Repeat expansion disorders

Abstract

An increasing number of repeat expansion disorders have been found to cause both rare and common neurological disease. This is exemplified in recent discoveries of novel repeat expansions underlying a significant proportion of several late-onset neurodegenerative disorders, such as CANVAS (cerebellar ataxia, neuropathy and vestibular areflexia syndrome) and spinocerebellar ataxia type 27B. Most of the 60 described repeat expansion disorders to date are associated with neurological disease, providing substantial challenges for diagnosis, but also opportunities for management in a clinical neurology setting. Commonalities in clinical presentation, overarching diagnostic features and similarities in the approach to genetic testing justify considering these disorders collectively based on their unifying causative mechanism. In this review, we discuss the characteristics and diagnostic challenges of repeat expansion disorders for the neurologist and provide examples to highlight their clinical heterogeneity. With the ready availability of clinical-grade whole-genome sequencing for molecular diagnosis, we discuss the current approaches to testing for repeat expansion disorders and application in clinical practice.

Keywords: CEREBELLAR ATAXIA; GENETICS; MOLECULAR BIOLOGY; NEUROGENETICS.

Figure 1. Repeat expansion disorders arise from short tandem repeats (STRs) that reside naturally within the genome (a). STRs can arise from any genic position and comprise repeat sequences of up to six base pairs in tandem. When expanded above a certain pathogenic threshold, they are associated with a repeat expansion disorder.While STRs can be intergenic, no pathogenic expansions of intergenic STRs have been characterised to date. UTR refers to untranslated regions of the gene. (b) Features of a repeat expansion that influence pathogenesis and are used to characterise a repeat expansion disorder. (c) Non-exhaustive list of repeat expansion disorders associated with neurological disease represented by their intragenic location. The repeat motif is listed after the disease in parentheses or collectively, within the triangles. n represents the number of repeat units. N in GCN represents any A/C/G/T nucleotide. The exonic repeat expansions are further partitioned by the probable translated product of the repeat-containing DNA into polypeptides, namely polyglutamine, polyglycine and polyalanine disorders. Unverricht-Lundborg disease is listed but is a dodecamer repeat and thus not strictly an expansion of an STR (each repeat unit is up to six base pairs). ALS, amyotrophic lateral sclerosis; CANVAS, cerebellar ataxia, neuropathy, vestibular areflexia syndrome; DRPLA, dentatorubral-pallidoluysian atrophy; FAME, familial adult myoclonic epilepsy; FTD, frontotemporal dementia; FXTAS, fragile X-associated tremor/ataxia syndrome; FRAXE, intellectual developmental disorder, X-linked; GDPAG, global developmental delay, progressive ataxia, and elevated glutamine; HDL-2, Huntington disease-like 2; IDD, intellectual development disorder; NIID, neuronal intranuclear inclusion disease; OPDM, oculopharygodistal myopathy; SCA, spinocerebellar ataxia. Created with BioRender.com.

Figure 2. General features of repeat expansion disorders. (a) Clinical heterogeneity is often present within the same repeat expansion: (i) Within FMR1, different clinical entities are associated with different repeat sizes. (ii) Within ATXN2, an intermediate range repeat expansion is associated with increased risk of ALS and a full range expansion causes spinocerebellar ataxia type 2. Therefore, accurate estimation of the repeat size is important for diagnosis. (iii) The C9orf72 hexanucleotide repeat expansion is a common genetic cause of both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) independent of the size of the repeat expansion. (b) Schematic showing the inverse relationship between age of disease onset and size of the repeat expansion. The severity of disease typically increases with increasing size of the repeat expansion. This is due to meiotic instability either in the paternal or maternal germline for that particular repeat, whereby n is the overall repeat size in the first generation and x is the number of repeats in addition to the n repeat size in each successive generation, in line with repeat instability associated with anticipation of the clinical presentation. (c) A population effect of repeat expansion disorders highlights genetic heterogeneity within one disease entity. This is exemplified in neuronal intranuclear inclusion disease (NIID). NIID is a clinical heterogeneous neurodegenerative condition characterised by eosinophilic neuronal intranuclear inclusions in both neuronal and non-neuronal cells. The NOTCH2NLC GGC repeat expansion is a common cause of NIID in individuals of East Asian ancestry who tend to have later-onset, milder disease presenting with cognitive decline or a neuromuscular phenotype. In Europeans, this repeat expansion is rare and disease is more severe, multisystemic and earlier in onset. (d). Convergence of different repeat expansion disorders on a common pathological feature. For example, ubiquitinated eosinophilic intranuclear inclusions are seen in brain tissue (and sometimes in antemortem skin biopsy) for a variety of different disorders including NIID and fragile X-associated tremor/ataxia syndrome (FXTAS). Created with Biorender.com.

Figure 3. Methods for detection of repeat expansions. (a) Evolution of methods used for detection of repeat expansions: from molecular, PCR-based approaches to bioinformatic tools applied to short-read exome or genome sequencing data to recent development of long-read sequencing technologies. (b) Clinical pathway for testing for repeat expansions within the National Health Service (NHS) Genomic Medicine Service (GMS). Note that the panel numbers and GMS pathway may be subject to change. The GMS website should be consulted for up-to-date panels. The panels provided here are for illustrative purposes of the principles of repeat expansion detection. DNA extracted from blood is processed for repeat expansion testing. If a particular repeat expansion disorder is suspected, then a standalone PCR with or without Southern blotting is requested if it falls into the remit of one of the listed disorders. If the clinical indication falls within any of 12 listed gene panels, then short-read whole-genome sequencing is carried out. Within each of the panels, integrated estimation of repeat size of a specific disorder within the panels listed by ExpansionHunter is provided. Further molecular confirmation is carried out if the bioinformatically estimated repeat size is above or falls just below the pathogenic threshold for repeat expansion. ALS, amyotrophic lateral sclerosis; CANVAS, cerebellar ataxia, neuropathy, vestibular areflexia syndrome; DRPLA, dentatorubral-pallidoluysian atrophy; FTD, frontotemporal dementia; FRDA, Friedreich’s ataxia; FXTAS, fragile X-associated tremor/ataxia syndrome; HLD-2, Huntington disease-like 2; SCA, spinocerebellar ataxia; ULD, Unverricht-Lundborg disease. Created with BioRender.com.