Barnyard 101: An Introduction to Transgenic Farm Animals
by Thomas M. Zinnen
"We have the ability today to probably transform any cell
type," said Carl Pinkert in his opening remarks to the Transgenic
Animals in Agriculture conference held August 24-28 in Tahoe City,
"But it's not just transfer of the gene, it's gene function
that's key," added Pinkert, a researcher at the University
of Alabama at Birmingham. He illustrated his point with a cartoon
of two mice. The first asks, "How do you like my new genes?"
The second replies with mores questions: "They're nice,
but are they expressed? Are they regulated properly? Are they
altered during inheritance?"
Pinkert noted that now nearly 20 years after the first transgenic
animal scientists can answer those questions.
Why Transgenic Animals?
Interest in transgenic animals originally fell into two broad
- Production efficiency of farm animals--the original area of interest among many university researcher.
- Molecular farming: using livestock to produce medicines, nutraceuticals and tissues for transplant to humans.
"Now most of the money is here (in molecular farming),"
said Pinkert, "and it's mostly corporate money."
Adding a Gene by Random Insertion
For basic research, Pinkert pointed out two different lines of
approach possible with transgenic cells: gain of function or
loss of function. To study gain of function merely requires addition
of a new gene.
Pinkert underscored the power of studying animals, not just cells.
"Anytime we tried to alter one trait with one gene,"
he said, "in the end the influences of that gene on other
traits was something we couldn't know until we had the animals."
During 1982-83 pioneering research featured transgenic mice given
a copy of the gene for human growth hormone. In this case, the
added gene inserted randomly in the mouse genome. It did not
insert at the mouse gene for growth hormone. The addition of
the gene for human growth hormone did not inactivate or "knock-out"
the genes for mouse growth hormone.
A picture of two mice side by side, one with the extra gene and
the other without, gave "visual impact of what this technology
might do," Pinkert pointed out. Comparing the size of the
mouse with the extra gene to the control was like comparing a
softball to a baseball.
But the control of the growth hormone gene's impact on the endocrine
system was not sufficient. This brought home the issue of qualitative
versus quantitative control of gene expression. Since then mouse
researchers have been wrestling with that next layer of problems:
how to control genes in time and tissue, "temporally and
Modifying Expression of an Existing Gene by Homologous Recombination
To study loss of function of an existing gene is a bit trickier.
It requires a system in which a modified or faulty version of
a gene is inserted by homologous recombination. The new version
inserts by trading places with the existing gene and "knocks
out" the existing version of the gene. In homologous recombination,
the inserted gene is not randomly inserted into the genome, but
rather it is targeted to insert at the site of the existing homologous
"Long life cycles of farm animals slow genetic analysis,"
said Pinkert. "That's why researchers use smaller, faster-breeding
animals such as mice as model systems to test their ideas and
their DNA constructs." Furthermore, the mouse is the only
model system that combines homologous recombination with cloning
to allow the study of modified genes in development of adult animals.
Currently, in livestock homologous recombination is possible
only with cells grown in tissue culture. This means a scientist
can study the effect of knocked-out genes only on the physiology
of the cell. The possible role of the gene in development from
embryo to adult cannot be tested without a system of cloning:
taking the original cell and growing an adult from it.
In 1997 the cloning of Dolly and engineering of Polly have combined
transgenesis and cloning. This combination was an essential step
in developing in livestock a system of homologous recombination
to modify existing genes. Now researchers are eagerly working
on making a system of homologous recombination to further test
their ideas with farm animals.
Once such a system is available, animal scientists will be able
to ask questions about the roles of genes in development from
embryo to adult using livestock, not just mice.
Regulatory Issues Leading to Commercialization
Researchers first produced transgenic farm animals in 1985, yet
as of 1997 Pinkert notes no products in the supermarket or at
the pharmacy are produced using such animals. A major reason
is the lack of a clear route to government approval. "Some
(regulatory issues) are still outstanding, affecting utility and
acceptance," he said. "Environmental impact is a huge
issue" especially for transgenic fish that would likely mingle
with their wild relatives.
Commercialization comes with a communication component. "The
reasons we put forth for transgenic animals influence public perception,"
Pinkert warned. He ended by noting that the public perception
of pioneering products such as BGH, PST and even FlavrSavr tomato
has affected governmental and corporate approach to reviewing
and commercializing products from transgenic animals.
Timeline of Animal Cloning and Gene Transfer
1891 first successful embryo transfer early 1900's in vitro embryo culture develops
1961 mouse embryo aggregation to produce chimeras
1966 first report of microinjection of mouse embryos
1973 foreign genes function after cell transfection
1974 development of teratocarcinoma cell transfer
1977 mRNA and DNA transferred to Xenopus eggs
1980 mRNA transferred into mammalian ova
1980-81 transgenic mice first documented
1981 transfer of ES cells derived from mouse embryos
1982 transgenic mice and a growth hormone phenotype
1983 tissue specific gene expression in transgenic mice
1985 transgenic domestic animals produced
1985 microinjection for transgenic pigs, sheep, rabbits, fish
1987 chimeric "knock-out" mice described
1987 retrovirus mediated: transgenic chicken
1989 targeted DNA integration & germline chimeric mice
1989 microinjection for transgenic cattle (Russia)
1989 first sperm mediated reports in farm animals
1991 microinjection for transgenic goats first refereed publication
1993 germline chimeric mice produced using co-culture
1996 ES cells used for nuclear transfer: sheep
1997 somatic cells from adult sheep used for cloning by nuclear
Gene Transfer Methods
Teratocarcinoma cell transfer
Microinjection of cells (oocytes) with DNA
ES (embryonic stem) cell transfer
Particle bombardment ("gene gun")