An artificial restriction DNA cutter for site-selective gene insertion in human cells

Pseudo-Complementary G:C Base Pair for Mixed Sequence dsDNA Invasion and Its Applications in Diagnostics (SARS-CoV-2 Detection)

Pseudo-complementary oligonucleotides contain artificial nucleobases designed to reduce duplex formation in the pseudo-complementary pair without compromising duplex formation to targeted (complementary) oligomers. The development of a pseudo-complementary A:T base pair, U^s:D, was important in achieving dsDNA invasion. Herein, we report pseudo-complementary analogues of the G:C base pair leveraged on steric and electrostatic repulsion between the cationic phenoxazine analogue of cytosine (G-clamp, C⁺) and N-7 methyl guanine (G⁺), which is also cationic. We show that while complementary peptide nucleic acids (PNA) form a much more stable homoduplex than the PNA:DNA heteroduplex, oligomers based on pseudo-C:G complementary PNA favor PNA:DNA hybridization. We show that this enables dsDNA invasion at physiological salt concentration and that stable invasion complexes are obtained with low equivalents of PNAs (2-4 equiv). We harnessed the high yield of dsDNA invasion for the detection of RT-RPA amplicon using a lateral flow assay (LFA) and showed that two strains of SARS-CoV-2 can be discriminated owing to single nucleotide resolution.

Applications of PNA-Based Artificial Restriction DNA Cutters

More than ten years ago, artificial restriction DNA cutters were developed by combining two pseudo-complementary peptide nucleic acid (pcPNA) strands with either Ce(IV)/EDTA or S1 nuclease. They have remarkably high site-specificity and can cut only one predetermined site in the human genome. In this article, recent progress of these man-made tools have been reviewed. By cutting the human genome site-selectively, desired fragments can be clipped from either the termini of chromosomes (telomeres) or from the middle of genome. These fragments should provide important information on the biological functions of complicated genome system. DNA/RNA hybrid duplexes, which are formed in living cells, are also site-selectively hydrolyzed by these cutters. In order to further facilitate the applications of the artificial DNA cutters, various chemical modifications have been attempted. One of the most important successes is preparation of PNA derivatives which can form double-duplex invasion complex even under high salt conditions. This is important for in vivo applications, since the inside of living cells is abundant of metal ions. Furthermore, site-selective DNA cutters which require only one PNA strand, in place of a pair of pcPNA strands, are developed. This progress has opened a way to new fields of PNA-based biochemistry and biotechnology.

Chemical modifications of artificial restriction DNA cutter (ARCUT) to promote its in vivo and in vitro applications

Recently, completely chemistry-based tools for site-selective scission of DNA (ARCUT) have been prepared by combining 2 strands of pseudo-complementary PNA (pcPNA: site-selective activator) and a Ce(IV)-EDTA complex (molecular scissors). Its site-specificity is sufficient to cut the whole human genome at one predetermined site. In this first-generation ARCUT, however, there still remain several problems to be solved for wider applications. This review presents recent approaches to solve these problems. They are divided into (i) covalent modification of pcPNA with other functional groups and (ii) new strategies using conventional PNA, in place of pcPNA, as site-selective activator. Among various chemical modifications, conjugation with positively-charged nuclear localization signal peptide is especially effective. Furthermore, unimolecular activators, a single strand of which successfully activates the target site in DNA for site-selective scission, have been also developed. As the result of these modifications, the site-selective scission by Ce(IV)-EDTA was achieved promptly even under high salt conditions which are otherwise unfavourable for double-duplex invasion. Furthermore, it has been shown that "molecular crowding effect," which characterizes the inside of living cells, enormously promotes the invasion, and thus the invasion seems to proceed effectively and spontaneously in the cells. Strong potential of pcPNA for further applications in vivo and in vitro has been confirmed.