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SSX: a multigene family with several members transcribed in normal testis and human cancer
1Ludwig Institute for Cancer Research, New York Branch, Memorial Sloan-Kettering Cancer Center, New York, USA.
Abstract:
Analysis of t(X;18) translocation in synovial sarcoma had previously led to the definition of the SSX2 gene, the fusion partner on chromosome X. Subsequent screening of testicular cDNA libraries identified 2 highly homologous genes, SSX1 and SSX3. Among these 3 genes, SSX2 has been found to be identical to HOM-MEL-40, which codes for an immunogenic tumor antigen expressed in various human cancers. SSX2 thus belongs to the family of cancer/testis (CT) antigens, i.e., immunogenic protein antigens with characteristic mRNA expression in normal testis and in cancer. To define additional CT antigens, we have immuno-screened a testicular cDNA expression library with an allogeneic serum from a melanoma patient, and both SSX2 and SSX3 were isolated. Further studies using 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 identified. All 5 members of the SSX family shared strong sequence homology, with nucleotide homology ranging from 88 to 95% and amino acid homology ranging from 77 to 91%. Genomic cloning of a prototype SSX gene (SSX2) showed that its coding region is encoded by 6 exons, and the shortened form of SSX4 cDNA represents an alternatively spliced product lacking the 5th exon. Analysis of SSX mRNA expression by gene-specific RT-PCR confirmed that all 5 SSX genes are expressed in testis. In addition, analysis of a panel of 12 melanoma cell lines showed strong mRNA expression of either SSX1 (3/12), SSX2 (3/12), SSX4 (1/12), or SSX5 (1/12), indicating variable activation of the genes in malignant cells.
Insights
Researchers identified five SSX genes (SSX1-5) as cancer/testis (CT) antigens. These genes show expression in the testis and variable activation in melanoma cells, highlighting their potential as immunogenic targets in cancer therapy.
Area of Science:
- Molecular Biology
- Immunology
- Oncology
Background:
- The SSX2 gene was identified through analysis of t(X;18) translocation in synovial sarcoma.
- SSX2 is identical to HOM-MEL-40, an immunogenic tumor antigen expressed in various human cancers.
- Cancer/testis (CT) antigens are immunogenic proteins with characteristic mRNA expression in normal testis and cancer.
Purpose of the Study:
- To identify and characterize novel cancer/testis (CT) antigens within the SSX gene family.
- To investigate the expression patterns of SSX genes in normal testis and melanoma cell lines.
Main Methods:
- Immuno-screening of a testicular cDNA expression library using melanoma patient serum.
- Screening of testicular cDNA libraries to identify homologous genes.
- Gene-specific RT-PCR for SSX mRNA expression analysis.
- Genomic cloning of the SSX2 gene to determine its exon-intron structure.
Main Results:
- Five members of the SSX gene family (SSX1-5) were identified, exhibiting high sequence homology.
- All five SSX genes are expressed in the testis.
- Variable mRNA expression of SSX1, SSX2, SSX4, and SSX5 was detected in a panel of 12 melanoma cell lines.
Conclusions:
- The SSX gene family represents a novel group of cancer/testis (CT) antigens.
- The identified SSX genes are potential targets for cancer immunotherapy due to their expression in tumors and normal testis.