Adeno-associated virus (AAV) technology is a significant advancement in gene therapy. This approach addresses the underlying genetic causes of various inherited diseases by delivering functional genes to cells. AAVs are naturally occurring viruses engineered into safe delivery vehicles, making them suitable for therapeutic applications. Their ability to introduce healthy genetic material into target cells provides a promising path towards long-term treatment outcomes. This technology is transforming how many genetic disorders are approached, moving beyond symptomatic relief to address root molecular issues.
Understanding Adeno-Associated Viruses
Adeno-associated viruses (AAVs) are small, naturally occurring viruses in the parvovirus family. In their wild state, they are non-enveloped and contain a single-stranded DNA genome, typically 4.7 to 4.8 kilobases long. AAVs are not associated with human diseases, often infecting individuals without symptoms. This lack of pathogenicity makes them appealing for medical applications.
Natural AAVs require co-infection with a “helper” virus, such as an adenovirus, to replicate. This dependency is a key characteristic. Scientists have identified at least 13 different natural AAV serotypes and over 100 variants, each with distinct preferences for infecting specific cell types or tissues. This diversity in tissue targeting is valuable for gene therapy.
In the laboratory, AAVs are engineered into safe and effective gene delivery vehicles. The viral genes for replication (Rep) and capsid formation (Cap) are removed from the AAV genome. These are replaced with the desired therapeutic genetic material, retaining only the inverted terminal repeats (ITRs) necessary for packaging and stability.
This modification renders AAVs replication-deficient, meaning they cannot multiply or cause illness. The engineered AAV, or recombinant AAV (rAAV), becomes a protein shell designed to transport a healthy gene. This ensures the AAV acts solely as a carrier, safely delivering corrective DNA to target cells without causing viral disease.
The Mechanism of Gene Delivery
AAV gene delivery begins with administering the engineered viral vector into the patient’s body, often intravenously or by direct injection into a specific tissue. The AAV’s outer protein shell, the capsid, binds to specific receptor molecules on target cells.
The AAV serotype determines which receptors it recognizes, guiding it to desired cells or tissues. After binding, the AAV particle is internalized by the cell within an endosome.
Inside the cell, the AAV must escape this internal bubble. Endosome acidification triggers a change in the AAV capsid, allowing the viral particle to break free and move deeper into the cell. The AAV then makes its way to the cell’s nucleus.
Upon reaching the nucleus, the AAV uncoats, shedding its protein shell and releasing its single-stranded DNA genome containing the therapeutic gene. The cell’s machinery converts this single-stranded DNA into a double-stranded, readable form. This introduced genetic information typically remains as an episome, a stable, circular piece of DNA that does not integrate into the host cell’s chromosomes.
The cell’s machinery then reads this genetic blueprint, transcribing it into messenger RNA (mRNA) and translating the mRNA into the therapeutic protein. This protein performs the function missing or defective due to the original genetic error. This non-integrating, episomal presence allows for stable, long-term production of the therapeutic molecule within the cell.
Therapeutic Applications and Progress
AAV technology treats a broad spectrum of genetic diseases by addressing their root causes. This involves delivering a functional gene copy to compensate for or replace a faulty one, allowing the body to produce the missing or defective protein. As of early 2024, eight AAV-based gene therapies have received approval from regulatory bodies like the U.S. Food and Drug Administration (FDA) and European Medicines Agency (EMA), marking a significant shift in medical treatment.
Luxturna (voretigene neparvovec) is approved for inherited retinal dystrophy caused by RPE65 gene mutations. This therapy delivers a healthy RPE65 gene directly to retinal cells, aiming to restore vision. Zolgensma (onasemnogene abeparvovec) treats Spinal Muscular Atrophy (SMA), a severe neuromuscular disorder. It introduces a functional SMN gene copy, which is deficient in SMA patients, improving motor neuron function and patient outcomes.
Elevidys (delandistrogene moxeparvovec-rokl) is approved for Duchenne muscular dystrophy (DMD) in certain pediatric patients. This therapy delivers a gene coding for a shortened, functional dystrophin protein, which is typically absent or non-functional in DMD, aiming to mitigate muscle degeneration. Roctavian (valoctocogene roxaparvovec) is approved for severe hemophilia A. It provides a functional gene for Factor VIII, the clotting protein deficient in these patients, reducing bleeding episodes.
Beyond approved treatments, AAV technology is under investigation for numerous other conditions. For neurological disorders like Parkinson’s, Alzheimer’s, Huntington’s, and Amyotrophic Lateral Sclerosis (ALS), AAVs are explored to deliver genes that could protect neurons or produce necessary enzymes. For example, in Aromatic L-amino acid decarboxylase (AADC) deficiency, AAV2-based therapy has shown improvements in motor and cognitive functions by delivering the AADC gene to the brain.
Ophthalmic conditions remain a focus, with trials exploring treatments for inherited retinal diseases like achromatopsia and retinitis pigmentosa, and age-related macular degeneration. In metabolic disorders, AAV vectors are evaluated for conditions like glycogen storage disease type IA and Wilson’s disease, aiming to correct enzymatic deficiencies or regulate metal accumulation. AAVs are also studied for lysosomal storage disorders, such as Hunter syndrome, to deliver genes that produce missing enzymes. Research continues to refine AAV vectors to enhance specificity, reduce immune responses, and expand applicability to more complex genetic diseases.
Safety Profile and Immune Considerations
AAV technology exhibits a favorable safety profile, largely because wild-type viruses are not known to cause human diseases. Engineered AAV vectors are replication-deficient, enhancing their safety by preventing uncontrolled multiplication. Despite this, the body’s immune system can still recognize the AAV vector, particularly its protein shell or capsid, as foreign, potentially triggering a response.
A common consideration is pre-existing antibodies in patients. Many individuals have been naturally exposed to wild-type AAVs, leading to antibodies that can neutralize the therapeutic AAV vector before gene delivery. Patients are screened for these antibodies, and treatment eligibility can depend on their levels, with thresholds varying by gene therapy product.
The immune system can also react to the AAV capsid or the newly produced therapeutic protein or impurities in the vector preparation. These immune responses can affect patient safety and the long-term effectiveness of the gene therapy. Higher doses of AAV vectors, especially when administered systemically or to sensitive organs like the brain, can lead to more pronounced immune reactions, including rapid antibody formation.
To manage immune considerations, various strategies are employed. Immunosuppressive medications, such as corticosteroids, are administered, often for a limited period following treatment, to dampen the immune response and allow the gene therapy to take effect. Researchers are modifying AAV capsids to make them less visible to the immune system, improving vector purification to remove empty capsids, and exploring alternative delivery routes to minimize systemic immune exposure.
The development of antibodies after initial treatment presents a challenge for redosing with the same AAV serotype, as these antibodies can persist for many years and neutralize subsequent doses. Continuous monitoring of patients for immune responses and side effects is standard practice, ensuring patient safety and informing future AAV-based therapies.