Large C9orf72 Hexanucleotide Repeat Expansions Are Seen in Multiple Neurodegenerative Syndromes and Are More Frequent Than Expected in the UK Population

Abstract

Hexanucleotide repeat expansions in C9orf72 are a major cause of frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). Understanding the disease mechanisms and a method for clinical diagnostic genotyping have been hindered because of the difficulty in estimating the expansion size. We found 96 repeat-primed PCR expansions: 85/2,974 in six neurodegenerative diseases cohorts (FTLD, ALS, Alzheimer disease, sporadic Creutzfeldt-Jakob disease, Huntington disease-like syndrome, and other nonspecific neurodegenerative disease syndromes) and 11/7,579 (0.15%) in UK 1958 birth cohort (58BC) controls. With the use of a modified Southern blot method, the estimated expansion range (smear maxima) in cases was 800-4,400. Similarly, large expansions were detected in the population controls. Differences in expansion size and morphology were detected between DNA samples from tissue and cell lines. Of those in whom repeat-primed PCR detected expansions, 68/69 were confirmed by blotting, which was specific for greater than 275 repeats. We found that morphology in the expansion smear varied among different individuals and among different brain regions in the same individual. Expansion size correlated with age at clinical onset but did not differ between diagnostic groups. Evidence of instability of repeat size in control families, as well as neighboring SNP and microsatellite analyses, support multiple expansion events on the same haplotype background. Our method of estimating the size of large expansions has potential clinical utility. C9orf72-related disease might mimic several neurodegenerative disorders and, with potentially 90,000 carriers in the United Kingdom, is more common than previously realized.

Screening of dementia genes by whole-exome sequencing in early-onset Alzheimer disease: input and lessons

“…For example, C9ORF72 GGGGCC hexanucleotide repeat expansions (which cannot be detected by sequencing), typically causing frontotemporal dementia -amyotrophic lateral sclerosis spectrum were found in patients with an AD phenotype or Parkinson's disease, but rarely in controls too, making this finding difficult to interpret. 28,29 It remains possible for these late-onset cases that AD pathology occurred independently of C9ORF72 expansions. 30 Similarly, at least one neuropathologically proven AD case was found to harbor a loss-offunction variant of GRN.…”

C9ORF72: grabbing a tiger by the tail

“…For example, does the number of repeats affect age at onset or disease severity (as in HD)? This has been difficult to assess due to technical limitations in the resolution power of southern blotting, though refined techniques now make determination of expansion length more accurate [6,10,62], but still no associations with age at onset or disease severity have been detected [17,62]. However, such observations are confounded by regional mosaicism, with reports that longer repeat lengths in cerebellum are associated with a longer, not shorter, duration (see Cooper-Knock).…”

“…These DPR proteins seem to be entirely specific to the expansion, not being seen in FTLD and ALS cases without expansions [39,40], though curiously are not specific for FTLD and ALS per se with rare appearances in cases with pathologically confirmed corticobasal degeneration [40,58], AD [12,29,35] but see [15], Creutzfeldt-Jakob disease [6], and even in apparently healthy controls [6,13]. This raises issues of penetrance, or even preclinical disease [6] (see Cooper-Knock, Mead). Given that proteinaceous accumulations have figured strongly in the pathogenesis of AD, Parkinson's disease, Creutzfeldt-Jakob disease, and in forms of FTLD associated with tau, TDP-43 and FUS, it is highly tempting to regard DPRs in this light, and attribute neurotoxic properties.…”

“…Indeed, DPR can consist of any or all of the five possible species [3,21,39,40,45,47,67,69]. These DPR proteins seem to be entirely specific to the expansion, not being seen in FTLD and ALS cases without expansions [39,40], though curiously are not specific for FTLD and ALS per se with rare appearances in cases with pathologically confirmed corticobasal degeneration [40,58], AD [12,29,35] but see [15], Creutzfeldt-Jakob disease [6], and even in apparently healthy controls [6,13]. This raises issues of penetrance, or even preclinical disease [6] (see Cooper-Knock, Mead).…”

C9orf72-related disorders: expanding the clinical and genetic spectrum of neurodegenerative diseases

“…It is possible to extend concepts obtained from the analysis of such populations to propose a proper overall analysis. Hexanucleotide repeat expansions in C9orf72 gene have been described in different contexts of neurodegenerative syndromes, ranging from classical bvFTD to sporadic Creutzfeldt-Jakob disease-like phenotype 39 . Few data regarding Brazilian patients with C9orf72-related disorders exist, but in general presenting with classical behavioural and extrapyramidal (focal dystonia, parkinsonism) compromise findings 40 .…”