228
Animal Biotechnology and Modeling
efFcient wool production, and enhanced
resistance to viral and bacterial diseases
(including development of ‘‘constitutive
immunity’’ or germ line transmission of
speciFc, rearranged antibody genes). Cu-
mulatively, it has become apparent from
these studies that greater knowledge of the
biology of animal development will be re-
quired before lines of domestic animals
with these desired characteristics can be
genetically engineered.
3.4
Domestic Animals as Bioreactors
The second general area of interest has
been the development of lines of trans-
genic domestic animals for use as biore-
actors. One of the main targets of these
so-called gene pharming efforts has in-
volved attempts to direct expression of
transgenes encoding biologically active hu-
man proteins. In such a strategy, the goal
is to recover from serum or from the
milk of lactating females large quantities
of functional proteins that have therapeu-
tic value. To date, expression of foreign
genes encoding
α
1
-antitrypsin, tissue plas-
minogen activator, clotting factor IX, and
protein C were successfully targeted to the
mammary glands of goats, sheep, cattle,
and/or swine.
Targeting expression of the cystic Fbro-
sis transmembrane conductance regulator
(C±TR) gene to the mammary epithelium,
in which the protein becomes associated
with the apical membranes of these cells,
has resulted in apocrine secretion of the
C±TR protein as a constituent of the milk
fat globule membrane. The demonstrated
utility of such a targeting strategy has led
to proposals for largescale production and
harvesting from milk of other classes of
important membrane-associated proteins,
such as hormone receptors, channel pro-
teins, and transport proteins.
Similarly, lines of transgenic swine and
mice have been created, which produce
human hemoglobin or speciFc circulating
immunoglobulins. The ultimate goal of
these efforts is to harvest proteins, from
the
serum
of
transgenic
animals,
for
use as important constituents of blood
transfusion
substitutes,
or
for
use
in
diagnostic testing.
3.5
Examples from Domestic and Miniature
Pigs
In contrast to gene transfer in mice, the
efFciency associated with the production
of transgenic livestock, including swine,
is quite low. However, two advantages of-
fered by swine over other domestic species
inc
ludeafavorab
leresponsetohormona
l
superovulation protocols (20–30 ova can
be collected on average) and a signiFcant
uterine capacity, in that swine are a litter-
bearing species. The highest transgenic
success rate in domestic animals has been
with outbred domestic pigs, while use of
inbred strains was reported to have lower
overall efFciency. Since the description
in 1985 of the Frst transgenic livestock
species, DNA microinjection has to had
been the only successful method identiFed
to produce germ line competent transgenic
livestock prior to nuclear transfer.
An initial problem encountered during
the creation of transgenic farm animal
species concerned the visualization of the
pronuclei or nuclei within the ova. In
lipid-dense swine ova, for example, the
cytoplasm is opaque and the nuclear struc-
tures are not discernible without some
type of manipulation. This was a critical
problem to overcome because transgenic
pigs were not produced following injection
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