Cloning and characterization of spliced fusion transcript variants of synovial sarcoma: SYT/SSX4, SYT/SSX4v, and SYT/SSX2v. Possible regulatory role of the fusion gene product in wild type SYT expression

Utility of Immunohistochemistry With Antibodies to SS18-SSX Chimeric Proteins and C-Terminus of SSX Protein for Synovial Sarcoma Differential Diagnosis

Synovial sarcoma is a relatively common soft tissue tumor characterized by highly specific t(X;18)(p11;q11) translocation resulting in the fusion of SS18 with members of SSX gene family. Typically, detection of SS18 locus rearrangement by fluorescence in situ hybridization or SS18 :: SSX fusion transcripts confirms the diagnosis. More recently, immunohistochemistry (IHC) for SS18-SSX chimeric protein (E9X9V) and C-terminus of SSX (E5A2C) showed high specificity and sensitivity for synovial sarcoma. This study screened a cohort of >1000 soft tissue and melanocytic tumors using IHC and E9X9V and E5A2C antibodies. Three percent (6/212) of synovial sarcomas were either negative for SS18-SSX or had scattered positive tumor cells (n=1). In these cases, targeted RNA next-generation sequencing detected variants of SS18 :: SSX chimeric transcripts. DNA methylation profiles of 2 such tumors matched with synovial sarcoma. A few nonsynovial sarcoma tumors (n=6) revealed either focal SS18-SSX positivity (n=1) or scattered positive tumor cells. However, targeted RNA next-generation sequencing failed to detect SS18 :: SSX transcripts in these cases. The nature of this immunopositivity remains elusive and may require single cell sequencing studies. All synovial sarcomas showed positive SSX IHC. However, a mosaic staining pattern or focal loss of expression was noticed in a few cases. Strong and diffuse SSX immunoreactivity was also seen in epithelioid sclerosing osteosarcoma harboring EWSR1 :: SSX1 fusion, while several sarcomas and melanocytic tumors including cellular blue nevus (5/7, 71%) revealed focal to diffuse, mostly weak to intermediate SSX staining. The SS18-SSX and SSX IHC is a useful tool for synovial sarcoma differential diagnosis, but unusual immunophenotype should trigger molecular genetic testing.

Undifferentiated round cell sarcomas with novel SS18-POU5F1 fusions

Despite significant recent advances in characterizing the molecular pathogenesis of undifferentiated round cell neoplasms, rare cases remain unclassified. Here, we report two distinctive undifferentiated round cell tumors occurring in young adults. One tumor presented intrabdominally and the other arose within the abdominal wall. One patient died of disease following local and distance recurrence, despite aggressive chemotherapy and radiotherapy. Morphologically, both tumors were similarly composed of primitive round to epithelioid cells arranged in nests, sheets, and trabecular patterns. The cytoplasm was scant and amphophilic, while the nuclei were round and uniform with brisk mitotic activity. Focal necrosis was present. Immunohistochemically, both tumors were variably positive for S100 and EMA, and one case focally expressed cytokeratin and TLE1. Targeted RNA sequencing revealed in both an identical SS18-POU5F1 fusion gene. Fluorescence in situ hybridization was performed which confirmed SS18 and POU5F1 gene rearrangements. Expression data, relative to over 200 other mesenchymal neoplasms that had undergone targeted RNA sequencing on the same platform, suggested the SS18-POU5F1 tumors cluster with EWSR1/FUS-POU5F1-positive myoepithelial tumors. In view of our limited sample size, additional studies are needed to characterize the breadth of clinical and pathologic findings in these neoplasms. In addition, further investigation is necessary to determine whether this entity represents a clinically aggressive and phenotypically undifferentiated variant of myoepithelial tumors, or perhaps an altogether novel category of undifferentiated round cell sarcoma.

The BAF complex in development and disease

The ATP-dependent chromatin remodelling complex BAF (= mammalian SWI/SNF complex) is crucial for the regulation of gene expression and differentiation. In the course of evolution from yeast to mammals, the BAF complex evolved an immense complexity with a high number of subunits encoded by gene families. In this way, tissue-specific BAF function and regulation of development begin with the combinatorial assembly of distinct BAF complexes such as esBAF, npBAF and nBAF. Furthermore, whole-genome sequencing reveals the tremendous role BAF complex mutations have in both neurodevelopmental disorders and human malignancies. Therefore, gaining a more elaborate insight into how BAF complex assembly influences its function and which role distinct subunits play, will hopefully give rise to a better understanding of disease pathogenesis and ultimately to new treatments for many human diseases.

Canonical Wnt/β-catenin signaling activation in soft-tissue sarcomas: A comparative study of synovial sarcoma and leiomyosarcoma

Background:

Previous studies have shown that aberrant activation of the Wnt/β-catenin pathway is associated with many malignant neoplasms. This includes some soft-tissue sarcoma phenotypes, most notably synovial sarcoma, implicating potential targets for novel molecular therapies.

Objective:

We investigate the level of Wnt/β-catenin pathway activation present in leiomyosarcomas relative to synovial sarcomas, using expression of LEF1 and β-catenin as surrogates.

Methods:

Cancer outlier profile analysis was performed on messenger RNA expression datasets in Oncomine (70 synovial sarcomas, 178 leiomyosarcomas). Results for LEF1 and β-catenin messenger RNA expression were reported in terms of median-centered intensity. Separate immunohistochemical studies were performed on tissue microarrays created from 77 synovial sarcomas and 89 leiomyosarcomas using antibodies to LEF1 and β-catenin. Tumors with unequivocal strong nuclear staining involving ⩾5% of cells were interpreted as positive.

