A Visual Compendium of Principal Modifications within the Nucleic Acid Sugar Phosphate Backbone

Nucleic acid chemistry is a huge research area that has received new impetus due to the recent explosive success of oligonucleotide therapy. In order for an oligonucleotide to become clinically effective, its monomeric parts are subjected to modifications. Although a large number of redesigned natural nucleic acids have been proposed in recent years, the vast majority of them are combinations of simple modifications proposed over the past 50 years. This review is devoted to the main modifications of the sugar phosphate backbone of natural nucleic acids known to date. Here, we propose a systematization of existing knowledge about modifications of nucleic acid monomers and an acceptable classification from the point of view of chemical logic. The visual representation is intended to inspire researchers to create a new type of modification or an original combination of known modifications that will produce unique oligonucleotides with valuable characteristics.

Evaluation of Chemically Modified Nucleic Acid Analogues for Splice Switching Application

In recent years, several splice switching antisense oligonucleotide (ASO)-based therapeutics have gained significant interest, and several candidates received approval for clinical use for treating rare diseases, in particular, Duchenne muscular dystrophy and spinal muscular atrophy. These ASOs are fully modified; in other words, they are composed of chemically modified nucleic acid analogues instead of natural RNA oligomers. This has significantly improved drug-like properties of these ASOs in terms of efficacy, stability, pharmacokinetics, and safety. Although chemical modifications of oligonucleotides have been discussed previously for numerous applications including nucleic acid aptamers, small interfering RNA, DNAzyme, and ASO, to the best of our knowledge, none of them have solely focused on the analogues that have been utilized for splice switching applications. To this end, we present here a comprehensive review of different modified nucleic acid analogues that have been explored for developing splice switching ASOs. In addition to the antisense chemistry, we also endeavor to provide a brief historical overview of the approved spice switching ASO drugs, including a list of drugs that have entered human clinical trials. We hope this work will inspire further investigations into expanding the potential of novel nucleic acid analogues for constructing splice switching ASOs.

Splice-Switching Antisense Oligonucleotides Targeting Extra- and Intracellular Domains of Epidermal Growth Factor Receptor in Cancer Cells

Cancer is one of the leading causes of death globally. Epidermal growth factor receptor is one of the proteins involved in cancer cell proliferation, differentiation, and invasion. Antisense oligonucleotides are chemical nucleic acids that bind to target messenger ribonucleic acid and modulate its expression. Herein, we demonstrate the efficacy of splice-modulating antisense oligonucleotides to target specific exons in the extracellular (exon 3) and intracellular (exon 18, 21) domains of epidermal growth factor receptor. These antisense oligonucleotides were synthesized as 25mer 2'-O methyl phosphorothioate-modified ribonucleic acids that bind to complementary specific regions in respective exons. We found that PNAT524, PNAT525, PNAT576, and PNAT578 effectively skipped exon 3, exon 18, and exon 21 in glioblastoma, liver cancer, and breast cancer cell lines. PNAT578 treatment also skipped partial exon 19, complete exon 20, and partial exon 21 in addition to complete exon 21 skipping. We also found that a cocktail of PNAT576 and PNAT578 antisense oligonucleotides performed better than their individual counterparts. The migration potential of glioblastoma cancer cells was reduced to a greater extent after treatment with these antisense oligonucleotides. We firmly believe that using these splice-modulating antisense oligonucleotides in combination with existing EGFR-targeted therapies could improve therapeutic outcomes.

Syntheses of Base-Labile Pseudo-Complementary SNA and l-aTNA Phosphoramidite Monomers

We previously synthesized phosphoramidite monomers bearing Boc-protected 2,6-diaminopurine (D) and 2-methyl-4-methoxybenzyl-protected 2-thiouracil (sU) as building blocks for the preparation of pseudo-complementary serinol nucleic acids (SNAs). Since SNA is stable under acidic conditions, an acid-deprotection step could be inserted into the work-up. However, as the 4,4'-dimethoxytrityl group was concurrently removed at this step, purification of SNA by reversed-phase HPLC was difficult. Here, we report the syntheses of SNA and acyclic l-threoninol nucleic acid (l-aTNA) phosphoramidite monomers with bis(phenoxyacetyl)-protected D and 4-acetoxybenzyl-protected sU, both of which can be deprotected under mild basic conditions. Using these monomers, we prepared pseudo-complementary SNA and l-aTNA in high yield using conventional oligonucleotide synthesis protocols. These monomers can be used for large-scale syntheses of SNAs and l-aTNAs.

