Non-Viral Vectors for Gene Transfer
Non-viral vectors were not the agents of choice for gene transfer
protocols but problems in clinical trials with recombinant viruses renewed
interest in this method of gene transfer. Non-viral vectors generally consist
of double-stranded recombinant DNA plas-mids alone or encapsulated in cationic
polymer or lipid-based formulations. Non-viral vectors offer several important
advantages over virus-based meth-ods for gene transfer (Table 5). Their
unlimited cloning capacity significantly expands the types of therapeutic
transgenes and expression cassettes that can be used for gene transfer.
Plasmids, unlike their viral counterparts, are generally non-immunogenic and
can easily be readministered multiple times without induction of a prohibitive
immune response. Non-viral vectors have a reduced capacity for inser-tional
mutagenesis and a limited ability to produce unwanted by-products in vivo due
to homologous recombination. Recombinant DNA plasmids for gene transfer are
also relatively easy to manipulate using standard techniques and do not require
specialized skills or equipment for large-scale production. They are also
inexpensive to produce, especially on a large scale in contrast to viral
vectors. Despite all the advantages that non-viral vectors have to offer, their
clinical utility is significantly hindered by low transduction efficiency,
which stems from non-specificuptake of the vector and poor delivery to the
therapeutic target. They also have a limited capacity to override cellular gene
silencing mechanisms and, as a result, cannot achieve sustained gene
expression. Refinement of delivery methods for non-viral gene transfer and
reconstruction of vectors at the molecular level have addressed these issues.
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