Evolution and Genetic Engineering - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 730

but in serum-free medium). Ideally, an overall optimization is most desirable and it is within
reach today.
14.7.6. Transgenic Animals
In some cases, proteins with necessary biological activity cannot be made in animal cell
culture. While posttranslational protein processes, such as N-linked glycosylation, can be
done in cell culture, other more subtle forms of posttranslational processing may not be
done satisfactorily. An alternative to cell culture is the use of transgenic animals. Animals
are engineered to express the protein and release it into specific fluids, such as milk or
urine. High concentrations of complex proteins can be achieved, and such approaches
can be cost effective for complex proteins. In these cases, the role for the bioprocess engi-
neer is in protein recovery and purification, although significant issues exist for agricul-
tural engineers and animal scientists in devising appropriate systems to obtain the
protein containing fluid (e.g. pig milking stations). While transgenic animals can be devel-
oped from many mammals, sheep, goats, and pigs are the primary species used
commercially.
There are significant limitations on the transgenic animal technology. In some cases, the
protein of interest will cause adverse health problems in the producing animal. The use of
animals raises safety concerns with respect to virus or prion transmission. The process to
generate and screen for high-producing animals is inefficient and costly (e.g. $100,000 for
a goat and $500,000 for a cow). Perhaps surprisingly, not all of the complex posttranslational
processing steps necessary to achieve the desired product occur when the protein is
expressed in milk, urine, or blood. Nonetheless, transgenic animals will be critical to produc-
tion of some proteins.
14.7.7. Transgenic Plants and Plant Cell Culture
Proteins, including many complex protein assemblies, such as antibodies and virus-like
particles (as vaccines), can be made inexpensively in plants. Transgenic plants offer many
potential advantages in addition to cost. Since plant viruses are not infective for humans,
there are no safety concerns with respect to endogenous viruses or prions. Scale-up is readily
accomplished by planting more acreage. The protein can be targeted for sterile, edible
compartments, either reducing the need for rigorous purification or making it an ideal
vehicle for oral delivery of a therapeutic protein. Indeed, development of edible vaccines
for use in developing countries is being actively pursued.
The disadvantages of transgenic plants are that expression levels are often low (1% of total
soluble protein is considered good), N-linked glycosylation is incomplete, and some other
mammalian posttranslational processing is missing. While inexpensive, with easy scale-up, it
takes 30 months to test and produce sufficient seed for unlimited commercial use. Such long
lead times are undesirable. Further, environmental control on field grown crops is difficult, so
the amount (and possibly quality) of the product can vary from time to time and place to place.
While many crops could be used, much of the commercial interest centers on transgenic
corn. Some corn products are used in medicinals, so there exist some FDA guidelines (e.g.
contamination with herbicides and mycotoxins), and there is considerable processing