CONTACT: JENNIFER BROWN
Iowa City IA 52242
(319) 335-9917; fax(319) 384-4638
Release: Sept. 16, 2002
UI team develops novel method to silence genes in mice
University of Iowa scientists have for the first time combined gene therapy
strategies and a process that interferes with gene expression to silence,
or turn off, genes in living animals (mice). The gene-silencing process, which
occurs naturally in many organisms, is known as RNA interference.
The UI study could provide a basis for new antiviral therapies and treatments
for certain inherited genetic diseases. The technique also could help researchers
understand the functions of newly identified genes. The study appears in the
Sept. 16 early online release from the journal Nature Biotechnology.
Normally, when a gene is turned on, or expressed, a series of events is
set in motion, which results in the production of a protein. However, RNA
interference disrupts gene expression by targeting an intermediate molecule
called mRNA for degradation. Specifically, a hairpin-shaped molecule called
small interfering RNA (siRNA) binds to mRNA, causing its removal. As a result,
little or no protein is produced and thus gene expression is silenced. Such
silencing could be useful to prevent the production of proteins that are harmful
to the body.
In particular, the ability to selectively turn off certain genes would be
useful in the treatment of many human diseases including viral diseases and
certain genetic disorders. However, in order to use RNA interference to silence
genes in specific tissues in the body, a way to instruct cells to make the
appropriate interfering molecule had to be developed.
Beverly Davidson, Ph.D., the Roy J. Carver Chair in Internal Medicine, and
colleagues, used their expertise to build a gene therapy vector to deliver
genetic information into cells of mice and thus direct those cells to make
specific siRNA molecules.
"Our experiments showed that we could decrease expression of genes
that are normally expressed in a mouse and decrease expression of genes that
are over-expressed in a transgenic mouse," Davidson explained. "This
is the first time this gene silencing technique has been used with a gene
transfer vector, and it is the first time it has been applied to animal tissues,
indicating that it might be useful for therapeutic purposes."
Specifically, the UI team silenced genes in the brains and livers of mice.
Davidson, who also is a UI professor in internal medicine, neurology, and
physiology and biophysics, suggested that one possible siRNA-based therapy
might be reducing the viral load of hepatitis in the liver. siRNA delivered
by gene therapy to liver cells would silence genes associated with the viral
The technique also could have important therapeutic implications for dominant
inherited diseases. These are genetic diseases where one mutated copy of a
gene (inherited from one parent) dominates the normal gene (inherited from
the other parent) and causes the genetic disorder in the offspring.
The team used siRNA to decrease gene expression in a cell model of a group
of dominantly inherited neurodegenerative diseases called the polyglutamine
expansion diseases. At least nine diseases, including several spinocerebellar
ataxias and Huntington's Disease, are caused by polyglutamine expansion.
The genetic defect in these diseases produces a mutated protein with an
abnormally long stretch of the amino acid, glutamine. Expansion of this polyglutamine
domain makes the protein toxic to brain cells, causing the protein to clump
together as aggregates.
The team used gene therapy to make a specific siRNA molecule in brain cells
that were producing a polyglutamine disease protein. The level of gene expression
and thus the amount of toxic protein was significantly reduced, and fewer
protein aggregates formed.
Davidson explained that knocking down the level of mutated protein in these
polyglutamine disorders could provide a way to prevent the damage caused by
these neurodegenerative diseases.
"We know from animal studies that if you modestly reduce levels of
the mutated gene copy, you have a dramatic impact on disease severity,"
The UI study showed that siRNA could distinguish between the mouse and human
version of a gene. Davidson added that the experiments lay the foundations
for investigators to precisely target siRNA to specific tissues or cell types,
so that interfering RNA is turned on only in those places.
"siRNA seems to work for many different genes in multiple cell types
and can be refined and optimized so that it knocks down the level of protein
expression from the targeted gene to very, very low levels," Davidson
said. "What's most exciting is the possibility of using this technique
for therapeutic purposes and for studies to understand the function of newly
identified genes and what their role is in the development of disease."
In addition to Davidson, the UI team included Haibin Xia, Ph.D., assistant
research scientist, and Qinwen Mao, Ph.D., research investigator, who are
lead authors of the study, and Henry Paulson, M.D., Ph.D., assistant professor
of neurology. The study was funded by the National Institutes of Health and
the Roy J. Carver Trust.
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the UI Roy J. and Lucille A. Carver College of Medicine and UI Hospitals and
Clinics and the patient care, medical education and research programs and
services they provide. Visit UI Health Care online at www.uihealthcare.com.