Snapshot: How does CAG tract length affect ataxia symptom onset?

The instructions our bodies need to grow and function are contained in our genes. These instructions are made up of tiny structures called nucleobases. There are four types of nucleobases in DNA: adenine (A), cytosine (C), guanine (G), thymine (T). By putting these four nucleobases in different orders and patterns, this writes the instructions for our body.

artists drawing of a blue DNA molecule
A cartoon strand of DNA. Image by PublicDomainPictures from Pixabay

Some of the genes contain long sections of repeating ‘CAG” instructions, called CAG tracts. Everyone has repeating CAG tracts in these genes, but once they are over a certain length they can lead to disease. Some ataxias are caused by this type of mutation, including SCA1, SCA2, SCA3, SCA6, SCA7 and SCA17. These are often called polyglutamine expansion disorders. This is because “CAG” gives the body instructions to make the amino acid glutamine. You can read more about what is polyglutamine expansion in our past Snapshot about that subject.

For each disorder caused by a CAG expansion mutation, the number of times the CAG is repeated in a particular gene will determine whether someone will develop the disease. Repeat lengths under this number will not cause symptoms and repeat lengths over the threshold will usually lead to ataxia. When someone undergoes genetic testing for ataxia, doctors will be able to tell them the length of these CAG tracts and whether they have a CAG repeat number in one of these genes that is over the threshold. This table gives a summary of different CAG expansion mutations that can lead to ataxia and how the length of the repeat affects age of onset.

 Affected Gene Normal
Repeat Size
Disease
Repeat Size
SCA1ATXN16-4439-88
SCA2ATXN215-3136-77
SCA3ATXN312-4055-86
SCA6CACNA1A 4-1821-33
SCA7ATXN74-3537-306
SCA12PPP2R2B4-3266-78
SCA17TBP25-4246-63

For SCA1, SCA2, SCA3, SCA6, and SCA7; longer CAG tracts are associated with earlier onset.

For SCA12, it is hard to predict the age of onset based on repeat length as SCA12 is so rare. Some individuals with long repeats don’t develop ataxia. One study found that longer CAG tract lengths are associated with earlier onset but that it does not affect the severity of symptoms.

For SCA17, Longer CAG tracts have separately been associated with an earlier age of onset and more severe cerebellar atrophy.

In general, people with longer repeat lengths in ataxia genes are likely to present with ataxia symptoms earlier in life. However, it is important to remember that there are many other factors involved. Other genes may have mutations that either worsen the progression of ataxia or protect against more severe symptoms. Therefore, in individual people, the length of the repeat is not always enough information to determine when that person will start showing symptoms, or how severe these symptoms will be.

If you would like more information about the genetic causes of SCAs, including information about genetic testing and what CAG repeat length might mean, take a look at these resources by the National Ataxia Foundation.

Snapshot written by Anna Cook and edited by Larissa Nitschke.

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Eyes: Windows to peek at brain function in spinocerebellar ataxias

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

Eye movement deficits occur ubiquitously in spinocerebellar ataxias, even at early disease states, highlighting their clinical importance.

Imagine the different motor movements that you make in your everyday life. Many people think of actions that we perform using our hands and legs, such as reaching for objects or walking. Let’s zoom in on a different task: catching a baseball. You need to know where the ball is going to land so you can run to that spot, then guide your arms while diving, if need be, to catch the ball. For this to work perfectly, you need to see and follow the ball. Your eyes enable you to track the ball while it is moving. How can your eyes keep the ball in focus while you are running at full speed towards the spot where you expect the ball to land? Your eyes are equipped with muscles which enable the eyes to move so as to keep the visual scene in focus. These eye movements, as demanded by the needs of the current scenario, in this case, catching a baseball, are indispensable for us to see the world without any hindrance.

Woman with hand in a "C" shape in front of her face. She's focusing in on her eye.
The eyes may provide a window into spinocerebellar ataxia, even before other symptoms show up. Photo by fotografierende on Pexels.com

Which brain region gives us the power to do this?

The cerebellum, or “little brain”, which enables one to move their arms and legs precisely, also controls the way we move our eyes. Therefore, it is logical to posit that when cerebellum goes awry, it may lead to eye movement abnormalities. Several previous studies have shown this to be true in many spinocerebellar ataxias (SCAs), where non-gait symptoms such as eye movement abnormalities have been found to accompany gait deficits in advanced stages of the disease. However, recent work from pioneers in clinical ataxia research at the Harvard Medical School have shown that eye movement abnormalities are also commonly present in SCAs even in pre-symptomatic states. This study emphasizes the critical need to better document the history of eye movement deficits and track them throughout the progression of the disease. This will help researchers to develop better rating scales for ataxia.

In this study, a population of SCA patients (134 individuals) who exhibited different types of SCA (including SCA1, SCA2, SCA3, SCA5, SCA6, SCA7, SCA8 and SCA17) were assessed for eye movement abnormalities at different stages of the disease, from pre-symptomatic (with no gait deficits) to advanced stages (those who use a wheel-chair). First, it was found that ~78% of all pre-symptomatic individuals exhibited eye movement deficits, and these deficits became even more common as the disease progressed, where every single person in advanced stages exhibited eye movement deficits. Second, when researchers examined the eye movements closely, they found that different types of ataxia might cause different kinds of eye movement deficits. However, these results are only suggestive because of the small population size of early-stage SCA individuals in this study, and the types of assessments used. Therefore, future studies will require a larger population size and a thorough quantitative analysis of specific types of eye movement deficits to help characterize eye movement abnormalities in SCAs. Finally, the Brief Ataxia Rating Scale (BARS), a recently designed simple clinical test for ataxia, was further improved in this study to account for the clinically observed eye movement deficits in SCAs. With such a nuanced metric, an improved BARS score was found to correlate with the stage, severity and duration of the disease irrespective of the type of ataxia.

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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.

dna-2358911_1280
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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