They say that a picture can be worth a thousand words.
This especially is true for describing
the structures of molecules that function to promote
cancer. Researchers at Johns Hopkins have built
a three-dimensional picture of an enzyme often mutated in
many types of cancers. The results,
published Dec. 14 in Science, suggest how the most
common mutations in this enzyme might lead to
cancer progression.
"Now that we have a better picture of the protein and
how it is altered in cancer, we can
envision development of mutation-specific inhibitors for
cancer therapy," said Victor Velculescu,
associate professor at the Johns
Hopkins Kimmel Cancer Center.
The enzyme known as PIK3CA is mutated frequently in
many cancers, including colon, brain,
stomach, breast and lung. Moreover, most of the reported
mutations occur in a few so-called hotspots
in the protein. All known mutations make PIK3CA more active
than normal, causing cells to divide more
frequently or faster than normal to give rise to cancer.
L. Mario Amzel, professor and director of Biophysics
and Biophysical Chemistry in the School of
Medicine, said, "We tried to guess how the enzyme's
activity was affected by the mutations based on
their locations along the length of the protein, but
without a 3-D structure, it's hard to do. It's like
having a puzzle but missing critical pieces."
The research team isolated purified PIK3CA and part of
another protein to which it normally
binds, grew crystals of the purified enzyme bound to its
partner and figured out its 3-D structure
using techniques that shoot X-rays through the protein
crystals. Using computers, they analyzed the
X-ray pattern and assembled a 3-D model of the enzyme. Onto
this model the researchers then
mapped all the cancer-associated mutations.
According to Sandra Gabelli, an instructor of
biophysics and biophysical chemistry, the
researchers originally suspected that the mutations somehow
interfered with the way PIK3CA
interacted with other proteins and parts of the cell and
therefore must be on the outside surface of
the enzyme. However, their results show that nearly all the
mutations map to regions within the
enzyme.
"Somehow," Amzel said, "these internal mutations must
cause the protein to subtly change how
it works and interacts with itself. It's an interesting
problem to solve, trying to figure out what slight
shape and structural changes can make an enzyme work
better; usually we're trying to figure out why
things stop working."
The researchers currently are unraveling the structure
of mutated PIK3CA so that they can
compare mutated to unmutated to better understand how the
mutations lead to cancer. Another goal
is to find drugs that can specifically interfere with
PIK3CA and turn it down, to develop cancer-
fighting therapies.
The research was funded by the Virginia and D.K.
Ludwig Fund for Cancer Research and the
National Institutes of Health.
Authors on the paper are Chuan-Hsiang Huang, Diana
Mandelker, Oleg Schmidt-Kittler, Yardena
Samuels, Kenneth W. Kinzler, Bert Vogelstein, Gabelli,
Velculescu and Amzel, all of Johns Hopkins.
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