DOI: 10.1097/01.BRS.0000042244.63751.D1
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PMID: 12567019
Issn Print: 0362-2436
Publication Date: 2003/02/01
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Excerpt
However, there are major scientific and technical questions and related problems with respect to the approaches used in gene therapy:
The use of nonviral vectors, such as liposomes or a “gene gun” approach, are far less effective than the use of viral vectors to transduce cells. In addition to low cellular uptake, currently available nonviral DNA delivery systems are unable to maintain prolonged gene expression.
Retroviral vectors (small RNA viruses) with the inserted gene, once inside the cell, are reverse-transcribed into double-stranded DNA, which then is randomly integrated into the genome of transduced cells. However, this integration may occur only during cell division; thus, only highly proliferative cells can be transduced with retroviral vectors using an ex vivo approach. Unfortunately, there is a little control over subsequent cell division or the viral gene expression once the transduced cells are implanted back into the autologous host.
In contrast to retroviruses, human adenoviruses, herpes simplex virus, and adeno-associated viruses can transduce both proliferating and nonproliferating cells. Therefore, these delivery systems can be used for either ex vivo or in vivo transduction of cells. However, except for the adeno-associated viruses, these viral vectors are not integrated into the genome, having a limited expression period, and practically all these viral vectors provoke the host’s immune system. The antiviral immune response, especially the production of virus-neutralizing antibodies, is the most frequent problem of adenovirus-based gene therapy. 4,5
Nevertheless, retroviral, adenoviral, and adeno-associated viral vectors are widely used as delivery systems in musculoskeletal research. 3,5,6 The reality is that despite successful experimental results, only a few successful clinical trials have been reported. The major drawback is our limited understanding as to how a selected gene (encoding, e.g., a growth factor, a cytokine, or an osteoinductive protein) is regulated. For example, while the proteins bind their specific and unique receptors, the ligand–receptor interaction may trigger a diverse signaling cascade and induce up-regulation or down-regulation of other genes. Similarly, a transcription factor may provoke a divergence in the regulation of multiple genes, and reciprocally, several transcription factors can modulate the expression of a single gene. 8 Although we know that some growth factors, cytokine inhibitors, or bone morphogenic proteins show efficacy in the treatment of musculoskeletal disorders, 3,7 introduction of their genes by viral delivery is still risky, and the in vivo expression of the gene (protein) cannot be controlled.
The article by Kim et al targets critical questions in gene therapy using human adenovectors: 1) how can an existing immune response, because of previous injection, or a “natural” adenovirus infection in humans, jeopardize the success of a subsequent gene therapy; and 2) how can this problem be eliminated? Several articles have been published largely by the same authors using human adenovirus (Ad5; retro-type 5, which lacks the Ea and E3 regions, and thus has reduced immunogenicity) for delivery of LIM mineralization protein-1 (LMP-1), a molecule involved in osteoblast differentiation.
