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Human synovial sarcoma has been shown to exclusively harbor the chromosomal translocation t(X;18) that produces the chimeric gene SYT-SSX. However, the role of SYT-SSX in cellular transformation remains unclear. In this study, we have established 3Y1 rat fibroblast cell lines that constitutively express SYT, SSX1, and SYT-SSX1 and found that SYT-SSX1 promoted growth rate in culture, anchorage-independent growth in soft agar, and tumor formation in nude mice. Deletion of the Nterminal 181 amino acids of SYT-SSX1 caused loss of its transforming activity. Furthermore, association of SYT-SSX1 with the chromatin remodeling factor hBRM͞hSNF2␣, which regulates transcription, was demonstrated in both SYT-SSX1-expressing 3Y1 cells and in the human synovial sarcoma cell line HS-SY-II. The binding region between the two molecules was shown to reside within the N-terminal 181 amino acids stretch (aa 1-181) of SYT-SSX1 and 50 amino acids (aa 156 -205) of hBRM͞hSNF2␣ and we found that the overexpression of this binding region of hBRM͞hSNF2␣ significantly suppressed the anchorageindependent growth of SYT-SSX1-expressing 3Y1 cells. To analyze the transcriptional regulation by SYT-SSX1, we established conditional expression system of SYT-SSX1 and examined the gene expression profiles. The down-regulation of potential tumor suppressor DCC was observed among 1,176 genes analyzed by microarray analysis, and semi-quantitative reverse transcription-PCR confirmed this finding. These data clearly demonstrate transforming activity of human oncogene SYT-SSX1 and also involvement of chromatin remodeling factor hBRM͞hSNF2␣ in human cancer. H uman synovial sarcoma is an aggressive soft tissue tumor that most commonly arises in the lower part of legs of young adults and for which there is currently no effective therapy (1). The chromosomal translocation t(X;18)(p11.2;q11.2) has been reported to be present at high frequency in synovial sarcomas (2-6). Isolation of genomic DNA in the vicinity of the break points has revealed that t(X;18) leads to fusion of the genes SYT localized at 18q11.2, and SSX localized at Xp11.2, respectively, thus resulting in expression of the chimeric protein SYT-SSX (7,8). Because expression of SYT-SSX has been observed in almost all cases of synovial sarcoma, this is considered to play a central role in tumorigenesis (9). However, there is no direct evidence yet for defining SYT-SSX as an oncogene.At the time of identification of SYT-SSX, the cDNA sequences of both SYT and SSX were reported (7, 8). The wild-type SYT protein is composed of 387 amino acids and possesses a QPGY domain that contains a characteristic 100-aa stretch that is rich in glutamine, proline, glycine, and tyrosine residues, as well as several putative SH2 and SH3 binding motifs (10). The SSX protein contains 188 amino acids, and its Nterminal region exhibits homology to the Kruppel-associated box (KRAB), which is found as a transcriptional repressor domain in Kruppel-type zinc finger protein (11). Except for the KRAB domain, no significant homol...

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