Efficacy and long-term safety of CRISPR/Cas9 genome editing in the SOD1-linked mouse models of ALS.

CRISPR/Cas9-mediated genome editing provides potential for therapeutic development. Efficacy and long-term safety represent major concerns that remain to be adequately addressed in preclinical studies. Here we show that CRISPR/Cas9-mediated genome editing in two distinct SOD1-amyotrophic lateral sclerosis (ALS) transgenic mouse models prevented the development of ALS-like disease and pathology. The disease-linked transgene was effectively edited, with rare off-target editing events. We observed frequent large DNA deletions, ranging from a few hundred to several thousand base pairs. We determined that these large deletions were mediated by proximate identical sequences in Alu elements. No evidence of other diseases was observed beyond 2 years of age in these genome edited mice. Our data provide preclinical evidence of the efficacy and long-term safety of the CRISPR/Cas9 therapeutic approach. Moreover, the molecular mechanism of proximate identical sequences-mediated recombination provides mechanistic information to optimize therapeutic targeting design, and to avoid or minimize unintended and potentially deleterious recombination events.