Silence the gene, save the cell: RNA interference as promising therapy for ALS

HD Lighthouse Contributing Editor's Comment: These results are encouraging. Using a lentiviral vector, researchers have successfully used RNAi therapy with a mouse model of ALS. Progression was delayed and survival time was lengthened dramatically. The disease wasn't cured, however, because the therapy didn't get to all the cells. Still, this is real progress.

What is really exciting about this study is that the researchers delivered copies of the normal gene while they were disrupting the mutated one, in the same vector! This research advance is important for the many people in Venezuela with two copies of the HD gene and the small number of individuals in other countries who also fall into this category! We don't want to leave anyone behind!!

What needs to be done is to 1) resolve safety concerns about this or any other vector used to deliver RNAi and 2) increase the efficiency of the delivery system.
--Marsha L. Miller, Ph.D.
Posted to the HDL: 18 Mar 2005

"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" - Patrick Aebischer, M.D.

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, and in the journal's advanced online publication March 13.

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 arrowing 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.

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 Raoul. 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 single lentiviralvector.

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."

The Nature Medicine abstract

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease resulting in the selective death of motor neurons in the brain and spinal cord. Some familial cases of ALS are caused by dominant mutations in the gene encoding superoxide dismutase (SOD1). The emergence of interfering RNA (RNAi) for specific gene silencing could be therapeutically beneficial for the treatment of such dominantly inherited diseases. We generated a lentiviral vector to mediate expression of RNAi molecules specifically targeting the human SOD1 gene (SOD1). Injection of this vector into various muscle groups of mice engineered to overexpress a mutated form of human SOD1 (SOD1(G93A)) resulted in an efficient and specific reduction of SOD1 expression and improved survival of vulnerable motor neurons in the brainstem and spinal cord. Furthermore, SOD1 silencing mediated an improved motor performance in these animals, resulting in a considerable delay in the onset of ALS symptoms by more than 100% and an extension in survival by nearly 80% of their normal life span. These data are the first to show a substantial extension of survival in an animal model of a fatal, dominantly inherited neurodegenerative condition using RNAi and provide the highest therapeutic efficacy observed in this field to date.

Source: Nat Med. 2005 Mar 13

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