Chimeric RNA and 2′-O, 4′-C-Ethylene-Bridged Nucleic Acids Have Stronger Activity Than Phosphorothioate Oligodeoxynucleotides in Induction of Exon 19 Skipping in Dystrophin mRNA

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.

The future of exon skipping for Duchenne muscular dystrophy

Antisense oligonucleotide (ASO)-mediated exon skipping can restore the open reading frame of dystrophin transcripts for Duchenne muscular dystrophy (DMD) patients. This allows production of internally deleted dystrophin proteins as found in the later onset, less severely progressive Becker muscular dystrophy. Currently, four ASOs that induce exon skipping and dystrophin restoration are approved for the treatment of DMD by the Food and Drug Administration (FDAthe regulatory agencies of the USA and Japan). However, approval was based on restoration of very small amounts of dystrophin and the approved ASOs apply to only a subset of patients. This expert perspective evaluates ways to improve ASO efficiency that are currently in or close to clinical trials, as well as ways to improve applicability of this mutation-specific approach.

Evaluating the Knockdown Activity of MALAT1 ENA Gapmers In Vitro

Antisense oligonucleotides (ASOs) are widely used for the identification of gene functions and regulation of genes involved in different diseases for therapeutic purposes. For in vitro evaluation of the knockdown activity of gapmer ASOs, we often use lipofection or electroporation to deliver gapmer ASOs into the cells. Here, we describe a method for evaluating the knockdown activity of gapmer ASOs by a cell-free uptake mechanism, termed as gymnosis, using MALAT1 gapmer ASOs modified with 2'-O-methoxyethyl RNA (2'-MOE) or 2'-O,4'-C-ethylene-bridged nucleic acid (ENA). This method is robust because it does not involve the use of any transfection reagent and has minimal effects on cell growth. Further, we describe a convenient technique for performing one-step reverse transcription and real-time qPCR using cell lysates without RNA extraction. Data for up to 96 samples can be obtained following these methods.

An improved synthesis of 2'-O,4'-C-ethylene nucleic acid (ENA) and thermodynamic studies of duplex formation containing the guanosine ENA unit

Oligonucleotides containing 2'-O,4'-C-ethylene nucleic acids (ENA) have been proven highly effective for antisense therapeutics. 2'-O,4'-C-Ethyleneguanosine and its phosphoramidite were previously obtained from 3,5-di-O-benzy1-4-C-(p-tolulenesulfonyloxyethyl)-1,2-di-O-acetyl-α-D-erythropentofuranose by glycosylation, but with limited efficiency. Using 3,5-di-O-benzy1-4-C-(2-t-butyldiphenylsilyloxyethyl)-1,2-di-O-acetyl-α-D-erythropentofuranose as an alternative substrate, we developed several methods to obtain 2'-O,4'-C-ethyleneguanosine derivatives with much higher yields than previously reported. These methods were also applicable for the synthesis of 2'-O,4'-C-ethyleneadenosine and 2'-O,4'-C-ethylene-5-methyluridine derivatives. Moreover, we investigated the thermodynamic benefit of DNA strands containing 2'-O,4'-C-ethyleneguanosines during duplex formation with complementary RNA. Only a single modification by the nucleoside resulted in a 10-fold greater binding constant of the DNA/RNA duplex.

Synthesis of ENA Nucleotides and ENA Oligonucleotides

2'-O,4'-C-Ethylene-bridged nucleic acid (ENA) is a sugar-modified oligonucleotide with an ethylene bridge between the 2'-oxygen and 4'-carbon of ribose. ENA not only has as high binding affinity to complementary RNA as conventional bridged/locked nucleic acid, but also has much higher nuclease resistance in plasma, which makes it a promising candidate for antisense therapeutics. This unit presents detailed protocols for the synthesis and characterization of ENA nucleosides and oligonucleotides. © 2018 by John Wiley & Sons, Inc.

