Fast-Track Pharmacy
New Drug Targets: An Array of Possibilities
Eric Hoffman sorts through a set of gene microarrays, and a
resulting series of charts and graphs, to identify abnormal patterns of gene
expression in DMD. |
Eric Hoffman, a molecular biologist at the Research Center for Genetic Medicine
in Washington, is at the forefront of high-throughput screens to identify new
drug targets for DMD treatment.
Hoffman was part of the team that discovered the dystrophin gene. Now, he and
others are realizing that a deficiency of dystrophin sets off a chain reaction
of harmful effects on other genes needed in muscle. Ultimately, the muscle's
overall pattern of gene activity or expression runs amok, with some
genes getting inappropriately turned on and others inappropriately turned off.
Identifying those incorrectly expressed genes — and finding drugs to correct
their expression — might provide an effective way to treat the disease, Hoffman
says.
Fishing for Genes: The Gene Microarray
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| To use the gene microarray as a snapshot of gene expression in
muscles affected by DMD, Hoffman starts with a muscle biopsy from a person or
animal with the disease. In a series of chemical procedures, the biopsy is
stripped down to yield a "soup" of active genes, and those genes are then
tagged with a fluorescent dye. |
When the soup is poured over a gene microarray, an active gene
in the soup sticks to a matching gene fragment on the microarray, creating a
fluorescent spot. An inactive gene (not present in the soup) leaves a blank
spot on the microarray. |
"What's critical is to tease apart all of these effects, and figure out which
ones are important," says Hoffman. "Which ones are directly caused by problems
with dystrophin? We need to build pathways that go from gene to gene. Then, we
can target these pathways with drugs."
As late as 1995, putting together this genetic jigsaw puzzle would have been
nearly impossible; back then, scientists could only look at the expression of
one or a few genes at a time. But thanks to a recent invention called a gene
microarray — a tiny chip of glass neatly arrayed with thousands of gene
fragments — it's now possible to get a panoramic snapshot of up to 10,000 genes
at nce (see "Fishing for Genes").
Using a set of gene microarrays, Hoffman recently scanned through some 6,000
genes, and found about 150 that stand out with highly abnormal expression
patterns in DMD.
"What we're doing now," Hoffman says, "is looking at all those genes, figuring
out what they do, and building the pathways that connect them together."
Hoffman says this process is the key to identifying what he calls pathway-directed
drugs — drugs that can compensate for abnormal pathways of gene
expression.
Sometimes, he says, there's an obvious fit between an abnormally expressed gene
and a pathway-directed drug.
One of the genes that lit up on Hoffman's microarrays turned out to be a
signpost for dendritic cells, immune cells that promote inflammation and
probably contribute to muscle damage in DMD. Oxatomide, a drug that inhibits
dendritic cell activity, is already being tested in clinical trials for DMD.
Fishing for Drugs
Christian Lorson helped design a system that allows rapid
screening through candidate SMA drugs by looking for a simple change in treated
cells. |
High-throughput screens designed to find drug treatments for spinal muscular
atrophy are starting to yield some promising "hits," says Brian Pollok of
Aurora Biosciences, the biotechnology company running the screens.
In SMA, the death of muscle-controlling nerve cells (motor neurons) in the
spinal cord leads to muscle weakness and wasting, causing death during infancy
in the most severe cases. Nearly all cases of SMA are caused by defects in the
SMN1 gene, which encodes an essential protein called survival motor neuron (SMN). Although everyone has a backup SMN gene called SMN2, it normally doesn't
produce enough protein to fully substitute for the missing SMN1 product.
Fortunately, experiments on mice have shown that a genetically engineered boost
of SMN2 can compensate for a deficiency of SMN1. Inspired by those results,
Aurora's ongoing screens are designed to identify drugs that can stimulate
SMN2.
MDA grantees Christian Lorson of Arizona State University in Tempe and Elliot
Androphy of Tufts University School of Medicine in Boston are helping design
those screens. For the screens to work, "the most important thing is developing
a 'read-out' method that will allow you to quickly scan the effects of a large
number of chemicals," says Lorson.
Toward that goal, Lorson and Androphy attached the SMN2 gene to a gene that
encodes green fluorescent protein (GFP), a green-glowing protein
naturally found in certain jellyfish. When the SMN2-GFP gene is inserted into
cells in culture, the cells (unlike the jellyfish) normally remain unassuming
and dim, barely visible under a microscope. But if the cells suddenly crank up
their levels of SMN2-GFP protein, they glow bright green.
This simple visual distinction — black vs. green — allows a rapid search for
drugs that can jump-start SMN2.
"You hope for several preliminary hits, knowing that the end product will be
whittled down to a handful of useful drugs. It's a hit-and-miss strategy," says
Lorson.
So far, the strategy is paying off. Using a similar method, Aurora has already
completed a preliminary screen for SMA drugs.
"We've screened half a million compounds [on cultured cells], and there are some
that show an effect," says Pollok. "Once we single out a really promising
compound, it will take between 18 months and five years to make it a drug and
evaluate it for safety," he says.  |
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