Written by Larissa Nitschke Edited by Dr. Gülin Öz
Researchers in the Netherlands uncover a new way to treat SCA3
Upon receiving a conclusive diagnosis of Spinocerebellar Ataxia (SCA), hundreds of questions can appear in a patient’s mind: What is Spinocerebellar Ataxia? Why am I affected? How will my symptoms progress? What is the ultimate prognosis? Thankfully, years of research have enabled us to answer many of these questions for patients affected by Spinocerebellar Ataxia Type 3 (SCA3), also known as Machado-Joseph Disease. Still, the most important question a patient could ask – How can I be healthy again? – has remained unanswered.
SCA3 is the most common form of Spinocerebellar Ataxia worldwide. It is passed down from generation to generation in affected families. Initial symptoms typically appear around midlife, but cases of much earlier and much later onset have been reported. At first, problems with movement coordination are the most noticeable, leading to an increase in stumbles and falls. At later stages, speech difficulties, muscle stiffness, and sleeping problems appear, leaving the patient fatigued during the day. The symptoms worsen over the course of 10 to 20 years, at which point affected individuals typically succumb to the disease. As with other SCAs, current options for SCA3 treatment are mainly limited to symptom management rather than treating the direct cause of the disease.
The genetic cause of SCA3 is the presence of excess copies of the DNA building blocks cytosine (C), adenine (A), and guanine (G) in the Ataxin-3 gene (Atxn3). Scientists refer to this type of mutation as an expansion of a triplet repeat, since the C, A, and G copies appear as sets of back-to-back CAGs. Because the CAG triplet is responsible for coding the amino acid glutamine (Gln or Q) in the Ataxin-3 protein, the repeat expansion results in an elongated glutamine (polyQ) tract. This faulty protein accumulates in cells and causes toxicity in specific regions of the brain. Since the 1994 discovery that SCA3 is caused by a polyQ expansion in Atxn3, scientists and physicians all over the world have been humbled by the question of how to help patients affected with SCA3. One specific angle of research has focused on the removal of the toxic protein altogether. However, one downside of this approach is that it would also cause the loss of normal Atxn3 function in patients. Atxn3 is critical for the degradation of unwanted proteins, which is necessary for the healthy functioning of all our body’s cells. It normally binds to little marks on proteins called ubiquitin chains (which tag proteins for removal), then cleaves these chains to facilitate the entry of proteins into the cell’s destruction machinery. Since treatment will need to be sustained over the span of a patient’s lifetime, the complete removal of Atxn3 might be harmful.
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 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.
Written by Dr. Terri M Driessen Edited by Dr. W.M.C. van Roon-Mom
Antisense oligonucleotides: a potential treatment for SCA3 that partially rescues SCA3 disease mouse models
Identifying new ways to slow down or delay neurodegenerative diseases has been a key research focus in the SCA field. There are many avenues that scientists can take to address this question. One method is to target the disease-causing protein: by lowering the levels of the disease-causing protein, scientists may be able to alter disease progression. These methods have recently been used in studies in other neurodegenerative disorders, like SCA2, Amyotrophic Lateral Sclerosis (ALS), and Huntington’s disease.
Prior work by the laboratory of Hank Paulson at the University of Michigan has suggested these methods may also work in SCA3. They used antisense oligonucleotides (ASOs) to lower the SCA3 disease-causing protein. ASOs are short DNA sequences that bind to specific pieces of RNA. When the ASOs bind to RNA, it is broken down and no protein is made. The Paulson laboratory designed ASOs that bind to ATXN3, which is the RNA associated with SCA3. These ASOs were able to lower the expression of mutant ATXN3 (Moore, et al. 2017). Importantly, they were capable of lowering the expression of mutant ATXN3 in both mouse models of SCA3 and SCA3 patient fibroblasts (Moore, et al. 2017). By removing the SCA3-causing protein from cells, they predicted that the cells would have a better chance at surviving.
This previous work was promising, but several questions remained. How long would one ASO treatment work? Would the ASO work even after the SCA3 mice started showing symptoms? Are there any obvious side effects, like increased inflammation, after ASO injection? And importantly, would lowering ATXN3 levels help with motor coordination problems in SCA3 mice?