Results:

Cancer outlier profile analysis demonstrated a higher level of LEF1 messenger RNA expression in synovial sarcomas than in leiomyosarcomas (p < 0.0001), but showed no significant difference in β-catenin messenger RNA expression (p = 0.868). Immunohistochemistry showed most synovial sarcomas had strong nuclear expression of LEF1 (79%) and β-catenin (84%), while a small minority of leiomyosarcomas had strong nuclear expression of LEF1 (5%) and β-catenin (6%).

Conclusion:

These results provide further evidence that aberrant activation of the Wnt/β-catenin pathway is present in most synovial sarcomas, but not in most leiomyosarcomas. While targeting the constituents of this pathway might be effective in the treatment of synovial sarcomas, it is not likely to be an effective strategy in the treatment of leiomyosarcomas.

Prognostic impact of the tumor immune microenvironment in synovial sarcoma

The association between the immune status within the tumor microenvironment and prognosis in synovial sarcoma is not well understood. We aimed to investigate the tumor immune microenvironment and analyze its prognostic impact for patients with synovial sarcoma. A total of 36 primary patients who were treated in our institution were retrospectively evaluated. Infiltration of lymphocytes (CD4+, CD8+, and FOXP3+), CD163+ macrophages, and expression of human leukocyte antigen (HLA) class I and programmed death ligand 1 (PD-L1) were evaluated by immunohistochemistry. Moreover, we investigated PD-L1 and programmed death ligand 2 (PD-L2) mRNA expression in 19 of the 36 cases, using real-time PCR. The Kaplan-Meier method was used to estimate overall survival and progression-free survival. Infiltration of lymphocytes and macrophages varied among the patients. Furthermore, the expression of HLA class I was negative or downregulated in 11 specimens. No PD-L1 expression was observed using immunohistochemistry. Moreover, although PD-L1 mRNA expression was observed in 18 of 19 specimens, the expression level was low. A higher infiltration of CD8+ or FOXP3+ lymphocytes in patients was associated with a favorable overall survival. In addition, a higher infiltration of CD163+ macrophages indicated a significantly worse overall and progression-free survival. Infiltration of CD4+ lymphocytes, HLA class I, PD-L1, and PD-L2 expression were not associated with patient prognosis. This represents the first report investigating the tumor immune microenvironment as a prognostic factor in synovial sarcoma, indicating that CD163+ macrophages are associated with tumor progression. Our results underscore the clinical significance of the tumor immune microenvironment in synovial sarcoma.

Primary synovial sarcoma of the scalp: Report of case with a clinicopathological and molecular cytogenetic study

Synovial sarcoma (SS) arising primarily in the scalp was scarcely reported. Here we described a subcutaneous mass in the right parietal region of the scalp of a 39year-old male patient. It has been present for 10years and enlarged recently. It was initially suspected as hemangioma by magnetic resonance imaging (MRI), but the histological examination revealed a sarcoma rich of spindle cells reminiscent of SS. Therefore, immunohistochemistry, florescence in site hybridization (FISH), reverse transcription-polymerase chain reaction (RT-PCR) and DNA sequencing were performed to determine the final diagnosis as biphasic SS. The detailed description of primary scalp SS might help differential diagnosis of scalp soft tumors.

SS18-SSX, the Oncogenic Fusion Protein in Synovial Sarcoma, Is a Cellular Context-Dependent Epigenetic Modifier

The prevalence and specificity of unique fusion oncogenes are high in a number of soft tissue sarcomas (STSs). The close relationship between fusion genes and clinicopathological features suggests that a correlation may exist between the function of fusion proteins and cellular context of the cell-of-origin of each tumor. However, most STSs are origin-unknown tumors and this issue has not yet been investigated in detail. In the present study, we examined the effects of the cellular context on the function of the synovial sarcoma (SS)-specific fusion protein, SS18-SSX, using human pluripotent stem cells (hPSCs) containing the drug-inducible SS18-SSX gene. We selected the neural crest cell (NCC) lineage for the first trial of this system, induced SS18-SSX at various differentiation stages from PSCs to NCC-derived mesenchymal stromal cells (MSCs), and compared its biological effects on each cell type. We found that the expression of FZD10, identified as an SS-specific gene, was induced by SS18-SSX at the PSC and NCC stages, but not at the MSC stage. This stage-specific induction of FZD10 correlated with stage-specific changes in histone marks associated with the FZD10 locus and also with the loss of the BAF47 protein, a member of the SWI/SNF chromatin-remodeling complex. Furthermore, the global gene expression profile of hPSC-derived NCCs was the closest to that of SS cell lines after the induction of SS18-SSX. These results clearly demonstrated that the cellular context is an important factor in the function of SS18-SSX as an epigenetic modifier.

