Adeno Associated Viruses also known in short as AAV are small viruses having single stranded DNA genome and belongs to parvovirus family. At specific chromosome 19 site, they can easily insert genetic material with almost 100% certainty. Using AAV does come with few disadvantages like it has low capacity of carrying DNA and finds it difficult to produce it. This virus however, is used as it is found to be non-pathogenic, with majority of the people across the globe carrying this harmless virus. Most people who are provided AAV treatment are found to not develop an immune system for eliminating this virus, including the successfully treated cells when compared to adenoviruses.

Trials & tests

Numerous trials are being conducted with AAV production and preparation, mainly with the aim to treat eye and muscle diseases, considered to be the two major tissues where this virus is found to be useful. Clinical trials also are initiated in those areas, where AAV vectors are being used for delivering the brain with the necessary genes. It is made possible, since quiescent (non-dividing) cells like neurons where there are expressed genomes for long time are likely to get infected by the AAV viruses. AAV infected cells are recognized by CD8+ immune cells as compromised ones and these cells are destroyed accordingly. Such actions are carried out by the type-2 virus outer coat or capsid.

It is on ssDNA (single stranded deoxyribonucleic acid) that the AAV vector systemis built, and is either negative or positive sensed, being around 4.7 kilobase in length. ITRs (inverted terminal repeats) are present in the genome at the DNA strand’s both ends, including two ORFs (open reading frames), cap and rep. The latter comprises of 4 overlapping genes that encodes the Rep proteins that is essential for AAV life cycle. The former comprises of capsid proteins like VP3, VP2 & VP1 in overlapping nucleotide sequences to interact together, thereby forming icosahedral symmetry capsid.

On the other hand, the ITR (Inverted Terminal Repeat) sequences have 145 bases each and named in this manner due to their symmetry, found to be essential for efficient AAV gene therapy genome multiplication. These sequences come with another property which is their capability to develop a hairpin, contributing to self priming to allow 2nd DNA strand’s primase independent synthesis. ITRs are necessary for AAV DNA integration within host cell genome as well as to rescue from the same including efficient AAV DNA encapsidation, combined with formation of fully assembled deoxyribonuclease resistant AAV particles.

ITRs with respect to gene therapy is perhaps the only sequence that is necessary in the cis, next to therapeutic gene, while packaging (rep) and structural (cap) are delivered in trans. There are established several methods with this particular assumption for efficient recombinant aav transduction vectors comprising of therapeutic or reporter gene.

For more details on Adeno Associated Viral vectors, one can log onto the leading portal https://www.genemedi.net/i/aav-packaging and get useful information about the same.