Evaluation and improvement of nuclease cleavage specificity
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 10 11 DNA sequences for their ability to be cleaved by active, dimeric nucleases, 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. 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 site-specific 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 site-specific nuclease comprises two monomers, each of which comprises a site-specific DNA binding domain and a cleavage domain; wherein each of the one or more double-stranded target sites of the dimeric site-specific nuclease comprises a 5′-[left-half site]-[spacer sequence]-[right-half site]-3′ (LSR) structure, and wherein the left-half site comprises a nucleic acid sequence that is bound by the site-specific DNA binding domain of one monomer of the dimeric site-specific nuclease and the right-half site comprises a nucleic acid sequence that is bound by the site-specific DNA binding domain of other monomer of the dimeric site-specific nuclease, and wherein the spacer sequence comprises a cleavage site of the dimeric site-specific nuclease;
(b) contacting the dimeric site-specific nuclease of step (a) with a library of candidate nucleic acid molecules, wherein each of the candidate nucleic acid molecules of the library comprises a concatemer containing multiple copies of the same DNA sequences linked in a series, wherein each copy of the same DNA sequences comprises one of the double-stranded target sites of the dimeric site-specific nuclease and a constant insert sequence, wherein the contacting step is done under conditions suitable for the dimeric site-specific nuclease to cut the candidate nucleic acid molecules once, twice, or multiple times, thereby releasing double-stranded fragments with 5′ overhangs from the concatemer, whereby each strand of the double-stranded fragments has a 5′ overhang, each of the double-stranded fragments comprises a first strand comprising the constant insert sequence flanked at the 5′ end by the right-half site and a part of the spacer sequence and a complementary strand of each of the double stranded fragments comprises the constant insert sequence flanked at the 5′ end by the left-half site and a part of the spacer sequence, wherein the double-stranded fragments with 5′ overhangs are released when the candidate nucleic acid molecules are cut twice by the dimeric site-specific nuclease;
(c) filling in the 5′ overhangs of the double-stranded fragments released in step (b), thereby creating fragments with blunt ends; and
(d) identifying the one or more double-stranded target sites of the dimeric site-specific nuclease of the double-stranded fragments with 5′ overhangs by determining the sequence of the fragments with blunt ends from step (c).
2. The method of claim 1 , wherein step (d) further comprises ligating sequencing adapters to the blunt ends of the fragments with blunt ends.
3. The method of claim 2 , wherein the method further comprises amplifying the fragments with blunt ends via a polymerase chain reaction (PCR) after said ligating the sequencing adapters to the blunt ends of the fragments with blunt ends.
4. The method of claim 1 further comprising a step of enriching the double-stranded fragments with 5′ overhangs released in step (b).
5. The method of claim 4 , wherein the step of enriching the double-stranded fragments with 5′ overhangs comprises a size fractionation of the double-stranded fragments with 5′ overhangs.
6. The method of claim 1 , further comprising compiling the one or more double-stranded target sites of the dimeric site-specific nuclease identified in step (d), thereby generating a double-stranded nuclease target site profile.
7. The method of claim 1 , wherein the dimeric site-specific nuclease is a therapeutic nuclease which cuts the one or more double-stranded target sites in a gene associated with a disease.
8. The method of claim 7 further comprising determining a maximum concentration of the therapeutic nuclease at which the therapeutic nuclease cuts only one of the double-stranded nuclease target sites, and does not cut more than 10, more than 5, more than 4, more than 3, more than 2, more than 1 of the double-stranded target sites in the candidate nucleic acid molecules.
9. The method of claim 1 , wherein the cleavage domain is a non-specific cleavage domain.
10. The method of claim 1 , wherein the dimeric site-specific nuclease cleaves a target sequence upon dimerization of the cleavage domain of each of the two monomers.
11. The method of claim 1 , wherein the site-specific DNA binding domain comprises a zinc finger.
12. The method of claim 11 , wherein the dimeric site-specific nuclease is a Zinc Finger Nuclease.
13. The method of claim 1 , wherein the site-specific DNA binding domain comprises a Transcriptional Activator-Like Element.
14. The method of claim 13 , wherein the dimeric site-specific nuclease is a Transcriptional Activator-Like Element Nuclease (TALEN).
15. The method of claim 1 , wherein the cleavage domain is a cleavage domain of an endonuclease.
16. The method of claim 15 , wherein the endonuclease is FokI.
17. The method of claim 1 , wherein the dimeric site-specific nuclease is a homing nuclease.