Gene Therapy Approaches Using Reproducible and Fully Penetrant Lentivirus-Mediated Endogenous Glioma Models - Gene Delivery and Therapy for Neurological Disorders - page 342

Implantable models utilizing xenografts or syngeneic glioma
cell lines have provided reproducible platforms of animal tumor
formation but often do not refl ect the stochastic etiology of tumor-
igenesis, reproduce characteristic histopathology of the disease, or
allow insertion of genetic lesions encountered in human tumors.
Also, transplantation models involving human tumor cells require
immunosuppression of the recipient animal. On the other hand,
endogenous brain tumors induced by expression of oncogenes
and/or inhibition of tumor suppressor genes in situ allow for
reproducible manipulation of oncogenic mutations found in
human glioma pathways. There is an increasing evidence that sev-
eral of these models faithfully recapitulate the molecular biology of
high-grade gliomas [ 4 ].
Databases of sequenced human high-grade gliomas have built
the understanding that gliomas involve genetic alterations in
three main pathways: (1) proliferation (receptor tyrosine kinase
(RTK)/phosphatidylinositol 3-kinase (PI3K)/HRAS-V12 axis),
(2) apoptosis (ARF/p53 axis), and (3) cell cycle progression
(RB/INK4 axis) [ 4 ]. Secondary GBM, i.e., those that arise from
primary low-grade gliomas, harnesses a distinct genetic pathway,
involving mutations in the cytosolic IDH1 protein, normally
involved in NADPH production [ 5 ]. The IDH1 mutation
(R132H) leads to a neo-catalytic metabolic activity leading to
hypermethylation which stalls neural cell differentiation [ 5 - 7 ].
Previous animal models have been built based on manipulations of
the above pathways, most using activation of the proliferation
pathway in order to reliably create tumors in mice and rats [ 8 - 16 ]
(Table 1 ). Rodent models of brain cancer based on the IDH1
pathway have been unsuccessful so far, perhaps due to species vari-
ation in IDH1 function.
There are multiple ways to create somatic mutations in
rodents. Initial models were built on RCAS (replication-compe-
tent avian sarcoma-leukosis) vectors, which required mice with
germline TVA (tumor virus A) retroviral receptors linked to cell-
type-specifi c promoters [ 9 , 10 , 12 , 13 , 16 ]. Canoll et al. used a
construct with replication-defi cient retrovirus plasmids and vesic-
ular stomatitis virus G (VSV-G) coats to somatically introduce
oncogenes into rodents [ 8 , 14 ]. The Sleeping Beauty [ 6 ] model
employs the SB transposase enzyme to transfer genetic material
from plasmids into host chromosomes after injection into neona-
tal mice lateral cerebral ventricles. Using various combinations of
human oncogenes and inhibitors of tumor suppressor function, they
were able to reliably induce spontaneous tumors resembling human
GBM [ 17 ]. Finally, lentivirus vectors have been used with increas-
ing frequency to express oncogenes (HRAS, KRAS, PDGF, AKT)
and to knock down tumor suppressors (p53 and INK4) [ 11 , 15 , 18 ].