Written by Dr. Ronald Buijsen Edited by Dr. Larissa Nitschke
O’Callaghan and colleagues show that novel therapeutic approaches to reduce the disease-causing protein in SCA1 do not increase the risk of developing cancer or Alzheimer’s disease in SCA1 mice.
People affected with Spinocerebellar Ataxia type 1 or SCA1 carry an expansion of a repetitive stretch of DNA in the ATXN1 gene. The expanded ATXN1 gene encodes an expanded ataxin-1 protein, which accumulates and causes toxicity in the brain. This causes motor coordination problems and premature lethality. So far, there is no treatment that slows, stops, or reverses SCA1 disease progression.
Still, several preclinical studies demonstrated that reducing ataxin-1 protein levels can improve the motor coordination deficits in SCA1 mouse models. One strategy to reduce ataxin-1 levels is the use of antisense oligonucleotides (ASO). These ASO treatments specifically cleave Atxn1 mRNA and lower ataxin-1 protein levels.
This study, published by the group of Dr. Harry Orr in 2018, showed that injection of ASOs into the brain of SCA1 mice improves motor deficits, prolonged survival, and reversed neurochemical abnormalities. However, lowering ataxin-1 protein levels might lead to altered expression of other proteins in the brain. This could impact the safety of this treatment strategy. Therefore, this follow-up study investigated whether lowering of ataxin-1 protein levels results in unwanted effects.
I nucleotidi anti-senso (anche noti come ASOs o AON, dall’inglese Antisense oligonucleotides) sono piccole molecole che possono essere usate per prevenire o alterare la produzione di proteine. Le proteine sono la forza lavoro della cellula, e dirigono la maggior parte dei processi cellulari. Le proteine sono prodotte in due fasi: nella prima un gene che codifica per una proteina viene convertito in una molecola che contiene specifiche istruzioni, l’RNA messaggero (mRNA). L’ mRNA trasferisce l’informazione contenuta nei geni al compartimento che assembla le proteine. Qui, l’mRNA è infine trasformato in proteina. Gli ASOs sono corte sequenze di DNA a singolo filamento, complementari alla sequenza di uno specifico mRNA. In base a diversi tipi di modifiche chimiche della loro sequenza, gli ASOs possono determinare due tipi di effetti sull’ mRNA complementare. Alcune modifiche fanno si che gli ASO distruggano l’mRNA e, di conseguenza, causano la perdita della proteina corrispondente. Altre modifiche, invece, permettono agli ASO di mascherare certi tratti dell’mRNA bersaglio, causando la produzione di una versione alterata della proteina.
La maggior parte delle Atassie spinocerebellari (dall’inglese Spinocerebellar Ataxias, SCAs) sono causate dall’accumulazione di una proteina tossica in una specifica regione del cervello. Per questo motivo, il principale obiettivo del trattamento delle SCAs con gli ASOs è inibire la produzione della proteina tossica. Un esempio di questa applicazione degli ASO è il lavoro del Prof. Harry Orr all’ Università del Minnesota. Il suo gruppo di ricerca studia l’Atassia spinocerebellare di tipo 1 (SCA1), causata dall’accumulo tossico della proteina Ataxina-1. Iniezioni di ASOs in modelli animali di SCA1 riducono i livelli di Ataxina-1 e migliorano l’incoordinazione motoria tipica della SCA1. Un altro modo di usare gli ASOs per il trattamento delle SCAs è la modifica dell’informazione trasmessa dall’mRNA per produrre una versione alterata della proteina. Questo approccio è stato testato nel caso della Atassia spinocerebellare di tipo 3 (SCA3), nella quale un’espansione nel gene Atxn3 rende la proteina Ataxina 3 tossica. Il gruppo del Dr. van Roon-Mom, in Olanda, per esempio, ha usato gli ASOs per rimuovere esclusivamente la porzione espansa della proteina Atxn3, lasciando intatta il resto della struttura proteica e la sua funzione.
