ArticlesA C9orf72 promoter repeat expansion in a Flanders-Belgian cohort with disorders of the frontotemporal lobar degeneration-amyotrophic lateral sclerosis spectrum: a gene identification study
Introduction
Amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) are two fatal neurodegenerative diseases without effective therapies. ALS is the most common neurodegenerative motor neuron disorder1 and FTLD has a similar prevalence to Alzheimer's disease in people younger than 65 years.2 These diseases are two extremes of a spectrum of clinically, pathologically, and genetically overlapping disorders,3 which suggests an overlap in disease mechanisms. Patients with ALS have reduced control of voluntary muscle movement due to progressive motor neuron degeneration in the motor cortex, brainstem, and spinal cord, resulting in muscle weakness and disturbances of speech, swallowing, or breathing. Up to 50% of patients with ALS have mild disturbances in executive function and some develop overt FTLD.4, 5 Symptoms of FTLD include behavioural, personality, and language disturbances and cognitive dysfunction caused by degeneration of frontal and temporal cortical neurons. Patients with advanced FTLD can also present with clinical signs of ALS.6 Although the diseases affect different neurons, TAR DNA-binding protein-43 (TDP-43) is a major constituent of neuronal deposits in both ALS and TDP-43-positive FTLD (FTLD-TDP), the most common pathological FTLD subtype.7, 8
Family-based linkage and population-based association studies have identified common genetic factors underlying ALS and FTLD. For example, mutations in the ALS genes TARDBP and FUS are occasionally noted in patients with FTLD8, 9, 10 and mutations in VCP (which is associated with an FTLD-related disorder) have been detected in ALS.11 Most striking is the finding that ALS and FTLD can occur within the same family or even the same patient. In more than 15 families worldwide, autosomal-dominant ALS and FTLD are causally linked with a major disease locus on chromosome 9 (ALSFTD2 locus).12, 13, 14, 15, 16, 17, 18 The minimal region linked in all these families is about 3·6 Mbp long and contains five known protein-coding genes (C9orf11, MOBKL2B, IFNK, C9orf72, and LINGO2; figure 1). Moreover, recent genome-wide association studies in ALS cohorts of different European origins have provided evidence for a major genetic risk factor in the same chromosome 9p region.19, 20, 21 We aimed to identify the disease-associated mutation from genetic studies of a family with FTLD-ALS conclusively linked to the 9p21 region and in an extended well characterised patient-control cohort from Flanders (Belgium).13 After initiation of our study, a Finnish study narrowed the associated region to a 232 kbp linkage disequilibrium block containing three known genes (MOBKL2B, IFNK, and C9orf72).19 Another such study implicated the same chromosomal region in an FTLD-TDP cohort,22 which was confirmed in other FTLD and FTLD-ALS cohorts.23 Two recent studies reported a GGGGCC repeat expansion in C9orf72 as the underlying genetic cause in chromosome 9 linked FLTD and ALS.24, 25
Section snippets
Participants and study design
The Flanders-Belgian cohort consisted of 337 patients with FTLD, 141 with ALS, and 23 with concomitant FTLD and ALS (FTLD-ALS).26, 27 Patients with FTLD were enrolled from 1998 onwards through an ongoing multicentre collaboration of neurology departments and memory clinics in Belgium (Hospital Network Antwerp Middelheim and Hoge Beuken, University Hospital Antwerp, University Hospitals Leuven, and University Hospital Ghent). Patients with ALS were recruited through the neuromuscular reference
Results
We excluded mutations in all five protein-coding genes in the 3·6 Mbp long minimal region (figure 1) through exon-based sequencing in family DR14. Subsequently, we sequenced 61 kbp of the most conserved non-coding sequences in the minimal region without finding patient-specific variants segregating with disease in family DR14. Whole-genome sequencing identified 120 new sequence variations in the minimal candidate region that segregated with disease, 37 of which were absent in controls.
We
Discussion
Previous genetic linkage and association studies have established a major locus for the spectrum of ALS and FTLD disorders on chromosome 9p21. In two reports,24, 25 a pathological GGGGCC repeat expansion in C9orf72 was shown to be the underlying genetic defect. Here, we report the independent finding of the same GGGGCC repeat expansion in patients with FTLD, FTLD-ALS, and ALS in a Flanders-Belgian cohort (panel). We identified several lines of evidence for the causative relation between the
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