Hypomethylation of the Xmrk oncogene promoter in melanoma cells of Xiphophorus

Melanoma formation in Xiphophorus is caused by the tissue-specific overexpression of the Xmrk oncogene. This gene arose by a non-homologous recombination event between the Xmrk proto-oncogene and an unrelated locus, D, during which the Xmrk transcribed regions were fused to new upstream sequences. Functional analyses of this newly acquired promoter revealed the presence of positive regulatory elements. Since these elements did not activate transcription in a melanoma-specific manner, the methylation state of the Xmrk oncogene promoter in different tissues was analyzed. Digestion with methylation sensitive restriction enzymes followed by PCR amplification demonstrated differential methylation in a melanoma cell line and non-melanoma tissue. The methylation state of single CpG-dinucleotides in genomic DNA from different cell types was investigated by bisulfite genomic sequencing. This analysis revealed a high degree of methylation of the Xmrk oncogene promoter in nontransformed tissues. In contrast, the DNA-region analyzed was completely unmethylated in the melanoma cell line PSM. This effect was oncogene-specific, since the Xmrk proto-oncogene showed CpG-methylation in these cells. Studies with melanoma tissue demonstrated that hypomethylation of the Xmrk oncogene promoter can also be found in DNA from these tumors. This correlation suggests that the methylation status of the promoter might play a role in the overexpression of the Xmrk oncogene in vivo.