Biomedical Engineering Reference
In-Depth Information
transcription factors, essential for DNA transcription. In addition, the enhancers
and the silencer elements in the DNA play important roles in controlling the DNA
expression level and specificity [187,188] .
Controlling these regions to control the transgene expression level is an impor-
tant obstacle to the gene delivery and must be overcome by using proper selection of
these elements.
The mRNA formed is exported to the cytoplasm and is then translated to the
desired therapeutic protein, using the cellular ribosome assembly. However, it is
necessary to control translation rate as well as transcription control to regulate the
gene expression [189,190] . For desired gene therapy, it is necessary to regulate the
expression of transgenes at an optimal level, keeping in mind the possibility of post-
translational modifications in proteins. To achieve this, the mRNA transcription pro-
cess should be controlled with proper promoters, because mRNA stability as well as
translation efficiency are regulated ultimately through the promoters by controlling
the end groups of mRNA [45] .
2.5 Biological and Immune Response as Barriers
(Especially for Viral Vectors)
In addition to extracellular and intracellular barriers to gene delivery, the biologi-
cal immune barriers are also observed in application of gene delivery. These barri-
ers are characteristically observed in viral gene delivery vectors. However, immune
responses to the DNA sequences with central unmethylated CpG motif and to the
vectors have also been observed [37-39] . These immunostimulatory (i.s.) DNA
sequences containing unmethylated CpG motifs stimulate the antigen-presenting
cells (APC) to express a full complement of costimulatory molecules and produces
cytokines including interleukin (IL)-12 and tumor necrosis factor (TNF)-. Although
IL-12 is key to their T helper cell (Th)1-promoting strong immune responses in
mammals, secretion of toxic levels of TNF- is harmful in that it promotes toxic
shock [190,191] . However, this barrier, because of the plasmid sequence immuno-
genicity, may be resolved by removing the sequence of TNF-, a toxic chemical
[190,191] .
Immune response to the DNA vectors is also an important barrier, especially
limiting the viral gene delivery. The immunogenicity of the viral vectors poses
one of the main obstacles to successful human gene-replacement therapy, unless
gene transfer vectors are applied to immune-privileged sites. Both innate and adap-
tive immunity work in concert against sustained gene transfer, but the functions
of patients' regulatory T cells (Tregs) and tolerogenic dendritic cells (DCs) could
potentially be harnessed to reduce these immune responses. The vectors can now
be suitably modified so as to reduce the risk of mutagenecity and carcinogenic-
ity during transgene expression and cell division, but still keep long-term gene
expression. Over the last few years, immunologists have gained an ever-increasing
knowledge of immunoregulatory pathways, especially those that prevent or dampen
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