Editors' ChoiceGenome Editing

Both sides of the CRISPR coin

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Science Translational Medicine  27 Feb 2019:
Vol. 11, Issue 481, eaaw5332
DOI: 10.1126/scitranslmed.aaw5332

Abstract

Examination of the long-term effects of genome editing reveals potential and perils.

The field of genome editing has been transformed by the emergence of approaches involving prokaryotic DNA sequences containing clustered regularly interspaced short palindromic repeats (CRISPR) within the last decade. Part of the appeal of CRISPR strategies is their potential to permanently alter the genome for therapeutic benefit after a single administration. However, the durability of CRISPR-induced functional effects has yet to be extensively tested, and thus the optimal delivery strategy as well as potential side effects remain unknown.

To address these issues, Nelson et. al. explored the long-term impact of virally delivered CRISPR designed to restore expression of the protein dystrophin in a mouse model of Duchenne muscular dystrophy (DMD). This combination of delivery mechanism and therapeutic target has substantial translational relevance: DMD is a highly-touted target for genome editing that has been the focus of considerable efforts involving a variety of technologies, including CRISPR, and the viral vectors used in these studies, adeno-associated viruses (AAVs), are used in US Food and Drug Administration–approved gene therapy products. The authors assessed effects of AAV-CRISPR delivered by intramuscular injection in adult mice as well as intravenous delivery in neonates. Although total genome editing in adults was not sustained between eight weeks and six months post-injection, neonates exhibited sustained editing in the heart, tibialis anterior, and diaphragm for up to one year. Consistent restoration of dystrophin expression was verified one year after AAV-CRISPR by immunoblot.

Critically, the authors then went on to evaluate potential side effects resulting from this intervention. After systemic delivery in neonates, genome editing was detected in several non-muscle tissues, including liver, spleen, kidney, brain, and testis. Further, although neonates did not exhibit detectable humoral immune response to AAV-CRISPR, intramuscularly and intravenously injected adults formed antibodies against the virally encoded enzymatic component of the therapy. Lastly, using an adapted deep sequencing approach to assess all possible editing outcomes in an unbiased manner, the authors found that unintended genome modifications were also persistent in treated mice.

This report establishes a new benchmark of persistence for therapeutic genome editing. In addition, it highlights crucial considerations that must be made to eventually enable clinical translation of this approach. Specifically, the immunogenicity of AAV-CRISPR in adults and the potential for unintended editing must be addressed and overcome. The combination of these findings should inform future studies toward bringing genome editing treatments for DMD and other genetic diseases closer to clinical reality.

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