Adeno Associated Virus (AAV) is a small parvovirus with a 4.7 kilobase single stranded DNA genome. As a method for delivering good genes into humans with defective genes, AAV looks like the next great hops for gene therapy.

More than 85% of adult humans are have AAV floating around in their system and there is no specific pathology for AAV in humans. So, nature has already conducted a safety study of AAV and determined it to be safe and nontoxic, unlike members of the Adenovirus or Retrovirus families.

AAV infects a wide variety of cells, including dividing and nondividing cells, so widespread delivery of a healthy gene isn't as much a problem as with other vectors. Once genetically altered, the vector will contain no viral DNA and only human DNA. Also once the DNA is integrated into the q arm of Chromosome 19, good levels of gene expression can be achieved.

But, AAV vectors have several important limitations. Genes greater than 4.5 kilobases cannot be inserted because of AAV's small size. This limits the application of AAV in treating diseases, but there are lots of genetic diseases whose genes are small enough to be packaged in AAV, such as Haemophilia B and Hurler's disease.

Multiple helper functions from adenoviruses are also required for replication. And, most importantly, AAV is one of the most difficult vectors to purify and manufacture at high titers. It is this hurdle that keeps more companies from using AAV as a vector for gene therapy. Two companies that have cleared this manufacturing hurdle include Cell Genesys and Avigen.

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