Written by Anna Cook and Dr. Alanna Watt Edited by Dr. Vitaliy V. Bondar
Scientists uncover a promising therapeutic avenue to treat spinocerebellar ataxia type 2 (SCA2).
Spinocerebellar ataxia type 2 (SCA2) is a progressive ataxia caused by a mutation in the ATXN2 gene. This mutation causes a tract of the amino acid glutamine in the ataxin 2 protein to expand, making it toxic to cells. This type of mutation – known as a polyglutamine expansion – is common to several neurodegenerative diseases, including Huntington’s Disease and several forms of ataxia. One treatment strategy that has been devised for polyglutamine diseases such as SCA2 is to remove the toxic protein from cells. And, in their tour de force SCA2 paper from 20171, this is precisely what Scoles and colleagues attempted to do. Removing protein levels is a particularly promising strategy for SCA2, since previous research from the authors of this paper has shown that a complete loss of healthy ataxin 2 protein in cells does not cause any major detectable behavioural consequences in mice2.
Removing a toxic protein from a cell is not a simple task; in fact, it has only been done a handful of times in models of neurodegeneration. One way to eliminate a protein in neurons is to cause the RNA that encodes it to be degraded before it can make the protein. Through a collaboration with a company that specializes in this approach — Ionis Pharmaceuticals — the authors created their own short RNA molecules that matched the sequence and therefore bound to regions in the specific RNA that encodes the protein ataxin 2. These small molecules are known as anti-sense oligonucleotides (ASOs), and once they bind to their partner, they recruit the cell’s waste system to degrade the RNA. Currently, ASO therapy is one of the most promising methods researchers have developed to eliminate toxic proteins for a wide range of degenerative diseases.
After designing many of these molecules, the authors screened 152 different ASOs to determine which were most effective at lowering levels of the toxic protein. ASOs were applied to skin cells that had been donated by SCA2 patients, and levels of mutated ataxin 2 protein were measured. By picking out the designs that caused the greatest decrease in ataxin 2 levels, the authors narrowed down the original group of potential ASOs to give a shortlist of promising candidates. The authors then chose one ASO (ASO7) to test in mouse models of SCA2.
Mouse models of diseases allow us to test how a treatment works in a living system and whether it is likely to be safe in humans. In this case, the authors used two different mouse models of SCA2, both of which produce expanded polyglutamine tracts in the ataxin 2 protein. Importantly, both show impaired motor coordination and cellular abnormalities in the brain that resemble those found in SCA2 patients. Since a single mouse model is not always a perfect predictor of how treatments will work in humans. By repeating experiments in two different mouse models of SCA2, the authors strengthened their results. If the treatment was effective in both mouse models, it would be more likely that these findings could lead to a therapy for SCA2 patients.
To determine whether the motor dysfunction seen in SCA2 mice would be affected by ASO treatment, the authors used a motor performance test called the rotarod. In the rotarod test, mice are placed on a slowly rotating rod and must use motor coordination and balance to stay on as the rod rotates. Problems with coordination are readily apparent on the rotarod, which is why it is often used in ataxia research. Both SCA2 mouse models had difficulty with this test and were not able to stay on the rotarod for long. However, when the authors injected ASO7 into SCA2 mice, they found that the mice could stay on the rotarod much longer than their untreated counterparts. This suggests that the removal of toxic ataxin 2 in these mice improved brain function. Mice were injected once and tested at different times after treatment, and the improvements were seen even 4 months after ASO injection. Importantly, these mice were injected with ASO at an age when motor symptoms were already evident in SCA2 mice (that is, when the disease is already progressing). This aligns well with the SCA2 timeline in humans, since most SCA2 patients are diagnosed after the onset of motor symptoms. Therefore, ASO therapy has the potential to help these patients.
In addition to looking at motor coordination, the authors probed the health of their neurons. Several studies have previously identified characteristic changes in the neurons in the brain that can signal that they are functioning poorly. Purkinje cells are the principal cells of the cerebellum and are known to be altered in human SCA23, as well as several other forms of ataxia. The authors probed the health of Purkinje cells, which they have previously shown to be abnormal in one of their SCA2 mouse models3. In this study, they found that the same ASO7 treatment that caused the mice to improve their motor coordination on the rotarod brought the health of Purkinje cells up to normal levels, which, at the cellular level, might account for some of the motor symptom improvement.
