A study has shown that suppressing a certain protein in a mouse model of amyotrophic lateral sclerosis (ALS) could markedly extend the animal’s life span. In one experiment, none of the untreated mice lived longer than 29 days, while some of the treated mice lived over 400 days.
This study was published in the journal Nature.
ALS is a disease in which the nerve cells in the brain and spinal cord degenerate, leading to wasting of the muscles. Patients gradually lose the ability to move, speak, eat or breathe, often leading to paralysis and death within two to five years. It is associated with environmental risk factors, such as old age and military service. In addition, mutations in certain genes can cause ALS. Exactly how ALS works is still poorly understood, but knowing which genes are involved can point researchers toward processes inside cells that would be good targets for drugs.
One indicator of ALS, as well as other neurodegenerative diseases, is clumps of protein in the brain. In ALS, these clumps, or aggregates, are made up of a protein called TDP-43. Eliminating TDP-43, and therefore the TDP-43 aggregates, might seem like a good way to prevent or cure ALS. But cells need TDP-43 to survive, so suppressing TDP-43 itself is not a good idea.
A different approach was needed. The researchers knew that a second protein, ataxin 2, helped cells survive when TDP-43 formed toxic clumps. Unlike TDP-43, ataxin 2 is not essential for a cell’s survival, making it a reasonable therapeutic target.
In a previous study, the team had shown that when ataxin 2 is suppressed or blocked in yeast cultures and fruit flies that carry the human TDP-43 gene, cells are more resistant to the potential toxic effects of the clumping TDP-43 protein.
In still another study, the scientists had shown that versions of the human ataxin 2 gene that resulted in a more stable ataxin 2 protein — and therefore more of the protein — increased the risk for developing ALS. The researchers reasoned that if mutations that increased the amount of ataxin 2 raised the risk of ALS, maybe lowering the amount of ataxin 2 would protect a person from ALS.
The scientists used genetically engineered mice whose neurons produced human TDP-43 protein at high levels. These mice exhibit some features that resemble human ALS, including a buildup of clumps of TDP-43 in their neurons. These mice also have difficulty walking and typically have life spans of no more than 30 days. They genetically engineered these ALS mice to have half the normal amount of ataxin 2, and also engineered other mice to completely lack the protein. The researchers found that with half the ataxin 2, the ALS-like mice survived much longer, and with no ataxin 2, the mice lived for hundreds of days.
The team next tried something that could have a more direct therapeutic value: treating mice with a type of DNA-like drug, designed to block the production of ataxin 2. These so called « antisense oligonucleotides » are strands of synthetic DNA that target a gene and block the expression of the protein that it encodes. Delivery of the antisense oligonucleotides to the nervous systems of some of the ALS mice enabled them to maintain their health much longer than the ALS mice treated with a placebo.
The scientists said the study showed that suppressing ataxin 2 delayed onset and slowed the progression of the ALS-like disease in mice that were not yet showing symptoms. Whether oligonucleotides or other protein-blocking treatments could reverse symptoms in mice that are already sick is another question. Because TDP-43 clumping occurs in nearly all ALS cases, targeting ataxin 2 could be a broadly effective therapeutic strategy, they say.
Paper: “Therapeutic reduction of ataxin-2 extends lifespan and reduces pathology in TDP-43 mice”
Reprinted from materials provided by Stanford University Medical Center.