SYT-SSX fusion proteins in synovial sarcomas: detection and characterization with new antibodies

Epigenetic features of human mesenchymal stem cells determine their permissiveness for induction of relevant transcriptional changes by SYT-SSX1

BACKGROUND

A characteristic SYT-SSX fusion gene resulting from the chromosomal translocation t(X;18)(p11;q11) is detectable in almost all synovial sarcomas, a malignant soft tissue tumor widely believed to originate from as yet unidentified pluripotent stem cells. The resulting fusion protein has no DNA binding motifs but possesses protein-protein interaction domains that are believed to mediate association with chromatin remodeling complexes. Despite recent advances in the identification of molecules that interact with SYT-SSX and with the corresponding wild type SYT and SSX proteins, the mechanisms whereby the SYT-SSX might contribute to neoplastic transformation remain unclear. Epigenetic deregulation has been suggested to be one possible mechanism.

METHODOLOGY/PRINCIPAL FINDINGS

We addressed the effect of SYT/SSX expression on the transcriptome of four independent isolates of primary human bone marrow mesenchymal stem cells (hMSC). We observed transcriptional changes similar to the gene expression signature of synovial sarcoma, principally involving genes whose regulation is linked to epigenetic factors, including imprinted genes, genes with transcription start sites within a CpG island and chromatin related genes. Single population analysis revealed hMSC isolate-specific transcriptional changes involving genes that are important for biological functions of stem cells as well as genes that are considered to be molecular markers of synovial sarcoma including IGF2, EPHRINS, and BCL2. Methylation status analysis of sequences at the H19/IGF2 imprinted locus indicated that distinct epigenetic features characterize hMSC populations and condition the transcriptional effects of SYT-SSX expression.

CONCLUSIONS/SIGNIFICANCE

Our observations suggest that epigenetic features may define the cellular microenvironment in which SYT-SSX displays its functional effects.

Immunostaining for SYT protein discriminates synovial sarcoma from other soft tissue tumors: analysis of 146 cases

Synovial sarcoma in its classic biphasic form can be distinguished readily from other soft tissue lesions; however, monophasic and poorly differentiated forms are diagnostically more problematic. For this reason, we assessed the efficacy of immunostaining for SYT and SSX1 proteins, the gene products resulting from unique synovial sarcoma translocation, to distinguish synovial sarcoma from other soft tissue lesions. A total number of 146 cases were analyzed, including 47 synovial sarcoma cases (all of which were verified by FISH to have t(X; 18) translocation and SYT-SSX fusion gene) and 99 soft tissue tumors of various types. A polyclonal IgG antibody against SYT was used to stain formalin-fixed paraffin embedded tissues. Forty-one out of 47 (87%) synovial sarcoma displayed strong positive nuclear staining (ranging from 80 to 90% of the tumor cells) for SYT antibody. Nineteen of 99 (19%) non-synovial sarcoma cases showed variable nuclear and cytoplasmic staining with SYT, which ranged from 20 to 60% of tumor nuclei, and included malignant peripheral nerve sheath tumor (5/25), solitary fibrous tumor (2/14), Ewing sarcoma (2/6), low grade fibromyxoid tumor (2/4), extraskeletal mesenchymal chondrosarcoma (2/6), gastrointestinal tumor (4/17), epithelioid sarcoma (2/2). The remaining non-synovial sarcomas were negative. This is the first study demonstrating SYT protein expression in tissue sections of synovial sarcoma. This method could provide an easy, rapid and widely applicable means of assisting in the diagnosis of synovial sarcoma, particularly when material and/or resources are unavailable for PCR or FISH-based testing. However, as variable weak staining for SYT may be encountered in a small percentage of non-synovial sarcoma sarcomas, a positive interpretation should be made only when the staining is strong, nuclear and present in the majority of cells.

Sarcomas: genetics, signalling, and cellular origins. Part 2: TET-independent fusion proteins and receptor tyrosine kinase mutations

Although the mechanisms that underlie sarcoma development are still poorly understood, the identification of non-random chromosomal translocations and receptor tyrosine kinase mutations associated with defined sarcoma types has provided new insight into the pathogenesis of these tumours. In Part 1 of the review (J Pathol 2007;213:4-20), we addressed sarcomas that express fusion genes containing TET gene family products. Part 2 of the review summarizes our current understanding of the implications of fusion genes that do not contain TET family members in sarcoma development, as well as that of specific mutations in genes encoding receptor tyrosine kinases (RTKs). The final section will serve as a summary of both reviews and will attempt to provide a synthesis of some of the emerging principles of sarcomagenesis.

Cancer/testis tumour-associated antigens: immunohistochemical detection with monoclonal antibodies

Cancer/testis tumour-associated antigens (C/T TAA) were the first human tumour-associated antigens to be characterised at the molecular level. Specific genes are expressed in the testis and in tumours of varying histological origin. The tissue expression pattern supports the notion that these antigens could be targets for active specific immunotherapy. Specific serological reagents have been developed and have helped to clarify biochemical characteristics of C/T TAA and to assess their distribution within clinical tumour samples. We review immunohistochemical evidence of the expression of C/T TAA known to be recognised by specific cytotoxic T lymphocytes. The emerging picture is consistent with a mostly heterogeneous expression in human cancers. These findings support the concept of multiantigenic tumour vaccine preparations. Moreover, the wide range of tumours in which C/T TAA have been detected urges further efforts to develop effective specific immunotherapeutic procedures.

