Figure 1.

MicroRNAs as oncogenes and tumor suppressors. Cancer is a very complex genetic disease characterized by alterations in oncogenic and tumor‐suppressor protein‐coding genes and microRNAs. The abnormalities found to influence the activity of miRNAs includes chromosomal rearrangements, genomic amplifications or deletions, mutations and hypermethylation. Activation of oncogenic microRNAs reduced the levels of proteins blocking proliferation and activating apoptosis; by contrast, inactivation of suppressor miRNAs is followed by accumulation of proteins that stimulates proliferation and decrease apoptosis. The two oncogenic paradigms found in gliomas, the interaction between the oncogenic miR‐21 and the suppressor protein coding gene PTEN and between the oncogenic cluster miR‐221‐222 and the p27^kip1 suppressor protein are schematically presented. The possibility that other miRNAs (such as members of let‐7 or miR‐29 families) are down‐regulated and, consequently oncogenic proteins (such as RAS) overexpressed, is also presented.