Splice-Switching ASOs: Mechanism & Design
How antisense oligonucleotides can reprogram RNA splicing — with lessons from Nusinersen, Eteplirsen, and the expanding splice modulation pipeline.
What Is Splice-Switching?
Most human genes contain multiple exons separated by introns. During RNA processing, the spliceosome decides which exons to include in the mature mRNA — a process called alternative splicing. This means a single gene can produce multiple protein isoforms with different functions.
Splice-switching ASOs work by binding to specific regulatory elements on the pre-mRNA — exonic splicing enhancers (ESEs), intronic splicing silencers (ISSs), or splice sites themselves — to alter which exons are included or excluded. The ASO doesn't destroy the RNA; it redirects how the RNA is processed.
This is fundamentally different from knockdown: instead of eliminating a protein, you're changing which version of the protein is made.
Two Core Strategies
Exon Inclusion
Block an intronic splicing silencer (ISS) or exonic splicing silencer (ESS) to promote inclusion of a normally skipped exon. This restores production of a functional protein.
Flagship Example
Nusinersen (Spinraza) — Blocks ISS-N1 in SMN2 intron 7, promoting exon 7 inclusion and restoring functional SMN protein in SMA patients.
Exon Skipping
Block an exonic splicing enhancer (ESE) or a splice site to force the spliceosome to skip a disease-causing exon. This can restore the reading frame in frameshift mutations.
Flagship Example
Eteplirsen (Exondys 51) — Skips exon 51 of dystrophin in DMD, restoring a truncated but partially functional protein.
The Regulatory Logic
Splicing decisions are controlled by a balance of enhancer and silencer elements, both exonic and intronic. These elements recruit RNA-binding proteins (RBPs) — like SR proteins and hnRNPs — that either promote or inhibit spliceosome assembly at nearby splice sites.
The key principle for ASO design:
Block an enhancer (ESE/ISE) → The exon loses support → Exon is skipped
Block a silencer (ESS/ISS) → The exon loses repression → Exon is included
This means successful splice-switching ASO design requires understanding where the regulatory elements are and which RBPs they recruit. Tools like ESEfinder, Human Splicing Finder, and SpliceAI can help map these elements, but interpreting the results in the context of a specific gene's biology requires expertise.
Design Considerations for Splice-Switching ASOs
Chemistry must be steric-blocking
Splice-switching ASOs must NOT recruit RNase H (which would destroy the RNA). Use fully modified designs (2'-MOE uniform, mixmers) or PMO chemistry.
Target site selection is everything
The ASO must overlap a functional regulatory element. Shifting even a few nucleotides can mean the difference between potent activity and no effect.
The "ASO walk" legacy
Historically, researchers tiled ASOs across a region and tested them all to find hotspots. This works but is expensive. Computational approaches can now narrow the search space significantly.
Dose-response matters
Splice modulation is often dose-dependent — you can achieve partial correction at lower doses. This creates a therapeutic window that pure knockdown approaches don't have.
Validation requires splice-specific readouts
RT-PCR with primers spanning the affected exon junction is the gold standard. Western blot confirms protein-level effects.
FDA-Approved Splice-Switching ASOs
| Drug | Target | Mechanism | Chemistry | Year |
|---|---|---|---|---|
| Nusinersen (Spinraza) | SMN2 | Exon 7 inclusion | 2'-MOE PS | 2016 |
| Eteplirsen (Exondys 51) | DMD exón 51 | Exon skipping | PMO | 2016 |
| Golodirsen (Vyondys 53) | DMD exón 53 | Exon skipping | PMO | 2019 |
| Viltolarsen (Viltepso) | DMD exón 53 | Exon skipping | PMO | 2020 |
| Casimersen (Amondys 45) | DMD exón 45 | Exon skipping | PMO | 2021 |
References
1. Hua Y et al. (2008) Antisense masking of an hnRNP A1/A2 intronic splicing silencer corrects SMN2 splicing in transgenic mice. Am J Hum Genet 82:834-848. PMID: 18371932
2. Aartsma-Rus A et al. (2009) Guidelines for antisense oligonucleotide design and insight into splice-modulating mechanisms. Mol Ther 17:548-553. PMID: 19436183
3. Finkel RS et al. (2017) Nusinersen versus sham control in infantile-onset SMA. N Engl J Med 377:1723-1732. PMID: 29091570
4. Cartegni L et al. (2003) ESEfinder: a web resource to identify exonic splicing enhancers. Nucleic Acids Res 31:3568-3571. PMID: 12824367
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