Starting in 2016, the above hybrid ‘Target-AID’ or ‘CRISPR Nickase System’ of cytosine deaminases was extensively used for developing a largenumber of plasmid vectors10, which were described as base editors(BEs). These BEs were successfully used for generating precise CG®TA mutations in mouse/ Xenopus/humanliving cells and embryos (Figure 3). In case of mouse and Xenopus, the modified embryos were also used for transplantation inpseudopregnant surrogate mothers for generating offsprings exhibiting alteredphenotypes (e.g., black to albino body colour). In plant systems also,protoplasts were successfully used for creating mutations in specific genes,and then regenerated into whole plants exhibiting desirable altered traits.
The progress made in the development and use of BEs was also summarizedin five articles that appeared in a special issue of NatureBiotechnology in May 2017 (see later). These five papers included papers on base editing in mouse embryos as well as incrops including rice, wheat, maize and tomato. The work on C®T base editing was followed by twoimportant reports that were simultaneously published in October 2017. In onereport, DNA adenine deaminases, which did not occur in nature were developed inthe laboratory using naturally occurring RNA adenine deaminases (used for tRNAprocessing), so that a series of adenine base editors (ABE) became available forconversion of adenine into inosine in a DNA molecule.
Thus it also becamepossible to replace A:T base pair by G:C (A:T®I:T®G:C; Figure3)11. In the second report, a technology for base editing of RNAtranscripts was published12. In this manner, CRISPR/Cas was modifiedand used for editing DNA/RNA molecules in the form of BEs and ABEs. This broughtmani-fold efficiencyand precision to gene editing technology.