Synovial sarcoma: from genetics to genetic-based animal modeling

Abstract

Synovial sarcomas are highly aggressive mesenchymal cancers that show modest response to conventional cytotoxic chemotherapy, suggesting a definite need for improved biotargeted agents. Progress has been hampered by the lack of insight into pathogenesis of this deadly disease. The presence of a specific diagnostic t(X;18) translocation leading to expression of the unique SYT-SSX fusion protein in effectively all cases of synovial sarcoma suggests a role in the etiology. Other nonspecific anomalies such as overexpression of Bcl-2, HER-2/neu, and EGFR have been reported, but their role in the pathogenesis remains unclear. Using gene targeting, we recently generated mice conditionally expressing the human SYT-SSX2 fusion gene from mouse endogenous ROSA26 promoter in chosen tissue types in the presence of Cre recombinase. These mice develop synovial sarcoma when SYT-SSX2 is expressed within myoblasts, thereby identifying a source of this enigmatic tumor and establishing a mouse model of this disease that recapitulates the clinical, histologic, immunohistochemical, and transcriptional profile of human synovial sarcomas. We review the genetics of synovial sarcoma and discuss the usefulness of genetics-based mouse models as a valuable research tool in the hunt for key molecular determinants of this lethal disease as well as a preclinical platform for designing and evaluating novel treatment strategies.

Fig. 1A−C

A diagram illustrates the proteins involved in synovial sarcoma-specific t(X;18) translocation: (A) SYT protein; (B) SSX protein; and (C) SYT-SSX fusion protein, which retains almost the entire SYT except the last eight amino acids, which are replaced with the last 78 amino acids of SSX. SNH = SYT N-terminal homology domain; QPGY = glutamine-, proline-, glycine-, and tyrosine-rich domain; SH2 BM = Src homology 2 binding motif; SH3 BM = Src homology 3 binding motif; KRAB = Kruppel-associated box; SSXRD = SSX repression domain. Although SYT, SSX, and the SYT-SSX fusion proteins are localized to the nucleus, there is no nuclear localization signal or DNA-binding domain associated with these proteins.

Fig. 2

A diagram illustrates the conditional expression of SYT-SSX2 in the mouse model of synovial sarcoma. The transcriptional stop signal includes the Pgk-Neo selection cassette followed by a polyadenylation signal that prevents transcription of downstream SYT-SSX2. Cre recombines the two LoxP sites placed in the same orientation and removes the transcriptional stop and allows transcription through the ROSA promoter. Transcription leads to formation of a bicistronic SYT-SSX2-IRES-EGFP message that is then translated into two separate proteins, SYT-SSX2 and EGFP. This strategy allows expression of SYT-SSX2 within investigator-specified tissue based on delivery of the Cre recombinase.

Fig. 3A–F

Photographs show the tumors in the mouse model of synovial sarcoma: (A) a tumor in the right forelimb; (B) a tumor arising in the neck; (C) gross features of a large tumor in the left forelimb; and (D) small tumors in the thoracic cage. (E) Small tumors in the thoracic cage (shown in D) are easily identified by fluorescence. (F) Histology on a representative mouse synovial sarcoma shows the presence of trapped skeletal muscle lending further proof of a myogenic origin. Consistent expression of the EGFP fluorescent protein within the mouse synovial sarcoma-like tumors is apparent by the intense green fluorescence and demonstrates the expression of SYT-SSX2 within these tumors. Absence of fluorescence in the surrounding tissue suggests only the tumors express SYT-SSX2.