Polyglutamine Diseases Archives |

Mutated ataxin-1 protein forms harmful, doughnut-shaped aggregates that are not easily destroyed

Written by Brenda Toscano Marquez Edited by Marija Cvetanovic

Insoluble clumps of mutated ataxin-1 capture essential proteins and trigger the creation of toxic reactive oxygen species.

All proteins produced by our cells consist of long chains of amino acids that are coiled and bent into a particular 3D structure. Changes in that structure can cause serious issues in a cell’s function, sometimes resulting in disease. Spinocerebellar ataxia type 1 (SCA1) is thought to be the result of one such structural change. The cause of SCA1 is a mutation that makes a repeating section of the ATXIN1 gene abnormally long. This repeated genetic code, “CAG,” is what encodes the amino acid glutamine in the resulting ataxin-1 protein. Therefore, in the cells of patients with SCA1, the Ataxin-1 protein is produced with an expanded string of glutamines, one after the other. This polyglutamine expansion makes the mutated ataxin-1 protein form clumps in many different types of cells – most notably, though, in the cells most affected in SCA1: the brain’s Purkinje cells.

Recent research suggests that these clumps, or “aggregates,” not only take up space in the cell, but that the act of ataxin-1 proteins clustering together might even be beneficial in early stages of disease (it’s possible that the proteins wreak less havoc when they’re in large clumps, rather than all floating around individually). However, another line of research suggests that ataxin-1 aggregates might also be toxic, triggering signals that lead to the cell’s death. As such, how exactly these aggregates affect the deterioration of cells has remained an important question in SCA1 research.

n a search for answers, an international team led by Stamatia Laidou designed a unique cell model of SCA1 to track the development of ataxin-1 aggregates. Their study, published in a recent paper, made use of normal human mesenchymal stem cells that had been engineered to make a modified version of the ataxin-1 protein. In these cells, ataxin-1 was produced not only with the SCA1-causing expansion, but also with a marker protein attached to its end. This marker, known as “green fluorescent protein” (GFP), is used extensively in biological research because it glows under fluorescent light.

doughnut with white and pink sprinkles — Laidou and colleagues have observed mutated ataxin-1 clumps that cause cell stress. Photo by Tim Gouw on Pexels.com

Using this to their advantage, Laidou and her team used a fluorescent microscope to follow the formation of ataxin-1 aggregates over the course of 10 days. The abnormal protein first started accumulating in the nucleus as small dots. As time went on, these dots started blending together, increasing in size. By ten days, the ataxin-1 aggregates had grown even more, forming a doughnut-shaped structure that occupied most of the cell’s nucleus – a crucial structure that houses the cell’s genetic information. These results were intriguing, since the accumulation of normal, non-expanded Ataxin-1 protein does not result in an aggregate with a doughnut shape.

Spotlight: The Truant Lab

Principal Investigator: Dr. Ray Truant

Location: McMaster University, Hamilton, Ontario, Canada

Year Founded: 1999

What disease areas do you research?

SCA1
SCA7
Huntington’s Disease
Parkinson’s Disease

What models and techniques do you use?

Human cell biology
High content screening
Biophotonics
Microscopy

Research Focus

What is your research about?

We are looking into the role of oxidative DNA damage as a trigger to diseases like ataxia and neurodegeneration. We examine the roles of the disease proteins (ataxin-1, ataxin-7, etc,) and genes which modify or change disease that are involved with DNA damage repair.

Why do you do this research?

We are looking at what triggers the very first steps of disease. If we can understand this, we can design a treatment to stop it from happening in the first place.

Research team of 10 holding a sign which reads "We are Ataxia Aware" — Group picture of the Truant Laboratory celebrating International Ataxia Awareness Day 2019.

Fun Lab Fact

All our fridges in the laboratory are named after Game of Thrones characters! (We have several proud nerds in the lab)

For More Information, check out the Truant Lab Website!

We have an open lab notebook blog where our post-doctoral fellow Dr. Tam Maiuri post updates on her experiments in real-time! We plan to launch an ataxia open notebook in Winter 2021.

Written by Ray Truant, Edited by Celeste Suart

New molecule can reverse the Huntington’s disease mutation in lab models

Written by Dr. Michael Flower Edited by Dr. Rachel Harding

Editor’s Note: This article was initially published by HDBuzz on March 13, 2020. They have graciously allowed us to build on their work and add a section on how this research may be relevant to ataxia. This additional writing was done by Celeste Suart and edited by David Bushart.

