Scientists at the Ecole Polytechnique Fédérale de Lausanne (EPFL) in 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.
In addition to silencing the mutated gene that causes ALS, the EPFL 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 harrowing disease has no cure and its pathogenesis is not very well understood.
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.