2 Retroviral Vectors

Abstract

The combination of the advancements in the knowledge of gene regulation and improvements in gene delivery into cells has resulted in significant progress in the field of human gene therapy during the past 15years. In the United States alone, there were more than 50 gene therapy protocols approved by the National Institutes of Health (NIH) during 1997. The gene transfer vectors involved in these clinical trials include vectors derived from retroviruses, plasmids, adenoviruses, pox-viruses, and adeno-associated viruses (AAV). Because of their many unique properties, vectors derived from retroviruses, especially from the Moloney murine leukemia virus (Mo-MLV), have been used extensively in a majority of clinical trials for somatic gene therapy (Miller 1992; Mulligan 1993; Crystal 1995). Each retroviral particle consists of an envelope on the outside, which is derived from the host plasma membrane during virus budding, and two single-stranded RNAs encapsidated in a nucleocapsid structure. Interaction between the virus-encoded envelope protein, which is embedded in the virion envelope, with cell surface receptors allows the virus to enter into the infected cells. On infection, both RNAs serve as templates to generate one copy of a linear double-stranded DNA in the cytoplasm via reverse transcription using virion-associated reverse transcriptase. The viral genomic DNA migrates into the nucleus and integrates into the host chromosome. The integration step is mediated by the virion-associated integrase and is a highly efficient and specific reaction that results in the generation of a proviral DNA that is colinear with its unintegrated form. The unique properties that make...