Repair of thalassemic human beta-globin mRNA in mammalian cells by antisense oligonucleotides

Anti-tumor activity of splice-switching oligonucleotides

Alternative splicing has emerged as an important target for molecular therapies. Splice-switching oligonucleotides (SSOs) modulate alternative splicing by hybridizing to pre-mRNA sequences involved in splicing and blocking access to the transcript by splicing factors. Recently, the efficacy of SSOs has been established in various animal disease models; however, the application of SSOs against cancer targets has been hindered by poor in vivo delivery of antisense therapeutics to tumor cells. The apoptotic regulator Bcl-x is alternatively spliced to express anti-apoptotic Bcl-x_L and pro-apoptotic Bcl-x_S. Bcl-x_L is upregulated in many cancers and is associated with chemoresistance, distinguishing it as an important target for cancer therapy. We previously showed that redirection of Bcl-x pre-mRNA splicing from Bcl-x_L to -x_S induced apoptosis in breast and prostate cancer cells. In this study, the effect of SSO-induced Bcl-x splice-switching on metastatic melanoma was assessed in cell culture and B16F10 tumor xenografts. SSOs were delivered in vivo using lipid nanoparticles. Administration of nanoparticle with Bcl-x SSO resulted in modification of Bcl-x pre-mRNA splicing in lung metastases and reduced tumor load, while nanoparticle alone or formulated with a control SSO had no effect. Our findings demonstrate in vivo anti-tumor activity of SSOs that modulate Bcl-x pre-mRNA splicing.

Exonic sequences provide better targets for antisense oligonucleotides than splice site sequences in the modulation of Duchenne muscular dystrophy splicing

Antisense-mediated exon skipping is currently the most promising therapeutic approach for Duchenne muscular dystrophy (DMD). The rationale is to use antisense oligonucleotides (AONs) to hide exons from the splicing machinery, causing them to be skipped from the mature mRNA. Thus, the mutated, out-of-frame dystrophin transcripts as seen in DMD are reframed, allowing the generation of internally deleted, partly functional dystrophin proteins, rather than prematurely truncated, nonfunctional ones. This approach is mutation specific, so multiple AONs targeting all internal DMD exons have been designed and tested. Here, we have retrospectively compared our own set of 156 exon-internal AONs and 256 AONs as present in patents and publications from Dr. Wilton (Australia), which includes exon-internal as well as splice site-targeting AONs. Effective AONs are significantly more often exon-internal and, as anticipated, have better thermodynamic properties. Comparison of splice site and exon-internal AONs revealed that exon-internal AONs are more efficient and target more predicted exonic splicing enhancer and less predicted exon splicing silencer sites, but also have better thermodynamic properties. This suggests that exons may be better AON targets than introns per se, because of their higher GC content, which generally will result in improved AON binding.

Repair of aberrant splicing in growth hormone receptor by antisense oligonucleotides targeting the splice sites of a pseudoexon

CONTEXT

The GH receptor (GHR) pseudoexon 6Psi defect is a frequent cause of GH insensitivity (GHI) resulting from a non-functioning GH receptor (GHR). It results in a broad range of phenotypes and may also be present in patients diagnosed as idiopathic short stature.

OBJECTIVE

Our objective was to correct aberrant GHR splicing and inclusion of 6Psi using exon-skipping antisense oligonucleotides (ASOs).

DESIGN AND SETTING

Three ASOs binding the 5' (ASO-5), 3' (ASO-3), and branch site (ASO-Br) of 6Psi were tested in an in vitro splicing assay and a cell transfection system. The wild-type (wt) and mutant (mt) DNA minigenes (wt- and mtL1-GHR6Psi-L2, respectively) were created by inserting the GHR 6Psi in a well-characterized splice reporter (Adml-par). For the in vitro splicing assay, the wt- and mtL1-GHR6Psi-L2 were transcribed into pre-mRNA in the presence of [alpha(32)P]GTP and incubated with ASOs in HeLa nuclear extracts. For the cell transfection studies, wt- and mtL1-GHR6Psi-L2 cloned into pcDNA 3.1 were transfected with ASOs into HEK293 cells. After 48 h, RNA was extracted and radiolabeled RT-PCR products quantified.

RESULTS

ASO-3 induced an almost complete pseudoexon skipping in vitro and in HEK293 cells. This effect was dose dependent and maximal at 125-250 nm. ASO-5 produced modest pseudoexon skipping, whereas ASO-Br had no effect. Targeting of two splice elements simultaneously was less effective than targeting one. ASO-Br was tested on the wtL1-GHR6Psi-L2 and did not act as an enhancer of 6Psi inclusion.

CONCLUSIONS

The exon-skipping ASO approach was effective in correcting aberrant GHR splicing and may be a promising therapeutic tool.

RNA targeting therapeutics: molecular mechanisms of antisense oligonucleotides as a therapeutic platform

Dramatic advances in understanding of the roles RNA plays in normal health and disease have greatly expanded over the past 10 years and have made it clear that scientists are only beginning to comprehend the biology of RNAs. It is likely that RNA will become an increasingly important target for therapeutic intervention; therefore, it is important to develop strategies for therapeutically modulating RNA function. Antisense oligonucleotides are perhaps the most direct therapeutic strategy to approach RNA. Antisense oligonucleotides are designed to bind to the target RNA by well-characterized Watson-Crick base pairing, and once bound to the target RNA, modulate its function through a variety of postbinding events. This review focuses on the molecular mechanisms by which antisense oligonucleotides can be designed to modulate RNA function in mammalian cells and how synthetic oligonucleotides behave in the body.

Alternative splicing: role of pseudoexons in human disease and potential therapeutic strategies

What makes a nucleotide sequence an exon (or an intron) is a question that still lacks a satisfactory answer. Indeed, most eukaryotic genes are full of sequences that look like perfect exons, but which are nonetheless ignored by the splicing machinery (hence the name 'pseudoexons'). The existence of these pseudoexons has been known since the earliest days of splicing research, but until recently the tendency has been to view them as an interesting, but rather rare, curiosity. In recent years, however, the importance of pseudoexons in regulating splicing processes has been steadily revalued. Even more importantly, clinically oriented screening studies that search for splicing mutations are beginning to uncover a situation where aberrant pseudoexon inclusion as a cause of human disease is more frequent than previously thought. Here we aim to provide a review of the mechanisms that lead to pseudoexon activation in human genes and how the various cis- and trans-acting cellular factors regulate their inclusion. Moreover, we list the potential therapeutic approaches that are being tested with the aim of inhibiting their inclusion in the final mRNA molecules.