Thiomorpholino oligonucleotides as a robust class of next generation platforms for alternate mRNA splicing

Recent advances in drug development have seen numerous successful clinical translations using synthetic antisense oligonucleotides (ASOs). However, major obstacles, such as challenging large-scale production, toxicity, localization of oligonucleotides in specific cellular compartments or tissues, and the high cost of treatment, need to be addressed. Thiomorpholino oligonucleotides (TMOs) are a recently developed novel nucleic acid analog that may potentially address these issues. TMOs are composed of a morpholino nucleoside joined by thiophosphoramidate internucleotide linkages. Unlike phosphorodiamidate morpholino oligomers (PMOs) that are currently used in various splice-switching ASO drugs, TMOs can be synthesized using solid-phase oligonucleotide synthesis methodologies. In this study, we synthesized various TMOs and evaluated their efficacy to induce exon skipping in a Duchenne muscular dystrophy (DMD) in vitro model using H2K mdx mouse myotubes. Our experiments demonstrated that TMOs can efficiently internalize and induce excellent exon 23 skipping potency compared with a conventional PMO control and other widely used nucleotide analogs, such as 2'-O-methyl and 2'-O-methoxyethyl ASOs. Notably, TMOs performed well at low concentrations (5-20 nM). Therefore, the dosages can be minimized, which may improve the drug safety profile. Based on the present study, we propose that TMOs represent a new, promising class of nucleic acid analogs for future oligonucleotide therapeutic development.

Phosphorothioate modification improves exon-skipping of antisense oligonucleotides based on sulfonyl phosphoramidates in mdx mouse myotubes

2'-O-Methyl (2'-OMe) antisense oligonucleotides (AOs) possessing a various number of 4-(trimethylammonio)butylsulfonyl or tosyl phosphoramidates (N+ and Ts-modifications, respectively) instead of a native phosphodiester linkage were designed to skip exon-23 in dystrophin pre-mRNA transcript in mdx mice myotubes. AOs bearing several zwitterionic N+ modifications in the sequence had remarkably increased thermal stability towards complementary mRNA in comparison with 2'-OMe-RNAs having negatively charged Ts and phosphorothioate (PS) linkages. However, only Ts-modified AOs exhibited a similar level of exon skipping in comparison with fully modified PS-containing 2'-OMe-RNA, whereas the exon skipping induced by N+ modified AOs was much lower with no exon-skipping detected for AOs having seven N+ modifications. The level of exon-skipping was improved once Ts and especially N+ moieties were used in combination with PS-modification, most likely through improved cellular and nuclear uptake of AOs. These results provide new insights on expanding the design of novel chemically modified AOs based on phosphate modifications.

Oligonucleotides containing 2'-O-methyl-5-(1-phenyl-1,2,3-triazol-4-yl)uridines demonstrate increased affinity for RNA and induce exon-skipping in vitro

The nucleotide monomer containing the 1-phenyl-1,2,3-triazole group attached to the 5-position of 2'-O-methyluridine is hereby presented together with a derivative further substituted with a p-sulfonamide group on the phenyl ring. Both were conveniently synthesised, and synergistic effect of the modifications were demonstrated when introduced into oligonucleotides and hybridised to complementary RNA. The combination of stacking of the phenyltriazoles and the conformational steering from the 2'-OMe group gave thermally very stable duplexes. Exon skipping in the distrophin transcript using 20-mer 2'-OMePS sequences with two phenyltriazoles introduced in different positions with and without the sulfonamide demonstrated efficient exon skipping but at the same level as the 2'-OMePS reference ASO.

