How epigenetics can explain human metastasis: a new role for microRNAs

The metastatic process is characterized by the dissemination of tumoral cells throughout the bloodstream to distal sites, where these transformed cells proliferate and give rise to secondary tumors, which are the principal cause of mortality in cancer patients. In recent years, a significant number of metastasis-related genes have been described, such as cadherins, laminins, heparan sulfates and protease and angiogenesis inhibitors, among others. However, the mechanisms by which these genes are altered in metastasis remain unclear, since genetic alterations occur rarely, despite their widespread downregulation. Epigenetic alterations, and specifically CpG island hypermethylation-associated silencing, can potentially explain the aberrant expression of many of these genes. The disruption of histone modifiers and chromatin-remodeling factors also contributes to the alteration of metastasis genes. Members of a new class of regulatory RNAs, microRNAs (miRNAs), have an important role in cancer and metastasis and are also regulated by epigenetic mechanisms in both malignancies. As we gain insight into the epigenetic mechanisms orchestrating all the metastatic steps, we broaden the therapeutic possibilities of epigenetic drugs, such as DNA demethylating drugs and histone deacetylase inhibitors, which can act upon metastasis-related genes and miRNAs, restoring their expression. In this review, the latest studies relating cancer epigenetics and metastasis are analyzed, and we emphasize the importance of miRNAs and their epigenetic regulation in tumoral progression.