Written by Dr Hannah Shorrock Edited by Larissa Nitschke
Repeat interruptions in SCA10 influence repeat tract stability and are associated with epileptic seizures
Multiple spinocerebellar ataxias (SCAs) are caused by repeat expansion mutations, but in some cases, these repeat expansions are interrupted. The presence of repeat interruptions can influence disease symptoms and how the repeat expansion behaves. This is the case for SCA10. Some patients with SCA10 have a series of repeat interruptions, which are referred to as an ATCCT repeat interruption motif. In SCA10 patients with this interruption motif, Dr. Ashizawa and his team found an increased risk of developing epileptic seizures and identified that the interruptions influence the local stability of the repeat expansion.
SCA10 is a dominantly inherited ataxia caused by an ATTCT repeat expansion in the Ataxin 10 gene (ATXN10). Unaffected individuals usually carry 9-32 ATTCT repeats, while SCA10 patients carry an expansion of up to 4500 repeats. SCA10 patients suffer from cerebellar ataxia, but some patients also have other symptoms, including epileptic seizures. Dr. Ashizawa and his team were interested in why some patients with SCA10 suffer from epileptic seizures, but others do not.
Initially, the group investigated whether the length of the ATXN10 repeat expansion correlated with epileptic seizures. They found no difference in repeat length between 37 SCA10 patients who developed epilepsy and 51 who did not. This shows that repeat length does not influence whether or not SCA10 patients develop epileptic seizures.
SCA10 patients with interruptions have greater risk of epilepsy
Next, the group was interested in whether repeat interruptions were associated with epilepsy in SCA10. In some patients with SCA10, the ATXN10 repeat expansion contains a complicated pattern of repeat interruptions consisting of stretches of ATTCT repeats with ATTTTCT and ATATTCT interruptions before a stretch of ATCCT repeats. If the repeat tract contains these interruptions, it is classified as positive for the ATCCT repeat interruption motif.
The group found that of 43 SCA10 patients who developed epilepsy, 23 had the ATCCT repeat interruption motif; however, out of 78 SCA10 patients who did not develop epilepsy, only 12 had the ATCCT interuption. Taking a different approach to the data, the group found that the majority of patients carrying the ATCCT motif either developed epilepsy themselves or had a first-degree family member with SCA10 and epileptic seizures. Both of these relationships were statistically significant. This showed that SCA10 patients carrying the ATCCT interruption motif have a greater risk of developing epilepsy. They are also more likely to have a family history of SCA10 with epileptic seizures.
Interruptions increase stability the region they are found in, but not for the whole gene
The SCA10 repeat can be highly variable in length between generations, meaning the length can easily change between one generation and the next. For this reason, Dr. Ashizawa and his team were interested in whether the ATCCT repeat interruption motif changed from one generation to the next.
In patients positive for the ATCCT interruption, the group looked at the length of the ATTCT repeats between the ATTTTCT and ATATTCT interruptions. They found that in the 13 transmissions of ATCCT-positive expansions from mother to child, the stretches of ATTCT repeats between the interruptions remained the same length. Similarly, in nine of the 11 paternal transmissions, they found no change in the number of ATTCT repeats between the interruptions, and only small changes in the other two paternal transmissions.
While only small changes in length were detected within the interrupted region, the overall length of the repeat expansion frequently changed by 100 or more repeats from one generation to the next. From this, the group concluded that the repeat interruptions prevent changes in repeat length in the region of the expansion where they are found. However, but that the interruptions do not prevent changes in the length of the repeat across the full ATXN10 expansion.
Where did these interruptions come from?
Finally, because it is thought that the SCA10 repeat expansion may have a single origin, the group investigated whether it is likely that the interruption motif may have a single origin within the SCA10 population. Based on the rate of change in the number of ATTCT repeats between interruptions and the number of different interruption structures found in this study, the group estimated that the minimum time to the most recent common ancestor of the interruption motif was 775 years ago. According to this model, after the ATCCT interruption motif originated in one individual at least 775 years ago, through the generations, it was passed on to every person positive for the interruption motif.
To classify individuals as ATCCT-positive or negative, and to count the number of ATTCT repeats between the ATTTTCT and ATATTCT interruptions the group used a technique called polymerase chain reaction (PCR). Using this technique, if a SCA10 patient was ATCCT-positive, multiple copies of a region of the ATXN10 expansion between the start of their expansion and the stretch of ATCCT repeats would be generated and visualized. If a SCA10 patient was ATCCT-negative, the PCR would not generate multiple copies of this region, and the assay would give a negative result.
This allows the researchers to classify SCA10 patients as ATCCT-positive or negative. For ATCCT-positive patients, this region of the repeat was then sequenced to allow the number of ATTCT repeats between interruptions to be counted. By looking at these genetic characteristics in families with multiple generations affected by SCA10, the group was able to study how the ATCCT interruption motif influences disease symptoms and local repeat length across generations.
Learning from SCA10 can teach us about other dieases
While further work is needed to understand the pathogenic mechanism of how the interruption motif influences disease presentation in SCA10, the studies performed here further our understanding of the role of interruptions in repeat expansion diseases in the wider context.
The presence of repeat interruptions and their effects on disease symptoms and repeat expansion stability is not unique to SCA10 among the spinocerebellar ataxias. In both SCA1 and SCA2, the presence of interruptions in non-expanded repeat tracts has been shown to prevent their expansion into the disease-causing range. When these interruptions are found on expanded alleles in SCA2, they have been associated with differences in how the disease presents, including a later age of onset. Associations between the presence of repeat interruptions and disease presentation have also been reported for other repeat expansion diseases including myotonic dystrophy and Huntington’s disease. Because repeat interruptions are present in many repeat expansion diseases, studies such as these by Dr. Ashizawa can help to inform our understanding of the effect of repeat interruptions on disease variability across many diseases.
Repeat interruption: A change in the sequence of a repeat expansion within the repeat tract. For example, the following repeat tract is a CAG repeat expansion containing a CAA interruption: CAG CAG CAG CAG CAG CAA CAG CAG CAG.
Conflict of Interest Statement
The author and editor declare no conflict of interest.
Citation of Articles Reviewed
Landrian I, McFarland KN, Liu J, Mulligan CJ, Rasmussen A, Ashizawa T (2017) Inheritance patterns of ATCCT repeat interruptions in spinocerebellar ataxia type 10 (SCA10) expansions. PLoS One 12: e0175958 https://doi.org/10.1371/journal.pone.0175958
McFarland KN, Liu J, Landrian I, Zeng D, Raskin S, Moscovich M, Gatto EM, Ochoa A, Teive HA, Rasmussen A, Ashizawa T (2014) Repeat interruptions in spinocerebellar ataxia type 10 expansions are strongly associated with epileptic seizures. Neurogenetics 15: 59-64 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4038098/