SCA3 Archives |

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.

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Gene Therapy Validated In Human SCA3 Stem Cells

Written by Dr. Marija Cvetanovic Edited by Dr. Sriram Jayabal

Research group in Michigan report the creation of the first NIH-approved human cell model that mirrors SCA3 disease features – cellular defects that, after gene therapy, show improvement

Spinocerebellar ataxia type 3 (SCA3) is a dominantly-inherited, late onset genetic disease that affects multiple brain regions. Affected individuals suffer from several symptoms, with impaired movement coordination being the most debilitating. SCA3 is caused by a mutation in the Ataxin-3 (ATXN3) gene. In unaffected individuals, ATXN3 typically has anywhere from 12 to 44 repeats of the genetic code “CAG;” however, in some people’s genetic code, the number of CAG repeats can become abnormally high. If this “repeat expansion” mutation causes the ATXN3 gene to have more than 56 CAG repeats, the person develops SCA3. Cells use repeating CAG sequences in their genome to make proteins with long tracts of the amino acid glutamine. In SCA3 cells, these “polyglutamine” (polyQ) tracts are abnormally long in the ATXN3 protein, which makes the protein more prone to form clumps (or “aggregates”) in the cell. The presence of these protein clumps in the cells of the brain is one of the hallmarks of SCA3.

Despite knowing the genetic cause of SCA3, it is still not known how this mutation affects cells on the molecular level. Having said that, several cellular and animal models have been developed in the past two decades to help study these underlying mechanisms. SCA3 models have not only helped to our increased understanding of the disease’s progression at all levels – molecular, cellular, tissue, and behavioral – but also helped move us closer to therapeutic interventions. For instance, recent studies using SCA3 mouse models have established that targeting ATXN3 with a form of gene therapy known as antisense oligonucleotide (ASO) treatment could very well be an effective strategy for improving the lives of patients. ATXN3-targeting ASOs cause the cells of the brain to produce less of the mutant ATXN3 protein and, when given to SCA3 mice, improved their motor function. These results strongly support the potential use of ASOs to treat SCA3. Still, it is important to see if this finding can be repeated in human neurons (a step that is needed to bring us closer to ASO clinical trials).

Female scientist in a while lab coat busy at work, we are looking at her from behind through some glass bottles — Image of a research scientist hard at work in the lab. Image courtesy of pxfuel.

Previous experience from unsuccessful clinical trials highlight the importance of determining the similarities and differences between humans and mice when it comes to disease. For instance, the SCA3 mutation does not naturally occur in mice; therefore, modeling SCA3 with mice usually requires additional genetic manipulation, which could lead to unexpected effects that we do not typically see in patients. In addition, we may miss important determinants of SCA3 pathology due to the inherent differences between humans and mice. For example, proteins that help contribute to SCA3 in human patients may simply not be present in mouse neurons (and vice versa). Because of such species differences, the therapeutic interventions that are effective in mice are not always as effective in humans.

SCA3 human neurons can help bridge the gap between rodent models and human patients, acting as a clinically relevant tool for looking into disease mechanisms and testing new therapies. Because we cannot remove a portion of an SCA3 patient’s brain to study the disease, these neurons must be created in a lab. Human neurons can be generated from induced pluripotent stem cells (iPSCs) or from human embryonic stem cells (hESCs). Induced pluripotent stem cells (iPSCs) are made from adult cells (usually blood or skin cells) that are reprogrammed to return to an embryo-like form (known as the “pluripotent” state). Just like during normal development, iPSCs can create many different types of cells, including neurons. One problem with this approach is that the process of reprograming can potentially change these cells in way that could affect how the disease presents. To avoid this issue, researchers can also create human neurons from human embryonic stem cells (hESCs), which are derived from embryos and are, therefore, naturally pluripotent. Because hESCs do not require reprograming, they are more likely to accurately model disease. However, they are more difficult to obtain and work with. The researchers in this study, led by Lauren Moore in Dr. Hank Paulson’s lab at the University of Michigan, used hESCs to generate the first ever National Institutes of Health (NIH)-approved SCA3 model using human cells.

