Abstract
Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant neurodegenerative disorder caused by an expansion of a polyglutamine tract within the ATXN1 gene. Normal alleles have been reported to range from 6 to 35 repeats, intermediate alleles from 36 to 38 repeats and fully penetrant pathogenic alleles have at least 39 repeats. This distribution was based on relatively few samples and the narrow intermediate range makes the accuracy of the repeat sizing crucial for interpreting and reporting diagnostic tests, which can vary between laboratories. Here, we examine the distribution of 6378 SCA1 chromosomes and identify a very late onset SCA1 family with a fully penetrant uninterrupted pathogenic allele containing 38 repeats. This finding supports the theory that polyQ toxicity is related to the increase of the length of the inherited tracts and not as previously hypothesized to the structural transition occurring above a specific threshold. In addition, the threshold of toxicity shifts to a shorter polyQ length with the increase of the lifespan in SCA1. Furthermore, we show that SCA1 intermediate alleles have a different behavior compared to the other polyglutamine disorders as they do not show reduced penetrance when uninterrupted. Therefore, the pathogenic mechanism in SCA1 is distinct from other cytosine-adenine-guanine (CAG) repeat disorders. Accurately sizing repeats is paramount in precision medicine and can be challenging particularly with borderline alleles. We examined plasmids containing cloned CAG repeat tracts alongside a triplet repeat primed polymerase chain reaction (TP PCR) CAG repeat ladder to improve accuracy in repeat sizing by fragment analysis. This method accurately sizes the repeats irrespective of repeat composition or length. We also improved the model for calculating repeat length from fragment analysis sizing by fragment analyzing 100 cloned repeats of known size. Therefore, we recommend these methods for accurately sizing repeat lengths and restriction enzyme digestion to identify interruptions for interpretation of a given allele’s pathogenicity.
Highlights
Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant progressive neurodegenerative disorder resulting in a loss of coordination and balance
There were 367 chromosomes that fall within the intermediate range (35–38 repeats) and out of these SCA1 diagnostic tests only one repeat polymerase chain reaction (PCR) product was resistant to digestion by the restriction enzyme SfaNI
In order to take into account for the distinct migration pattern of CAG repeats by fragment analysis capillary electrophoresis, Triplet Repeat Primed PCR (TP PCR) CAG repeats were used as a ladder to improve the accuracy of sizing CAG repeats by fragment analysis
Summary
Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant progressive neurodegenerative disorder resulting in a loss of coordination and balance. Normal alleles have less than 35 repeats (Ranum et al, 1994; Goldfarb et al, 1996), whilst a subject with 44 repeats has been reported as being asymptomatic at the age of 66 years due to the presence of cytosineadenine-thymine (CAT) interruptions (Goldfarb et al, 1996). For alleles of 39–44 repeats to be considered pathogenic they must contain an uninterrupted stretch of CAG of at least 39 repeats (Menon et al, 2013). These ranges, have been defined using very small sample sizes and there is a need for refining them with a view to precision medicine
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