Target Discovery and Validation Reviews and Protocols

Transgenic Models 181
vectors can direct synthesis of these of these proteins. The major limit is the
availability of the proteins having a potent transdominant negative action without
any side effect.
3.4 Genetic Ablation
Inducing the specific destruction of a cell type in a given tissue at a well
defined step of development may give interesting information on the function
of these cells. This technique, known as genetic ablation, has been used for
more than 15 yr, although not at a high frequency. The limit of this technique
is to obtain a potent and specific destruction of the targeted cells. Genes coding
for toxins are being used for this purpose. Ideally, the action of the cytotoxin
should be induced in a precise manner. Two models can exemplify this
approach. One aims at destroying chondrocytes at different stages of development.
For this purpose, a first line of mice harbored a transgene containing PGK
promoter active in all cell types associated to a mutated version of TK gene no
longer able to induce infertility in mice. A neor gene and a transcription terminator
separated the PGK promoter and the TK gene. The TK gene was not
expressed in these mice. The first line of mice was crossed with another line
expressing the Cre recombinase specifically in chondrocytes. The neor gene
was deleted in chondrocytes of hybrid mice. Fialuridine (FIAU), a drug that is
transformed in a cytotoxic compound in cells expressing the TK gene, was
injected. The injection of FIAU induced chondrocyte destruction at different
steps of development (102). Another model is based on the use of diphtheria
toxin. This protein is extremely potent and its presence in targeted cells must be
strictly restricted to the period of genetic ablation. Transgenic mice expressing
specifically the toxin receptor in a chosen cell type were generated. Injection of
the toxin at any step of the life of the animals induced the destruction of the
cells (103).
3.5. Vectors for Conditional Transgene Expression
Ideally, in a number of situations, inducers not acting on host genes should
selectively and reversibly activate transgenes. This process avoids that a possibility
that a natural transgene inducer stimulates or inhibits simultaneously
some host genes, lowering the relevance of the model.
Several systems allowing specific activation of transgenes are available. They
are all based on the use of chimeric transcription factors containing one region
sensitive to the inducer and another region able to stimulate the associated gene.
The most frequently used system is based on the use of tetracycline and derivatives
as inducers and tetracycline repressor fused to an animal transcription factor
(104,105). These systems use murine promoters modulated by the fusion
transcription factor. One of the limits of these systems is that the promoter