Epigenetic reprogramming as a key contributor to melanocyte malignant transformation

Abstract

Melanoma progression requires deregulation of gene expression by currently uncharacterized epigenetic mechanisms.A mouse model based on changes in cell microenvironment was developed by our group to study melanocyte malignanttransformation. Melanoma cell lines (4C11- and 4C11+) were obtained as result of 5 sequential anchorage blockadesof non-tumorigenic melan-a melanocytes. Melan-a cells submitted to 4 de-adhesion cycles were also established (4C),are non-tumorigenic and represent an intermediary phase of tumor progression. The aim of this work was to identifyfactors contributing to epigenetic modifications in early and later phases of malignant transformation induced byanchorage impediment. Epigenetic alterations occur early in tumorigenesis; 4C cell line shows changes in global andgene-specific DNA methylation and histone marks. Many histone modifications differ between melan-a, 4C, 4C11- (nonmetastaticmelanoma cell line) and 4C11+ (metastatic melanoma cell line) which could be associated with changes ingene and microRNA expression. These epigenetic alterations seem to play a key role in malignant transformation sincemelanocytes treated with 5-Aza-2'-deoxycytidine before each anchorage blockade do not transform. Some epigeneticchanges seem to be also responsible for the maintenance of malignant phenotype, since melanoma cell lines (4C11- and4C11+) treated in vitro with 5-Aza-2'-deoxycytidine or Trichostatin A showed reduction of tumor growth in vivo. Changesin gene expression reflecting cell adaptation to new environment were also observed. We propose a model in whichsustained microenvironmental stress in melanocytes results in epigenetic reprogramming. Thus, after adaptation, cellsmay acquire epigenetic marks that could contribute to the establishment of a malignant phenotype. (AU)