Snapshot: What is Gene Therapy?

Gene therapy is using nucleic acids to treat a genetic disorder.  These nucleic acids can be designed in a variety of ways to achieve the same therapeutic outcome. Gene therapy tools can be used to correct a mutant gene by one of three ways:

  1. Expressing a healthy copy of a gene
  2. Silencing or inactivating the mutant gene transcript
  3. Using genome editing tools to repair or turn-off the mutated gene.
computer desk laptop stethoscope
Photo of a stethoscope by Negative Space on Pexels.com

How is gene therapy used?

Monogenic disorders, like some spinocerebellar ataxias (SCAs), are excellent targets for gene therapy approaches. Gene therapies are currently being used throughout ataxia research for studying disease mechanisms and for preclinical therapeutic application.

Overview of how gene therapy works. First, Package the healthy gene, RNAi, or gene editing tools into the AAV (can also deliver as naked DNA or in a nanoparticle). Second, Inject the packaged AAV into the tissue of interest. Third, AAV will enter the cell and release the genetic material. The cell will become healthy by either 1) expressing the normal gene, 2) repressing the mutant RNA, or by 3) correcting the mutant gene.
Overview of gene therapy, designed by Stephanie Coffin using Biorender.

One gene therapy approach for rescuing SCA1 phenotypes involves overexpressing a healthy gene, ataxin-1-like, which competes with the mutant ATXN1 protein for complex formation. This work, conducted by Keiser and colleagues in 2016, showed phenotypic rescue in a mouse model of SCA1.

There are two common technologies for silencing or inactivating disease genes: RNA interference (RNAi) or antisense oligonucleotides (ASOs). RNAi strategies utilize small RNA molecules to knock down the expression of target mutant RNA transcripts, while ASOs are DNA molecules used to knock down or correct mutant RNA transcripts. Both therapeutic approaches are being pursued in SCAs. For example, Carmo and colleagues in 2013 showed that using RNAi against the SCA3 disease gene, ATXN3, could longitudinally decrease mutant ATXN3 levels. See the SCAsource snapshot on ASOs for further information about their use in SCAs.

The most common genome editing tool is the CRISPR/Cas9 system, which uses an RNA guide to direct the Cas9 nuclease to the region of the genome to be edited. One can then knockout that gene or correct the mutant gene. It is early days for this technology as a potential therapeutic option due to the challenges of delivery and the risk of off-target editing.

How is gene therapy delivered?

One of the most difficult aspects of gene therapy is how to deliver these various molecules to the cells of interest. One of the most common delivery methods is through viral delivery.  The “drug” nucleic acid is transferred into the disease cells by a vector, which is a virus that has been modified to remove viral components. The most common viral vectors for gene therapies currently are adeno-associated viruses (AAVs). Other delivery methods include non-viral vectors such as naked DNA and nanoparticles.

How long-lasting is gene therapy?

Viral delivery of gene therapy products provides a longitudinal expression of the nucleic acid, while naked DNA and nanoparticles express the nucleic acid drug transiently, thus typically requiring ongoing treatment.

If you would like to learn more about gene therapy, take a look at these resources by the National Institutes of Health and KidsHealth.

Snapshot written by Stephanie Coffin and edited by Dr.Hayley McLoughlin.

Snapshot: What is a biomarker?

A biomarker is any biological-based measurement that provides useful information regarding a person’s health. For example, blood test results showing increased glucose levels can be used as a biomarker for diabetes. A blood test showing an increased white blood cell count is a biomarker for infection. There are many sources of biomarkers beyond blood biomarkers. MRI, CT, and x-ray scans are all examples of imaging biomarkers. Scored motor assessments can also be used as biomarkers. For example, police use the field sobriety test as a biomarker for alcohol consumption.

Biomarkers can be used to:

  • Diagnose an existing disease or predict a patient’s prognosis.
  • Track disease progression.
  • Determine whether experimental drugs prevent, improve, or slow progression of disease within clinical trials.
close up photo of a measuring tape on a white background, with the end fading off into the distance.
Biomarkers act like a measure tape for diseases. Photo by Pixabay on Pexels.com

What are current biomarkers for spinocerebellar ataxias (SCAs)?

