Herpes Simplex Virus 1 (HSV-1)-Based Vectors - Viral Vector Approaches in Neurobiology and Brain Diseases - page 75

the cytochrome P450 4B1, a potent bioactivating “suicide” gene,
with the EGFP marker gene. Amplicon-mediated delivery of the
fusion protein, which converts cyclophosphamide (CPA) into toxic
metabolites, to tumor cells was successfully demonstrated and, in
addition, a strong bystander effect, mediated by cell-to-cell contact,
was observed [ 179 ]. Amplicons were also used to transduce both
TK and cytosine deaminase, followed by treatment with ganciclo-
vir and 5-fl uorocytosine (5-FC), in rat 9 L gliosarcoma and in
human Gli36 glioma cells [ 180 ].
In order to improve effi ciency and safety of cancer gene thera-
pies, efforts at specifi cally targeting proliferating cells were made in
glioma models. The HSV-1 immediate-early protein ICP0 pos-
sesses E3-ubiquitin ligase activity [ 181 ] and can induce the degra-
dation of centromeric proteins [ 182 ]. Amplicons expressing the
HSV-1 ICP0 were used to infect human glioblastoma Gli36 cells
and well-established models of nondividing cells, such as primary
cultures of either rat cardiomyocytes or brain cells. Results showed
that ICP0 induced a strong cytostatic effect and signifi cant cell death
in Gli36 cells. In contrast, neither cell death nor any evidence of
ICP0-induced toxicity was observed in both primary cultures of
noncycling cells. These observations suggest that ICP0 has gene
therapy potential and could be the fi rst member of a new family of
cytostatic proteins that could be used to treat cancers [ 183 ].
In order to target the invasive activity of malignant glioma
cells, an amplicon vector expressing the tissue inhibitor of metallo-
proteinase-2 was used. Results suggested that this strategy is poten-
tially useful to treat malignant brain tumors [ 184 ]. A different
approach used amplicons expressing siRNA in order to mediate
posttranscriptional silencing of the epidermal growth factor recep-
tor (EGFR). Infected human glioblastoma cells with knockdown
for EGFR expression displayed growth inhibition both in culture
and in athymic mice [ 185 ].
Another used strategy is to target tumor cells via transcriptional
control of therapeutic genes. Ho et al. constructed a glioma-specifi c
and cell cycle-regulated amplicon carrying the glial fi brillary acidic
protein (GFAP) enhancer/promoter element, plus a cell cycle-spe-
cifi c regulatory element from the cyclin A promoter. Transgenic
activity was mediated in a cell-type-specifi c and cell cycle-dependent
manner, both in vitro and in vivo in glioma-bearing animals [ 186 ].
Antitumor effi cacy of this vector system was assessed using the pro-
apoptotic proteins (FasL- and Fas-Associated protein with a Death
Domain, FADD), both in vitro and in vivo [ 187 , 188 ].
Effi ciency of the tumor necrosis factor-related apoptosis-
inducing ligand (TRAIL) in Gli36 cells and in subcutaneous glioma
was evaluated upon delivery of this molecule using amplicons
[ 189 ]. In cultured cells, TRAIL induced apoptosis by 24 h postin-
fection. In addition, TRAIL-treated gliomas reduced in size over a
period of 4 weeks, demonstrating the effi ciency of TRAIL delivery