SSX - A MULTIGENE FAMILY WITH SEVERAL MEMBERS TRANSCRIBED IN NORMAL TESTIS AND HUMAN CANCER

Analysis of t(X; 18) translocation in synovial sarcoma had previously led to the definition of the SSX2 gene, the fusion partner on chromoso me X. Subsequent screening of testicular cDNA libraries identified 2 h ighly homologous genes, SSX1 and SSX3. Among these 3 genes, SSX2 has b een found to be identical to HOM-MEL-40, which codes for an immunogeni c tumor antigen expressed in various human cancers. SSX2 thus belongs to the family of cancer/testis (CT) antigens, i.e., immunogenic protei n antigens with characteristic mRNA expression in normal testis and in cancer. To define additional CT antigens, we have immune-screened a t esticular cDNA expression library with an allogeneic serum from a mela noma patient, and both SSX2 and SSX3 were isolated. Further studies us ing testicular cDNA and SSX probes defined 2 new members of this gene family, SSX4 and SSX5, while a shorter cDNA variant of SSX4 was also i dentified. All 5 members of the SSX family shared strong sequence homo logy, with nucleotide homology ranging from 88 to 95% and amino acid h omology ranging from 77 to 91%. Genomic cloning of a prototype SSX gen e (SSXZ) showed that its coding region is encoded by 6 exons, and the shortened form of SSX4 cDNA represents an alternatively spliced produc t lacking the 5th exon. Analysis of SSX mRNA expression by gene-specif ic RT-PCR confirmed that all 5 SSX genes are expressed in testis. In a ddition, analysis of a panel of 12 melanoma cell lines showed strong m RNA expression of either SSX1 (3/12), SSX2 (3/12), SSX4 (1/12), or SSX 5 (1/12), indicating variable activation of the genes in malignant cel ls. (C) 1997 Wiley-Liss, Inc.