwannomas. The 2011 Schwannomatosis Update was organized by the Children's Tumor Foundation (www.ctf.org) and held in Los Angeles, CA, from June 5-8, 2011. This article summarizes the highlights presented at the Conference and represents the "state-of-the-field" in 2011. Genetic studies indicate that constitutional mutations in the SMARCB1 tumor suppressor gene occur in 40-50% of familial cases and in 8-10% of sporadic cases of schwannomatosis. Tumorigenesis is thought to occur through a four-hit, three-step model, beginning with a germline mutation in SMARCB1 (hit 1), followed by loss of a portion of chromosome 22 that contains the second SMARCB1 allele and one NF2 allele (hits 2 and 3), followed by mutation of the remaining wild-type NF2 allele (hit 4). Insights from research on HIV and pediatric rhabdoid tumors have shed light on potential molecular pathways that are dysregulated in schwannomatosis-related schwannomas. Mouse models of schwannomatosis have been developed and promise to further expand our understanding of tumorigenesis and the tumor microenvironment. Clinical reports have described the occurrence of intracranial meningiomas in schwannomatosis patients and in families with germline SMARCB1 mutations. The authors propose updated diagnostic criteria to incorporate new clinical and genetic findings since 2005. In the next 5 years, the authors expect that advances in basic research in the pathogenesis of schwannomatosis will lead toward clinical investigations of potential drug therapies.

Expression of SMARCB1 (INI1) mutations in familial schwannomatosis

Genetic changes in the SMARCB1 tumor suppressor gene have recently been reported in tumors and blood from families with schwannomatosis. Exon scanning of all nine SMARCB1 exons in genomic DNA from our cohort of families meeting the criteria for 'definite' or 'presumptive' schwannomatosis previously revealed constitutional alterations in 13 of 19 families (68%). Screening of four new familial schwannomatosis probands identified one additional constitutional alteration. We confirmed the presence of mRNA transcripts for two missense alterations, four mutations of conserved splice motifs and two additional mutations, in less conserved sequences, which also affect splicing. Furthermore, we found that transcripts for a rare 3'-untranslated region (c.*82C > T) alteration shared by four unrelated families did not produce splice variants but did show unequal allelic expression, suggesting that the alteration is either causative itself or linked to an unidentified causative mutation. Overexpression studies in cells lacking SMARCB1 suggest that mutant SMARCB1 proteins, like wild-type SMARCB1 protein, retain the ability to suppress cyclin D1 activity. These data, together with the expression of SMARCB1 protein in a proportion of cells from schwannomatosis-related schwannomas, suggest that these tumors develop through a mechanism that is distinct from that of rhabdoid tumors in which SMARCB1 protein is completely absent in tumor cells.

Genetic predisposition to peripheral nerve neoplasia: diagnostic criteria and pathogenesis of neurofibromatoses, Carney complex, and related syndromes

Neoplasms of the peripheral nerve sheath represent essential clinical manifestations of the syndromes known as the neurofibromatoses. Although involvement of multiple organ systems, including skin, central nervous system, and skeleton, may also be conspicuous, peripheral nerve neoplasia is often the most important and frequent cause of morbidity in these patients. Clinical characteristics of neurofibromatosis type 1 (NF1) and neurofibromatosis type 2 (NF2) have been extensively described and studied during the last century, and the identification of mutations in the NF1 and NF2 genes by contemporary molecular techniques have created a separate multidisciplinary field in genetic medicine. In schwannomatosis, the most recent addition to the neurofibromatosis group, peripheral nervous system involvement is the exclusive (or almost exclusive) clinical manifestation. Although the majority of cases of schwannomatosis are sporadic, approximately one-third occur in families and a subset of these has recently been associated with germline mutations in the tumor suppressor gene SMARCB1/INI1. Other curious syndromes that involve the peripheral nervous system are associated with predominant endocrine manifestations, and include Carney complex and MEN2b, secondary to inactivating mutations in the PRKAR1A gene in a subset, and activating mutations in RET, respectively. In this review, we provide a concise update on the diagnostic criteria, pathology and molecular pathogenesis of these enigmatic syndromes in relation to peripheral nerve sheath neoplasia.

