Viral Vectors: Adeno—Associated Viruses

ADENO—ASSOCIATED VIRAL VECTORS are derived from parvoviruses and used as a tool for delivering genetic material in gene therapy. Such therapy frequently uses stem cells because of their self—renewing properties, which eliminate the need for repeated delivery of therapeutic genes. Adeno—associated virus (AAV) causes no known illness and triggers only mild immune responses. The virus also infects both dividing and nondividing cells. Because stem cells are often quiescent, or nondividing, for long periods until activation, these vectors could provide a method for delivering therapeutic genes to engineer cells capable of proliferation and repairing damaged tissues and organs. AAV can also integrate into the genome of targeted cells, thus leading to long—term expression. However, the amount of integration is often small, which means the use of lentivirus may be more appropriate if continual growth of the stem cells is required. Preparing large quantities of the vector is difficult, however, and limits the effectiveness of the vector in gene therapy. Some safety risks exist as well.

AAV belongs to the parvovirus family—small viruses with single—stranded DNA genomes. To establish productive infections producing new viral particles, AAV requires the presence of a helper virus to stimulate target cell division, as replication of the AAV genome is dependent on actively dividing cells. The first discovered helper virus was adenovirus, hence the name adeno—associated virus, although herpes virus was later found to associate with AAV in the same manner. Coinfection with helper viruses allows AAV to replicate episomally, or independent of integration into the host chromosome, and produce viral proteins. In the absence of a helper virus, AAV typically integrates into human chromosome 19. The virus then remains latent until the host cell is infected with a helper virus. When this coinfection occurs, the AAV genome replicates and establishes a productive infection that results in viral shedding.

AAV genetic integration is stable, as the virus typically integrates near chromosome 19 in humans. This stability gives AAV an advantage over other vectors derived from retroviruses, for instance. With many retroviral vectors, integration is unpredictable. In rare cases in which vectors integrate near oncogenes—genes that control cell development—transcription is interrupted and the result is uncontrolled growth, potentially leading to cancer. The stable integration tendencies characteristic of AAV vectors eliminate this hazard.

Another advantage is that AAV elicits relatively mild responses in humans. No known illness is [Page 608]associated with the virus, which is ubiquitous in the environment. This makes AAV a fairly safe therapy tool, should the engineered vector resort back to a replication—competent form. Were a len—tiviral vector, such as that derived from human immunodeficiency virus, to resort back to a replication—competent form in a vector recipient, the result would be fatal disease.

Furthermore, unlike adenovirus, AAV is only weakly immunogenic. The virus does not stimulate strong inflammatory responses from the host immune system and elicits only low levels of antibodies.

AAV as a gene therapy vector has been associated with one fatality. In July 2007, a patient in a trial treating inflammatory arthritis died after receiving a gene therapy product delivered using AAV—derived vector. The vector was carrying a gene for tumor necrosis factor receptor to reduce inflammation and was injected into the patient's joint. The patient was one of roughly 100 subjects enrolled in the trial and was the only individual who experienced serious side effects.

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