Synovial sarcoma: defining features and diagnostic evolution

Synovial sarcoma (SS) is a malignant mesenchymal neoplasm with variable epithelial differentiation, with a propensity to occur in young adults and which can arise at almost any site. It is generally viewed and treated as a high-grade sarcoma. As one of the first sarcomas to be defined by the presence of a specific chromosomal translocation leading to the production of the SS18-SSX fusion oncogene, it is perhaps the archetypal "translocation-associated sarcoma," and its translocation remains unique to this tumor type. Synovial sarcoma has a variety of morphologic patterns, but its chief forms are the classic biphasic pattern, of glandular or solid epithelial structures with monomorphic spindle cells and the monophasic pattern, of fascicles of spindle cells with only immunohistochemical or ultrastructural evidence of epithelial differentiation. However, there is significant morphologic heterogeneity and overlap with a variety of other neoplasms, which can cause diagnostic challenge, particularly as the immunoprofile is varied, SS18-SSX is not detected in 100% of SSs, and they may occur at unusual sites. Correct diagnosis is clinically important, due to the relative chemosensitivity of SS in relation to other sarcomas, for prognostication and because of the potential for treatment with specific targeted therapies in the near future. We review SS, with emphasis on the diagnostic spectrum, recent immunohistochemical and genetic findings, and the differential diagnosis.

Loss of SS18-SSX1 inhibits viability and induces apoptosis in synovial sarcoma

BACKGROUND

Most synovial sarcomas contain a chromosomal translocation t(X;18), which results in the formation of an oncoprotein SS18-SSX critical to the viability of synovial sarcoma.

QUESTIONS/PURPOSES

We (1) established and characterized three novel synovial sarcoma cell lines and asked (2) whether inhibition of SS18-SSX1 decreases cell viability in these cell lines; and (3) whether reduction in viability after SS18-SSX1 knockdown is caused by apoptosis. After identifying a specific posttranscriptional splice variant in our cell lines, we asked (4) whether this provides a survival benefit in synovial sarcoma.

METHODS

Cells lines were characterized. SS18-SSX1 knockdown was achieved using a shRNA system. Cell viability was assessed by WST-1 analysis and apoptosis examined by caspase-3 activity.

RESULTS

We confirmed the SS18-SSX1 translocation in all cell lines and identified a consistent splicing variant. We achieved successful knockdown of SS18-SSX1 and with this saw a significant reduction in cell viability. Decreased viability was a result of increased apoptosis. Reintroduction of the exon 8 sequence into cells reduced cell viability in all cell lines.

CONCLUSIONS

We confirmed the presence of the SS18-SSX1 translocation in our cell lines and its importance in the survival of synovial sarcoma. We have also demonstrated that reduction in cell viability is related to an increase in apoptosis. In addition, we have identified a potential mediator of SS18-SSX function in exon 8.

CLINICAL RELEVANCE

SS18-SSX represents a tumor-specific target in synovial sarcoma. Exploitation of SS18-SSX and its protein partners will allow us to develop potent tumor-specific therapeutic agents.

Truncated SSX protein suppresses synovial sarcoma cell proliferation by inhibiting the localization of SS18-SSX fusion protein

Synovial sarcoma is a relatively rare high-grade soft tissue sarcoma that often develops in the limbs of young people and induces the lung and the lymph node metastasis resulting in poor prognosis. In patients with synovial sarcoma, specific chromosomal translocation of t(X; 18) (p11.2;q11.2) is observed, and SS18-SSX fusion protein expressed by this translocation is reported to be associated with pathogenesis. However, role of the fusion protein in the pathogenesis of synovial sarcoma has not yet been completely clarified. In this study, we focused on the localization patterns of SS18-SSX fusion protein. We constructed expression plasmids coding for the full length SS18-SSX, the truncated SS18 moiety (tSS18) and the truncated SSX moiety (tSSX) of SS18-SSX, tagged with fluorescent proteins. These plasmids were transfected in synovial sarcoma SYO-1 cells and we observed the expression of these proteins using a fluorescence microscope. The SS18-SSX fusion protein showed a characteristic speckle pattern in the nucleus. However, when SS18-SSX was co-expressed with tSSX, localization of SS18-SSX changed from speckle patterns to the diffused pattern similar to the localization pattern of tSSX and SSX. Furthermore, cell proliferation and colony formation of synovial sarcoma SYO-1 and YaFuSS cells were suppressed by exogenous tSSX expression. Our results suggest that the characteristic speckle localization pattern of SS18-SSX is strongly involved in the tumorigenesis through the SSX moiety of the SS18-SSX fusion protein. These findings could be applied to further understand the pathogenic mechanisms, and towards the development of molecular targeting approach for synovial sarcoma.

Reversible disruption of mSWI/SNF (BAF) complexes by the SS18-SSX oncogenic fusion in synovial sarcoma

Recent exon sequencing studies have revealed that over 20% of human tumors have mutations in subunits of mSWI/SNF (BAF) complexes. To investigate the underlying mechanism, we studied human synovial sarcoma (SS), in which transformation results from the translocation of exactly 78 amino acids of SSX to the SS18 subunit of BAF complexes. We demonstrate that the SS18-SSX fusion protein competes for assembly with wild-type SS18, forming an altered complex lacking the tumor suppressor BAF47 (hSNF5). The altered complex binds the Sox2 locus and reverses polycomb-mediated repression, resulting in Sox2 activation. Sox2 is uniformly expressed in SS tumors and is essential for proliferation. Increasing the concentration of wild-type SS18 leads to reassembly of wild-type complexes retargeted away from the Sox2 locus, polycomb-mediated repression of Sox2, and cessation of proliferation. This mechanism of transformation depends on only two amino acids of SSX, providing a potential foundation for therapeutic intervention.

