Viral Vectors: Adenovirus

ADENOVIRAL VECTORS ARE derived from the adenovirus family and are used in gene therapy as tools for delivering genetic material. Gene therapy often targets stem cells because of their self—renewing properties, which could increase procedural efficiency by eliminating the necessity of readmin—istering therapeutic genes. Viral vectors such as adenovirus provide a way to deliver genetic material to these cells, thus allowing researchers and physicians to engineer cells that express selected therapeutic characteristics. Although adenovi—ruses are unable to integrate into host cell chromosomes, these viruses can infect both dividing and nondividing stem cells.

Adenoviruses are a viral family known for infecting membranes of the respiratory, intestinal, and urinary tract, in addition to membranes of the eye. Disease is relatively mild, and symptoms vary, but it can include respiratory disease, gastroenteritis, and urinary tract and eye infections. Among viruses, adenoviruses have a simple genetic composition and consist of linear, double—stranded DNA.

[Page 609]As viral vectors, adenoviruses offer several advantages. The engineered vectors can transduce, or deliver genetic material to, a broad range of cell types, including dividing and nondividing cells. This characteristic is important because stem cells are often quiescent, or nondividing, for long periods of time until activation. Adenoviral vectors therefore provide a way to deliver therapeutic genes and engineer cells capable of proliferating and possibly repairing damaged tissues. Because adenoviruses cause relatively mild disease, derived vectors are fairly safe, even should the vector revert to a replication—competent form. The vectors also produce high titers, so low volumes yield high expression in targeted host cells.

The major flaw of adenoviruses is their inability to integrate into the host cell chromosome. This characteristic limits the role of adenoviral vectors in stem cell research, as expression in dividing cells is progressively lost. In this way, lentivirus, which integrates and continuously expresses transduced genes, derives more efficient vectors.

Adenoviruses are known to trigger strong cellular and humoral immune responses. The inflammatory responses produced by cytokines, lymphocytes, and antibodies can inhibit transgene expression, thus rendering the vector ineffective. Methods potentially combating inflammatory effects include engineering vectors to remove immunogenic capacity and administering immu—nosuppressive drugs. Because adenoviruses are prevalent in the environment, past exposure, which is relatively common, helps reduce these inflammatory responses.

As is often the case with viral vectors, certain health risks are associated with use of adenoviral vectors in clinical trials. In 1999, the death of a teenager involved in a trial at the University of Pennsylvania prompted safety reviews of adenoviruses as gene therapy vectors.

The patient, one of 19, received high doses of genetically modified adenovirus vectors as a therapy for ornithine transcarbamylase deficiency, a condition impairing the body's ability to metabolize nitrogen. Practically all patients in the study experienced flu—like symptoms, but the teenager died from systemic shock, the result of an adenovirus—induced immune response.

Adenoviruses are made replication incompetent through engineering that removes key genetic sequences. One technique involves inactivating the El region, which controls genes regulating cell growth. A genetically engineered adenoviral vector could, however, potentially revert to a replication—competent form. This might happen if a vector recombines, or receives a piece of DNA, from a wild—type adenovirus infecting the vector recipient when gene therapy is administered. This event could generate a replication—competent adenovirus capable of causing disease. However, although some viral vectors, such as lentivirus—a group of diseases including human immunodeficiency disorder—can cause fatal disease, symptoms of adenoviral infection are comparatively mild.

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