Ligase-mediated programmable genomic integration (L-PGI): an efficient site-specific gene editing system that overcomes the limitations of reverse transcriptase-based editing systems

Since their discovery, CRISPR/Cas9 systems have been repurposed for programmable targeted genomic editing. This has led to unprecedented advancement of gene editing for therapeutic benefit. Initial uses of CRISPR/Cas9 were focused on gene disruption via DNA cleavage, but significant engineering led to systems for single base editing as well as insertion, deletion and manipulation of short stretches of genomic sequences using nicking Cas9 and RT-based methods. These technologies allowed safer and more precise editing but were limited to small corrections and showed significantly reduced efficiencies in nondividing cells, presenting difficulty for translation to in vivo therapies. To find an alternate editing strategy that could address these shortcomings, we revisited the mechanism of DNA nicking by nCas9. nCas9 nicking creates a free 5’ phosphate group and a 3’ hydroxyl group on the complementary strand of the target sequence. Under ordinary conditions in the cell these ends are re-joined by endogenously expressed ligases to repair DNA back to wild-type. If, however, a DNA fragment containing the desired edit were present, ligation of the nicked genomic DNA with the delivered fragment could result in gene editing. We demonstrate that optimization of each component and introduction of a chemically modified high affinity splinting DNA allows a variety of ligase-based edits, including longer edits not efficient with RT-based systems, at high efficiencies and fidelities that minimize genomic byproducts in both dividing and nondividing cells as well as in vivo in adult mice. Here we present the first therapeutically relevant ligation-based programmable gene editing technology, L-PGI.