by What are Antisense Oligonucleotides?
Antisense Oligonucleotides (ASOs) are short, synthetic strands of DNA or RNA that are designed to bind to RNA through complementary base pairing. Through this binding, ASOs are able to interfere with the production of pathogenic proteins by modulating RNA function. ASOs are generally between 18-30 nucleic acid bases long and are constructed of molecule analogs that provide favorable pharmacokinetic properties.
Mechanism of Action
There are several ways in which ASOs can interfere with RNA function and production of disease-causing proteins:
RNA Degradation: ASOs can be designed to recruit RNase H, an endonuclease enzyme that degrades the RNA strand of RNA-DNA duplexes. By binding to pre-mRNA, ASOs can induce targeted RNase H cleavage and degradation of the pre-mRNA, preventing translation into protein.
Translation Blocking: Through steric blocking of pre-mRNA splicing or ribosomal scanning, ASOs can prevent splicing of introns or translation of mRNA into protein without degrading the RNA target.
Nonsense Mutation Suppression: Certain genetic disorders are caused by premature stop codons, also known as nonsense mutations. ASOs can induce readthrough of these stop codons, allowing translation past the mutation site and production of full-length, functional proteins.
Splicing Modulation: Interaction of ASOs with pre-mRNA can modify splicing by either promoting exon inclusion or skipping through complementary base pairing with splice site sequences. This can restore the correct open reading frame in certain genetic diseases.
Therapeutic Applications
Due to their ability to precisely modulate gene expression and correct mRNA splicing defects, ASOs have shown great promise as treatments for various human genetic disorders:
Spinal Muscular Atrophy (SMA): SMA is caused by loss of the SMN1 gene and low levels of the survival motor neuron (SMN) protein. ASOs targeting an intronic splicing silencer induce inclusion of SMN2 exon 7 during splicing and partial restoration of SMN protein levels, providing clinical benefits in several SMA trials.
Duchenne Muscular Dystrophy (DMD): DMD is caused by dystrophin gene mutations resulting in deletion or duplication of exons. Exon-skipping ASOs mask regions flanking specific dystrophin exons during pre-mRNA splicing, inducing readthrough of the mutation and production of a partially functional dystrophin protein. Multiple exon-skipping ASOs are approved for use in DMD.
Hereditary Transthyretin Amyloidosis: This disease results from mutations in the transthyretin (TTR) gene that destabilize the TTR tetramers and cause them to misfold into amyloid deposits. ASOs designed to induce targeted degradation of TTR pre-mRNA through RNase H activity have received approval based on reductions in TTR levels and disease progression.
Future Therapeutic Potential
Beyond currently approved applications, new ASO therapeutics are being explored for a wide range of rare diseases:
Myotonic Dystrophy: Similar to DMD, ASOs are targeting the aberrant expansions in noncoding RNAs responsible for myotonic dystrophy types 1 and 2 with the goal of normalizing mRNA splicing and protein function.
Huntington’s Disease: Preclinical studies demonstrate ASO modulation of huntingtin mRNA splicing and translation to reduce levels of the toxic huntingtin protein involved in pathogenesis.
Transthyretin-Related Amyloidosis: Additional TTR-lowering ASOs are in late-stage testing to treat more prevalent, non-hereditary forms of TTR amyloidosis.
Potential also exists for ASO treatments of neurodegenerative diseases like ALS, Parkinson’s, and Alzheimer’s through strategies like silencing of aggregation-prone proteins or modulating disease-associated RNAs and splicing factors. With continued advancements, ASOs hold great promise to precisely target the RNA basis of many inherited and acquired human diseases.
Safety Considerations
Like other oligonucleotide therapeutics, ASOs have the potential for off-target effects through interactions with non-targeted RNAs or immune stimulation. Thorough preclinical safety testing is required to identify potential toxicities. In the clinic, close monitoring is important during dose titration periods to identify toxicity risks and determine maximum tolerated doses. To date, approved ASO therapies have shown generally favorable benefit-risk profiles. As the field continues to progress, lessons from early clinical experience can help inform design of safer, more optimized ASOs.
Future Directions
Beyond natural chemistry modifications to improve targeting and physicochemical properties, ongoing efforts seek to expand the multifaceted therapeutic functionality of ASOs. Areas of active investigation include:
– Conjugation of ASOs to targeting moieties like peptides or antibodies for tissue- or cell-specific delivery.
– Development of multitarget “cocktail” ASO formulations to simultaneously modulate multiple pathogenic RNAs or pathways.
– Engineering of switchable or conditional ASOs whose activity can be externally regulated through small molecule triggers.
– Combination therapies utilizing ASOs with gene editing, gene therapy or protein replacement approaches for additive or synergistic effects.
With their high specificity and proven clinical validation, ASOs represent a major new frontier for genetic medicines. As delivery hurdles are overcome and combination strategies emerge, they have immense potential to transform the treatment landscape across dozens of inherited and acquired disorders. With further development, ASOs may become one of the most effective classes of drugs to emerge in recent decades.
