Often, gene therapy reviews concentrate upon specific therapeutic modalities-particularly either viral vector-mediated or a nonviral approach. Here, we draw together a comprehensive array of knowledge across the field of genetic therapy for genetic neurological disease. The sections on preclinical and clinical application of viral vectors are followed by sections on RNA-based therapies and then by antisense oligonucleotide approaches also in preclinical and clinical settings. We present a separate section on gene editing strategies and conclude with a section elaborating on the neurosurgical techniques and the expertise required for clinical application of many of these technologies. SIGNIFICANCE STATEMENT: Genetic therapies have significant potential to treat life-limiting neurological diseases. This review examines the different approaches, clinical successes, and considerations on how to deploy them.

The pathological huntingtin ( HTT ) trinucleotide repeat underlying Huntington’s disease (HD) continues to expand throughout life. Repeat length correlates both with earlier age at onset (AaO) and faster progression, making slowing its expansion an attractive therapeutic approach. Genome-wide association studies have identified candidate variants associated with altered AaO and progression, with many found in DNA mismatch repair (MMR) associated genes. We examine whether lowering expression of these genes affects the rate of somatic expansion in human ex vivo models using HD iPSCs and HD iPSC-derived striatal neurons. We have generated a stable CRISPR interference HD iPSC line in which we can specifically and efficiently lower gene expression from a donor carrying over 125 CAG repeats. Lowering expression of each member of the MMR complexes MutS (MSH2, MSH3 & MSH6), MutL (MLH1, PMS1, PMS2 & MLH3) and LIG1 resulted in characteristic MMR deficiencies. Reduced MSH2, MSH3 and MLH1 slowed repeat expansion to the largest degree, while lowering either PMS1, PMS2 and MLH3 slowed it to a lesser degree. These effects were recapitulated in iPSC derived striatal cultures where MutL factor expression was lowered. Here, reducing the expression of MMR factors by CRISPRi to levels typically reached by current therapeutics effectively slows the pathogenic expansion of the HTT CAG repeat tract. We highlight members of the MutL family as potential therapeutic targets to slow repeat expansion with the aim to delay onset and progression of HD, and potentially other repeat expansion disorders exhibiting somatic instability. GRAPHICAL

Talpid3 is a basal body protein required for the formation of primary cilia, an organelle involved in signal transduction. Here, we asked if Talpid3 has a role in the regulation of differentiation and/or self-renewal of ES cells and whether cells lacking cilia due to a deletion in Talpid3 can be reprogrammed to induced pluripotent stem (iPS) cells. We show that mouse embryonic limb fibroblasts which lack primary cilia with a targeted deletion in the Talpid3 (Ta3) gene can be efficiently reprogrammed to iPS cells. Furthermore, vector-free Ta3−/− iPS cells retain ES cell features and are able to self-renew. However, both Ta3−/− iPS and ES cells are unable to form visceral endoderm and differentiate poorly into neurons. The observed defects are not a consequence of reprogramming since Ta3−/− ES cells also exhibit this phenotype. Thus, Talpid3 and primary cilia are required for some differentiation events but appear to be dispensable for stem cell self-renewal and reprogramming.