Recurrent and novel SS18-SSX fusion transcripts in synovial sarcoma: description of three new cases

Synovial sarcoma (SS) is an aggressive type of tumor, comprising approximately 10 % of soft tissue sarcomas. Over 90 % of SS cases are characterized by the t(X;18)(p11.2;q11.2) translocation, which results mainly in the formation of oncogenic SS18-SSX1 or SS18-SSX2 fusions. In a typical SS18-SSX fusion transcript, exon 10 of SS18 is fused to exon 6 of SSX1/2. However, several variant fusion transcripts have been already described. In the present study, we examined the fusion transcript type in a series of 40 primary untreated SS tumor specimens using reverse transcription polymerase chain reaction and fluorescence in situ hybridization assay. We detected SS18-SSX1 transcript in 22 (55 %) patients and SS18-SSX2 transcript in 17 (42.5 %) patients, while in one patient, none of SS18-SSX1/2 fusion transcripts were identified. Among the cases under study, two tumors carried novel SS18-SSX1 and SS18-SSX2 variant translocations that were allegedly created by an alternative splicing, and in additional case, an unusual translocation variant previously described by other group was found. Our data suggest that alternative splicing may play an important role in novel fusion transcript formation, and additionally we show that it may be a recurrent event in SS. Furthermore, we describe the first case of a complex rearrangement possibly linking SS to REPS2 gene.

Downstream and intermediate interactions of synovial sarcoma-associated fusion oncoproteins and their implication for targeted therapy

Synovial sarcoma (SS), an aggressive type of soft tissue tumor, occurs mostly in adolescents and young adults. The origin and molecular mechanism of the development of SS remain only partially known. Over 90% of SS cases are characterized by the t(X;18)(p11.2;q11.2) translocation, which results mainly in the formation of SS18-SSX1 or SS18-SSX2 fusion genes. In recent years, several reports describing direct and indirect interactions of SS18-SSX1/SSX2 oncoproteins have been published. These reports suggest that the fusion proteins particularly affect the cell growth, cell proliferation, TP53 pathway, and chromatin remodeling mechanisms, contributing to SS oncogenesis. Additional research efforts are required to fully explore the protein-protein interactions of SS18-SSX oncoproteins and the pathways that are regulated by these partnerships for the development of effective targeted therapy.

INI1-deficient tumors: diagnostic features and molecular genetics

Significant progress has been made in understanding the molecular genetic alterations involved in sarcomagenesis. Cytogenetic and molecular studies have identified nonrandom genetic abnormalities, including tumor suppressor gene inactivation. Mutations, deletions, and other somatic alterations in the tumor suppressor gene INI1 (hSNF5; SMARCB1), which encodes a subunit of the SWI/SNF chromatin remodeling complex, were first described in the malignant rhabdoid tumor of infancy. Since then, INI1 has also been implicated in the pathogenesis of additional tumor types including renal medullary carcinomas and epithelioid sarcomas and a subset of epithelioid malignant peripheral nerve sheath tumors, myoepithelial carcinomas, and extraskeletal myxoid chondrosarcomas. As varied as this group appears, they all show loss of INI1 protein expression, a propensity for rhabdoid cytomorphology, and sometimes other overlapping immunohistochemical and histologic findings. We will review the clinicopathologic features of these tumor types and emphasize the clinical utility of INI1 immunohistochemistry in differential diagnosis.

The benefits of molecular pathology in the diagnosis of musculoskeletal disease : part I of a two-part review: soft tissue tumors

Bone and soft tissue metabolic and neoplastic diseases are increasingly characterized by their molecular signatures. This has resulted from increased knowledge of the human genome, which has contributed to the unraveling of molecular pathways in health and disease. Exploitation of this information has allowed it to be used for practical diagnostic purposes. The aim of the first part of this two-part review is to provide an up-to-date review of molecular genetic investigations that are available and routinely used by specialist musculoskeletal histopathologists in the diagnosis of neoplastic disease. Herein we focus on the benefits of employing well characterized somatic mutations in soft tissue lesions that are commonly employed in diagnostic pathology today. The second part highlights the known somatic and germline mutations implicated in osteoclast-rich lesions of bone, and the genetic changes that disturb phosphate metabolism and result in a variety of musculoskeletal phenotypes. Finally, a brief practical guide of how to use and provide a molecular pathology service is given.

Novel SYT-SSX fusion transcript variants in synovial sarcoma

Synovial sarcoma (SS) is characterized by the t(X;18)(p11.2;q11.2) chromosomal translocation detected in >95% of cases. Through this translocation, one of the SYT genes, SYT4 on chromosome 18, is fused to one of the SSX genes on chromosome X. SYT4-SSX1 is the most common fusion subtype, present in approximately two thirds of the cases, followed by SYT4-SSX2 and, very rarely, SYT4-SSX4. Variant fusion transcripts occur less often, and most of the reported cases are the result of small insertions. Described here is a novel fusion variant containing a small deletion resulting in an alternative reading frame of the SSX part of the fusion gene. This fusion transcript may provide further insight into the oncogenic function of the SSX partner of the fusion gene.

Cytosolic SYT/SS18 isoforms are actin-associated proteins that function in matrix-specific adhesion

SYT (SYnovial sarcoma Translocated gene or SS18) is widely produced as two isoforms, SYT/L and SYT/S, that are thought to function in the nucleus as transcriptional coactivators. Using isoform-specific antibodies, we detected a sizable pool of SYT isoforms in the cytosol where the proteins were organized into filamentous arrays. Actin and actin-associated proteins co-immunoprecipitated with SYT isoforms, which also co-sedimented and co-localized with the actin cytoskeleton in cultured cells and tissues. The association of SYT with actin bundles was extensive yet stopped short of the distal ends at focal adhesions. Disruption of the actin cytoskeleton also led to a breakdown of the filamentous organization of SYT isoforms in the cytosol. RNAi ablation of SYT/L alone or both isoforms markedly impaired formation of stress fibers and focal adhesions but did not affect formation of cortical actin bundles. Furthermore, ablation of SYT led to markedly impaired adhesion and spreading on fibronectin and laminin-111 but not on collagen types I or IV. These findings indicate that cytoplasmic SYT isoforms interact with actin filaments and function in the ability cells to bind and react to specific extracellular matrices.

