Method for synthesizing a nuclease with reduced off-site cleavage
Endonucleases play an essential role in genetic engineering and molecular biology. A major barrier to the clinical adoption of nucleases and engineered nucleases remain the overall lack of specificity and activity. Off-site cleavage, cleavage at loci other than the target loci, typically occurs resulting in mutations, unexpected gene-knockouts, or translocations. Provided herein are systems and methods for identifying the off-site cleavage loci and predicting the activity of engineered endonucleases for a given genome. It is expected that these tools and methods will be useful for designing nucleases and other related DNA binding domains (e.g. TAL effectors) for genomic therapy and engineering.
1. A method for synthesizing a nuclease with reduced off-site cleavage comprising:
scanning complete genomic sequence data for the off-target cleavage locations of each nuclease from a group of nucleases to return off-target cleavage locations in the genome, wherein scanning comprises iteratively comparing a user-provided target site to the genomic sequence to identify mismatches between the target site and the genomic sequence, wherein the target site comprises a left half and a right half;
locating all potential cleavage sites of each nuclease including mismatches in the left and right half target sites,
assigning a score to each nuclease based upon the returned off-target cleavage locations indicative of the predictive likelihood of off-target cleavage wherein the score is calculated using the algorithm
SCORE H =( N MAX +1− N L ) 2 +( N MAX +1− N R ) 2
wherein N L and N R are the number of mismatches in the left and right half target sites respectively and N MAX is the maximum number of mismatches allowed per half site;
ranking the nucleases using the score calculated based upon the off-target cleavage locations, wherein a higher score indicates a nuclease with more potential off-target cleavage sites compared to a nuclease with a lower score;
synthesizing the nuclease with the lowest number of predicted off-target cleavage locations;
contacting the synthesized nuclease with the genomic sequence;
and assaying the genomic sequence after nuclease treatment to ensure specific genome editing,
wherein scanning the genomic sequence, locating cleavage sites, assigning a score, and ranking the off-target cleavage locations are performed in a computer system.
2. The method of claim 1 , wherein the nuclease uses a guide RNA strand, such as the CRISPR, and/or Cas9 systems.
3. The method of claim 1 , further comprising the step of designating the off-target cleavage location as being in an exon, intron, promoter or regulatory, or intergenic region.