MBNL1 binds GC motifs embedded in pyrimidines to regulate alternative splicing

Muscleblind-like 1 (MBNL1) regulates alternative splicing and is a key player in the disease mechanism of myotonic dystrophy (DM). In DM, MBNL1 becomes sequestered to expanded CUG/CCUG repeat RNAs resulting in splicing defects, which lead to disease symptoms. In order to understand MBNL1’s role in both the disease mechanism of DM and alternative splicing regulation, we sought to identify its RNA-binding motif. A doped SELEX was performed on a known MBNL1-binding site. After five rounds of SELEX, MBNL1 selected pyrimidine-rich RNAs containing YGCY motifs. Insertion of multiple YGCY motifs into a normally MBNL1-independent splicing reporter was sufficient to promote regulation by MBNL1. MBNL1 was also shown to regulate the splicing of exon 22 in the ATP2A1 pre-mRNA, an exon mis-spliced in DM, via YGCY motifs. A search for YGCY motifs in 24 pre-mRNA transcripts that are mis-spliced in DM1 patients revealed an interesting pattern relative to the regulated exon. The intronic regions upstream of exons that are excluded in normal tissues relative to DM1, are enriched in YGCY motifs. Meanwhile, the intronic regions downstream of exons that are included in normal tissues relative to DM1, are enriched in YGCY motifs.

Future alternative therapies for β-thalassemia

β-thalassemia is an inherited disorder due to mutations found in the β-globin gene, leading to anemia and requiring sporadic or chronic blood transfusions for survival. Without proper chelation, β-thalassemia results in iron overload. Ineffective erythropoiesis can lead to iron overload even in untransfused patients who are affected by β-thalassemia intermedia. Better understanding of the molecular biologic aspects of this disorder has led to improvements in population screening and prenatal diagnosis, which, in turn, have led to dramatic reductions in the number of children born with β-thalassemia major in the Mediterranean littoral. However, as a consequence of decreases in neonatal and childhood mortality in other geographical areas, β-thalassemia has become a worldwide clinical problem. A number of unsolved pathophysiological issues remain, such as ineffective erythropoieis, abnormal iron absorption, oxidative stress, splenomegaly and thrombosis. In the last few years, novel studies have the potential to introduce new therapeutic approaches that might reduce these problems and limit the need for blood transfusion.

Gene therapy in thalassemia and hemoglobinopathies

Sickle cell disease (SCD) and ß-thalassemia represent the most common hemoglobinopathies caused, respectively, by the alteration of structural features or deficient production of the ß-chain of the Hb molecule. Other hemoglobinopathies are characterized by different mutations in the α- or ß-globin genes and are associated with anemia and might require periodic or chronic blood transfusions. Therefore, ß-thalassemia, SCD and other hemoglobinopathies are excellent candidates for genetic approaches since they are monogenic disorders and, potentially, could be cured by introducing or correcting a single gene into the hematopoietic compartment or a single stem cell. Initial attempts at gene transfer of these hemoglobinopathies have proved unsuccessful due to limitations of available gene transfer vectors. With the advent of lentiviral vectors many of the initial limitations have been overcame. New approaches have also focused on targeting the specific mutation in the ß-globin genes, correcting the DNA sequence or manipulating the fate of RNA translation and splicing to restore ß-globin chain synthesis. These techniques have the potential to correct the defect into hematopoietic stem cells or be utilized to modify stem cells generated from patients affected by these disorders. This review discusses gene therapy strategies for the hemoglobinopathies, including the use of lentiviral vectors, generation of induced pluripotent stem cells (iPS) cells, gene targeting, splice-switching and stop codon readthrough.

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.

Antisense therapeutics for neurofibromatosis type 1 caused by deep intronic mutations

Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder affecting 1:3,500 individuals. Disease expression is highly variable and complications are diverse. However, currently there is no specific treatment for the disease. NF1 is caused by mutations in the NF1 gene, approximately 2.1% of constitutional mutations identified in our population are deep intronic mutations producing the insertion of a cryptic exon into the mature mRNA. We used antisense morpholino oligomers (AMOs) to restore normal splicing in primary fibroblast and lymphocyte cell lines derived from six NF1 patients bearing three deep intronic mutations in the NF1 gene (c.288+2025T>G, c.5749+332A>G, and c.7908-321C>G). AMOs were designed to target the newly created 5' splice sites to prevent the incorporation of cryptic exons. Our results demonstrate that AMO treatment effectively restored normal NF1 splicing at the mRNA level for the three mutations studied in the different cell lines analyzed. We also found that AMOs had a rapid effect that lasted for several days, acting in a sequence-specific manner and interfering with the splicing mechanism. Finally, to test whether the correction of aberrant NF1 splicing also restored neurofibromin function to wild-type levels, we measured the amount of Ras-GTP after AMO treatment in primary fibroblasts. The results clearly show an AMO-dependent decrease in Ras-GTP levels, which is consistent with the restoration of neurofibromin function. To our knowledge this is the first time that an antisense technique has been used successfully to correct NF1 mutations opening the possibility of a therapeutic strategy for this type of mutation not only for NF1 but for other genetic disorders.

Targeted correction of a thalassemia-associated beta-globin mutation induced by pseudo-complementary peptide nucleic acids

β-Thalassemia is a genetic disorder caused by mutations in the β-globin gene. Triplex-forming oligonucleotides and triplex-forming peptide nucleic acids (PNAs) have been shown to stimulate recombination in mammalian cells via site-specific binding and creation of altered helical structures that provoke DNA repair. However, the use of these molecules for gene targeting requires homopurine tracts to facilitate triple helix formation. Alternatively, to achieve binding to mixed-sequence target sites for the induced gene correction, we have used pseudo-complementary PNAs (pcPNAs). Due to steric hindrance, pcPNAs are unable to form pcPNA–pcPNA duplexes but can bind to complementary DNA sequences via double duplex-invasion complexes. We demonstrate here that pcPNAs, when co-transfected with donor DNA fragments, can promote single base pair modification at the start of the second intron of the beta-globin gene. This was detected by the restoration of proper splicing of transcripts produced from a green fluorescent protein-beta globin fusion gene. We also demonstrate that pcPNAs are effective in stimulating recombination in human fibroblast cells in a manner dependent on the nucleotide excision repair factor, XPA. These results suggest that pcPNAs can be effective tools to induce heritable, site-specific modification of disease-related genes in human cells without purine sequence restriction.