The proto-oncoprotein SYT (SS18) controls ATP release and regulates cyst formation by polarized MDCK cells

The SYT proto-oncoprotein (also known as SS18) is a gene expression regulator conserved across species. Although its biological function is still unknown, the importance of SYT as a housekeeping protein is illustrated by the lethal phenotype of SYT-null embryos. Notably, SYT is a component of the synovial sarcoma-associated translocation product, the SYT-SSX oncogene. SYT was previously reported as a mediator of cell adhesion. In the present study we show that SYT possesses distinct domains that control MDCK cyst formation in three-dimensional collagen cultures. While the carboxy-half of SYT, the QPGY domain, is required for cyst growth, the amino-terminal region appears to exert on this process a regulatory effect. Further analysis suggested that the purinergic G protein-coupled P2Y receptor signaling is involved in SYT-induced cystogenesis. Activation of this cascade is due to facilitation of ATP release in the extracellular space of polarized MDCK cells by SYT. These studies allow us to begin to understand the vital role of SYT in controlling epithelial morphogenesis and might explain the lethality of its loss in the developing embryo.

A novel variant of SYT-SSX1 fusion gene in a case of spindle cell synovial sarcoma

Synovial sarcoma (SS) is a rare soft-tissue tumor that affects children and young adults. It is characterized by chromosomal translocation t(X;18)(p11.2;q11.2), which results in the fusion of the gene SYT on chromosome 18 with SSX genes on chromosome X. Heterogeneity within SS fusion junctions is rare. We report a case of a 9-year-old boy with a high-grade spindle cell sarcoma. Reverse transcriptase-polymerase chain reaction revealed a characteristic translocation of SSs. However, this sarcoma showed a longer-than-expected PCR product after gel electrophoresis. Direct sequencing of the product disclosed a novel SYT/SSX1 fusion transcript. Detection of fusion transcripts is useful for diagnostics of SS. In each case, when considering this diagnosis on the morphologic grounds an attempt to analyze the translocation using PCR should be made, including the recognition of its uncommon variants.

Detection of SS18-SSX fusion transcripts in formalin-fixed paraffin-embedded neoplasms: analysis of conventional RT-PCR, qRT-PCR and dual color FISH as diagnostic tools for synovial sarcoma

Synovial Sarcoma consistently harbors t(X;18) resulting in SS18-SSX1, SS18-SSX2 and rarely SS18-SSX4 fusion transcripts. Of 328 cases included in our study, synovial sarcoma was either the primary diagnosis or was very high in the differential diagnosis in 134 cases: of these, amplifiable cDNA was obtained from 131. SS18-SSX fusion products were found in 126 (96%) cases (74 SS18-SSX1, 52 SS18-SSX2), using quantitative and 120 by conventional reverse transcriptase-polymerase chain reaction (RT-PCR). One hundred and one cases in a tissue microarray, analyzed by fluorescence in situ hybridization (FISH), revealed that 87 (86%) showed SS18 rearrangement: four RT-PCR positive cases, reported as negative for FISH, showed loss of one spectrum green signal, and 15 cases had multiple copies of the SS18 gene: both findings are potentially problematic when interpreting results. One of three cases, not analyzed by RT-PCR reaction owing to poor quality RNA, was positive by FISH. SS18-SSX1 was present in 56 monophasic and 18 biphasic synovial sarcoma: SS18-SSX2 was detected in 41 monophasic and 11 biphasic synovial sarcoma. Poorly differentiated areas were identified in 44 cases (31%). There was no statistically significant association between biphasic, monophasic and fusion type. Five cases were negative for SS18 rearrangement by all methods, three of which were pleural-sited neoplasms. Following clinical input, a diagnosis of mesothelioma was favored in one case, a sarcoma, not otherwise specified in another and a solitary fibrous tumor in the third case. The possibility of a malignant peripheral nerve sheath tumor could not be excluded in the other two cases. We concluded that the employment of a combination of molecular approaches is a powerful aid to diagnosing synovial sarcoma giving at least 96% sensitivity and 100% specificity but results must be interpreted in the light of other modalities such as clinical findings and immunohistochemical data.

Molecular Diagnosis of Sarcomas: Chromosomal Translocations in Sarcomas

Context.—Sarcomas are rare, numerous in type, and often difficult to definitively classify. Work in the last 2 decades has revealed that a significant subset of sarcomas are associated with specific chromosomal translocations producing chimeric (fusion) genes that play a role in the sarcomas' biology and are helpful in their differential diagnosis.

Objective.—To briefly review the sarcomas associated with specific translocations presenting Ewing sarcoma and synovial sarcoma as archetypes and to further explain how cytogenetic and molecular biologic approaches are being used in the diagnosis of sarcomas.

Data Sources.—This work is based on a selected review of the relevant medical and scientific literature and our extensive experience with molecular testing in sarcomas.

