Snapshot: What is an antisense oligonucleotide (ASO/AON)?

Antisense Oligonucleotides (also known as ASOs or AONs) are small molecules that can be used to prevent or alter the production of proteins. Proteins are the workforce of the cell, taking care of most cellular processes. They are generally made in a two-step process: first, a specific protein-coding gene is converted into an instruction file, called the messenger RNA (mRNA). The mRNA carries the information from that gene to the compartment of the cell that builds proteins. There, the mRNA’s information then gets converted into the protein. ASOs are short single stranded pieces of DNA that match the complementary sequence of a specific mRNA. Based on the type of chemical modifications, the ASO can have two different effects on the mRNA. Some modifications of ASOs trigger the destruction of the mRNA. This will result in the loss of the corresponding protein. Other modifications can mask only certain parts of the mRNA leading to a modified version of the protein.

diagram showing how ASOs can bind or snip mRNA to create alternative forms of protein or prevent protein synthesis
How ASOs work in the human body. Image by Larissa Nitschke, Created with BioRender.

The majority of Spinocerebellar Ataxias (SCAs) are caused by the accumulation of toxic proteins in certain regions of the brain. The primary goal of ASO treatments for SCAs is therefore to prevent the production of the toxic protein altogether. One example is work from Dr. Harry Orr’s group at the University of Minnesota. His lab studies Spinocerebellar Ataxia Type 1 (SCA1), which is caused by the toxic accumulation of the Atxn1 protein. Injections of ASOs into a SCA1 animal model decreased Atxn1 levels and rescued the SCA1 motor incoordination symptoms. Another way of using ASOs as treatment for SCAs is the modification of the mRNAs information to produce a modified version of the protein. This approach has been tested in Spinocerebellar Ataxia Type 3 (SCA3), in which an expansion in the Atxn3 gene renders the Atxn3 protein toxic. The van Roon-Mom group from the Netherlands, for instance, used ASOs to only remove the expansion from Atxn3 while leaving the remaining protein structure and function intact.

Both studies as well as other studies performed for additional SCAs are highlighting the potential use of ASOs as therapeutics for SCAs. While ASO research for SCA is mostly in the pre-clinical phase, ASO treatment for other diseases, including Duchenne muscular dystrophy and spinal muscular atrophy, have already gained approval by the US Food and Drug Administration (FDA). Further clinical trials will need to be performed to measure the therapeutic benefit of ASOs in SCA patients.

If you would like to learn more about antisense oligonucelotides, take a look at this article in HDBuzz about ASOs in development for Huntington’s Disease.

Snapshot written by Larissa Nitschke edited by Dr. Hayley McLoughlin

 

 

A Combined Approach to Treatment: Targeting PAKs in SCA1

Written by Carrie A. Sheeler  Edited by Dr. Marija Cvetanovic

Group 1 p21-associated kinases (PAKs) present a new avenue for SCA1 research.

Spinocerebellar ataxia type 1 (SCA1) is caused by a specific mutation in the Ataxin1 gene, which causes the protein that’s made from that gene (also called Ataxin1) to have an abnormally elongated polyglutamine (polyQ) tract. This leads to dysfunction and death in the affected cells of the brain (predominantly Purkinje neurons in the cerebellum), which causes symptoms in patients that include a progressive worsening of coordination and balance. While there is currently no cure for SCA1, several studies suggest that lowering the amount of Ataxin1 protein in the brain may delay the onset of the disease and decrease the severity of symptoms. This leads us to an important question: how do we most effectively decrease the amount of Ataxin1 in SCA1 patients? One paper recently published by Bondar and colleagues suggests that a multi-pronged approach could be the most effective means of reducing this toxic protein.

close up of two people shaking hands
Evidence suggests combining two potential treatments has a greater impact than each treatment on its own. Photo by rawpixel.com on Pexels.com

The amount of any specific protein in the body can be altered by either decreasing the amount of protein produced or increasing the rate at which cells break those proteins down. Proteins are made using messenger RNA (mRNA), which is created following specific instructions found in DNA. Decreasing the production or stability of mRNA decreases the amount of corresponding protein made. One way to target the mRNA that causes production of a specific protein is with antisense oligonucleotides (ASOs). ASOs are designed to target specific mRNA sequences by binding to them directly. Binding of ASOs to mRNA causes those molecules to be marked for destruction within the cell. Proteins in the body are also regularly recycled, but without the blueprints to build a new protein, cells cannot replenish the protein supply it loses over time. So, if Ataxin1 mRNAs are targeted and destroyed by ASO treatment, the amount of Ataxin1 in our cells would theoretically decrease.

Some proteins can also be altered by other proteins, creating another way that their stability, shape, and function can be regulated. This leads us to the other way we can alter the amount of a specific protein in a cell: regulating the regulators. In terms of SCA1, this could mean removing a protein that helps stabilize Ataxin1 or increasing the production of a protein that breaks Ataxin1 down. Previous research has identified several proteins of interest that regulate Ataxin1 protein stability, including several kinases. Kinases are a class of proteins that transfer a phosphate group from adenosine triphosphate (ATP) to another protein in the cell. The addition of this phosphate group acts as an energy source to the receiving protein, altering its stability or how it interacts with other molecules in the cell (usually by causing it to change its shape). Recently, Bondar and colleagues have identified a new potential regulator of Ataxin1: a group of proteins known as p21-activated kinases (PAKs) (Bondar et al 2018).

Continue reading “A Combined Approach to Treatment: Targeting PAKs in SCA1”

Snapshot: What is an ion channel?

