To identify novel genes associated with ALS, we undertook two lines of investigation. We carried out a genome-wide association study comparing 20,806 ALS cases and 59,804 controls. Independently, we performed a rare variant burden analysis comparing 1,138 index familial ALS cases and 19,494 controls. Through both approaches, we identified kinesin family member 5A (KIF5A) as a novel gene associated with ALS. Interestingly, mutations predominantly in the N-terminal motor domain of KIF5A are causative for two neurodegenerative diseases: hereditary spastic paraplegia (SPG10) and Charcot-Marie-Tooth type 2 (CMT2). In contrast, ALS-associated mutations are primarily located at the C-terminal cargo-binding tail domain and patients harboring loss-of-function mutations displayed an extended survival relative to typical ALS cases. Taken together, these results broaden the phenotype spectrum resulting from mutations in KIF5A and strengthen the role of cytoskeletal defects in the pathogenesis of ALS.

Our results indicate C9orf72 haploinsufficiency could be a contributing factor in the spectrum of ALS/FTLD neurodegenerative disorders. Loss of function of the zebrafish orthologue of zC9orf72 expression in zebrafish is associated with axonal degeneration of motor neurons that can be rescued by expressing human C9orf72 mRNA, highlighting the specificity of the induced phenotype. These results reveal a pathogenic consequence of decreased C9orf72 levels, supporting a loss of function mechanism of disease.

Mutations in the TAR DNA Binding Protein gene (TARDBP), encoding the protein TDP-43, were identified in amyotrophic lateral sclerosis (ALS) patients. Interestingly, TDP-43 positive inclusion bodies were first discovered in ubiquitin-positive, tau-negative ALS and frontotemporal dementia (FTD) inclusion bodies, and subsequently observed in the majority of neurodegenerative disorders. To date, 47 missense and one truncating mutations have been described in a large number of familial (FALS) and sporadic (SALS) patients. Fused in sarcoma (FUS) was found to be responsible for a previously identified ALS6 locus, being mutated in both FALS and SALS patients. TARDBP and FUS have a structural and functional similarity and most of mutations in both genes are also clustered in the C-terminus of the proteins. The molecular mechanisms through which mutant TDP-43 and FUS may cause motor neuron degeneration are not well understood. Both proteins play an important role in mRNA transport, axonal maintenance, and motor neuron development. Functional characterization of these mutations in in vitro and in vivo systems is helping to better understand how motor neuron degeneration occurs. This report summarizes the biological and clinical relevance of TARDBP and FUS mutations in ALS. All the data reviewed here have been submitted to a database based on the Leiden Open (source) Variation Database (LOVD) and is accessible online at www.lovd.nl/TARDBP, www.lovd.nl/FUS.