2'-O-Methyl RNA/Ethylene-Bridged Nucleic Acid Chimera Antisense Oligonucleotides to Induce Dystrophin Exon 45 Skipping

Duchenne muscular dystrophy (DMD) is a fatal muscle-wasting disease characterized by dystrophin deficiency from mutations in the dystrophin gene. Antisense oligonucleotide (AO)-mediated exon skipping targets restoration of the dystrophin reading frame to allow production of an internally deleted dystrophin protein with functional benefit for DMD patients who have out-of-frame deletions. After accelerated US approval of eteplirsen (Exondys 51), which targets dystrophin exon 51 for skipping, efforts are now focused on targeting other exons. For improved clinical benefits, this strategy requires more studies of the delivery method and modification of nucleic acids. We studied a nucleotide with a 2′-O,4′-C-ethylene-bridged nucleic acid (ENA), which shows high nuclease resistance and high affinity for complementary RNA strands. Here, we describe the process of developing a 2′-O-methyl RNA(2′-OMeRNA)/ENA chimera AO to induce dystrophin exon 45 skipping. One 18-mer 2′-OMeRNA/ENA chimera (AO85) had the most potent activity for inducing exon 45 skipping in cultured myotubes. AO85 was administered to mdx mice without significant side effects. AO85 transfection into cultured myotubes from 13 DMD patients induced exon 45 skipping in all samples at different levels and dystrophin expression in 11 patients. These results suggest the possible efficacy of AO-mediated exon skipping changes in individual patients and highlight the 2′-OMeRNA/ENA chimera AO as a potential fundamental treatment for DMD.

Enhancement of exon skipping in mdx52 mice by 2′-O-methyl-2-thioribothymidine incorporation into phosphorothioate oligonucleotides

Incorporation of 2′-O-methyl-2-thioribothymidine (s²T_m) into antisense oligoribonucleotides significantly enhanced the exon skipping activity in Duchenne muscular dystrophy model mice.

Overview of alternative oligonucleotide chemistries for exon skipping

The chemistry of the oligonucleotide backbone is crucial to obtaining high activity in vivo in exon skipping applications. Apart from the ability to bind strongly and sequence-specifically to pre-mRNA targets, the type of backbone also influences cell delivery, in vivo pharmacology, bio-distribution, toxicology, and ultimately the therapeutic use in humans. Reviewed here are classes of oligonucleotide commonly used for exon skipping applications, namely negatively charged backbones typified by RNA analogues having 2'-O-substitution and a phosphorothioate linkage and charge-neutral backbones such as PNA and PMO. Also discussed are peptide conjugates of PNA and PMO that enhance cellular and in vivo delivery and their potential for drug development. Finally, the prospects for development of other analogue types in exon skipping applications are outlined.

Optimizing RNA/ENA chimeric antisense oligonucleotides using in vitro splicing

A molecular therapy for Duchenne muscular dystrophy (DMD) that converts dystrophin mRNA from out-of-frame to in-frame transcripts by inducing exon skipping with antisense oligonucleotides (AOs) is now approaching clinical application. To exploit the broad therapeutic applicability of exon skipping therapy, it is necessary to identify AOs that are able to induce efficient and specific exon skipping. To optimize AOs, we have established an in vitro splicing system using cultured DMD myocytes. Here, we describe the process of identifying the best AO.Cultured DMD myocytes are established from a biopsy sample and the target exon is chosen. A series of AOs are designed to cover the whole target exon sequence. As AOs, we use 15-20-mer chimeric oligonucleotides consisting of 2'-O-methyl RNA and modified nucleic acid (2'-O, 4'-C-ethylene-bridged nucleic acid). Each AO is transfected individually into cultured DMD myocytes, and the resulting mRNA is analyzed by reverse transcription-PCR. The ability of each AO to induce exon skipping is examined by comparing the amount of cDNA with and without exon skipping. If necessary, having roughly localized the target region, another set of AOs are designed and the exon skipping abilities of the new AOs are examined. Finally, one AO is determined as the best for the molecular therapy.Our simple and reliable methods using an in vitro splicing system have enabled us to identify optimized AOs against many exons of the DMD gene.