Vivo-Morpholinos: a non-peptide transporter delivers Morpholinos into a wide array of mouse tissues

We have developed a new transporter structure that provides effective delivery of Morpholino antisense oligomers into a wide variety of tissues in living mice. This transporter comprises a dendritic structure assembled around a triazine core which serves to position eight guanidinium head groups in a conformation effective to penetrate cell membranes. This transporter structure is conjugated to a Morpholino oligomer to form a delivery-enabled product referred to as a Vivo-Morpholino. Vivo-Morpholinos are shown to effectively enter and function within cultured cells in the presence of 100% serum using a rigorous positive test system based on correction of a defined splicing error in a pre-messenger RNA. In addition, Vivo-Morpholinos are demonstrated to enter into a wide variety of tissues in a similar positive test system in transgenic mice, as evidenced by correction of the targeted splicing error in all tissues assessed, including near-complete splice correction in the small intestine, colon, stomach, liver kidney, and a number of muscles. Finally, Vivo-Morpholinos, which target the exon-skipping of exon 23 harboring a premature termination codon in the mdx mouse model, effectively restore the reading frame of dystrophin and restore expression of a functional dystrophin protein.

Therapeutic potential of splice-switching oligonucleotides

Alternative splicing enables a single pre-messenger RNA transcript to yield multiple protein isoforms, making it a major contributor to the diversity of the proteome. While this process is essential for normal development, aberrations in alternative splicing are the cause of a multitude of human diseases. Methods for manipulating alternative splicing would thus be of therapeutic value. Chemically modified antisense oligonucleotides that alter alternative splicing by directing splice site selection have been developed to achieve this end. These splice-switching oligonucleotides (SSOs) have been applied to correct aberrant splicing, induce expression of a therapeutic splice variant, or induce expression of a novel therapeutic splice variant in a number of disease-relevant genes. Recently, in vivo efficacy of SSOs has been reported using animal disease models, as well as in results from the first clinical trial.

Modification of HER2 pre-mRNA alternative splicing and its effects on breast cancer cells

The oncogene HER2 is overexpressed in a variety of human tumors, providing a target for anti-cancer molecular therapies. Here, we employed a 2'-O-methoxyethyl (MOE) splice switching oligonucleotide, SSO111, to induce skipping of exon 15 in HER2 pre-mRNA, leading to significant downregulation of full-length HER2 mRNA, and simultaneous upregulation of Delta15HER2 mRNA. SSO111 treatment of SK-BR-3 cells, which highly overexpress HER2, led to inhibition of cell proliferation and induction of apoptosis. The novel Delta15HER2 mRNA encodes a soluble, secreted form of the receptor. Treating SK-BR-3 cells with exogenous Delta15HER2 protein reduced membrane-bound HER2 and decreased HER3 transphosphorylation. Delta15HER2 protein thus has similar activity to an autoinhibitory, natural splice variant of HER2, Herstatin, and to the breast cancer drug Herceptin. Both SSO111 and Delta15HER2 may be potential candidates for the development of novel HER2-targeted cancer therapeutics.

RNA repair restores hemoglobin expression in IVS2-654 thalassemic mice

Repair of beta-globin pre-mRNA rendered defective by a thalassemia-causing splicing mutation, IVS2-654, in intron 2 of the human beta-globin gene was accomplished in vivo in a mouse model of IVS2-654 thalassemia. This was effected by a systemically delivered splice-switching oligonucleotide (SSO), a morpholino oligomer conjugated to an arginine-rich peptide. The SSO blocked the aberrant splice site in the targeted pre-mRNA and forced the splicing machinery to reselect existing correct splice sites. Repaired beta-globin mRNA restored significant amounts of hemoglobin in the peripheral blood of the IVS2-654 mouse, improving the number and quality of erythroid cells.

Advances in antisense oligonucleotide development for target identification, validation, and as novel therapeutics

Antisense oligonucleotides (As-ODNs) are single stranded, synthetically prepared strands of deoxynucleotide sequences, usually 18–21 nucleotides in length, complementary to the mRNA sequence of the target gene. As-ODNs are able to selectively bind cognate mRNA sequences by sequence-specific hybridization. This results in cleavage or disablement of the mRNA and, thus, inhibits the expression of the target gene. The specificity of the As approach is based on the probability that, in the human genome, any sequence longer than a minimal number of nucleotides (nt), 13 for RNA and 17 for DNA, normally occurs only once. The potential applications of As-ODNs are numerous because mRNA is ubiquitous and is more accessible to manipulation than DNA. With the publication of the human genome sequence, it has become theoretically possible to inhibit mRNA of almost any gene by As-ODNs, in order to get a better understanding of gene function, investigate its role in disease pathology and to study novel therapeutic targets for the diseases caused by dysregulated gene expression. The conceptual simplicity, the availability of gene sequence information from the human genome, the inexpensive availability of synthetic oligonucleotides and the possibility of rational drug design makes As-ODNs powerful tools for target identification, validation and therapeutic intervention. In this review we discuss the latest developments in antisense oligonucleotide design, delivery, pharmacokinetics and potential side effects, as well as its uses in target identification and validation, and finally focus on the current developments of antisense oligonucleotides in therapeutic intervention in various diseases.

