Design of Transgene

Assuming that a transgenic approach is likely to provide a model of the disorder
in question, the next step is to design a suitable vector for the expression of the
gene. A key element of the expression vector is the promoter. A few promoters that
direct expression to craniofacial tissues have been characterized in sufficient detail to
be useful. Table 1 describes these promoters and their expression patterns.
In general, the use of a promoter that mimics the expression of the endogenous gene
is ideal. This allows for the maximum precision in the modeling of the disorder. However,
a more general promoter can also be informative. As has been clearly demonstrated
from work in Drosophila and vertebrates, the ectopic expression of a gene can
show whether the gene has a dominant effect on cell fate or morphogenetic processes.
This information may prove helpful in the analysis of the pathophysiological mechanism.
For example, in Boston craniosynostosis, we used the CMV promoter to gener-ally overexpress Msx2 (9) and the Msx2 promoter to overexpress Msx2 at the sites of its
normal expression (9a). We showed that in general, CMV-driven overexpression
resulted in a craniosynostosis-like phenotype as well as ectopic cranial bone. Specific
overexpression under the control of Msx2 caused only the craniosynostosis-like condition.
These data suggested that Msx2 is sufficient to induce cells to an osteogenic fate
and thus provided a hypothesis as to how a dominant active mutation in Msx2 could
lead to craniosynostosis.