Transcriptional Characterization of Glioma Neural Stem Cells Diva ...

1.3 Primary and Secondary Glioblastomas Introduction
activation [426,549]. Thus, GEMMs have been instrumental so far in the
molecular understanding of the causes of human gliomagenesis. Mutations in
Phosphatase and tensin homolog (PTEN), TP53 and Retinoblastoma 1 (RB1)
have all been tested in mouse models expressing Cre specifically in the brain
and their phenotypical effects have all been cell-type as well as developmental-
stage specific [100]. The use of the Cre-lox system allows specific recombination
events to occur in genomic DNA. The Cre protein is a site-specific DNA re-
combinase that catalyses the recombination of DNA between specific loxP sites
that have a directional core sequence between them. When cells express Cre,
the DNA is cut at both loxP sites and the result of the recombination depends
on whether the lox sites are located on the same chromosome and whether in
an inverted or direct repeat fashion. Same chromosome inverted lox sites pro-
duce an insertion, while direct repeats cause a deletion. Different chromosome
lox sites may cause translocation events [371].
Glioma Cell Lines Cancer cell lines have been the historical standard
both for exploring the biology of human tumours and as preclinical models
for screening of potential therapeutic agents [261]. Due to the inherent diffi-
culty in establishing and maintaining primary tumour cell cultures, established
cell lines have been traditionally used to characterize the genomic aberrations
identified in primary tumours [275]. However, it is possible that the genetic
aberrations accumulated by repeatedly passaging the cells in vitro may cause
their phenotypic characteristics to bear little resemblance with the primary
human tumour [66,261].
The laboratory of Howard Fine has attempted a systematic genomic survey
of five of the most commonly used glioma cell lines - A172, Hs683, T98G,
U251, and U87 - for the purpose of evaluating their similarity to primary
gliomas [275]. Their research, conducted with high-density Single Nucleotide
Polymorphism (SNP) arrays, showed that established glioma cell lines and pri-
mary tumour have significant differences in both genomic alterations and gene
expression, indicating that glioma cell lines may not be an accurate representa-
tion or model system for primary gliomas [275]. The differences observed in the
biological phenotype of glioma cell lines compared with primary tumours are
further confounded by the lack of molecular hallmarks in serially passaged cell
lines, such as the over-expression of EGFR, the silencing of cyclin-dependent
kinase inhibitor (CDKN) CDKN2A and the loss of PTEN [207,445]. This ex-
plains why in vitro and in vivo cancer cell line-based preclinical therapeutic
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