Entrambi gli studi, così come altri studi portati avanti per altre SCAs, hanno evidenziato il potenziale uso degli ASOs come strumenti terapeutici per le SCAs. Mentre la ricerca sugli ASOs per le SCAs è per lo più nella fase preclinica, il trattamento con gli ASO per altre malattie, come la Distrofia Muscolare di Duchenne e l’atrofia muscolare spinale, è stato già approvato dall’ente statunitense Food and Drug Administration. Ulteriori studi clinici saranno necessari per misurare il beneficio terapeutico degli ASOs in pazienti di SCAs.
Per saperne di più sugli oligonucleotidi antisenso, leggi questo articolo alla pagina HDBuzz sugli ASOs in via di sviluppo per la malattia di Huntington.
“2 minuti di Scienza” scritto da Dr. Larissa Nitschke, revisionato da Dr. Hayley McLoughli, tradotto in italiano da Dr. Antonia De Maio. Pubblicato per la prima volta il 31 Maggio 2019.
Written by Dr. Ambika Tewari Edited by Dr. Sriram Jayabal
Targeting phosphatases in the cerebellum can correct miscommunication in multiple models of ataxia.
The cerebellum is essential for motor coordination and consists of the coordinated activity of different types of cells. Purkinje cells are one of the most fascinating cell types in the cerebellum. They have an elaborate network of branches called dendrites, where a neuron receives communication from other neurons. It is one of the most complex branching systems seen across all neurons in the entire brain. Each one of these branches has many points of contact with other branches called axons. Each axon is part of a neuronal structure that allow communication between neurons. These axons are from different cell types and allow information to be transferred to Purkinje cells.
Due to this branching complexity, Purkinje cells receive many messages or inputs. This represents different pieces of sensory information to ensure that movements are precisely timed. Purkinje cells must integrate and process this information. This produces motor behaviors like walking, writing, playing a musical instrument, and many more. Any alteration to the processing of this information will result in cerebellum dysfunction; in fact, Purkinje cells have gained attention because they undergo progressive deterioration in most ataxias.
Neurons, including Purkinje cells, communicate with other neurons using electrical signals known as action potentials or spikes. Firing rate, defined as the number of spikes within a defined period of time, is thought to be an important feature of this communication, which is critical for coordinating muscle movements. Therefore, a lower firing rate in Purkinje cells would signal a faulty communication between Purkinje cells and their targets. This has devastating consequences as seen in many ataxias.
For instance, in an earlier study, a group of authors found that the firing rate of Purkinje cells was decreased in mouse models of three different Spinocerebellar ataxias (SCAs): SCA1, SCA2, and SCA5. They further explored whether there was a common reason underlying the decreased firing rate. They found that a protein named Missing in Metastasis (MTSS1), was important for Purkinje cells to effectively communicate with each other. Mice engineered to have no MTSS1 protein had a decreased firing rate and difficulty walking and maintaining their balance.
In every cell in the body, including brain cells, there are numerous proteins that perform different functions. The concerted effort of all are needed for the cell to perform its intended duty. Some of these proteins are maintained in the cell in an inactive form and are activated when they are required in the cell and inhibited when they are not. This highly regulated system aims to maintain precise levels of proteins in each cell, while simultaneously conserving energy. Each cell has many ways of activating/inactivating a protein. A specialized group of proteins known as kinases and phosphatases, adds and removes phosphate groups to and from proteins respectively, thereby altering their active/inactive forms which then changes their interactions with other proteins. MTSS1 is one such protein that inhibits the activity of a group of kinases known as Src family of non-receptor tyrosine kinases (SFKs).
A genome is a person’s complete set of DNA which provides the instructions to make and maintain their body’s functions. Throughout the entire genome, there are genetic differences between individuals known as single nucleotide polymorphisms or SNPs (pronounced “snips”). These variations may be unique or may occur in many people. Normally, these SNPs do not directly cause diseases. But SNPs can sometimes be associated with diseases, and can provide interesting and potentially important information. A genome-wide association study (GWAS) looks at the genomes of many individuals to identify these variations, with the goal of linking more of these variations to particular diseases.