Antisense oligonucleotide induced dystrophin exon 45 skipping at a low half-maximal effective concentration in a cell-free splicing system

Antisense oligonucleotides (AOs) can facilitate the expression of internally deleted dystrophin in dystrophin-deficient Duchenne muscular dystrophy (DMD) by correcting the reading frame of the pre-mRNA with AO-mediated exon skipping. An antisense 18-mer 2'-O-methyl RNA/ethylene-bridged nucleic acid chimera AO targeting exon 45 of the dystrophin gene, AO85, can induce exon 45 skipping efficiently in cultured cells. AO85 is expected to facilitate dystrophin expression in 8%-9% of all DMD patients. Here, we examined the kinetics of AO85-mediated exon 45 skipping in a cell-free splicing system. In vitro transcribed pre-mRNAs containing dystrophin exon 45 and part of its flanking introns within a hybrid minigene were incubated with HeLa cell nuclear extract, and the resultant mRNAs were amplified by semiquantitative reverse transcriptase-polymerase chain reaction. Time-course analysis revealed that the splicing process fitted well to first order kinetics. Addition of AO85 produced an extra spliced product, deleting exon 45 (Δexon 45), indicating AO85-mediated exon 45 skipping. Production of Δexon 45 increased linearly with increasing concentrations of AO85, reaching a maximum of nearly 80% of the transcripts. The half-maximal effective concentration (EC(50)) of AO85 was 58.0 nM. The percentage of Δexon 45 among the transcripts decreased inversely with the pre-mRNA concentration; Lineweaver-Burk plotting revealed a competitive fashion of AO85 action. The low EC(50) indicates high potential of AO85 for clinical application.

Antisense applications for biological control

Although Nature's antisense approaches are clearly impressive, this Perspectives article focuses on the experimental uses of antisense reagents (ASRs) for control of biological processes. ASRs comprise antisense oligonucleotides (ASOs), and their catalytically active counterparts ribozymes and DNAzymes, as well as small interfering RNAs (siRNAs). ASOs and ribozymes/DNAzymes target RNA molecules on the basis of Watson-Crick base pairing in sequence-specific manner. ASOs generally result in destruction of the target RNA by RNase-H mediated mechanisms, although they may also sterically block translation, also resulting in loss of protein production. Ribozymes and DNAzymes cleave target RNAs after base pairing via their antisense flanking arms. siRNAs, which contain both sense and antisense regions from a target RNA, can mediate target RNA destruction via RNAi and the RISC, although they can also function at the transcriptional level. A considerable number of ASRs (mostly ASOs) have progressed into clinical trials, although most have relatively long histories in Phase I/II settings. Clinical trial results are surprisingly difficult to find, although few ASRs appear to have yet established efficacy in Phase III levels. Evolution of ASRs has included: (a) Modifications to ASOs to render them nuclease resistant, with analogous modifications to siRNAs being developed; and (b) Development of strategies to select optimal sites for targeting. Perhaps the biggest barrier to effective therapies with ASRs is the "Delivery Problem." Various liposomal vehicles have been used for systemic delivery with some success, and recent modifications appear to enhance systemic delivery, at least to liver. Various nanoparticle formulations are now being developed which may also enhance delivery. Going forward, topical applications of ASRs would seem to have the best chances for success. In summary, modifications to ASRs to enhance stability, improve targeting, and incremental improvements in delivery vehicles continue to make ASRs attractive as molecular therapeutics, but their advance toward the bedside has been agonizingly slow.

Comparison between properties of 2'-O,4'-C-ethylene-bridged nucleic acid (ENA) phosphorothioate oligonucleotides and N3'-P5' thiophosphoramidate oligonucleotides

Synthesis and properties of an oligonucleotide uniformly modified with 2'-O,4-C-ethylene-bridged nucleic acid (ENA) units were compared with those of GRN163, which is modified with N3'-P5' thiophosphoramidates, with the sequence targeting human telomerase RNA subunit. Although an ENA phosphorothioate oligonucleotide, ENA-13, could be synthesized using ENA phosphoramidites on a 100-mg scale, synthesis of GRN163 was very hard even on a 1-micomol scale. In view of both stability of the duplex formation with complementary RNA and the efficiency of cellular uptake by endocytosis, ENA-13 was superior to GRN163. These findings suggest that ENA-13 has useful properties for antisense therapeutic application.

