Amyotrophic lateral sclerosis (ALS) is the most common type of adult-onset motor neuron disease, characterized by rapid progression, projected fast grow of the number of patients, and genetic and clinical heterogeneity (age at onset, site of onset, disease aggressiveness and spread, etc.). In spite of the high level of heritability of ALS, a monogenic etiology can be found in <15% of overall patients, indicating that a considerable proportion of ALS disease genes and mutations remain to be identified. The probability of detecting a genetic cause of disease in sporadic ALS patients correlates with the presence of extreme phenotypes – e.g., exceptionally early onset or very aggressive or mild disease. Finally, apart from genetic factors within the nuclear genome, alterations of the mitochondrial (mt) genome (e.g., changes in the mtDNA mutational load) have been found to influence clinical expression or penetrance in other neurodegenerative disorders, but have never been investigated in ALS. The observed clinical variability of ALS manifestation and aggressiveness can at least partially be explained by the multitude of hitherto recognized and currently unknown genetic causes, risk factors, and disease modifiers, each of which represents a potential therapeutic avenue and influences present and future clinical decisions. The overarching goal of this proposal is to uncover novel genetic causes and modifiers of ALS by nuclear and mitochondrial genome sequencing in large and well clinically characterized cohorts of ALS patients by three complementary approaches. In Objective 1, we will search for (novel) genetic causes of ALS by utilizing a whole-genome sequencing (WGS) approach in three large ALS cohorts, consisting of 200 patients each; with very early age at onset, extremely fast, or extremely slow disease progression, respectively. In Objective 2, we will leverage the WGS datasets obtained from patients with extreme clinical manifestations from Objective 1 in order to identify genetic factors that influence the penetrance and/or the phenotypic expression of ALS. Finally, in Objective 3, we aim to screen mtDNA from 200 patients by ultra-deep sequencing in order to determine whether changes in the mtDNA mutational load influence age at onset and progression in ALS. The applicants have extensive and complementary expertise in clinical genetics, genomics, and large-scale data analysis and access to an optimal and unique biospecimen repository.