Sustained dystrophin expression induced by peptide-conjugated morpholino oligomers in the muscles of mdx mice

Cell-penetrating peptides (CPPs), containing arginine (R), 6-aminohexanoic acid (X), and/or beta-alanine (B) conjugated to phosphorodiamidate morpholino oligomers (PMOs), enhance their delivery in cell culture. In this study, the potency, functional biodistribution, and toxicity of these conjugates were evaluated in vivo, in EGFP-654 transgenic mice that ubiquitously express the aberrantly spliced EGFP-654 pre-mRNA reporter. Correct splicing and enhanced green fluorescence protein (EGFP) upregulation serve as a positive readout for peptide-PMO (PPMO) entry into cells and access to EGFP-654 pre-mRNA in the nucleus. Intraperitoneal injections of a series of PPMOs, A-N (12 mg/kg), administered once a day for four successive days resulted in splicing correction in numerous tissues. PPMO-B was highly potent in the heart, diaphragm, and quadriceps, which are key muscles in the treatment of Duchenne muscular dystrophy. We therefore investigated PPMO M23D-B, designed to force skipping of stop-codon containing dystrophin exon 23, in an mdx mouse model of the disease. Systemic delivery of M23D-B yielded persistent exon 23 skipping, yielding high and sustained dystrophin protein expression in body-wide muscles, including cardiac muscle, without detectable toxicity. The rescued dystrophin reduced serum creatinine kinase to near-wild-type levels, indicating improvement in muscle integrity. This is the first report of oligonucleotide-mediated exon skipping and dystrophin protein induction in the heart of treated animals.

Correction of a splice-site mutation in the beta-globin gene stimulated by triplex-forming peptide nucleic acids

Splice-site mutations in the beta-globin gene can lead to aberrant transcripts and decreased functional beta-globin, causing beta-thalassemia. Triplex-forming DNA oligonucleotides (TFOs) and peptide nucleic acids (PNAs) have been shown to stimulate recombination in reporter gene loci in mammalian cells via site-specific binding and creation of altered helical structures that provoke DNA repair. We have designed a series of triplex-forming PNAs that can specifically bind to sequences in the human beta-globin gene. We demonstrate here that these PNAs, when cotransfected with recombinatory donor DNA fragments, can promote single base-pair modification at the start of the second intron of the beta-globin gene, the site of a common thalassemia-associated mutation. This single base pair change was detected by the restoration of proper splicing of transcripts produced from a green fluorescent protein-beta-globin fusion gene. The ability of these PNAs to induce recombination was dependent on dose, sequence, cell-cycle stage, and the presence of a homologous donor DNA molecule. Enhanced recombination, with frequencies up to 0.4%, was observed with use of the lysomotropic agent chloroquine. Finally, we demonstrate that these PNAs were effective in stimulating the modification of the endogenous beta-globin locus in human cells, including primary hematopoietic progenitor cells. This work suggests that PNAs can be effective tools to induce heritable, site-specific modification of disease-related genes in human cells.

Doxycycline-controlled splicing modulation by regulated antisense U7 snRNA expression cassettes

Many diseases affect pre-mRNA splicing, and alternative splicing is a major source of proteome diversity and an important mechanism for modulating gene expression. The ability to regulate a specific splicing event is therefore desirable; for example, to understand splicing-associated pathologies. We have developed methods based on modified U7 snRNAs, which allow us to induce efficient skipping or inclusion of selected exons. Here, we have adapted these U7 tools to a regulatable system that relies on a doxycycline-sensitive version of the Krüppel-associated box (KRAB)/KAP1 transcriptional silencing. Co-transduction of target cells with two lentiviral vectors, one carrying the KRAB protein and the other the regulatable U7 cassette, allows a tight regulation of the modified U7 snRNA. No leakage is observed in the repressed state, whereas full induction can be obtained with doxycycline in ng ml(-1) concentrations. Only a few days are necessary for a full switch, and the induction/repression can be repeated over several cycles without noticeable loss of control. Importantly, the U7 expression correlates with splicing correction, as shown for the skipping of an aberrant beta-globin exon created by a thalassaemic mutation and the promotion of exon 7 inclusion in the human SMN2 gene, an important therapeutic target for spinal muscular atrophy.

Antisense masking of an hnRNP A1/A2 intronic splicing silencer corrects SMN2 splicing in transgenic mice

Survival of motor neuron 2, centromeric (SMN2) is a gene that modifies the severity of spinal muscular atrophy (SMA), a motor-neuron disease that is the leading genetic cause of infant mortality. Increasing inclusion of SMN2 exon 7, which is predominantly skipped, holds promise to treat or possibly cure SMA; one practical strategy is the disruption of splicing silencers that impair exon 7 recognition. By using an antisense oligonucleotide (ASO)-tiling method, we systematically screened the proximal intronic regions flanking exon 7 and identified two intronic splicing silencers (ISSs): one in intron 6 and a recently described one in intron 7. We analyzed the intron 7 ISS by mutagenesis, coupled with splicing assays, RNA-affinity chromatography, and protein overexpression, and found two tandem hnRNP A1/A2 motifs within the ISS that are responsible for its inhibitory character. Mutations in these two motifs, or ASOs that block them, promote very efficient exon 7 inclusion. We screened 31 ASOs in this region and selected two optimal ones to test in human SMN2 transgenic mice. Both ASOs strongly increased hSMN2 exon 7 inclusion in the liver and kidney of the transgenic animals. Our results show that the high-resolution ASO-tiling approach can identify cis-elements that modulate splicing positively or negatively. Most importantly, our results highlight the therapeutic potential of some of these ASOs in the context of SMA.

Therapeutic applications of aptamers

Aptamers constitute a new class of oligonucleotides that have gained therapeutic importance. With the approval of the first aptamer drug, pegaptanib, interest in this class of oligonucleotides, often referred to as 'chemical antibodies', has increased. This article discusses aptamers in relation to other oligonucleotide molecules such as antisense nucleotides, short inhibitory sequences, ribozymes and so on. The development of pegaptanib is looked at from the point of view of the challenges faced in converting aptamers into therapeutic molecules. Cases of other aptamers, which show promise as drugs, are discussed in slightly greater detail. Comparison with antibodies and small molecules, which have hitherto held monopoly in this area, is also made.

