Study of N-acetylcysteine in the Treatment of Patients With the m.3243A>G Mutation and Low Brain Glutathione Levels

N-Acetylcysteine (NAC), an anti-oxidant, will be studied to investigate the effects on brain glutathione levels, cognitive skills, motor skills, and quality of life. A group of 18 participants will take either 1800, 3600 or 5400 mg per day of N-acetylcysteine (NAC) for 3 months in this dose escalation study. The investigators want to determine first if the 3600 mg dose per day is safe and might provide some efficacy. If the 3600 mg dose is safe, then additional participants will be treated with 5400 mg per day of NAC, for up to a total of 18 participants. If the 3600 mg per day dose is unsafe, then participants will be treated with the 1800 mg per day dose. Data from this pilot study will be used to determine the most safe and effective dose of NAC for a future clinical trial.