Antisense Therapy Explained: How Blocking mRNA Can Treat Genetic Disorders

Antisense therapy has proven to be effective at treating previously untreated genetic disorders including Duchenne muscular dystrophy and familial hypercholesterolemia. The therapy has also demonstrated promising results in Phase III clinical trials for amyotrophic lateral sclerosis (ALS).

What is antisense therapy, and how are antisense oligonucleotides used to treat genetic disorders?

Background

Genetic Disorders and Proteins

Genetic disorders are diseases caused by abnormal changes in our DNA sequence (mutations). Many diseases have a genetic basis, with mutations either being a direct cause or one of many contributors to a disease’s proliferation.

Some people are born with genetic disorders, acquiring mutations from one or both parents, while others acquire them during their lifetime due to mistakes made by their own cells or exposure to viruses, radiation, or mutagenic chemicals. Most mutations do not result in genetic disorders.

The reason why mutations can affect biological processes is because our DNA provides our cells with the blueprints necessary to build proteins, which are complex molecules responsible for carrying out the chemical reactions that occur within our bodies.

Humans are believed to have 25,000 unique proteins (some copied trillions of times throughout our bodies) that have very specific tasks and functions pertaining to growth, maintenance, structure, metabolism, immune defense, and much more. It follows that a mutation, which creates an error in the genetic instructions to create a specific protein, can have profound impacts on our health.

Genetic disorders that cause the creation of harmful proteins are notoriously difficult to treat. New genes can be introduced into cells to result in the creation of non-mutated proteins, but it is not yet possible to completely stop the production of specific proteins.

This limitation even applies with the recently discovered CRISPR-Cas9 gene editing technology, which can add, remove, inhibit, and activate genes–but not in all cells of the body, meaning some cells will still produce the harmful target protein. Therefore, gene therapy that could inhibit the expression of harmful mutated genes would benefit patients with such disorders.

Antisense Therapy

How It Works

When cells use our DNA’s instructions to build new copies of a protein, it must first be processed into a form that can be read by the ribosome, which is the site of protein synthesis in our cells. Messenger RNA (mRNA) is the final form into which a part of the DNA sequence is processed before the ribosome uses its instructions to build new proteins.

Antisense oligonucleotides (ASOs) are strands of DNA or RNA that are complementary to an mRNA strand that encodes for a mutated protein. Due to this complementary nature, the ASO and the faulty mRNA strand will bind together. This prevents the ribosome from ever translating the specific mutated mRNA strand into the harmful, mutated protein that is the basis of the target genetic disorder.

Implications and Discussion

Many genetic disorders are caused by single mutated proteins that have harmful effects. Some of the most serious neurodegenerative diseases like Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis (ALS) are caused by harmful mutated proteins.

Using ASOs to stop these proteins from being built by our cells can offer significant therapeutic effects in patients with this kind of genetic disorder.

For example, a mutation in the gene that encodes for the huntingtin protein causes the protein to take on an elongated shape. When metabolized, these mutated huntingtin proteins bind together and accumulate into increasingly large deposits in the brain, eventually becoming large enough to affect normal brain function. This is the basis for Huntington’s disease. Using ASOs to decrease expression of the mutated Huntingtin protein could provide therapeutic effects.

Antisense therapies could also treat diseases by inhibiting non-mutant proteins. For hypertriglyceridemia (excess triglycerides), ASOs could be used to inhibit the production of the APOC3 gene which encodes for a protein that regulates triglyceride metabolism.

Certain cancers could also be targeted by ASOs, as they could be used to block the production of proteins that facilitate the growth of a cancerous or precancerous mass of cells.

Solely using the antisense oligonucleotide is around 50% effective at preventing synthesis of a target protein. However, when combined with an enzyme that degrades the complex between the mRNA and ASO, this synthesis-blocking efficacy reaches 95%. This can slow the progression of or provide lasting relief from symptomatic disorder.

Limitations

Antisense therapy can not be used for all genetic disorders. Only those which are caused by a single protein mutated into a harmful form could theoretically be treated by the therapy. Also, stopping the production of an implicated protein could have unexpected side effects due to the discontinuation of normal functions of the protein. In one available ASO therapy, nusinersen (Spinraza), patients experienced varied side effects including increased risk of respiratory infection, congestion, constipation, and stunted growth in children–potentially related to the decreased presence of the target protein.

Another limitation of oligonucleotides is that it is very difficult to deliver them to the interior of our cells. However, surrounding them with fatty particles, like what is used to surround the mRNA in COVID-19 vaccines, can protect them from degradation and help them enter our cells. Though, it can still prove difficult to deliver antisense agents to places like the brain, where a drug must make it through the difficult-to-permeate ‘firewall’ that is the blood-brain barrier. For nusinersen (Spinraza), which has a target protein in the central nervous system, the antisense agent must be injected directly into the spinal canal.

In general, antisense therapy research faces an uphill battle. Since the prospect of using ASOs as drugs was first conceived by Harvard scientists in 1978, less than 10 antisense therapies have been approved by the FDA–the first was approved in 1998. Many antisense therapies have failed in the early phases of clinical trials due to low efficacy. Ionis Pharmaceuticals is the most notable biotechnology company researching antisense therapy, with nine current antisense drug candidates reaching Phase III trials as of June 2022.

Whether ASOs will play a wide role in the treatment of genetic disorders has yet to be determined, though recent innovations in drug delivery systems as well as dozens of such therapies being in advanced clinical trials makes them more promising than ever.