Lentiviral Vectors for Gene Delivery to the Nervous System - Gene Delivery and Therapy for Neurological Disorders - page 34

elements [ 81 ] have been used and might also optimize the target-
ing strategy restricting transduction to the tissue of interest. The
use of mammalian promoters can result in long-term physiological
levels of transgene expression [ 8 ]. Glial fi brillary acidic protein
(GFAP), synapsin 1 [ 82 - 84 ], neuronal specifi c enolase (NSE)
[ 85 ], tyrosine hydroxylase/neurofi lament [ 86 ], and the prion pro-
moters [ 87 ] are some examples of powerful neuronal promoters
which have been used in adenoviral and lentiviral systems.
Cytomegalovirus (CMV), platelet growth factor- ʲ , and CMV/

chicken -actin promoters have also been tested in AAV and lenti-

viral vector systems for CNS targeting, where specifi city achieved
from each can vary between different CNS regions [ 84 ].
Alternatively, incorporation of drug-inducible systems (e.g.,
tetracycline, ecdysone, mifepristone [ 88 , 89 ]) in lentiviral vectors
can better control the levels and timing of gene expression [ 90 ].
Such lentiviral systems have been generated and used to mediate
regulated gene delivery in Parkinson's disease [ 90 , 91 ]. Additionally,
it is also possible to achieve cell type specifi c expression through
microRNA (miR) regulation [ 92 , 93 ]. When shRNA molecules
are incorporated into a lentiviral vector, they are able to inhibit
mRNA by RNA interference [ 94 ]. Moreover, control might be
exerted by the inclusion of matrix scaffolding domains which place
sequences in transcriptionally favored regions of the nucleus, and
insulating elements which can protect promoters from the infl u-
ences of vector and genomic sequences [ 95 ].
As described, transgene expression can be achieved using both
integrating and non-integrating LVs. There is, however, a low risk
of activation of proto-oncogenes or dedifferentiation following
integration, as most transduced neural cells are terminally differen-
tiated [ 4 ]. Concerns associated with random integration and inser-
tional oncogenesis can be alleviated with the use of integration
defi cient lentiviral vectors (IDLV) that are defi cient in integrase
activity, the enzymatic activity required to catalyze integration into
the host cell genome [ 96 ]. IDLV can be used for short-term
expression of a gene of interest in dividing cells. As far as non-
dividing target cells are concerned, specifi cally brain, liver, or ret-
ina, IDLV can induce relatively stable transgene expression
[ 97 - 99 ]. Particularly, in short-term studies, it has been demon-
strated that use of IDLV resulted in successful transduction of the
rat striatum and retrograde transport to the dopaminergic neurons
of the substantia nigra [ 99 ].
Moreover, IDLV can be used in conjunction with zinc-fi nger
nuclease (ZFNs) hybrid technology to achieve site-specifi c gene
editing/correction or addition at the targeted chromosomal loci
[ 100 ] in which integration is believed to be safe, which could
minimize or overcome the risks associated with random integra-
tion. Despite the fact that this technology is promising, requires

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