What is AAV Gene Therapy

What is Gene Therapy

Gene therapy refers to the introduction of exogenous normal genes into target cells to correct or compensate for diseases caused by defective and abnormal genes, thereby achieving the purpose of treating diseases. For a long time, traditional large/small molecule drugs mainly work on protein targets, which also makes their indications limited. Especially for hereditary diseases where genetic factors are the main cause, they cannot effectively exert their therapeutic effects. The number of people suffering from such gene-related diseases is actually not small. For example, rare diseases have as many as 350 million patients worldwide, and 80% of rare diseases are single-gene genetic diseases.

For these genetic diseases, the current treatment methods are limited, and they are basically treated from the perspective of improving symptoms, that is, treating the symptoms but not the root cause. The patient's disease will continue to progress and seriously affect the quality of life, and even eventually lead to death. In this context, gene therapy emerged as the times require, which can treat diseases from the root cause. Although gene therapy has relevant clinical application cases 20 years ago, it has only been widely studied and clinically tested in recent years.

Adeno-Associated Virus (AAV)

Adeno-associated virus is a non-enveloped single-stranded DNA virus that cannot replicate autonomously. It has the following advantages:

High safety: AAV virus does not cause disease during the reproduction process of the human body, and the immune response to the human body is small, so AAV gene therapy is safer.

Long expression time: The genes carried by the AAV virus can be expressed in human cells for several years, thus achieving the purpose of long-term treatment.

Wide range of applications: AAV gene therapy can be used to improve and treat many different types of genetic diseases.

Currently, recombinant adeno-associated virus vectors (rAAV) are mainly used in gene therapy clinical trials. It is modified from non-pathogenic wild-type AAV, retaining only two inverted terminal repeats (ITRs) to replace the original genome with the target gene, maximizing the capacity to carry the target gene and reducing its immunogenicity and cytotoxicity.

The purified AAV viral vector can be used to infect cells. When infecting cells, the AAV capsid protein binds to specific receptors on the cell surface, activating intracellular signaling pathways, which in turn triggers AAV to enter the cell through receptor-mediated endocytosis, with the assistance of endosomes, Golgi apparatus and other organelles. Enters the cell nucleus, and the capsid protein is degraded by the proteasome.

Once the AAV viral vector enters the nucleus, it will uncoat and release its single-stranded genome, and convert it into a double-stranded DNA template, on which the transgene will be transcribed and translated to achieve the expression of the target gene.

Different serotypes of AAV have different tissue targeting specificities. Currently, 13 subtypes and nearly 200 variants of AAV have been discovered, and are gradually used in the research and development of genetic drugs. The process of AAV entering cells relies on the recognition of AAV capsid protein by glycosylated receptors on the cell surface, so the AAV capsid protein determines its tissue targeting specificity. New tissue-prone AAV serotypes are generated by modifying and mutating the capsid protein sequence of AAV. Currently, 13 natural serotypes of AAV (AAV1-13) have been discovered, with nearly 200 variants, each with different characteristics and tissue tropism, which can target different tissues. Currently, AAV2, AAV5, AAV8 and AAV9 are commonly used in clinical practice.