There are multiple biomarkers that are commonly used for patients with ataxia. DNA sequencing from saliva or blood samples of undiagnosed patients with ataxia symptoms can be used to diagnose or rule out SCAs caused by known genetic mutations. The Scale for the Assessment and Rating of Ataxia (SARA) scoring is a common motor assessment used to measure and track severity of ataxia-related balance and coordination issues in patients. MRI scans and other brain imaging techniques can be used to examine brain abnormalities or loss of brain cells.

Why do we need better biomarkers for SCAs?

In an ideal clinical trial, a patient would receive the potential treatment and then undergo a simple assessment (i.e. give a blood sample) shortly after that could conclusively determine whether the drug is working. Thankfully, many potential ataxia treatments are currently in development or are already being tested in clinical trials for patients with SCAs. Unfortunately, we currently do not have an easy, cheap, and sensitive way to measure whether ataxia symptoms are worsening or improving in a relatively short amount of time.

How can we identify better biomarkers for the SCAs?

Researchers are actively seeking better biomarkers for SCAs in animal and cell models of ataxia. There are also multiple ongoing “Natural History” and biomarker clinical trials that focus on different types of SCA diseases. These clinical studies aim to improve our understanding of the SCAs and identify new biomarkers to improve ataxia diagnosis and drug development. These studies may track patients over months or years, and can involve multiple tests, including blood or cerebrospinal fluid samples, brain imaging, or SARA scoring.

If you would like to learn more about biomarkers, take a look at these resources by the ALS Association and News Medical.

Snapshot written by Dr. Lauren Moore and edited by Dr. Gulin Oz.

Snapshot: How do clinicians measure the severity of ataxia in patients?

Coordination of smooth and effective movements is essential in daily tasks, such as speaking or walking. The ability to successfully orchestrate these movements is commonly referred to as “motor coordination”. While SCA patients can generally initiate movements with their bodies, their ability to execute these in a smooth and precise fashion is impaired. For instance, motor incoordination can be seen in a patient with ataxia’s inability to walk in a straight line, or in the difficulty they experience when swallowing. These and other motor function problems can greatly impact daily life. Assessing how well a patient can perform these movements provides an indication of how affected they are by the disease.

Black pencil lying on top of paper that has scoring chart on it
Photo by Pixabay on Pexels.com

Unlike what’s measured with more familiar medical tests, such as blood pressure or levels of blood sugar, human movement cannot be quantified easily with clear numbers. To address this, multiple rates scales have been developed to help measure standardize motor coordination examinations. One of these scales is the Scale for the Assessment and Rating of Ataxia (SARA). An experienced clinician (typically a neurologist) evaluates a patient’s ability to perform a series of tasks (such as standing and walking) and then, using the SARA, assigns a score for each task. The process takes about 15-20 minutes, and typically involves the following tests:

Continue reading “Snapshot: How do clinicians measure the severity of ataxia in patients?”

Snapshot: What is the Cerebellum?

The cerebellum, often referred to as the “little brain”, is part of the brain that is located behind the cerebrum (forebrain). The cerebellum accounts for about 10% of the brain’s volume. Despite occupying a small volume, the cerebellum contains more than half of the neurons in the brain. Most of the evolutionary research with respect to the brain has been focused on the forebrain; however, recent evidence suggests that the expansion of the size of the cerebellum might have given humans an edge with respect to higher behavioral functions, such as the use of tools. Therefore, the cerebellum has played a vital role during evolution, and this suggests an indispensable function for the human cerebellum.

cartoon diagram of the human brain, with the cerebelum coloured in pink
Diagram of the human brain, with the cerebellum highlighted in pink. Picture courtesy of Wikimedia Commons.

What does the cerebellum do?

For several decades, scientists believed that the main role of the cerebellum was to maintain posture and balance, to fine-tune motor movements, and to enforce motor learning. If you think about performing a certain movement (these thoughts happen in the forebrain), the cerebellum compares these “movement plans” with what movements were actually made and corrects for errors if there were any. This fine-tuning makes movements precise and is critical for making voluntary movements such as walking, running, or speaking. Therefore, it is with the help of the cerebellum that we learn to get better at throwing a curveball, riding a bike, or learning any other complex motor tasks.

