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Building a better mouse
A Yale geneticist and a Chinese lab are creating the Amazon.com of medical research animals
May/June 2010
by Margot Sanger-Katz ’02
Margot Sanger-Katz ’02
is senior staff editor of the Yale Alumni Magazine.
Mice like to bury
marbles. If you give a laboratory mouse a handful of marbles, it will often
bury a few into the bedding in its cage. But one particular mouse in Yale
genetics professor Tian Xu’s lab will bury marbles all day long. That mouse is
missing a gene. Having seen it at work, Xu and his colleagues now think the
gene may be related to behavioral problems like obsessive-compulsive disorder.
Another mouse of Xu’s
appears to have the mouse equivalent of male-pattern baldness. It grows lovely
white fur all over its body, but its head is bare. Studying the gene this mouse
is missing could help scientists understand what causes some men to lose the
hair on their heads.
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Humans and mice share about 99% of their genes.
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Tian Xu ’90PhD, who has
developed an innovative system for easily producing mice missing single genes,
is raising thousands of mutant mice in a laboratory complex in China—mice that
don’t grow properly, mice that have kidney disease, mice with neurological
problems, mice that lack sex appeal—in hopes that researchers around the world
can use those mice to better understand human development and disease. His goal
is a “functional map” of the mouse genome, linking every gene in the mouse to
its function in the species.
Such a tool has never
been created for a mammal, and it could have major implications for human
medicine. Humans and mice share about 99 percent of their genes, making mice,
which are small, inexpensive to raise, and (relatively) short-lived, an ideal
animal model for studying human disorders and testing possible treatments.
Biologists have been studying mice for decades, using them to learn about
physical and mental development, cancer, and heart disease, among other
subjects. The scientists who developed the first system for disabling single
genes in mice won the Nobel Prize in Medicine in 2007.
Over the last two decades,
scientists who needed mice missing specific genes have used gene splicing and
other methods to engineer them for lab work. Texas A&M Health Science
Center professor Richard Finnell, for instance, studies spina bifida, a birth
defect that can lead to lifelong paralysis. In 2006, he developed a mouse with
damage to a gene called FKBP8, which controls the development of the spinal
column. (He calls the mouse “Stacey.”) He has used its descendants ever since
in his research on the environmental factors that contribute to the birth
defect.
All told, scientists
have so far succeeded in knocking out about a fifth of the 24,000 genes that
have been mapped in the mouse genome. Xu’s breakthrough, developed at Fudan
University in Shanghai, where he has an adjunct faculty appointment, was in
creating a simple and inexpensive system for disabling different genes quickly
and efficiently—a sort of assembly line for mutants. In the first 18 months of
his project, he’d already reached the 5,000-gene mark that took previous
scientists 20 years.
Instead of the
painstaking work of splicing coded bits into mouse DNA, placing the manipulated
DNA into embryonic stem cells, and then implanting those cells into pregnant
mouse wombs—a process that can take an individual lab as long as a year and
cost more than $50,000—Xu can breed a mouse with a disabled gene simply by
mating two specially prepared mice. One of the mice has a piece of genetic
material called a transposon, which can jump within the genome of the
reproductive cells, inserting itself at random to disable an existing gene. The
other makes an enzyme that activates the jumping gene.
Breed the two together,
and each of their offspring will be born with one inactive gene, a different
gene in each. The process doesn’t require molecular biologists, just skilled
technicians who can care for the mice and run basic tests to determine which
gene is missing.
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Xu has figured out a way to color-code the mutant mice.
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Transposons are common
in plants and insects, and scientists have used a similar technology to create
mutants in insect species. But before Xu, no one had been able to find a
transposon that would work efficiently in mammals. And, in a clever twist, he
has figured out a way to color-code the mice so that technicians can tell
instantly whether a mouse is normal or a mutant: the mutant mice carry a gene
that makes them glow pink under an ultraviolet lamp.
The mutants are bred in
a lab in China large enough to house 150,000 mice at a time. Overseen by about
150 Chinese lab technicians and scientists—and U.S. researchers via webcam—the
lab is breeding its way through the mouse genome. Xu expects to disable nearly
every mouse gene in a few years, at a fraction of the cost required to complete
the project the traditional way.
