GENERATION HD2. A Study to Evaluate the Safety, Biomarkers, and Efficacy of Tominersen Compared With Placebo in Participants With Prodromal and Early Manifest Huntington's Disease

Inclusion Criteria: DB Period: * HD gene expansion mutation carrier status with a cytosine-adenine-guanine-age product (CAP) score of 400-500 inclusive * Either: * Prodromal HD (defined as Diagnostic Confidence Level (DCL) 2 to 3, Independence Scale (IS) ≥70, and TFC ≥8); Or * Early manifest HD (defined as DCL 4, IS ≥70, and TFC ≥8) * Total body weight \> 40 kilograms (kg) and a body mass index (BMI) within the range of 18-32 kilograms per meter square (kg/m\^2) * Study companion OLE Period: * Participants must have completed the DB treatment period * Participants must remain in the DB Safety follow-up period until OLE period starts Exclusion Criteria: DB Period: * Current or previous use of an antisense oligonucleotide (ASO) (including small interfering ribonucleic acid \[RNA\]) or any HTT lowering therapy (including tominersen) * Anti-platelet or anticoagulant therapy within 14 days prior to screening or anticipated use during the study, including, but not limited to, aspirin (unless ≤ 81 milligrams per day \[mg/day\]), clopidogrel, dipyridamole, warfarin, dabigatran, rivaroxaban, apixaban, and heparin * History of gene therapy, cell transplantation, or brain surgery * Hydrocephalus * Pregnancy or breastfeeding, or intention of becoming pregnant during the study or within 5 months after the final dose of study drug * History of attempted suicide or suicidal ideation with plan (i.e., active suicidal ideation) that required hospital visit and/or change in level of…