BE3 Cytosine Base Editor

First cytosine base editor enabling C-to-T conversion at protospacer positions 4–8 without double-strand DNA breaks. Achieves ~30% editing efficiency with ~1.1% indels in human cells using APOBEC1 deaminase fused to Cas9 D10A nickase. Essential reference for benchmarking against optimized editors (BE4max, ABEmax) and for studying CBE mechanism and constraints.

Length: 1000 bp(333 aa)

Type: Cytosine Base Editor

Conversion: C-to-T

Editing window: Positions 4-8

Origin: Rat APOBEC1 cytidine deaminase

Architecture

APOBEC1 cytidine deaminase fused to Cas9 D10A nickase with uracil glycosylase inhibitor (UGI). Nickase creates single-strand break opposite edited base to bias repair toward edited strand.

Characteristics

Converts C to T at positions 4-8 within protospacer with average 30% efficiency in human cells. Single UGI domain protects edited uracil from excision. Indel formation averages 1.1% across genomic targets.

Applications

Pioneering tool for C-to-T transition mutations in research applications. Validated across multiple mammalian cell lines for proof-of-concept base editing. Foundation for understanding base editing mechanisms and constraints.

Limitations

Lower efficiency than fourth-generation editors like BE4max. Can generate bystander edits at multiple cytosines within editing window. Higher indel rates and C-to-non-T byproducts than optimized successors.

Literature References

  1. Komor et al. (2016). Programmable editing of a target base in genomic DNA without double-stranded DNA cleavage. Nature - Komor 2016 BE3