Mice, made to order.
At laboratories across the country, biologists create mice with a specific
genetic makeup to assist colleagues hunting for the causes of human diseases.
The rodents are transgenic, meaning that scientists have taken a foreign
gene - sometimes human, sometimes mouse - and transferred it, said Dr. Sonia
Pearson-White, director of the University of Virginia's Transgenic Mouse Core
Facility.
"Transgenic is, you're adding something, a different color, activity or an
oncogene," which causes cancer, she said.
The U.Va. center, and a similar lab at Virginia Commonwealth University,
also creates so-called "knock-out" mice, which lack one or more of their
80,000 or so genes.
The mouse is now one of the most frequently used mammals in medical and
behavioral research. Early in the 20th century, its potential to shed light on
the genetic study of human diseases was recognized by scientists who
established the Jackson Laboratory, based in Bar Harbor, Maine, which remains
a well-known mouse farm.
Not only is the mouse genome about the same size as the human
(approximately 3.1 billion base pairs), the two share virtually the same set
of genes. Because the DNA sequence of the mouse could help identify and study
human genes, the same federal project that sequenced the human genome went on
to do the same for the mouse, finishing up this spring.
"Because mice and humans share many of the same fundamental biological and
behavioral processes, this animal is one of the most significant laboratory
models for human disease," the former director of the National Institutes of
Health, Dr. Harold Varmus, has said.
Mice are valued "fuzzy test tubes" for their fecundity and relatively low
maintenance costs. They come of age about eight weeks after birth, and can
produce several pups in a single litter.
In recent years, their shared mammalian inheritance with humans has proved
particularly helpful in transgenic research, which allows scientists to study
human genes in action - or even inaction, when they've been knocked out in
specific groups of cells.
Recognizing that tremendous potential, VCU in January opened a facility to
produce transgenic and knock-out mice. Directed by geneticist Dr. Jolene J.
Windle, the lab produces animals to reveal the actions of a wide array of
genes, including those involved in lungs, bone and skin as well as the nervous
system.
"Of mice and men" takes on new meaning when a human gene that causes breast
cancer can be injected into mouse embryos and then studied by cancer
specialists eager to uncover how the tumors begin and how to stop them.
"There's not a field in biomedical research where these tools can't be
used," said Windle.
"These animals make it possible," Pearson-White said, to study human
ailments in an animal model "as close to people as reasonably, ethically
feasible."
Dr. Tim Bender, a U.Va. microbiologist, says the transgenic mice are
crucial to his work studying gene regulation of white blood cells called
lymphocytes. Pearson-White's lab recently produced for him a line of mice that
overexpress a cancer-causing gene called "myb."
Without the gene, mice can't make blood, and don't survive. Until recently,
transgenic mice in which myb (pronounced "mib") was overexpressed resulted in
mice in whom myb was on all the time, killing the animals. New technology is
allowing Pearson-White and Bender to make transgenic mice in which myb's
expression is controlled "in space and time," Bender said.
With the new mice, Bender will be able to control when and where myb is
turned on. The animal will develop normally with myb turned off, and then will
be given a drug that will act like a pharmaceutical remote control to turn on
myb in certain lymphocytes.
"At this point, we don't know that it's going to work," Bender said. Still,
the myb mice have the potential to shed new light on the gene's involvement in
human diseases such as colon cancer and breast carcinomas.
"Without Sonia's facility, that work can't be done," Bender said.
Pearson-White began the lab at U.Va. in 1992, when she realized that
techniques allowing researchers to put genes of one species into another would
lend itself to her work studying genes that control skeletal muscle
development.
The technology was just a couple of years old, but Pearson-White knew "it
was incredibly powerful."
She went to the Cold Spring Harbor Laboratory in New York to learn how to
inject foreign genes, called DNA constructs, into mouse embryos, transfer them
to surrogate mothers, and raise them in an environment free from germs,
barking dogs or cats.
Since then, the U.Va. lab has created more than 500 "founder lines" of mice
from 181 DNA constructs for 32 Virginia investigators, she said. Each line has
about 40 mice.
In its freshman year, the VCU facility has provided mice for seven labs,
and is working with more investigators on how to use the made-to-order mice in
other research, Windle said.
"A lot of what we do is looking at gene regulation," Pearson-White said.
Cancer researchers need to understand how and what turns "on" an oncogene, and
may someday use that knowledge to guide them in developing gene therapy that
can treat patients.
Costs vary from lab to lab, but at U.Va., for about $3,405, the researcher
receives some 30 to 50 mouse pups, of which about 15 percent are transgenic.
Taking a mouse embryo and inserting new genes is not the same as a car
company changing a part in its latest model. Biology, Pearson-White says, is
particular and has its own rules.
"It is hard," she says. "There are so many steps that could all go wrong."
Too much foreign DNA, for example, can be lethal. And the chemicals that a
researcher uses to produce the foreign DNA also can be fatally contaminated.
"The magic is you can do it at all," she says.
The process of creating transgenic mice is basically the same. At U.Va.,
DNA injection begins with a "starter" mouse from the Jackson Lab, a breed that
is one-quarter black and three-quarters brown. The lab has a stable of "stud"
males, who despite vasectomies romance the female mice at night, usually
around midnight.
Eggs from the females, which have been given hormones, are harvested every
morning from the fallopian tubes. The embryos are removed, and the biological
mother sacrificed.
The embryos are transferred to a dish containing a soupy mixture of
growth-promoting chemicals. Narrow-tipped pipettes, filled with foreign DNA,
are attached to a microscope, and a technician viewing the dish manipulates
them, poking the embryos cautiously and releasing the DNA.
A single technician does this 200 to 300 times a day, in the early
afternoon when the embryos are still young. The engineered embryos are then
implanted into the oviduct of a white surrogate mouse, and the pups are born
in about three weeks.
Not all pups carry the transgene. A sliver of tail is taken from each to
check its DNA to determine which are transgenic. Some will be merely "mosaic,"
in which only a small subset of cells carry the transgene, which then will not
transmit to offspring.
Knock-out mice have a better success rate - about a third - in part because
they are implanted in the uterus, rather than the fallopian tubes,
Pearson-White said.
Three weeks after birth, after weaning, the mice are transferred again,
this time to the client-researcher elsewhere on the U.Va. campus.INFORMATION U.Va. Transgenic Mouse Core Facility:
http://www.healthsystem.virginia.edu
VCU Transgenic Mouse Core Facility:
http://www.vcu.edu/mcc/research_info/massey_sr_transgenic_mouse.htm

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