Conclusions.—In addition to, and complementing, the traditional diagnostic methods of examination of hematoxylin-eosin stained slides, immunohistochemistry, and sound clinical-pathologic correlation, additional cytogenetic and molecular biologic methods are being increasingly utilized and relied on in sarcoma pathology. These methods include chromosomal karyotyping, fluorescence in-situ hybridization, spectral karyotyping, and polymerase chain reaction– based methods for demonstrating specific chromosomal translocations and fusion genes. Understanding the basis of these methods and their application is critical to better provide accurate and validated specific diagnoses of sarcomas.

A variant of the SYT-SSX2 fusion gene in a case of synovial sarcoma

Synovial sarcoma is a malignant soft tissue tumor harboring a tumor-specific fusion gene, SYT-SSX, of which exon 10 of the SYT gene is fused to exon 6 of the SSX gene is the common form. Here we report a case of synovial sarcoma with a novel form of the SYT-SSX2 fusion transcript, in which 75 bases were inserted at the common fusion junction. Computer analyses revealed that 15 bases were from intron 10 of the SYT gene, and 10 from the end of intron 4, and 50 from exon 5 of the SSX2 gene. Precise analyses of genomic breakpoints in SYT and SSX2 loci revealed that the reciprocal translocation creating the fusion gene was associated with a large deletion in both loci. The structure of SYT-SSX2 suggests that the fusion transcript in this case was created using a cryptic splicing acceptor site 15 bases upstream of the genomic fusion point, incorporating intronic sequences in mature mRNA. Reexamination of two variant SYT-SSX2 genes reported previously revealed that unknown sequences inserted at the common junction points were derived from intron sequences, as in the present case.

Synovial sarcoma translocation (SYT) encodes a nuclear receptor coactivator

We previously cloned and characterized a novel RNA-binding motif-containing coactivator, named coactivator activator (CoAA), as a thyroid hormone receptor-binding protein-interacting protein using a Sos-Ras yeast two-hybrid screening system. A database search revealed that CoAA is identical with synovial sarcoma translocation (SYT)-interacting protein. Thus, we hypothesized that SYT could also function as a coactivator. Subsequently, we isolated a cDNA encoding a larger isoform of SYT, SYT-long (SYT-L), from the brain and liver total RNA using RT-PCR. SYT-L possesses an additional 31 amino acids in its C terminus compared with SYT, suggesting that these two SYT isoforms may be expressed from two mRNAs produced by alternative splicing of a transcript from a single gene. By Northern blot analysis, we found that SYT-L mRNA is expressed in several human embryonic tissues, such as the brain, liver, and kidney. However, we could not detect SYT-L in adult tissues. Glutathione-S-transferase pull-down studies showed that SYT binds to the C-terminus of CoAA, but not to the coactivator modulator. Both isoforms of SYT function as transcriptional coactivators of nuclear hormone receptors in a ligand- and dose-dependent manner in CV-1, COS-1, and JEG-3 cells. However, the pattern of transactivation was different between SYT and SYT-L among these cells. SYT synergistically activates transcription with CoAA. In addition, SYT activates transcription through activator protein-1, suggesting that SYT may function as a general coactivator. These results indicate that SYT activates transcription, possibly through CoAA, to interact with the histone acetyltransferase complex.

Diagnostik des Synovialsarkoms

Synovial sarcoma diagnosis and differential diagnostic distinction from other spindle cell sarcomas may be difficult. In these cases the detection of the t(X;18) translocation by FISH and RT-PCR is diagnostically extremely helpful. This study was aimed at the question whether or not simultaneous use of both methods is required for evidence of t(X;18) translocation.Paraffin-embedded tumour specimens from 53 patients were included in the study which were considered to be possible synovial sarcomas on the basis of histological aspect and immunohistochemical profile. Detection of t(X;18) was performed using FISH and RT-PCR simultaneously. Nuclei and amplifiable RNA could be isolated from 39 of the 53 included cases (75%). In 72% of these 39 cases FISH and RT-PCR showed identical negative or positive results. The remainder of the cases (28%) showed either a typical PCR product or a positive FISH signal.In conclusions FISH could be confirmed by typical PCR products and is therefore qualified as an internal quality control. Nevertheless tumour biological and methodical reasons have an important influence on both methods. Consequently in difficult cases simultaneous FISH and RT-PCR analysis is necessary for a clear evidence of t(X;18) translocation.

Detection of SYT-SSX rearrangements in synovial sarcomas by real-time one-step RT-PCR

Synovial sarcomas are aggressive tumors of adolescent and young adults that account for up to 10% of soft tissue sarcomas. Cytogenetically, they are characterized by translocation t(X;18), which is found in more than 95% of tumors. In most cases, it results in fusion of the SYT gene with the SSX1 or SSX2 gene, thus creating SYT-SSX1 or SYT-SSX2 rearrangement. The 2 types of gene fusion have been correlated with histologic variants and prognosis of synovial sarcomas. In this study, we developed a simple and rapid method for the simultaneous detection of SYT-SSX1 and SYT-SSX2 rearrangements by using a LightCycler real-time one-step reverse transcriptase polymerase chain reaction (RT-PCR) technology (Roche). Oligonucleotide probes were designed so that the donor probe would span a fusion point and the acceptor probe would be complementary to the SSX1 sequence but have 2 nucleotide mismatches with SSX2 sequence. Such a design allows simultaneous amplification of 2 types of rearrangement in the same reaction but distinguishes them based on differences in melting temperature detected by melting curve analysis after PCR. With this method, 27 tumors (9 synovial sarcomas and 18 nonsynovial sarcomas) were studied and showed SYT-SSX1 rearrangement in 6 cases and SYT-SSX2 in 3 cases. These results had complete correlation with the finding of conventional RT-PCR and direct sequencing. In conclusion, we have developed a fast, accurate, and simple method for the detection of 2 major types of SYT-SSX rearrangement by using LightCycler RT-PCR and melting curve analysis.

