| The CRISPR/Cas(Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR associated)system could induce efficient DNA editing at target sites and has become the most widely used genome editing tool in various organisms.However,undesired byproducts and chromosomal instability resulting from DNA double strand breaks(DSBs)generated by CRISPR-Cas,also raise many concerns.To overcome these issues,base editors(BEs)and prime editors(PEs)were developed by fusing deaminase enzymes or reverse transcriptase with Cas9 nickase.Two classes of base editors were mainly included:cytosine base editors(CBEs)and adenine base editors(ABEs).ABEs mediate efficient A·T-to-G·C conversions without creating DSBs and requiring the DNA donor template,and could theoretically correct nearly half of the pathogenic point mutations,showing great potential in the treatment of genetic diseases.However,the applications of ABEs are still hindered by undesired editing efficiency,limited editing scopes,and off-targeting effects.To further expand the application scopes of adenine base editing tools,we developed a new near PAMless ABE with high editing activity but low off-target activity based on ABEmax-SpRY,and preliminarily explored its potentials in editing disease-associated mutations.We firstly engineered 8e-SpRY with high editing activities by replacing the adenine deaminase domain of ABEmax-SpRY with TadA-8e,then four 8e-SpRY variants were developed by introducing mutations or Cas-embedding strategy.Among them,CE-8eSpRY,generated by inserting TadA-8e into the tolerant site of SpRY-Cas9,induced a little higher editing activities than 8e-SpRY;V106W-SpRY,engineered by installing V106W mutation in deaminase domain,mediated a little lower editing activities relative to 8eSpRY;while the other two 8e-SpRY variants exhibited significantly reduced editing activities in comparison to 8e-SpRY.Further analysis revealed that three 8e-SpRY variants with comparable activities displayed higher editing activities at sites with NRN(R refers to A or G)than with NYN(Y refers to C or T)PAMs,in line with SpRY nuclease;moreover.8e-SpRY variants performed efficient editing in a wider activity window(positions 3-10)than conventional ABEs(positions 4-7),enabling a wider range of nucleotides editable.We next compared the off-target effects among developed SpRY-ABEs.In terms of RNA off-target effects.CE-8e-SpRY induced the lowest off-target editing at transcriptome level;when it concerns to sgRNA-dependent DNA off-targets,CE-8e-SpRY exhibited lower editing activities than 8e-SpRY;as to the sgRNA-independent DNA off-targets,CE8e-SpRY only generated a little higher unguided DNA editing than SpRY-Cas9 nuclease.Therefore,CE-8e-SpRY was considered as the best SpRY-derived ABE with high on-target but low off-target editing activities.Then,CE-8e-SpRY was adopted to edit the disease-relevant mutations in cells.Results suggested CE-8e-SpRY could induce efficient editing at sites which were outside the activity windows of conventional ABEs,moreover,specific editing requirements,like precise editing,or editing with the highest efficiency or specificity,could be achieved through sgRNA optimization.Finally,to enable efficient editing in vivo,we optimized the dual AAV delivery system of CE-8e-SpRY(Split-CE)and found Split573-CE displayed higher editing activities.Besides,the promotor of Split573-CE was also optimized to improve the tissue specificity and P3 promotor might have a better liver specificity for Split-CE.In summary,we developed a new SpRY-ABE with high on-target but low off-target editing activities,named CE-8e-SpRY.CE-8e-SpRY could expand the editing scope of adenine base editing tools and meet different requirements raised by target genome editing.Moreover,our optimized AAV-P3-CE-8e-SpRY system laid the foundation for efficient somatic editing in liver. |
