Biomedical Engineering Reference
In-Depth Information
vectors were delivered by intramuscular injection. While intramuscular injection
resulted in highly efficient gene transfer, this only occurred locally, in the injection
area [
62
]. For a disease like DMD, the number of injections required to treat a
patient would be staggering. This is one of the many hurdles facing gene therapy
approaches for treating global muscle diseases. Therefore, a muscle-targeted
systemic delivery approach is desirable.
Many viruses, including retroviruses, lentiviruses, adenoviruses, adeno-asso-
ciated viruses (AAV) have all been tested in preclinical models of muscular
dystrophy with limited success [
61
]. The largest hurdle to overcome has been the
large size of the dystrophin gene, preventing packaging in most viral vectors. This
has been partially overcome through the use of so-called mini-dystrophin, which
lacks a large portion of the rod domain, yet is still functional [
63
]. However, just as
the current treatments for muscular dystrophy are aimed at improving quality of
life, gene therapy could take a similar approach. For example, AAV vector-
mediated delivery of a dominant negative mutant of IkB Kinase b, a critical kinase
for inflammatory macrophage activation, has been shown to increase muscle
regeneration and decrease necrosis in locally treated skeletal muscle of aged mdx
mice, a murine DMD model [
64
].
Recombinant AAV (rAAV) vector mediated gene transfer has shown promise
in recent years. AAV can infect both dividing and nondividing cells, an attractive
characteristic for gene delivery to post-mitotic muscle fibers. Additionally, unlike
lentiviruses, AAV does not insert into the genome, and unlike adenoviruses, AAV
has a low immunogenicity [
62
]. In the case of DMD the viruses should be targeted
to the appropriate tissue, skeletal, and cardiac muscle. There are many different
serotypes of AAV, each with a unique tropism, some with a natural tropism
towards muscle [
62
]. Despite these positive attributes, problems remain for rAAV
gene delivery. For example, AAV serotype 2 (AAV2) has been extensively
investigated and is the only serotype approved for clinical trials in humans.
However, a large percentage of the population has pre-existing, neutralizing
antibodies to this serotype [
65
]. In order to overcome a humoral response, inno-
vation has led to the creation of chimeric viral capsids, which have the dual benefit
of targeting specific tissues and reducing immunogenicity [
66
].
6.3.5 Combining Biological Therapeutics
Given the pros and cons of the numerous biological strategies discussed thus far, it
is reasonable to hypothesize that a combination of two or more approaches might
give the most successful therapeutic results. By combining cell or gene therapy
with
biomaterial
science,
many
groups
have
developed
novel
combinatorial
regenerative therapies.
Recently, by altering the elasticity of growth surfaces, conditions designed to
more closely resemble a stem cell niche have been used to expand muscle stem
cells in vitro without loss of ''potency'' [
67
]. Extending this concept in vivo,
