Viral Vectors: Lentivirus

LENTIVIRAL VECTORS ARE derived from a genus of retroviruses and used as a tool in gene therapy for administering genetic material. Gene therapy frequently uses stem cells because of their self—renewing properties, which allow one—time, as opposed to repeated, delivery of therapeutic genes. Len—tiviruses are unique for their ability to integrate into the genome of both dividing and nondividing cells. Because stem cells are often quiescent, or nondividing, for long periods until activation, len—tiviral vectors provide a way to deliver therapeutic genes and engineer cells capable of proliferating and repairing damaged cells comprising tissues and organs. Although lentivirus offers advantages over other viral vectors, its use in therapy can present potential risks.

Lentiviruses are characterized by persistent infection, and viruses proliferate within host cells undetected for extended periods of time. Like other retroviruses, lentiviruses are composed of RNA and, on infecting a host, are reverse—transcribed into DNA that is then integrated into the host chromosome and passed to subsequent host cell progeny. These qualities—the ability to transfer genetic material to, or transduce, various cell types and the ability to integrate and continuously express transduced genes—make retroviruses desirable viral vectors.

Lentiviruses became widely used as vectors beginning in the late 1990s. Before this time, therapeutic vectors were typically derived from Moloney murine leukemia virus (MMLV), a ret—rovirus, or adenoviruses—a family of viruses that cause respiratory tract infections and gastroenteritis. These two vectors had shortcomings: Because adenoviruses are unable to integrate into host cell chromosomes, expression is only short term; although MMLV is capable of integration, the virus must take advantage of the nuclear membrane breakdown that occurs during division to associate with host cell chromosomes, and consequently, it only infects dividing cells. Lentiviruses, however, can integrate into nondividing cells and cells that divide slowly. This trait is a result of a preintegration complex consisting of RNA, proteins, and enzymes including integrase, which allows lentiviruses to transport genetic material through intact nuclear pores.

By far the most common lentiviral vector is human immunodeficiency virus (HIV). The first use of a lentiviral vector in clinical trials occurred in 2003 after the IRxSYS Corporation, a private biotechnology company, received approval from the U.S. Food and Drug Administration to initiate phase 1 clinical trials treating HIV and AIDS. Aside from HIV, vectors have also been derived from a multitude of other lentiviruses, including feline immunodeficiency virus, Jembrana disease virus, equine infectious anemia virus, simian immunodeficiency virus, bovine immunodeficiency virus, caprine arthritis—encephalitis virus, and visna virus.

Although retroviral vectors provide useful tools for gene therapy, they also pose potential health risks. In 2003, the U.S. Food and Drug Administration temporarily suspended all active gene therapy trials using retroviral vectors to deliver genetic material to hematopoietic stem cells. The decision was made after several children developed conditions resembling leukemia following a French gene therapy trial treating X—linked severe combined immunodeficiency disease. The ban lasted from January to April. The French trial used MMLV, an oncoretrovirus, as opposed to a lentivirus. Oncoret—roviruses insert randomly and can integrate near oncogenes, which regulate cell growth. Subsequent uncontrolled growth potentially leads to cancer. The French trials raised concern that lentiviruses and [Page 611]other retroviruses used as vectors could randomly insert and activate oncogenes in the same way.

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