Since it’s discovery adeno-associated virus has helped move science and research forward
due to it’s excellence as a gene delivery system.
This small virus has revolutionized the usage of viral vectors for gene therapy and transgenic
expression with its many features that make it a suitable candidate for these tasks.
Although it's usage as a viral vector is still relatively new compared to other tried methods,
AAV features many significant advantages that other vectors lack.
Adeno-Associated virus possesses a variety of features that make it a truly remarkable
gene delivery system.
AAV has a linear single strand DNA genome of approximately 4.7 kb in length, with two
145 nucleotide - long inverted terminal repeats.
The AAV genome does not encode a polymerase and instead relies on cellular polymerases
for its replication.
It's entire genome only encodes viral replication and capsid genes.
These two genes encode all non-structural and structural proteins, for replication regulation
and capsid structure respectively.
The capsid proteins of the AAV assemble into a near-spherical protein shell of 60 subunits
AAV was first discovered in 1965 as a contaminant of adenovirus (Ad) preparations, which is
subsequently how it got its name.
At a size of approximately 22 nm, the AAV is one of the smallest non-enveloped capsid
viruses discovered.
AAV is also exceptional for its very low pathogenicity in comparison to its near universal distribution
in human populations.
This virtually non-existent pathogenicity of AAV can be attributed to its inability
to replicate on its own - it requires a co-infector to be able to replicate and cause a productive
infection in the body.
Even then at worst the adeno-associated virus causes a very mild immune response - making
it a great candidate for gene therapy due to the low damage it can potentially cause.
When AAV infects a human cell alone, it's gene expression program is auto-repressed
and latency is ensued by integration of the virus into a 2kb region of human chromosome
19 called AAVS1.
This allows the virus to stay in a lysogenic or "dormant" state in the cell or infected
tissue until a helper virus such as adenovirus is brought into the infected cells.
Once a helper virus is introduced AAV can enter a lytic cycle and replicate along with
the adenovirus.
E1a, E1b55k, E2a, E4orf6 and Viral Associated genes from the adenovirus are the known “helper”
genes required for AAV replication.
Although almost all AAV serotypes can easily infect almost every kind of cell in the body,
AAV is unique from other viral vectors due to it's natural tropism toward specific cell
types in the body.
Tropism in the case of AAV means it's natural preference to infect and thrive in specific
tissue types, and this specificity is determined by the capsid serotype.
AAV serotype 2 has been the most extensively studied and presents a natural tropism toward
skeletal muscles, neurons, vascular smooth muscle cells, and hepatocytes.
Other AAV serotypes have been studied as well, and found themselves to be excellent gene
vectors for various other tissue types in the body including: AAV1 has been found to
excel in gene delivery to vascular-endothelial cells, as well as the retina, heart and lung.
AAV5 also exhibits a tropism toward vascular endothelial cells but more importantly is
efficient in transducing astrocytes.
AAV6 has been found to be excellent in transducing airway epithelial cells, as well as hepatocytes.
AAV7 is has been found to be excellent in transducing murine smooth muscle cells, just
like AAV1 and AAV5.
AAV8 and AAV3 both show a natural tropism toward hepatocytes and are excellent in transducing
them.
AAV4 has shown tropism toward both kidney and heart cells.
Serotypes 1 through 9 are available at abm and can be ordered for lab use.
As was mentioned before, Adeno-Associated Virus is a very promising viral vector, and
an important candidate for highly efficient gene editing.
The advantages to using adeno-associated virus over other viral vectors is: AAV infects both
dividing and quiescent cells, which allows genetic material to be delivered to a highly
diverse range of cell types.
Modern day AAV can persist in cells in an extra-chromosomal form making it an attractive
tool for gene therapy.
AAV can cause long term expression in non-dividing cells, as AAV is not diluted until a host
cell divides.
AAV has negligible pathogenecity and induces very mild immune response.
AAV vectors have been used in over 117 clinical trials worldwide to date, including a number
of promising results in trials for Leber's Congenital Amaurosis, Hemophilia, congestive
heart failure, lipoprotein lipase deficiency, and Parkinson's disease Although it does present
many advantages, use of the virus is not without some disadvantages.
It's major drawback is its very small genome size; in comparison to Adenovirus AAV is 5
to 10x smaller in size.
This greatly restricts the size of the gene that can be inserted into an AAV viral vector,
as the transgene itself can only be less that 4.5 kb in length.
abm offers a wide range of AAV products, and services which can meet a broad range of experimental
requirements.
Abm offers libraries of open reading frame (ORF) in human, mouse and rat genes, with
over 30,000 individual ORF products, which are available in both AAV viral vector or
as packaged virus.
abm also offers siRNA and microRNA for gene knowckdown appliactions.
These products are also available in both packaged viruses or AAV viral vectors.
Our AAV products come with a wide range of customization options: (1) promoters including
CMV, PGK, EF1a. (2) reporter choice including luciferase or GFP, (3) AAV 1-11 serotype options
, and (4) custom AAV titrations.
In addition abm can also perform AAV custom services, which include Custom AAV cloning,
and packaging services.
abm also offers a Custom AAV Titration - qPCR Assay, which include master mixes, reagent
mixes and all other necessary components to perform AAV titration analysis.