The molecular biology of vestibular schwannomas and its association with hearing loss: a review

Hearing loss is the most common symptom in patients with vestibular schwannoma (VS). In the past, compressive mechanisms caused by the tumoral mass and its growth have been regarded as the most likely causes of the hearing loss associated with VS. Interestingly, new evidence proposes molecular mechanisms as an explanation for such hearing loss. Among the molecular mechanisms proposed are methylation of TP73, negative expression of cyclin D1, expression of B7-H1, increased expression of the platelet-derived growth factor A, underexpression of PEX5L, RAD54B, and PSMAL, and overexpression of CEA. Many molecular mechanisms are involved in vestibular schwannoma development; we review some of these mechanisms with special emphasis on hearing loss associated with vestibular schwannoma.

Atypical teratoid rhabdoid tumors (ATRTs): the British Columbia's Children's Hospital's experience, 1986-2006

As "atypical teratoid rhabdoid tumors" (ATRTs) may mimic "small round blue cell tumors" (SRBCT), we reexamined our ATRT experience focusing upon INI-1 immunohistochemistry (IHC). All high-grade pediatric brain tumors occurring from 1986-2006 at our institution underwent INI-1 IHC. Clinicopathologic data from each INI-1 immunonegative case were reviewed. Additional genetic, epigenetic and IHC analyses (including interrogation of INI-1 and CLDN6) were performed on a subset of the INI-1 immunonegative cases. Twelve INI-1 IHC negative tumors were identified retrospectively, of which only two previously carried the diagnosis of ATRT. Overall, the clinicopathologic and genetic data supported the assertion that all 12 cases represented ATRT. Unexpectedly, three long-term survivors (4.2, 7.0 and 8.5 years) were identified. As hypothesized, "teratoid" and "rhabdoid" histologic features were relatively infrequent despite gross total resections in some cases. Methylation specific polymer chain reaction (PCR) (MSP) revealed a uniform methylation pattern across all cases and gene promoters tested (ie, MGMT, HIC1, MLH3 and RASSF1); notably, all cases demonstrated unmethylated MGMT promoters. Our data demonstate that a primitive non-rhabdoid histophenotype is common among ATRTs and highlights the diagnostic importance of INI-1 IHC. Epigenetically, the MGMT promoter is usually unmethylated in ATRT, suggesting that potential temozolomide-based chemotherapy may be of limited efficacy.

Epithelioid malignant peripheral nerve sheath tumor arising in a schwannoma, in a patient with "neuroblastoma-like" schwannomatosis and a novel germline SMARCB1 mutation

Epithelioid malignant peripheral nerve sheath tumors arising in preexisting schwannomas are extremely rare. We report an unusual example occurring in a patient with multiple schwannomas (schwannomatosis), all but 1 of which showed "neuroblastoma-like" histology. By immunohistochemistry, both the epithelioid malignant peripheral nerve sheath tumor and the schwannomas showed a complete loss of the Smarcb1 protein. Subsequent genetic evaluation revealed the presence of a novel germline mutation in the SMARCB1/INI1 gene in the patient and in 3 of her children, 2 of whom were diagnosed with atypical teratoid/rhabdoid tumors of the brain.

Tumors of central and peripheral nervous system associated with inherited genetic syndromes

There are several genetic syndromes that predispose to the development of tumors of the nervous system. In the present study, we provide a review of the tumors that are associated with neurofibromatosis type 1, neurofibromatosis type 2, tuberous sclerosis complex, von Hippel-Lindau disease, Li-Fraumeni syndrome, Cowden disease, Turcot syndrome, nevoid basal cell carcinoma syndrome (Gorlin syndrome) and rhabdoid predisposition syndrome, which are the most common.

