Target Discovery and Validation Reviews and Protocols

Transgenic Models 183
then used to generate chimeric mice. The expression of the marker gene (often
a hybrid containing β-galactosidase and neomycin-resistant genes) indicates
that the vector was randomly integrated within a gene. This gene can easily be
identified. Interesting models can be obtained in this way if a correlation can be
established between the expression of the gene trap vector in a given tissue and
particular health problems in the animals. A series of gene trap vectors have
been designed to identify genes expressed in ES cells or in differentiated tissues,
coding for secreted proteins or activated by specific inducers such as hormones
or growth factors (107–110). Banks of ES cells in which gene trap
vectors have been introduced are available (111).
4. Transgenic Models: Case Studies
Very diverse models of transgenic animals are currently generated.
Established lines of transgenic animals are available as conventional lines, particularly
for transgenic mice used to study some human diseases.
Several databases are available. Information on one database recently launched
can be accessed at http://www.biomedels.net/, http://www.ebi.ac.uk/biomedels,
and http://research.bmn.com/mkmdj. Animals are available in The Jackson
Laboratories (Bar Harbor, ME) and EMMA (European Mouse Mutant Archive)
as well as in private companies. The number of models is very large and some of
them are described in the present review. They have been chosen to exemplify the
potency and the flexibility of transgenesis in this field.
4.1. Preparation of Recombinant Proteins
Recombinant proteins are prepared to study their biological properties and
use as pharmaceuticals. Several production systems are being used or are in
development: bacteria, yeast, animal cells, and transgenic plants and animals.
Each system offers advantages and disadvantages on a case-by-case basis (112).
Milk from transgenic animals is one the most mature systems (2). Before being
transferred to farm animals (rabbits, goats, sheep, pigs, or cows) for the large
production of proteins, gene constructs are evaluated in model transgenic mice.
This approach indicates whether the construct expresses efficiently the transgene
in milk and whether the foreign protein in milk in properly posttranscriptionally
modified.
Some recombinant proteins prepared in Chinese hamster ovary (CHO)
cells or in milk of transgenic animals are not properly glycosylated,
γ-carboxylated, or cleaved to generate mature molecules. This limited glycosylation
may be attributable to the low level of maturation enzymes in
producing cells. CHO cells overexpressing some glycosylating enzymes
secrete more completely glycosylated proteins. Transgenic mice overexpressing
the furin gene in their milk can cleave human protein C precursors