Comparative analysis of antisense oligonucleotide sequences for targeted skipping of exon 51 during dystrophin pre-mRNA splicing in human muscle

Duchenne muscular dystrophy (DMD) is caused by mutations in the dystrophin gene that result in the absence of functional protein. In the majority of cases these are out-of-frame deletions that disrupt the reading frame. Several attempts have been made to restore the dystrophin mRNA reading frame by modulation of pre-mRNA splicing with antisense oligonucleotides (AOs), demonstrating success in cultured cells, muscle explants, and animal models. We are preparing for a phase I/IIa clinical trial aimed at assessing the safety and effect of locally administered AOs designed to inhibit inclusion of exon 51 into the mature mRNA by the splicing machinery, a process known as exon skipping. Here, we describe a series of systematic experiments to validate the sequence and chemistry of the exon 51 AO reagent selected to go forward into the clinical trial planned in the United Kingdom. Eight specific AO sequences targeting exon 51 were tested in two different chemical forms and in three different preclinical models: cultured human muscle cells and explants (wild type and DMD), and local in vivo administration in transgenic mice harboring the entire human DMD locus. Data have been validated independently in the different model systems used, and the studies describe a rational collaborative path for the preclinical selection of AOs for evaluation in future clinical trials.

Enhancement of SMN2 exon 7 inclusion by antisense oligonucleotides targeting the exon

Several strategies have been pursued to increase the extent of exon 7 inclusion during splicing of SMN2 (survival of motor neuron 2) transcripts, for eventual therapeutic use in spinal muscular atrophy (SMA), a genetic neuromuscular disease. Antisense oligonucleotides (ASOs) that target an exon or its flanking splice sites usually promote exon skipping. Here we systematically tested a large number of ASOs with a 2'-O-methoxy-ethyl ribose (MOE) backbone that hybridize to different positions of SMN2 exon 7, and identified several that promote greater exon inclusion, others that promote exon skipping, and still others with complex effects on the accumulation of the two alternatively spliced products. This approach provides positional information about presumptive exonic elements or secondary structures with positive or negative effects on exon inclusion. The ASOs are effective not only in cell-free splicing assays, but also when transfected into cultured cells, where they affect splicing of endogenous SMN transcripts. The ASOs that promote exon 7 inclusion increase full-length SMN protein levels, demonstrating that they do not interfere with mRNA export or translation, despite hybridizing to an exon. Some of the ASOs we identified are sufficiently active to proceed with experiments in SMA mouse models.

Antisense-mediated exon skipping: a versatile tool with therapeutic and research applications

Antisense-mediated modulation of splicing is one of the few fields where antisense oligonucleotides (AONs) have been able to live up to their expectations. In this approach, AONs are implemented to restore cryptic splicing, to change levels of alternatively spliced genes, or, in case of Duchenne muscular dystrophy (DMD), to skip an exon in order to restore a disrupted reading frame. The latter allows the generation of internally deleted, but largely functional, dystrophin proteins and would convert a severe DMD into a milder Becker muscular dystrophy phenotype. In fact, exon skipping is currently one of the most promising therapeutic tools for DMD, and a successful first-in-man trial has recently been completed. In this review the applicability of exon skipping for DMD and other diseases is described. For DMD AONs have been designed for numerous exons, which has given us insight into their mode of action, splicing in general, and splicing of the DMD gene in particular. In addition, retrospective analysis resulted in guidelines for AON design for DMD and most likely other genes as well. This knowledge allows us to optimize therapeutic exon skipping, but also opens up a range of other applications for the exon skipping approach.

U7 snRNA-mediated correction of aberrant splicing caused by activation of cryptic splice sites

A considerable fraction of mutations associated with hereditary disorders and cancers affect splicing. Some of them cause exon skipping or the inclusion of an additional exon, whereas others lead to the inclusion of intronic sequences or deletion of exonic sequences through the activation of cryptic splice sites. We focused on the latter cases and have designed a series of vectors that express modified U7 small nuclear RNAs (snRNAs) containing a sequence antisense to the cryptic splice site. Three cases of such mutation were investigated in this study. In two of them, which occurred in the PTCH1 and BRCA1 genes, canonical splice donor sites had been partially impaired by mutations that activated nearby intronic cryptic splice donor sites. Another mutation found in exonic region in CYP11A created a novel splice donor site. Transient expression of the engineered U7 snRNAs in HeLa cells restored correct splicing in a sequence-specific and dose-dependent manner in the former two cases. In contrast, the third case, in which the cryptic splice donor site in the exonic sequence was activated, the expression of modified U7 snRNA resulted in exon skipping. The correction of aberrant splicing by suppressing intronic cryptic splice sites with modified U7 is expected be a promising alternative to gene replacement therapy.

Achieving targeted and quantifiable alteration of mRNA splicing with Morpholino oligos

This work represents the first guide for using steric-block antisense oligos as tools for effective and targeted modification of RNA splicing. Comparison of several steric-block oligo types shows the properties of Morpholinos provide significant advantages over other potential splice-blocking oligos. The procedures and complications of designing effective splice-blocking Morpholino oligos are described. The design process requires complete pre-mRNA sequence for defining suitable targets, which usually generate specific predictable messengers. To validate the targeting procedure, the level and nature of transcript alteration is characterized by RT-PCR analysis of splice modification in a beta-globin splice model system. An oligo-walking study reveals that while U1 and U2 small nuclear RiboNucleoProtein (snRNP) binding sites are the most effective targets for blocking splicing, inclusion of these sites is not required to achieve effective splice modifications. The most effective targeting strategy employs simultaneously blocking snRNP binding sites and splice-junctions. The work presented here continues to be the basis for most of the successful Morpholino oligos designed for the worldwide research community to block RNA splicing.

Dendritic alpha,epsilon-poly(L-lysine)s as delivery agents for antisense oligonucleotides

PURPOSE

To evaluate the potential use of dendritic alpha,epsilon-poly(L-lysine)s (DPL) for the efficient cellular delivery of antisense oligonucleotides.

METHODS

A series of dendritic alpha,epsilon-polylysines of various generations were prepared. Their physical properties and the ability to form complex with oligonucleotide were investigated by polyacrylamide gel electrophoresis, capillary zone electrophoresis (CZE), agarose gel electrophoresis, fluorescence titration and atomic force microscopy (AFM). The efficiency to deliver oligonucleotide to HeLa cells, stably transfected with plasmid pLuc/705, was evaluated by using antisense splicing correction assay and confocal microscopy.

RESULTS

DPLs formed the complexes with antisense oligonucleotide with modest cytotoxicity. The charge ratio of oligonucleotide to DPL and the size (generation) of DPLs were all critical variables for the antisense effect. Compared to low generation DPLs, high generation DPLs were more effective in delivering oligonucleotide into cells.

