Martijn Kedde

MicroRNAs (miRNAs) are small noncoding RNA molecules of 20-24 nucleotides that regulate gene expression. In animals, miRNAs form imperfect interactions with sequences in the 3' Untranslated region (3'UTR) of mRNAs, causing translational inhibition and mRNA decay. In contrast, plant miRNAs mostly associate with protein coding regions. Here we show that human miR-148 represses DNA methyltransferase 3b (Dnmt3b) gene expression through a region in its coding sequence. This region is evolutionary conserved and present in the Dnmt3b splice variants Dnmt3b1, Dnmt3b2, and Dnmt3b4, but not in the abundantly expressed Dnmt3b3. Whereas overexpression of miR-148 results in decreased DNMT3b1 expression, short-hairpin RNA-mediated miR-148 repression leads to an increase in DNMT3b1 expression. Interestingly, mutating the putative miR-148 target site in Dnmt3b1 abolishes regulation by miR-148. Moreover, endogenous Dnmt3b3 mRNA, which lacks the putative miR-148 target site, is resistant to miR-148-mediated regulation. Thus, our results demonstrate that the coding sequence of Dnmt3b mediates regulation by the miR-148 family. More generally, we provide evidence that coding regions of human genes can be targeted by miRNAs, and that such a mechanism might play a role in determining the relative abundance of different splice variants.

MicroRNAs (miRNAs) are genes involved in normal development and cancer. They inhibit gene expression by associating with 3'-Untranslated regions (3'UTRs) of messenger RNAs (mRNAs), and are thought to regulate a large proportion of protein coding genes. However, it is becoming apparent that miRNA activity is not necessarily always determined by its expression in the cell. MiRNA activity can be affected by RNA-binding proteins (RBPs). For example, the RNA-binding protein HuR associates with the 3'UTR of the CAT1 mRNA after stress, counteracting the effect of miR-122. Second, we found that the expression of an RNA-binding protein called Dead end (Dnd1) prohibits the function of several miRNAs by blocking the accessibility of target mRNAs.(2) Dnd1 function is essential for proper development of primordial germ cells (PGCs) in zebrafish and mammals, indicating a crucial role for RBP/miRNA interplay on 3'UTRs of mRNAs in developmental decisions. In this perspective we discuss the interplay between RBPs and miRNAs in the context of germ cells and review current observations implicating RBPs in miRNA function.