Antisense Oligonucleotide-Mediated Splice Switching: Potential Therapeutic Approach for Cancer Mitigation

Splicing is an essential process wherein precursor messenger RNA (pre-mRNA) is reshaped into mature mRNA. In alternative splicing, exons of any pre-mRNA get rearranged to form mRNA variants and subsequently protein isoforms, which are distinct both by structure and function. On the other hand, aberrant splicing is the cause of many disorders, including cancer. In the past few decades, developments in the understanding of the underlying biological basis for cancer progression and therapeutic resistance have identified many oncogenes as well as carcinogenic splice variants of essential genes. These transcripts are involved in various cellular processes, such as apoptosis, cell signaling and proliferation. Strategies to inhibit these carcinogenic isoforms at the mRNA level are promising. Antisense oligonucleotides (AOs) have been developed to inhibit the production of alternatively spliced carcinogenic isoforms through splice modulation or mRNA degradation. AOs can also be used to induce splice switching, where the expression of an oncogenic protein can be inhibited by the induction of a premature stop codon. In general, AOs are modified chemically to increase their stability and binding affinity. One of the major concerns with AOs is efficient delivery. Strategies for the delivery of AOs are constantly being evolved to facilitate the entry of AOs into cells. In this review, the different chemical modifications employed and delivery strategies applied are discussed. In addition to that various AOs in clinical trials and their efficacy are discussed herein with a focus on six distinct studies that use AO-mediated exon skipping as a therapeutic strategy to combat cancer.

Antisense Oligonucleotide Modified with Disulfide Units Induces Efficient Exon Skipping in mdx Myotubes through Enhanced Membrane Permeability and Nucleus Internalization

We have found that antisense oligonucleotides and siRNA molecules modified with repeat structures of disulfide units can be directly introduced into the cytoplasm and exhibit a suppressive effect on gene expression. In this study, we analyzed the mechanism of cellular uptake of these membrane-permeable oligonucleotides (MPONs). Time-course analysis by confocal microscopy showed that the uptake of MPONs from the plasma membrane to the cytoplasm reached 50 % of the total uptake in about 5 min. In addition, analysis of the plasma membrane proteins to which MPONs bind, identified several proteins, including voltage-dependent anion channel. Next, we analyzed the behavior of MPONs in the cell and found them to be abundant in the nucleus as early as 24 h after addition with the amount increasing further after 48 and 72 h. The amount of MPONs was 2.5-fold higher than that of unmodified oligonucleotides in the nucleus after 72 h. We also designed antisense oligonucleotides and evaluated the effect of MPONs on mRNA exon skipping using DMD model cells; MPONs caused exon skipping with 69 % efficiency after 72 h, which was three times higher than the rate of the control. In summary, the high capacity for intracytoplasmic and nuclear translocation of MPONs is expected to be useful for therapeutic strategies targeting exon skipping.

Design and In Vitro Evaluation of Splice-Switching Oligonucleotides Bearing Locked Nucleic Acids, Amido-Bridged Nucleic Acids, and Guanidine-Bridged Nucleic Acids

Our group previously developed a series of bridged nucleic acids (BNAs), including locked nucleic acids (LNAs), amido-bridged nucleic acids (AmNAs), and guanidine-bridged nucleic acids (GuNAs), to impart specific characteristics to oligonucleotides such as high-affinity binding and enhanced enzymatic resistance. In this study, we designed a series of LNA-, AmNA-, and GuNA-modified splice-switching oligonucleotides (SSOs) with different lengths and content modifications. We measured the melting temperature (T_m) of each designed SSO to investigate its binding affinity for RNA strands. We also investigated whether the single-stranded SSOs formed secondary structures using UV melting analysis without complementary RNA. As a result, the AmNA-modified SSOs showed almost the same T_m values as the LNA-modified SSOs, with decreased secondary structure formation in the former. In contrast, the GuNA-modified SSOs showed slightly lower T_m values than the LNA-modified SSOs, with no inhibition of secondary structures. We also evaluated the exon skipping activities of the BNAs in vitro at both the mRNA and protein expression levels. We found that both AmNA-modified SSOs and GuNA-modified SSOs showed higher exon skipping activities than LNA-modified SSOs but each class must be appropriately designed in terms of length and modification content.

Nonenzymatic polymerase-like template-directed synthesis of acyclic L-threoninol nucleic acid

Evolution of xeno nucleic acid (XNA) world essentially requires template-directed synthesis of XNA polymers. In this study, we demonstrate template-directed synthesis of an acyclic XNA, acyclic L-threoninol nucleic acid (L-aTNA), via chemical ligation mediated by N-cyanoimidazole. The ligation of an L-aTNA fragment on an L-aTNA template is significantly faster and occurs in considerably higher yield than DNA ligation. Both L-aTNA ligation on a DNA template and DNA ligation on an L-aTNA template are also observed. High efficiency ligation of trimer L-aTNA fragments to a template-bound primer is achieved. Furthermore, a pseudo primer extension reaction is demonstrated using a pool of random L-aTNA trimers as substrates. To the best of our knowledge, this is the first example of polymerase-like primer extension of XNA with all four nucleobases, generating phosphodiester bonding without any special modification. This technique paves the way for a genetic system of the L-aTNA world.