“There’s a significant
advantage to doing this stuff in China, because it’s just so cheap,” says Colin
Fletcher, program manager for the National Institutes of Health’s (NIH) Mouse
Knockout Project, which has funded part of Xu’s research. The project is also
funding other scientists to knock out 8,500 genes in mouse stem cells using
more-conventional technologies; its goal is to make a complete “library” of
knockout mouse stem cells available to researchers. In China, Fletcher says,
Xu’s work sees cost savings in building construction and lab equipment, but particularly
in labor. A technician who would earn a salary of more than $30,000 in the
United States can be hired for about $4,500 in China.
The breeding facility
occupies two of the four buildings in a huge complex at Fudan University in
Shanghai, the Fudan-Yale Biomedical Research Center. The first building, with
about 25,000 square feet of mouse space, went up in just six months, Xu says,
and the other buildings followed quickly. The fourth and largest addition,
still under construction, will add 100,000 square feet of space, most of it for
mice. “This really made it possible to do it large-scale,” Xu says, marveling
at the speed of the Chinese construction effort. The complex not only houses
his project, but also aids Fudan faculty research and offers training for
talented graduate students and postdocs—some of whom Yale may eventually
attract to do research here.
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Yale has created several such collaborations with Chinese universities.
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The collaboration with
Fudan was Xu’s own suggestion, in part because he received his undergraduate
training in genetics there. He grew up in China, the son of intellectuals who
suffered under the Cultural Revolution, and is quick to credit Yale for giving
him opportunities—when he was a graduate student on fellowship who spoke little
English and when he was an assistant professor with a novel idea for a project
on mice. But he says he is also eager to give back to Fudan, a school with
highly regarded biology departments and numerous Yale connections. (Xu, who is
the vice chair of the Yale genetics department and holds a prestigious
five-year grant as a Howard Hughes Medical Institute investigator, is the
director of the Fudan-Yale Center.)
In its
post-tercentennial push to globalize its research, Yale has created several
such collaborations with Chinese universities. The projects fulfill President Richard
Levin’s mission to “become a truly global university,” says Fawn Wang of Yale’s
Office for International Affairs, who is tasked with setting up the Yale-China
partnerships. But they also allow Yale researchers to complete major research
on the cheap. Xu’s mouse buildings were built with grants from the Chinese
government; Yale brought $1 million in NIH grant funding for the research, but
didn’t raise donor gifts or contribute any money from its general fund. “This
is such a world-class university,” Wang says. “They can use our name to apply
for money.”
Although costs are low
for the lab at Fudan, Xu says he has been careful to ensure that the lab
maintains the same high standards for animal care that would be employed in a
Yale facility. He works at the facility for about one week out of every month
and says the mouse areas are climate-controlled, roomy, and constantly
monitored. Fudan is responsible for animal care, but Yale’s contract with Fudan
specifies that it has to meet NIH animal care standards. As with other
subcontractors, Yale does not undertake on-site inspections. But a
representative of Yale’s Institutional Care and Animal Use Committee toured the
facility with President Levin when it opened, Xu says, and pronounced it better
than some animal labs in New Haven. He considers it an important visit, “so we
were not attacked for running a sweatshop.”
Breeding the mice is
just the first step. Because such a small portion of the genome is currently
understood, Xu’s next project is figuring out what effect each disabled gene
will have on the development, health, and behavior of the mice. And diagnosing
the problems is a complex business, because Xu’s method of knocking out genes
is random. Unlike the molecular biologists who disable a selected gene because
they think it may be related to diabetes or cancer, Xu has no way to control
which genes will be deactivated by his breeding process. So when each mouse is
born, he needs to test it for a whole battery of different abnormalities.
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The mouse hospital will soon be a part of Yale’s new West Campus.
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Back at Yale, Xu is at
work creating the “most advanced mouse hospital on earth.” Parts of that
hospital are already functional, in his current lab at the Boyer Center for
Molecular Medicine. A miniature CT (computed tomography) scanner sits in the
corner of a laboratory room, along with a hot plate (used to detect pain
response) and other mouse-testing equipment. But the mouse hospital will soon
be a part of Yale’s new West Campus, where substantial mouse vivarium space
will be devoted to Xu’s project. “I was trained as a PhD in genetics to become
a mouse doctor,” Xu jokes.
