Target Discovery and Validation Reviews and Protocols

186 Houdebine
and stroma. Several other gene modifications in mice have been carried out to
decipher the different mechanisms in the development and the differentiation of
the mammary gland (123).
4.4. Models for Cystic Fibrosis
Cystic fibrosis results from mutations of the CFTR gene. However, several
hundred mutations have been found in this gene, but only some of them cause
a severe pathogenicity. In contrast, it is known that the most potent mutations,
namely, the ∆F 508 mutation, have not the same effect in all patients. Thus, the
cooperation with other genes may enhance or attenuate the effect of the mutation.
Transgenic mice harboring the ∆F 508 mutation in their CFTR gene have
been independently generated by several groups. In no single case did the models
seem fully relevant. CFTR is a chloride channel that is less active and not
properly transported to plasma membrane in mutants. Mice have other chloride
channels, and their lungs are only weakly affected by the CFTR gene knockout.
Yet, these animals show intestinal disorders typical of cystic fibrosis. Rabbit
and sheep are expected to be better models for cystic fibrosis because their
CFTR genes are more similar to the human CFTR gene. Experiments are underway
to generate rabbits and sheep with the mutated CFTR gene, but the major
hurdle is that the cloning technique by nuclear transfer must be implemented.
The siRNA approach might be an alternative requiring less laborious techniques.
However, inactivation of mRNAs by siRNAs is often not complete,
and the residual expression of mouse CFTR might diminish the relevance of
the model.
4.5. Models Requiring Several Transgenes
In many cases, a mutated gene increases the probability of a disease developing
but is insufficient alone. This situation is seen with the breast cancer BRCA1
and BRCA2 genes in human breast cancer. Several transgenes must then be
expressed simultaneously to generate the pathology. The cooperative effect of two
genes is sometimes unexpected. The transgenic mice used to study amyotrophic
lateral sclerosis exemplify this point. The crossing of two lines of mice, one harboring
the superoxide dismutase-1 gene and the other gene for a neurofilament
subunit gave much more relevant models than each line separately (124).
Occasionally, the effect of a knockout gene may be compensated by the
action of a very different gene, revealing a nonanticipated role of the second
gene. Mice in which the LAM2 gene has been knocked out suffer from congenital
muscular dystrophy. The addition of the agrin gene known to be
involved in the formation of neuromuscular junctions restored muscular function
(125). Alzheimer’s disease (AD) is complex, and it occurs after a long