Epigenetic alterations along melanoma genesis

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A melanoma genesis model, consisting of non-tumorigenic melanocytes (melan-a lineage), melanocytes corresponding to intermediate phases of tumor progression (2C and 4C cell lines) and different tumorigenic melanoma cell lines (4C3, 4C11, Tm1 and Tm5), were established in our laboratory after submitting a non-tumorigenic murine melanocyte lineage, melan-a, to sequential anchorage impediment cycles. We have been interested in determining the contribution of epigenetic mechanisms in this melanocyte malignant transformation model. Progressive differences in global DNA methylation level and intracellular homocysteine concentrations were observed throughout the process resulting in melan-a malignant transformation. All melan-a-derived cell lines presented genomic hypomethylation, with tumorigenic melanoma cells presenting the highest DNA hypomethylation level, as analyzed by 5-MeC content. In addition, these cell lines showed reduced intracellular homocysteine levels, suggesting an imbalance of methionine cycle that could be affecting methylation reactions. Significant differences were also found in DNA methyltransferase 3b and EZH2 histone methyltransferase expression during melanoma genesis. Lost of dnmt3b expression and decrease in EZH2 protein expression were observed in melanoma cells. Significant decrease in the methylation level of repetitive DNA sequences, like IAP and A-repeats, were found only in more aggressive melanoma cell lines. These epigenetic alterations, involving both DNA methylation and histone modifications, seem to be relevant in the tumorigenic phenotype, since Tm5 melanoma cell line treated in vitro with 5-aza-2-deoxycitidine (5-Aza-CdR) and Trichostatin A (TSA) showed a significant reduction in tumor development in vivo. These results show that a perturbation of epigenetic events occurs along the malignant transformation process, and may play an important role in the acquisition of a malignant phenotype.
 Supported by FAPESP (06/61293-1) and CNPq.