A collaborative team of scientists from Canada and Japan have identified a small molecule which can change the CAG-repeat length in different lab models of Huntington’s disease.

CAG repeats are unstable

Huntington’s disease is caused by a stretch of C, A and G chemical letters in the Huntingtin gene, which are repeated over and over again until the number of repeats passes a critical limit; at least 36 CAG-repeats are needed to result in HD.

In fact, these repeats can be unstable, and carry on getting bigger throughout HD patients’ lives, but the rate of change of the repeat varies in different tissues of the body.

In the blood, the CAG repeat is quite stable, so an HD genetic blood test result remains reliable. But the CAG repeat can expand particularly fast in some deep structures of the brain that are involved in movement, where they can grow to over 1000 CAG repeats. Scientists think that there could be a correlation between repeat expansion and brain cell degeneration, which might explain why certain brain structures are more vulnerable in HD.

a print out of genetic information show as a list of A,T, C, and G letters — The CAG repeat of the huntingtin gene sequence can be changed to include more and more repeats, in a process called repeat expansion. This can also happens in some ataxia related genes. Image credit: “Gattaca?” by IRGlover is licensed under CC BY-NC 2.0

But why?

This raises the question, what is it that’s causing the CAG repeat to get bigger? It seems to be something to do with DNA repair.

We’re all exposed continually to an onslaught of DNA damage every day, from sunlight and passive smoking, to ageing and what we eat. Over millions of years, we’ve evolved a complex web of DNA repair systems to rapidly repair damage done to our genomes before it can kill our cells or cause cancer. Like all cellular machines, that DNA repair machinery is made by following instructions in certain genes. In effect, our DNA contains the instructions for repairing itself, which is quite trippy but also fairly cool.

“What is it that’s causing the CAG repeat to get bigger? ”

We’ve known for several years that certain mouse models of HD have less efficient systems to repair their DNA, and those mice have more stable CAG repeats. What’s more, deleting certain DNA repair genes altogether can prevent repeat expansion entirely.

But hang on, isn’t our DNA repair system meant to protect against mutations like these?? Well normally, yes. However, it appears a specific DNA repair system, called mismatch repair, sees the CAG repeat in the huntingtin gene as an error, and tries to repair it, but does a shoddy job and introduces extra repeats.

Why does this matter?

There’s been an explosion of interest in this field recently, largely because huge genetic studies in HD patients have found that several DNA repair genes can affect the age HD symptoms start and the speed at which they progress. One hypothesis to explain these findings is that slowing down repeat expansion slows down the disease. What if we could make a drug that stops, or even reverses repeat expansion? Maybe we could slow down or even prevent HD.

Continue reading “New molecule can reverse the Huntington’s disease mutation in lab models”

Snapshot: What does dominant ataxia mean?

Ataxias can occur due to a multitude of reasons. One way a patient might acquire ataxia is from an accident or an injury – not as a result of genetics. On the other hand, a patient could also inherit a specific mutation (a genetic defect, in other words) from one or both of their parents. In this case, the ataxia is called “hereditary.” Hereditary ataxias can be further classified as either “dominant” or “recessive.”

What is a dominantly-inherited disorder?

Most genes in our body have two copies: one that we inherit from our mother, and one that we inherit from our father. Dominantly-inherited disorders are diseases in which a mutation in just one copy of a gene is enough to cause disease. When both copies of a gene need to be mutated to cause symptoms, the disorder is known as “recessive” (learn more in the Snapshot on recessive ataxias). For a patient with a dominantly-inherited ataxia, this means that there is a 1-in-2 chance that their children will inherit the disease-causing mutation (assuming that their spouse is unaffected). If both spouses are affected by the same dominantly-inherited disease, this chance increases to 3-in-4. In cases where the child inherits both mutant copies of the gene, the symptoms are often more severe than when a single copy is inherited.

Visual depiction of paragraph above — How dominant disorders are inherited. Illustration by Larissa Nitschke, created with BioRender.

Which ataxias are dominantly-inherited?

The most well-known ataxias with dominant inheritance patterns are the Spinocerebellar Ataxias (SCAs), such as SCA1, SCA2, SCA3, SCA6, and SCA7. Each disease is caused by defects in a different gene. Due to the high similarity in symptoms among all ataxias, genetic testing is often required to determine the exact gene mutation and type of ataxia a patient has.

