The Roles of miR-26, miR-29, and miR-203 in the Silencing of the Epigenetic Machinery during Melanocyte Transformation.

Cláudia Regina Gasque Schoof,Alberto Izzotti,Luciana Dos Reis Vasques,Miriam Galvonas Jasiulionis

doi:10.1155/2015/634749

Abstract

The epigenetic marks located throughout the genome exhibit great variation between normal and transformed cancer cells. While normal cells contain hypomethylated CpG islands near gene promoters and hypermethylated repetitive DNA, the opposite pattern is observed in cancer cells. Recently, it has been reported that alteration in the microenvironment of melanocyte cells, such as substrate adhesion blockade, results in the selection of anoikis-resistant cells, which have tumorigenic characteristics. Melanoma cells obtained through this model show an altered epigenetic pattern, which represents one of the first events during the melanocytes malignant transformation. Because microRNAs are involved in controlling components of the epigenetic machinery, the aim of this work was to evaluate the potential association between the expression of miR-203, miR-26, and miR-29 family members and the genes Dnmt3a, Dnmt3b, Mecp2, and Ezh2 during cells transformation. Our results show that microRNAs and their validated or predicted targets are inversely expressed, indicating that these molecules are involved in epigenetic reprogramming. We also show that miR-203 downregulates Dnmt3b in mouse melanocyte cells. In addition, treatment with 5-aza-CdR promotes the expression of miR-26 and miR-29 in a nonmetastatic melanoma cell line. Considering the occurrence of CpG islands near the miR-26 and miR-29 promoters, these data suggest that they might be epigenetically regulated in cancer.

Highlights

Epigenetic modifications play a pivotal role in modulating gene expression during development and tissue differentiation, in the establishment and maintenance of genomic imprinting, and in X chromosome inactivation in female mammals
Since microRNAs are predicted to regulate the expression of 60% of all coding genes [28] including the epigenetic machinery, they are good candidates for promoting aberrant expression of epigenetic machinery, contributing to the deregulation of epigenetic marks during cell transformation. Using this murine melanoma model [26, 27], we demonstrate that Dnmt3a and Dnmt3b are downregulated in nonmetastatic (4C3−) and metastatic (4C3+) melanoma cell lines, concomitant with the overexpression of miR-29b and miR-29c, which are known Dnmt3a and Dnmt3b regulators [30]. miR-26a, which targets Ezh2 [31], a histone methyltransferase belonging to the Polycomb family, was found to be downregulated in a nonmetastatic melanoma cell line (4C3−) but upregulated in 4C3+
In a previous study, using a murine melanoma model obtained through sequential anchorage blockade cycles in melan-a melanocytes, Molognoni and coworkers [27] demonstrated alterations in the expression of genes involved in the epigenetic machinery and changes in DNA methylation levels in premalignant melanocytes, suggesting that epigenetic reprogramming occurs prior to malignant transformation in these cells

Summary

Introduction

Epigenetic modifications play a pivotal role in modulating gene expression during development and tissue differentiation, in the establishment and maintenance of genomic imprinting, and in X chromosome inactivation in female mammals. The most commonly studied type of epigenetic modification is the DNA methylation of CpG dinucleotides. DNA methylation is regulated through a family of DNA methyltransferases (Dnmt) comprising 5 members. Three of these proteins mediate the addition of a methyl group predominantly to DNA cytosines of CpG dinucleotide sequences. Dnmt is ubiquitously expressed at high levels in proliferating cells and is involved in the maintenance of DNA methylation. Dnmt3a and Dnmt3b promote de novo DNA methylation and are highly expressed at the early stages of development and in embryonic stem cells

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Results