SS18-SSX fusion genes resulting from a
chromosomal translocation t(X;18)(p11.2;q11.2) are a genetic hallmark of
synovial sarcoma. Although such cytogenetic or molecular aberrations have mostly been detected by fluorescence in situ hybridization or reverse transcription-polymerase chain reaction, the expression of SS18-SSX has been poorly investigated at a cellular or tissue level. In this study,
biotinylated tyramide (BT)-based in situ hybridization (ISH) was performed to detect SS18-SSX transcripts using
formalin-fixed,
paraffin-embedded tissues from 15
synovial sarcomas.
Digoxigenin-labeled
cRNA probes flanking the fusion points of SS18-SSX1 and SS18-SSX2 were generated by in vitro transcription, and hybridized signals were detected by a
streptavidin-
biotin complex method after chemical enhancement with BT. The localizations of signals were compared with the immunohistochemical expressions of epithelial or neuroectodermal markers and those of cell adhesion including cytokeratins (
CAM5.2, AE1/AE3, CK7),
epithelial membrane antigen,
E-cadherin,
beta-catenin, c-erbB-2 (HER2/neu), CD56, and
claudin-1. The ISH signals of the SS18-SSX transcripts were identified in 13
synovial sarcomas, and their fusion types correlated with those determined by reverse transcription-polymerase chain reaction. In biphasic
tumors, the ISH signals tended to localize to epithelial areas, whereas spindle-cell areas or monophasic fibrous
tumors showed a less intense or focal expression pattern. Notably, the expression patterns of AE1/AE3, CK7, and c-erbB-2 often colocalized with the ISH signals (7 of 11 cases positive for each marker). Our results suggest that BT-based ISH can be used as a molecular technique for the detection of SS18-SSX using
formalin-fixed,
paraffin-embedded tissues.