Project description:Current base editors use DNA deaminases, including cytidine deaminase in cytidine base editor (CBE) or adenine deaminase in adenine base editor (ABE), to facilitate transition nucleotide substitutions. Combining CBE or ABE with glycosylase enzymes can induce limited transversion mutations. Nonetheless, a critical demand remains for base editors capable of generating alternative mutation types, such as T>G corrections. In this study, we leveraged pre-trained protein language models to optimize a uracil-N-glycosylase (UNG) variant with altered specificity for thymines (eTDG). Notably, after two rounds of testing fewer than 50 top-ranking variants, more than 50% exhibited over 1.5-fold enhancement in enzymatic activities. When eTDG was fused with nCas9, it induced programmable T-to-S (G/C) substitutions and corrected db/db diabetic mutation in mice (up to 55%). Our findings not only establish orthogonal strategies for developing novel base editors, but also demonstrate the capacities of protein language models for optimizing enzymes without extensive task-specific training data.
Project description:Adenine and cytosine base editors (ABEs and CBEs) represent a new genome editing technology that allows the programmable installation of A-to-G or C-to-T alterations on DNA. We engineered Streptococcus pyogenes Cas9-based adenine and cytosine base editor (SpACE) that enables efficient simultaneous introduction of A-to-G and C-to-T substitutions in the same base editing window on DNA.
Project description:CRISPR-Cas base editor technology enables targeted nucleotide alterations and is being rapidly deployed for research and potential therapeutic applications. The most widely used base editors induce DNA cytosine (C) deamination with rat APOBEC1 (rAPOBEC1) enzyme, which is targeted by a linked Cas protein-guide RNA (gRNA) complex. Previous studies of cytosine base editor (CBE) specificity have identified off-target DNA edits in human cells. Here we show that a CBE with rAPOBEC1 can cause extensive transcriptome-wide RNA cytosine deamination in human cells, inducing tens of thousands of C-to-uracil (U) edits with frequencies ranging from 0.07% to 100% in 38% - 58% of expressed genes. CBE-induced RNA edits occur in both protein-coding and non-protein-coding sequences and generate missense, nonsense, splice site, 5’ UTR, and 3’ UTR mutations. We engineered two CBE variants bearing rAPOBEC1 mutations that substantially decrease the numbers of RNA edits (reductions of >390-fold and >3,800-fold) in human cells. These variants also showed more precise on-target DNA editing and, with the majority of gRNAs tested, editing efficiencies comparable to those observed with wild-type CBE. Finally, we show that recently described adenine base editors (ABEs) can also induce transcriptome-wide RNA edits. These results have important implications for the research and therapeutic uses of base editors, illustrate the feasibility of engineering improved variants with reduced RNA editing activities, and suggest the need to more fully define and characterize the RNA off-target effects of deaminase enzymes in base editor platforms. This SuperSeries is composed of the SubSeries listed below.