|Number of repeats||Counts (alleles)||Percentage|
Overview of repeats
ATXN8OS is the opposite strand of the ATXN8 gene that transcribes an mRNA that is associated with the spinocerebellar ataxia type 8 (SCA8) (Daughters et al., 2009). CTG repeat expansions in the 3′-UTR of ATXN8OS are leading to a toxic gain-of-function protein and causing the disease (Daughters et al., 2009).
The cut-off values for pathogenic length of repeats is debatable as many studies have shown the pathogenic range to be above 80 repeats, whereas one of the biggest ones so far has reported that intermediate alleles (45-109 CTGs) were found in almost similar frequencies in both controls and patients (Schöls et al., 2003). However, there are also reports of many healthy individuals who had a repeat length over 91 CTGs (Zeman et al., 2004). This shows the overlap in repeat length of individuals with and without SCA8 and it is recommended to interpret genotyping results in clinical context (Gupta & Jankovic, 2009). In general, the majority of studies show a repeat length at least 80 for affected individuals but at least 50 should be expected for a SCA8 patient (Gupta & Jankovic, 2009). In over 99% healthy controls the range has been shown to be between 2 and 37 repeats (Brusco et al., 2002). Based on the studies, we have specified here an intermediate range between 38-79.