Yun You, an associate
research scientist at Yale who is managing the mouse testing effort, says the
diagnostic work begins even before the mutant mice are born. About 15 percent
of genes appear to be so vital to survival that mice without them die in utero.
Some of those that survive will have visible problems: they won’t grow
normally, or they have patchy fur, or they grow tusks. Others require more
advanced testing. Every mouse has its blood drawn and its heart rhythm scanned.
The mice get colonoscopies, X-rays, and CT scans. Motor control is tested on
tiny balance beams. Mouse memory is tested using special mazes. Behavior tests
look for depression, fearfulness, obsessive-compulsive disorders, and trouble
with breeding. Xu hopes this method will reveal many more genes of medical
importance than would have been discovered the old way, by guesswork.
As the lab learns more
about the function of each gene, Xu and his colleagues will publish scientific
papers and enter the information about each mouse into a central Internet
database. That website is already live with about 1,200 known genes. Scientists
can use the site as a sort of catalog, then contact Xu with an order if a
particular mutant would aid their research. Xu wants to create a shopping-cart
system, so that scientists can search for traits and genes and order frozen
embryos or live mice in a few simple steps.
“They’re all
available,” says Jon Soderstrom, managing director of the Yale Office of Cooperative
Research, who is helping Xu patent the mice and set up a for-profit corporation
to manage the mouse-selling effort. “They’ll all be cataloged, and you’ll be
able to go online and click through and put your order through.”
That concept is similar
to a website the NIH has built for its knockout mouse project, but is vastly
different from how knockout mice were obtained for research in the past. Until
recently, there has been no comprehensive database listing knockouts and no
reliable way for scientists to put in requests, even for those mice that were
ready. Though Yale’s plan is to make money from mouse distribution, the
project’s overall goal is to make mice widely available, at lower prices than
those of most labs that currently offer mutant mice to researchers.
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Xu creates live breeding pairs; the NIH library will provide modified stem cells.
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Stefan Somlo, a
nephrologist who works across the street from Xu, read an early published
report about Xu’s mutants, and found a mouse that was perfect for his own work
on a common inherited kidney disease that causes cysts and kidney failure. He
hopes the transposon-created mouse will help him find out whether therapies
that turn the gene on and off will influence the progress of the disease. “This
is an enabling technology for us,” he says.
As NIH scientists rush
towards a similar goal—a complete library of knockout mice—it is unclear
whether Xu’s mice will become the favored research animals or simply one of
several options for scientists. But if it works, Xu’s massive experiment could
serve as a demonstration project for how to catalog an entire genome quickly.
He’s already shown that a similar transposon technology can be used to create
mutant rats, and he believes it could work in other mammal species that may be
preferable for some kinds of research. Moreover, Xu says his mice are likely to
be less expensive to use because his method creates live breeding pairs. The
NIH library will provide modified stem cells that are many complex steps away
from living research animals, because scientists must transfer them into mouse
embryos that they then breed for several generations.
Research has also shown
that different mutant technologies can yield subtly different effects, and Xu’s
mice may be favored by certain scientists. “Our work is complementary,” says
Kent Lloyd, a veterinarian and research physiologist at the University of
California–Davis who is working on the NIH knockout project.
Xu is also carving out
his own project in the mouse genome. He knows that most of the mice he creates
will benefit other researchers, but he’s taken an interest in a few mutants
they’ve already uncovered. One particularly promising mutant has trouble
absorbing nutrients in its digestive system. It is born with chronic diarrhea
and ultimately dies after suffering many of the same symptoms seen in starving
children. Xu thinks the mouse could serve a double value. By determining how to
improve the efficiency of its digestive system, Xu hopes he can help develop
treatments to mitigate starvation in parts of the world where food is scarce.
He also hopes the mouse’s inefficient digestive system will help him find ways
for humans who eat junk food and live sedentary lives to avoid the ravages of
diabetes and heart disease. It’s a very American question for a Chinese mouse.  |
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