A novel case of synovial sarcoma of the kidney: impact of SS18/SSX analysis of renal hemangiopericytoma-like tumors

We report a new case of synovial sarcoma of the kidney. The patient underwent nephrectomy because of a large tumor in the right kidney. The histologic diagnosis was hemangiopericytoma. Less than 1 year after primary surgery the patient was reoperated due to massive local recurrence. Histology now revealed a poorly differentiated tumor tissue with hemangiopericytoma-like features. Immunostainings showed immunoreactivity to cytokeratin, epithelial membrane antigen, and vimentin. The tumor was negative to CD34 and factor VIII. The tumor cell proliferation, assessed by Ki-67, was high. RT-PCR analysis and sequence analysis demonstrated the presence of SS18/SSX2 fusion gene. Review of the histologic specimens from the original tumors confirmed hemangiopericytoma-like morphology. The new diagnosis was poorly differentiated synovial sarcoma. At the time of reoperation, lung metastases were detected radiologically, reflecting a very aggressive phenotype. To our knowledge, this is the third case of poorly differentiated synovial sarcoma of the kidney. Common for all these three cases is the hemangiopericytoma-like histology and a very aggressive clinical behavior. These circumstances accentuate the impact of SS18/SSX analysis in diagnosis of renal hemangiopericytoma-like tumors.

A new human cell line, PDSS-26, from poorly differentiated synovial sarcoma, with unique chromosomal anomalies

Permanent synovial sarcoma cell lines are invaluable tools for understanding of the biology of this tumor. The present study reports the establishment of a new human cell line, PDSS-26, derived from a surgical specimen of a poorly differentiated synovial sarcoma. PDSS-26 has a doubling time of a 72 hours and grows as a monolayer of spindle cells that retain immunoreactivity for bcl-2 and vimentin. Karyotypic analysis revealed a rearrangement involving chromosomes 17 and 18, at the breakpoints q11.2 and q11.2, respectively, as the only structural aberrations. Analysis by reverse transcriptase polymerase chain reaction showed the presence of the SYT-SSX1 fusion transcript in both the primary tumor and the cell line. Cytoplasmic PTEN staining was detected by immunohistochemistry in both the PDSS-26 cell line and in original tumor, whereas no mutation was identified by automatic sequencing. Thus, PDSS-26 cells could be useful for future functional studies.

Splicing isoform of SYT-SSX fusion protein accelerates transcriptional activity and cell proliferation

The human SYT-SSX gene has two splicing isoforms (type N and I), the latter of which contains an additional insertion of 93 bases. In the present study, we found increased transcriptional activity of the SYT-SSX type I protein in luciferase assay. When the SYT-SSX cDNAs were transfected to NIH3T3 cells, the type I transformant grew faster than the type N transformant. Furthermore, we evaluated the isoform ratio of the SYT or SYT-SSX transcripts in various tissues. Our results suggest that the SYT-SSX type I protein plays a critical role in the tumorigenesis of synovial sarcomas through increased transcriptional activity.

Molecular genetics of pediatric soft tissue tumors: clinical application

The application of molecular genetics to pediatric soft tissue tumors has grown tremendously over the last decade. It has resulted in the identification of novel genes that have provided us with an increased understanding of oncogenesis. Furthermore, these findings have identified diagnostic and potentially prognostic factors for patient management. Molecular diagnostic techniques, such as reverse transcription PCR (RT-PCR) and fluorescence in situ hybridization (FISH), have become important tools for evaluating pediatric soft tissue tumors. By detecting characteristic fusion genes, these techniques have greatly increased the diagnostic accuracy of histopathological classification. One of the exciting promises of the development of these molecular techniques is their ability to detect micrometastasis and minimal residual disease. Monitoring of minimal residual disease in pediatric soft tissue tumors by quantitative RT-PCR may provide important prognostic information. Furthermore, the potential development of targeted therapy based on the understanding of the molecular pathology of a specific soft tissue tumor may complement existing treatments and improve disease outcome.

Pathologic and molecular aspects of soft tissue sarcomas

This article retains the conventional approach to the classification of soft tissue sarcomas, dividing them into several major histogenetic categories based on their overall microscopic appearance, tissue differentiation pattern, and biologic potential. The author advocates a multimodal approach, in which four distinctive data sets--clinical, radiographic, microscopic, and, in some cases, molecular--are considered to establish the diagnosis and treatment plan. Such step-wise analysis is more likely to lead to consistency and accuracy as compared with an intuitive approach based on fragmentary data. The author describes individual lesions of soft tissue as clinicopathologic entities and believes that they can be more accurately diagnosed and appropriately treated with the help of data generated by a multidisciplinary team. In addition, this article emphasizes the need to use emerging molecular techniques that can provide important clues for both diagnosis and prognosis.