Chemical and Structural Implications of 1‘,2‘- versus 2‘,4‘- Conformational Constraints in the Sugar Moiety of Modified Thymine Nucleosides

In order to understand how the chemical nature of the conformational constraint of the sugar moiety in ON/RNA(DNA) dictates the duplex structure and reactivity, we have determined molecular structures and dynamics of the conformationally constrained 1',2'-azetidine- and 1',2'-oxetane-fused thymidines, as well as their 2',4'-fused thymine (T) counterparts such as LNA-T, 2'-amino LNA-T, ENA-T, and aza-ENA-T by NMR, ab initio (HF/6-31G** and B3LYP/6-31++G**), and molecular dynamics simulations (2 ns in the explicit aqueous medium). It has been found that, depending upon whether the modification leads to a bicyclic 1',2'-fused or a tricyclic 2',4'-fused system, they fall into two distinct categories characterized by their respective internal dynamics of the glycosidic and the backbone torsions as well as by characteristic North-East type sugar conformation (P = 37 degrees +/- 27 degrees , phi(m) = 25 degrees +/- 18 degrees ) of the 1',2'-fused systems, and (ii) pure North type (P = 19 degrees +/- 8 degrees , phi(m) = 48 degrees +/- 4 degrees ) for the 2',4'-fused nucleosides. Each group has different conformational hyperspace accessible, despite the overall similarity of the North-type conformational constraints imposed by the 1',2'- or 2',4'-linked modification. The comparison of pK(a)s of the 1-thyminyl aglycon as well as that of endocyclic sugar-nitrogen obtained by theoretical and experimental measurements showed that the nature of the sugar conformational constraints steer the physicochemical property (pK(a)) of the constituent 1-thyminyl moiety, which in turn can play a part in tuning the strength of hydrogen bonding in the basepairing.

Conformationally constrained 2'-N,4'-C-ethylene-bridged thymidine (aza-ENA-T): synthesis, structure, physical, and biochemical studies of aza-ENA-T-modified oligonucleotides

The 2'-deoxy-2'-N,4'-C-ethylene-bridged thymidine (aza-ENA-T) has been synthesized using a key cyclization step involving 2'-ara-trifluoromethylsufonyl-4'-cyanomethylene 11 to give a pair of 3',5'-bis-OBn-protected diastereomerically pure aza-ENA-Ts (12a and 12b) with the fused piperidino skeleton in the chair conformation, whereas the pentofuranosyl moiety is locked in the North-type conformation (7 degrees < P < 27 degrees, 44 degrees < phi m < 52 degrees). The origin of the chirality of two diastereomerically pure aza-ENA-Ts was found to be due to the endocyclic chiral 2'-nitrogen, which has axial N-H in 12b and equatorial N-H in 12a. The latter is thermodynamically preferred, while the former is kinetically preferred with Ea = 25.4 kcal mol-1, which is thus far the highest observed inversion barrier at pyramidal N-H in the bicyclic amines. The 5'-O-DMTr-aza-ENA-T-3'-phosphoramidite was employed for solid-phase synthesis to give four different singly modified 15-mer antisense oligonucleotides (AONs). Their AON/RNA duplexes showed a Tm increase of 2.5-4 degrees C per modification, depending upon the modification site in the AON. The relative rates of the RNase H1 cleavage of the aza-ENA-T-modified AON/RNA heteroduplexes were very comparable to that of the native counterpart, but the RNA cleavage sites of the modified AON/RNA were found to be very different. The aza-ENA-T modifications also made the AONs very resistant to 3' degradation (stable over 48 h) in the blood serum compared to the unmodified AON (fully degraded in 4 h). Thus, the aza-ENA-T modification in the AON fulfilled three important antisense criteria, compared to the native: (i) improved RNA target affinity, (ii) comparable RNase H cleavage rate, and (iii) higher blood serum stability.

Inhibition of VEGF mRNA by 2'-O,4'-C-ethylene-bridged nucleic acids (ENA) antisense oligonucleotides and their influence on off-target gene expressions

We investigated 2'-O,4'-C-ethylene-bridged nucleic acids (ENA) antisense oligonucleotides (AONs) for vascular endothelial growth factor (VEGF) in human lung carcinoma A549 cells. An ENA/DNA gapmer AON with RNase H-mediated activity was virtually stable in rat plasma and exhibited more than 90% inhibition of VEGF mRNA production. Moreover, 22 genes that are likely to bind to the AON were found in the GenBank database by BLAST and CLUSTAL W searches. Three of these genes were actually inhibited by the ENA AON. In shorter ENA AONs with fewer matched sequences of these genes, inhibitiory activities were decreased and off-target effects were improved. These results indicate that ENA AONs act in a sequence-specific manner and could be used as effective antisense drugs.

