Global Repression of Cancer Gene Expression in a Zebrafish Model of Melanoma Is Linked to Epigenetic Regulation

We have established a model of melanoma progression in zebrafish through the generation of transgenic lines specifically expressing oncogenic human HRAS in the melanocytic lineage. In these tumors we have carried out quantitative expression analysis of several putative cancer genes, from known and predicted cancer gene lists. In particular, we analyzed 39 out of 101 putative cancer genes identified with a bioinformatics approach and selected for the low frequency of duplication and the high connectivity in protein networks. Data obtained by real-time polymerase chain reaction analysis from zebrafish melanoma tissue shows that the expression of many cancer genes is downregulated in zebrafish melanomas, whereas only cell cycle genes are upregulated. To understand whether this trend is due to global repression of gene expression associated to a repressive chromatin state, we investigated whether changes of histone methylation were detectable in our melanoma model. We found substantial differences in the levels of H3K9me3, H4K20me2, H3K27me3, H3K4me3, and H3R2me2a immunostaining in melanoma tissue when compared with normal skin. Thus our analysis suggests that in our model, like in human melanoma, important changes occur to the methylation status of histones. Although the outcome of these changes is still unknown, they could be responsible for the global repression of gene expression through epigenetic regulation shown in this study.