Silence the gene, save the cell: RNA interference as promising therapy for ALS
13 March 2005
Scientists at the Ecole Polytechnique Fédérale de Lausanne (EPFL)
Switzerland have used RNA interference in transgenic mice to silence a
mutated gene that causes inherited cases of amytrophic lateral
sclerosis (ALS), substantially delaying both the onset and the
progression rate of the fatal motor neuron disease. Their results will
be published in the April issue of Nature Medicine, and in the
journal's advanced online publication March 13.
In addition to silencing the mutated gene that causes ALS, the
researchers were able to simultaneously deliver a normal version of the
gene to motor neuron cells using a single delivery mechanism. "This is
the first proof of principle in the human form of a disease of the
nervous system in which you can silence the gene and at the same time
produce another normal form of the protein," notes Patrick Aebischer,
EPFL President and a co-author of the study.
ALS is a progressive neurological disease that attacks the motor
neurons controlling muscles. Although its victims retain all their
mental faculties, they experience gradual paralysis and eventually lose
all motor function, becoming unable to speak, swallow or breathe. Known
also as Lou Gehrig's disease, from the baseball player who succumbed to
it, this arrowing disease has no cure and its pathogenesis is not very
An estimated 5,000 Americans are diagnosed with ALS every year,
and most of these cases are "sporadic", with no identifiable cause.
About 5-10% of ALS cases are inherited. Of these, 20% have been linked
to any of more than 100 mutations in the gene that expresses the
superoxide dismutase enzyme (SOD1).
These SOD1 mutations are "toxic gain-of-function mutations,"
meaning that the protein expressed by the mutated gene has, in addition
to all its normal cellular functions, some additional function that
makes it toxic to the cell. "Any mutation to the SOD1 gene is fatal to
motor neuron cells," Aebischer notes. Recent research also indicates
that mutant SOD1 gene expression in neighboring glial cells is also
implicated in motor neuron death.
Lead author Cedric Raoul and colleagues targeted the cause of the
disease by using RNA interference to silence the defective gene,
preventing it from expressing the SOD1 protein.
RNA interference is part of a complex cellular housekeeping
process that protects cells from invading viruses or other genetic
threats. It works by interrupting messenger RNA as it transfers the
genetic code for a protein from the nucleus to the site in the cell
where the protein is synthesized.
To trigger RNA interference and silence a gene, short bits of
double-stranded RNA are introduced in the cell, where they bind with
matching sections of messenger RNA. The cell identifies the resulting
messenger RNA strand as faulty and chops it up. As a result, the
genetic blueprint isn't delivered and the protein never gets made.
"Gene silencing is an example of using "molecular scissors" at its most advanced level," Raoul explains.
Raoul and colleagues used RNA interference to reduce levels of
mutant SOD1 protein in the spinal cords of transgenic ALS mice (mice
bred to express the human SOD1 gene). Short strands of RNA that
targeted multiple mutated and normal forms of the human SOD1 gene were
delivered in a specially engineered lentivirus. Expression of the SOD1
protein was knocked down in the affected motor neurons and neighboring
glial cells, and both the onset and the rate of progression of the
disease in the treated mice were substantially reduced. In addition,
the mice showed a significant improvement in neuromuscular function.
"This is the first demonstration of therapeutic efficacy in vivo
of RNA interference-mediated gene silencing in an ALS model," notes
Because the normal form of the SOD1 protein may be necessary for
the survival or function of adult human motor neurons, the Swiss
researchers designed a gene replacement technology that allows the
knock-down of all mutant SOD1 forms while permitting the expression of
a normal type SOD1 protein that is resistant to RNA interference-based
silencing. Both these effects are expressed long-term via delivery by a
Aebischer is optimistic about the future of gene silencing as a
potential therapy, particularly in incurable progressive neurological
diseases such as ALS. "I would not be surprised to see, in the next ten
years, this technology used for treating diseases of the nervous
system, particularly diseases that involve toxic gain-of-function, such
as inherited forms of Parkinson's disease or Huntington's disease,"
notes Aebischer. "But it's important to note that the safety of
delivering lentiviral vectors to the nervous system will have to be
carefully examined prior to treating patients."