Gene therapy retroviruses

X CREA", CRIP Adenovirus transformed HEK Mouse NIH3/MoIoney leukemia Epithelial Spherical Gene therapy Retrovirus packaging... 

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]. 

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... 

Bai J, Rossi J, Akkina R (2001) Multivalent anti-CCR ribozymes for stem cell-based HIV type 1 gene therapy. AIDS Res Hum Retroviruses 17 385-399 Bai J, Sui J, Zhu RY, TaUarico AS, Gennari F, Zhang D, Marasco WA (2003) Inhibition of Tat-mediated transactivation and HIV-1 replication by human anti-hCychnTl intrabodies. J Biol Chem 278 1433-1442... 

Morgan RA, Baler-BitterUch G, Ragheb JA, Wong-Staal E, GaUo RC, Anderson WE (1994) Further evaluation of soluble CD4 as an anti-HIV type 1 gene therapy demonstration of protection of primary human peripheral blood lymphocytes from infection by HIV type 1, AIDS Res Hum Retroviruses 10 1507-1515... 

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. 

Gene therapy offers another potential avenue to fix the defective gene. The therapy itself is by no means straightforward. In a French gene therapy trial, three boys with SCID were treated using retrovirus-based gene therapy. They later developed cancer and one died of leukemia. Reviews showed that the retrovirus inserted near oncogenes and promoted development of cancer. 

Early gene therapy protocols were principally of the ex vivo type and relied on recombinant retroviruses as gene transfer vehicles. Retroviruses can accomplish effective gene transfer to target cells despite being rendered replication-incompetent by genomic deletions. Flowever, a variety of limitations have restricted their use to achieve in vivo gene transfer. 

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. 

Retrovirus vectors were subjected to the first clinical trial on human gene therapy to correct adenosine deaminase (ADA) deficiency (32). White-blood cells isolated from patients were infected ex vivo with an MLV-based vector expressing ADA and a neomycin marker gene. After selection with G418, neomycin-resistant cells were isolated and reintroduced into patients. The treatment improved the physical condition of the patients and the ADA-containing provirus was stable in the blood for several years. 

Actions Adenosine deaminase stimulator, genetically engineered autologous cell therapy, immunomodulator, retrovirus-based gene therapy... 

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