Approaching the age of clinical therapy for spinocerebellar ataxia type 1

Written by Dr. Marija Cvetanovic Edited by Dr. Maxime W. Rousseaux

New research (published Nov. 2018) reveals promising potential genetic therapy for SCA1.

A research team comprised of scientists from academia and industry have tested a new treatment for Spinocerebellar ataxia type 1 (SCA1), bringing disease-modifying therapy one step closer to the clinic. SCA1 is a dominantly-inherited ataxia that is currently untreatable. Symptoms of the disease include progressive loss of balance, slurring of speech, difficulties with swallowing and coughing, mild cognitive impairments, and depression. With a life expectancy after diagnosis of only 10-15 years, SCA1 is one of the fastest-progressing SCAs: after symptoms first appear, patients typically have just over a decade before these symptoms become so severe that they cause death (often due to respiratory failure). In 1993, collaborative efforts from the laboratories of Drs. Harry T. Orr and Huda Y. Zoghbi discovered that SCA1 is caused by the expansion of a CAG repeat somewhere in a patient’s DNA. CAG repeats cause a polyglutamine expansion in the protein that the mutated gene encodes; in this case, the group later identified that this had occurred in Ataxin-1 (ATXN1), the gene that encodes the ATXN1 protein. The SCA1 mouse models that Drs. Orr and Zoghbi generated (and graciously shared with the scientific community) have allowed for significant advances in the understanding of SCA1 pathogenesis over the years. Now, they provide preclinical evidence of a promising therapy to alter the progressive motor deficits and fatal outcome of SCA1.

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Spinocerebellar Ataxia Type 1 is Caused by a Trinucleotide DNA Repeat

Written by Hillary Handler  Edited by Dr. David Bushart

How researchers found that SCA1 is caused by an expanded, repetitive DNA sequence – a discovery that has allowed for accurate SCA1 diagnosis and more focused research strategies

Before the true genetic basis of Spinocerebellar Ataxia Type 1 (SCA1) was discovered, researchers and medical doctors noticed that SCA1 causes motor dysfunction, death of specific types of brain cells, and premature death in affected patients. By assessing health outcomes in multiple families affected by SCA1, scientists also recognized that the disease is inherited in an autosomal dominant manner. This means that each person with an SCA1 diagnosis has a 50% chance of passing the disease to each of his or her children. In addition, researchers noticed that affected members of SCA1 families displayed a disease feature called anticipation: a trend of increasing symptom severity and earlier age-of-onset as the disease is passed from generation to generation. Despite these discoveries, the specific genetic mutation responsible for causing SCA1 had not yet been identified or described. Determining the genetic cause of an inherited disease is critical for allowing accurate diagnosis of the condition. Furthermore, understanding the genetics of SCA1 would provide researchers with important clues about disease pathology that could help with designing and developing treatments.

Researcher looking through a microscope
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One of the groups that sought to identify the specific genetic cause of SCA1 was led by Dr. Harry Orr. These researchers published their findings in a landmark 1993 paper (Nature Genetics, 1993), which described the process by which they made their discovery. First, a technique called “linkage analysis” was used to determine the general location of the SCA1 gene within the human genome. By tracking how SCA1 is inherited relative to other, well-characterized genetic locations, the team was able to narrow their search to a small portion of chromosome 6’s short arm known as region 6p22-6p23. The researchers also noted that anticipation is often indicative of a particular DNA feature known as a trinucleotide repeat. To determine if a trinucleotide repeat was indeed causing SCA1, these scientists used DNA cloning and screening techniques within the identified region of chromosome 6. These experiments identified a CAG trinucleotide repeat within the SCA1 genomic target region of DNA.

