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Chapter 1
barriers for the use of genetically engineered mouse tumor models in preclinical TCR gene
therapy studies (Table 4). In these mice, a latent allele is phenotypically wild type until
stimulated in a tissue and time-specific manner with exogenous chemicals or viruses (111).
These models often rely on the use of site-specific-recombinases to control the mutation
of the genome. The most commonly used system is based on bacteriophage Cre-loxP with
Cre recombinase recognizing a pair of inverted repeat DNA elements (LoxP sites) resulting
in irreversible inversion or deletion of a sequence in between LoxP sites (119). Traditionally,
alleles of interest become constitutively activated or inactivated in the transgenic mice by
Cre recombinase under the control of a tissue-specific promoter. To achieve conditional gene
expression, Cre can be delivered via a virus administrated directly to the organ of interest
where it will result in a genetic mosaic in the target tissue. An example is the use of adenoviral
vector to express oncogenic K-RAS in a lung cancer model (120). Other possibilities to control
gene expression are a tamoxifen-inducible Cre system (CreERT), an Flp-FRT system and Tet-
On or Tet-Off systems. The CreERT system is based on a fusion protein of Cre and oestrogen
response elements. Upon administration of tamoxifen (or analogues) to the mouse, CreERT
recombinase becomes activated (121). A mouse model was developed in which a CreERT
gene was introduced under control of the tyrosine promoter, and Cre became activated in
melanocytes only upon application of tamoxifen. As a result of Cre activation both expression
of oncogenic BRAF and deletion of the tumor suppressor PTEN were induced and mice
developed malignant melanoma (102). An alternative site-specific recombinase system
is the Flp-FRT system based on the Flp recombinase (122). In a mouse model of soft tissue
sarcoma the combination of Cre-loxP and Flp-FRT systems allowed spatiotemporal control
of distinct genetic events, namely the sequential genetic mutation of common cancer genes
p53 and K-RAS (123). In addition to site-specific-recombinase systems, reversible control of
gene expression is achieved with Tet-On and Tet-Off systems, and as long as mice are fed
with tetracycline (or analogues, often doxycycline) the gene of interest is switched on or off
(124). In a doxycyline-inducible oncogenic H-RAS mouse model of melanoma, withdrawal
of doxycyline and consequent inactivation of oncogenic H-RAS resulted in melanoma regression
(125). There are a few conditional models in which tumors express a defined T cell
antigen, such as a melanoma model expressing the mouse germ line gene P1A (118) and a
lung cancer model expressing SIYRYYGL (126). Recently, DuPage and colleagues used a very
elegant model of lung cancer expressing defined T cell antigens to study the interaction of
the immune system with ‘naturally arising’ tumors (103). In these mice, lung adenocarcinoma
was initiated by inhalation of lentiviral vectors expressing Cre recombinase. To induce tumors
that express tumor neo-antigens, the lentiviral vector expressed SIYRYYGL and two antigens
from Ovalbumin (103). Next to the study of endogenous immune responses to tumors, these
models are valuable to assess and better understand parameters of TCR gene therapy.
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