CONCLUSIONS

High generation DPL-oligonucleotide complexes were moderately effective for delivery antisense oligonucleotide. The complex formation provides a promise for in vivo therapeutic application of DPLs or their derivatives in the delivery of gene or oligonucleotide.

Antisense oligonucleotide-induced alternative splicing of the APOB mRNA generates a novel isoform of APOB

BACKGROUND

Apolipoprotein B (APOB) is an integral part of the LDL, VLDL, IDL, Lp(a) and chylomicron lipoprotein particles. The APOB pre-mRNA consists of 29 constitutively-spliced exons. APOB exists as two natural isoforms: the full-length APOB100 isoform, assembled into LDL, VLDL, IDL and Lp(a) and secreted by the liver in humans; and the C-terminally truncated APOB48, assembled into chylomicrons and secreted by the intestine in humans. Down-regulation of APOB100 is a potential therapy to lower circulating LDL and cholesterol levels.

RESULTS

We investigated the ability of 2'O-methyl RNA antisense oligonucleotides (ASOs) to induce the skipping of exon 27 in endogenous APOB mRNA in HepG2 cells. These ASOs are directed towards the 5' and 3' splice-sites of exon 27, the branch-point sequence (BPS) of intron 26-27 and several predicted exonic splicing enhancers within exon 27. ASOs targeting either the 5' or 3' splice-site, in combination with the BPS, are the most effective. The splicing of other alternatively spliced genes are not influenced by these ASOs, suggesting that the effects seen are not due to non-specific changes in alternative splicing. The skip 27 mRNA is translated into a truncated isoform, APOB87SKIP27.

CONCLUSION

The induction of APOB87SKIP27 expression in vivo should lead to decreased LDL and cholesterol levels, by analogy to patients with hypobetalipoproteinemia. As intestinal APOB mRNA editing and APOB48 expression rely on sequences within exon 26, exon 27 skipping should not affect APOB48 expression unlike other methods of down-regulating APOB100 expression which also down-regulate APOB48.

Molecular therapies in beta-thalassaemia

The beta-thalassaemias have a major global impact on health and mortality. Allogeneic haemopoietic stem cell transplantation is the only approach that may lead to a cure but this approach is not available to most patients. The mainstay treatment for the majority remains life-long blood transfusion in combination with a rigorous regime of iron chelation. Improved understanding of the pathophysiology and molecular basis of the disease has provided clues for more effective strategies that aim to correct the defect in beta-globin chain synthesis at the primary level or redress the alpha/beta-globin chain imbalance at the secondary level. Improved understanding of the molecular basis of the disease complications, such as iron overloading, has also provided clues for potential molecular targets at the tertiary level.

The potential of oligonucleotides for therapeutic applications

Viral-derived particles have been widely used and described in gene therapy clinical trials. Although substantial results have been achieved, major safety issues have also arisen. For more than a decade, oligonucleotides have been seen as an alternative to gene complementation by viral vectors or DNA plasmids, either to correct the genetic defect or to silence gene expression. The development of RNA interference has strengthened the potential of this approach. Recent clinical trials have also tested the ability of aptamer molecules and decoy oligonucleotides to sequestrate pathogenic proteins. Here, we review the potential of oligonucleotides in gene therapy, outline what has already been accomplished, and consider what remains to be done.

Reflections on the European Union Eurythron Network Meeting "Molecular Control of Erythropoiesis," September 22-23, 2005, Istituto Superiore di Sanità, Rome, Italy

Red blood cells (RBCs) mediate oxygen transport throughout the body, a function that is essential for life. RBCs are continuously produced via a process called erythropoiesis. Anemias (insufficient numbers of functional RBCs), caused by failure of erythropoiesis, are a major cause of disease worldwide. Hereditary anemias constitute the most common human genetic disorders; they have no effective cure yet. The European research training network Eurythron follows a multidisciplinary approach to clarify the important molecular mechanisms in normal and pathological erythropoiesis, with a view to develop novel therapies to cure the anemias. The aim is to generate a comprehensive molecular description of mechanisms governing specification of hematopoietic stem cells in embryogenesis, lineage commitment, differentiation, and postmitotic maturation of RBCs. We report on the Eurythron meeting in Rome, in which novel approaches in stem cell and erythroid cell biology, including in vitro expansion of primary cells, biochemistry of receptor/signal transduction complexes and transcription factors, and (epi)genetics, were discussed.

Analysis of prostate-specific membrane antigen splice variants in LNCap cells

The prostate-specific membrane antigen (PSMA), a product of the folate hydrolase (FOLH1) gene, is highly expressed as a largely extracellular membrane-anchored protein in malignant prostate tissues and in nonprostatic tumor neovasculature. Treatment of prostate cancer LNCap cells with spliceswitching oligonucleotides (SSOs) modulated splicing of FOLH1 pre-mRNA from the full-length PSMA splice variant to three splice variants: the cytoplasmic PSM', alternatively spliced at exon 1, and the previously unexamined PSMADelta6 and PSMADelta18 variants, which lack exons 6 and 18, respectively. Application of SSOs decreased membrane PSMA levels and increased PSM', PSMADelta6, and PSMADelta18 transcripts. As a result, PSM' protein was translocated to the cytoplasm, and switching to PSMADelta6 and PSMADelta18 downregulated PSMA expression. NAALADase assays showed that PSM' retained enzymatic activity. PSMADelta6 and PSMADelta18 were not active, presumably due to a change in a reading frame that eliminated the NAALDase active site or the dimerization domain or both in these proteins.

Inhibition of HIV-1 replication by oligonucleotide analogues directed to the packaging signal and trans-activating response region

HIV possesses a remarkable capacity for mutational escape from therapeutics that target the viral proteins and enzymes. Inhibitory strategies aimed at highly conserved nucleic acid sequences within the genome are an attractive alternative. However, it has proven difficult to achieve an effective level of therapeutic at the appropriate site within the cell. Oligonucleotide delivery is a rapidly advancing field. We have investigated oligonucleotide analogues as steric-block therapeutics against two highly conserved regions of the HIV-1 genome. In the study we show that 2'0-methyl/locked nucleic acid oligonucleotides against the packaging signal and trans-activating response regions of HIV can inhibit replication of the virus.

