Open-label Extension (OLE) Study of WVE-004 in Patients With C9orf72-associated Amyotrophic Lateral Sclerosis (ALS) and/or Frontotemporal Dementia (FTD)

Inclusion Criteria: * Participants successfully completed the Phase 1b/2a study with WVE-004, WVE-004-001. * Participant has the ability and is willing to provide informed consent prior to any study procedures. In instances where signed written informed consent is unable to be obtained it is acceptable for the participant to provide consent with legally authorized representative signing on the participant's behalf. * In the opinion of the Investigator, the participant is able to tolerate all study procedures, is willing to comply with all other protocol requirements, and tolerated study drug in the parent study. * Participant is willing to practice highly effective contraception for the duration of the study and for 5 months after the last dose of study drug if the participant or their partner are of childbearing potential. In addition, willingness to forego sperm or ova (egg) donation for the duration of the study and 5 months after completion of the study. * Participant has identified a study partner(s) for the duration of the study. Exclusion Criteria: * Participant has a clinically significant medical finding on the physical examination other than C9orf72-associated ALS or FTD that, in the judgment of the Investigator or Sponsor, will make the participant unsuitable for participation in and/or completion of the trial procedures. * Participant received any other investigational drug, biological agent, or device within 1 month or 5 half-lives of study agent, whichever is longer. Participant received an investigational oligonucleotide within the past 6 months or 5 half-lives of the drug, whichever is longer. * Participant has a positive hepatitis B surface antigen or hepatitis C antibody test. * Participant who are pregnant (as determined by a serum pregnancy test) or breast feeding at the Screening visit, or plans to become pregnant during the trial. * Participants deemed to be at significant risk for suicidal behavior, based on Investigator assessment and/or active suicidal ideation. * Participant has a bone, spine, bleeding (e.g., hemophilia, Von Willebrand disease, or liver disease), or other disorder that exposes the participant to a risk of injury or unsuccessful lumbar puncture (LP). * Participant received prior treatment with viral or cellular-based gene therapy. * Participant anticipates using antiplatelet or anticoagulant therapy during the course of the study. Participants who received antiplatelet or anticoagulant therapy must complete a washout period before the Screening visit. * Participant was noncompliant, in the opinion of the Investigator or Sponsor, when participating in study WVE-004-001. * Participant is directly or indirectly involved in the conduct and administration of this trial as an Investigator, sub-investigator, trial coordinator, or other trial staff member, or the participant is a first-degree family member, significant other, or relative residing with one of the above persons involved directly or indirectly in the trial.