Gene therapy progress and prospects: Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a severe muscle wasting disorder affecting 1/3500 male births. There is currently no effective treatment, but gene therapy approaches are offering viable avenues for treatment development. The last 10 years have seen the development of a number of strategies and tools for muscle gene therapy. However, the major hurdle has been the inability to deliver vectors at high enough efficiency via a systemic route. The last 2-3 years (reviewed here) have seen unrivalled progress in efficient systemic delivery of viral and non-viral gene transfer agents and antisense oligonucleotides. This progress, coupled with the successful completion of the first gene therapy clinical trial for DMD, has led to three more clinical trials planned for the immediate future.

Redirecting Splicing to Address Dystrophin Mutations: Molecular By-pass Surgery

Mutations in the dystrophin gene that prevent synthesis of a functional protein lead to Duchenne muscular dystrophy (DMD), the most common serious childhood muscular dystrophy. The major isoform is produced in skeletal muscle and the size of the dystrophin gene and complexity of expression have posed great challenges to the development of a therapy for DMD. Considerable progress has been made in the areas of gene and cell replacement, yet it appears that any potential therapy for DMD is still some years away. Other approaches are being considered, and one that has generated substantial interest over the last few years is induced exon skipping. Antisense oligonucleotides have been used to block abnormal splice sites and force pre-mRNA processing back to the normal patterns. This approach is re-interpreted to address the more common dystrophin mutations, where normal splice sites are targeted to induce abnormal splicing, resulting in specific exon exclusion. Selected exon removal during processing of the dystrophin pre-mRNA can by-pass nonsense mutations or restore a disrupted reading frame arising from genomic deletions or duplications. Attributes of the dystrophin gene that have hampered gene replacement therapy may be regarded as positive features for induced exon skipping, which may be regarded as a form of by-pass surgery at the molecular level. In humans, antisense oligonucleotides have been more generally applied to down-regulate specific gene expression, for the treatment of acquired conditions such as malignancies and viral infections. From interesting in vitro experiments several years ago, the dystrophin exon-skipping field has progressed to the stage of planning for clinical trials.

Dystrophin expression in the mdx mouse after localised and systemic administration of a morpholino antisense oligonucleotide

BACKGROUND

Duchenne and Becker muscular dystrophies are allelic disorders arising from mutations in the dystrophin gene. Duchenne muscular dystrophy is characterised by an absence of functional protein, while Becker muscular dystrophy is usually caused by in-frame deletions allowing synthesis of some functional protein. Treatment options are limited, and we are investigating the potential of transcript manipulation to overcome disease-causing mutations. Antisense oligonucleotides have been used to induce specific exon removal during processing of the dystrophin primary transcript and thereby by-pass protein-truncating mutations. The antisense oligonucleotide chemistry most widely used to alter pre-mRNA processing is 2'-O-methyl-modified bases on a phosphorothioate backbone.

METHODS

The present studies evaluate 2'-O-methylphosphorothioate, peptide nucleic acid and morpholino antisense oligonucleotides in the mdx mouse model of muscular dystrophy, which has a nonsense mutation in exon 23 of the dystrophin gene.

RESULTS

We demonstrate dystrophin expression in mdx mouse tissues after localised and systemic delivery of a morpholino antisense oligonucleotide designed to target the dystrophin exon 23 donor splice site.

CONCLUSIONS

The stability of the morpholino structural type, and the fact that it can be delivered to muscle in the absence of a delivery reagent, render this compound eminently suitable for consideration for therapeutic exon skipping to address dystrophin mutations.