Detection and induction of CTLs specific for SYT-SSX-derived peptides in HLA-A24(+) patients with synovial sarcoma

To investigate the immunogenic property of peptides derived from the synovial sarcoma-specific SYT-SSX fusion gene, we synthesized four peptides according to the binding motif for HLA-A24. The peptides, SS391 (PYGYDQIMPK) and SS393 (GYDQIMPKK), were derived from the breakpoint of SYT-SSX, and SS449a (AWTHRLRER) and SS449b (AWTHRLRERK) were from the SSX region. These peptides were tested for their reactivity with CTL precursors (CTLps) in 16 synovial sarcoma patients using HLA-A24/SYT-SSX peptide tetramers and also for induction of specific CTLs from four HLA-A24(+) synovial sarcoma patients. Tetramer analysis indicated that the increased CTLp frequency to the SYT-SSX was associated with pulmonary metastasis in synovial sarcoma patients (p < 0.03). CTLs were induced from PBLs of two synovial sarcoma patients using the peptide mixture of SS391 and SS393, which lysed HLA-A24(+) synovial sarcoma cells expressing SYT-SSX as well as the peptide-pulsed target cells in an HLA class I-restricted manner. These findings suggest that aberrantly expressed SYT-SSX gene products have primed SYT-SSX-specific CTLps in vivo and increased their frequency in synovial sarcoma patients. The identification of SYT-SSX peptides may offer an opportunity to design peptide-based immunotherapeutic approaches for HLA-A24(+) patients with synovial sarcoma.

Expression of SSX genes in the neoplastic cells of Hodgkin's lymphoma

The cancer/testis antigen (CTA) group of tumor-associated proteins have been reported to be expressed in various cancers and in adult testis but they are essentially not found in any other normal adult nonneoplastic tissues. Prompted by the frequent detection of SSX1 in a previous comprehensive expression profile of the Hodgkin's lymphoma (HL) cell line L428, we analyzed SSX expression by nonnested reverse-transcription polymerase chain reaction (RT-PCR) in 4 HL cell lines (L428, L540, HD-MY-Z, and KM-H2) and 32 tumor samples of HL. The cellular localization of SSX expression in the tumor samples was further analyzed by in situ hybridization (ISH). All 4 HL cell lines were positive by RT-PCR using SSX consensus primers. Using primers specific to individual SSX genes, all 4 cell lines expressed multiple SSX family members. Five tumor samples (15.6%) were positive by RT-PCR using SSX consensus primers and direct sequencing of the RT-PCR products showed that 4 of 5 expressed more than 1 SSX family member. ISH confirmed that SSX expression originated in HL cells in all 5 RT-PCR-positive tumor samples. Furthermore, ISH demonstrated SSX-positive HL cells in 6 of 11 cases (55%) that were negative by RT-PCR. Our results suggest that members of the SSX family of CTA are expressed in most HL. This subset of HL may be a candidate for immunotherapy approaches directed at SSX proteins.

Fusions of the SYT and SSX genes in synovial sarcoma

Synovial sarcomas are high grade spindle cell tumors that are divided into two major histologic subtypes, biphasic and monophasic, according to the respective presence or absence of a well-developed glandular epithelial component. They contain in essentially all cases a t(X;18) representing the fusion of SYT (at 18q11) with either SSX1 or SSX2 (both at Xp11). Neither SYT, nor the SSX proteins contain DNA-binding domains. Instead, they appear to be transcriptional regulators whose actions are mediated primarily through protein-protein interactions, with BRM in the case of SYT, and with Polycomb group repressors in the case of SSX. Ongoing work on the SYT-SSX fusion and synovial sarcoma should yield a variety of data of broader biological interest, in areas such as BRM and Polycomb group function and dysfunction, transcriptional targets of SYT-SSX proteins and their native counterparts, differential gene regulation by SYT-SSX1 and SYT-SSX2, control of glandular morphogenesis, among others.

Detection of SYT-SSX fusion gene in peripheral blood from a patient with synovial sarcoma

This report describes a case involving a 22-year-old pregnant woman with synovial sarcoma in the thigh. The patient recognized an elastic hard mass accompanied by a dull pain in the anteromedial portion of the right thigh. Magnetic resonance imaging delineated a deep soft-tissue mass measuring 9 x 7 x 6 cm. Histologic diagnosis of poorly differentiated synovial sarcoma was made based the results of an open biopsy. In this patient, the SYT-SSX fusion gene transcript was detected by nested polymerase chain reaction (PCR) in the peripheral blood collected before biopsy. Two months after wide local resection of the tumor, multiple lung metastases developed. This is the first reported case in which tumor cells were detected by nested PCR in the peripheral blood of a patient with synovial sarcoma. These findings suggest that circulating tumor cells should be monitored because they may serve as a prognostic indicator for synovial sarcoma.