One of the most important features of neurons (Purkinje cells, for example), is that they are capable of electrical communication. Think of the last time you saw a TV intro or movie montage with a depiction of the brain on a microscopic level – though it’s technically invisible to the naked eye, that ‘spark’ you can see traveling down a portion of the neuron is actually not too far from reality. One of the most common ways to describe an active neuron, in fact, is to say that it’s “firing.” Essentially, when a neuron is activated, it ‘fires off’ an electrical impulse that is transmitted down a long, slender extension known as the axon. The axon ends where the next neuron in the circuit begins, and when the impulse arrives at that point, it initiates a series of events that allows the signal to jump to the next cell.

cartoon of neuron delivering an electrical impulse
An electrical impulse traveling down a neuron. Photo courtesy of Wikimedia.

This electrical signal is made possible by molecular machines known as ion channels. These proteins span the cell membrane, which is the barrier between the interior and exterior of the cell. When they receive a certain signal, the channel opens, allowing ions – atoms that carry an electrical charge, such as sodium, potassium, and calcium – to pass through. There are many types of proteins that allow the transport of small molecular components, but the source of a neuron’s electrical capabilities is that its channels specifically allow ions to pass into or out of the cell. Though a single ion’s charge is quite small, the large number of ions that are exchanged when a neuron’s channels open makes for a significant electrical effect – enough to produce an electrical signal that allows neurons to communicate with one another, giving us the ability to think, move, and interact with our environment.

diagram of an ion channgel in the closed, open, and inactivated state.
Cartoon of an ion channel in different states. Photo courtesy of Wikimedia.

Though the mutations that cause SCAs typically occur in genes that are expressed in every cell of the body, disease is usually restricted to the brain. One theory about why this is the case is that these SCA-related genes are necessary for the health and maintenance of ion channels in certain brain tissues – namely, the cerebellum and brainstem. At any rate, there is evidence that the electrical activity of these brain regions is abnormal in many SCAs, which strongly suggests that ion channels play a critical role in these disorders.

If you would like to learn more about ion channels, take a look at this Encyclopaedia Britannica article.

Snapshot written by Logan Morrison edited by Dr. David Bushart

 

 

Blurred lines: how spinocerebellar ataxia type 7 impacts vision

Written by Siddharth Nath Edited by Dr. Ray Truant

Spinocerebellar ataxia type 7 (SCA7) is unique amongst the SCAs in that it involves an organ besides the brain – the eye. Rather than problems with movement, the first hint that something may be wrong for SCA7 patients is often a subtle change in vision. Research done by Dr. Al La Spada in the early 2000s helps explain how and why this happens. 

It’s not all in your head

The spinocerebellar ataxias (SCAs) are, for the most part, similar in how they affect the body. They cause disordered movement (ataxia), trouble with speech (dysarthria), trouble swallowing (dysphagia), and other neurological symptoms. This holds true for all of the polyglutamine-expansion SCAs except for SCA7. In SCA7, doctors have long observed that patients report problems with vision, and in some cases may be entirely blind. Interestingly, these symptoms often appear ahead of any other signs that the patient might have a chronic illness, suggesting that SCA7 affects the eye before it begins to affect the brain.

In the early 2000s, while at the University of Washington, Dr. Al La Spada conducted research into how SCA7 affects the eye. He and his team set out to understand why patients with this disease experience a loss of vision.

Close up photo of a human eye from the side. The eye is hazel in colour.
Close up of a human eye. Photo by Pixabay on Pexels.com

Continue reading “Blurred lines: how spinocerebellar ataxia type 7 impacts vision”

Snapshot: What is Polyglutamine Expansion?

The information that allows the normal development and functioning of each human being is coded in DNA, which exists in all cells of the body. Several successive segments of DNA make up a gene, with the human body containing approximately 20,000. Every gene has a different arrangement of DNA segments and itself codes for a protein with a specific function. Genes code for proteins in the sequence of their DNA: combination of DNA sequences “code” for different protein precursors called amino acids. Thus, information from DNA (“genes”) codes for amino acids, which come together to form proteins, who function to maintain the normal well-being of the body.

A small number of genes have a small segment of DNA that is repeated successively, usually a couple dozen times, for unknown reasons. When the respective protein is formed, it also possesses a repetition of the same amino acid, corresponding to the repeated DNA segment. These repetitions in proteins have the prefix “poly”, meaning that the amino acids are repeated multiple times in a row, causing an “expansion” in the protein. One of the most common repeated amino acids is called glutamine: hence the name, polyglutamine.

Diagram showing how multiple CAG triplet repeats code for replicates of glutamine to be inserted into a protein
Photo courtesy of NHS HEE Genomics Education Programme.

When there is an increase in the number of repetitions of these segments in DNA, we say that an expansion of the polyglutamine has occurred. When the number of glutamines is increased sufficiently, a disease can develop: we call these disorders “polyglutamine diseases”. Some examples of diseases caused by this polyglutamine expansion are Huntington’s disease, SCA1, SCA2, SCA3, SCA6, and SCA7. The difference between all these diseases is that the expansion of the DNA segment that causes the polyglutamine occurs in different genes. Since these genes are distinct, the way that this expansion interferes with the normal body functioning is also different, giving rise to altered clinical presentations and courses. Moreover, it has been well established that, the larger the number of times that the segment is repeated, the more severe the disease will be. Finally, it has also been observed that throughout each generation, abnormally increased segments tend to become even bigger, making the disease worse.

The discovery of this mechanism of disease has been very important for scientists, since it allows for a “molecular diagnosis” of the disease. Armed with this understanding, research is now focused on understanding this process and finding ways to block the negative effects of polyglutamine expansion.

If you would like to learn more about polyglutamine expansion, take a look at this article.

Snapshot written by Jorge Diogo Da Silva, edited by Dr. Maxime Rousseaux