EVALUATION AND IMPROVEMENT OF NUCLEASE CLEAVAGE SPECIFICITY

摘要

Engineered nucleases (e.g., zinc finger nucleases (ZFNs), transcriptional activator-like effector nucleases (TALENs), and others) are promising tools for genome manipulation and determining off-target cleavage sites of these enzymes is of great interest. We developed an in vitro selection method that interrogates 1011 DNA sequences for their ability to be cleaved by active, dimeric nulceases, e.g., ZFNs and TALENs. The method revealed hundreds of thousands of DNA sequences, some present in the human genome, that can be cleaved in vitro by two ZFNs, CCR5-224 and VF2468, which target the endogenous human CCR5 and VEGF-A genes, respectively. Analysis of the identified sites in cultured human cells revealed CCR5-224-induced mutagenesis at nine off-target loci. Similarly, we observed 31 off-target sites cleaved by VF2468 in cultured human cells. Our findings establish an energy compensation model of ZFN specificity in which excess binding energy contributes to off-target ZFN cleavage and suggest strategies for the improvement of future nuclease design. It was also observed that TALENs can achieve cleavage specificity similar to or higher than that observed in ZFNs.

主权项

1. A method for identifying a target site of a nuclease, the method comprising
(a) providing a nuclease that cuts a double-stranded nucleic acid target site and creates a 5′ overhang, wherein the target site comprises a [left-half site]-[spacer sequence]-[right-half site] (LSR) structure, and the nuclease cuts the target site within the spacer sequence; (b) contacting the nuclease with a library of candidate nucleic acid molecules, wherein each nucleic acid molecule comprises a concatemer of a sequence comprising a candidate nuclease target site and a constant insert sequence, under conditions suitable for the nuclease to cut a candidate nucleic acid molecule comprising a target site of the nuclease; (c) filling in the 5′ overhangs of a nucleic acid molecule that has been cut twice by the nuclease and comprises a constant insert sequence flanked by a left half-site and cut spacer sequence on one side, and a right half-site and cut spacer sequence on the other side, thereby creating blunt ends; (d) identifying the nuclease target site cut by the nuclease by determining the sequence of the left-half site, the right-half-site, and/or the spacer sequence of the nucleic acid molecule of step (c).