| Summary: | The efficient delivery of therapeutic genes and appropriate gene expression are
the crucial issues for clinically relevant gene therapy. Viruses are naturally evolved
vehicles that efficiently transfer their genes into host cells. This ability made them
desirable for engineering virus vector systems for the delivery of therapeutic genes.
Among various vector systems, herpes simplex virus (HSV) vectors represent an
attractive delivery system, since these vectors have high gene transfer efficiency and
mediate high expression of therapeutic genes.
Although HSV has been shown to infect most cell types, they were restricted from
mature skeletal muscle tissue. As a result, research involving the use of this vector for
muscle-directed gene therapy was hampered. Previous studies indicated that the loss of
infectivity may be due, at least in part, to the development of the basal lamina throughout
the course of muscle maturation. Enzymatic disruptions of the basal lamina showed
moderate increases in levels of infection, although marked toxicity with such procedures
resulted. To initiate infection, HSV normally attaches to cell surface heparan sulfate,
which stabilizes the virus such that it can interact with secondary protein receptors
required for entry into host cells. Our studies revealed a downregulation of heparan
sulfate biosynthesis during skeletal muscle maturation. Furthermore, infectivity could be
restored by exposing mature skeletal myofibers to low concentrations of the
glycosaminoglycan analog, dextran sulfate (DS). This molecule appears to act as a
surrogate receptor to stabilize the virus at the myofiber surface such that HSV can engage
additional receptors. This demonstration that the basal lamina is not an absolute block to
HSV infection is remarkable because it allows for the nondestructive targeting of HSV to
mature myofibers and greatly expands the usefulness of this vector for the treatment of
inherited and acquired diseases.
In light of these results, dextran sulfate was further examined for its ability to
target cancer cells in a systemic model of delivery. Cancer cells typically display altered
glycosaminoglycan profiles, similar to mature skeletal muscle tissue. Vascular delivery
of oncolytic HSV and DS significantly delayed tumor growth, with 25% of the animals
cured following treatment. Although, DS did not act to stimulate infection of cancer
cells, its ability to alter the hemodynamic properties of the animal system in favor of viral
accumulation at tumor portals was key. Surprisingly, viral replication was not necessary
for antitumor efficacy and relatively low amounts of virus could result in marked
oncolysis. Furthermore, immunohistochemistry revealed infection of tumor vasculature
alongside very limited infection of surrounding tumor tissue. Taken together, the tumor
vasculature is likely the major target for oncolytic HSV in a systemic delivery model of
cancer. Thereby efforts to further enhance delivery of oncolytic HSV to tumor
vasculature by incorporating targeting peptides and using antiangiogenic viruses have
been successful.
Understanding the biology of gene therapy systems is crucial to developing the
most efficient and specific systems suited for individual disease applications. Increased
insights into the entry and trafficking of gene therapy systems in animal models
facilitates two approaches to developing appropriate therapies for individual applications.
First, understanding more clearly the biology of currently available systems permits a
more judicious choice of applications. Second, this also forms the basis for development
of advanced delivery systems with increased efficiency, stability and targeting specificity.
Therefore studies that provide insight into why biological therapies succeed and fail not
only allow for a better understanding of animal and vector systems, but they allow us to
exploit this knowledge to improve our arsenal of standard protocols of care for disease.
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