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La huntingtine: un nouvel acteur dans l’arsenal de la réparation de l’ADN

Écrit par Dr. Ambika Tewari, Edité par Dr. Mónica Bañez-Coronel, Traduction française par: L’Association Alatax, Publication initiale: 22 novembre 2019

Des mutations dans la protéine huntingtine altèrent la réparation de l’ADN, causant des dommages importants à l’ADN et une expression génétique modifiée.

Notre génome regroupe l’intégralité de notre matériel génétique, qui contient les instructions pour fabriquer les protéines essentielles à tous les processus de l’organisme. Chaque cellule de notre corps, des cellules de la peau qui constituent une barrière de protection essentielle, des cellules immunitaires qui nous protègent des espèces envahissantes et des cellules du cerveau qui nous permettent de percevoir et de communiquer avec le monde contient du matériel génétique. Au début du développement de chaque espèce de mammifère, il existe une prolifération massive de cellules qui permet le développement d’un embryon au stade une cellule à un corps fonctionnel contenant des trillions de cellules. Pour que ce processus se déroule de manière efficace et fiable, les instructions contenues dans notre matériel génétique doivent être transmises avec précision pendant la division cellulaire et son intégrité maintenue pendant toute la durée de vie de la cellule afin de garantir son bon fonctionnement.

De nombreux obstacles entravent la séquence complexe et hautement orchestrée d’événements au cours du développement et du vieillissement, provoquant des altérations pouvant entraîner un dysfonctionnement cellulaire et une maladie. Les sources de dommages à l’ADN internes et externes bombardent constamment le génome. Les rayonnements ultraviolets et l’exposition à des agents chimiques sont des exemples de sources externes, tandis que les sources internes incluent les processus cellulaires pouvant découler, par exemple, des sous-produits réactifs du métabolisme.

Heureusement, la nature a mis au point un groupe spécial de protéines, appelées protéines de réparation et de réparation de l’ADN, qui permettent aux détecteurs de détecter les messages erronés. Ces protéines spécialisées garantissent que les dommages aux molécules d’ADN qui codent nos informations génétiques ne sont pas transmis à la nouvelle génération de cellules lors de la division cellulaire ou lors de l’expression de nos gènes, protégeant ainsi notre génome. De nombreux troubles génétiques sont causés par des mutations du matériel génétique. Cela conduit à un ARN ou une protéine dysfonctionnel avec peu ou pas de fonction (perte de fonction) ou à un ARN ou une protéine avec une fonction entièrement nouvelle (gain de fonction). Étant donné que les protéines de réparation de l’ADN jouent un rôle crucial dans l’identification et le ciblage des erreurs commises dans le message, il va de soi que toute altération du processus de réparation de l’ADN pourrait conduire à une maladie. Dans cette étude, Rui Gao et ses collègues, par le biais d’une vaste collaboration, ont cherché à comprendre le lien qui existe entre la réparation de l’ADN modifiée et la maladie de Huntington.

Un dessin de molécules d'ADN bleues. — Un dessin de molécules d’ADN.

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Concevoir une stratégie thérapeutique unique pour traiter plusieurs types d’ataxie spinocérébelleuse

Écrit par Dr David Bushart, Édité par Dr Hayley McLoughlin, Traduction française par: L’Association Alatax, Publication initiale: 3 janvier 2020

Une stratégie de traitement nouvellement proposée pourrait être efficace contre plusieurs formes d’ataxie spinocérébelleuse et d’autres troubles associés aux répétitions CAG.

Lors de la réception d’un diagnostic initial d’ataxie spinocérébelleuse (SCA), un essaim de questions peut pénétrer dans l’esprit du patient. Bon nombre de ces questions porteront probablement sur la façon de gérer et de traiter leur maladie. Quels traitements sont actuellement disponibles pour traiter la SCA? Que puis-je faire pour réduire les symptômes? Le SCA a-t-il un remède, et sinon, les chercheurs sont-ils sur le point d’en trouver un ?

Les patients et les membres de la famille qui lisent SCASource peuvent être en mesure de répondre à certaines de ces questions.