Ossifying fibromyxoid tumor of soft parts: a clinicopathologic, proteomic, and genomic study

Ossifying fibromyxoid tumor (OFMTs) of soft parts is a rare soft tissue and bone tumor of borderline malignancy displaying an uncertain line of differentiation. The existence of fully malignant OFMT is controversial. To better understand the natural history and line of differentiation taken by OFMT, we studied 46 cases by light microscopic, immunohistochemical (IHC), genomic, proteomic, and fluorescence in situ hybridization (FISH) methods. Cases were classified according to the 2003 Folpe and Weiss system. Clinical and follow-up information was obtained. IHC for S-100 protein, desmin, epithelial membrane antigen (EMA), cytokeratins, smooth muscle actin (SMA), INI-1, neurofilament protein (NFP), CD56d excitatory amino acid transporter-4 (EAAT4), and MUC4 was performed on formalin-fixed, paraffin-embedded (FFPE) tissues. Gene expression profiling and proteomic studies were conducted on FFPE tissues from 13 and 5 cases, respectively. FISH for INI-1 was performed on 10 cases. The 46 tumors arose in 29 men and 17 women (median age, 52 y; range 39 to 63 y) and involved the proximal (N=17) and distal extremities (N=13), head and neck (N=9), and trunk (N=5). Median tumor size was 5.4 cm (range, 1.0 to 21.0 cm). Cases were classified as typical OFMT (26 of 46, 57%), atypical OFMT (5 of 46, 11%), and malignant OFMT (15 of 46 cases, 32%). Clinical follow-up (27 cases, median 55 months' duration) showed all patients with typical and atypical OFMT to be alive without disease. Adverse events, including 3 local recurrences, 3 metastases, and 3 deaths, were seen only in malignant OFMT. IHC results were as follows: S-100 protein (30 of 41, 73%), desmin (15 of 39, 38%), cytokeratin (4 of 35, 11%), EMA (5 of 32, 16%), SMA (2 of 34, 6%), INI-1 (lost in mosaic pattern in 14 of 19, 74%), EAAT4 (31 of 39, 80%), MUC4 (3 of 14, 21%), NFP (8 of 10, 80%) and CD56 (6 of 14, 43%). Gene expression profiling showed typical and malignant OFMTs to cluster together, distinct from schwannian tumors. Proteomic study showed expression of various collagens, S-100 protein, and neuron-related proteins. FISH showed INI-1 deletion in 5 of 7 (71%) cases. We conclude that malignant OFMTs exist and may be recognized by the previously proposed criteria of Folpe and Weiss. Expression of neuron-related markers, in addition to Schwann cell and cartilage-associated markers, suggests a "scrambled" phenotype in OFMTs. Loss of INI-1 or other genes on 22q is likely important in the pathogenesis of these rare tumors.

Superficial neurofibromas in the setting of schwannomatosis: nosologic implications

First described in the past decade, schwannomatosis is a syndrome distinct from neurofibromatosis 2 (NF2). It is characterized by the development of multiple schwannomas, sparing the vestibular division of cranial nerve VIII, and may also predispose to develop meningiomas. We report two female patients, a 27 and a 44 years old who developed multiple peripheral schwannomas, but without involvement of the vestibular nerves, satisfying clinical criteria for schwannomatosis. Lack of vestibular nerve involvement was confirmed with MRI using an internal auditory canal protocol with 3 mm thick slices in both patients after age 30. Both patients developed a small neurofibroma in axillary subcutaneous tissues and a diffuse cutaneous neurofibroma of the left buttock, respectively. This report highlights that superficial neurofibromas may arise in the setting of schwannomatosis, which may have implications for the diagnostic criteria of this unique syndrome. In particular, the presence of a cutaneous neurofibroma in a patient with multiple schwannomas should not lead to a diagnosis of NF2.

Polyradiculopathies from schwannomatosis

We describe a case of schwannomatosis presenting as radicular pain and numbness in multiple radicular nerve distributions. There were multiple peripheral nerve tumors detected by magnetic resonance imaging (MRI) at the left vestibular nerve, cauda equina, right radial nerve, thoracic paraspinal nerve, and brachial plexi. Several resected tumors have features of schwannomas, including hypercellular Antoni A areas, hypocellular Antoni B areas, Verocay bodies, and hyalinized blood vessels. The specimens are also positive for immunohistochemical staining for INI1 with diffuse nuclear staining. The findings are consistent with sporadic form of schwannomatosis. This case highlights the importance of using MRI and INI1 immunohistochemistry to differentiate familial schwannomatosis, neurofibromatosis 2 (NF2)-associated schwannomatosis, and sporadic schwannomatosis.