Evaluation of DNA segments in 2′-modified RNA sequences in designing efficient splice switching antisense oligonucleotides

Synthetic antisense oligonucleotides (ASOs) have emerged as one of the most promising therapeutic approaches. So far, nine ASO drugs have received approval for clinical use, and four of them are based on splice-switching principles demonstrating the impact of ASO-mediated splice modulation. Notably, three among them (Exondys 51, Vyondys 53 and Viltepso) are based on phosphorodiamidate morpholino (PMO) chemistry whereas Spinraza is based on 2′-O-methoxyethyl phosphorothioate (2′-MOE PS) chemistry. Although systemic delivery of PMOs has displayed a good safety profile even at high doses, the 2′-O-methyl phosphorothioate modified (2′-OMe PS) ASO drug candidate (drisapersen) failed due to safety issues. The potency of 2′-modified RNA for splice-switching needs to be further improved by novel design strategies for broad applicability. Towards this goal, in this study, we evaluated the potential of incorporating DNA segments at appropriate sites in 2′-OMe PS and 2′-MOE PS ASOs to induce exon skipping. For this purpose, a four-nucleotide DNA segment was systematically incorporated into a 20-mer 2′-OMe PS and 2′-MOE PS ASO designed to skip exon 23 in mdx mouse myotubes in vitro. Our results demonstrated that 2′-modified RNA PS ASOs containing four or less PS DNA nucleotides at the 3′-end yielded improved exon 23 skipping efficacy in line with fully modified ASO controls. Based on these results, we firmly believe that the present study opens new avenues towards designing splice modulating ASOs with limited chemical modifications for enhanced safety and therapeutic efficacy.

We evaluated the potential of 2′-modified RNA antisense oligonucleotides (ASOs) incorporated with DNA segments to induce exon skipping. Results demonstrated that ASOs with 4 or less DNA nucleotides at the 3′-end induce more efficient exon skipping compared with the control.

Intrastrand backbone-nucleobase interactions stabilize unwound right-handed helical structures of heteroduplexes of L-aTNA/RNA and SNA/RNA

Xeno nucleic acids, which are synthetic analogues of natural nucleic acids, have potential for use in nucleic acid drugs and as orthogonal genetic biopolymers and prebiotic precursors. Although few acyclic nucleic acids can stably bind to RNA and DNA, serinol nucleic acid (SNA) and L-threoninol nucleic acid (L-aTNA) stably bind to them. Here we disclose crystal structures of RNA hybridizing with SNA and with L-aTNA. The heteroduplexes show unwound right-handed helical structures. Unlike canonical A-type duplexes, the base pairs in the heteroduplexes align perpendicularly to the helical axes, and consequently helical pitches are large. The unwound helical structures originate from interactions between nucleobases and neighbouring backbones of L-aTNA and SNA through CH-O bonds. In addition, SNA and L-aTNA form a triplex structure via C:G*G parallel Hoogsteen interactions with RNA. The unique structural features of the RNA-recognizing mode of L-aTNA and SNA should prove useful in nanotechnology, biotechnology, and basic research into prebiotic chemistry.

Antisense Oligonucleotides as Potential Therapeutics for Type 2 Diabetes

Type 2 diabetes (T2D) is a chronic metabolic disorder characterized by persistent hyperglycemia resulting from inefficient signaling and insufficient production of insulin. Conventional management of T2D has largely relied on small molecule-based oral hypoglycemic medicines, which do not halt the progression of the disease due to limited efficacy and induce adverse effects as well. To this end, antisense oligonucleotide has attracted immense attention in developing antidiabetic agents because of their ability to downregulate the expression of disease-causing genes at the RNA and protein level. To date, seven antisense agents have been approved by the United States Food and Drug Administration for therapies of a variety of human maladies, including genetic disorders. Herein, we provide a comprehensive review of antisense molecules developed for suppressing the causative genes believed to be responsible for insulin resistance and hyperglycemia toward preventing and treating T2D.

