Investigation of C9orf72 in 4 Neurodegenerative Disorders

Amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) are fatal neurodegenerative syndromes that belong to the same clinicopathological spectrum.1,2 Frontotemporal lobar degeneration is a primary dementia characterized by early behavioral, language, and extrapyramidal changes, while symptoms of ALS are the result of the degeneration of motor neurons. Both syndromes may occur within the same family or even the same patient. Previously, linkage analyses revealed a 3.7-Mb region on 9p21 associated with familial ALS/FTLD,3–10 and genome-wide association studies suggested a major risk factor in the same locus for sporadic ALS and FTLD.11–15 Recently, 2 research groups independently explained this locus by a pathological noncoding hexanucleotide (G4C2)30–1600 repeat expansion in the chromosome 9 open reading frame 72 (C9orf72) gene of unknown function.16,17 Based on the allele frequencies in cases vs controls, the first studies suggested that expansions with more than 30 repeats should be considered pathological, while alleles with less than 20 repeats are wild type.16 However, a reliable cutoff for the pathological alleles remains to be established by additional studies (eg, segregation, neuropathological, or functional studies). Furthermore, the contribution of intermediate-size alleles (20–29 repeats) to disease pathology has not yet been evaluated. The expansion is the most frequent cause of ALS and FTLD identified to date. In the Finnish population, 46% of patients with familial ALS, 21% of patients with sporadic ALS, and 29% of patients with sporadic FTLD have the expansion.16 DeJesus-Hernandez et al17 reported the expansion in 24% of patients with familial ALS, 4% of patients with sporadic ALS, and 12% of patients with familial FTLD. In the Flanders-Belgian cohort, the mutation was observed in 47% of patients with familial ALS, 5% of patients with sporadic ALS, and 16% of patients with familial FTLD.18 The pathological mechanism associated with the expansion is currently unknown except that the expansion leads to a 50% reduction of C9orf72 messenger RNA expression,17,18 and the brain pathology in mutation carriers is associated with possibly toxic nuclear RNA foci, as well as TAR DNA-binding protein 43 (TDP-43) and p62 inclusions.17,19 The clinical phenotype appears to be highly heterogeneous in reported mutation carriers.20 Following the discovery of this novel mutation, many questions are yet to be addressed. What is the expansion frequency in other ALS/FTLD cohorts? Are there any clinical features that can discriminate between patients with and without the C9orf72 expansion? What is a reliable cutoff for the pathological repeat number? What is the role of alleles with intermediate repeat sizes or variability in the region flanking the G4C2 repeat? Could the expansion account for other neurodegenerative diseases, such as Alzheimer disease (AD) or Parkinson disease (PD)? These questions were investigated by the current study. The expansion frequency was estimated in a comprehensive case-control sample set consisting of 2224 individuals (patients with FTLD, ALS, AD, and PD and healthy controls). To our knowledge, this is the first case-control study using a 2-step genotyping strategy that allowed for the analysis of genotype information for the alleles with less than 50 repeats. Clinical data were analyzed to understand the phenotype spectrum observed in mutation carriers.