Long-offset paired nicking-based efficient and precise strategy for in vivo targeted insertion.

Clustered regularly interspaced short palindromic repeat (CRISPR)-based targeted insertion of DNA fragments holds great promise for gene therapy. However, designing highly efficient and precise integration of large DNA segments in somatic cells while avoiding unpredictable products remains challenging. Here, we devised a novel long-offset paired nicking target integration (LOTI) strategy, which enhances the capacity of Cas9 nickase (Cas9n) in targeted gene integration in somatic cells, yielding higher knock-in (KI) efficiency compared with classical nickase-based approaches. The underlying repair mechanism involves the DNA repair proteins Rad51 and Rad52, and Ligase I/III. Moreover, we achieved efficient KI of at least 1.5-kb gene fragments in hepatocytes and recovery 55% FIX activity in a hemophilia B mouse model using only one-dose plasmid DNA delivery. Compared with the Cas9-based strategy, LOTI reduces off-target activity and restricts the formulation of unwanted insertions and deletions (indels) at the target site. Thus, LOTI provides a precise and efficient strategy for gene integration in somatic cells in vivo.