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Adeno-Associated Virus

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The utilisation of adeno-associated viruses (AAV) has revolutionised the practice of viral vectors for gene therapy and transgenic expression due to its significant advantages as a gene delivery system. Simply put, AAV is a protein shell that surrounds a small DNA genome of around 4.7kb in length [1].

AAV Entering Host Cell

 

  1. The vector, with the transgene inside, binds to the cell membrane and continues to enter the nucleus.
  2. The vector capsid disintegrates, therefore releasing the DNA from the AAV.
  3. The DNA is then transcribed into RNA.
  4. The RNA then moves out of the nucleus, and back into the cytoplasm.
  5. The desired protein is created by translating instructions from the RNA.

AAVs do not encode a polymerase, instead, relying on a cellular preliminary system as well as the plasmid containing two inverted terminal repeats (ITRs). Promoters are specific to organisms and tissues and are required to ensure the desired gene expression [2].

There are also methods of enhancing the expression of a gene, namely cis-regulatory elements, for example, WPRE (Woodchuck Posttranscriptional Regulatory Element) and Poly A are often found in transfer plasmids, just up from the final ITR. WPRE helps transcribed RNA form a tertiary structure, as well as assisting nuclear export to the cytoplasm [3-4]. Similarly, Poly A aids export to the cytoplasm, this process is critical as can be seen in the ‘AAV Entering Host Cell’ diagram in stages 3 to 4 . If the RNA is not extracted to the cytoplasm, then the desired protein will not be synthesized, thus treatment will be unsuccessful. It additionally stabilises RNA by protecting it from degradation and aids RNA translation, both of these assist expressions of the desired gene [5-6]. Additional information on cis-regulatory elements Poly A and WPRE can be found in the list of references at the bottom of the webpage.

One vital characteristic of AAV that makes it considerably more advantageous than more traditional methods is its lack of pathogenicity. Pathogenicity itself can be defined by how much damage can be caused to host cells, and since AAVs cause negligible harm, it makes it an ideal vector. This, in turn, means that unwell patients with heavily occupied immune systems may still be able to undergo gene therapy, thus reducing the impact of symptoms.

   
AAV Serotype   
   
Tissue tropism   
   
CNS   
   
Retina   
   
Lung   
   
Liver   
   
Pancreas   
   
Kidney   
   
Heart   
   
Muscle   
   
AAV1   
   
✔   
   
✔   
       
✔   
     
✔   
   
✔   
   
AAV2   
     
   
     
   
     
   
   
   
AAV3   
     
✔   
   
✔   
   
✔   
       
✔   
 
   
AAV4   
   
   
   
   
           
   
 
   
AAV5   
   
✔   
   
✔   
   
✔   
     
✔   
     
   
AAV6   
   
   
     
   
   
   
       
   
   
   
   
AAV7   
         
✔   
         
✔   
   
AAV8   
     
   
     
   
   
   
       
   
   
AAV9   
   
✔   
     
✔   
   
✔   
       
✔   
   
✔   
   
AAV-DJ   
     
   
   
   
   
   
     
   
   
   
AAV-DJ/8   
     
✔   
     
✔   
         
✔   
   
AAV-Rh10   
   
   
     
   
   
   
       
   
   
   
   
AAV-PHP.B   
   
✔   
             
✔   
   
✔   
   
AAV8-PHP.eB   
   
   
               
   
   
AAV-PHP.S   
   
✔   
             
✔   
   
✔   

Another advantage of AAV is that it has the potential to affect both dividing and quiescent (non-dividing) meaning it impacts a wide range of cells. In addition to this, AAVs often result in long-term expression in quiescent cells as the viral DNA isn’t diluted until the cell proliferates [7].

There are multiple different strains of AAV, currently, a total of 12 have been discovered, all with very little current research or application. These variants can be manipulated in vivo to create custom vectors as well as custom-designed AAV capsids. Although, AAV 2 currently is the most studied and utilised in gene therapy due to its tropism to the most commonly desired cells. It has also proven dependable in cases of ovarian cancer treatments without any effect on healthy host cells [8].
 

2BScientific has a range of products, including a selection of AAV strains, anti-AAV antibodies and anti-AAV replicase.
2BScientific is passionate about ensuring that customers get the precise, required product and have a friendly and knowledgeable team ready to answer any technical queries.