Although SCA2 is caused by a mutation in the ATXN2 gene that encodes the ataxin 2 protein, ataxin 2 is not the only protein that is affected in SCA2. Indeed, the authors have previously shown that other proteins have abnormal levels of expression in the brains of SCA2 mice as the disease progresses4. It is not currently understood whether this arises because of changes in the ATXN2 gene, the ataxin 2 protein, or both. What is known, however, is that some of the proteins that are changed in SCA2 are likely to be altered in a similar way in other forms of ataxia. The authors showed that ASO7 treatment not only lowered the expression of the mutant ataxin 2, but also led to a normalization of expression of these other proteins that are abnormal in SCA2.
This careful and rigorous study by Scoles and colleagues suggests that reducing toxic ataxin 2 on its own, even when animals are already sick, is sufficient to improve cellular health and motor coordination. This suggests that eliminating toxic protein might be an effective way to try to improve ataxia in SCA2. More specifically, it suggests that reducing toxic protein with ASOs is a promising therapeutic avenue for human SCA2 patients. In this study, ASOs were delivered to the brain by a single brain injection that had a rather long-lasting effect (up to 4 months, and perhaps even longer). Remarkably, another article that was published in the same issue of this scientific journal showed something similar in a mouse model of amyotrophic lateral sclerosis (ALS, or Lou Gehrig’s disease): using this same ASO (ASO7) in ALS mice reduced their levels of ataxin 2, which also improved their motor symptoms5. This suggests that the same ASO treatment could be used for multiple neurodegenerative diseases.
The first ASO approved for use in people is the drug Nusinersen (marketed as Spinraza). This is an ASO treatment for spinal muscular atrophy (SMA), an infant-onset devastating motor neuron disease. Nusinersen treatment is costly, and invasive, needing to be injected directly into the nervous system (through intrathecal injection into the spinal cord), although it remains in brain fluid for 5 months or more. However, these concerns are outweighed by the striking improvements it produces in human SMA patients6,7. Taken together, these results in SCA2 mice combined with other advances in ASO treatment suggests that hopefully, we may be on the cusp of a new era for the treatment of neurodegenerative diseases.
ASO: Anti-sense oligonucleotide. These small molecules bind to RNA and prevent the production of the protein that the RNA produces.
RNA: Ribonucleic acid. This molecule copies the information encoded in genes (which are made of DNA) and functions as a blueprint for making proteins in a cell.
Conflict of Interest Statement
The authors and reviewer declare no conflict of interest.
Citation of Article Reviewed
Scoles, D.R., et al. Antisense oligonucleotide therapy for spinocerebellar ataxia type 2 Nature 544, 362-366 (2017). (https://www.ncbi.nlm.nih.gov/pubmed/28405024)
- Scoles, D.R., et al. Antisense oligonucleotide therapy for spinocerebellar ataxia type 2. Nature 544, 362-366 (2017).
- Kiehl, T.R., et al. Generation and characterization of Sca2 (ataxin-2) knockout mice. Biochem Biophys Res Commun 339, 17-24 (2006).
- Hansen, S.T., Meera, P., Otis, T.S. & Pulst, S.M. Changes in Purkinje cell firing and gene expression precede behavioral pathology in a mouse model of SCA2. Hum Mol Genet 22, 271-283 (2013).
- Dansithong, W., et al. Ataxin-2 regulates RGS8 translation in a new BAC-SCA2 transgenic mouse model. PLoS Genet 11, e1005182 (2015).
- Becker, L.A., et al. Therapeutic reduction of ataxin-2 extends lifespan and reduces pathology in TDP-43 mice. Nature 544, 367-371 (2017).
- Finkel, R.S., et al. Treatment of infantile-onset spinal muscular atrophy with nusinersen: a phase 2, open-label, dose-escalation study. Lancet 388, 3017-3026 (2016).
- Finkel, R.S., et al. Nusinersen versus Sham Control in Infantile-Onset Spinal Muscular Atrophy. N Engl J Med 377, 1723-1732 (2017).