The Adeno-associated virus (AAV) is an extremely simple organism. First off, it consists of only two genes located on a single-stranded DNA chain tucked inside its capsid. The virus is an icosahedral, which is one of the easier shapes for a virus to be. It is only 20-25 nm in diameter (nm, not ?m), so it is extremely tiny.
AAV is classified as a parvoviridae, but can be further defined as a dependovirinae. Basically, AAV can not cause a cell to replicate on its own, like other viruses. Instead, it needs either an adenovirus or a herpesvirus infecting the same cell in order to replicate. The co-infection by these helper viruses will trigger the lytic cycle. If these cells are not present, the AAV will simply integrate itself into the host’s genome. AAV is present is approximately 90% of the human population. However, on the bright side, AAV is non-pathogenic, and the majority of the population has antibodies which can quickly neutralize the virus.
So, if AAV is so small and simple, what’s so special about it? AAV is used as a viral delivery vehicle for gene therapy. Since about 90% of the population has antibodies against this virus, scientists must modify the virus so that it can bypass the antibodies. A modified, non-replicating virus is also known as a “gutless virus.” Other viruses, such as the adenovirus, can also be used as a delivery vehicle for gene therapy – but there are several advantages for using the AAV instead. AAV does not stimulate inflammation in the host, nor does it elicit antibodies against itself. Also, AAV can enter non-dividing cells and integrate itself into one specific spot in the genome of its host (chromosome 19 in humans).
However, since AAV is so small (can only hold 4.7kb), in order for the gene therapy to be successful, two AAV vectors must be present simultaneously. The first vector must carry the protein to activate gene transcription (but can not bind to DNA itself), a protein that binds to the DNA sequence in the promoter of the gene of interest, and also promoters that allow continuous expression of the two genes. The second vector must carry the identical promoters of the gene of interest, and the actual gene of interest itself.
Using AAV as a viral delivery vehicle has had impressive lab results so far. Insulin-Dependent Diabetes Mellitus and Hemophilia B have both been cured in mice using this method. Treatment for ALS in mice has also shown great promise. The AAV-2 serotype has also been shown to kill cancer cells without harming the healthy ones (in mice). AAV has also been used to increase levels of erythropoietin (EPO) within mice and monkeys.
References
http://www.stanford.edu/group/virus/parvo/2004oleary/
http://medicine.ucsd.edu/gt/AAV.html
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/G/GeneTherapy2.html
Fradkin, Lee G. (et. al). Principles of Tissue Engineering (2nd Edition). Gene-Based Therapeutics. 2000. (pgs. 385-405).
Dr. Roger Brown. Tissue Engineering. Class Notes.
*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.
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