Retroviruses gene therapy vectors

To circumvent this problem, vectors that are based on lentiviruses have been developed. In contrast to prototypic retroviruses, lentiviruses do not require cell division for integration. Gene-therapy vectors have been developed from a broad spectrum of lentiviruses including human immunodeficiency vims (HIV), simian and feline immunodeficiency vims as well as visna/maedi vims. The most widely used lentiviral vector system is based on HIV-1. These vectors can efficiently transduce a broad spectrum of dividing and nondividing cells including neurons, hepatocytes, muscle cells, and hematopoietic stem cells [1,2]. 

PER.C6 cells have been used extensively for the production of gene therapy vectors. This cell line was derived from the immortalization of human embryonic retina cells through the use of adenovirus El gene (Fallaux et al., 1998). This cell line has been well characterized since its establishment, and no retroviruses or adventitious viruses have been detected in it. This cell line is easily adapted to different growth conditions and stably produces high levels of recombinant proteins. 

Since the OTC trial, there has been rapid advancement in vector development with marked improvements in the safety (and efficacy) of new viral delivery systems. There is also a better understanding of the immunological responses to gene therapy vectors. The adverse events in the SCID trial are more recent. They appear to be a likely result of the choice of vector (retrovirus) and the ex vivo selection strategy used to modify the affected stem cells of the SCID patients. The risk benefit ratio for gene therapy treatment of these children is still deemed favorable in light of the fact that without treatment, they would not be alive. 

Retrovirus- and lentivims-detived vectors are used in approximately one quarter of all gene-therapy trials (Table 1). The gene-therapy trial in patients suffering... 

Some 24 per cent of all gene therapy clinical trials undertaken to date have employed retroviral vectors as gene delivery systems. Retroviruses are enveloped viruses. Their genome consists of ssRNA of approximately 5-8 kb. Upon entry into sensitive cells, the viral RNA is reverse transcribed and eventually yields double-stranded DNA. This subsequently integrates into the host cell genome (Box 14.1). The basic retroviral genome contains a minimum of three structural genes ... 

The ability of such retroviruses to (a) effectively enter various cell types and (b) integrate their genome into the host cell genome in a stable, long-term fashion, made them obvious potential vectors for gene therapy. 

Retroviruses display a number of properties/characteristics that influence their potential as vectors in gene therapy protocols. These may be summarized as follows ... 

Gene Therapies. The types of vectors that have been used or proposed for gene transduction include retrovirus, adenovirus, adeno-associated viruses, other viruses (e.g., herpes, vaccinia, etc.), and plasmid DNA. Methods for gene introduction include ex vivo replacement, drug delivery, marker studies, and others and in vivo, viral vectors, plasmid vectors, and vector producer cells. 

There is a wide variety of vectors used to deliver DNA or oligonucleotides into mammalian cells, either in vitro or in vivo. The most common vector systems are based on viral [retroviruses (9, 10), adeno-associated virus (AAV) (11), adenovirus (12, 13), herpes simplex virus (HSV) (14)] andnonviral [cationic liposomes (15,16), polymers and receptor-mediated polylysine-DNA] complexes (17). Other viral vectors that are currently under development are based on lentiviruses (18), human cytomegalovirus (CMV) (19), Epstein-Barr virus (EBV) (20), poxviruses (21), negative-strand RNA viruses (influenza virus), alphaviruses and herpesvirus saimiri (22). Also a hybrid adenoviral/retroviral vector has successfully been used for in vivo gene transduction (23). A simplified schematic representation of basic human gene therapy methods is described in Figure 13.1. 

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