The discovery of Adeno-Associated Virus (AAV) is yet another example of, “chance favoring the prepared mind.” In 1965, Dr. R. Wayne Atchison, was researching the viral etiology of human cancers. While checking an adenovirus strain for purity before injecting it into hamster subjects, he noticed some contaminants that consistently appeared on the electron micrograph. Since the contaminants were uniform in size and appearance, and he was able to discern an icosahedral shape, Dr. Atchison theorized that the contaminant was a virus. Further, since the virus was smaller than any previously known virus, he knew his discovery would be novel if it were a virus. Later Dr. Atchison was able to prove that he had indeed discovered a new virus.
AAV was termed the defective virus, because it was incapable of replication of either infectious or non-infectious virons without the co-infection of a helper adenovirus or herpesvirus. AAV is one of the smallest known viruses at only 18-26 nm in diameter and is a single stranded DNA virus with a genome of approximately 4.7 Kb. The genomes are unique in that 50% of the virons exhibit – sense DNA, while the other 50% exhibit + sense, it is the + sense DNA viral particles that are the infectious ones, after the + strain is incorporated into the host cell’s DNA, the – strands simply bind as complimentary DNA. Though AAV depends on a helper virus for replication, it inhibits the replication of the helper virus. For this reason AAV ends up outnumbering its helper virus.
AAV does not encode enzymes that cleave the host chromosome. Instead, it must rely on double stranded breaks (DSb’s) in the host cells DNA which have been repaired via non-homologous end-joining (NHEJ). The more damaged the host cells DNA, the more often AAV is able to integrate. This process is mediated by Ku, DNA-dependent protein kinase, a catalytic subunit (DNA-PKcs) and a ligase. However, it takes more than a DSB to integrate AAV into a host chromosome. AAV must stall the NHEJ event long enough to be integrated into the host chromosome. It does this by binding to Ku, thus inactivating it. As long as Ku is bound to AAV, it is unable to repair the chromosome, and AAV binds to both free ends of the host chromosome. It is thought that due to AAV’s propensity for NHEJ’s, AAV does not actually cause mutations, but seeks them out in order to integrate itself into the genome, and actually help repair it.
Hemophilia is just one of the diseases in which AAV gene therapy is proposed to help. Because hemophilia is the result of a single gene mutation, if that gene could be replaced with a fully functioning copy, or if the gene could be repaired, the individual would no longer have hemophilia. Replacement genes have already been designed, and the vectors are now in clinical trials with lab animals.5 One draw back to this type of gene therapy is that the virus will randomly insert itself at any available DBS, which may in turn activate some unwanted peripheral genes such as oncogenes. This may be overcome by careful selection of promoters, and use of an insulator to allow more space between the AAV and the chromosomal gene.
References
Atchison DW; “Discovery of Adenovirus-Associated Virus” Infectious Disease and Microbiology; Graduate School of Public Health, University of Pittsburgh Vol. 1 No. 2 Dec 1998
Buchen-Osmond, C. “50.1.3.0.001 Adeno-associated virus 1” International Committee on Taxonomy of Viruses. 15 June 2004. 02 March 2006. <http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/50130001.htm>
Russell DW. “AAV loves an active genome.” News and Views. Nature, Genetics. Vol. 34, 241-242 (2003)
“Researching New Hemophilia Treatments.” Causes of Hemophilia. 29 Sep 2005. NCERx Inc. 03 Mar 2006. <http://www.causes-of-hemophilia.com/html/hemophilia-research.php3>
*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.
Return to Missouri S&T Microbiology HomePage Go to DJW's HomePage
This Document is maintained by djwesten@ mst.edu
