by What are Antisense Oligonucleotides?
Antisense Oligonucleotides (ASOs) are short synthetic strands of DNA or RNA that are designed to bind to target messenger RNA (mRNA) through complementary base pairing. By binding to mRNA, ASOs can modify gene expression through several distinct mechanisms. Depending on the sequence and chemical modifications, ASOs can induce mRNA degradation by RNase H or inhibit translation by steric hindrance. This selective suppression of target genes makes ASOs a promising platform for treating many genetic diseases.
Mechanisms of Action
There are two main mechanisms through which ASOs alter gene expression – RNA degradation and steric blocking.
RNA degradation is mediated by ASOs containing a “gapmer” design, which has a central region complementary to mRNA flanked by wings composed of DNA-like bases. When an ASO binds to mRNA, it recruits the endogenous RNase H enzyme which degrades the mRNA strand of the duplex, leading to target gene suppression.
Alternatively, “steric-blocking” ASOs are fully modified to be resistant to RNase H. They bind to mRNA through Watson-Crick base pairing but don’t catalyze degradation. Instead, they sterically obstruct translation machinery from accessing the mRNA, inhibiting protein production from that transcript without degrading it. This allows for reversible silencing of target genes.
Therapeutic Applications
The ability to selectively suppress disease-causing genes makes ASOs applicable across a wide range of genetic disorders. Here are some of their promising therapeutic applications:
Duchenne Muscular Dystrophy (DMD): ASOs are being developed to “skip” specific exons during pre-mRNA splicing in DMD patients, restoring the reading frame and preventing loss of the dystrophin protein. Several exon skipping ASOs are approved or in late-stage testing.
Neurological Disorders: ASOs targeting genes linked to amyotrophic lateral sclerosis, spinal muscular atrophy, and Huntington’s disease could potentially slow or stop disease progression by reducing levels of toxic proteins.
Hepatic Disorders: Targeting expression of genes involved in cholesterol synthesis like APOC3 or PCSK9 using ASOs has shown rapid, robust reductions in cholesterol and promise for treating atherosclerosis.
Metabolic Disorders: Genes linked to rare metabolic disorders like Hereditary Transthyretin Amyloidosis or Familial Chylomicronemia Syndrome are candidates for ASO suppression therapy to lower toxic protein or enzyme levels.
Exploring New Targets and Combinations
As the mechanistic understanding and delivery methods for ASOs continue to improve, more disease targets are being explored. Combination therapies are also emerging as a strategy.
One exciting application is using ASOs to upregulate protective genes rather than just downregulate toxic ones. By targeting microRNAs or certain mRNAs with antisense sequences, expression of potentially therapeutic proteins can be increased.
Combining ASOs with other drugs may offer synergistic benefits. For example, co-administering an ASO with a small molecule to treat a recessive disorder by knocking out just one disease allele instead of both. Or pre-treating tumors with an ASO before CAR-T cell therapy to enhance antigen presentation.
ASOs also show promise as part of combination antiviral therapies, with some programs testing HIV or SARS-CoV-2 targets. Their selective gene modulation ability gives them enhanced specificity over traditional broad antivirals.
Delivering on the Potential
Improving delivery techniques will be key to leveraging ASOs against more systemic diseases. Early IV or subcutaneous formulations are mostly confined to the liver, but newer delivery platforms aim to distribute ASOs throughout the body. These include cell-penetrating peptides, exosomes, and encapsulation in lipid nanoparticles.
Refinements to ASO chemistry continue as well. Next-generation modified bases provide better gene knockdown potency and increased duration of effect with fewer doses. And alternative backbone structures offer improved cell penetration and resistance to enzymatic degradation.
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.
