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. Author manuscript; available in PMC: 2021 Aug 5.
Published in final edited form as: Gene Genome Ed. 2021 Apr 24;1:100005. doi: 10.1016/j.ggedit.2021.100005

Figure 1: Overview of DNA base editing technologies.

Figure 1:

A) DNA base editors are generally protein fusion constructs consisting of a catalytically impaired Cas nuclease (dCas9, dCas12, or Cas9n) linked to a ssDNA-specific nucleobase modifying enzyme. (1) Binding of a base editor to its target locus forms a DNA/RNA hybrid R-loop, which exposes the target nucleotide X in a stretch of ssDNA outside of the Cas protein. (2) The ssDNA nucleobase modifying enzyme now chemically modifies its target base X into X*, a noncanonical nucleotide that serves as a mutagenic intermediate. (3) X* is further processed by cellular repair or replication machinery, resulting in its conversion to P. For CBEs, X•Z, X*•Z and P•Q are C•G, U•G, and T•A, respectively. For ABEs, they are A•T, I•T, and G•C, respectively. B) The ssDNA nucleobase modifying enzymes used in CBEs are cytidine deaminases, which chemically modify deoxycytidine to deoxyuridine via hydro-deamination chemistry. C) Adenosine deaminases are used in ABEs, which chemically modify deoxyadenosine to deoxyinosine via hydro-deamination chemistry. D) The unique DNA damage intermediate harnessed for base editing, featuring a modified nucleotide(X*)-containing mismatch and a nick located 3 to 14 nucleotides away in the 5’ direction from the mismatched site on the unmodified strand. E) DdCBEs are constructed by fusing two halves of a dsDNA cytidine deaminase DddA to separate TALE proteins that bind at neighboring “half sites”.