Molecular diagnostics of CNS embryonal tumors

Tremendous progress has recently been made in both molecular subgrouping, and the establishment of prognostic biomarkers for embryonal brain tumors, particularly medulloblastoma. Several prognostic biomarkers that were initially identified in retrospective cohorts of medulloblastoma, including MYC and MYCN amplification, nuclear β-catenin accumulation, and chromosome 17 aberrations have now been validated in clinical trials. Moreover, molecular subgroups based on distinct transcriptome profiles have been consistently reported from various groups on different platforms demonstrating that the concept of distinct medulloblastoma subgroups is very robust. Well-described subgroups of medulloblastomas include tumors showing wingless signaling pathway (Wnt) activation, and another characterized by sonic hedgehog pathway activity. Two or more additional subgroups were consistently reported to contain the vast majority of high-risk tumors, including most tumors with metastatic disease at diagnosis and/or large cell/anaplastic histology. Several years ago, atypical teratoid rhabdoid tumor (AT/RT) was recognized as a separate entity based on its distinct biology and particularly aggressive clinical behavior. These tumors may occur supra or infratentorially and are usually found to have genetic alterations of SMARCB1 (INI1/hSNF5), a tumor suppressor gene located on chromosome 22q. Subsequent loss of SMARCB1 protein expression comprises a relatively specific and sensitive diagnostic marker for AT/RT. For CNS primitive neuroectodermal tumors (CNS PNETs), a consistent finding has been that they are molecularly distinct from medulloblastoma. Furthermore, a distinct fraction of CNS PNETs with particularly poor prognosis only occurring in young children was delineated, which was previously labeled ependymoblastoma or embryonal tumor with abundant neuropil and true rosettes (ETANTR) and which is morphologically characterized by the presence of multilayered "ependymoblastic" rosettes. This group of tumors shows a unique cytogenetic abnormality not seen in other brain tumors: focal amplification of a micro-RNA cluster at chromosome 19q13.42, which has never been found to be amplified in other CNS PNETs, medulloblastoma or AT/RT. In summary, these consistent findings have significantly contributed to our ability to sub-classify embryonal brain tumors into clinically and biologically meaningful strata and, for some of the subgroups, have led to the identification of specific targets for future development of molecularly targeted therapies.

What's new in neurofibromatosis? Proceedings from the 2009 NF Conference: new frontiers

The NF Conference is the largest annual gathering of researchers and clinicians focused on neurofibromatosis and has been convened by the Children's Tumor Foundation for over 20 years. The 2009 NF Conference was held in Portland, Oregon from June 13 to June 16, 2009 and co-chaired by Kathryn North from the University of Sydney and The Children's Hospital at Westmead, Sydney, Australia; and Joseph Kissil from the Wistar Institute, Philadelphia. The Conference included 80 platform presentations in 9 sessions over 4 days; over 100 abstracts presented as posters; and three Keynote presentations. To date, there have been tremendous advances in basic research in the pathogenesis of neurofibromatosis, and more recently in progress toward identifying effective drug therapies and the commencement of neurofibromatosis clinical trials. The NF Conference attendees have significantly increased (doubling from 140 in 2005 to 280 attending in 2009) with a significant increase in attendance of physicians and clinical researchers. Correspondingly the NF Conference scope has expanded to include translational research, clinical trials and clinical management issues while retaining a core of basic research. These themes are reflected in the highlights from the 2009 NF Conference presented here.

