Amyotrophic lateral sclerosis (ALS) is a devastating and progressive neurodegenerative disease characterized by loss of upper and lower motor neurons in the brainstem, spinal cord and cerebral cortex. ALS leads to muscle wasting, paralysis and, ultimately, death from respiratory failure within 3-5 years of symptom onset. There is no cure for ALS and current treatments only slow the progression of disease. Mutations that confer toxic function(s) to the Cu/Zn superoxide dismutase 1 (SOD1) gene are responsible for nearly 25% of inherited ALS cases and are the most common genetic cause for this disorder. These mutations are thought to impart neurotoxicity to motor neurons. Indeed, production of pathogenic SOD1 protein in mice results in a late- onset, progressive neurodegenerative disease that closely mimics the hallmarks of ALS. Moreover, numerous studies have shown that mutant SOD1 drives neurodegeneration in a non-cell autonomous manner by which pathogenic astrocytes, oligodendrocytes and microglia are toxic to motor neurons. The destructive impact of ALS underscores the urgent need for the development of new therapies that effectively treat the underlying cause of this disorder. Gene therapy holds great promise for the treatment of many human diseases and is a potentially powerful approach for combating neurodegeneration. To date, proof-of-principle studies have indicated that viral vector-mediated silencing of the mutant SOD1 gene in motor neurons and astrocytes can increase lifespan in mouse models of ALS. However, these approaches have been limited by the incomplete nature of the treatment. The use of site-specific DNA endonucleases for therapeutic purposes represents a potentially paradigm shifting opportunity to address ALS from the perspective of gene therapy. Unlike conventional methods, which only address disease symptoms, engineered nucleases are capable of correcting the underlying cause of the disorder, thereby permanently eliminating the symptoms via genome modification. The goal of the proposed research is to develop a gene therapy for ALS based on nuclease-mediated knockout of the SOD1 gene in vivo. Targeted disruption of mutant SOD1 via genome editing, in conjugation with delivery of a replacement SOD1 gene, will be used to delay the onset of paralysis, improve motor function, and extend survival in mouse models of ALS. Adeno-associated virus 9 (AAV9), which crosses the blood-brain barrier in neonatal mice via systemic injection, will be used to deliver the genome editing cargo to motor neurons and astrocytes. To ensure efficient delivery to ALS-affected cells, directed evolution will be performed to generate new AAV vectors with enhanced targeting capabilities. These studies will demonstrate the feasibility of therapeutic genome editing for treatment of ALS and lay the groundwork for future clinical translation.