Length of Uninterrupted CAG, Independent of Polyglutamine Size, Results in Increased Somatic Instability, Hastening Onset of Huntington Disease

Abstract

Huntington disease (HD) is caused by a CAG repeat expansion in the huntingtin (HTT) gene. Although the length of this repeat is inversely correlated with age of onset (AOO), it does not fully explain the variability in AOO. We assessed the sequence downstream of the CAG repeat in HTT [reference: (CAG)n-CAA-CAG], since variants within this region have been previously described, but no study of AOO has been performed. These analyses identified a variant that results in complete loss of interrupting (LOI) adenine nucleotides in this region [(CAG)n-CAG-CAG]. Analysis of multiple HD pedigrees showed that this LOI variant is associated with dramatically earlier AOO (average of 25 years) despite the same polyglutamine length as in individuals with the interrupting penultimate CAA codon. This LOI allele is particularly frequent in persons with reduced penetrance alleles who manifest with HD and increases the likelihood of presenting clinically with HD with a CAG of 36-39 repeats. Further, we show that the LOI variant is associated with increased somatic repeat instability, highlighting this as a significant driver of this effect. These findings indicate that the number of uninterrupted CAG repeats, which is lengthened by the LOI, is the most significant contributor to AOO of HD and is more significant than polyglutamine length, which is not altered in these individuals. In addition, we identified another variant in this region, where the CAA-CAG sequence is duplicated, which was associated with later AOO. Identification of these cis-acting modifiers have potentially important implications for genetic counselling in HD-affected families.

On the wrong DNA track: Molecular mechanisms of repeat-mediated genome instability

“…In other words, a loss of AGG interruption likely predisposes CGG repeat tracts to large-scale expansions (149,152). Generally, the stabilizing role of interruptions is common for expandable repeats and has been reported for GAA (153,154), CAG (38,128,129,(155)(156)(157)(158)(159)(160)(161)(162), CCTG (54,58,(163)(164)(165), and other repeats in somatic tissues, intergenerational transmission, and experimental systems.…”

Gene expression profiles complement the analysis of genomic modifiers of the clinical onset of Huntington disease

“…Recent studies have started to make progress in identifying modifier genetic variants, both for HD, 7–11 and other neurological disorders. 12–14 For example, genetic variants in the interrupting sequence between the pathogenic CAG repeat and the polymorphic CCG repeat have been shown to influence age of onset of HD patients, and are particularly relevant for patients that carry reduced penetrance alleles.…”

“…12–14 For example, genetic variants in the interrupting sequence between the pathogenic CAG repeat and the polymorphic CCG repeat have been shown to influence age of onset of HD patients, and are particularly relevant for patients that carry reduced penetrance alleles. 10, 11 Additionally, genome-wide association studies (GWAS) have identified trans -modifiers of clinical age of onset in HD, 7, 8, 10 and have highlighted the important role of DNA repair genes in modulating age of onset, potentially through altering the somatic instability of the CAG repeat.…”

Repeat length increases disease penetrance and severity in C9orf72 ALS/FTD BAC transgenic mice

“…Our generation of an isogenic series of C9-BAC mice with 800, 500 or 50 repeats has allowed us to directly demonstrate that longer repeat tracts result in earlier onset of behavioral abnormalities and decreased survival compared to shorter alleles. These results, combined with the somatic instability seen in patients, suggest that targeting DNA repair pathways may be a viable approach for mitigating disease in C9orf72 ALS/FTD (Jones et al, 2017, Wright et al, 2019. While reducing repeat tract length in our mice did reduce disease penetrance, several C9-50 mice did show ALS/FTD phenotypes.…”

“…While some expansion carriers remain asymptomatic into their 90s, the frequency of reduced penetrance is not yet clear because patients with symptoms are much more likely to be seen by a physician and genetically tested compared to asymptomatic mutation carriers , Murphy, Arthur et al, 2017. Repeat length and somatic repeat instability, which are known to contribute to Huntington disease and other repeat expansion disorders (Jones, Houlden et al, 2017, Lee, Wheeler et al, 2015, Wright, Collins et al, 2019, may contribute to the reduced disease penetrance of C9orf72 ALS/FTD, differences in age-of-onset and the wide-ranging clinical effects of the C9orf72 expansion mutation , Murphy et al, 2017. However, because of ascertainment bias, technical difficulties in measuring repeat length and somatic instability in patients, it is challenging to study the effects of repeat length as a modifier of C9orf72 ALS/FTD (Hsiung, DeJesus-Hernandez et al, 2012, Van Mossevelde, van der Zee et al, 2017a.…”