Target Discovery and Validation Reviews and Protocols

184 Houdebine
more extensively. Transgenic mice are valuable models to define genes to be
added to farm animals (112).
Pharmaceutical recombinant proteins sometimes induce formation of antibodies
in patients. This antibody production may take place because the
recombinant proteins have slightly different structure, namely, in the carbohydrate
content. Transgenic animals and essentially mice expressing a
recombinant protein in any of their tissues are models to study the immunogenicity
of the recombinant protein. Indeed, the recombinant protein belongs
to the self of the animals, which mimics the situation of patients treated by
the protein.
4.2. Xenografting
Using animal, essentially pig, organs for humans would save the lives of
many patients but will not be possible until the strong rejection mechanisms are
not controlled. In the 1990s, an understanding of some of the major rejection
mechanisms urged the generation of transgenic animals harboring genes having
an anticomplement effect. Mice, rats, and rabbits were used for this purpose
before transgenic pigs were prepared (113). Kidneys from transgenic pigs
expressing human DAF and CD59 genes could be kept intact for a few days
after having been grafted to experimental monkeys. This finding validated the
transgenic approach to study rejection mechanisms and to obtain pig organs to
be potentially grafted to humans.
Other transgenic models are regularly obtained by different academic groups
and by private companies to study rejection mechanisms. The most advanced
project is based on transgenic pigs in which the α-galactosyl transferase gene
has been knocked out. This knock out was achieved using the homologous
recombination and cloning techniques described above (114,115). Homozygous
pigs are healthy, and their kidneys grafted to monkeys immunosuppressed by
classical methods could survive more than 2 mo without any degradation,
whereas control organs were destroyed after at most a few days. The expression
of nonpathogen retrovirus in pigs must be controlled to prevent any transmission
to patients (116). Interestingly, strains of pigs not expressing the retrovirus have
been found (117).
4.3. Models for Embryo Development and Organogenesis
Transgenesis is extensively used to study development mechanisms. Gene
addition, knockout, and knockdown are essential tools to determine gene function
in development. Gene coding for proteins giving colors to cells are currently
used. β-Galactosidase is one of these genes. The blue color resulting from the
enzymatic activity of this protein allows the positioning of cells in which a given
promoter is active. β-Galactosidase suffers from several limitations. Cells must