Gene therapy is a promising approach to treat various genetic diseases by delivering therapeutic genes into the target cells. However, gene delivery is not a trivial task, as it requires overcoming several biological barriers, such as immune recognition, cellular uptake, endosomal escape, nuclear entry, and gene expression. To overcome these challenges, researchers have developed various types of gene delivery vectors, such as viral and non-viral vectors.
Among the viral vectors, adeno-associated virus (AAV) is one of the most widely used and successful gene delivery vehicles. AAV is a small, non-enveloped virus that can infect both dividing and non-dividing cells and can mediate long-term gene expression without integrating into the host genome. AAV has a low immunogenicity and a broad tropism, meaning that it can infect various types of cells and tissues. Moreover, AAV can be engineered to display different surface proteins or peptides to enhance its targeting specificity and efficiency.
However, AAV also has some limitations, such as a small packaging capacity of about 4.7 kb, a potential risk of insertional mutagenesis, and a pre-existing immunity in some individuals. Therefore, researchers are constantly exploring new ways to improve AAV vectors for gene therapy applications.
One of the strategies to optimize AAV vectors is to use non-human primates (NHPs) as animal models. NHPs are closely related to humans in terms of physiology, anatomy, immunology, and genetics, and thus can provide more relevant and predictive data for human gene therapy trials. NHPs can also be used to evaluate the safety, efficacy, biodistribution, and immunogenicity of AAV vectors in vivo.
In this blog post, we will review some of the recent advances in using NHPs as models for AAV-mediated gene therapy. We will focus on three main areas: (1) developing novel AAV serotypes or variants with improved transduction properties; (2) engineering tissue-specific or inducible promoters to control gene expression; and (3) applying CRISPR-Cas9 technology to achieve precise genome editing with AAV vectors.
Besides delivering healthy genes to replace defective ones, AAV-based gene therapy can also be used to achieve other goals, such as enhancing gene expression, silencing gene expression, or editing gene sequences. To accomplish these tasks, researchers have been developing novel AAV serotypes or variants with improved transduction properties, engineering tissue-specific or inducible promoters to control gene expression, and applying CRISPR-Cas9 technology to achieve precise genome editing with AAV vectors.
One of the ways to improve AAV transduction efficiency and specificity is to engineer novel AAV serotypes or variants that can better target the desired cell types and evade the immune system. For example, researchers have used directed evolution, rational design, or peptide display methods to create new AAV capsids that have enhanced tropism for certain tissues, such as the brain, the liver, the heart, or the retina. Some of these novel AAV capsids have also shown reduced immunogenicity and increased stability compared to natural AAV serotypes.
Another way to improve AAV gene therapy is to engineer tissue-specific or inducible promoters that can regulate when and where the transgene is expressed. For example, researchers have used promoters that are activated by certain stimuli, such as light, temperature, or drugs, to control the timing and level of transgene expression. Alternatively, researchers have used promoters that are specific to certain cell types, such as neurons, glia, or photoreceptors, to restrict the transgene expression to the target cells. These strategies can help reduce unwanted side effects and increase therapeutic efficacy of AAV gene therapy.
A third way to improve AAV gene therapy is to apply CRISPR-Cas9 technology to achieve precise genome editing with AAV vectors. CRISPR-Cas9 is a powerful tool that can introduce targeted mutations or corrections in the genome by using a guide RNA (gRNA) and a Cas9 nuclease. Researchers have used AAV vectors to deliver the gRNA and Cas9 components into target cells, and achieved successful genome editing in various animal models of human diseases. For example, researchers have used AAV-CRISPR-Cas9 to correct mutations in genes associated with Duchenne muscular dystrophy, cystic fibrosis, hemophilia, and retinitis pigmentosa. However, there are also some challenges and limitations of using AAV-CRISPR-Cas9, such as off-target effects, immune responses, and packaging constraints.
