Characterization of XAGE-1b, a short major transcript of cancer/testis-associated gene XAGE-1, induced in melanoma metastasis.

Suppression subtractive hybridization, comparing mRNA expression profiles of common nevocellular nevi and melanoma metastases, was used to identify potential markers of melanoma progression. From the metastases we isolated XAGE-1b, a 470 bp transcript of the XAGE-1 gene. In general, expression of XAGE-1b was much more prominent than expression of the longer XAGE-1 transcript, isolated from Ewing's sarcoma. The XAGE-1b open-reading frame codes for a putative protein of 81 amino acids, harboring a functional bipartite nuclear localization signal and a C-terminal acidic transcription-activation-like domain. On the nucleotide level, XAGE-1b has a 50% homology with members of the GAGE family. However, homology between the corresponding proteins is weak. Expression of XAGE-1b in normal tissues was mainly restricted to testis, while placenta and brain were sporadically positive. In human tumor cell lines as well as in human tumor lesions, expression was most frequently found in melanocytic tumors and Ewing's sarcoma. In the different stages of melanocytic tumor progression, expression was exclusively seen in melanoma metastases (38%; n = 61), while all tested common and atypical nevi (n = 10) as well as primary melanomas (n = 8) were negative. Upregulation of expression after treatment with demethylating agent 5-aza-2'-deoxycytidine was detected in 1 of 4 human melanoma cell lines tested. The XAGE-1 gene consists of 4 exons and is located on chromosome Xp11.21-Xp11.22. After transfection into COS cells, the corresponding protein can direct the coupled fluorescent protein to the nucleus, showing a distinct speckled staining aspect. Our data imply the nuclear cancer/testis-associated XAGE-1b to be a marker for late melanocytic tumor progression.