Download New Technologies Yield Insight Into How Cells Go Awry

New Technologies
Yield Insight Into
How Cells Go Awry
The discovery, so far, of approximately 50 oncogenes and tumor suppressor genes has wrought important,
yet incremental victories in the war
against cancer. The problem is that trying to understand such a complex disease in terms of individual genetic
defects is like the five blind men seeking to comprehend an elephant.
In what has been dubbed the postgenomics revolution, several technologies promise to allow researchers to
view the entire elephant, instead of a
leg here and a trunk there. Rather than
searching for single genes in tumor
cells, researchers will be able to make
side by side comparisons of normal and
cancerous cells, or of different stages of
malignancy, to see which genes are expressed in one state while silent in the
other.
For example, "If you find a gene in
normal epithelium that is absent in tumor, that might be a tumor suppressor
gene," said Mark Adams, Ph.D., investigator at the Institute for Genomic Research, Gaithersburg, Md.
New Insights
More importantly, researchers believe that the patterns of gene expression in cancer cells will give them new
insights into how and why cancer cells
go awry, and how better to intervene.
Two of the techniques are ways of
detecting messenger RNA in samples
of cellular extract. The third, known
as proteome technology, is a modernized version of an old technique using
2-dimensional gels.
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Researchers anticipate discovering a
slew of cancer-related genes, from
which may emerge the maps of the pathways of transformation. The techniques
should also illuminate drug action
mechanisms, and rationalize the search
for new therapeutics.
"If you treat a cancer cell with interferon or TNF, you are going to be able
to see what the global effect on a large
number of genes is," explained Paul
Meltzer, M.D., Ph.D., head of the Section on Molecular Genetics, National
Center for Human Genome Research.
"That takes you towards a better understanding of how these important biological pathways in cell growth really
work."
"The greatest impact will be on development of complex diagnostics to
guide therapeutic use," said Leigh Anderson, Ph.D., president of Large Scale
Biology Corp., a biotechnology company in Rockville, Md. "Different tumors may be related to different
patterns of oncogenes."
"It might be possible to identify
prognostic markers for drug sensitivity, resistance, and for metastatic and
non-metastatic cancers," said Kenneth
Kinzler, Ph.D., associate professor of
oncology, Johns Hopkins Oncology
Center, Baltimore. Working with one
of the RNA techniques known as
SAGE, he and Bert Vogelstein, M.D.,
also professor of oncology at Johns
Hopkins, are characterizing expression
patterns of colon and pancreatic cancer
cells.
Prognosis and diagnosis are only
the beginning. The genomics revolution will refine oncologists' crude
tools. A better understanding of how
sex hormones influence cancer might
permit doctors to prevent testosterone
from fanning the flames of prostate cancer without turning off the sex drive
and feminizing men, said Otis
Brawley, M.D., senior investigator in
the National Cancer Institute's Division of Cancer Prevention and Control.
How Drugs Work
Besides a better understanding of
how cancer arises, researchers also expect to gain a clearer picture of how
drugs work. This would greatly enhance the search for new drugs.
Proteins that wax or wane when the
cell is exposed to a drug are usually
playing an important role, either therapeutically, or as part of toxic reactions,
explained Anderson.
"We studied chemopreventive compounds with Vernon Steele, [Ph.D., program director of the chemoprevention
branch, NCI]," said Anderson. "A
group of those compounds are analogs
of the drug Oltipraz, which is known to
protect against many carcinogens in rodent test systems. The potency [of the
analogs] was very closely correlated
with the appearance of certain proteins
in the livers of the treated rats."
"We then selected the protein that
was most strongly induced," said Anderson. It turned out to be aflatoxin aldehyde reductase, an enzyme that
metabolizes the eponymous fungus, and
has proven protective against aflatoxin
in animal studies. (Human trials are under way in China, where aflatoxin is implicated in liver cancer.)
Journal of the National Cancer Institute, Vol. 88. No. 5, March 6, 1996
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Studies of the antihistamine methapyrilene, and a benign analog, pyrilamine, further support the hypothesis
that expression patterns have predictive
value. In 1980, methapyrilene was
pulled from pharmacy shelves after researchers discovered it caused cancer in
100% of exposed rats. "We looked at
the effects of both of these compounds
in rodent liver," said Anderson. "Methapyrilene causes significant changes in
abundances of more than 100 proteins.
Pyrilamine causes none."
Improving Therapeutic Windows
"One of the principal things to be
learned is how to do the most good
with the least harm," said Anderson.
"You can get a lot of information just
by comparing how many proteins are
affected by a series of analogous drugs.
If they all have similar potency, there
is pretty good reason to believe the
best one is the one that causes the fewest changes."
In its quest for new drugs, NCI's Developmental Therapeutics Program has
screened 55,725 new compounds
against 60 cell lines from different
types of cancer. Recently, the program's John Weinstein, M.D., Ph.D.,
has teamed up with Anderson in using
the updated proteome technology to develop a database of expression patterns
from each of the cell lines. This will
make it possible to gain a better understanding of how anti-cancer compounds
work.
For example, a compound's impotence against all cell lines expressing
the MDR-1 gene, which codes for a
membrane pump that removes certain
compounds from the cell, would reflect
its removal by that pump.
Expression studies also offer the opportunity to determine just how predic-
tive particular in vitro and animal models really are, said Norman Anderson,
Ph.D., chairman and chief scientist at
Large Scale Biology, and father of
Leigh. The greater the similarity of expression patterns, the more predictive
the model, he said. For the animal and
in vitro models to be predictive, their
expression patterns should be similar to
those in human tumors, particularly the
expression patterns of proteins that are
targets of drugs.
But Weinstein and Leigh Anderson
recently compared expression patterns
of some of the cell lines used in the
NCI screen. Expression patterns sometimes appeared more similar among different cancer types than among cells
from one cancer type in different systems, leading Norman Anderson to
question the predictive value of NCI's
models.
Edward A. Sausville, M.D., Ph.D.,
NCI's associate director for developmental therapeutics, asserted that the
program's screening remains "a defensible first pass at looking at whether a
compound may be active in a clinical
situation." Of the thousands of compounds that have undergone initial
screening to date, 31 compounds or
classes of analogs have been selected
for development, and Investigational
New Drug applications have been filed
for five of these.
Resolving Theoretical Questions
Nongenotoxic cancers are one of the
mysteries of cancer research. These are
cancers whose cells do not show any
genetic mutation and which are caused
by compounds that generally only promote cancer. Such cancers are well
known in animal models, but to her
knowledge, none has been proven to
occur in humans, said Lucy Anderson,
Journal of the National Cancer Institute, Vol. 88, No. 5, March 6, 1996
Ph.D., chief of the perinatal carcinogenesis section of NCI.
No one has yet determined the molecular biology of nongenotoxic carcinogenesis. A variety of simple
mechanisms have been proposed,
such as out-of-control cell division, or
interference with programmed cell
death, said Leigh Anderson, "but it's
extremely unclear whether any of
those simple mechanisms will explain
the diverse effects of non-genotoxic
carcinogens.
Proteome studies show that nongenotoxic carcinogens cause far
greater changes in gene expression
than "almost all other classes of
agents we've looked at," said Leigh
Anderson, possibly due to the size of
the disturbance.
Beyond this question, the new technologies may open up whole new areas
of inquiry, both in oncology specifically, and in molecular biology generally. For they allow researchers not
only to determine which proteins are active, but where in the cell they are active, according to the Andersons. This
can be done simply by dividing the cell
into different component parts prior to
conducting the studies.
If sequencing the genome is like
gathering anatomical data, then postgenomics can be likened to physiology,
said Leigh Anderson. Understanding
how it all fits together will require the
kinds of skills that software designers
have, he predicted, adding that they
would contribute to this new era in
the manner that physicists and chemists helped usher in molecular biology. Then, it will become possible to
model how cells function, and to understand, at the deepest level, why
they go awry.
— David Holzman
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