What can these types of studies tell us?
Scientists have gathered plenty of information from GWAS. Once these genetic variations are identified, researchers can use this information to learn more about how diseases occur and affect certain people. For example, GWAS have successfully identified genetic variations that can contribute to diabetes, obesity, and heart disease.
These kinds of studies can also help with creating personalized medicine – where different strategies can be used by doctors to treat patients based on their genetic makeup. This can allow doctors to give patients the most effective treatments, while limiting bad side effects.
How are these kinds of studies conducted?
Researchers typically look at two groups of people: individuals with the disease that is being studied, and people without the disease. DNA is obtained from people in each group to be studied, typically through a blood sample, or skin cells. In order for these studies to work, researchers try to look at as many people as possible. It is a big task, and requires not just hundreds, but thousands of participants! This allows researchers to be confident in the conclusions that they make.
In the early 2000’s, researchers mapped out the complete human genome. Since then, more genetic information from more people have been catalogued. Databases have been created that make it easier for researchers to compare new genomes to ones that have already been sequenced. This makes it quicker and easier to identify genetic variations and how they can contribute to disease.
What has GWAS taught us about SCA?
Some forms of Spinocerebellar ataxia (SCAs) are members of a larger group of diseases known as polyglutamine diseases. This group of diseases are caused by an abnormally long stretch of repetitive segments in the DNA. Scientists have identified that more repeats generally correspond with earlier disease onset, however, this is not always the case. Therefore, scientists have established that disease onset may be affected by other things, such as their environment or other parts of their genome or genetic factors. If researchers can identify these genetic factors, it could improve how these diseases are treated.
The cells in your body are equipped with machinery that helps identify and repair damage to DNA that occurs thousands of times a day from normal cellular processes or the environment (such as sun damage). A few years ago, GWAS revealed that genes involved in these pathways could affect SCA disease onset, and this opened up a new and exciting route of discovery for scientists! Many scientists are currently exploring this route, and more will be done in the coming years to see if we can find new therapies.
Written by Dr. Sriram Jayabal Edited by Dr. Ray Truant
A potential new pathway for SCA17: gene therapy that in mice restores a critical protein deficit protects brain cells from death in SCA17.
Neurodegenerative ataxias are a group of brain disorders that progressively affect one’s ability to make fine coordinated muscular movements. This makes is difficulty for people with ataxia to walk. Spinocerebellar ataxia type 17 (SCA17) is one such late-onset neurological disease which typically manifests at mid-life. The life expectancy after symptoms first appear is approximately 18-20 years. Besides ataxia, SCA17 can cause a number of other symptoms ranging from dementia (loss of memory), psychiatric disorders, dystonia (uncontrollable contraction of muscles), chorea (unpredictable muscle movements), spasticity (tightened muscles), and epilepsy.
Brain imaging and post-mortem studies have identified that the cerebellum (often referred to as the little brain) is one of the primary brain regions that is affected. That being said, other brain regions such as the cerebrum (cortex or the big brain) and brainstem (distal part of the brain found after the cerebellum) could undergo degeneration. Further, the genetic mutation that leads to SCA17, is a CAG-repeat expansion mutation, similar to several other forms of ataxias. In most other ataxias, where the function of the mutated protein is unknown. However in SCA17, the function of the mutated protein, TATA-box binding protein, is very well understood. Despite this unique advantage, we are yet to completely understand how the mutant gene leads to SCA17. This is why current treatment strategies often focus on treating the symptoms, but not the underlying cause.
SCA17 mutation leads to Purkinje cell death
Researchers from China have shed more light on how the mutant gene causes SCA17. TATA-box binding protein is a transcription initiation factor is a protein that turns on the production of RNA from genes. It is widely found across the brain including the cerebellum. TATA-box binding protein controls the amount of protein manufactured from several genes. This raised a very important question: pertinent not only to SCA17 but also more generally to several SCAs – why is that the cerebellar neurons, especially the most sensitive neuron, the Purkinje cells die?