PerspectiveGenome Editing

When genome editing goes off-target

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Science  19 Apr 2019:
Vol. 364, Issue 6437, pp. 234-236
DOI: 10.1126/science.aax1827

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Summary

Editing DNA in eukaryotic cells with CRISPR-based systems has revolutionized the genome engineering field. Cas (CRISPR-associated) endonucleases are directed to a particular location in the genome by a short guide RNA, providing an easily programmable strategy to target any section of DNA. As of now, two CRISPR-based approaches can introduce targeted, permanent edits. DNA cleavage with the Cas endonuclease facilitates small insertions or deletions of nucleotides that can disable the targeted gene (1). A second modified “base editor” system can generate precise single-base mutations in the targeted DNA (2). For both approaches, it is imperative that DNA modifications are made in the intended region (“on-target”) and not elsewhere in the genome (“off-target”). On pages 286, 289, and 292 of this issue, Wienert et al. (3), Zuo et al. (4), and Jin et al. (5), respectively, describe methods that identify off-target activities, which will be invaluable in therapeutic contexts as well as for stringent evaluation of future iterations of gene-editing tools.