AAV or Adeno Associated Virus is a virus that is not currently known to cause disease. The virus’ lack of infection causing ability is further strengthened by the fact that only a mild immune response is often seen associated with the virus. The virus is 20nm in size and is a non-enveloped replication defective virus.
Adeno Associated Virus infects certain primate species and humans. AAV can be used with gene therapy vectors to infect both quiescent and dividing cells. It does not integrate into the host cell’s genome and manages to persist in the extra-chromosomal state. However, some of the virally carried genes have been noted to become integrated in the host genome in AAV native virus. Because of these features, AAV is extremely useful in the creation of gene therapy viral vectors. It is also useful in creating isogenic (in the lab) human disease models.
A study of AAV’s DNA shows that it might have some role in promoting infertility in human males because it is commonly found in abnormal semen samples. However, there is no causal link to this role at this time. Apart from this, the pathology of AAV shows nothing. Gene therapists have been studying wild types of Adeno Associated Virus because of its lack of infection causing ability (pathogenicity). It is considerably more predictable than various other retroviruses because of the virus’ ability to integrate in human chromosome 19’s host cell genomes and infect non-dividing cells. Random genome incorporations with AAV are rarely seen. AAV’s immunogenicity is pretty low as well, with no defined cytotoxic response and only some neutralizing antibodies have been noticed.
The major disadvantages of AAV include the limited cloning capacity and unsuitability of large genes in standard vectors. However, various options are being explored and research is being conducted to overcome this limitation. Altered AAV version, or scAAV, has been created by gene therapists by packaging two shorter but complimentary strands instead of a single AAV strand, thereby leading to quicker expression. However, scAAV has half the coding capability of AAV and is more immunogenic.
Throughout the world, 117 clinical trials have been conducted by using AAV vectors. Positive results have been seen in relation to Parkinson’s disease, heart failure (congestive), Chylomicronemia, and Hemophilia. Three different trials have also shown a promising result of AAV’s safety, treatment and efficacy against LCA. Various other selected trials have ended or are ongoing. In prostate cancer, treatment trials are at Phase III currently but there has been no direct AAV administration to patients yet. They are all ex vivo, thus far. Trials for Batten’s, Canavan’s, muscular dystrophy and arthritis are still ongoing and are at Stage I.
Structure and Serotypes
The AAV genome’s structure is built of negative or positive sensed 4.7k long ssDNA. Both the ends of the strand of the DNA of the genome have ITRs and ORFs (cap and rep). Each of the ITR sequences has 145 bases and they have a certain amount of symmetry, allowing them to form hairpin structures. The genome’s left has p5 and p19 promoters from which two different length mRNAs are produced. These can be spliced to synthesize Rep proteins Rep78, 68, 52 and 40. The Cap proteins VP1, 2, and 3, are found on the right with one (p40) promoter.
A total of 11 serotypes for AAV have been described which can infect anything from cells to tissue types. The most extensively examined serotype is Serotype 2 which has presented tropism towards hepatocytes, cells of vascular smooth muscle, neurons and skeletal muscles. According to a study, Serotype 2 (AAV2) has shown positive results against cancer cells. It kills off the cancerous cells and does not harm healthy cells. Other serotypes have also shown some positive results. For example, AAV6 shows promise in infecting epithelial cells of the airway and AAV8 causes transduction in hepatocytes. Neural tropism has been presented by AAV serotypes in the brain. It has been observed that the receptors which bind the AAV serotypes have a significant influence.