Heterogeneous expression of the SSX cancer/testis antigens in human melanoma lesions and cell lines

The SSX genes, located on the X chromosome, encode a family of highly homol ogous nuclear proteins. The SSX1 and SSX2 genes were initially identified a s fusion partners of the SYT gene in t(X;18)-positive synovial sarcomas, Re cently, however, it was Pound that these two genes, as well as the highly h omologous SSX4 and SSX5 genes, are aberrantly expressed in different types of cancers, including melanomas. Because normal SSX expression has been det ected only in the testis and, at very low levels, the thyroid, these protei ns are considered as new members of the still growing family of cancer/test is antigens. These antigens are presently considered as targets for the dev elopment of cancer immunotherapy protocols. In the present study, we develo ped a monoclonal antibody Pound to recognize SSX2, SSX3, and SSX4 proteins expressed in formaldehyde-fixed and paraffin embedded tissues. This antibod y was used to investigate SSX expression in normal testis and thyroid, beni gn melanocytic lesions, melanoma lesions, and melanoma fell lines. SSX nucl ear expression in the testis was found to be restricted to spermatogenic ce lls, mainly spermatogonia. Of 18 melanoma cell lines analyzed, 9 showed SSX RNA and protein expression, although heterogeneously and at variable level s. Treatment of an SSX-negative cell line with 5-aza-2'-deoxycytidine, a de methylating agent, led to SSX RNA and protein expression, indicating a role for methylation in transcription regulation. Thirty-four of 101 primary an d metastatic melanoma cases and 2 of 24 common nevocellular and atypical ne ws cases showed SSX nuclear staining. Again, SSX expression was heterogeneo us, ranging from widespread to scarce. Our findings stress the importance o f assessing the a priori SSX expression status of melanoma cases that mag b e selected for immunotherapeutic trials.