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Key to Triplet Repeat Brain Diseases Opens Door for New Way to Understand, Treat Genetic Diseases

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Duke Health News 919-660-1306

DURHAM, N.C. -- As reported in the March issue of Nature
Medicine, Duke University Medical Center researchers have found
a possible key to brain disorders such as Huntington's disease,
as well as discovering a new concept of how mutant genes may
produce disease.

The scientists discovered that these "triplet repeat"
disease genes produce proteins that errantly stick to an enzyme
crucial to the production of energy in brain cells. They
believe that because this enzyme cannot, therefore, do its job,
brain tissue will malfunction and may, over years, die. This
would result in fatal physical and intellectual deterioration,
such as that seen in Huntington's disease patients.

Furthermore, their findings may answer why the number of
genetic repeats in these diseases determines how severe
symptoms will be: More repeats produce a protein that holds
this "energy" enzyme more tightly, producing widespread nerve
cell failure in the brain.

The work offers biomedical researchers a new way to
understand, and to potentially treat these genetic diseases,
the Duke scientists say.

"This is the first explanation of what these proteins
produced by triplet repeat genes may be doing in nerve cells,"
said neurologist and principal investigator Dr. Warren
Strittmatter. "It provides a new paradigm for inherited
neurologic disease, and that is the repeat region of these
proteins binds other proteins critical to glucose and oxygen
metabolism in the brain. And the larger the repeat region, the
tighter they bind."

Authors of the study are from the same team that discovered
the major gene linked to Alzheimer's disease. In many ways,
"this finding may be more pivotal than our earlier work," said
Dr. Allen Roses, scientific director of the Deane Laboratory at
Duke, where the experiments were conducted. "We are beginning
to cross the difficult area between gene discovery and abnormal
functions of the brain, and to open the door to target drugs,"
he said.

The primary researchers involved in the discovery are Duke
neurologists Strittmatter, James Burke and Jeffery Vance. Other
collaborators include Jan Enghild and Margaret Martin, both
from Duke, and geneticists Yuh-Shan Jou and Richard Myers from
Stanford University School of Medicine.

Abnormal "triplet repeat" genes constitute the molecular
equivalent of stuttering. They repeat the same sequence of
genetic units, called nucleotides, over and over far more than
do normal genes. While a normal gene might repeat the three
nucleotides cytosine, adenine and guanine (CAG) fewer than 30
times, genes that produce Huntington's disease and other such
brain disorders might repeat the sequence between 40 and 100
times. As the repetition grows, the disease becomes more
severe.

The Duke study explains why the number of nucleotide repeats
in such diseases relates to the severity of symptoms, the
researchers say. The nucleotide sequence CAG determines that
the amino acid glutamine will be inserted into the protein
during its translation from genetic information to protein in
the brain cell. The greater the number of CAG repeats in the
gene, the more glutamines will be added, resulting in a
structurally different protein that binds strongly to a
particular energy enzyme. As these crucial enzymes become
disabled, increased numbers of neurons malfunction and may die,
destroying brain tissue.

The researchers have found this effect in two neurological
diseases -- Huntington's and Haw River Syndrome. They believe
the effect also works for at least three other rare
neurological disorders -- Kennedy's syndrome, Machado-Joseph
disease, and Spinocerebellar atrophy I -- that have the same
expanded CAG repeat.

"Traditional dogma is that mutant genes produce proteins
that either don't work at all or don't work well," said Burke,
the first author. "However, we found the proteins produced by
these genes interact in a different way, based on the number of
repeated amino acids, producing devastating consequences."

"The novel insight is that this extra domain produced by
triple repeats acts in the same way across all these diseases,"
Vance said. "Even though we don't know what the normal function
is of each of the disease genes, the triple repeats are causing
all the proteins to bind to this single enzyme. Scientists have
usually only thought about how a gene mutation produces a
change in that protein that results in a disease. This is an
observation that provides a mechanism for a group of
diseases."

If drugs could block the protein interaction the researchers
described, then the discovery may offer treatment options,
according to Roses.

The investigators became interested in the action of
repeated gene sequences in brain tissue two years ago, when
they discovered Haw River Syndrome (also known as DRPLA), an
inherited neurological disorder that affects residents of a
rural North Carolina community.

The scientists found that the syndrome was like Huntington's
disease in that it was attributed to genes that acquire excess
CAG nucleotide sequences.

Although the scientists do not know what the roles of the
proteins produced by Haw River or any of the triplet repeat
diseases are, "Our hypothesis is that these different proteins
are all doing something similar, which is binding to the same
protein," Vance said. In other words, the abnormal repeat in
the DNA is adding a "docking area" domain in the protein, which
allowed substances that bind loosely to the normal protein to
stick more tightly to the enlarged disease protein.

To test that theory, the Duke team made artificial strings
of glutamines, either 20 glutamines, found in normal proteins,
or 60 glutamines, found in the mutant proteins. They then mixed
these glutamine strings with brain proteins to see what
proteins bound. They found that one brain protein,
glyceraldehyde-3 phosphate dehydrogenase (GAPDH), bound to the
long glutamine string better than to the short glutamine
string. GAPDH, found in all cells, is known to produce energy,
along with a number of other important functions.

In a second experiment, brain proteins were incubated with
GAPDH. Huntingtin CQ protein and DRPLA protein bound to GAPDH,
while other brain proteins did not.

While he can hypothesize that binding of GAPDH may start the
pathway of errant protein interactions, Strittmatter said he
can't yet define the cascade of events that produces the
disorders. "The brain is entirely dependent on glucose
metabolism, and this is a protein that is important in that
pathway," he said. "But because GAPDH is expressed in every
cell, not just in brain cells, there has to be another level of
complexity that we don't understand."

"This is at least an initial insight into the molecular
genesis of these diseases," Burke said. "It is a novel
interaction that may explain the increase in severity with
increasing repeat size."

The researchers point out that their theory of triplet
repeat diseases would not apply to those that contain
stuttering sequences in the "junk" area of the genome, since
those areas do not encode for protein production. The genetic
disease called myotonic dystrophy is an example of such a
disorder.

The interdisciplinary research under Roses was a key to the
discovery, Vance said. "We are a group of clinicians,
geneticists and molecular biologists. Only by working across
disciplines could we have come up with such an understanding,"
he said.

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