Evaluation and improvement of nuclease cleavage specificity
Engineered nucleases 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 10 11 DNA sequences for their ability to be cleaved by nucleases. The method revealed hundreds of thousands of DNA sequences 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.
1. A method comprising
(a) providing a dimeric nuclease that cuts one or more double-stranded target sites of a double-stranded nucleic acid and creates a 5′ overhang on the double-stranded nucleic acid, wherein the dimeric nuclease comprises two nuclease monomers, and wherein each of the target sites comprises
(i) a left-half site, wherein the left-half site comprises a nucleic acid sequence that is bound by one monomer of the dimeric nuclease;
(ii) a right-half site, wherein the right-half site comprises a nucleic acid sequence that is bound by other monomer of the dimeric nuclease; and
(iii) a spacer sequence between the left-half site and the right-half site;
wherein the left-half site, the spacer sequence, and right-half site forms a 5′-[the left-half site]-[the spacer sequence]-[the right-half site]-3′ (LSR) structure, and the cleavage site of the dimeric nuclease is located within the spacer sequence;
(b) contacting the dimeric nuclease with a library of candidate nucleic acid molecules, wherein each of the candidate nucleic acid molecules comprises a concatemer containing multiple copies of identical DNA sequences, a plurality of the target sites and multiple constant insert sequences, each of the constant insert sequences is located between two target sites of the plurality of the target sites, and each of the multiple copies of the identical DNA sequences comprises a target site of the plurality of the target sites and a constant insert sequence of the multiple constant insert sequences, under conditions suitable for the dimeric nuclease to cut a candidate nucleic acid molecule of the library of candidate nucleic acid molecules; thereby generating one or more candidate nucleic acid molecules cut once, twice and multiple times by the dimeric nuclease, wherein the candidate nucleic acid molecules cut twice by the dimeric nuclease comprise a 5′ overhang and the constant insert sequence flanked by the left half-site and a part of the spacer sequence from one of the plurality of the target sites, and flanked by the right half-site and a part of the spacer sequence from another of the plurality of the target sites,
(c) filling in the 5′ overhang of each of the one or more candidate nucleic acid molecules cut twice by the dimeric nuclease, thereby creating one or more candidate nucleic acid molecules with blunt ends; and
(d) identifying the one or more target sites of the one or more candidate nucleic acid molecules cut twice by the dimeric nuclease by determining the sequence of the one or more candidate nucleic acid molecules with blunt ends created in step (c).
2. The method of claim 1 , wherein step (d) further comprises ligating sequencing adapters to the blunt ends of the one or more candidate nucleic acid molecules with blunt ends and amplifying and/or sequencing the one or more candidate nucleic acid molecules with blunt ends.
3. The method of claim 2 , wherein said amplifying the one or more candidate nucleic acid molecules with blunt ends is performed via PCR after said ligating the sequencing adapters.
4. The method of claim 1 , further comprises a step of enriching more than one of the candidate nucleic acid molecules with blunt ends.
5. The method of claim 4 , wherein the step of enriching the more than one of the candidate nucleic acid molecules with blunt ends comprises a size fractionation of the more than one of the candidate nucleic acid molecules with blunt ends.
6. The method of claim 5 , wherein the size fractionation is done by gel purification.
7. The method of claim 1 , wherein the nuclease is a homing endonuclease.
8. The method of claim 1 , further comprising compiling more than one of the target sites identified in step (d), thereby generating a nuclease target site profile.
9. The method of claim 1 , wherein the nuclease is a therapeutic nuclease which cuts one or more specific nuclease target sites in a gene associated with a disease.
10. The method of claim 9 further comprising determining a maximum concentration of the therapeutic nuclease at which the therapeutic nuclease cuts the specific nuclease target sites, and does not cut more than 10 of the specific nuclease target sites in a genome.
11. The method of claim 9 further comprising determining a maximum concentration of the therapeutic nuclease at which the therapeutic nuclease cuts the specific nuclease target sites, and does not cut more than 5 of the specific target sites in a genome.
12. The method of claim 9 further comprising determining a maximum concentration of the therapeutic nuclease at which the therapeutic nuclease cuts the specific nuclease target sites, and does not cut more than 2 of the specific target sites in a genome.
13. The method of claim 9 further comprising determining a maximum concentration of the therapeutic nuclease at which the therapeutic nuclease cuts a specific nuclease target site of the specific nuclease target sites, and does not cut more than 1 of the specific target sites in a genome.
14. The method of claim 9 further comprising determining a maximum concentration of the therapeutic nuclease at which the therapeutic nuclease cuts a specific nuclease target site of the specific nuclease target sites, and does not cut any additional specific nuclease target site of the specific nuclease target sites in a genome.
15. The method of claim 1 , wherein the nuclease comprises an unspecific nucleic acid cleavage domain.
16. The method of claim 1 , wherein the nuclease comprises a FokI cleavage domain.
17. The method of claim 1 , wherein the nuclease comprises a nucleic acid cleavage domain that cleaves a target sequence upon dimerization of the nucleic acid cleavage domain.
18. The method of claim 1 , wherein the nuclease comprises a binding domain that specifically binds a nucleic acid sequence.
19. The method of claim 18 , wherein the binding domain comprises a zinc finger.
20. The method of claim 19 , wherein the binding domain comprises at least 2 zinc fingers.
21. The method of claim 19 , wherein the binding domain comprises at least 3 zinc fingers.
22. The method of claim 19 , wherein the binding domain comprises at least 4 zinc fingers.
23. The method of claim 19 , wherein the binding domain comprises at least 5 zinc fingers.
24. The method of claim 18 , wherein the binding domain comprises a Transcriptional Activator-Like Element.
25. The method of claim 1 , wherein the nuclease is a Zinc Finger Nuclease.
26. The method of claim 1 , wherein the nuclease is a Transcriptional Activator-Like Element Nuclease (TALEN).