Adeno-associated Virus Gene Expression Vector
The adeno-associated virus (AAV) vector system is a popular and versatile tool for in vitro and in vivo gene delivery. AAV is effective in transducing many mammalian cell types, and, unlike adenovirus, has very low immunogenicity, being almost entirely nonpathogenic in vivo. This makes AAV the ideal viral vector system for many animal studies.
An AAV vector is first constructed as a plasmid in E. coli. It is then transfected into packaging cells along with helper plasmids, where the region of the vector between the two inverted terminal repeats (ITRs) is packaged into live virus. Any gene(s) placed in-between the two ITRs are introduced into target cells along with the rest of viral genome.
The wild-type AAV genome is a linear single-stranded DNA (ssDNA) with two ITRs forming a hairpin structure on each end. It is therefore also known as ssAAV. In order to express genes on ssAAV vectors in host cells, the ssDNA genome needs to first be converted to double-stranded DNA (dsDNA) through two pathways: 1) synthesis of second-strand DNA by the DNA polymerase machinery of host cells using the existing ssDNA genome as the template and the 3' ITR as the priming site; 2) formation of intermolecular dsDNA between the plus- and minus-strand ssAAV genomes. The former pathway is the dominant one.
AAV genomic DNA forms episomal concatemers in the host cell nucleus. In non-dividing cells, these concatemers can remain for the life of the host cells. In dividing cells, AAV DNA is lost through the dilution effect of cell division, because the episomal DNA does not replicate alongside host cell DNA. Random integration of AAV DNA into the host genome can occur but is extremely rare. This is desirable in many gene therapy settings where the potential oncogenic effect of vector integration can pose a significant concern.
A major practical advantage of AAV is that in most cases AAV can be handled in biosafety level 1 (BSL1) facilities. This is due to AAV being inherently replication-deficient, producing little or no inflammation, and causing no known human disease.
Many strains of AAV have been identified in nature. They are divided into different serotypes based on different antigenicity of the capsid protein on the viral surface. Different serotypes can render the virus with different tissue tropism (i.e. tissue specificity of infection). When our AAV vectors are packaged into virus, different serotypes can be conferred to the virus by using different capsid proteins for the packaging. The table below lists different AAV serotypes and their tissue tropism.
||Smooth muscle, CNS, lung, retina, pancreas, heart, liver
||Smooth muscle, CNS, liver, kidney, retina
||Smooth muscle, liver, lung
||CNS, retina, lung, kidney
||Smooth muscle, CNS, lung, retina
||Smooth muscle, heart, lung, adipose, liver
||Smooth muscle, retina, CNS, liver
||Smooth muscle, CNS, retina, liver, pancreas, heart, kidney, adipose
||Smooth muscle, lung, liver, heart, pancreas, CNS, retina, testes, kidney
||Smooth muscle, lung, liver, heart, pancreas, CNS, retina, kidney
||Liver, heart, kidney, spleen
For further information about this vector system, please refer to the papers below.
Our AAV vector system is optimized for high copy number replication in E. coli, high-titer packaging of live virus, efficient transduction of host cells, and high-level transgene expression. This viral vector can be packaged into virus using all known capsid serotypes, is capable of very high transduction efficiency, and presents low safety risk.
Safety: AAV is the safest viral vector system available. AAV is inherently replication-deficient, and is not known to cause any human diseases.
Low risk of host genome disruption: Upon transduction into host cells, AAV vectors remain as episomal DNA in the nucleus. The lack of integration into the host genome can be a desirable feature for in vivo human applications, as it reduces the risk of host genome disruption that might lead to cancer.
High viral titer: Our AAV vector can be packaged into high titer virus. When AAV virus is obtained through our virus packaging service, titer can reach >1013 genome copy per ml (GC/ml).
Broad tropism: A wide range of cell and tissue types from commonly used mammalian species such as human, mouse and rat can be readily transduced with our AAV vector when it is packaged into the appropriate serotype. But some cell types may be difficult to transduce, depending on the serotype used (see disadvantages below).
Effectiveness in vitro and in vivo: Our vector is often used to transduce cells in live animals, but it can also be used effectively in vitro.
Small cargo space: AAV has the smallest cargo capacity of any of our viral vector systems. AAV can accommodate a maximum of 4.7kb of sequence between the ITRs, which leaves ~4.2 kb cargo space for user's DNA of interest.
Difficulty transducing certain cell types: Our AAV vector system can transduce many different cell types including non-dividing cells when packaged into the appropriate serotype. However, different AAV serotypes have tropism for different cell types, and certain cell types may be hard to transduce by any serotype.
Technical complexity: The use of viral vectors requires the production of live virus in packaging cells followed by the measurement of viral titer. These procedures are technical demanding and time consuming relative to conventional plasmid transfection. These demands can be alleviated by choosing our virus packaging services when ordering your vector.
5' ITR: 5' inverted terminal repeat. In wild type virus, 5' ITR and 3' ITR are essentially identical in sequence. They reside on two ends of the viral genome pointing in opposite directions, where they serve as the origin of viral genome replication.
Promoter: The promoter that drives your gene of interest is placed here.
Kozak: Kozak consensus sequence. It is placed in front of the start codon of the ORF of interest to facilitate translation initiation in eukaryotes.
ORF: The open reading frame of your gene of interest is placed here.
SV40 late pA: Simian virus 40 late polyadenylation signal. It facilitates transcriptional termination of the upstream ORF.
3' ITR: 3' inverted terminal repeat. See description for 5’ ITR.
Ampicillin: Ampicillin resistance gene. It allows the plasmid to be maintained by ampicillin selection in E. coli.
pUC ori: pUC origin of replication. Plasmids carrying this origin exist in high copy numbers in E. coli.
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