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James Bradner, MD
Scientists have devised an innovative way to disarm a key protein
considered to be "undruggable," meaning that all previous efforts to
develop a drug against it have failed.
Their discovery, published in the November 12 issue of Nature,
lays the foundation for a new kind of therapy aimed directly at a
critical human protein — one of a few thousand so-called transcription
factors — that could someday be used to treat a variety of diseases,
especially multiple types of cancer.
"There is a pressing need for drugs that target transcription
factors, both for use as scientific tools in the laboratory and as
therapies in the clinic," said senior author James Bradner, MD, a
Harvard chemical biologist and oncologist at the Dana-Farber Cancer
Institute and an associate member of the Broad Institute of MIT and
"Our work brings us a step closer toward that goal for a protein with
major roles in cancer, cardiovascular disease and stem cell biology."
If human physiology is like a puppet show, then transcription factors
pull the puppet strings. They bind to DNA and turn genes on or off,
setting in motion genetic cascades that control how normal cells grow
and develop. They also help maintain tumor growth, underscoring their
importance as cancer drug targets.
Yet transcription factors are counted among the most difficult
molecules to neutralize with a drug — in fact, no such drugs are
Based on his work as an oncologist, Bradner became deeply interested
in a human protein called NOTCH. The gene encoding this protein is often
damaged, or mutated, in patients with a form of blood cancer, known as
T-ALL or T-cell acute lymphoblastic leukemia.
Abnormal NOTCH genes found in cancer patients remain in a state of constant activity, switched on all
the time, which helps to drive the uncontrolled cell growth that fuels
tumors. Similar abnormalities in NOTCH also underlie a variety of other
cancers, including lung, ovarian, pancreatic and gastrointestinal
Even with this deep scientific knowledge, drugs against NOTCH — or
any other transcription factor — have traditionally been extremely
difficult, if not impossible, to develop. Most current drugs take the
form of small chemicals (known as "small molecules") or larger-sized
proteins, both of which have proven impractical to date for disabling
A few years ago, Bradner and his colleagues hatched a different idea
about how to tame the runaway NOTCH protein. Looking closely at its
structure as well as the structures of its partner proteins, they
noticed a key protein-to-protein junction that featured a helical shape.
"We figured if we could generate a set of tiny little helices we
might be able to find one that would hit the sweet spot and shut down
NOTCH function," said Bradner.
Creating and testing these helices involved a team of
interdisciplinary researchers, including Greg Verdine, Erving Professor
of Chemistry at Harvard University and director of the Chemical Biology
Initiative at Dana-Farber Cancer Institute, as well as scientists at
Brigham and Women's Hospital and the Broad Institute's Chemical Biology
Program, which is directed by Stuart Schreiber.
Verdine invented a drug discovery technology that uses chemical
braces or "staples" to hold the shapes of different protein snippets.
Without these braces, the snippets (called "peptides") would flop
around, losing their three-dimensional structure and thus their
Importantly, cells can readily absorb stapled peptides, which are
significantly smaller than proteins. That means the peptides can get to
the right locations inside cells to alter gene regulation.
"Stapled peptides promise to significantly expand the range of what's
considered 'druggable,'" said Verdine, who is a co-senior author of the
study and an associate member of the Broad Institute. "With our
discovery, we've declared open season on transcription factors and other
intractable drug targets."
After designing and testing several synthetic stapled peptides, the
research team identified one with remarkable activity. Not only could it
bind to the right proteins and reach the right places inside cells, it
also showed the desired biological effect: the ability to disrupt NOTCH
Moreover, experiments in cultured cells as well as in mice proved the
peptide's ability to limit the growth of cancer cells fueled
exclusively by NOTCH. Interestingly, these effects are also seen at the
level of gene activity or "expression."
The researchers looked at the expression levels of genes across the
genome, in both cells and mice treated with the peptide, and observed
markedly reduced expression of genes that are controlled directly and
indirectly by NOTCH. These results offer some early insights into how
the peptide works at a molecular level.
In addition to the potential therapeutic applications to NOTCH-dependent cancers, the Nature study also forms the basis of a general strategy for taking aim at other transcription factors.
"A variety of key transcription factors assemble in a manner similar
to NOTCH," said first author Raymond Moellering, a graduate student in
Harvard University's Department of Chemistry and Chemical Biology who
works with both Verdine and Bradner. "Our approach could offer a
template for targeting many other master regulators in cancer."
This work was supported by the Leukemia and Lymphoma Society, the
American Association for Cancer Research, the American Society of
Hematology, the Harvard & Dana-Farber Program in Cancer Chemical
Biology, the NIH and other funding organizations.
Dana-Farber Cancer Institute (www.dana-farber.org)
is a principal teaching affiliate of the Harvard Medical School and is
among the leading cancer research and care centers in the United States.
It is a founding member of the Dana-Farber/Harvard Cancer Center
(DF/HCC), designated a comprehensive cancer center by the National
Cancer Institute. It is the top ranked cancer center in New England,
according to U.S. News & World Report, and one of the
largest recipients among independent hospitals of National Cancer
Institute and National Institutes of Health grant funding.
The Eli and Edythe L. Broad Institute of MIT and Harvard was founded
in 2003 to empower this generation of creative scientists to transform
medicine with new genome-based knowledge. The Broad Institute seeks to
describe all the molecular components of life and their connections;
discover the molecular basis of major human diseases; develop effective
new approaches to diagnostics and therapeutics; and disseminate
discoveries, tools, methods and data openly to the entire scientific
Founded by MIT, Harvard and its affiliated hospitals, and the
visionary Los Angeles philanthropists Eli and Edythe L. Broad, the Broad
Institute includes faculty, professional staff and students from
throughout the MIT and Harvard biomedical research communities and
beyond, with collaborations spanning over a hundred private and public
institutions in more than 40 countries worldwide. For further
information about the Broad Institute, go to www.broadinstitute.org.