Target Discovery and Validation Reviews and Protocols

168 Houdebine
Fig. 2. Major methods to introduce genes into oocytes and embryos.
high expression may be because lentiviral vectors integrate preferentially in
genome regions containing genes and often within transcribed regions. It
remains that lentiviral vectors cannot harbor more than 8 or 9 kb of foreign DNA
and that the regulatory regions present in the viral long terminal repeat (LTRs)
may interfere with the promoters associated to the genes of interest. This interference
may reduce the cell specificity of transgene expression. Sophisticated
vectors inactivate LTR enhancers, but their manipulation is more complex.
In some circumstances, it may be interesting to obtain a transient expression
of foreign genes in the whole cells of the organism or in a few cell types only.
This end can be achieved by using adenoviral vectors. These vectors are extensively
studied for gene therapy in humans. Methods to introduce foreign genes
in the adenoviral genome have been standardized. Highly efficiency and safe
vectors can be prepared. They remain stable for a few days without being integrated
into host genome. Transient expression experiments should determine
whether a particular gene has an effect. In a positive effect, classical transgenic
animals may be generated to study the function of the transgene. This approach
may enhance the chance of creating relevant models. Adenoviral vectors were
used to express human growth hormone gene transiently and at a high rate in
goat milk (22). In this case, the method seems suitable to predict whether