Biomedical Engineering Reference
In-Depth Information
receptors (Rosen 2009 ), intracellular delivery and trafficking is not a challenge in
the case of growth factor delivery, as long as the proteins are freely available in the
extracellular matrix to the target cells.
Gene delivery provides a potential alternative to address some of the shortfalls
of protein delivery. Compared to bolus protein delivery, gene delivery scaffolds can
be designed in such a way that DNA released over several weeks can to be taken
up by cells gradually (Shea et al. 1999 ). The burst release kinetics associated with
a bolus protein injection are also absent in a gene delivery system. The kinetics of
growth factor expression after gene delivery may better mimic natural growth fac-
tor production during fracture healing, since a relatively constant amount of pro-
tein is expected to be produced by the cells and the level of the produced protein
could be lower. Since in situ expressed growth factor concentrations are closer to
physiological levels, there may potentially be less bone ectopic to the site that must
be remodelled. Similar to protein delivery, however, gene delivery is not without
its challenges. Unlike protein therapies, the therapeutic genes not only need to
enter the cell but also undergo the appropriate intracellular trafficking and must be
delivered to the nucleus for transcription. A carrier usually facilitates cellular
uptake of the protein-coding expression vector, and forms a complex with the DNA
containing the gene of interest, or is an attenuated virus harbouring the gene of
interest. Viruses are generally considered the most effective agent for gene expres-
sion but their safety profile is worrisome. The risks of inflammation, immune
response and insertion mutagenesis make the use of viruses justifiable in only life-
threatening diseases (such as cancer) and not in wide-spread clinical applications.
However, their high transfection efficiency offers insight to the barriers facing non-
viral delivery.
3.2
Molecular Features Making Viral Gene Delivery Effective
Fostered by millions of years of evolution, viruses are masters of intracellular
delivery of nucleic acid. Their extraordinary efficiency permits effective gene deliv-
ery to a multitude of cell types while providing an effective model of gene delivery
for emulation with non-viral carriers. The general paradigm for engineering viruses
into useful experimental reagents and therapeutics requires disabling their replica-
tion and pathogenicity while forcing them to express a desired genetic material.
The usual method for this involves separating the viral genome into two (or more)
DNA molecules (Verma and Weitzman 2005 ); one ( the vector construct ) that car-
ries the desired foreign sequences and any cis -acting viral elements necessary for
transcription and packaging the foreign DNA into viral particles, and the other ( the
packaging construct , which is either a plasmid or a genomically-inserted cassette
of a packaging cell line) that encodes viral structural proteins and any other proteins
essential for the production of viral particles. Genes that are deemed non-essential
or associated with pathogenicity are excised to maximize the space available for
foreign sequences, as well as to prevent deleterious effects on infected cells in the
Search WWH ::




Custom Search