DNA nanostructure-based siRNA delivery systems

With the approval of multiple nucleic acid drugs for clinic use by FDA, we witness a revival of gene therapy in recent years. As one of the most important drug candidates, small interfering ribonucleic acid (siRNA) plays an essential role in gene silencing. Although a large variety of siRNA delivery systems have been developed, lack of efficient way to deliver siRNA to target tissue remains as a hurdle that retards the translation of siRNA for clinic use. Different from traditional cationic liposome and polymer based nano-delivery systems that load and compress the siRNA by electrostatic interaction, self-assembled DNA nanostructures can be equipped with functional nucleic acids by DNA hybridization, which further serve as vehicles for siRNA delivery. In this mini-review, first we introduce the concept of RNA interference (RNAi) based gene silencing and emphasize the importance. It is well known that siRNA can disturb the process of translating, silence genes, and further inhibit the expression of corresponding proteins via RNAi. However, naked siRNA is not stable during the circulation. Besides that, it is difficult for siRNA itself to enter the cells, demanding proper gene vehicles to assist its cellular and systemic delivery. Unlike traditional cationic carriers that are usually toxic to cells, DNA nanostructures have been verified with excellent biocompatibility and biodegradability. Along with the rapid development of DNA nanotechnology and tremendous DNA-based nanostructures that have been assembled, more attentions have been paid on using DNA nanostructures as new carriers for siRNA delivery. Subsequently, we systematically summarize the recent progress of DNA-based siRNA delivery systems, including their designs, structures, functions, and various applications. For instance, DNA nanocage is one representative carrier used for siRNA delivery. Lee once assembled DNA tetrahedron with 6 siRNAs on each strut and folate ligands on its surface, achieving enhanced efficiency of siRNA delivery for both in vitro and in vivo. Sleiman group also reported a DNA "nanosuitcase" to encapsulate siRNA and release it upon specific trigger. By virtue of DNA origami, Ke et al. designed DNA nanoparticles to transport siRNA and further investigated the morphology influence on cellular uptake efficiency. Moreover, taking advantage of rolling-circle amplification (RCA) method, new DNA-based delivery systems with different shapes, including Y-DNA structure and periodic DNA nanoribbons, were developed for siRNA delivery. Except barely loading functional siRNA on DNA nanostructures, protecting siRNA from degradation in the new delivery systems is also important. Recently, we also reported a crosslinked DNA hydrogel platform for siRNA delivery, in which siRNA can embedded inside the nanogel to avoid the enzymatic degradation. Both in vivo and in vitro results revealed that the crosslinked nanogel had excellent delivery efficiency and antitumor effect in an siRNA-based therapy. Despite great advances have been achieved, several problems remain to be solved in using DNA nanostructure for siRNA delivery. Lastly, we discuss the main challenges in this field, including the stability, immunogenicity, targeting capability, cost of the delivery vehicles and make a brief prospect. Once these problems are nicely addressed, we believe that DNA-based gene vectors will take a huge step toward practical use in clinic.