Adeno-Associated Vectors for Gene Delivery to the Nervous System - Gene Delivery and Therapy for Neurological Disorders - page 2

ability of some serotypes (AAV2, AAV9) to cross the blood-brain
barrier. Lentivirus-based vectors are also playing an increasingly sig-
nificant role in CNS-directed gene therapy, and they have the
advantage of possessing a larger packaging capacity [ 2 ]. However,
lentiviral vectors do not naturally infect cells of the CNS and also
have the potential for insertional mutagenesis associated with their
natural integration in the host cell genome. Modern vectors have
been modified in an effort to eliminate integration into the host
genome [ 3 ]. Adenoviral vectors have also been applied to neuro-
logical disorders, but there are still some obstacles that need to be
overcome before these vectors can be used extensively in the
CNS. Because of its high immunogenicity, efforts have been made
to exploit other serotypes or even nonhuman adenoviruses, to avoid
any potential reaction because of pre-existing immunity [ 2 , 4 ]. This
chapter will focus primarily on AAV vectors and their applications.
AAV vectors are derived from a nonpathogenic, helper depen-
dent, single-stranded DNA parvovirus. The virion is small
(~20 nm), consisting of an icosahedral capsid comprised of three
proteins VP1, VP2, and VP3. These proteins facilitate binding of
the virion to the cell surface receptor which varies between sero-
types. The genome of a typical AAV is approximately 4.7 kb and
contains two genes rep and cap , surrounded by inverted terminal
repeats (ITRs), elements essential for genome replication and
packaging. In recombinant AAV (rAAV), the viral genes are deleted
and replaced by the gene of interest flanked by the ITRs, the only
cis sequences required for the generation of rAAV. The capsid pro-
teins and helper virus functions are supplied in trans [ 5 ].

1.1 AAV for Gene
Therapy

Gene therapy holds the potential to offer a “cure” for inherited
diseases which are caused by a single genetic mutation in the
human DNA, often responsible for a significant reduction in life
span and/or quality of life. Diseases caused by a single error in a
single gene such as hereditary pancreatitis and cystic fibrosis repre-
sent approximately 1 % of all births. It is possible that gene therapy
for these diseases could become a reality by using vectors to replace
the defective gene in the patient's cells to restore function. It could
also be used to treat other more complex, multifactorial diseases
such as Parkinson's disease and ALS where the cause of onset is
potentially due to a synergistic effect of genetic and/or environ-
mental factors [ 6 ]. Currently, introducing a specific gene into the
host cells and inducing controlled expression in a useful and safe
manner over an extended period of time poses many challenges.
Viruses have become the vectors of choice for gene delivery to
mammalian cells. AAV vectors have been identified as the current
most promising gene delivery candidate for serious nonlethal dis-
eases in need of long-term treatment. Most of the viral genes are
removed from these viruses to prevent replication of the virus and
to reduce its immunogenicity.