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DNA Damage Repair: A New SCA Disease Paradigm

Written by Dr. Laura Bowie Edited by Dr. Hayley McLoughlin

Researchers use genetics to find new pathways that impact the onset of polyglutamine disease symptoms

The cells of the human body are complex little machines, specifically evolved to fulfill certain roles. Brain cells, or neurons, act differently from skin cells, which, in turn, act differently from muscle cells. The blueprints for all of these cells are encoded in deoxyribonucleic acid (DNA). To carry out the instructions in these cellular blueprints, the DNA must be made into ribonucleic acid (RNA), which carries the instructions from the DNA to the machinery that makes proteins. Proteins are the primary molecules responsible for the structure, function, and regulation of the body’s organs and tissues. A gene is a unit of DNA that encodes instructions for a heritable characteristic – usually, instructions for a making a particular protein. If there is something wrong at the level of the DNA (known as a mutation) then this can translate to a problem at the level of the protein. This could alter the function of a protein in a detrimental manner – possibly even rendering it totally non-functional.

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Artist representation of a DNA molecule. Image courtesy of gagnonm1993 on Pixabay.

DNA is made up of smaller building blocks called nucleotides. There are four different nucleotides: cytosine (C), adenine (A), guanine (G), and thymine (T). Polyglutamine diseases, such as the spinocerebellar ataxias (SCAs) and Huntington’s disease (HD), are caused by a CAG triplet repeat gene expansion, which leads to the expansion of a polyglutamine tract in the protein product of this gene (MacDonald et al., 1993; Zoghbi & Orr, 2000). Beyond a certain tract length, known as the disease “threshold,” the length of this expansion is inversely correlated with age at disease onset. In other words, the longer this expansion is, the earlier those carrying the mutation will develop disease symptoms. However, scientists have determined that onset age is not entirely due to repeat length, since individuals with the same repeat length can have different age of disease symptom onset (Tezenas du Montcel et al., 2014; Wexler et al., 2004). Therefore, other factors must be involved. These factors could be environmental, genetic, or some combination of both.

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Molecular Mechanism behind Purkinje Cell Toxicity in SCA1 Uncovered

Written by Dr. Chandrakanth Edamakanti   Edited by Dr. Hayley McLoughlin

Recent study decodes the protein signature of toxic Purkinje cells, finding that Purkinje cell mTORC1 signaling is impaired in SCA1.

Spinocerebellar ataxia type 1 (SCA1) is a late onset cerebellar neurodegenerative disorder caused by a mutation (in this case, an abnormal polyglutamine stretch) in the Ataxin-1 gene. People with this condition experience problems with coordination and balance, a set of symptoms known as ataxia. The protein produced by this faulty gene, ATXN1, is particularly toxic to the Purkinje cells, the sole output neurons of the cerebellum. However, the reason behind the selective toxicity of Purkinje cells in SCA1 is unknown.

The main focus of this article is to address this question. It is the first study to find the protein signature of toxic Purkinje cells in SCA1 mice. In the end, the authors identified widespread protein changes that are associated with Purkinje cell toxicity.

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Image of scientific laboratory. Photo by Martin Lopez on Pexels.com

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Protein kinase C to the Rescue in Spinocerebellar Ataxias

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

Protein kinase C: one protein that may help to protect against cerebellar neuronal dysfunction & death in spinocerebellar ataxias

Among the estimated 86 billion brain cells (known as “neurons”) in the human body (Azevedo et al., 2009), there is a small population of cells called Purkinje neurons. Though they only constitute a modest ~14-16 million cells, (Nairn et al., 1989), death or dysfunction in Purkinje neurons can cause you to lose your ability to walk coherently – a clinical symptom known as “ataxia.” This is because Purkinje neurons are the major work horse of the cerebellum, which is the part of the brain that fine-tunes our movement. While different types of hereditary spinocerebellar ataxias (SCAs) are caused by mutations in different genes, they all exhibit one thing in common: Purkinje neurons undergo severe degeneration. Neither the reasons for this selective vulnerability of Purkinje neurons in ataxia, nor how to increase their resistance to degeneration, are clear.

Three cartoon brains
Image courtesy of the The Internet Archive/Nielsen Malaysia

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