Bien que les scientifiques soient conscients de certaines des causes génétiques sous-jacentes de la SCA et que les patients puissent grandement bénéficier de l’exercice et de la physiothérapie, il n’existe malheureusement aucune thérapie médicamenteuse actuelle qui puisse traiter efficacement ces maladies.

Cependant, c’est une période très excitante dans la recherche sur les SCA, car les chercheurs travaillent dur pour développer de nouvelles stratégies de traitement pour plusieurs des SCA les plus courants. Beaucoup de ces thérapies nouvellement proposées sont spécialisées pour traiter un sous-type génétique spécifique de SCA (par exemple SCA1, SCA3, etc.), ce qui permettrait à ces thérapies d’être très spécifiques. Cependant, ces efforts spécialisés soulèvent une autre question : serait-il possible de traiter différents types de SCA avec la même stratégie thérapeutique ?

sketch of a human brain and spinal cord across a blue background — Croquis de l’artiste d’un cerveau humain. Image reproduite avec l’aimable autorisation de Pixabay.

C’est précisément ce que les chercheurs ont voulu déterminer dans une étude récente, rédigée par Eleni Kourkouta et ses collègues. Ce groupe de chercheurs a utilisé une technologie appelée oligonucléotides antisens (souvent en abrégé ASO, ou AON), pour se demander si un seul ASO pourrait être utilisé pour traiter plusieurs troubles neurologiques qui ont différentes causes sous-jacentes. Actuellement, la plupart des technologies ASO dépendent de notre capacité à cibler sélectivement des gènes spécifiques causant des maladies, ce qui permet à l’ASO de reconnaître et d’agir uniquement sur le gène spécifique qui cause l’ataxie. Une fois reconnus, ces ASO peuvent recruter des machines cellulaires qui abaissent les niveaux d’ARN du gène pathogène, limitant ainsi considérablement la quantité de protéines pathogènes produites (en savoir plus dans notre aperçu de l’ARN, qu’est-ce que l’ARN?). Cette stratégie a le potentiel d’être très efficace pour traiter les SCA associés à l’expansion de la polyglutamine (polyQ).

Cependant, le type de technologie ASO décrit ci-dessus n’est pas le seul moyen de réduire les niveaux des protéines pathogènes dans SCA. Dans cet article, Kourkouta et ses collègues utilisent un type différent d’ASO avec un mécanisme d’action différent, ce qui réduit également les niveaux de la protéine pathogène dans deux SCA différents.

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Working with cerebellar ataxia

Written by Dr. David Bushart Edited by Dr. Sriram Jayabal

How can employment be made more accessible for ataxia patients? What barriers exist? A study of workers and non-workers with ataxia analyzes the benefit of employment, as well as how to reduce risk of injury.

A job can often become part of a person’s identity. When people meet for the first time, one of the first questions that often comes up is “what do you do for work?” While this question can be harmless, it can also be frustrating to non-workers, particularly to those who are actively looking for employment. This may include some patients with cerebellar ataxia.

It can be difficult to manage disease symptoms alongside the stress of a job. However, some patients may find that including a job as part of their routine can be helpful for physical and mental wellness. In these cases, it is important for ataxia patients to have access to fair employment. Despite these benefits, finding a job can prove quite challenging, and unfortunately, ignorant assumptions about the capabilities of workers with ataxia may make finding employment even harder. How can employment be made more accessible to ataxia patients who wish to work?

two people shaking bands over a business agreement — Photo by fauxels on Pexels.com

Determining the work capabilities of ataxia patients

Helping ataxia patients find work might have a significant benefit on their overall quality-of-life. Researchers in Italy designed a study to get a better idea about the capabilities of workers with ataxia and the barriers to employment that they face. The research team, led by Alberto Ranavolo, interviewed both workers and non-workers with ataxia. Importantly, the patients interviewed for this study had been diagnosed with different types of ataxia, including dominantly-inherited ataxias, Friedrich’s ataxia, and other ataxias with unknown causes. Within this group, 24 were currently workers and 58 were non-workers at the time of the study. This allowed the researchers to determine how characteristics such as age, gender, education, and duration of symptoms might impact the ability to work.

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