As we reach the end of the first month back from break, I think it’s a nice time to continue on our journey into the world of gene therapy. My last blog entry focused on why the adeno-associated virus (AAV) capsid is such an important part of gene therapy, and this time, we’ll focus on how different types of AAV capsid affect therapy efficacy and what researchers are doing to find new types of AAV capsids.
AAV was first discovered in the 1960s by groups of scientists when they found small particles riding along with adenovirus cultures. In 1982, the first AAV serotype, AAV2, was cloned. (For a more in-depth review of AAV history, click here: Birth of a New Therapeutic Platform: 47 Years of Adeno-associated Virus Biology From Virus Discovery to Licensed Gene Therapy). However, it was soon discovered that AAV2 displays natural tropism towards certain tissues. What this means is that once injected into the blood stream, AAV2 automatically moves toward specific areas of the body; in this case, the CNS and kidneys. (For more information on serotype tissue specificity, click here: Adeno-associated Virus (AAV) Guide). This specificity would prove to be a double-edged sword. On the one hand, tissue specificity reduces risks of systemically expressing genes and reduces immunogenicity; on the other hand, this means that unless new capsid types are found to target an organ system, diseases of that system would be impenetrable to gene therapy.
Fortunately, soon after the isolation of AAV2, several other natural AAV serotypes were discovered (such as the, ‘Novel adeno-associated viruses from rhesus monkeys as vectors for human gene therapy’). These new serotypes were found to have drastically different tropisms from AAV2, which opened up new doors towards creating novel therapies that were more specific, and less immunogenic. However, as hard-working scientists go, you knew that they weren’t just going to let a couple new dozen natural serotypes be discovered. Researchers began to tinker with mixing capsids together to generate new hybrid AAVs, in the hopes of creating new capsids that combined properties that would make them even more efficient. Soon, hybrid capsids like AAV2/5 were created, combining their organ specificities and AAV2’s already well-established production efficiency.
The idea of discovering new AAVs in nature and combining them with other natural AAVs to create hybrids sounds all fun and exciting, but we must take a moment to think. What if we can create an environment that drives AAV evolution towards specific tropisms and/or improved production efficiency? What if we force AAV to adapt to certain set conditions in the lab and create new AAV serotypes that are better than those that evolved in nature? Within the past two decades, the idea of using directed evolution to create new serotypes of AAV has exploded in the field of gene therapy, and multiple methods have been described in literature to achieve this goal (here is a rather well known one: Directed evolution of adeno-associated virus yields enhanced gene delivery vectors). The basic idea of directed evolution is simple: subject AAV to a hostile environment, collect the survivors, let them grow, and repeat until the hardiest one is left. Thus, it became possible to not just find AAV in nature and hybridize them, but it became possible to speed up nature to make our own AAV serotypes.
In summary, with the discovery of new natural AAV serotypes, the hybridisation of these natural serotypes, and by incorporating the directed evolution of AAV; we have created an entire host of AAV agents that are specific to organ systems (and sometimes even to specific cells!). At the same time, it also became possible to develop new techniques that made AAV production costs much cheaper as multiple new serotypes grew much faster in culture. Next time, we will take a look at the beginning of the era of human gene therapy trials.
Author: Danning Li
Danning Li completed his BSc. majoring in Physiology at McGill University. Afterwards, he worked for two years on developing a gene replacement therapy for Canavan Disease, a rare inherited leukodystrophy, at the Horae Gene Therapy Center at the University of Massachusetts Medical School. Now a medical student at Schulich, he wants to bring attention to the interesting genetic therapies that will become available in the not so distant future.