Dataset Information

Unexpected frequency of the pathogenic AR CAG repeat expansion in the general population.

ABSTRACT: CAG repeat expansions in exon 1 of the AR gene on the X chromosome cause spinal and bulbar muscular atrophy, a male-specific progressive neuromuscular disorder associated with a variety of extra-neurological symptoms. The disease has a reported male prevalence of approximately 1:30 000 or less, but the AR repeat expansion frequency is unknown. We established a pipeline, which combines the use of the ExpansionHunter tool and visual validation, to detect AR CAG expansion on whole-genome sequencing data, benchmarked it to fragment PCR sizing, and applied it to 74 277 unrelated individuals from four large cohorts. Our pipeline showed sensitivity of 100% [95% confidence interval (CI) 90.8-100%], specificity of 99% (95% CI 94.2-99.7%), and a positive predictive value of 97.4% (95% CI 84.4-99.6%). We found the mutation frequency to be 1:3182 (95% CI 1:2309-1:4386, n = 117 734) X chromosomes-10 times more frequent than the reported disease prevalence. Modelling using the novel mutation frequency led to estimate disease prevalence of 1:6887 males, more than four times more frequent than the reported disease prevalence. This discrepancy is possibly due to underdiagnosis of this neuromuscular condition, reduced penetrance, and/or pleomorphic clinical manifestations.

SUBMITTER: Zanovello M

PROVIDER: S-EPMC10316764 | biostudies-literature | 2023 Jul

REPOSITORIES: biostudies-literature

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Unexpected frequency of the pathogenic AR CAG repeat expansion in the general population.

Zanovello Matteo M Ibáñez Kristina K Brown Anna-Leigh AL Sivakumar Prasanth P Bombaci Alessandro A Santos Liana L van Vugt Joke J F A JJFA Narzisi Giuseppe G Karra Ramita R Scholz Sonja W SW Ding Jinhui J Gibbs J Raphael JR Chiò Adriano A Dalgard Clifton C Weisburd Ben B Hanna Michael G MG Greensmith Linda L Phatnani Hemali H Veldink Jan H JH Traynor Bryan J BJ Polke James J Houlden Henry H Fratta Pietro P Tucci Arianna A

Brain : a journal of neurology 20230701 7

CAG repeat expansions in exon 1 of the AR gene on the X chromosome cause spinal and bulbar muscular atrophy, a male-specific progressive neuromuscular disorder associated with a variety of extra-neurological symptoms. The disease has a reported male prevalence of approximately 1:30 000 or less, but the AR repeat expansion frequency is unknown. We established a pipeline, which combines the use of the ExpansionHunter tool and visual validation, to detect AR CAG expansion on whole-genome sequencing ...[more]

PMID: 36797998

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Project description:Huntington's disease is an inherited neurodegenerative disorder caused by a CAG repeat expansion that encodes a polyglutamine tract in the huntingtin (HTT) protein. The mutant CAG repeat is unstable and expands in specific brain cells and peripheral tissues throughout life. Genes involved in the DNA mismatch repair pathways, known to act on expansion, have been identified as genetic modifiers; therefore, it is the rate of somatic CAG repeat expansion that drives the age of onset and rate of disease progression. In the context of an expanded CAG repeat, the HTT pre-mRNA can be alternatively processed to generate the HTT1a transcript that encodes the aggregation prone and highly pathogenic HTT1a protein. This may be a mechanism through which somatic CAG repeat expansion exerts its pathogenic effects, as the longer the CAG repeat, the more HTT1a and HTT1a is produced. The allelic series of knock-in mouse models, HdhQ20, HdhQ50, HdhQ80, HdhQ111, CAG140 and zQ175 with polyglutamine expansions of 20, 50, 80, 111, 140 and ∼190, can be used to model the molecular and cellular consequences of CAG repeat expansion within a single neuron. By western blot of cortical lysates, we found that mutant HTT levels decreased with increasing CAG repeat length; mutant HTT was only 23 and 10% of wild-type levels in CAG140 and zQ175 cortices, respectively. To identify the optimal bioassays for detecting the full-length HTT and HTT1a isoforms, we interrogated the pairwise combinations of seven well-characterized antibodies on both the 'homogeneous time-resolved fluorescence' and 'Meso Scale Discovery' platforms. We tested 32 assays on each platform to detect 'full-length mutant HTT', HTT1a, 'total mutant HTT' (full-length HTT and HTT1a) and 'total full-length HTT' (mutant and wild type). None of these assays recapitulated the full-length mutant HTT levels as measured by western blot. We recommend using isoform- and species-specific assays that detect full-length mutant HTT, HTT1a or wild-type HTT as opposed to those that detect more than one isoform simultaneously. Our finding that as the CAG repeat expands, full-length mutant HTT levels decrease, while HTT1a and HTT1a levels increase has implications for therapeutic strategies. If mutant HTT levels in cells containing (CAG)200 are only 10% of wild-type, HTT-lowering strategies targeting full-length HTT at sequences 3' to Intron 1 HTT will predominantly lower wild-type HTT, as mutant HTT levels in these cells are already depleted. These data support a therapeutic strategy that lowers HTT1a and depletes levels of the HTT1a protein.

Dataset Information

Unexpected frequency of the pathogenic AR CAG repeat expansion in the general population.

Publications

Unexpected frequency of the pathogenic AR CAG repeat expansion in the general population.

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