"Another way doctors could use CRISPR is to assist in regenerating
tissue within damaged organs. Epstein ultimately wants to place
embryonic stem cells that have developed into cardiac muscle cells back
into the heart. But the main danger with this lies in accidentally
injecting any non-cardiac cells. "If you put a cell into the heart meant
to make a tooth or a hair, it might cause a tumor," said Epstein.
So
instead of blindly inserting a group of cells hoping they are all
cardiac muscle, he is using CRISPR to insert marker genes - such as a
gene that includes a glowing, green fluorescent indicator - to be able
to clear out every other non-heart cell in mouse models.
Earlier
methods of performing genomic surgery had barriers of high costs and low
flexibility that kept many researchers from adopting them.
"Then
CRISPR started coming out, and since then it has absolutely exploded,"
said biologist Montserrat Anguera of Penn's School of Veterinary
Medicine. "CRISPR seems to be the easiest and fastest way for labs to
edit the genome."
She studies how embryonic stem cells develop
into specialized cells within organs such as the liver or heart. Using
CRISPR, she can delete regions of the stem cell genome to help decipher
their function in human development.
Bao first began his work with
sickle-cell disease using older systems such as zinc finger nucleases,
but has since switched to CRISPR - and he is a believer.
"I call them nanoscissors - a truly amazing tool," Bao said.
Anguera
joined Penn's faculty a year and a half ago after a postdoctoral
fellowship at Harvard Medical School, where CRISPR-guided research
flourishes. Both she and Epstein hope to grow Penn's community of users,
now just a handful.
The Broad Institute, a Harvard-MIT biomedical
research collaborative, holds the patent for the tool's components and
methods. Its main inventor? A 32-year-old neuroscientist, Feng Zhang.
Early last year, Zhang and his team were the first to demonstrate the
system's search-and-edit capabilities in the cells of mice and humans.
Researchers
at the University of California, Berkeley, first used CRISPR to make
targeted DNA cuts. After more work by Zhang, CRISPR has become the
powerful tool seen today, with a proven record in many animals and
plants.
"The analogy would be like the search-and-replace function
in Microsoft Word," Zhang said. "In the genome, we don't have a
biological search function, so we use a specific string of letters."
This
string of 20 letters, or bases, is used as a template to search for a
specific matching section of the genome, which is no small feat. In
humans, double-stranded DNA is made up of three billion base pairs. But
once you engineer the special 20-letter-long code, or guide RNA, the
CRISPR system will target the desired gene by searching along the DNA
until it makes a match. Once locked in, an enzyme then acts like DNA
scissors and cuts the two strands.
Because many CRISPRs can act at
once, you can delete whole regions or cut and paste from different
areas of the genome. The tool can also exploit the cell's natural DNA
repair mechanism and weave a new piece of genetic code into a gap.
Zhang has cofounded a Cambridge, Mass.-based start-up, Editas Medicine, to develop new treatments using CRISPR.
"In
areas of research, CRISPR can help us understand and identify genetic
mutations that can lead to disease," he said. "Clinically in the long
run, we might be able to use it to repair mutations."
Zhang also
notes that real-world applications abound beyond medicine too: making
better crops or biofuels. But experts say it will be years before
genomic surgery comes to a hospital near you.
"It may take 10
years, could be shorter," Bao said. "The most important challenge is
off-target cutting - not only cutting where you want to cut, but also at
other locations you don't want."
For instance, if 19 out of 20
letters match up, the guide RNA may still bind and cause the enzyme to
cut the wrong gene. Zhang is working on ways to make the search more
stringent, such as increasing the number of letters.
And even when
the correct edits are made, an organ could have other issues.
Researchers recently regenerated damaged heart muscle in monkeys using
an older gene editing technique, and found that the animals had episodes
of irregular heartbeats after the procedure.
Still, Epstein remains optimistic about CRISPR's future, and predicts it will be available to patients within a decade.
"It's not pie-in-the-sky anymore, it's real," he said. "Changes can occur in surprising, quantum leaps."
meerinkim@gmail.com
857-205-6920
+on+Twitter+2014-07-23+21-01-02.png)
Articles