Alternative splicing: therapeutic target and tool

Alternative splicing swells the coding capacity of the human genome, expanding the pharmacoproteome, the proteome that provides targets for therapy. Splicing, both constitutive and regulated forms, can itself be targeted by conventional and molecular therapies. This review focuses on splicing as a therapeutic target with a particular emphasis on molecular approaches. The review looks at the use of antisense oligonucleotides, which can be employed to promote skipping of constitutive exons, inhibit inappropriately activated exons, or stimulate exons weakened by mutations. Additionally this manuscript evaluates methods that reprogram RNAs using reactions that recombine RNA molecules in trans. Preliminary, but exciting, results in these areas of investigation suggest that these methods could eventually lead to treatments in heretofore intractable ailments.

As antisense RNA gets intronic

Recent work describing the transcriptional output of the human genome points to the existence of a significant number of non-coding RNA transcripts coming from intronic regions, with a fraction of these being oriented antisense relative to the protein-coding mRNA of the known gene. In this article, we survey the main findings of the large-scale expression analysis projects that led to the identification of antisense intronic messages and which demonstrate their ubiquitous expression in the human genome. We review the current knowledge on long, unspliced, intronic antisense transcripts, a new class of non-coding RNAs, recently described by our group to be correlated with the degree of tumor differentiation in prostate cancer, which we postulate is involved in the fine tuning of gene expression in eukaryotes. Possible mechanisms of antisense intronic transcript biogenesis and function in gene expression regulation are discussed, as is their involvement in human diseases. While there is still no conclusive evidence demonstrating a functional role for these long, intronic antisense messages, the far-reaching implications of their existence for the mechanisms regulating gene expression certainly warrant further experimentation.

Epigenetic manipulation of gene expression: a toolkit for cell biologists

Cell biologists have been afforded extraordinary new opportunities for experimentation by the emergence of powerful technologies that allow the selective manipulation of gene expression. Currently, RNA interference is very much in the limelight; however, significant progress has also been made with two other approaches. Thus, antisense oligonucleotide technology is undergoing a resurgence as a result of improvements in the chemistry of these molecules, whereas designed transcription factors offer a powerful and increasingly convenient strategy for either up- or down-regulation of targeted genes. This mini-review will highlight some of the key features of these three approaches to gene regulation, as well as provide pragmatic guidance concerning their use in cell biological experimentation based on our direct experience with each of these technologies. The approaches discussed here are being intensely pursued in terms of possible therapeutic applications. However, we will restrict our comments primarily to the cell culture situation, only briefly alluding to fundamental differences between utilization in animals versus cells.

Retargeting mobile group II introns to repair mutant genes

Retroposable elements such as retroviral and lentiviral vectors have been employed for many gene therapy applications. Unfortunately, such gene transfer vectors integrate genes into many different DNA sequences and unintended integration of the vector near a growth-promoting gene can engender pathological consequences. For example, retroviral vector-mediated gene transfer induced leukemia in 2 of 11 children treated for severe combined immunodeficiency, raising significant safety issues for gene transfer strategies that cannot be targeted to specific sequences. Here, we examine the use of a mobile retroposable genetic element that can be targeted to introduce therapeutic sequences site specifically into mutant genes. The data demonstrate that the mobile group II intron from Lactococcus lactis can be targeted to insert into and repair mutant lacZ (approved gene symbol GLB1) and beta-globin (approved gene symbol HBB) genes with high efficiency and fidelity in model systems in bacteria. These results suggest that these mobile genetic elements represent a novel class of agents for performing targeted genetic repair.

Towards a therapeutic inhibition of dystrophin exon 23 splicing in mdx mouse muscle induced by antisense oligoribonucleotides (splicomers): target sequence optimisation using oligonucleotide arrays

BACKGROUND

The activity of synthetic antisense oligonucleotides (splicomers) designed to block pre-mRNA splicing at specific exons has been demonstrated in a number of model systems, including constitutively spliced exons in mouse dystrophin RNA. Splicomer reagents directed to Duchenne muscular dystrophy (DMD) RNAs might thus circumvent nonsense or frame-shifting mutations, leading to therapeutic expression of partially functional dystrophin, as occurs in the milder, allelic (Becker) form of the disease (BMD).

METHODS

Functional and hybridisation array screens have been used to select optimised splicomers directed to exon 23 of dystrophin mRNA which carries a nonsense mutation in the mdx mouse. Splicomers were transfected into cultured primary muscle cells, and dystrophin mRNA assessed for exon exclusion. Splicomers were also administered to the muscles of mdx mice.

RESULTS

Oligonucleotide array analyses with dystrophin pre-mRNA probes revealed strong and highly specific hybridisation patterns spanning the exon 23/intron 23 boundary, indicating an open secondary structure conformation in this region of the RNA. Functional screening of splicomer arrays by direct analysis of exon 23 RNA splicing in mdx muscle cultures identified a subset of biologically active reagents which target sequence elements associated with the 5' splice site region of dystrophin intron 23; splicomer-mediated exclusion of exon 23 was specific and dose-responsive up to a level exceeding 50% of dystrophin mRNA, and Western blotting demonstrated de novo expression of dystrophin protein at 2-5% of wild-type levels. Direct intramuscular administration of optimised splicomer reagents in vivo resulted in the reappearance of sarcolemmal dystrophin immunoreactivity in > 30% of muscle fibres in the mdx mouse

CONCLUSIONS

These results suggest that correctly designed splicomers may have direct therapeutic value in vivo, not only for DMD, but also for a range of other genetic disorders.

Use of antisense oligonucleotides in functional genomics and target validation

With the completion of sequencing of the human genome, a great deal of interest has been shifted toward functional genomics-based research for identification of novel drug targets for treatment of various diseases. The major challenge facing the pharmaceutical industry is to identify disease-causing genes and elucidate additional roles for genes of known functions. Gene functionalization and target validation are probably the most important steps involved in identifying novel potential drug targets. This review focuses on recent advances in antisense technology and its use for rapid identification and validation of new drug targets. The significance and applicability of this technology as a beginning of the drug discovery process are underscored by relevant cell culture-based assays and positive correlation in specific animal disease models. Some of the antisense inhibitors used to validate gene targets are themselves being developed as drugs. The current clinical trials based on such leads that were identified in a very short time further substantiate the importance of antisense technology-based functional genomics as an integral part of target validation and drug target identification.

