Target Discovery and Validation Reviews and Protocols

Transgenic Models 189
(142). Genes whose expression is amplified in mammary tumors (c-myc, crbB2,
and cyclin D1) have an oncogenic effect when used as transgenes. Crossing mice
harboring different oncogenes and having knocked-out genes has made it possible
to determine the cooperative actions of some of these genes.
Mice having knocked-out cyclin D1 gene are insensitive to neu and ras oncogenes,
whereas Wnt-1 and c-myc genes have kept their capacity to trigger mammary
tumors. These data help to search for new drugs capable of inhibiting neu,
ras, and cyclin D1 genes (143,144).
The stability of mammary epithelial cells is highly dependent on the extracellular
matrix. Development of mammary tumors and metastasis are correlated
with local degradation of some of the extracellular matrix components.
Degradation of the extracellular matrix is achieved by metalloproteinases, which
are controlled by specific inhibitors. The role of different genes involved in
extracellular matrix synthesis and degradation is being studied using transgenic
mouse models. A recent study has pointed out the role of Akt gene, which delays
mammary gland involution. This effect is mediated by the prolonged presence of
one of the metalloproteinase inhibitors (145). A better understanding of these
phenomena may lead to the identification of new drugs preventing metastasis.
It is agreed that tumor cells derive from a single cell in which all the mutations
required for cancer development occurred, thereby explaining in part why
cancer development is a slow process at the early steps and becomes rapid in
time. The classical transgenic models do not take into account the fundamental
clonal origin of cancers. Specific gene constructs capable of sporadically activating
oncogenes have been designed (146). The technique makes it possible to
activate k-ras oncogene randomly and at a low frequency (147). This new
approach is expected to reflect more precisely the process of tumor formation
under natural conditions.
4.7. Models for Infectious Diseases
Many pathogens have species specificity, and conventional laboratory animals
may not be relevant models. The limitation is often because of the lack of
appropriate receptors for the pathogens in the animal cells. Receptor genes can
then be transferred to animals to be preferentially expressed in tissues, which
are the normal target of the pathogens.
Listeria monocytogenes is responsible for severe foodborne infections. A
surface protein of these bacteria, called internalin, has been generated that recognizes
the intestinal host receptor E-cadherin gene exclusively in the enterocytes
of the small intestine. This recognition was achieved by using the
promoter from the rat FABP-1 gene. These mice were sensitive to infection by
L. monocytogenes and constitute an excellent model for the study of this infectious
pathogen. Interestingly, E-cadherin is not expressed in the tight junction