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.
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.
Written by Dr. David D. Bushart Edited by Dr. Carolyn J. Adamski
How one research group worked to identify previously unknown causes of SCA13, and what we can learn from their strategy.
With so many different causes of cerebellar ataxia, how are doctors able to make an accurate diagnosis? This is an extremely important question for doctors, research communities, and patients. For doctors, knowing the underlying genetic cause for a case of ataxia is critical not only for formulating a more specific treatment plan, but also for performing informed genetic screens to determine if a patient’s family members are at risk for developing ataxia. For researchers, knowing what causes a certain type of ataxia allows for the development of new treatment strategies. And for patients, an accurate diagnosis can, importantly, provide peace-of-mind.
Unfortunately, getting to this point of diagnosis can still be a difficult task in a lot of cases – up to 20 percent of ataxia cases do not have a confirmed genetic cause (Hadjivassiliou et al., Journal of Neurology, Neurosurgery, and Psychiatry 2016). This is clearly an area for improvement in the field of ataxia research. Fortunately, many research groups are making efforts to improve our knowledge of the many different causes for cerebellar ataxia, how frequently they appear, and how we might be able to better treat them.
Though there are many studies that are continuously being performed and are constantly improving our knowledge of the specific causes of cerebellar ataxia, this summary will focus on the work of one group (Figueroa et al., PLoS One 2011). The research team, led by Dr. Stefan Pulst at the University of Utah, sought to better identify the frequency of different genetic mutations causing SCA13, a rare, dominantly-inherited form of spinocerebellar ataxia caused by mutations in a gene called KCNC3.
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.