Written by Carrie A. Sheeler Edited by Dr. Ronald A.M. Buijsen
RNAi reduces levels of disease-causing Ataxin-1 in SCA1 model mice, easing symptoms of disease when injected both before and after symptom onset.
Lowering the amount of the disease-causing mutant Ataxin-1 protein in affected cells and tissues improves symptoms of disease in spinocerebellar ataxia type 1 (SCA1) mouse models. Like patients with SCA1, mouse models exhibit worsening coordination and degeneration of neurons, beginning in adulthood. Previous work has used genetic manipulation before disease onset (Zu et al 2004). This prevents or delays the onset of disease in SCA1 mouse models. When this is done soon after the onset of symptoms, associated markers of disease are reversed. This suggests that there is a window of time after symptoms start wherein mutant Ataxin-1 can be targeted to improve patient outlook. The 2016 paper by Keiser and colleagues seeks to further study this effect, using RNA interference as a strategy to reduce disease-causing levels of Ataxin-1. As there is no current treatment for Ataxin-1, this is an important step towards assessing possible treatment strategies that could be useful in patients.
Current strategies seek to decrease the amount of Ataxin-1 made in cells by targeting messenger RNA (mRNA)- the blueprints for proteins in a cell- for destruction. RNA interference (RNAi) is one such method which harnesses normal cellular processes to degrade specific mRNAs. In Keiser’s 2016 paper, a modified virus carrying a short sequence of DNA is injected into the brain of a mouse with SCA1. When this virus is injected, the DNA sequence enters the cells of nearby brain regions and stops the production of specific mRNA. In this case, it is Ataxin-1 mRNA that is specifically targeted. As Ataxin-1 mRNA are destroyed, the amount of Ataxin-1 protein made in the cell decreases.
The 2016 paper by Keiser and colleagues builds on several previous publications which targeted Ataxin-1 levels with RNAi. These showed that using this method to reduce Ataxin-1 could prevent cerebellar and brain stem degeneration (Xia et al 2004, Keiser et al 2014) and improve motor coordination (Keiser et al 2014). The 2016 paper builds on this by using a newly made virus to replicate previous work. This both confirms that the virus works as expected and supports previous findings that using this technique in pre-symptomatic mice delays symptom onset. A moderate dose of the virus improves motor performance (Figure 1D), corrects the imbalance of certain metabolites (Figure 2C), and prevents the death of Purkinje cells- a subtype of neuron that is lost in both human and mouse SCA1 (Figure 3 A and B). These effects were seen despite some lingering presence of Ataxin-1 in cells, suggesting that a partial reduction of Ataxin-1 protein is enough to improve symptoms.
In the latter half of the paper, the same virus is used in post-symptomatic mice to assess whether RNAi can reverse symptoms. The researchers again found lasting improvements in mice treated with the moderate doses of virus. Areas of improvement included motor coordination (Figure 5C), slowed Purkinje cell loss (Figure 5C-D, Figure 6A-B), and a correction of the imbalance of metabolites associated with Ataxin-1-mediated degeneration (Figure 5E). Not only does this support previous findings that reducing Ataxin-1 protein levels improves SCA1 symptoms, but it also demonstrates that RNAi is a method to do so. It is important to note that in both pre- and post-symptomatic treatment experiments show less effect at the highest dose of virus. The cause of this was not explored within the context of this paper, but there is some suggestion that activation of local immune cells- astrocytes and microglia- could be playing a role. Local inflammation occurred at the site of injection in both experiments and extended to other areas of the brain at the highest dose. Since the moderate dose effectively reduced Ataxin-1 and ameliorated symptoms without increasing inflammation, it is likely that this should be considered an effective range for later work.
Decreasing the amount of Ataxin-1 is a promising potential treatment for SCA1. The 2016 paper by Keiser and colleagues demonstrates evidence that partial reduction of Ataxin-1 protein levels is sufficient to improve motor coordination and alleviate signs of degeneration in the cerebellum. This is effective when applied before the onset of symptoms as well as up to 1-2 weeks after symptoms appear. While this is very promising, the mouse model used in this set of experiments expresses mutant Ataxin-1 only in the Purkinje cells of the cerebellum. To fully test the therapeutic potential of this method, the next step would be to test this in a different mouse model which expresses expanded Ataxin-1 throughout the brain and body. This would allow for the assessment of a wider variety of symptoms, including breathing impairments (Orengo et al 2018) and decreased life span. Should this method improve these non-cerebellar aspects of the disease, it would make a strong case as a potential treatment for SCA1 in patients.
RNAi: A method for targeting specific proteins for destruction in the cell.
mRNA: “Messenger” RNA. Chains of nucleic acids that serve as blueprints for protein creation. They are called messengers because they carry the information encoded in DNA to areas outside of the nucleus. They are the RNA molecules that ‘translate’ the genetic code from DNA so that proteins can be built based on that code.
Modified Virus: The repurposing of a viral particle so that it delivers specific pre-planned molecules to a cell.
Metabolite: Different forms of a protein that occur as a result of different cellular functions
Cerebellum: The area of the brain responsible for motor coordination and balance. A primary area of pathology in the spinocerebellar ataxias.
Conflict of Interest Statement
The authors and editor declare no conflict of interest.
Citation of Article Reviewed
Keiser MS, Monteys AM, Corbau R, Gonzalez-Alegre P, Davidson BL. RNAi prevents and reverses phenotypes induced by mutant human ataxin-1. Ann Neurol. 2016. doi:10.1002/ana.24789 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5115960/)
Keiser MS, Boudreau RL, Davidson BL. Broad therapeutic benefit after RNAi expression vector delivery to deep cerebellar nuclei: Implications for spinocerebellar ataxia type 1 therapy. Mol Ther. 2014. doi:10.1038/mt.2013.279
Orengo JP, van der Heijden ME, Hao S, Tang J, Orr HT, Zoghbi HY. Motor neuron degeneration correlates with respiratory dysfunction in SCA1. Disease models & mechanisms. 2018 Feb 1;11(2):dmm032623. doi:10.1242/dmm.032623
Xia H, Mao Q, Eliason SL, et al. RNAi suppresses polyglutamine-induced neurodegeneration in a model of spinocerebellar ataxia. Nat Med. 2004. doi:10.1038/nm1076
Zu T. Recovery from Polyglutamine-Induced Neurodegeneration in Conditional SCA1 Transgenic Mice. J Neurosci. 2004. doi:10.1523/jneurosci.2978-04.2004