Antisense morpholino-oligomers directed against the 5' end of the genome inhibit coronavirus proliferation and growth

Conjugation of a peptide related to the human immunodeficiency virus type 1 Tat represents a novel method for delivery of antisense morpholino-oligomers. Conjugated and unconjugated oligomers were tested to determine sequence-specific antiviral efficacy against a member of the Coronaviridae, Mouse hepatitis virus (MHV). Specific antisense activity designed to block translation of the viral replicase polyprotein was first confirmed by reduction of luciferase expression from a target sequence-containing reporter construct in both cell-free and transfected cell culture assays. Peptide-conjugated morpholino-oligomers exhibited low toxicity in DBT astrocytoma cells used for culturing MHV. Oligomer administered at micromolar concentrations was delivered to >80% of cells and inhibited virus titers 10- to 100-fold in a sequence-specific and dose-responsive manner. In addition, targeted viral protein synthesis, plaque diameter, and cytopathic effect were significantly reduced. Inhibition of virus infectivity by peptide-conjugated morpholino was comparable to the antiviral activity of the aminoglycoside hygromycin B used at a concentration fivefold higher than the oligomer. These results suggest that this composition of antisense compound has therapeutic potential for control of coronavirus infection.

Delineation of the mechanisms of aberrant splicing caused by two unusual intronic mutations in the RSK2 gene involved in Coffin-Lowry syndrome

Coffin-Lowry syndrome (CLS) is caused by mutations in the RSK2 gene encoding a protein kinase of the Ras signalling pathway. We have studied two point mutations which cause aberrant splicing but do not concern the invariant GT or AG nucleotides of splice sites. The first, an A-->G transition at position +3 of the 5' splice site of exon 6, results in vivo and in vitro in exon skipping and premature translation termination. The natural 5' splice site, although intrinsically weak, is not transactivated under normal conditions. Consequently, replacement of an A/U by a G/U base pairing with U1 snRNA reduces its strength below a critical threshold. The second mutation, an A-->G transition 11 nt upstream of exon 5, creates a new AG near the natural 3' splice site. In vitro this synthetic 3' AG is used exclusively by the splicing machinery. In vivo this splicing event is also observed, but is underestimated because the resulting RSK2 mRNA contains premature stop codons which trigger the nonsense-mediated decay process. We show that a particular mechanism is involved in the aberrant splicing of exon 5, implying involvement of the natural 3' AG during the first catalytic step and the new 3' AG during the second step. Thus, our results explain how these mutations cause severe forms of CLS.

Antisense-induced multiexon skipping for Duchenne muscular dystrophy makes more sense

Dystrophin deficiency, which leads to severe and progressive muscle degeneration in patients with Duchenne muscular dystrophy (DMD), is caused by frameshifting mutations in the dystrophin gene. A relatively new therapeutic strategy is based on antisense oligonucleotides (AONs) that induce the specific skipping of a single exon, such that the reading frame is restored. This allows the synthesis of a largely functional dystrophin, associated with a milder Becker muscular dystrophy phenotype. We have previously successfully targeted 20 different DMD exons that would, theoretically, be beneficial for >75% of all patients. To further enlarge this proportion, we here studied the feasibility of double and multiexon skipping. Using a combination of AONs, double skipping of exon 43 and 44 was induced, and dystrophin synthesis was restored in myotubes from one patient affected by a nonsense mutation in exon 43. For another patient, with an exon 46-50 deletion, the therapeutic double skipping of exon 45 and 51 was achieved. Remarkably, in control myotubes, the latter combination of AONs caused the skipping of the entire stretch of exons from 45 through 51. This in-frame multiexon skipping would be therapeutic for a series of patients carrying different DMD-causing mutations. In fact, we here demonstrate its feasibility in myotubes from a patient with an exon 48-50 deletion. The application of multiexon skipping may provide a more uniform methodology for a larger group of patients with DMD.

Oligonucleotides and derivatives as gene-specific control agents

The current achievement of genome sequence projects of a dozen eukaryote organisms (including human genome) and the development of functional genomics are providing the basic knowledge required to utilize gene-specific reagents for both basic understanding of cell physiology and therapeutical development. The field of chemical genomics has the ambitious goal of designing molecules that could act selectively on every single gene or gene product in a cell and in vivo. The progress in oligonucleotide-based approaches will be the topic of this review, however, other nucleic acid- and SELEX-based approaches as well as high sequence-specific low molecular weight DNA-specific ligands will also be discussed.

Optimization of functional efficacy of phosphorothioate-modified oligonucleotides in a human CD8+ T-cell ex vivo expansion model

Antisense oligodeoxyribonucleotides (ODNs) can specifically inhibit gene expression, but their application to fresh human CD8+ T cells is limited by poor spontaneous uptake (<2%). We have examined and optimized the uptake of phosphorothioate-modified oligodeoxyribonucleotides (PS-ODNs) into these cells in an ex vivo expansion model. Optimal antisense treatments were found to be, for fresh CD8+ T cells, 1 micro m PS-ODNs complexed with lipofectin (LF), which resulted in 35% uptake and 10 micro m PS-ODNs in the absence of LF, for cultured cells, which resulted in 95% uptake. The delivered antisenses were functional, as determined by the inhibition of protein expression. In this respect, partially phosphorothioate-modified ODNs (PS-ODNs-P) were twice as effective as completely modified (PS-ODNs-C), and the antisense specific for the cap site showed the highest protein suppression of those tested (68%). Uptake mechanisms were also investigated. To our knowledge, this is the first optimization of the delivery of antisense oligonucleotides into human CD8+ T cells. This protocol could be used to study the function of a particular gene in cytotoxic T lymphocytes and also by those looking for a method to deliver short interfering RNA into cell lines to specifically suppress a gene of interest.

Therapeutic potential of antisense nucleic acid molecules

Elucidation of many disease-related signal transduction and gene expression pathways has provided unparalleled opportunities for the development of targeted therapeutics. The types of molecules in development are increasingly varied and include small-molecule enzyme inhibitors, humanized antibodies to cell surface receptors, and antisense nucleic acids for silencing the expression of specific genes. This Perspective reviews the basis for various antisense strategies for modulating gene expression, including RNA interference, and discusses the prospects for their clinical use.