An Extension Study to Assess the Long-Term Safety, Tolerability, Pharmacokinetics, and Effect on Disease Progression of BIIB067 Administered to Previously Treated Adults with Amyotrophic Lateral Sclerosis Caused by Superoxide Dismutase 1 Mutation

Key Inclusion Criteria:

• Must have diagnosis of superoxide dismutase 1-amyotrophic lateral sclerosis (SOD1-ALS), and must have completed the End of Study Visit for either Parts A, B, or C of Study 233AS101 (NCT02623699) (i.e., were not withdrawn).
• If taking riluzole, participant must be receiving a stable dose for ≥30 days prior to Day 1.
• If taking edaravone, participant must have initiated edaravone ≥60 days (2 treatment cycles) prior to Day 1. Edaravone may not be administered on dosing days during this study.
• Medically able to undergo the study procedures, and to adhere to the visit schedule at the time of study entry, as determined by the Investigator.
• For female participants of childbearing potential must agree to practice effective contraception during the study and be willing and able to continue contraception for 5 months after their last dose of study treatment.
• Participants from Study 233AS101 Parts A and B must have a washout ≥16 weeks between the last dose of study treatment received in Study 233AS101 and the first dose of BIIB067 received in the current Study 233AS102.

Key Exclusion Criteria:

• History of allergies to a broad range of anesthetics.
• Presence of risk for increased or uncontrolled bleeding and/or risk of bleeding that is not managed optimally and could place a participant at an increased risk for bleeding during or after a Lumbar Puncture (LP) procedure. These risks could include, but are not limited to, anatomical factors at or near the LP site (e.g., vascular abnormalities, neoplasms, or other abnormalities) and underlying disorders of the coagulation cascade, platelet function, or platelet count (e.g., hemophilia, Von Willebrand's disease, liver disease).
• Presence of an implanted shunt for the drainage of CSF or an implanted central nervous system (CNS) catheter.
• Prior or current treatment with small interfering ribonucleic acid (RNA), stem cell therapy, or gene therapy.
• Treatment with another investigational drug, biological agent (excluding BIIB067), or device within 1 month or 5 half-lives of study agent, whichever is longer.
• Current or anticipated need, in the opinion of the Investigator, of a diaphragm pacing system (DPS) during the study period.
• Current or recent (within 1 month) use, or anticipated need, in the opinion of the Investigator, of copper (II) (diacetyl-bis(N4-methylthiosemicarbazone)) or pyrimethamine.
• Female participants who are pregnant or currently breastfeeding.
• Current enrollment in any other interventional study.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply