Contact: Cherie Winner, 509-335-4846, cwinner@wsu.edu
Molecular Mata Hari
"What we're trying to do is evolution in a test tube," says molecular biologist Margaret Black (right), here with assistant Karina Villa-Romero.
Margaret Black designs weapons that make their targets
self-destruct. She's not a military strategist or explosives
expert, though, but a molecular biologist working to perfect a way
to trick cancer cells into killing themselves.
Her approach is called suicide gene therapy. It works by a sneaky
route that even a fabled spy like Mata Hari could appreciate. In
conventional chemotherapy, the patient is given a drug that kills
any cells in the body that are replicating their DNA. Suicide gene
therapy works by infiltrating cancer cells and getting them to make
the drug that will do them in.
Best of all, says Black, suicide gene therapy can minimize the
nasty side effects commonly suffered by patients on
chemotherapy.
A patient on suicide gene therapy receives two substances: a
prodrug, which by itself has little effect; and a suicide gene that
codes for an enzyme that converts the prodrug into a toxic form of
the drug.
Black, who is an associate professor in the Department of
Pharmaceutical Sciences, says most gene therapists are working on
how to target cancer cells specifically, so the suicide gene
doesn't end up in normal cells. Her research focuses on what
happens once the gene and prodrug get into the cell. She's trying
to make a form of the gene that is more deadly for cancer cells and
that produces less spillover of toxin from cancer cells to normal
cells.
She does that by making mutations in the natural form of the
enzyme's gene and then screening for mutant enzymes that can work
with just a tiny amount of prodrug.
"What we're trying to do is evolution in a test tube," says Black.
"Evolution is generally something that happens one step at a time,
one mutant at a time. What we're trying to do is make leaps at a
time by introducing many mutations simultaneously."
One approach is to hit the gene with random mutations. That works,
but it's inefficient. Black prefers to target areas of the gene she
thinks might shift the enzyme's activity without destroying it
altogether. Then she puts the mutant genes into bacterial cells and
uses a two-step screening process to find the mutants that are most
likely to succeed as suicide genes. First she identifies the
mutants that still have a functional enzyme.
The second test is a bit tricky. The mutants she wants-those best
able to turn the harmless prodrug into the deadly drug-will die
during the test. Black keeps samples of each mutant alive and
healthy in other containers until the test is completed. Those
whose brethren die when fed the prodrug move on to more detailed
analysis of their mutant enzyme.
The screening process looks reasonable on a diagram. Then you see
the numbers she's dealing with. In one series of experiments, Black
and her students screened more than a million mutants. They found
two that were good candidates as therapeutic agents.
"It's a numbers game," she shrugs. "There's a point where you can
go crazy doing this. My students will tell you that, because
they're in the process of screening a lot."
In work recently reported in Science Magazine, Black and several
colleagues at the Fred Hutchinson Cancer Research Center and the
University of Washington described a new method to streamline the
process. They used a special computer program to predict the
effects of mutations at various points in the enzyme, and came up
with a triple mutant that looked promising. Black is now testing
the new weapon for its ability to kill cells from a rat brain
tumor.
Black didn't start out as an espionage agent in the cancer wars.
Her main interest has been the enzymes involved in DNA
synthesis-how they work, and how their structure relates to their
function. The cancer connection came about in 1992, when she read a
paper describing the use of one of her subject enzymes as a suicide
gene. "I immediately took my mutants and said, 'We can do better
with these.' It totally shifted in two minutes what I was doing
already. It was just a little bit of an extension, and it made a
big difference."