Upon hearing the term “virus,” it is the common trail of thought to picture the destructive manipulation of cells through a rapid replication process. Since diseases are now able to thrive in the body’s weakened state, such dismantling of the body’s immune system can lead to pathogenesis. Long story short, this perception of a virus is attached to a negative connotation where the virus harmfully invades an organism to manipulate the metabolic equipment that resides in cells. These viruses effectively inject their DNA/RNA information into the cell and commence the production of proteins that create more replications. Such significant shifts in the cell’s activity largely disrupt homeostasis and can permanently damage the cell. The growing virus hijacks more and more cells to amass more and more copies, thus destroying tissues and eventually organs. Different viruses may target different areas. For instance, the notorious Influenza virus targets the area involving the nose, throat, and lungs. Cells that line lung airways are susceptible to viral attack, and the body pays a toll by fending the virus off and destroying any remains of the virus. This is where the common symptoms of the flu arise. Overall, viruses can generally be characterized as aggressive, damaging particles that can leave the body weak and devastated.
However, a special type of virus exists—one that does not involve the uncontrollable hijacking of defenseless cells.
Adeno-Associated Virus Discovery
The Adeno-Associated Virus (AAV) was initially discovered around 50 years ago. Scientists Bob Atchinson, M. David Hoggan, and Wallace Rowe uncovered the new virus particles while researching the established adenoviruses, which are very common viruses that elicit symptoms of an ordinary cold. This new virus was established to be a member of the Parvoviridae virus family since it consisted of single-stranded DNA. When the basic background research of this virus was underway, it was clear that it was unique; there was a drastic difference in virus behavior that was not consistent with other adenoviruses. AAV did not replicate within a cell culture—the main function associated with a virus! Unlike many other viruses, AAV did not execute a replication spree that manipulated and destroyed cells; it simply did not replicate. It was eventually found that AAV was able to execute standard virus replication while being introduced into a cell with other adenoviruses, or “helper-viruses,” concurrently. The researchers concluded that viral pathogenesis (virus leading to disease) was not possible with this virus due to the inability of replication.
Around 15 years after the initial discovery, more research elucidated the details of the virus’s basic genetic information. Namely, it was confirmed that the virus can manufacture up to 100,000 particles per infected cell when paired with a helpervirus. This set the path for vector research.
AAV as a Vector
With such distinct features, it was clear that this virus could be utilized in a therapy-based manner. Yet another striking aspect of the virus strain was the size of the genome. The AAV genome is extremely small and consists of around three genes. This feature, along with the previously mentioned features of non-existent pathogenesis, controllable viral replication, and general minimized risk, led to AAV’s use as a recombinant vector. In other words, scientists sought out to use AAV to deliver non-native genetic information as a therapeutic method in patients with genetic diseases. Such a preferable genome allowed for very effective genetic editing.
Modern Gene Therapy with AAV
In modern-day AAV-based gene therapy, scientists have managed to create an efficient way of editing the virus vector to their liking. Simply put, the single strand of DNA within the virus is cut apart. The middle part of the DNA strand is removed, but the ends of the strand are left as they are necessary for gene transfer later on. New foreign DNA containing therapeutic genes is placed to fill in the strand and spliced with the two ends to join the pieces together.
Following this is the actual vector process. As mentioned previously, AAV requires the presence of another virus in order to replicate. This “helpervirus” certainly plays a role in gene therapy. Both the edited AAV vector, as well as a compatible helpervirus, are combined with a bacterium. This effectively creates what is known as a plasmid (one plasmid being the AAV vector with the bacterium, and another plasmid being the helpervirus with the bacterium), which is a genetic formation that is capable of replication and can be used to easily affect genes and gene expression. Once both plasmids are introduced to the target cell, many AAV particles are produced within the cell and thus effectively activate the therapeutic gene, which creates the protein needed to potentially resolve a disease or problem within the given tissue.
AAV therapy has already been proven to be capable of relieving the symptoms of several types of diseases and issues that have been problematic to many patients. AAV is also behind some of the very few Food and Drug Association (FDA) approved gene therapies, which means that the benefits substantially outweigh the risks.
An example of such a therapy is “Luxturna,” an AAV-based therapy used to treat a type of retinal disease that is passed down through inheritance. Over time, blindness can result in a patient as a result of this disease. This disease involves the RPE65 gene, which normally contains the instructions to produce the RPE65 protein, a necessary protein for vision. Mutations in this gene cause a lack of RPE65 proteins, which can negatively affect the function of RPE cells that contain the proteins. These cells are responsible for photoreception, and when a large amount of RPE cells become dysfunctional, loss of vision can be imminent.
AAV is used as a vector for delivering unmutated RPE65 genes to the RPE cells and producing the RPE65 proteins that were initially lacking. The vector is sent inside the body via eye injection. Once the vector reaches the targeted RPE cells, functional cell numbers return to normal, and vision can be restored effectively.
AAV has certainly been a breakthrough in the past and is serving as an effective therapy for problems that were considered unsolvable a few years back. For the future, it is clear that the next steps are to utilize AAV vectors as solutions to more genetic problems and diseases and to ensure that these therapies are FDA approved. With further research into AAV vectors, many diseases with temporary fixes can be treated with more effective solutions. AAV is truly quite a unique virus, and applications can certainly be maximized in the future.
Brian Caballo, Youth Medical Journal 2020
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