Since the discovery of the C9orf72 repeat expansion as the most common genetic cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis, it has increasingly been associated with a wider spectrum of phenotypes, including other types of dementia, movement disorders, psychiatric symptoms and slowly progressive FTD. Prompt recognition of patients with C9orf72-associated diseases is essential in light of upcoming clinical trials. The striking clinical heterogeneity associated with C9orf72 repeat expansions remains largely unexplained. In contrast to other repeat expansion disorders, evidence for an effect of repeat length on phenotype is inconclusive. Patients with C9orf72-associated diseases typically have very long repeat expansions, containing hundreds to thousands of GGGGCC-repeats, but smaller expansions might also have clinical significance. The exact threshold at which repeat expansions lead to neurodegeneration is unknown, and discordant cut-offs between laboratories pose a challenge for genetic counselling. Accurate and large-scale measurement of repeat expansions has been severely hindered by technical difficulties in sizing long expansions and by variable repeat lengths across and within tissues. Novel long-read sequencing approaches have produced promising results and open up avenues to further investigate this enthralling repeat expansion, elucidating whether its length, purity, and methylation pattern might modulate clinical features of C9orf72-related diseases.
|Number of pages||8|
|Journal||Journal of Neurology, Neurosurgery and Psychiatry|
|Publication status||Published - 1 May 2021|
Bibliographical noteFunding Information:
ELvdE, HS and JCVS are supported by two Memorabel grants from Deltaplan Dementie (The Netherlands Organisation for Health Research and Development and Alzheimer Nederland; grant numbers 7330550813 and 733050103), The Bluefield Project to Cure Frontotemporal Dementia, the Dioraphte foundation (grant number 1402 1300) and the European Joint Programme-Neurodegenerative Disease Research (JPND, PreFrontALS). MvB is supported by the Muscular Dystrophy Association (MDA), ALS Association and National Institutes of Health (NIH) grants R21 NS110994 and R21 NS099631.
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