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.

In search of a common pathway leading to motor dysfunction in cerebellar ataxias

Written by Dr. Carolyn J. Adamski Edited by Dr. Judit M Perez Ortiz

A research group uncovers a drug target to potentially correct motor phenotypes across several cerebellar ataxias.

When someone is diagnosed with spinocerebellar ataxia (SCA), their symptoms may look very similar despite the fact that different genes are causing the disease. There are over 35 genes known to cause cerebellar ataxia, each of which are studied by scientists to try to understand the ways in which they can each lead to disease. Increasingly, scientists are beginning to appreciate that perhaps it would be helpful to find commonalities between the different SCAs to identify treatment options that could help more SCA patients. The emerging picture is that the genes causing cerebellar ataxia are all vital to the health and function of neurons. Studies like these are currently being conducted all over the world. One group focused on MTSS1, a critical component of neuronal function. They made the exciting discovery that a handful of other genes known to cause cerebellar ataxia were doing so, at least in part, through MTSS1. This study uncovered a common network between cerebellar ataxia genes. Their hope is that someday clinicians will be able to treat many cerebellar ataxias with one therapy.

wooden pole with a wooden arrow pointing to the left
A photo of a road sign giving direction. Could MTSS1 be the pathway sign pointing towards ataxia? Photo by Jens Johnsson on Pexels.com

One approach scientists use to understand a gene’s function is to remove it from the genome, typically in mice, and observe what happens. This group reported that when they removed MTSS1, mice were not able to walk as well as healthy mice. This defect got progressively worse with age. What they observed in these mice looked very similar to what patients with cerebellar ataxia experience. Because there are a few areas of the brain important for walking, the authors wanted to make sure this was due to defects in the cerebellum. Neurons in the cerebellum missing MTSS1 were there, but they were unable to effectively communicate with other neurons in the brain and were slowly dying. When a neuron in the cerebellum fails to communicate the right message, things like poor coordination of body movement happen.

After establishing that removal of MTSS1 causes disease, this group went back to the literature and found that MTSS1 was a fundamental regulator of a pathway known to be critical for communication between neurons. They looked in the mice lacking MTSS1 and found that this pathway was abnormally in “overdrive”. They immediately started looking for ways to correct this. They hoped that by correcting this major pathway, they could help the neurons to more effectively communicate body movements again. Eventually, they found a compound that could specifically dial this pathway down. They gave this drug to the mice lacking MTSS1 and used a number of tests to examine their every movement. To their surprise, they were unable to tell the difference between normal healthy mice and those lacking MTSS1 and treated with the compound. In other words, the compound was able to help the ataxia in these mice. This was an exciting result indeed!

Continue reading “In search of a common pathway leading to motor dysfunction in cerebellar ataxias”

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.

Mitochondrial impairments identified in SCA7 mouse model and patient cells

Written by Dr. Colleen A. Stoyas Edited by Dr. Monica Banez 

Duke University researchers have found that altered cellular metabolism and mitochondrial dysfunction play a central role in spinocerebellar ataxia type 7 (SCA7), a result that has therapeutic implications for this disease.

Spinocerebellar ataxia type 7 (SCA7) is a dominantly-inherited ataxia characterized by retinal degeneration and cerebellar atrophy. As retinal degeneration advances, patients experience progressive central vision loss. Atrophy (i.e., cell loss) in the cerebellum causes a progressive loss of balance, as the cerebellum is the region of the brain that controls coordinated movement and motor learning. SCA7 patients also experience difficulty speaking and swallowing in later stages of the disease. Symptoms can manifest at any age, though the disease is particularly aggressive when found in infants and children. SCA7 is caused by an expansion mutation in the Ataxin-7 (ATXN7) gene, which produces a protein containing extra repeats of the amino acid glutamine. These additional glutamines make the protein fold in an incorrect shape. Much like an umbrella turned inside-out, this protein, once it loses its shape, does not work in the way it’s meant to. Dr. Albert La Spada has previously shown that the ataxin-7 protein is necessary for the expression of genes that are central to the normal function of the eye – particularly, the retina. Now, his group has provided evidence that abnormal cellular metabolism underlies the brain changes observed in SCA7.

Mice whose brains carry the SCA7 mutation model the juvenile forms of this disease. Using this mouse model, the La Spada group observed changes in the network and physical size of the brain’s mitochondria. Mitochondria are the cell’s “power plants,” and are responsible for the chemical reactions (known as cellular metabolism) that generate the energy our cells need to function. Cellular metabolism is assessed by measuring metabolites, which are the products of these chemical reactions. The La Spada group’s researchers identified dysfunction in the mitochondria in SCA7 due to an underlying decrease in one specific metabolite: NAD+.

microscope and sample slide
Photo by Pixabay on Pexels.com

Short for nicotinamide adenine dinucleotide, NAD+ is necessary for proper mitochondrial function. A general reduction of NAD+ occurs as humans age, as well as in a host of other neurodegenerative disorders (many of which exhibit mitochondrial dysfunction). This recent recent by Dr. La Spada and his team has shown that NAD+ is also reduced in mitochondria in SCA7.

Continue reading “Mitochondrial impairments identified in SCA7 mouse model and patient cells”

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?”