A Study to Evaluate the Safety and Efficacy of PTC518 in Participants With Huntington's Disease (HD)

Key Inclusion Criteria: * Genetically confirmed HD diagnosis with a cytosine-adenine-guanine (CAG) repeat length from 40 to 50, inclusive Eligibility for HD-ISS Stage 2 Group (Parts A, B, and C): * A Unified Huntington's Disease Rating Scale (UHDRS)-Independence Scale (IS) score of 100 * A UHDRS Total Functional Capacity (TFC) score of 13 * A score between 0.18 and 4.93 inclusive on the normed version of the HD prognostic index (PINHD) Eligibility for HD-ISS Mild Stage 3 Group (Parts D, E, and F): * A UHDRS Total Functional Capacity (TFC) score of 11 or 12, or a UHDRS TFC score of 13 with an UHDRS IS score of \<100 Key Exclusion Criteria: * Receipt of an experimental agent within 90 days or 5 half-lives prior to Screening or anytime over the duration of this study, ribonucleic acid (RNA)- or deoxyribonucleic acid (DNA)-targeted HD-specific investigational agents such as antisense oligonucleotides, cell transplantation, or any other experimental brain surgery * Any history of gene therapy exposure for the treatment of HD * Participation in an investigational study or investigational paradigm (such as exercise/physical activity, cognitive therapy, brain stimulation, etc) within 90 days prior to Screening or anytime over the duration of this study * Any medical history of brain or spinal disease that would interfere with the lumbar puncture process safety assessments * Any medical history or condition that would interfere with the ability to complete the protocol-specifie…