Double-stranded DNA-templated cleavage of oligonucleotides containing a P3'->N5' linkage triggered by triplex formation: the effects of chemical modifications and remarkable enhancement in reactivity

Nanostructured DNA for the delivery of therapeutic agents

DNA and RNA, the nucleic acids found in every living organism, are quite crucial, because not only do they store the genetic information, but also they are used as signals through interaction with various molecules within the body. The nature of nucleic acids, especially DNA, to form double-helix makes it possible to design nucleic acid-based nanostructures with various shapes. Because the shapes as well as the physicochemical properties determine their interaction with proteins or cells, nanostructured DNAs will have different features in the interaction compared with single- or double-stranded DNA. Some of these unique features of nanostructured DNA make ways for efficient delivery of therapeutic agents to specific targets. In this review, we begin with the factors affecting the properties of nanostructured DNA, followed by summarizing the methods for the development of nanostructured DNA. Further, we discuss the characteristics of nanostructured DNA and their applications for the delivery of bioactive compounds.

Capillary electrophoresis-systematic evolution of ligands by exponential enrichment selection of base- and sugar-modified DNA aptamers: target binding dominated by 2'-O,4'-C-methylene-bridged/locked nucleic acid primer

Chemically modified DNA aptamers specific to human α-thrombin were obtained from oligodeoxyribonucleotide (ODN) libraries by using a capillary electrophoresis-systematic evolution of ligands by exponential enrichment (CE-SELEX) method. These libraries contained 2'-O,4'-C-methylene-bridged/linked bicyclic ribonucleotides (B/L nucleotides) in the primer region and/or C5-modified thymidine bearing N(6)-ethyladenine (t) in the nonprimer region. Modified DNA aptamers showed high binding affinities to the target, with dissociation constants (Kd) values in the range of subnanomolar to several ten nanomolar levels. The introduction of base modification significantly suppressed the frequency of G-quadruplex motifs, which are often seen in thrombin-binding DNA aptamers. The resulting alternatives contained the 10-mer consensus sequence t5Gt2G2, which is frequently found in modified DNA aptamers with subnanomolar protein binding affinities. Furthermore, some base- and sugar-modified DNA aptamers with the 12-mer consensus sequence t2G2tC(A/G)A2G2t displayed binding activities that were dependent on the presence of B/L nucleotides in the primer region. Such aptamers were interestingly not recovered from a natural DNA library or from DNA libraries modified with either B/L nucleotides or t's. This emerging characteristic binding property will enable the creation of a direct selection methodology for DNA-based molecular switches that are triggered by chemical conversion of B/L nucleotides introduced to constant sequence regions in ODN libraries.

2',4'-BNA/LNA aptamers: CE-SELEX using a DNA-based library of full-length 2'-O,4'-C-methylene-bridged/linked bicyclic ribonucleotides

DNA-based aptamers that contain 2'-O,4'-C-methylene-bridged/linked bicyclic ribonucleotides (B/L nucleotides) over the entire length were successfully obtained using a capillary electrophoresis systematic evolution of ligands by exponential enrichment (CE-SELEX) method. A modified DNA library was prepared with an enzyme mix of KOD Dash and KOD mutant DNA polymerases. Forty 2'-O,4'-C-methylene bridged/locked nucleic acid (2',4'-BNA/LNA) aptamers were isolated from an enriched pool and classified into six groups according to their sequence. 2',4'-BNA/LNA aptamers of groups V and VI bound human thrombin with K(d) values in the range of several 10 nanomolar levels.

Oligonucleotide-templated reactions for sensing nucleic acids

Oligonucleotide-templated reactions are useful for applying nucleic acid sensing. Various chemistries for oligonucleotide-templated reaction have been reported so far. Major scientific interests are focused on the development of signal amplification systems and signal generation systems. We introduce the recent advances of oligonucleotide-templated reaction in consideration of the above two points.

Cleavage of oligonucleotides containing a P3'→N5' phosphoramidate linkage mediated by single-stranded oligonucleotide templates

Double-stranded DNA (dsDNA) templates can hybridize to and accelerate cleavage of oligonucleotides containing a P3'→N5' phosphoramidate (P-N) linkage. This dsDNA-templated cleavage of P-N linkages could be due to conformational strain placed on the linkage upon triplex formation. To determine whether duplex formation also induced conformational strain, we examined the reactivity of the oligonucleotides with a P-N linkage in the presence of single-stranded templates, and compared these reactions to those with dsDNA templates. P-N oligonucleotides that are cleaved upon duplex formation could be used as probes to detect single-stranded nucleic acids.