How can a patient prevent passing on a dominantly-inherited disorder to their children?

There are multiple options to prevent passing on the disease to your child if you are affected by a hereditary ataxia. One potential option is to perform in vitro fertilization (IVF), a technology that is used the conceive embryos outside the human body. The embryos can be screened for genetic mutations, allowing only the healthy embryos to be implanted into the uterus.

If you are affected by a hereditary ataxia and want to prevent having a child with ataxia, it is recommended to talk to your physician and genetic counselor regarding reproductive options.

If you would like to learn more about in vitro fertilization and embryo screening, please take a look at these resources by the University of Pennsylvania. If you want to learn more about dominant ataxia, take a look at these resources by the National Organization for Rare Disorders and Ataxia Canada.

Snapshot written by Larissa Nitschke and edited by Dr. Marija Cvetanovic.

A Potential Treatment for Universal Lowering of all Polyglutamine Disease Proteins

Written by Frida Niss Edited by Dr. Hayley McLoughlin

One drug to treat them all: an approach using RNA interference to selectively lower the amount of mutant protein in all polyglutamine diseases. Work by a group in Poland shows initial success in Huntington’s Disease, DRPLA, SCA3/MJD, and SCA7 patient cells.

Can one drug treat nine heritable and fatal disorders? Polyglutamine diseases are disorders in which a gene encoding a specific protein is expanded to include a long CAG repeat. This results in the protein having a long chain of the amino acid glutamine, which disturbs the ability of the protein to fold itself and interact correctly with other proteins. This type of malfunctioning protein would normally be degraded by the cell, but in the case of polyglutamine proteins this seems unusually difficult. This causes a gradual build-up of faulty protein that disrupts several cellular pathways, eventually leading to cell death in sensitive cells. Currently there is only symptomatic treatment of these fatal diseases available, and they do not slow down the disease progression. One promising line of research is investigating the possibility of lowering the amount of these disease proteins using RNA interference.

RNA interference is the method by which a gene is silenced through a manipulation of a natural defense mechanism against viruses. When a virus attacks, it tries to inject DNA or RNA like particles to hijack the cell’s machinery for its own survival. To defend itself, the cell uses the RNA interference pathway, where the protein Dicer slices the DNA/RNA into smaller pieces and loads it into the RNA-induced silencing complex (RISC complex). The RISC complex finds all DNA/RNA particles in the cell with the same sequence and destroys them, effectively hamstringing the virus.

This machinery can be co-opted as a potential tool for treating neurodegenerative diseases caused by harmful mutant proteins. By inserting a small interfering RNA (siRNA), we can target the mRNA that codes for the harmful protein and trick the RISC complex into degrading it. In polyglutamine diseases, this has been successful when the mutant mRNA possesses a small mutation called a single nucleotide polymorphism (SNP). However, when an siRNA is delivered to a cell using a vector, which is a circular piece of DNA carrying genetic material, the Dicer protein tends to process the siRNA in unpredictable ways. This means that the treatment may not always be selective, and can end up targeting the normal protein as well. Moreover, not all patients have the same SNPs, so several drugs for every disease might be needed.

A pipette transfering liquid between small centifuge tubes — Close up picture of scientific research being conducted in a laboratory. Image courtesy of the University of Michigan SEAS.

In the paper by Kotowska-Zimmer and colleagues they have used short hairpin RNAs (shRNAs) targeting the CAG repeat tract itself instead of siRNAs targeting SNPs around the CAG repeat tract. shRNAs fold themselves like a hairpin when transcribed, and this loads them into the RISC complex through a somewhat different pathway, with less degradation along the way than conventional siRNAs. The second part that is different to other RNA interference strategies in this study is that the shRNA does not completely match the CAG repeat, but contains mismatches. This means that the RISC complex cannot actually cut and degrade the mRNA, and ends up simply sitting on the CAG repeat tract instead. The longer the repeat tract, the more RISC complexes can fit on the tract and block translation. Using this type of RNA interference Kotowska-Zimmer and colleagues have tried to lower the expression of huntingtin, atrophin-1, ataxin-3 and ataxin-7 proteins in cellular models of the corresponding polyglutamine diseases.

Continue reading “A Potential Treatment for Universal Lowering of all Polyglutamine Disease Proteins”