Is that all the cerebellum does?

Well, scientists used to think so. Over the past two decades, new evidence has made scientists to re-evaluate their thoughts about the cerebellum. Scientists now believe that the role of the cerebellum extends beyond fine-tuning motor movements, and likely includes cognitive functioning and certain reward-seeking behaviors. However, this aspect of cerebellar function is still being studied and there is a lot for scientists to uncover.

What happens when the cerebellum is damaged?

The cerebellum is one of the primary culprits in many types of cerebellar ataxia, where the damaged cerebellum forces the affected individuals to gradually lose their ability to walk. Therefore, it is imperative to better understand how the cerebellum contributes to ataxia to provide better treatment for patients. Apart from ataxia, the cerebellum may also contribute to other disorders such as dystonia, Parkinson’s disease, Huntington’s disease, Alzheimer’s disease, and autism spectrum disorders. Therefore, understanding what happens when the cerebellum goes awry is critical for improving the quality-of-life for patients all over the globe.

If you would like to learn more about the cerebellum, take a look at these resources by the Khan Academy and BrainFacts.org.

Snapshot written by Dr. Sriram Jayabal and edited by Dr. David Bushart.

Snapshot: What are Mouse Models?

If you are thinking of a dressed-up mouse walking on a ramp and posing for pictures, then you are thinking wrong! Mouse models – as the name indicates – serve as a “model” for human diseases. Mice, similar to many mammals, can develop diseases. These include cancers, diabetes, and cardiovascular problems. Over the past century, mice have been used to study not only these naturally-occurring mouse diseases, but also disorders which do not typically affect mice. For instance, thanks to advancements in genetic engineering, scientists have generated mice that develop Alzheimer’s disease and cystic fibrosis. These mouse models are then used as tools to help scientists study the underlying causes of human diseases and, ultimately, create better treatments.

gloved hands holding a white mouse with red eyes
Photo by Pixabay on Pexels.com

Why can’t we study the disease in humans?

In humans, we can only obtain a snapshot of disease at a given time. This is particularly true for neurodegenerative diseases, where one cannot simply isolate parts of the brain to study them. In addition, an individual suffering from neurodegenerative disease is often not aware of their condition – and are not even diagnosed – until they are symptomatic. As a result, it is impossible to understand how an individual developed the disease over time, which can make it quite difficult to determine a proper therapeutic course. To best understand the cause and progression of neurodegenerative disorders, researchers generate animal models. These models, often using mice, are informed by genetic susceptibility and environmental risk and mimic the clinical course of a human disease.

Why use a mouse as a model?

There are several advantages to using the mouse as a model:

  • Humans share more than 95% of their genome with mice, which means disease-causing mutations in mice have an effect that is similar to what occurs in human disease.
  • The mouse genome is well-studied and completely mapped (i.e., all genes have been identified), which makes it easier for researchers to manipulate genes in mice to study most human diseases.
  • The life span of mice is short, which allows the disease to be studied at an accelerated pace.
  • The size and ease in handling of mice makes preliminary testing of potential drug treatments a fairly simple process.
  • The maintenance of mouse models is much cheaper than larger organisms.

Why are mouse models vital?

Mouse models are indispensable for a better understanding of human disease and for the development of effective treatments. Mouse models of deafness, for instance, have played a vital role in identifying the specific genes that are responsible for inherited hearing loss. Similarly, mouse models have also enabled scientists to model complex neurological diseases such as Alzheimer’s disease, Huntington’s disease, and several different types of ataxia. These models have greatly improved our understanding of neurodegenerative disease and brought us one step closer to the development of effective treatments. Because of this, the quest to identify a mouse model that mimics every aspect of a human disease – such as a specific type of spinocerebellar ataxia – continues to this day. These mice are instrumental for scientists to develop new treatments for disease and, ultimately, a cure.

If you would like to learn more about mouse mondels, take a look at these resources by the Jackson Laboratory and National Institutes of Health.

Snapshot written by Dr. Sriram Jayabal edited by Dr. Maxime Rousseaux