Novel Disulfide-Bridged Bioresponsive Antisense Oligonucleotide Induces Efficient Splice Modulation in Muscle Myotubes in Vitro

Splice-modulating antisense therapy has shown tremendous potential in therapeutic development in recent years with four FDA-approved antisense drugs since 2016. However, an efficient and nontoxic antisense oligonucleotide (AO) delivery system still remains as a major obstacle in nucleic acid therapeutics field. Vitamin-E (α-tocopherol) is an essential dietary requirement for human body. This fat-soluble compound is one of the most important antioxidants which involves in numerous biological pathways. In this study, for the first time, we explored the scope of using α-tocopherol-conjugated bioresponsive AOs to induce splice modulation in mouse muscle myotubes in vitro. Our results showed that the bioresponsive construct efficiently internalized into the cell nucleus and induced exon 23 skipping in mdx mouse myotubes. Based on our exciting new results, we firmly believe that our findings could potentially benefit toward establishing a delivery approach to advance the field of splice-modulating AO therapy.

Development of Novel Chemically-Modified Nucleic Acid Molecules for Efficient Inhibition of Human MAPT Gene Expression

The hyperphosphorylation of the microtubule-associated protein tau (MAPT) has been implicated in various neurological diseases, including Alzheimer’s disease. It has been hypothesized that the reduction of MAPT would result in depolymerizing neurofibrillary tangles and could be a potential strategy for the treatment of Alzheimer’s disease and other tauopathies. In this study, we report the development of novel DNAzymes and splice-modulating antisense oligonucleotides (AOs) for the efficient inhibition of MAPT. We designed and synthesized a range of DNAzymes and 2ʹ-O-methyl (2’-OMe)-modified AOs on a phosphorothioate (PS) backbone targeting various exons across the MAPT gene transcript. Our results demonstrated that RNV563, an arm-loop-arm-type DNAzyme targeting exon 13, and an AO candidate AO4, targeting exon 4, efficiently downregulated MAPT RNA expression by 58% and 96%, respectively. In addition, AO4 also reduced the MAPT protein level by 74%. In line with our results, we believe that AO4 could be used as a potential therapeutic molecule for Alzheimer’s disease and other tauopathies.

Alpha-l-Locked Nucleic Acid-Modified Antisense Oligonucleotides Induce Efficient Splice Modulation In Vitro

Alpha-l-Locked nucleic acid (α-l-LNA) is a stereoisomeric analogue of locked nucleic acid (LNA), which possesses excellent biophysical properties and also exhibits high target binding affinity to complementary oligonucleotide sequences and resistance to nuclease degradations. Therefore, α-l-LNA nucleotides could be utilised to develop stable antisense oligonucleotides (AO), which can be truncated without compromising the integrity and efficacy of the AO. In this study, we explored the potential of α-l-LNA nucleotides-modified antisense oligonucleotides to modulate splicing by inducing Dmd exon-23 skipping in mdx mouse myoblasts in vitro. For this purpose, we have synthesised and systematically evaluated the efficacy of α-l-LNA-modified 2'-O-methyl phosphorothioate (2'-OMePS) AOs of three different sizes including 20mer, 18mer and 16mer AOs in parallel to fully-modified 2'-OMePS control AOs. Our results demonstrated that the 18mer and 16mer truncated AO variants showed slightly better exon-skipping efficacy when compared with the fully-23 modified 2'-OMePS control AOs, in addition to showing low cytotoxicity. As there was no previous report on using α-l-LNA-modified AOs in splice modulation, we firmly believe that this initial study could be beneficial to further explore and expand the scope of α-l-LNA-modified AO therapeutic molecules.

A Quencher-Free Linear Probe from Serinol Nucleic Acid with a Fluorescent Uracil Analogue

With the goal of developing a quencher-free probe composed of an artificial nucleic acid, the fluorescent nucleobase analogue 5-(perylenylethynyl)uracil (^Pe U), which was incorporated into totally artificial serinol nucleic acid (SNA) as a substitute for thymine, has been synthesized. In the context of a 12-mer duplex with RNA, these fluorophores reduce duplex stability slightly compared with that of an SNA without ^Pe U modification; thus suggesting that structural distortion is not induced by the modification. If two ^Pe Us were incorporated at separate positions in an SNA, the fluorescent emission at λ≈490 nm was clearly enhanced upon hybridization with complementary RNA. A quencher-free SNA linear probe containing three ^Pe Us, each separated by six nucleobases, has been designed. Detection of target RNA with high sensitivity and discrimination of a single-base mismatch has also been demonstrated.