Schwannomatosis associated with multiple meningiomas due to a familial SMARCB1 mutation

Schwannomatosis (MIM 162091) is a condition predisposing to the development of central and peripheral schwannomas; most cases are sporadic without a clear family history but a few families with a clear autosomal dominant pattern of transmission have been described. Germline mutations in SMARCB1 are associated with schwannomatosis. We report a family with multiple schwannomas and meningiomas. A SMARCB1 germline mutation in exon 1 was identified. The mutation, c.92A>T (p.Glu31Val), occurs in a highly conserved amino acid in the SMARCB1 protein. In addition, in silico analysis demonstrated that the mutation disrupts the donor consensus sequence of exon 1. RNA studies verified the absence of mRNA transcribed by the mutant allele. This is the first report of a SMARCB1 germline mutation in a family with schwannomatosis characterized by the development of multiple meningiomas.

Claudin 6 is a positive marker for atypical teratoid/rhabdoid tumors

Atypical teratoid/rhabdoid tumors (AT/RTs) are highly aggressive pediatric brain tumors characterized by the presence of rhabdoid cells and negative immunostaining for INI1 (BAF47). Histogenesis is unknown and diagnosis can be challenging because of their extreme morphological and immunophenotypic heterogeneity. Currently no signature markers other than INI1 loss have been identified. To search for possible candidate proteins of interest in AT/RTs, Affymetrix GeneChip microarrays were utilized to investigate nine AT/RTs vs. 124 other tumor samples. The most distinctive gene identified was claudin 6 (CLDN6), a key component of tight junctions. CLDN6 showed moderate or higher mRNA expression in eight of nine AT/RTs, with little to no expression in 114 of 115 other tumors. Average expression was 38-fold higher in AT/RTs vs. other samples. Immunohistochemical (IHC) staining of 33 tumor specimens found positive membrane staining in seven of seven AT/RTs, and was negative in 26 of 27 other brain tumor samples. Notably, none of the 16 medulloblastomas/primitive neuroectodermal tumors showed IHC staining for CLDN6. IHC staining results closely matched the level of mRNA expression detected by microarray. CLDN6 may be a useful positive marker to help further identify AT/RTs for diagnostic and treatment purposes.

Familial occurrence of schwannomas and malignant rhabdoid tumour associated with a duplication in SMARCB1

BACKGROUND

The role of germline and somatic SMARCB1 gene mutations in malignant rhabdoid tumour (MRT) predisposition is well known. Germline SMARCB1 mutations have also recently been identified in a subset of individuals with schwannomatosis. Surprisingly, MRT predisposition and schwannomatosis have never been reported to co-occur in a family. The correlation between genotype and phenotype for mutations in SMARCB1 has not been determined.

RESULTS

We have identified a germline 2631 bp duplication that includes exon 6 of SMARCB1 in a unique family with a four generation history of MRT predisposition and schwannomatosis. This duplication segregates with disease in individuals affected with both conditions, linking MRT predisposition and schwannomatosis as components of the same syndrome in this family.

CONCLUSION

The unique combination of tumours that result from the duplication described in this report may provide important clues about the mechanisms that influence the phenotype associated with a given SMARCB1 mutation.

Alterations in the SMARCB1 (INI1) tumor suppressor gene in familial schwannomatosis

Schwannomatosis is a third major form of neurofibromatosis that has recently been linked to mutations in the SMARCB1 (hSnf5/INI1) tumor suppressor gene. We analyzed the coding region of SMARCB1 by direct sequencing and multiplex ligation-dependent probe amplification (MLPA) in genomic DNA from 19 schwannomatosis kindreds. Microsatellite markers in the SMARCB1 region were developed to determine loss of heterozygosity (LOH) in associated tumors. We detected four alterations in conserved splice acceptor or donor sequences of exons 3, 4 and 6. Two alterations that likely affect splicing were seen in introns 4 and 5. An additional four alterations of unclear pathogenicity were found to segregate on the affected allele in eight families including two non-conservative missense alterations in three families. No constitutional deletions or duplications were detected by MLPA. Nine of 13 tumors examined showed partial LOH of the SMARCB1 region consistent with 'second hits.' Alterations were detected in tumors both with and without somatic NF2 gene changes. These findings support the hypothesis that SMARCB1 is a tumor suppressor for schwannomas in the context of familial disease. Further work is needed to determine its role in other multiple and single tumor syndromes.