Building CRISPR Gene Therapies for the Central Nervous System

Importance: Gene editing using clustered regularly interspaced short palindromic repeats (CRISPR) holds the promise to arrest or cure monogenic disease if it can be determined which genetic change to create without inducing unintended cellular dysfunction and how to deliver this technology to the target organ reliably and safely. Clinical trials for blood and liver disorders, for which delivery of CRISPR is not limiting, show promise, yet no trials have begun for central nervous system (CNS) indications. Observations: The CNS is arguably the most challenging target given its innate exclusion of large molecules and its defenses against bacterial invasion (from which CRISPR originates). Herein, the types of CRISPR editing (DNA cutting, base editing, and templated repair) and how these are applied to different genetic variants are summarized. The challenges of delivering genome editors to the CNS, including the viral and nonviral delivery vehicles that may ultimately circumvent these challenges, are discussed. Also, ways to minimize the potential in vivo genotoxic effects of genome editors through delivery vehicle design and preclinical off-target testing are considered. The ethical considerations of germline editing, a potential off-target outcome of any gene editing therapy, are explored. The unique regulatory challenges of a human-specific therapy that cannot be derisked solely in animal models are also discussed. Conclusions and Relevance: An understanding of both the potential benefits and challenges of CRISPR gene therapy better informs the scientific, clinical, regulatory, and timeline considerations of developing CRISPR gene therapy for neurologic diseases.