The “finally” isn’t just hyperbole. So-called knockout mice,
engineered by “turning off”
selected genes, have been around
since 1989. Knock-in mice, whose
DNA is spiked with foreign genes—
often human—came a few years
later. Because of the genetic and
physiological similarities between
mice and humans, these mice
have become widely used to study
the function of genes and to model
human diseases including cancer,
heart disease, diabetes, neurological disorders, and obesity. The
inventors of the knockout mouse
were awarded the Nobel Prize in
Physiology or Medicine in 2007.
Rats are even more physiologically similar to humans; their size
makes certain experiments easier,
and their bigger brains make
them a better model for studying
neurological conditions. But for
almost two decades, efforts to
genetically engineer rats via
the standard mouse method—
manipulating and culturing
embryonic stem cells, implanting
them in an embryo, and breeding
two more generations of animals
to produce a mutant strain—
failed. That changed a few years
ago. Over a decade of research, a
company named Sangamo Bio-Sciences developed a technology
called zinc-finger nucleases
(ZFNs), which it planned to use
in therapeutic applications for
humans. In 2008, Sigma obtained
exclusive rights to use zinc fingers, which are synthetic
enzymes, for R&D applications
and animal models, and in July
2009, its researchers, along with
collaborators from other labs,
hailed the creation of the first targeted knockout rats in the prestigious journal Science.
You can think of ZFNs as scissors that snip DNA at any location
you choose. (Genes are essentially
lined up in a row along t wo t wisting strands of DNA.) Say you want
to knock out a gene called p53,
which inhibits the growth of
tumors. (A rat with fast-growing
tumors is a valuable tool for testing potential tumor-shrinking
drugs.) You program the ZFNs to
find the front end of that gene,
inject the ZFNs into the nuclei of
one-cell rat embryos, and transfer
the modified embryos into a foster
mother. At three weeks old, the
resulting rat pups are tested for the
knockout. DNA has a self-repair
mechanism that will “tape up” the
cut about 90% of the time, resulting in a normal rat. The other 10%
of pups will have the desired
knockout. These animals are then
bred with one another to create a
colony that will consistently pass
on the mutation. SAGE has produced knock-in rats the same way.
What may be most
remarkable—or, if you’ve seen
Planet of the Apes, disturbing—
about SAGE’s zinc-finger technol-
ogy is that it seems to work not just
in rats and mice but in any animal
that produces an embryo. At press
time, SAGE was preparing to
announce the first successful
effort to produce a knockout rabbit,
an animal widely used for ocular
and cardiovascular research. Pigs,
used in studies on skin conditions
and cystic fibrosis, are another
potential platform. Researchers
collaborating with SAGE have also
expressed interest in working
with monkeys.