BACE1 Inhibition Using 2'-OMePS Steric Blocking Antisense Oligonucleotides

Amyloid beta-peptide is produced by the cleavage of amyloid precursor protein by two secretases, a β-secretase, beta-site amyloid precursor protein cleaving enzyme 1 (BACE1) and a γ-secretase. It has been hypothesised that partial inhibition of BACE1 in individuals with a high risk of developing Alzheimer’s disease may be beneficial in preventing cognitive decline. In this study, we report the development of a novel antisense oligonucleotide (AO) that could efficiently downregulate the BACE1 transcript and partially inhibit BACE1 protein. We designed and synthesised a range of 2’-OMethyl-modified antisense oligonucleotides with a phosphorothioate backbone across various exons of the BACE1 transcript, of which AO2, targeting exon 2, efficiently downregulated BACE1 RNA expression by 90%. The sequence of AO2 was later synthesised with a phosphorodiamidate morpholino chemistry, which was found to be not as efficient at downregulating BACE1 expression as the 2’-OMethyl antisense oligonucleotides with a phosphorothioate backbone variant. AO2 also reduced BACE1 protein levels by 45%. In line with our results, we firmly believe that AO2 could be used as a potential preventative therapeutic strategy for Alzheimer’s disease.

Enhancement of exon skipping activity by reduction in the secondary structure content of LNA-based splice-switching oligonucleotides

PAGE and UV melting analysis revealed that longer LNA-based splice-switching oligonucleotides (SSOs) formed secondary structures by themselves, reducing their effective concentration. To avoid such secondary structure formation, we introduced 7-deaza-2'-deoxyguanosine or 2'-deoxyinosine into the SSOs. These modified SSOs, with fewer secondary structures, showed higher exon skipping activities.

Systematic evaluation of 2'-Fluoro modified chimeric antisense oligonucleotide-mediated exon skipping in vitro

Antisense oligonucleotide (AO)-mediated splice modulation has been established as a therapeutic approach for tackling genetic diseases. Recently, Exondys51, a drug that aims to correct splicing defects in the dystrophin gene was approved by the US Food and Drug Administration (FDA) for the treatment of Duchenne muscular dystrophy (DMD). However, Exondys51 has relied on phosphorodiamidate morpholino oligomer (PMO) chemistry which poses challenges in the cost of production and compatibility with conventional oligonucleotide synthesis procedures. One approach to overcome this problem is to construct the AO with alternative nucleic acid chemistries using solid-phase oligonucleotide synthesis via standard phosphoramidite chemistry. 2′-Fluoro (2′-F) is a potent RNA analogue that possesses high RNA binding affinity and resistance to nuclease degradation with good safety profile, and an approved drug Macugen containing 2′-F-modified pyrimidines was approved for the treatment of age-related macular degeneration (AMD). In the present study, we investigated the scope of 2′-F nucleotides to construct mixmer and gapmer exon skipping AOs with either 2′-O-methyl (2′-OMe) or locked nucleic acid (LNA) nucleotides on a phosphorothioate (PS) backbone, and evaluated their efficacy in inducing exon-skipping in mdx mouse myotubes in vitro. Our results showed that all AOs containing 2′-F nucleotides induced efficient exon-23 skipping, with LNA/2′-F chimeras achieving better efficiency than the AOs without LNA modification. In addition, LNA/2′-F chimeric AOs demonstrated higher exonuclease stability and lower cytotoxicity than the 2′-OMe/2′-F chimeras. Overall, our findings certainly expand the scope of constructing 2′-F modified AOs in splice modulation by incorporating 2′-OMe and LNA modifications.

Optimization of 2',4'-BNA/LNA-Based Oligonucleotides for Splicing Modulation In Vitro

Antisense oligonucleotide-mediated splicing modulation is an attractive strategy for treating genetic disorders. In 2016, two splice-switching oligonucleotides (SSOs) were approved by the FDA. To date, various types of novel artificial nucleic acids have been developed, and their potential for splicing modulations has been demonstrated. To apply these novel chemistries to SSOs, it is necessary to determine the appropriate design for each artificial nucleic acid such as the length of the SSO and number of modifications. In this protocol, we focus on SSOs modified with 2'-O,4'-methylene-bridged nucleic acid (2',4'-BNA)/locked nucleic acid (LNA), which is an artificial nucleic acid that shows extremely high binding affinity to target RNA strands. We describe our typical protocol for the optimization of 2',4'-BNA-based SSOs.

