Rolling circle replication for engineering drug delivery carriers.

The technique to generate long concatenated nucleic acids capable of constructing nano-, microand macro-scopic structures is a powerful platform for designing drug d elivery carriers. Rolling circle replication (RCR), including DNA and RNA replication, is a process found in some bacteriophages or viroids for replicating the DNA or RNA genomes. In vitro versions of this natural phenomenon are the rolling circle amplification (RCA) [1] and rolling circle transcription (RCT) [2] that enabled cost-efficient synthesis of concatenated DNA and RNA. Φ29 DNA polymerase, the most popular DNA polymerase for RCA, is capable to replicating the full-length genome of Φ29 bacteriophage [3]. The superb RCA performance of Φ29 DNA polymerase is attributed to its high processivity and strand displacement ability, making it possible to generate long consecutive DNA products even in the presence of topologically complicated DNA templates. In addition, Φ29 DNA polymerase reacts in an isothermal condition that obviates the need for a thermal cycler for the reaction. Other isothermal polymerases that can be employed in RCA include Bst DNA polymerase [4], Vent exo-DNA polymerase [1] and so on. For a typical RCA, the three major components are the DNA polymerase, a single stranded DNA (ssDNA) template and a ssDNA primer. The fact that the product is an amplification of the ssDNA template leads to extensive study of RCA for signal amplification in biodetection [1]. Recently, the polymeric property of RCA products has attracted a lot of attention due to the development of DNA nanotechnology [5]. The programmability of the DNA template makes RCA a highly versatile platform to generate DNA particles or gels for biomedical applications. Functional DNA sequences, such as aptamers and DNAzymes, can be incorporated into the RCA products for applications including targeted drug delivery or bioimaging. Similar to RCA, RCT generates periodic RNA products from a ssDNA template. A typical RCT reaction requires only two major components, a DNA-dependent RNA polymerase (generally T7 RNA polymerase) and a ssDNA template containing the binding site of the RNA polymerase [2]. The high programmability of the DNA template makes RCT easy to manipulate. The flexible base-pairing rules of RNA, such as noncanonical base pairing, make RNA nanostructures more versatile and thermally stable than their DNA counterparts, giving RNA protein-like diversity in functions [6]. Functional RNA molecules such as aptamers, ribozyme, miRNA and siRNA have greatly expanded the toolbox for designing RNA carriers for drug delivery. In this editorial, we highlight recent advances in using RCR techniques for engineering DNAand RNA-based carriers for Rolling circle replication for engineering drug delivery carriers Wujin Sun Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill & North Carolina State University, Raleigh, NC 27695, USA and Division of Molecular Pharmaceutics, Center for Nanotechnology in Drug Delivery, UNC Eshelman School of Pharmacy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA