Conditional Regulation of Gene Expression by Ligand-Induced Occlusion of a MicroRNA Target Sequence

Researchers sought to develop a compact, programmable, and non-immunogenic system for the conditional regulation of gene expression in mammalian cells. By leveraging the cell's endogenous RNA interference (RNAi) machinery, they aimed to create an 'on switch' that activates a target gene only in the presence of a specific small-molecule ligand.

The team designed RNA switches that incorporate a microRNA target sequence (miR-T) into the 3' untranslated region (UTR) of a gene of interest. In the absence of a ligand, the miR-T is accessible to the RNA-induced silencing complex (RISC), which suppresses gene expression. The switch includes a tetracycline-binding aptamer and a competing strand. When the ligand (tetracycline) binds the aptamer, it induces a conformational change that promotes the formation of a stem structure, effectively masking or 'occluding' the miR-T. This occlusion prevents RISC from binding, thereby allowing the gene to be expressed.

The researchers evaluated various architectures, including 3'-type and 5'-type designs, and found that combining these into 'dual-type' switches significantly improved performance. By optimizing the length of the competing strand, introducing transition mismatches, and adding 'lock-stem' sequences to stabilize the ligand-bound conformation, they achieved up to 19-fold regulation of luciferase expression in HeLa cells. The system was successfully generalized to other microRNAs, such as miR-122, and demonstrated 7- to 11-fold regulation of therapeutic proteins, including HIV-1 entry inhibitors, when delivered via adeno-associated virus (AAV) vectors.

This approach offers a versatile alternative to existing gene-regulation systems. Because these switches are small, they are well-suited for AAV-based gene therapy, where packaging capacity is limited. Furthermore, because they utilize endogenous cellular machinery rather than foreign proteins, they are less likely to trigger immune responses in patients. The ability to program these switches to respond to different microRNAs and ligands provides a flexible platform for precise, dose-dependent control of therapeutic transgenes.