Intraperitoneal administration of phosphorothioate antisense oligodeoxynucleotide against splicing enhancer sequence induced exon skipping in dystrophin mRNA expressed in mdx skeletal muscle

Antisense oligodeoxynucleotide against the splicing enhancer sequence (SES) in exon 19 of the dystrophin gene have been shown to induce exon 19 skipping and promote the expression of internally deleted dystrophin by correcting the translational reading frame in the cultured Duchenne muscular dystrophy (DMD) myocytes with the deletion of exon 20. Transfection of the antisense oligodeoxynucleotide, therefore, has been proposed as a promising means for therapeutic modification of dystrophin mRNA of DMD, a fatal disorder caused by defects in the dystrophin gene. A systemic delivery method targeting the large number of diseased muscles remains to be established for clinical application of antisense oligodeoxynucleotide. In this study, we investigated capability of oligodeoxynucleotide transfer into the skeletal muscles of mdx mouse, a mouse model of DMD. Thirty-one mer phosphorothioate oligodeoxynucleotide complementary to the SES of dystrophin exon 19 was intraperitoneally administered to mdx mice without any carrier. Histochemical study disclosed that fluorescence-labeled oligodeoxynucleotide appeared in the nuclei of femoral skeletal muscle cell at the second day after injection of 20 mg/kg BW oligodeoxynucleotide, and still visible at 14th day. Reverse transcription (RT)-PCR analysis of dystrophin transcript in these cells disclosed that a proportion of it showed skipping of exon 19 from second to seventh day after injection. These results showed that the intraperitoneally administered oligodeoxynucleotide could be transfected to nucleus of mdx skeletal muscle without any carrier and was able to induce exon skipping in vivo.

A G-to-A transition at the fifth position of intron-32 of the dystrophin gene inactivates a splice-donor site both in vivo and in vitro

The splicing pattern of pre-mRNA is unpredictable in genes harboring a single-nucleotide change within the consensus sequence of a splice-donor site. In the dystrophin gene, a transition from G to A at the fifth position of intron-32 (4518+5G > A) has been reported as a polymorphism within the consensus sequence or a mutation identified in Duchenne muscular dystrophy (DMD). Here, we report both in vivo and in vitro evidence that shows inactivation of the splice-donor site caused by this mutation. In one Japanese DMD case, two novel dystrophin mRNAs were identified in the patient's lymphocytes, one with a 98 bp deletion of the 3' end of exon-32 (dys32-98) and the other with a 28 bp intron retained between exons 32 and 33 (dys32 + 28). Genomic sequencing disclosed a single-nucleotide change from G to A at the fifth position of intron-32 (4518+5G > A). To demonstrate in vitro the inactivation of this splice-donor site by this nucleotide change, mini-dystrophin genes comprising three exons harboring either normal or mutant intron-32 sequences were expressed in HeLa cells, and the splicing products were analyzed by reverse-transcription PCR amplification. A normal transcript consisting of three exons was obtained from the normal construct. From the mutant, we obtained one product containing a 98 bp deletion at the 3' end of exon-32, indicating complete inactivation of the native splice-donor site. Thus, both in vivo and in vitro experiments demonstrate that 4518+5G > A causes a splicing error leading to transcript termination; it did not behave like a silent polymorphism. Our results indicate that the in vitro splicing system is a powerful tool for determining the underlying mechanism of a disease-causing mutation in a splicing consensus sequence.

Chimeric RNA/ethylene-bridged nucleic acids promote dystrophin expression in myocytes of duchenne muscular dystrophy by inducing skipping of the nonsense mutation-encoding exon

Editing of dystrophin mRNA by induction of exon skipping, using antisense oligonucleotides, has been proposed as one way to generate dystrophin expression in Duchenne muscular dystrophy (DMD) patients. Here, antisense chimeric oligonucleotides consisting of RNA and a new modified nucleic acid are tested for activity to induce skipping of an exon containing a nonsense mutation. In a Japanese DMD case, a nonsense mutation (R1967X) due to a single nucleotide change in exon 41 of the dystrophin gene (C5899T) was identified. Oligonucleotides consisting of 2'-O-methyl RNA and a new 2'-O,4'-C-ethylene-bridged nucleic acid (ENA) were designed to bind the mutation site of exon 41, and their ability to induce exon 41 skipping in dystrophin mRNA was evaluated. Finally, among the specific oligonucleotides tested, an 18-mer RNA/ENA chimera was found to have the strongest activity, inducing exon 41 skipping in nearly 90% of dystrophin mRNA. Accordingly, nearly 90% of cultured myocytes were shown to be dystrophin positive by immunohistochemical analysis. Western blot analysis disclosed the presence of nearly normal-sized dystrophin up to 1 week after the transfection. Our results suggest that an RNA/ENA chimera can be used to express dystrophin in DMD.