In gene therapy, Adeno-Associated Viruses (AAVs) function as a biological delivery service for genetic material. The effectiveness of this delivery hinges on tropism, the innate tendency of a virus to target particular cells or tissues. Among the various AAVs, serotype 9 (AAV9) is a prominent tool due to its widespread and valuable targeting capabilities.
AAV9’s Primary Target Tissues
AAV9’s natural tropism enables it to reach several tissue types after a single injection into the bloodstream, a method known as systemic administration. This is a significant advantage for treating diseases that affect multiple parts of the body. The tissues AAV9 primarily targets include the central nervous system, the heart, skeletal muscles, and the liver.
A noteworthy feature of AAV9 is its capacity to cross the blood-brain barrier (BBB). The BBB is a highly selective membrane that controls which substances can pass from the blood into the brain and spinal cord. AAV9’s ability to traverse this barrier represents a major step forward for treating neurological disorders.
Beyond the nervous system, AAV9 demonstrates a strong affinity for muscle tissues. It targets cardiomyocytes, the cells that make up the heart muscle, and is also effective at transducing skeletal muscle cells. This makes it well-suited for developing gene therapies for inherited heart conditions and muscular dystrophies.
Like many AAV serotypes, AAV9 is also taken up by liver cells (hepatocytes). This can be advantageous when the liver is the target for a gene therapy, such as in treating metabolic disorders. However, this liver tropism must be carefully considered during treatment design, as high concentrations of the vector can lead to unwanted side effects.
The Cellular Entry Mechanism
AAV9’s tissue-targeting ability is dictated by molecular interactions at the cellular level. The virus’s outer protein shell, the capsid, has structures that function like keys designed to fit into specific receptors on host cells. This mechanism ensures the virus binds to and enters only certain types of cells.
For AAV9, the primary receptor it recognizes is a complex sugar molecule known as a glycan with a terminal galactose unit. The high density of this galactose-containing receptor on cells in the central nervous system, heart, and skeletal muscles primarily determines AAV9’s tropism.
Once attached to the galactose receptor, the AAV9 particle is internalized by the cell through a process called endocytosis, where the cell membrane envelops the virus. Inside the cell, the virus must then travel to the nucleus. There, the genetic material it carries can be released to perform its function.
Implications for Systemic Gene Therapy
The broad tropism of AAV9 has profound implications for gene therapy. It enables a systemic approach where a therapy administered into the bloodstream can address genetic defects in multiple, hard-to-reach tissues simultaneously. This is important for complex genetic disorders that manifest with symptoms throughout the body.
A prime example of AAV9’s impact is in the treatment of Spinal Muscular Atrophy (SMA), a devastating genetic disease. SMA is caused by a faulty gene that leads to the loss of motor neurons and progressive muscle weakness. The therapy, Zolgensma, uses an AAV9 vector to deliver a functional copy of the human SMN1 gene.
AAV9 is an ideal delivery vehicle for SMA because it can cross the blood-brain barrier to reach motor neurons while also targeting skeletal muscle cells. When administered intravenously to infants with SMA, the vector delivers the corrective gene to these tissues. This approach addresses the root genetic cause of the disease from a single dose, transforming the treatment landscape for SMA.
Factors Modifying AAV9 Tropism
While AAV9 has a natural tropism, its effectiveness can be influenced by several factors. These variables alter where the vector goes and how well it performs, which are important considerations in developing AAV9-based gene therapies. Understanding these factors allows for better optimization of treatments.
The route of administration modifies the vector’s biodistribution. Systemic intravenous delivery results in widespread distribution, while a direct intrathecal injection into the spinal fluid can concentrate the AAV9 vector within the central nervous system. This method increases the dose to the brain and spinal cord while reducing exposure in other organs.
A patient’s immune history is another modifying factor. Many people have pre-existing antibodies against AAVs from natural exposure, and for AAV9, it is estimated that around 45% of the population has them. These neutralizing antibodies can bind to the AAV9 vector and render the therapy ineffective, so patients are screened for this immunity before treatment.
To overcome limitations and refine targeting, scientists are working on capsid engineering. This involves making small changes to the AAV9 capsid protein. The goal is to create new variants that can better evade the immune system or have an enhanced tropism for specific cell types while being detargeted from others, like the liver.