Written by Dr. Hannah K Shorrock Edited by Dr. Judit M Perez Ortiz
How one team uncovered the first SCA known to be caused by a CTG repeat expansion mutation
Identifying the gene that causes a type of ataxia not only gives patients and their families a clearer diagnosis and prognosis, but also allows scientists to model the disease. Through genetic animal models of ataxia, researchers can study how a single mutation causes a disease and how we can try to slow, halt, or even reverse this process. It is this path through research that may eventually lead from gene discovery to the development of effective therapies.
The gene that causes spinocerebellar ataxia type 8 (SCA8) was first described in a research article published in 1999. Since then, many research articles on SCA8 have been published, including research into the DNA repeat expansions that cause the ataxia, the cellular processes that lead to ataxia, and the development of multiple animal models of SCA8. Together, these move the scientific community further along the road of research.
Back to the start
SCA8 was discovered when a research group led by Dr. Laura Ranum identified an unexpected abnormality in the DNA of a mother and a daughter, both of whom were affected with a previously undescribed type of dominantly-inherited ataxia. Each of them had an abnormally expanded DNA sequence of 80 CTG repeats within a gene that had not previously been linked to ataxia. By performing genetic screens on large families affected by genetically undefined ataxia, the researchers were able to identify an additional seven families that also had this repeat expansion in their DNA. One of those families consisted of seven generations within which 84 individuals were clinically evaluated for spinocerebellar ataxia symptoms and tested for the CTG repeat expansion. As it turned out, all members of this family with signs of ataxia tested positive for the CTG repeat expansion. Such large families allow researchers to perform linkage analyses to assess if there is a tendency for ataxia and the repeat expansion to be inherited together. By studying the genetics behind a disease in different families, the research group confirmed that this abnormal CTG expansion was responsible for causing this novel form of spinocerebellar ataxia. The researchers termed this disease SCA8, numerically following SCA7.
Overall, the research group found that in patients affected by SCA8, there were 107 to 127 CTG repeats in the genetic expansion. In contrast, the number of CTG repeats in a large population of unaffected individuals was significantly lower. Through the process of identifying SCA8 CTG expansions in eight different families, the researchers were also able to show that SCA8 is not caused by other genetic factors that are near the CTG expansion. They did this by analyzing the structure of the regions around the CTG expansion and found that they were not the same between the different families.
By analyzing the clinical information of the patients in the study, the researchers were able to clinically define SCA8 in terms of the symptoms, onset, and progression of the disease. The group found that the most common initial symptoms were dysarthria (difficulty articulating words), mild aspiration, and balance instability. Patients first noticed these symptoms in adulthood, between 18 and 65 years of age. When evaluated by clinicians, patients exhibited limb and gait ataxia (incoordination), limb spasticity (stiffness), reduced ability to perceive vibration, and nystagmus (involuntary eye movements). While the patients examined in the study generally showed a reasonably slow disease progression, severely affected individuals could no longer walk by their thirties to fifties. Magnetic resonance imaging (MRI) of these patients revealed cerebellar atrophy, a finding that suggests loss of cells within the cerebellum, and is consistent with the clinical evaluations and the symptoms experienced by the patients.
The genetic ins and outs of SCA8
In many cases, scientists and clinicians can show that a particular DNA sequence (genetic variant) causes a specific disease. In some diseases, however, not everyone with that genetic variant will also develop the disease. This concept is called “penetrance”, or how well a DNA sequence associated with a disease can predict that the person will clinically manifest the disease. SCA8 appears to exhibit reduced penetrance: that is, not everyone who has the abnormal CTG expansion will develop SCA8. This is due to several quirks of the CTG expansion. First off, there is the length of the expansion. This research group showed that SCA8 penetrance is affected by the CTG repeat length: the longer the repeat, the more likely the individual is to develop symptoms of SCA8. Interestingly, the researchers also identified that the CTG repeat was more likely to have increased in length if inherited from the mother and more likely to have decreased in length if inherited from the father. This is known as a “maternal penetrance bias.” In other words, the SCA8 CTG expansion is more likely grow in length when it is passed down from a mother with the repeat expansion than when it is passed down from a father with the repeat expansion. This was a novel finding, because the repeat expansions previously identified in SCAs display the opposite (i.e., paternal) bias.
Typically, DNA is “read” in one direction, called the sense direction, to generate an RNA that can be translated later into functional proteins in the cell. Occasionally, however, the DNA is read in “reverse”, or the antisense direction; this is the case for the SCA8 repeat expansion. This means that the SCA8 expansion is not present in the sense RNA but is present in the antisense RNA. Interestingly, neither the sense nor antisense RNAs in the SCA8 gene are translated into functional proteins. While the functions of the RNAs themselves remain unknown, the group thought that the antisense RNA likely manipulated the levels of the sense RNA sequence. This process could be affected by the abnormal SCA8 repeat expansion with the potential to lead to SCA8 symptom development.
This report was the first to identify a spinocerebellar ataxia caused by a CTG repeat expansion. Notably, while the SCAs identified before SCA8 are caused by abnormal CAG repeat expansions (which are translated into polyglutamine tracts in proteins), SCA8 was the first example of a dominant SCA identified that is not caused by a CAG mutation. SCA8 is clinically similar to other spinocerebellar ataxias and shares an overall pathology typical of SCAs. However, the genetic abnormality in SCA8 is more similar to myotonic dystrophy type 1 (DM1), a multisystemic disease with different clinical manifestation than many SCAs. Before this discovery, DM1 was the only disease known to be caused by a CTG expansion. The identification of the genetic cause of SCA8 has since enabled researchers to develop genetic models of the disease, which have begun to provide insights into the cellular processes that lead to SCA8 symptoms and have opened up potential routes of therapy development.
Dominantly Inherited: Inheriting an abnormal gene from one parent will cause the disease
Dysarthria: Difficulty speaking, resulting from problems controlling muscles involved in speech.
Mild aspiration: Entry of food, drink, stomach contents or secretions into the respiratory tract
Nystagmus: A condition involving the loss of eye movement control.
Penetrance: The proportion of individuals carrying a particular genetic variant that also show the characteristic phenotype
Gene: A unit of heredity made up of DNA that fully or partially controls the development of specific traits.
Conflict of Interest Statement
Dr Hannah K Shorrock, who wrote this piece, now works as a postdoctoral associate in the Ranum lab, but did not work on this specific project. Dr Judit M Perez Ortiz, who edited this piece, completed her graduate studies in an SCA1 research laboratory and did not work on this project.
Citation of Article Reviewed
Koob M.D., et al., An untranslated CTG expansion causes a novel form of spinocerebellar ataxia (SCA8). Nature Genetics, 1999 Apr; 21(4): p. 379-384. (https://www.ncbi.nlm.nih.gov/pubmed/10192387)