Internal ribosome entry site (IRES)-mediated translation as a putative candidate mechanism to mRNA-based therapies

Untranslated regions in the messenger RNA (mRNA) are susceptible to the interaction of regulatory elements including proteins or non-coding RNA molecules, being an important hotspot to the study of gene expression regulation. Internal Ribosome Entry Sites (IRES) are secondary structures usually located on the 5’UTR of an mRNA molecule that can recruit the ribosome during initiation of protein synthesis without the involvement of the cap structure. This mechanism tends to appear when the cell is under stress conditions, which might include the presence of oncogenes, growth factors or proteins involved in programmed cell death. This work focuses on the human AGO1, an important key player for RNA-mediated gene silencing, and whether its 5’UTR is capable of driving cap-independent translation initiation. To achieve this goal, a construct containing the 5’UTR of human AGO1 was cloned, taking advantage of a bicistronic vector containing two reporter genes, Renilla luciferase (RLuc) and firefly luciferase (FLuc), the last one cloned downstream from the 5’UTR. We performed luminometry assays to assess the relative translation efficiency of FLuc, which is under the control of AGO1 5’UTR. The results showed that human AGO1 5’UTR mediates a cap-independent eIF4G-dependent mechanism of translation initiation enhanced by a free 5’ end. Also, we saw that this alternative mechanism is maintained, and even enhanced, under stress conditions, such as the knock-down of eukaryotic initiation factor 4E, the protein that directly binds to the cap structure. We are currently investigating what is the minimal sequence required for IRES-mediated translation. Combining these results with emerging RNA-based therapies will be helpful to develop novel strategies to prevent and treat disorders, such as cancer, involving dysregulation of AGO1 translation.