Systematic Screening of Commonly Used Commercial Transfection Reagents towards Efficient Transfection of Single-Stranded Oligonucleotides

Non-viral vector-mediated transfection is a core technique for in vitro screening of oligonucleotides. Despite the growing interests in the development of oliogonucleotide-based drug molecules in recent years, a comprehensive comparison of the transfection efficacy of commonly used commercial transfection reagents has not been reported. In this study, five commonly used transfection reagents, including Lipofectamine 3000, Lipofectamine 2000, Fugene, RNAiMAX and Lipofectin, were comprehensively analyzed in ten cell lines using a fluorescence imaging-based transfection assay. Although the transfection efficacy and toxicity of transfection reagents varied depending on cell types, the toxicity of transfection reagents generally displayed a positive correlation with their transfection efficacy. According to our results, Lipofectamine 3000, Fugene and RNAiMAX showed high transfection efficacy, however, RNAiMAX may be a better option for majority of cells when lower toxicity is desired. The transfection efficacy of Lipofectamine 2000 was compromised by its high toxicity, which may adversely affect its application in most cells. We firmly believe that our findings may contribute to the future In vitro delivery and screening of single-stranded therapeutic oligonucleotides such as antisense oligonucleotides, antimiRs, and DNAzymes.

Synthesis, Characterization, and Function of an RNA-Based Transfection Reagent

A synthetic 8-mer, amphipathic, trans-acting poly-2'-O-methyluridylic thiophosphate triester RNA element (2'-OMeUtaPS) can be prepared using solid-phase synthesis protocols. 2'-OMeUtaPS efficiently mediates the delivery of uncharged polyA-tailed phosphorodiamidate morpholino (PMO) sequences in HeLa pLuc 705 cells, as evidenced by flow cytometry measurements. In this cell line, 2'-OMeUtaPS-mediated transfection of an antisense polyA-tailed PMO sequence induces alternative splicing of an aberrant luciferase pre-mRNA splice site, leading to restoration of functional luciferase, as quantitatively measured using a typical luciferase assay. 2'-OMeUtaPS is also potent at delivering an uncharged antisense polyA-tailed PMO sequence in muscle cells of the mdx mouse model of muscular dystrophy; targeting the polyA-tailed PMO sequence against a splice site of the pre-mRNA encoding mutated dystrophin triggers an alternate splicing event that results in excision of the mutated exon (exon 23) from the pre-mRNA and production of functional dystrophin, as demonstrated by agarose gel electrophoresis. © 2018 by John Wiley & Sons, Inc.

An amphipathic trans-acting phosphorothioate RNA element delivers an uncharged phosphorodiamidate morpholino sequence in mdx mouse myotubes

An efficient method for the delivery of uncharged polyA-tailed phosphorodiamidate morpholino sequences (PMO) in mammalian cells consists of employing a synthetic 8-mer amphipathic trans-acting poly-2'-O-methyluridylic thiophosphate triester element (2'-OMeUtaPS) as a transfection reagent. Unlike the dTtaPS DNA-based element, this RNA element is potent at delivering polyA-tailed PMO sequences to HeLa pLuc 705 cells or to myotube muscle cells. However, much like dTtaPS, the 2'-OMeUtaPS-mediated internalization of PMO sequences occurs through an energy-dependent mechanism; macropinocytosis appears to be the predominant endocytic pathway used for cellular uptake. The transfected PMO sequences induce alternate splicing of either the pre-mRNA encoding luciferase in HeLa pLuc 705 cells or the excision of exon 23 from the pre-mRNA encoding dystrophin in myotube muscle cells of the mdx mouse model of muscular dystrophy with an efficiency comparable to that of commercial cationic lipid reagents but without detrimental cytotoxicity.

Nucleobase-modified antisense oligonucleotides containing 5-(phenyltriazol)-2′-deoxyuridine nucleotides induce exon-skipping in vitro

We investigated the potential of nucleobase-modified antisense oligonucleotides to induce exon-skipping, and found that 5-(phenyltriazol)-2′-deoxyuridine-modified antisense oligonucleotides induced efficient exon-skipping in vitro.