The SSX gene family: characterization of 9 complete genes

Human SSX genes comprise a gene family with 6 known members. SSX1, 2 and 4 have been found to be involved in the t(X;18) translocation characteristically found in all synovial sarcomas. Four (SSX1, 2, 4 and 5) are known to be expressed in a subset of tumors and testis, and anti-SSX antibodies have been found in sera from cancer patients. SSX antigens are thus typical cancer-testis (CT) antigens. To identify additional SSX family members, we isolated and characterized human genomic clones homologous to a prototype SSX cDNA. We also searched public databases for sequences similar to SSX. This identified 3 additional SSX genes, SSX7, 8, 9, and also completed the sequence of the formerly partially defined SSX6 gene. In addition to these novel SSX genes, several SSX pseudogenes were identified. With the exception of 1 pseudogene, all SSX genomic SSX sequences map to chromosome X. Among normal tissues, SSX7 mRNA was present only in testis, whereas SSX6, 8 and 9 were not detected in any normal tissue. SSX6 and 7 were expressed in 1 of 9 melanoma cell lines tested, whereas SSX8 and 9 expression was not detected in any tumor tissue or cell lines tested. SSX1, 2, 4 and 5 mRNA expression can be induced in cell lines by 5-aza-2-deoxycytidine or Trichostatin A. These agents also induce SSX6, but not SSX3, 7, 8 or 9 in the tumor cell lines tested, indicating that mechanisms other than methylation or histone acetylation may be responsible for the repressed state of some SSX genes.

Chromosomal translocations and sarcomas

This review examines how the identification of tumor-specific translocations and fusion proteins has advanced the basic scientific and clinical understanding of sarcomas. Recent genetic advances, including the ASPL-TFE3 fusion of alveolar soft part sarcoma, the JAZF1-JJAZ1 fusion of endometrial stromal sarcoma, and HMGIC fusions in liposarcoma, are discussed. Next, the review addresses the ways in which molecular genetic data have influenced diagnostic and prognostic paradigms. For example, recent studies describe the detection of occult tumor cells and the identification of primary renal neoplasms that are genetically related to alveolar soft part sarcoma. In addition, the review discusses potential therapies based on the targeting of sarcoma-specific fusion proteins. These reports describe the potential use of Gleevec (STI571) for dermatofibrosarcoma protuberans and the use of tumor-specific fusion proteins as potential targets for immunotherapy. Finally, basic scientific findings are reviewed that elucidate, for example, the aberrant functions of SYT-SSX in chromatin remodeling and of EWS-FLI1 in transcription and mRNA splicing. These and other emerging models of tumorigenesis will help identify new therapeutic targets.

Co-existence of SYT-SSX1 and SYT-SSX2 fusions in synovial sarcomas

The chromosomal translocation t(X;18)(p11.2;q11.2) is tightly linked to the tumorigenesis of synovial sarcoma. Through this translation the SYT gene on chromosome 18 is fused with a testis/cancer antigen gene on the X chromosome, generating either a SYT-SSX1, SYT-SSX2, or less often a SYT-SSX4 fusion gene. It has been anticipated that the individual synovial sarcoma carries only one of these variants, however, in this study we demonstrated that SYT-SSX1 and SYT-SSX2 co-exist in a significant proportion of the cases. From 121 SYT-SSX positive primary tumors, co-expression of SYT-SSX1 and SYT-SSX2 was seen in 12 cases (10%), which were characterized in further detail both at the RNA, DNA and chromosomal level. In all 12 cases the SYT-SSX1 and SYT-SSX2 fusions resulted in identical SYT-SSX fusion transcripts. However, at the genomic level the translocations were different, and most likely occurred between variable intronic sites in the target genes. By interphase FISH analyses of 10 cases SYT-SSX2 translocations were found to be the most abundant in all but one of the cases, in which SYT-SSX1 was predominating. The findings reveal a new heterogenous feature of synovial sarcoma, accounting for approximately 10% of all cases, which may shed light on the molecular genetic mechanisms behind translocations in general, and on the etiology of synovial sarcoma in particular.

A novel type of SYT/SSX fusion: methodological and biological implications

Synovial sarcoma (SS) is a rare soft-tissue tumor that affects children and young adults. It is characterized by the chromosomal translocation t(X;18)(p11.2;q11.2), which results in the fusion of the SYT gene on chromosome 18 with a SSX gene on chromosome X. In the majority of cases, SYT is fused to exon 5 of SSX1 (64%), SSX2 (36%), or, rarely, SSX4. A novel fusion transcript variant deriving from the fusion of SYT to exon 6 of SSX4 gene (SYT/SSX4v) was found coexpressed in one of the previously reported SYT/SSX4 cases. In the present investigation, we describe a new SS case that was previously shown to be negative for SYT/SSX1 and SYT/SSX2 expression by conventional reverse transcription polymerase chain reaction (RT-PCR) methods. By redesigning and optimizing the RT-PCR protocol, we were able to detect SYT/SSX4v as the sole fusion transcript expressed in this tumor sample. This finding suggests that this novel fusion gene, which involves exon 6 of SSX only, is sufficient to keep the transforming function conferred by the SYT/SSX translocation of SS. In about 3% of morphologically, ultrastructurally, and immunohistochemically defined SS, the SYT/SSX fusion transcript is not detected using conventional RT-PCR. Here we demonstrate that optimization of the RT-PCR method is important for detecting different and unexpected SYT/SSX variants, which otherwise could be overlooked. Using nine cases of SS in which SYT/SSX fusion transcripts were not detected by conventional RT-PCR methods, we demonstrate the presence of SYT/SSX transcripts in two cases using the proposed RT-PCR approach. Applications of optimized RT-PCR can contribute to reduce false-negative SYT/SSX SS cases reported in literature.